NATIONAL
CONFERENCE ON
KMNtN
Management
Nitrogen in
Irrigated
Agriculture
MAY 15-18, 1978
Sacramento, California
Sponsored by
U.S. Environmental Protection Agency
National Science Foundation
University of California
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PROCEEDINGS NATIONAL CONFERENCE ON
MANAGEMENT OF NITROGEN IN IRRIGATED
AGRICULTURE
Edited by:
P. F. Pratt
Professor of Soil Science
Department of Soil and Environmental Sciences
University of California
Riverside, California 92521
Sponsored by:
U.S. National Science Foundation
U.S. Environmental Protection Agency
The University of California
May 15-18, 1978
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The opinions expressed herein are those of
the authors and do not necessarily represent
official policies of the sponsoring organizations
Printed in the United States of America
Library of Congress Catalog Card No. 78-56958
Copies may be obtained at a cost of $13 each from:
University Bookstore
University of California
Riverside, California 92521
Published by:
The Department of Soil and Environmental Sciences
University of California
Riverside, California 92521
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PREFACE
Traditionally, the main interests in management of nitrogen in
irrigated agriculture have been related to agronomic and economic
matters. Passage of the Federal Water Pollution Control Act in 1965
broadened the scope of interest in nitrogen management substantially
by introducing environmental concerns. It soon became apparent, how-
ever, that there were some important gaps in our knowledge that would
have to be closed in order to deal effectively with these expanded
interests. If nitrogen were to be managed in a manner that would
meet both the traditional needs of growers and the environmental con-
cerns of the general public, new information would have to be develop-
ed by research. Specifically, the extent of nitrogen losses from
irrigated lands and the mechanisms involved were not adequately under-
stood to develop best management practices.
In the intervening years, numerous grants for research on these
matters were made by the U.S. Environmental Protection Agency's
Office of Research and Development, and by the National Science
Foundation to universities, federal government agencies, and private
research groups. Linked with these funds was the understanding that
the research programs would include a technology transfer activity
for the benefit of potential users. Results of the research have
been published in various journals and have been presented at meet-
ings with local or regional focus.
These Proceedings represent a National Conference on Nitrogen
Management in Irrigated Lands, designed to bring together users
representing a diversity of interests throughout the country for an
integrated review of the findings from the comprehensive national
research effort. The accumulated information needs to be utilized
as fully as possible by the many individuals and organizations making
decisions in current water quality planning. This will help ensure
the emergence of appropriate and viable solutions to our water quality
iii
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problems — solutions that are sensitive to both local and national
needs and priorities.
The program planning committee for the Conference consisted of
the following:
R. L. Branson
Cooperative Extension
University of California
Riverside, CA
D. R. Fox
CK M Hill
Redding, CA
Carl E. Franzoy
Agricultural Technology Co.
Tempe, AZ
A. G. Hornsby
U.S. Environmental Protection
Agency
Ada, OK
8, D. Meyer
Cooperative Extension
University of California
Davis, CA
L. K. Porter
ARS-USDA
Fort Collins, CO
P. F. Pratt
Dept. of Soil and Environ-
mental Sciences
University of California
Riverside, CA
R. S. Rauschkolb
Cooperative Extension
University of California
Davis, CA
W. C. White
The Fertilizer Institute
Washington, D.C.
Norman Whittlesey
Dept. of Agricultural
Economics
Washington State University
Pullman, WA
The financial support of the National Science Foundation through
Grant No. ENV76-10283 AOl and the tj.S. Environmental Protection Agency
through Grant No. R805394-1, is hereby gratefully acknowledged. With-
out this support the Conference could not have been held and these
proceedings could not have been produced. We are indebted to each
author for submitting manuscripts and for making presentations during
the Conference. We appreciate the help of Winnie Gillette and Art
Laag in the preparation of these proceedings.
P. F. Pratt
R. L. Branson
April 1978
University of California
Riverside, CA 92521
iv
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RESOURCE MANAGEMENT AS A MEANS OF POLLUTION
CONTROL IN IRRIGATED AGRICULTURE
The development of effective pollution control strategies re-
quires an adequate understanding of the factors which affect the pollu-
tant parameters in the systems we desire to protect. The control of
nitrogen as a pollutant in the environment is a matter of concern since
it is an essential element of life processes and as such the consequences
of any control strategy should be carefully examined. The nitrogen cycle
is one of the principle bio-chemical processes which supports animal and
plant life on the "spaceship earth" on which we reside. Left ro the
elements of nature a balance has developed which provides continuity
of life in many forms.
Through modern technology and understanding of science, mankind
has exerted a degree of control over the natural system to benefit the
perceived needs of man. His ability to provide foodstuffs in excess of
individual needs is a prime example of how he has learned to manipulate
the natural system. However, inherent in this ability to control the
system is the responsibility to manage it properly both for presently
perceived needs and for needs of future generations. This is the
challenge we face — what actions should we take and what are the con-
sequences if we act or do not act.
Obviously, the answers are not easy to obtain since there are
conflicting needs to resolve. Yet decisions will be made which will
affect all of society. A prudent approach would be to acquire a
working knowledge of the system we are trying to manage; develop an
understanding of how management options affect the system; and formu-
late alternative management strategies which allow us to objectively
weigh the impact of action or inaction.
This conference was conceived to provide you with an opportunity
to better understand the role of nitrogen in irrigated agriculture.
The Environmental Protection Agency, chartered to enhance the quality
v
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of the environment, is actively seeking an understanding of what
can be controlled and what should be controlled to provide for the
nation's present and future needs. Through coordination of research
efforts among federal and state agencies, attention can be focused
on specific problems to develop appropriate solutions. This
Conference is one such endeavor to provide not only research results
but also opportunity to utilize the researchers as resource persons
by the various publics represented here at the Conference.
Arthur G. Hornsby
Soil Scientist
Office of Research and Development
U.S. Environmental Protection Agency
Ada, Oklahoma 74820
vi
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NATIONAL INTEREST IN NITRATES IN WATERS
When the National Science Foundation organized its program on
Research Applied to National Needs (RANN) in 1971, it sought to en-
courage research in areas of science having application in more than
one of the Federal mission agencies. It was felt that research funds
could be used efficiently by having a common source of funding for
investigations spanning the interests of multiple agencies. Moreover,
it was recognized that agency missions can sometimes clash, as in the
cases where economy of production and protection of environmental quality
come into conflict. These are situations in which a comprehensive re-
search approach can be useful, an approach that seeks to harmonize, as
well as possible, society's need for the production of goods and the
production of a clean and healthy environment. Yet the needed compre-
hensive approach is ever in danger of falling apart if it has to depend
upon the coordination of Isolated projects supported by different
agencies, each working under its own legislative mandate.
Early in its program in support of applied research, the National
Science Foundation was aware of public concern about nitrate pollution
of water supplies. The Foundation therefore encouraged the development
of proposals to combine the agricultural and environmental aspects of
research on soil nitrate in a unified approach designed to harmonize
the objectives of economic crop production and environmental protection.
Successful initiatives from Washington University in St. Louis and from
the University of California were the response to the Foundations' in-
terest in this area. The Washington University project, recently com-
pleted , dealt with conditions in the midwestern cornbelt.
The California project, under the leadership of Dr. Parker F.
Pratt has involved the Berkeley, Davis and Riverside campuses of the
university. The results of this six-year program now in its final
stage, provide much of the input to this conference on nitrate in the
irrigated lands of the West. The National Science Foundation is pleased
vii
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to join with the Environmental Protection Agency in sponsoring this
Conference to assist in the utiJ ization of the research it has
supported along with other related investigations to be reported
here. It is the hope of the Foundation that its support of work
in this area will have helped to show how an integrated research
effort can help to find acceptable compromises between environmental
protection and the use of nitrogen fertilizers for economic crop
produc tion.
Richard A. Carrigan
National Science Foundation
Washington, D.C. 20550
viii
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TABLE OF CONTENTS
Page
THE INDISPENSABLE ROLE OF NITROGEN IN
AGRICULTURAL PRODUCTION
R. A. Olson 1
NITROGEN FORMS AND CYCLING IN RELATION
TO WATER QUALITY
Ronald G. Menzel • 33
OVERVIEW OF NITROGEN IN IRRIGATED
AGRICULTURE
R. S. Rauschkolb 53
SOURCES OF NITROGEN FOR CROP UTILIZATION
L. S. Murphy 61
MINERALIZATION, IMMOBILIZATION AND
NITRIFICATION
F. E. Broadbent 109
REMOVAL OF NITROGEN BY VARIOUS IRRIGATED
CROPS
T. C. Tucker and R. D. Hauck 135
VOLATILE LOSSES OF NITROGEN FROM SOIL
D. E. Rolston 169
LEACHING OF NITRATE FROM SOILS
B. L. McNeal and P. F. Pratt 195
EFFECT OF WATER MANAGEMENT ON NITRATE
LEACHING
J. Letey, J. W. Biggar, L. H. Stolzy,
and R. S. Ayers 231.
MONITORING WATER FOR NITROGEN LOSSES
FROM CROPLANDS
Kenneth K. Tanji 251
ESTIMATING THE INFLUENCE OF SOIL RESIDENCE
TIME ON EFFLUENT WATER QUALITY
W. A. Jury 265
ix
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X
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TABLE OF CONTENTS (Continued)
Page
USE OF MATHEMATICAL RELATIONSHIPS TO
DESCRIBE THE BEHAVIOR OF NITROGEN IN
THE CROP ROOT ZONE
J. M. Davidson and P. S. C. Rao 291
DIAGNOSTIC TECHNIQUES USED TO IDENTIFY
OPTIMUM LEVELS OF NITROGEN FERTILIZATION
FOR IRRIGATED CROPS
T. L. Jackson. 321
ECONOMIC CONCEPTS RELATED TO CONTROLLING
NON-POINT SOURCE POLLUTION STEMMING FROM
AGRICULTURE
Norman K. Whittlesey and Paul W. Barkley 333
A CASE STUDY — NITRATES IN THE UPPER
SANTA ANA RIVER BASIN IN RELATION TO
GROUNDWATER POLLUTION
R. S. Ayers 355
AN ECONOMIC METHODOLOGY FOR EVALUATING
"BEST MANAGEMENT PRACTICES" IN THE SAN
JOAQUIN VALLEY OF CALIFORNIA
Gerald L. Horner, Daniel J. Dudek,
and Robert B. Mo.Kusick 369
NITROGEN BALANCES FOR THE SANTA MARIA
VALLEY
L. J. Lund, J. C. Ryden, R. J.
Miller, A. E. Laag, and W. E. Bendixen 395
ECONOMIC IMPACTS OF CONTROLLING
NITROGEN CONCENTRATION AND OTHER
WATER QUALITY DETERMINANTS IN THE
YAKIMA RIVER BASIN
George H. Pfeiffer and Norman
K. Whittlesey 415
XI
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xii
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THE INDISPENSABLE ROLE OF NITROGEN
IN AGRICULTURAL PRODUCTION
R. A. Olson—^
ABSTRACT
The indispensable role of nitrogen in food and fiber production
for the world's people cannot be disputed. There is no substitute
for nitrogen in its essential roles as a component of the chlorophyll
and protein constituents of crop plants, The quantity required for
obtaining an economic yield of most crops exceeds that of all other
soil-derived essential nutrients. The advent of relatively cheap fer-
tilizer nitrogen in the 1950's caused radical increases in yields
obtainable with most crops in the developed countries and provided
the spark that ignited the Green Revolution in many of the Less
Developed Countries in the 1960's as well. Its preeminence in the food
production chain notwithstanding, nitrogen has been subjected recently
to more critical surveillance than any other element in agriculture
by reason of energy expended in its conversion into fertilizers, its
monetary cost to the farmer, and its potential role as environmental
pollutant. The economic and environmental problems can be minimized,
however, by matching rate and timing of applied nitrogen with the
amount likely to be provided by the soil during the growing season and
with the water regime afforded. The agricultural sector must achieve
this matching objective promptly if it is not to be condemned by the
rest of society in the long term.
—^Department of Agronomy, University of Nebraska, Lincoln, Nebraska
68583.
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NATURE AND OCCURRENCE
Nitrogen is one of the most plentiful of all the elements
known to be essential for plant growth. It constitutes 78% of the
atmosphere as elemental nitrogen (N^) gas and is present in substan-
tial quantities combined with other elements in the sedimentary
rocks of the earth's crust and in the organic fraction of the soil.
The latter constitutes the major natural reservoir of nitrogen likely
to become available for crops over the long term. A representative
figure for the surface soil in a native stand of grass or forest,
never cultivated, might be in the order of 3% organic matter of
which approximately 5% would be nitrogen. Of course there are soils
developed in old lake beds that can range up to nearly 100% organic
matter (peat), and on the other extreme desert soils that contain
barely perceptible amounts. By knowing the weight of the soil, the
few thousand kg nitrogen/ha-30 cm is readily calculated. Through
microbial activity from 2 to 4% of this total is released as mineral
ammonium and nitrate available to plants over the short time interval
of a growing season.
So the growing plant may be likened to the ship-wrecked sailor
with oceans of water about but nary a drop to drink. There are more
than 67,000 metric tons of nitrogen/ha in the atmosphere above and
even larger amounts in the rocks below, none of which is useable by
the plant due to the inert elemental state of that above and the in-
accessibility of that below. In essence, there is a larger total
amount present in the biosphere but with proportionately less in
fixed forms available for immediate crop production relative to crop
needs than exists with any other essential plant nutrient. Nitrogen
contained in the grain of most commonly grown crops exceeds the com-
bined total of all other soil-derived essential nutrients with the
exception of carbon, hydrogen and oxygen. As a consequence nitrogen is
the most extensively deficient element for optimum crop production
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world-wide (Donald et al, 1963) and greatly exceeds all others in
tonnage applied as fertilizer today.
ROLE OF NITROGEN IN PLANT GROWTH
There is no substitute for nitrogen in the essential roles it
performs in plant growth. The all-important function of energy
intake from solar radiation effected by chlorophyll could not be
served were it not for the nitrogen component which completes that
remarkable photoreceptor molecule (Figure 1). Nitrogen is also one
of the necessary constituents of amine acids, the building blocks of
protein so essential to the nutrition of animals and man. The
proteins formed in the plant afford functional roles in the plant's
growth as well, many of which are enzymes controlling metabolic
processes including the synthesis of protein. In the case of cereals,
threshold levels of protein exist below which grain does not form.
Adequacy of nitrogen in plant cells is needed for the utilization
of carbohydrates produced by photosynthesis. Under conditions of
deficiency, excess carbohydrate deposition occurs in vegetative cells
with resulting cell thickening, less protoplasm formation and ulti-
mately reduced plant succulence. Sufficiency of nitrogen to Lhe plant
is mandatory not only for luxuriant top growth but also for the
development of an extensive and vigorous root system. Thus, Holt and
Fisher (1960) measured an approximate 20% increase in the mass of roots
under nitrogen fertilized Coastal Bermuda grass as compared to un-
fertilized grass, and Kmoch et al (1957) reported notably deeper root
development from applied nitrogen for extraction of existing deep sub-
soil moisture by winter wheat as did Olson et al (1964b) for corn and
grain sorghum.
3
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HjOCH
CH,CH
—ch3 in chlorophyll a
CHO in chlorophyll b
ch3
^ch3
CH,
Figure 1. Structural formula of the chlorophyll molecule.
4
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CROP REQUIREMENTS
Figure 2 summarizes approximate quantities of nitrogen and other
major nutrients present in good yields five major agricultural
crops. These cannot be taken as precise values for the several
elements since nutrient composition of crops varies substantially
with differences in soil nutrient availability, plant genotype and
environmental conditions from one place to another. It will be noted
first in these approximations that the amount, of nitrogen removed/
crop generally exceeds that of all other nutrients, especially in
the case of grain crops where the forage is returned to the soil.
The second observation of significance is the 110 to 170 kg nitrogen/
ha removed by the harvested portion of the crop, excepting the legumes
(in this case soybeans) which contain much greater amounts but which
fix much of the nitrogen on their own. With annual removals of this
magnitude by non-legumes it is evident that total depletion of the
native nitrogen in the surface 30 cm of a medium textured virgin soil
of 3% organic matter content would be accomplished in an approximate
50-year period if there were no replenishment.
Not only is the total quantity of nitrogen released during a
growing season important, but the rate of release assumes prime signif-
icance during the crop's grand period of growth. The corn crop pro-
ducing some 10,000 kg/ha and utilizing around 170 kg nitrogen/ha
absorbs almost 3% of the amount/day during the two weeks just before
silking, or about 4 kg nitrogen/ha daily. Similar amounts are utilized
by wheat during the maximum vegetative stage and somewhat more by soy-
beans (Figure 3 and Hanway, 1962). Accordingly, the crop manager must
assure that nitrogen is plentifully available throughout this period
of high demand without affording excessive quantities that might ulti-
mately contribute to environmental pollution.
Much more will be presented on nitrogen removal by irrigated
crops in a subsequent paper.
5
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300—(
Potato**
40,000 kg/ha
Soybean•
4,000 fag/ha
Harvested portion
CORN
200-
10,000 kg/h<
4,000 kg/ha
(¦••d «. lint)
Wheat
5,000 kg/ha
100-
UL
o»
Figure 2. Nutrients contained in harvested portion and residues of
good yields of major agricultural crops.
6
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•320
/
M0+ */ ^l~-
o/
o / •'
• - so
/
o
/
/
J40 - " ^ ,
/' ^ / A
i' / ®/
\
I /
»• * 1
z / 1 I
160 ^ ' ,
I '
"* I /
I I
t
I
/
/
/
/ '
o '
/ /.
I I
I I
I /
/
__-0"
—1 1 ¦ i 1 J —i
April May June July August Sept. Oct.
GROWTH PERIOD
Figure 3. Rate of uptake of nitrogen by winter wheat, corn and
soybeans under Nebraska environmental conditions.
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NITROGEN SUPPLYING MEASURES FOR CROP PRODUCTION
Organic Methods
Supplemental nitrogen for benefitting crop growth has been
achieved through the centuries of time by the growth of a leguminous
crop and by the addition of animal manures and human sewage to the
soil. During the Golden Age of Greece, several hundred years B.C.,
authors wrote of the great value of manures for preserving the
productivity of the land, and in ancient Rome the relative value of
several leguminous plants for soil Improvement was generally known.
Well into the 20th century farmers world-wide depended primarily
on manures and legumes in rotation for supplying the nitrogen require-
ments of crops after high levels of native soil fertility had been
depleted. Beginning with the chemical age of agriculture in the
years immediately following World War II, however, greatly modified
traditional concepts of soil fertility maintenance developed in the
industrialized countries. During the earlier organic age, as now,
there was never more than about 15 to 20% of the nitrogen required
for crop production available from the animal manure source (CAST,
1977) while the leguminous green manure crop occupies the land for a
year or more when neither food nor fiber crop could be grown. Also
the legume's heavy water requirement precluded its use in the drier
climatic regions.
Advent of Fertilizer Nitrogen
Technological advances hastened by the urgencies of war resulted
in facilities for fixed nitrogen synthesis which were released for
peaceful uses at the end of World War II. A burgeoning world popula-
tion accompanying enhanced health protection measures and the recog-
nition of expanding world food requirements prompted a tremendous
growth in fertilizer nitrogen manufacturing facilities. With the
resulting substitution of fertilizer nitrogen for the old traditional
8
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nitrogen sources, fertilizer nitrogen consumption soon came to
exceed the tonnages of other major nutrients (Table 1).
Table 1. Consumption of fertilizer nitrogen, phosphate
and potash in the world, 1938-76.
v _ Consumption of
P205 k20
million nit of nutrient*
1938-39
2.6
3.6
2.8
1953-54
5.2
6.3
5.7
1959-60
9.7
9.7
8,6
1964-65
16.8
13.8
12.1
1967-68
24.2
16.7
14.1
1973-74
38.7
24.2
20. 7
1975-76
40.9
26.2
24.7
* mt = metric tons
Note that nitrogen consumption has increased over 15 times in the
37 year period up to 1976 while use of the two other major nutrients
increased about 8 times, such that ratio of nitrogen-phosphate-potash
expressed as N-P20<.-K20 changed from .29-.40-.31 to .44-.30-.26.
Approximately one-fourth of the total nitrogen used in 1975-76 was
consumed in the U.S. The crop itself cannot distinguish between
the various incremental sources, organic or inorganic, since virtually
all of that applied is eventually converted to ammonium or nitrate
form in the soil, if not so applied, and these are the forms
primarily absorbed by plants.
Efforts to depict balance sheets of nitrogen input/output with
the changing times have been tenuous at best. Few absolute measure-
ments have been made on items like rainfall fallout, leaching,symbiotic
and nonsymbiotic nitrogen fixation in a very limited number of eco-
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systems prior to the 'environmental awakening' of the 1960's, and
none at all of denitrification. Thus the summation presented in
Table 2 for the U.S. by Parr (1972) can be taken only at face value,
but the values probably are within 25% of the actual.
Table 2. Balance sheet of nitrogen in the United States: estimated
changes from 1930 to 1980 on harvested crot>land
(Parr, 1972).
Nitrogen, millions of tons*
Item 1930 ~ 1947 1969" 1980
Inputs of nitrogen from
1.
Fertilizer nitrogen
0.3
0.7
6.8
12.2
2.
Nitrogen
fixed by legumes
1.7
1.7
2.0
2.5
3.
Nitrogen
fixed nonsymbiotically
1.0
1.0
1.0
1 .0
4.
Barnyard
manure
1.9
1.3
1.0
0.9
5.
Roots of
of crops
unharvested portions
1.1
1.5
2.5
3.0
6.
Rainfall
0.8
1.0
1.5
1.8
Total
6.8
7.2
14.8
21.4
;moval of nitrogen by
1.
Harvested crops
4.6
6.5
9.5
13.0
2.
Erosion
5.0
4.0
3.0
2.0
3.
Leaching
of soil nitrogen
4.0
3.0
2.0
1.5
4.
Leaching
of fertilizer nitrogen
0
0
?
1.2
5.
Denitrification
?
?
?
2.2
Total
13.6
13.5
14.5
19.9
* The data are English tons (2,000 pounds/ton).
The uncertainties notwithstanding it is quite clear that fertilizer
nitrogen has taken over as the predominant input, representing around
60% of the total inputs in 1980 while removal in harvested crops will
10
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account for 2/3 of the total removal. These figures correspond
with respective values of and 1/3 in 1930. The primary enigma
in recent times and in the forward projection is with the denitri-
fication removal. It is also this author's opinion that those
responsible for the summation have been overly optimistic about the
continuing reduction in eroded nitrogen with time and on the increas-
ing amounts of symbiotically fixed nitrogen.
Much research was conducted in the developed countries for estab-
lishing effective fertilizer nitrogen practices for maximum yields
under varied soil and cropping conditions from 1950 onwards, and
demonstrations of the principles realized were carried to developing
countries through fertilizer development programs. Only China among
more advanced countries with major food requirements has resisted
the trend toward predominant inorganic nitrogen use. As late as 1976
the Chinese with their populous agrarian society were deriving some
75% of the nitrogen utilized in agricultural production from
organic sources (Johnson and Beemer, 1977). But even these pragmatic
organic farmers will soon be satisfying a major portion of crops'
needs by the inorganic nitrogen route on completion of a dozen large
capacity nitrogen plants now building that will supply in the order
of 3 million tons of fixed nitrogen annually.
The expanding U.S. population during the 50 years prior to .1930
was balanced by conversion of virgin lands to crop lands for provid-
ing national food and fiber needs. Since that year of maximum culti-
vated acres, however, there has been a reduction of almost 10% in
the nation's cropland while population has nearly doubled (Figure 4).
This anomaly was made possible by the advent of the chemical fertilizer
industry subsequent to World War II and most particularly by the
tremendous growth in fertilizer nitrogen use. The additional pro-
duction afforded has not only met the needs of the nation but has
contributed more than any other national product toward the balance
of trade in international commerce, representing almost one-fourth of
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200
500
400
ISO
c
o
i
z
o
5 100
200
100
1920
1900
1940
(••0
Yl Aft
Figure 4. Relation between cultivated land, population and
fertilizer nitrogen consumption in the U.S., 1880-
1976. Fertilizer use is in millions of metric tons.
12
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all U.S. exports in the late 1970's at some $22 to 24 billion dollars
per annum. A very substantial portion of this income can be attribu-
ted directly to fertilizer nitrogen. Without fertilizer nitrogen the
U.S. housewife would likely be paying at least twice what she does for
groceries today.
Crop Response to Fertilizer Nitrogen
Response data of the type presented in Figure 5 show the dramatic
effect that fertilizer nitrogen has had on world food and fiber pro-
duction in the past generation. No other agricultural innovation
during this period has contributed as much toward satisfying the needs
of a food hungry world. Note the quadrupled average corn yields in
the midwestern state of Nebraska with growth in fertilizer nitrogen
consumption. Of course other technological advances contributed
including improved hybrids, better control of pests, advanced methods
of crop culture, expanded irrigation, etc., but the yield enhancement
could not have resulted without the nitrogen input.
The success achieved by the miracle varieties of the Green
Revolution was made possible by the arrival of relatively cheap
fertilizer nitrogen on the scene in the 1960 "s according to the
movement's acknowledged father (Borlaug, 1971). Without this input
along with improved watering and pest control practices the new
varieties were little if any better than those used traditionally by
peasant farmers. Wright (1972), for example, in summarizing a series
of coordinated experiments on fall-seeded spring wheats of CYMMIT
origin in India noted doubled yields from the appropriate nitrogen
treatment. The data also demonstrate response to notably greater
rates of nitrogen by dwarf varieties than by the tall types investi-
gated. A 120 kg/ha rate would have been most economic for the dwarf
types in this study whereas no more than 60 kg/ha was needed for the
tall types with respective yield levels of approximately 43 and 34
quintals/ha. He also reported similarities in response to nitrogen
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540
420
Fertilizer N
1,000 tons
Average yield of corn
s
Anhydrous Ammonia
Nitrogen Solutions
Ammonium Nitrate
Mixtures
Urea & Am. Sulfate
-.-..i"1 j?'
o
£
V
300
180
1952
1956
"J r
1960 1964
1972
Figure 5. Average yield of corn in Nebraska in relation to state-
wide consumption of fertilizer nitrogen, 1952-1972.
14
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rate in many regions where high-yielding varieties have been intro-
duced with explanation that the wheat is generally grown on formerly
desert soils, now irrigated, which were inherently of very low
organic matter and nitrogen contents.
The required nitrogen for the new varieties of rice from IRRI
also doubled and even trebled the practical yield potential of the
traditional varieties. Note the large response of I&8 and IR20
(Figure 6) to nitrogen rates in the order of 150 kg/ha in the dry season
of the Philippines and 90 kg in the wet season, in contrast with the
lack of response of the Peta, a native variety selected through prior
times for its performance on tropical soils of existing low fertility
status (Brady,. 1975).
Acceptance of the fertilizer nitrogen source was forthright in
the more humid and. irrigated regions with the high value/cost ratios
that could be demonstrated and the minimal risk involved. Perhaps the
greatest deterrent was the old-line agronomist, who insisted that phas-
phating the legumes to harvest a large nitrogen yield was still the
logical route to follow. Farmer acceptance was much slower, however,
in the semi-arid cropping regions where taller, greener plants didn't
necessarily portend greater yields of grain. With moisture supply
for the crop eves critical it was a matter of balancing the highly
growth stimulating nitrogen practice against soil moisture storage and
rainfall probabilities. Thus* data of Raaig and Rhoades (1963)
suggested likely economic response of winter wheat to 22 kg nitrogen/ha
for each 7 cm stored moisture in the rooting profile at. planting of
soil, with nominal nitrogen capacity in a region of 15 cm growing
season, rainfall. Figure ? also demonstrates the critical need for
effective nitrogen management practice in dry-farming wheat production
of western Nebraska where use of only slightly aore than optimum
nitrogen rates can have depressive effects on grain yield and economic
return. Higher than economic rates for grain sorghum did not depress
yields, however, arid for the reason that summer side-dress application
15
-------
It 20
IR20
IRS
'•O, Peto
WET SEASON
DRY SEASON
90
90
60
120
ISO 0
N applied, kg / ha
120
ISO
60
Figure 6. Effect of nitrogen level on grain yield of three rice
varieties during wet and dry seasons at four Philippine
locations averaged over six cropping seasons, 1968-73
(Brady, 1975).
16
-------
W. WHEAT
G. SORGHUM
Yield Inc.
«KgN/kia>0 23 & & ife"
®IM>7.5 8j0 95 10.2 IQ4 ia5
Figure 7. The response of winter wheat and grain sorghum to
fertilizer nitrogen in dryfarming, average of 54
experiments with spring applied nitrogen on winter
wheat and 10 experiments with side-dressed nitrogen
on grain sorghum, Nebraska, 1959-1967.
17
-------
was employed which did not stimulate excessive vegetative growth.
It has been estimated that fertilizer nitrogen is responsible
for 33% of the total agricultural production in the U.S. today
(CAST, 1977). In some regions where crops of high nitrogen demand
are grown and where native levels of soil phosphorus and potassium
are high, as in the western corn belt of the U.S., fertilizer
nitrogen accounts for an even greater portion, perhaps as much as 50%
of current production.
Improving the Efficiency of Fertilizer Nitrogen
The 2% of the total natural gas consumption that is presently in-
volved in fertilizer nitrogen production in the U.S. affords manifold
returns in calories of food energy. Few will contest its worth.
There may be contention by the turn of the century, however, when the
total energy demand for fertilizer nitrogen per annum may reach 10% of U.S.
gas consumption, with total fertilizer nitrogen production costs of
2/
$30 billion.— Should this indeed come to pass the country's citizenry
will demand maximum efficiency in use of the product irrespective of
environmental considerations.
Rate. The tremendous impact of fertilizer nitrogen on world
crop production notwithstanding, there is a generally recognized need
for improving the efficiency of its use. In dry regions the rate em-
ployed must be balanced with the moisture likely to be available
throughout the growing season so that overstimulation of early growth
does not result in yield reduction at season's end. Even with plenti-
ful moisture it is necessary to keep nitrogen supplied to the plant
within reasonable bounds for small grains if lodging and consequent
yield loss are not to occur, for sugar crops so that excessive vege-
2/
— Frederick Ausubel. Increasing crop yield by increasing the amount
and efficiency of biological nitrogen fixation by means of recombinant
DNA technology. Dept. of Biology, Harvard Univ.
18
-------
tative growth does not take the place of sugar set, for tree fruit
crops to assure good appearance and keeping quality of the fruit,
and for many crops in the control of stalkrots and other plant
diseases.
Determination of an effective rate requires a series of
fertilizer nitrogen rate experiments with any crop under considera-
tion. Commensurate measurements must be made on soil nitrogen avail-
ability from native and residual fertilizer nitrogen sources in the
crop rooting zone so that the acquired response data can be used for
predictive purposes. A subsequent paper treats the soil nitrogen
evaluation matter in detail.
Excessive rates of fertilizer nitrogen have been employed locally,
especially for some of the high-value vegetable and fruit crops and
particularly in association with irrigated culture. But heavier than
necessary quantities have been employed as well in the Corn Belt of
the U.S. Illinois, for example, applied 29% more nitrogen than was
harvested in crops in 1970 (Welch, 1972), and Nebraska used nearly
45% more nitrogen than was harvested in 1968 (Figure 8). Such appli-
cation rates totally disregard nitrogen release that can be expected
from soil reserves, fallout in rain and the various natural fixation
processes. The excesses can be attributed in large part to the over-
production and depressed market value of fertilizer nitrogen in the
late 1960's, and do show how farmers will insure against any possibil-
ity of shortage if the cost for nitrogen is low.
Time and Method of Application. Appropriate time and method of
applying fertilizer nitrogen is determined especially by growth
characteristics of the crop, climatic factors, soil properties and
nature of the nitrogen carrier. As a very general rule, efficiency
in use is enhanced if the major portion of the applied nitrogen can
be programmed to become positionally available to the root system
just prior to the period of maximum need by the crop. At the same
time the active root system limits the amount that can escape the root
19
-------
Lt_
I2C
JD
OOfi Replaced
CL
cc
/O— -Q- 0
¦Or-
Q_
—of
k20
-ft -? y
s g. s
O
'60
YEARS
'56
Figure 8. Proportion of harvested crop nutrients replaced by ferti-
lizer, Nebraska, 1952-72. In Olson, R. A. 1974. Ferti-
lizer nitrogen: Mainstay for agriculture. Farm, Ranch
and Home Quart., vol. 20:20-22. Univ. of Nebr. Coll. Agr.,
Lincoln, Nebraska.
20
-------
zone by leaching. So it is that summer sidedressing of corn under
irrigated and humid cropping conditions commonly results in most
efficient response to the nitrogen by the current crop and greatest
residual remaining in the soil for use by the next crop (Olson et al,
1964a; Welch et al, 1971; Herron et al, 1971; Miller et al, 1975).
Good evidence exists, furthermore, that nitrogen taken up relatively
late in the growth period is channeled more directly to grain with
less immobilization in the vegetative portions of the crop (Gass et
al, 1971). A crop like cool season grass grown for seed, on the other
hand, must absorb a major portion of the nitrogen required very early
in the growing season to obtain maximum seed yield. Timing of nitro-
gen availability is especially critical for a crop like sugar beets
which requires a high level of nitrogen availability throughout the
first 3/4 of the growing season but essentially a deficiency of nitro-
gen during the final quarter to assure maximum sugar set (Alexander
et al, 1954; Rhoades and Harris, 1954).
In the very humid regions and with very sandy soils of low water
holding capacity it may prove necessary to apply nitrogen in two or
more increments to reduce the leaching potential to acceptable levels.
For example, it has become common practice to apply nitrogen in incre-
ments throughout the irrigation season where corn is now grown on
sands under center pivot irrigation systems.
The chemical carrier of the nitrogen source will often dictate
what is effective time and method of application. Anhydrous ammonia,
for example, is equally effective when fall applied after soil temper-
ature has dropped below 40F to that summer sidedressed for a summer
crop like corn, while a carrier like ammonium nitrate could suffer
serious leaching losses during the late fall, winter and early spring
months from fall application. Placement method is correspondingly dic-
tated by the chemical carrier involved, a prime example being the
necessity of incorporating urea to limit ammonia volatilization losses.
Environmental Considerations. As with other chemicals intro-
21
-------
duced into agricultural production systems, virtually all of the
attendant research was directed toward the impact on the target crop
without regard for the surrounding ecosystem or those adjacent. It
required a 'Silent Spring' (Rachel Carson, 1962) to focus inter-
national attention on effects that the chemical treatment might have
beyond the intended target. The ensuing years have brought forth a
modified perspective of most economic return from the fertilizer nitro-
gen with minimum contamination of the environment (Viets and Hageman,
1971). Many facts have been elaborated concerning nitrogen in the
environment during the interim while at the same time much fantasy
has been written by self-styled environmentalists clouding the issue.
Debate will continue on the magnitude of environmental contamination
by fertilizer nitrogen until much corroborative research has been
completed across many and varied ecosystems. In view of the number of
documented situations where nitrogen from fertilizer sources has con-
taminated ground water (Ward, 1970; Bingham et al, 1971; Ayres and
Branson, 1973; Muir et al, 1973), however, it becomes mandatory that
farmers derive maximum efficiency in crop use to minimize the pollu-
tion hazard and, at the same time, serve their own best economic
interests.
Converse to the detrimental features to be treated in subsequent
papers, fertilizer nitrogen use provides a number of environmental
benefits. Most obvious, of course, is the enhanced agricultural pro-
ductivity afforded as previously elaborated. After all, what environ-
mental quality is more important to man than a sufficient food supply?
Related is the enhanced protein and vitamin contents of food grain
crops that commonly accompany nitrogen fertilization for optimum
yield. A second environmental benefit is the improvement in the
physical, biological, and chemical character of soils from judicious
fertilizer use, coming from the larger organic return and greater
root proliferation of thrifty, well-fertilized crops. Farming effi-
ciency is improved by fertilizer nitrogen, thirdly, allowing increased
22
-------
production on fewer acres while permitting retirement of erosive, low
productivity land to permanent vegetation (again, see Figure 4).
Fourth, by producing greater vegetative and root growth, applied
nitrogen is one of the most effective of all erosion control measures.
Finally, of special significance in regions of limited moisture avail-
ability is the enhanced water use efficiency with provision of suffi-
cient nitrogen for the crop. Conservancy in water results when yield
is increased by added nitrogen since the added yield is accomplished
with only a little more water consumed than is used by the non-treated
crop (Viets, 1962; Olson et al, 1964b).
Influence of Applied Nitrogen on Crop Quality
The most commonly recognized benefit of applied nitrogen to crop
quality is that of increased protein content of the food grains, of
especial import to the developing countries where around 2/3 of the
protein in the daily diet comes from cereals in contrast with the 1/5
from that source in the U.S. (F.A.O., 1968). Much of the past
research on production practices for these crops has concerned itself
with grain yield, and little more than casual notice was paid to the
crude protein in the grain. The milling and baking industries of the
developed countries orient their purchasing practices to specific
regions where soil and climatic environment produce the protein levels
required. Recently, however, world protein needs have become increas-
ingly apparent. Moreover, within individual countries some regions
of traditionally high protein grain can no longer supply the expected
product by reason of soil depletion and protein dilution imposed by
higher yielding varieties.
There was no general agreement on benefit to protein of cereal
grains from nitrogen fertilization until the period following World
War II. The majority of fertilizer trials prior to that time employed
very low rates of nitrogen, as in the order of 10 to 30 kg/ha, for
which today we would predict limited if any protein increase. Thus,
23
-------
field experiments at the Rothamsted experiment station through the
late 1930's showed no clear-cut effects from nitrogen fertilizaion
on grain protein (Russel and Watson, 1940). Most recent literature
on fertilization of cereals, however, shows increasing protein con-
tent of the grain with increasing nitrogen rate, at least above some
minimal rate which increases yield at the expense of protein. The
rate of protein increase from that point onward is usually a near
straight line function with rate of nitrogen until some protein
maximum for the varieties employed is reached, normally well in
excess of the rate required for maximum grain yield (Figure 9).
Most studies indicate that applying nitrogen in the spring for
winter wheat results in a higher grain protein content than results
from fall application with a slight yield advantage as well in the
semi-arid wheat producing areas (Hucklesby et al, 1971; Olson et al,
1964a). Protein percentage tends to become progressively higher as
time of nitrogen application is delayed, providing precipitation is
received to carry the nitrogen to the root system. With foliar
treatments nitrogen application may be delayed so that the entire
expressed nitrogen effect is on protein and not at all on yield
(Finney et al, 1957). Protein increases ranging from 2 to 4% have
thus been noted from spraying urea solution in three increments to
a modest total rate of 50 kg nitrogen/ha on winter wheat, the most
benefit deriving from the treatment made at early bloom state
(Sadaphal and Das, 1966) .
Delayed application of economic rates of nitrogen for corn has
also been more effective for grain and protein yields than earlier
treatment (Figure 10). Submerged rice as well benefits more from
fertilizer nitrogen applied during the growing season than at plant-
ing or before in terms of total nitrogen uptake and utilization for
grain protein (Beachell et al, 1972; DeDatta et al, 1972; FAO/IAEA,
1970). Application shortly before primordial initiation has proved
optimum for maximum protein production with a given nominal nitrogen
24
-------
RESIDUAL NO3N
45-90
-•>135
"~ 90-135
<45
NEBRASKA-CORN
I7EXPTS. 1961-65
RESIDUAL NO3N
z >135
—<3 90-135
45-90
<45
< 6
-------
750-
650
550-
,71 450
350
250
NEBRASKA
14 IRRIGATED EXPT.
CORN
CONTROL
FALL
SPRING
SUMMER SIDEDRESS
pSSD
90
APPLIED N, kg/ha
Figure 10. Average crude protein and grain yields in 14 irrigated
corn experiments in Nebraska as influenced by time and
rate of fertilizer nitrogen application (F = fall, S =
spring, SD = summer sidedress application of nitrogen
fertilizer).
26
-------
rate for representative soils having sufficient nitrogen reserve to
get the plant well started. Thus it appears that for all cereals
the latest possible application of nitrogen commensurate with crop
stage of development and growing conditions that permit ready nitro-
gen uptake, provides for maximum protein yields.
The leaf crops such as lettuce and spinach are markedly en-
hanced in succulence and marketability when produced with sufficient
nitrogen. The grower in this case must employ the quantity of ferti-
lizer nitrogen required for this attribute commensurate with keeping
the nitrate in the plant tissue within acceptable levels for human
nutrition. On the other side of the ledger there is documentary
evidence of decreased shelf life of various vegetables and fruits,
of increased contents of undesired free amino acids, of decreased
ascorbic acid and potassium contents of produce, and of loss in flavor
of some commodities with excessive use of fertilizer nitrogen (Schuphan,
1972) .
Other quality aspects, pro and con, related to fertilizer nitro-
gen exist for most commercial crops exceeding the bounds of this paper
for elaboration. Suffice it to say, the nitrogen nutritional needs
of the world's present man and animal population would be woefully
undersupplied without an inorganic fertilizer input.
27
-------
LITERATURE CITED
Alexander, J. T., Clyde C. Schmer, Louis P. Orleans, and R. H. Cotton.
1954. The effect of fertilizer applications on leaf and yield
of sugar beets. Proc. Am. Soc. Sugar Beet Technol. 8:370-379.
Ayers, R. S. and R. L. Branson (eds.). 1973. Nitrates in the upper
Santa Ana River Basin in relation to groundwater pollution.
Calif. Agr. Exp. Sta. Bull. 861: 60 pp.
Beachell, Henry M., Gurdev S. Khush, and Bienvendo 0. Juliano.
1972. Breeding for high protein content in rice. J[n Rice
Breeding: 419-428. I.R.R.I., Los Banos, Philippines.
Bingham, F. T., S. Davis, and E. Shade. 1971. Water relations, salt
balance, and nitrate leaching losses of a 960-acre citrus water-
shed. Soil Sci. 112:410-418.
Borlaug, Norman E. 1971. Mankind and civilization at another cross-
road. McDougall Memorial Lecture. FAO General Conf., 16th
Session, Rome.
Brady, Nyle C. 1975. Rice responds to science. Iji Crop Product-
ivity—Research Imperatives. Mich. St. Univ., Charles F.
Kettering Found., NSF(RANN), ERDA, USDA and USAID.
Carson, Rachel. 1962. Silent Spring. Fawcett Publications, Inc.,
Greenwich, Conn.
CAST. 1977. Energy use in agriculture: now and for the future.
Counc. Agri. Sci. Tech. Report #68: 28 pp.
DeDatta, S. K., W. N. Obcemea, and R. K. Jana. 1972. Protein content
of rice grain as affected by N fertilizer and some triazines
and substituted ureas. Agron. J. 64:785-788.
Donald, Leroy, Harvey J. Stangel, and John Pesek. 1963. Advances in
knowledge of nitrogen fertilization in the USA since 1950. Iri
M. H. McVickar et al (eds.) Fertilizer Technology and Usage.
Soil Sci. Soc. Amer., Madison, Wis.
F.A.0. 1968. Production yearbook. Rome, Italy.
F.A.O./I.A.E.A. 1970. Rice fertilization. Tech. Rept. Ser. //108:
177 pp.
28
-------
Finney, K. F., J. W. Meyer, F. W. Smith, and H. C. Fryer. 1957.
Effect of foliar spraying of Pawnee wheat with urea solutions
on yield, protein content and protein quality. Agron. J.
49:341-347.
Gass, W. B., G. A. Peterson, R. D. Hauck, and R. A. Olson. 1971.
Recovery of resj^ual nitrogen by corn from various soil depths
as measured by N tracer techniques. Soil Sci. Soc. Amer. Proc.
35:290-294.
Hanway, J. J. 1962. Corn growth and compositon in relation to soil
fertility. II. Uptake of N, P, and K and their distribution
in different plant parts during the growing season. Agron. J.
54:217-222.
Herron, G. M.f A. F. Dreier, A. D. Flowerday, W. L. Colville, and
R. A. Olson. 1971. Residual mineral N accumulation in soil
and its utilization by irrigated corn. Agron. J. 63:322-327.
Holt, E. C. and F. L. Fisher. 1960. Root development of Coastal
Bermuda grass with high N fertilization. Agron. J. 52:593-596.
Hucklesby, D. P., G. M. Brown, S. E. Howell, and R. H. Hageman.
1971. Late spring applications of N for efficient utilization
and enhanced production of grain protein of wheat. Agron. J.
63:274-276.
Johnson, Virgil A. and Halsey L. Beemer, Jr. (eds.). 1977. Soil
and crop management and related factors. In Wheat in the
People's Republic of China. CSCPRC Report #6:1-26. National
Academy of Sciences, Washington, D.C.
Kmoch, H. G., R. E. Ramig, and F. E. Koehler. 1957. Root develop-
ment of winter wheat as influenced by soil moisture and nitrogen
fertilization. Agron. J. 49:20-25.
Miller, H. F., John Kavanaugh, and G. W. Thomas. 1975. Time of N
application and yields of corn in wet, alluvial soils. Agron.
J. 67:401-404.
Muir, John, Edwin C. Seim, and R. A. Olson. 1973. A study of factors
influencing the nitrogen and phosphorus contents of Nebraska
waters. Jour. Env. Qual. 2:466-470.
Olson, R. A., A. F. Dreier, C. Thompson, K. Frank, and P. H. Grabouski.
1964a. Using fertilizer nitrogen effectively on grain crops.
Nebr. Agr. Exp. Sta. SB 479: 42 pp.
29
-------
Olson, R. A., C. A. Thompson, P. H. Grabouski, D. D. Stukenholtz,
K. D. Frank, and A. F. Dreier. 1964b. Water requirement of grain
crops as modified by fertilizer use. Agron. J. 46:427-432.
Parr, J. F. 1972. Chemical and biochemical considerations for
maximizing the efficiency of fertilizer nitrogen. Soils Bull.
16:53-86.
Ramig, Robert E. and H. F. Rhoades. 1963. Interrelationships of
soil moisture level at planting time and nitrogen fertilization
on winter wheat production. Agron. J. 55:123-127.
Rhoades, H. F. and L. Harris. 1954. Cropping and fertilization
practices for the production of sugar beets in western Nebraska.
Proc. Am. Soc. Sugar Beet Technol. 8:71-80.
Russel, E. J. and D. J. Watson. 1940. The Rothamsted field experi-
ments on the growth of wheat. Imp. Bur. Soil Sci., Tech. Comm.
40: 163 pp.
Sadaphal, M. N. and N. P. Das. 1966. Effect of spraying urea on
winter wheat. Agron. J. 58:137-141.
Schuphan, W. 1972. Effects of the application of inorganic and
organic manures on the market quality and on the biological
value of agricultural products. Soils Bull. 16:198-224.
F.A.0., Rome.
Viets, Frank G., Jr. 1962. Fertilizers and the efficient use of
water. Advan. Agron. 14:223-264.
Viets, Frank G., Jr. and Richard H. Hageman. 1971. Factors affect-
ing the accumulation of nitrate in soil, water, and plants.
U.S.D.A., A.R.S. Agr. Handbook No. 413, 63 pp.
Ward, P. C. 1970. Existing levels of nitrates in waters-the California
situation. In Nitrate and Water Supply: Source and Control.
Twelfth Sanit. Eng. Conf. Proc., Engin. Pub. Off., Univ. Illinois,
Urbana, 111.: 14-26.
Welch, F. 1972. More nutrients are added to soil than are hauled
away in crops. Illinois Research Vol. 14:3-4.
Welch, L. F., D. L. Mulvaney, M. G. Oldham, L. V. Boone and J. W.
Pendleton. 197x. Corn yields with fall, spring and sidedress
nitrogen. Agron. J. 63:119-123.
30
-------
Wright, B. C. 1972. Fertilization of fall-planted spring wheat
under irrigation. Internat. Winter Wheat Conf. Proc.: 59-62.
USDA, USAID, Univ. of Nebr., Ankara, Turkey.
31
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NITROGEN FORMS AND CYCLING IN RELATION TO WATER QUALITY--^
2/
Ronald G. Menzel—
ABSTRACT
The intended use of water determines the significance of various
forms of nitrogen in water quality. Upper limit concentrations have
been recommended for several forms of nitrogen in water used for
public water supplies, freshwater aquatic life, marine aquatic life,
and agricultural uses. The concentrations are lowest for cyanide,
and increase in the order ammonium, nitrite, and nitrate. Neverthe-
less, nitrate is more often of concern in water quality than are the
first three forms of nitrogen.
The recommended upper limit concentration for nitrate-nitrogen
in public water supplies is 10 mg/1. The actual concentration in
ground water exceeds this limit in some areas, and may be increasing.
Because of the slow movement of ground water, increasing concentra-
tions may take many years to correct.
Nitrate is depleted from surface waters by plant uptake, and
may cause excessive growth of algae or other plants. This is seldom
a problem in streams, but may become one in impoundments, estuaries,
and near-shore ocean areas.
Much of the total nitrogen in surface waters occurs in particu-
late or dissolved organic forms. Plant growth is often excessive
when the total nitrogen concentration exceeds 0.5 mg/1 if other
nutrients are adequately supplied and growth conditions are favor-
—^Contribution from the Water Quality Management Laboratory, Science
and Education Administration, USDA, Durant, Oklahoma 74701.
2/
— Soil Scientist
33
-------
able. Nitrogen supply limits growth in many ocean areas and in a
few lakes, mainly eutrophic ones. Nitrogen fixation by blue-green
algae tends to correct nitrogen deficiencies. Denitrification by
bacteria tends to correct excesses. These features indicate that
high nitrogen concentrations may be an effect, rather than a cause,
of eutrophication.
INTRODUCTION
Nitrogen occurs in a variety of chemical forms in all natural
waters. Some are readily soluble plant nutrients, such as nitrate
and ammonium ions. Some may be highly toxic, such as cyanide or
nitrite, which form occasionally during animal or plant decomposition.
Some are organic compounds, varying in complexity from simple amino
acids to nucleic acids. A few pesticide compounds contain nitrogen.
The kinds and amounts of nitrogen forms in water are closely
connected with the growth and decay of organisms. Transformations
among nitrogen compounds are often carried out biologically. Since
biological activity depends on such factors as light intensity, flow
velocity, and depth of water, it will be convenient to discuss water
quality separately for ground water, streams, impoundments, and
marine environments. In each environment, the prevalent forms, trans-
formations, and important water quality effects of nitrogen are
somewhat different.
RECOMMENDED UPPER LIMIT CONCENTRATIONS
Dissolved forms of nitrogen have the most direct effects upon
water quality. For some of these forms, upper limit concentrations
in water have been recommended, always in relation to specific uses
of water. The limits recommended in Water Quality Criteria 1972 by
34
-------
a Committee of the National Academy of Sciences - National Academy
of Engineering are summarized in Table 1.
Table 1. Summary of water quality criteria for common forms of
nitrogen. (from Water Quality Criteria 1972).
Form of
nitrogen
Type of Water Use*
I
II III
IV
Recommended Maximum Concentration
(mg N/liter)
100+
Nitrate
10
Nitrite
1
10+
Ammonium
0.5
0.02t O.Olt
Cyanide
0.2
0.005t 0.005t
*
I Public water supplies.
II Freshwater aquatic life.
Ill Marine aquatic life.
IV Agricultural uses, including livestock watering and irrigation.
+
For livestock water.
X Nitrogen as unionized ammonia or hydrocyanic acid.
The highest concentrations in Table 1 are shown for nitrate in
public water supplies and for livestock watering. The hazard from
nitrate arises principally from its possible reduction to nitrite in
the digestive tract, especially in infant humans and ruminant animals.
Upper limit concentrations were not set for water supporting aquatic
life. However, the Committee concluded that as little as 0.36 mg/
liter of nitrogen as nitrate could produce enough organic matter to exhaust
the oxygen content of marine water. They considered this an excessive
concentration.
The limiting concentrations of nitrite are based on its effect
in the blood stream when water is drunk by humans or animals. Nitrite
combines with the hemoglobin of red blood cells so that they cannot
35
-------
transport oxygen. Nitrite concentrations are too low in most natural
waters to be of concern for freshwater or marine aquatic life.
Therefore, upper limit concentrations of nitrite were not set for
these water uses.
Ammonia is quickly formed from all kinds of animal excrement,
and dissolves in water to form ammonium ions. The recommended upper
limit in public water supplies is based on the facts that ammonium
may indicate other types of pollution, and that it requires additional
chlorination for water treatment. Fish are sensitive to unionized
ammonia, which means that the toxicity of a given ammonium concentra-
tion increases rapidly as pH values increase above 8. The percentage
of unionized ammonia is 3% at pH 8, 10% at pH 8.5, and 30% at pH 9.
Cyanide is highly toxic to fish, the toxicity increasing rapidly
at pH values below 8. This indicates that the hydrogen cyanide mole-
cule is the effective toxicant. The Committee indicated that indus-
trial wastes were the principal sources of cyanide in waters. Some
forage grasses and other plants contain hydrogen cyanide after freez-
ing or other stresses (Heath et al, 1973). Such plants mav at times
release hydrogen cyanide into nearby bodies of water.
The Committee considered many other compounds in relation to
water quality. Among the nitrogen containing compounds considered
were a number of pesticides. Most nitrogen containing pesticides
degrade in water, eventually forming ammonium and then nitrate.
However, the amounts of these compounds added to water in pesticides
are very small compared with other sources or forms of nitrogen.
NITROGEN IN GROUND WATER QUALITY
Nitrate is the major form of nitrogen in ground water. Three
extensive ground water surveys are summarized in Table 2.
36
-------
Table 2. Amounts of nitrogen found as nitrate, ammonium, and
nitrite in eround water samples.
Source of water
and reference
Number of
samples
Nitrogen (mg/1) in the form of
Nitrate Ammonium Nitrite
Swedish wells,
Wiklander (1977)
179
Ave. 3.2
Max. 177.0
0.1
56.5*
0.016
13.4*
Wisconsin dairy wells
Crabtree (1972)
59
Ave. 9.9
Max. 27.3
2.0
5.1
0.02
0.30
Nebraska irrigation wells 364
Olson et al
(1973)
Ave. 3.1
Max.
No
0.34 analysis
* Not included in the average.
They show average concentrations ranging from 3 to 10 mg/1 of nitrogen
as nitrate. Generally less than 1 mg/1 of nitrogen occurs as ammonium
and less than 0.1 mg/1 as nitrite. In Sweden exceptionally high con-
centrations of all forms of nitrogen were found in two old unused wells
close to farm houses. Occasional high concentrations of nitrite in
Wisconsin occurred mostly in shallow dug wells immediately after heavy
rains.
Nitrate appears to be quite stable in ground water. The filter-
ing action of soil removes bacteria and most organic matter so that
there can be little biological activity that might remove or trans-
form the nitrate. Thus, in the Arroyo Grande Basin near San Luis
Obispo, Stout and Burau (1967) followed seasonal increments of nitrate
down several cropped soil profiles.
Nitrite and ammonium, on the other hand, are not as stable as
nitrate. Feth (1966) discussed two California studies showing that
neither ion travelled far after injection into shallow ground water.
The presence of nitrite or ammonium in ground water usually indicates
surface contamination.
Concentrations of nitrate higher than the recommended limit for
37
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public water supplies have been observed In ground water in California
and several midwestern states. Ward (1970) discussed two California
surveys, the first a state-wide survey conducted in 1954 and 1955. Of
541 wells sampled, 33 had water containing more than 10 mg/1 of
nitrate-nitrogen. These high nitrate wells were located mostly in
the south central and interior basins of Southern California. During
1961, a second survey was made of 1.00 domestic wells in each of eight
Southern California counties. Samples were collected in January, April,
July, and October. Of the 800 wells, the number containing water with
more than 10 mg/1 of nitrate-nitrogen ranged from 49 in January to 34
in July. Minor forms of nitrogen were also determined and were generally
found in the following decreasing order: ammonium, organic nitrogen,
and nitrite.
In addition to the concern about its effects in drinking water,
excessive nitrate in ground water moves into surface waters in some
areas. Biggar and Corey (1969) estimated that 451 of the total nitro-
gen entering Lake Mendota in Wisconsin moved as nitrate in ground-
water. Movement of nitrate from irrigated areas into the San Luis
drain in the San Joaquin Valley has received much study (Miller and
Smith, 1976). The role of nitrate in eutrophication of streams, lakes,
and estuaries will be disfcussed in subsequent sections.
Nitrate concentrations in ground water are increasing in some
areas. Adriano et al (1971) noted concentrations averaging 54 mg/1
of nitrate-nitrogen at the top of the water table in the dairy area
between Riverside and Los Angeles. Deeper domestic wells in the
same area yielded water averaging only 6 mg/1 of nitrate-nitrogen.
The concentration should increase as the nitrate at the top of
the water table mixes into the body of ground water. In a study
of 379 Nebraska irrigation wells, Olson et al (1973) sampled the
same wells in 1961 and 1971. The average concentration of nitrate-
nitrogen increased from 2.4 to 3.1 mg/1 in 10 years. In a study of
one fertilized agricultural site in southern England Young et al
38
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(1977) projected an increase in nitrate-nitrogen concentration in
the ground water from 4 mg/1 in 1970 to 14 mg/1 in 2000.
Viets and Hageman (1971) pointed out that the hazard from nitrate
contamination depends greatly on the residence time of ground water.
Where residence time is short, as in some humid regions, nitrate would
probably not accumulate and corrective measures could be effected
rather quickly if it did. Where residence times are long, as in some
Midwestern and Plains States, nitrate contamination might not become
evident for decades after the process had started. An equally long
time would be required to correct it.
NITROGEN IN STREAM WATER QUALITY
Flow is an essential characteristic of streams that is highly
relevant to water quality. Nitrogen and other nutrients do not
accumulate in flowing streams, but are only detained temporarily in
sediments and vegetation. Typical streams change from fast, turbulent
upstream flows to slower, sometimes laminar downstream flows (Hynes,
1970). Typical vegetation changes accordingly from attached algae
and other plants adapted to running water to drifting algae and
plants that root in fine sediments. Dissolved oxygen concentrations
are always near saturation upstream, but may sometimes decrease
markedly downstream.
The forms of nitrogen in streams reflect those in the source
waters, as modified by biological activity. In the Kaskaskia River
of Illinois, Smith et al (1975) reported concurrent nitrate-
nitrogen concentrations in the river and a major tile outlet into
it over a 30-month period. Monthly average concentrations ranged
from 2 to 12 mg/1 in the river and from 5 to 12 mg/1 in the tile
outlet. The concentrations in the river and tile waters were nearly
equal except during July, August, and September, when concentrations
in the river were much lower than in the tile outlet. Relatively
39
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low flows, averaging about 30 1/sec, and warm water temperatures
favored biological activity in the river during these months. Feth
(1966) reported that in most United States streams, nitrate concen-
trations were highest between December and April, and lowest between
May and November. Other forms of nitrogen were not reported in
these studies.
In many upstream waters, the forms of nitrogen correspond well
with thosein land surface runoff. This might be expected considering
that streams generally carry suspended soil particles and decomposing
plant residues. Similar microbial transformations of nitrogen occur
in moist soils and in streams. Typical concentrations of different
forms of nitrogen in small streams and in land surface runoff are
summarized in Table 3. Although the examples are few and not directly
comparable, the dissolved nitrogen in streams is predominantly
nitrate whereas runoff sometimes contains more nitrogen as ammonium
than as nitrate. Another distinguishing characteristic of land runoff
is the occasionally high concentration of nitrogen carried on soil
particles. Concentrations of particulate nitrogen in streams decrease
as the larger soil particles settle out of the flowing water.
Removal of nutrients by vegetation growing in and along streams
has not been studied extensively. McColl (1974) showed that ammonium,
but not nitrate, was quickly removed by attached filamentous algae
and entrapped sediment in a small New Zealand forest stream. Manny
and Wetzel (1973), working on an 8 km reach of Augusta Creek in
Michigan, showed daytime decreases in nitrate, ammonium, and labile
forms of dissolved organic nitrogen compared with their nighttime
concentrations. The relative decreases were about 20% for nitrate,
50% for ammonium, and 30% for organic nitrogen. Concentrations of
all forms generally decreased slightly in the downstream direction.
Manny and Wetzel concluded that ground water influx and vegetative
uptake of the nitrogen compounds produced the observed changes.
Microbiological transformations of nitrogen occurring in streams
40
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Table 3. Amounts of nitrogen found as nitrate, ammonium, nitrite, dissolved
organic, and particulate forms in land runoff and streams.
Source of water
and reference
Typical ranges of nitrogen concentration (rog/1) in the form of
Dissolved
Nitrate Ammonium Nitrite Organic Particulate
Upper Danube
Vollenweider (1971)
1.12-1.43 0.25-0.51 0.025-0.04
—*
Nebraska streams
Olson, Seim,
and Muir (1973)
0.1-1.1
0.04-0.36
Augusta Creek, Michigan 0.63-0.98 0.02-0.10
Manny and Wetzel (1973)
0.22-0.34
Runoff, aspen-birch
forest
Timmons et al (1977)
0.08-0.33 0.07-0.55
0.97-2.33**
Runoff, southern pine
forest
Schreiber, Duffy,
and McClurkin (1976)
0.03-0.10 0.72-1.52
Runoff, Oklahoma cropland 0.3-1.4 0.05-0.20 0.01-0.04 — 1.4-3.7**
Olness et al (1975)
Runoff, Iowa cropland 0.9-4.5 0.8-3.2 — — 0.4-100
Schuman et al (1973)
Runoff, Minnesota 0.2-5 0.3-.1 — 0.3-1.2 0-180
cropland
Burwell, Timmons,
and Holt (1975)
* Dash means this form of nitrogen was not determined.
**These values include dissolved organic nitrogen.
41
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have been nicely summarized by Rhenheimer (1974). Many of the same
bacteria that are present in soils convert organic nitrogei. to
ammonium, ammonium to nitrite, or nitrite to nitrate. Numerous bac-
teria can convert organic nitrogen to ammonium. They multiply rapidly
whenever proteins become available, as in additions of sewage or
manure. The optimum temperature for this conversion is 30-35C, but
it continues even under ice with increased bacterial populations com-
pensating somewhat for their reduced activity. Specialized bacteria
convert ammonium to nitrite, and nitrite to nitrate. The dissolved
oxygen required for these conversions is usually available in streams.
Activity of the nitrite-forming bacteria practically censes at water
temperature below IOC. Activity of the nitrate-forming bacteria is
not as sensitive to low temperatures but may be more sensitive to low
oxygen supply. Microbial denitrification (reduction of nitrate to
gaseous nitrogen) and nitrogen fixation (reduction of gaseous nitrogen
to ammonium) do not appear to be important in stream waters.
The transformations of nitrogen in stream sediments and in stream-
bank soils in relation to water quality have not been studied exten-
sively. For small streams, these transformations may be more signifi-
cant than those that take place in the water. Edwards and Rolley
(1965) showed high rates of denitrification of sediment cores from
English rivers.
Undesirable plant growth in streams is not easily related to ex-
cessive concentrations of nitrogen compounds. In various cases, growth
may be limited by shading, turbidity, flow conditions, or low concen-
tration of other plant nutrients. Patrick (1973) found that concen-
trations of nitrogen, mainly as nitrate, up to 2.5 mg/1 did not
produce excessive algae growth. Similarly, Moore (1977) found that
density of attached filamentous algae was only slightly influenced by
nitrate-nitrogen concentrations up to 5 mg/1 in a small English stream.
The density of algae attached to mud or stones was controlled mainly
by light intensity and water velocity. None of the factors, tempera-
42
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ture, pH, nor levels of dissolved phosphate, silica or nitrate had
any major effect on the density of algae attached to plants.
NITROGEN IN IMPOUNDMENT WATER QUALITY
Impoundment of water in lakes, ponds, or reservoirs permits
greater recycling of nitrogen and other nutrients than is possible
in streams. Through the growth and decay of organisms, the supply
of nitrogen compounds in an impoundment may be reused several times
during a year. Algae and other aquatic plants absorb most of the
nitrate and ammonium from impoundments when growing conditions are
favorable. Dead plant and animal bodies settle to the bottom, where
microbiological decay gradually releases ammonium back into the
water. In deep water the cycle may be completed within the water
column.
Thus, the forms of nitrogen in impoundments reflect biological
activity to a greater extent than do the forms of nitrogen in
streams. The forms vary with season and oxygen depletion of the
water. This subject is discussed in an informative chapter on
nitrogen in "Limnology" by Wetzel (1975) .
Organic forms of nitrogen predominate in many impoundments. Con-
centrations are usually in the range of 0.1 to 1 mg/1 of organic
nitrogen. In summer, much organic nitrogen is contained in algae
and other organisms. However, on the average, more organic nitrogen
occurs as dissolved amino acids, peptides, or more complex compounds
excreted from living or decaying organisms. The amount of dissolved
organic nitrogen in impounded water is often greater than the amount
of inorganic nitrogen.
The forms of inorganic nitrogen in impoundments depend greatly
on the presence of dissolved oxygen. When oxygen is exhausted, as
it may be in deep unmixed water or under prolonged ice cover,
nitrate is reduced to nitrite and gaseous nitrogen. Ammonium
43
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released from decaying organic matter under these conditions is not
oxidized. More than 5 mg/1 of ammonium-nitrogen may accumulate
near the sediments in eutrophic impoundments. Concentrations of
nitrite-nitrogen rarely exceed 0.1 mg/1. These accumulations of
ammonium and nitrite are alleviated in spring and fall in most
impoundments when the surface and deep waters reach the same tempera-
ture and mix freely. At such times the deep waters receive oxygen,
which allows ammonium and nitrite to be oxidized to nitrate. Nitrate
is usually the predominant form in surface water with concentrations
in winter and early spring ranging up to 4 mg/1 of nitrate nitrogen.
However, summer algae growth reduces the concentrations in many
impoundments to less than 0.01 mg/1 of nitrate-nitrogen.
The water leaving an impoundment usually contain." less nitrogen
than the water entering it. Olson et al (1973) found nitrate-
nitrogen concentrations of 0.5-1.2 mg/1 entering, and 0.0-0.1
mg/1 leaving three Nebraska reservoirs. Ammonium-nitrogen concentra-
tions were about the same entering and leaving, 0.14-0.31 mg/1.
Similar results were reported from a central Missouri reservoir by
Rausch and Schreiber (1977) . They also measured the retention,
or trapping of phosphate and sediment in the reservoir. Thus, im-
poundments may be useful in management of several aspects of water
quality.
The major input of nitrogen into reservoirs is usually in stream-
flow. But, in some lakes, such as Lake Mendota, groundwater inflow
is the main source of nitrogen (Biggar and Corey, 1969).
Nitrate or ammonium dissolved in rainfall and nitrogen fixation
are other natural sources of nitrogen that may be important in some
impoundments. The total nitrogen dissolved as nitrate and ammonium
in rainfall usually ranges from 0.5 to 2 mg/1 (Junge, 1958). These
concentrations are similar to those found in streams, and higher than
those found in most impoundments. Assuming a concentration of 1 mg/1
and rainfall amounting to 80 cm annually, the deposition of dissolved
44
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nitrogen amounts to 8 kg/ha. Biggar and Corey (1969) estimated that
rainfall contributed 17% of the nitrogen entering Lake Mendota.
Nitrogen fixation and denitrification seem to play an enhanced
role in the nitrogen balance of eutrophic impoundments. Annual fixa-
2
tion of nitrogen by blue-green algae amounted to 0.1 g/m in moderately
productive Lake Windermere of. England (Home and Fogg, 1970) and 0.4
2
g/m in eutrophic Clear Lake of California (Home and Goldman, 1972).
These amounts were less than 1% of the total nitrogen input to Lake
Windermere, but were 43% of the input to Clear Lake. Nitrogen fixa-
tion provided 14% of the nitrogen input to moderately eutrophic Lake
Mendota in Wisconsin (Biggar and Corey, 1969). Denitrification is
carried out by many bacteria in the absence of dissolved oxygen.
Brezonik and Lee (1968) estimated that denitrification removed 11%
of the nitrogen input to Lake Mendota. Significant losses of nitro-
gen, amounting to 44% of the nitrogen regenerated from decaying
organic matter, were noted in Lake Erie and presumed to be a result
of denitrification (Burns and Ross, 1972). Hutchinson (1957) believed
that significant ammonia volatilization from some lakes was possible
during the fall mixing of surface and deep waters.
Thus, it appears that nitrogen fixation and denitrification tend
to establish a self-regulating balance of nitrogen with other nutrients
in eutrophic impoundments. Sehindler (1977) pointed out that nitrogen-
fixing blue-green algae develop in lakes that are fertilized with
phosphate. This compensates for the relative deficiency of nitrogen.
Denitrification provides a mechanism for eliminating excess accumula-
tions of nitrogen. Neither nitrogen fixation nor denitrification are
thought to be significant in unenriched lakes (Hutchinson, 1957).
Algae or plant growth in impoundments is related to the maximum
nitrate-nitrogen concentrations that occur in early spring. Vollenweider
(1971) indicates that lakes with very low productivity contain less
than 0.2 mg/1 and eutrophic lakes contain from 0.5 to 1.5 mg/l of
nitrate-nitrogen. Because of the self-regulating feature just mentioned,
45
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the nitrate concentrations may be as much the effect of lnke pro-
ductivity as its cause.
Nitrate additions alone do not increase algal growth in the
majority of lake waters. In samples of water from 49 widely dis-
tributed U.S. lakes, algal growth was limited by phosphate in 35,
nitrate in 8, and other factors in 6 (Miller et al, 1974). The
nitrate-limited lakes were mainly eutrophic lakes located
in western states. In 29 lakes, addition of nitrate and phosphate in-
creased growth more than addition of phosphate alone.
Little is known with certainty about the effects of nitrogen
compounds on competition among plants in impoundments. Schindler
(1977) showed that enrichment with nutrients other than nitrogen
favors growth of blue-green algae. He therefore suggested that adding
nitrate to keep a nutrient balance might favor the growth of more
desirable green algae. Much research remains to be done in this area.
NITROGEN IN MARINE WATER QUALITY
Nitrogen is more frequently a limiting element for growth in
marine environments than it is in streams or fresh water impound-
ments. We will consider briefly some nitrogen relations in the open
ocean, in estuaries, and in near-shore areas.
Nitrate is the predominant form of nitrogen in deep ocean water.
Concentrations generally range from 0.3 to 0.6 mg/1 of nitrate-
nitrogen (Sverdrup et al., 1942). The concentrations of nitrite- and
ammonium-nitrogen rarely exceed 0.05 mg/1. The highest concentrations
of nitrite are found only in a thin layer at depths from 100 to 400
meters. Concentrations of ammonium are more uniform through the
water column.
In surface ocean water, biological uptake depletes the supply of
inorganic nitrogen. Concentrations of particulate and dissolved or-
ganic nitrogen range widely from 0.0014 to 0.14 mg/1 (Sverdrup et al,
46
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1942). The higher concentrations are found where deep ocean water
wells up to the surface, as along the Southern California coast or the
western coasts of South America and Africa. Gradual incorporation of
nitrogen into organisms, their death, and their sedimentation lowers
the content of all forms of nitrogen in surface water of the open
ocean. Recycling of organic nitrogen is necessary to support new
growth in these areas. Ryther and Dunstan (1971) pointed out that a
slower recycling rate for nitrogen than for phosphorus causes nitrogen
to limit growth in much of the ocean.
Estuaries and near shore areas receive nutrients from land runoff.
Concentrations of inorganic and organic nitrogen approach those in
streams and impoundments in some areas. Up to 1 mg/1 of nitrate-nitrogen
is observed in parts of San Francisco Bay (Bain and Hoag, 1972).
Along the south shore of Long Island, areas receiving runoff from duck
farms developed dense blooms of algae (Ryther and Dunstan, 1971).
Nitrogen was found to be the limiting nutrient for the algae in this
case. Phosphorus and other nutrients were relatively more abundant
in the runoff.
Nitrogen fixation and denitrification may be significant in marshy
areas (Nixon et al, 1976). They are thought to be less important in
the ocean in the short run (Ryther and Dunstan, 1971), although they
may regulate the balance of nitrogen with other nutrients in the ocean
over geologic time.
There is no evidence that nitrogen levels can change species compo-
sition in the marine environment. Frey (1977) found that nitrogen and
phosphorus levels controlled the total growth of marine algae, but that
micro-nutrients and vitamins had more effect on species composition.
Carpenter and Guillard (1971) found that several species of marine
algae developed strains adapted to different concentrations of nitrate-
nitrogen.
47
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SUMMARY AND CONCLUSIONS
The intended use of water determines the .significance of various
forms of nitrogen in water quality. Upper limit concentrations have
been recommended for several forms of nitrogen in water used for
public water supplies, freshwater aquatic life, marine aquatic life, and
agricultural uses. The concentrations are lowest for cyanide, and
increase in the order ammonium, nitrite, and nitrate. Nevertheless,
nitrate is more often of concern in water quality than are the first
three forms of nitrogen.
The recommended upper limit concentration for nitrate-nitrogen
in public water supplies is 10 mg/1. The actual concentration in ground
water exceeds this limit in some areas, and may be increasing. Because
of the slow movement of ground water, increasing concentrations may
take many years to correct.
Nitrate is depleted from surface waters by plant uptake, and may
cause excessive growth of algae or other plants. This is seldom a
problem in streams, but may become one in impoundments, estuaries,
and near-shore ocean areas.
Much of the total nitrogen in surface water occurs in particulate
or dissolved organic forms. Plant growth is often excessive when the
total nitrogen concentration exceeds 0.5 mg/1 if other nutrients are
adequately supplied and growth conditions are favorable. Nitrogen
supply limits growth in many ocean areas and in a few lakes, mainly
eutrophic ones. Nitrogen fixation by blue-green algae tends to
correct nitrogen deficiencies. Denitrification by bacteria tends to
correct excesses. These features indicate that high nitrogen concen-
trations may be an effect, rather than a cause, of eutrophication.
48
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salt in soils and ground waters from land disposal of dairy
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Olson, R. A., E. C. Seim, and J. Muir. 1973. Influence of agricultural
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OVERVIEW OF NITROGEN IN IRRIGATED AGRICULTURE-'
2/
R. S. Rauschkolb
Events of the recent past have been moving agriculture rapidly
towards the point where it can no longer afford the luxury of being
concerned with production of food and fiber alone. One of the prin-
ciple reasons for the change in attitudes has been the Federal Water
Pollution Control Act Amendment of 1972, commonly referred to as
Public Law 92-500. Within this law there is a section dealing with
areawide planning, Section 208. Under this section each state is to
develop waste water management strategies which indicate methods for
control or treatment of all point and non-point sources of pollution
within an area. Specific outputs resulting from Section 208 area-
wide planning include a regulatory program to control or treat all
point and non-point pollution sources, including in-place or accumu-
lated pollution sources. This represents only one of the many outputs
that are expected from area 208 planning, but it is one that seems to
be the most important with respect to nitrogen management in irrigated
agriculture.
Another factor, which will have an impact on the management of
nitrogen, is the cost of energy. As fossil fuel becomes more costly
and less available, the cost of fertilizer nitrogen will substant-
ially increase which may result in the grower having to become more
- An abstract of this short paper is not provided because it is
essentially an abstract to the introduction to a Report from R. S.
Rauschkolb to the RSKERL, EPA (A. G. Hornsby), Ada, Oklahoma
74820, on Nitrogen Management in Irrigated Agriculture.
2 /
—Cooperative Extension, University of California, Davis, Califor-
nia 95616.
53
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efficient in the utilization of his fertilizer resources by virtue
of cost. This does not appear at present to be a particularly
strong motivating force for achieving efficiency because demand for
food is going to increase as population increases, and there is no
alternative to food production to meet the demand. Therefore, as
cost of production increases, the increased cost will be passed on
3 /
to thfc consumer. According to Stout—, the on-farm requirement for
nitrogen to maintain a diet comparable to what the U.S. population
is accustomed is about 82 kg/nitrogen/capita/yr. This is the amount
of nitrogen required to provide the animal and plant protein we
currently consume, and it also includes inefficiencies and various
losses inherent in the agricultural food production system. Based
on 1977 population this means about 18 million metric tons of farm-
site nitrogen; about 50% of this nitrogen demand is met by manufactured
commercial fertilizer nitrogen. The remainder of the nitrogen demand
is made up by biological nitrogen fixation and recycling of nitrogen
released from organic matter as well as natural chemical fixation.
Improvement in efficiencies and reduction of losses are certain
to occur. It is felt these will be compensated for by increased
population so that the farm-site nitrogen demand may remain fairly
steady for a few years. However, the percentage of the farm-site
nitrogen requirement met by the use of commercial fertilizers will
in all likelihood increase.
Although this quantity of nitrogen seems quite large it is very
small indeed in comparison to the total amount of nitrogen present
in the earth and its atmosphere. The quantity is estimated by
Delwiche (] 970) to be on the order of 23 x 10^ metric tons of
nitrogen. Using various estimates of the amount of nitrogen being
fixed on a global basis, less than 1 millionth of a percent of the total
nitrogen is being recycled. The amazing aspect of this is that in
3 /
— Stout, P. R. 1971. Agricultural requirements for nitrogen
fertilizers in the USA. Unpublished report, University of Califor-
nia, Davis, California 95616.
54
-------
spite of all the nitrogen that is present in the soil, water, air,
plant and animal system, the general case in an agricultural system
is for nitrogen to be deficient with respect to the amount of avail-
able nitrogen required for maximum plant growth. Consequently, there
is the need to add nitrogen in order to produce the food and fiber
needed.
The use of nitrogen fertilizers also has some beneficial effects
with respect to utilization of our land and water resources. In a
recent article on the environmental benefits of intensive crop pro-
duction, Barrons (1971) shows a decrease in the number of acres
used for plant production from approximately 365 million to about
290 million in a period from 1930 to 1970. During the same period,
the population in the United States grew from approximately 120 to
slightly more than 200 million. There was an increased food demand
that was met by utilizing less acres for production of food and fiber.
In the same article Barrons compared the number of acres required for
the production of 17 crops in the 1938 to 1940 period to meet the
production required for the 1968-1970 period for these same crops.
Although other factors such as improved management, improved varieties,
and pest control played an important role in the increased production,
the widespread use of nitrogen fertilizer had a major impact on the
increased production per unit area of land or per unit input of water.
Consequently, it has substituted for increased development of water
supplies and arable land. The number of acres saved, as indicated by
Barrons, was approximately 292 million acres. In spite of the many
benefits which can be ascribed to the use of nitrogen fertilizers
there is ample evidence to indicate that inefficient utilization of
our nitrogen fertilizer resources can lead to polLutian of surface and
ground water supplies. And, some of the pollution is attributable to
the natural presence of nitrogen in vast quantities in our surroundings.
There has been considerable debate in recent years among scien-
tists, environmentalists, and others in state and national institutions
55
-------
and agencies with respect to the best methods to employ for minimizing
the nitrogen pollution potential coming from various sources. The
debate has centered around two possible mechanisms for minimizing
nitrogen pollution potential, and these are (1) treatment of the
discharge waters or (2) control of inputs in order to minimize nitro-
gen output. For domestic and industrial waste water discharge where
the discharge occurs at an identifiable point and where the volumes
are relatively small, at least in comparison with the volume of
water used in agriculture, it appears that treatment of waste water
is the most appropriate method to minimize nitrogen pollution potential.
In an agricultural system the situation is very different in that there
are two types of discharges and volumes are much greater. One is an
identifiable point source such as might occur in discharge of effluent
from a drainage tile and the other system is a diffuse non—point
discharge and is typified by water percolating below the root zone
to an underground water table or by subsurface flow to return to
the surface waters. Although the technology exists for treatment of
the point source discharges, it is generally agreed that the volumes
of water which must be handled make this an economical impracticable
alternative. Because of the diffuse nature of percolating water and
the virtual impossibility of collecting these waters for treatment,
it is generally agreed, or at least conceded, that treatment of this
non-point sources of nitrogen pollution is not a viable alternative.
Monitoring of nitrogen concentration in percolating waters has
been suggested as one technique for evaluating nitrogen pollution
potential of these percolating waters. The idea being that if un-
satisfactory levels were detected then corrections could be made in
the amount of nitrogen input. However, one of the problems inherent
in using this technique is the extreme variability which is known to
occur with nitrogen concentration in soils. In a recent study in
California, funded by the Research Applied to National Needs Division
of the National Science Foundation, it was found by Rible et al (1976)
56
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that in order to be within 20% of the mean nitrate-nitrogen concentration
when there were 9 samples taken per hole, it would require anywhere
from 5 to 20 holes per site to reach that level of precision. They
found the number of holes that were required varied depending upon irri-
gation method, crop type and soil texture. Even if one could afford
the cost incurred to reach the level of precision indicated, that level
of precision is not satisfactory as a method for determing whether
excessive nitrogen inputs have been made or not. In addition, that
level of precision is thought by some to be no better than what might
be achieved through nitrogen management and water management to control
nitrogen pollution potential. Furthermore, it is even expected that
some control measures will result in a reduction of cost rather than
an increase in cost.
So, it appears that the least costly, most effective method for
minimizing nitrogen pollution potential in irrigated agriculture is to
employ management techniques which result in the greatest crop output
for each unit of input of nitrogen and water. In addition to being an
effective means of minimizing nitrogen pollution potential, this
technique also allows the use of a simple method to monitor the
effectiveness of control measures. By use of figures that are easily
obtainable from existing farm records it is possible to determine the
crop output per unit of nitrogen input. However, the variations which
occur in an agricultural system and in the management variables that
may be employed in an agricultural system will determine the peak
efficiency that may be attained. Using this approach, peak operational
efficiency can be the goal which will lead to greater nitrogen use
efficiency with a concomittant reduction in the nitrogen pollution
potential.
An understanding of the factors which are operating in the
agricultural plant production system is essential in order to under-
stand how various techniques of nitrogen and water management may be
employed on a site specific basis to reduce the nitrogen pollution
57
-------
potential. This approach takes into account the variation that occurs
within the agricultural system and applies management technology to
achieve a minimization or elimination of unwanted entry of nitrogen
into surface and ground water resources. There are the independent
system variables such as soil, crop, irrigation methods, nitrogen
cycle, and environment (SCINE) which defines the system within which
management techniques can be employed. Although it is recognized that
not all these major factors delineating the system are entirely
independent of one another they are sufficiently independent of one
another as to warrant their separate consideration in attempting to
learn how to adapt management alternatives to mitigate unwanted effects.
Another aspect of the system is the dependent management variables
which can be employed within a given set of conditions to increase
nitrogen use efficiency. These management variables are placement,
equipment, rate, source, irrigation management and energy (organic
matter) which allows the assignment of an acronym PER SITE. This
assists in recognition of those management variables which are
functioning within a given set of conditions for the agricultural
system and at the same time connotes the importance of considering
the site specific factors which determines how each of the management
techniques will be employed. A feature of the management techniques
and indeed the problem that makes their application difficult is that
they are so greatly interdependent. Learning how to utilize combina-
tions of management variables to have an impact on reducing nitrogen
pollution potential is the challenge facing those charged with
mitigating possible pollution effects.
Methods of implementing on-farm control measures for nitrogen
management in irrigated agriculture will require field personnel with
an understanding of soil, watei and plant relationships who can guide
the farmer in the application of control measures which provide
flexibility, ease of implementation and are adaptable to a wide
range of conditions and managerial abilities. Agriculture does not
58
-------
exist independently from all other segments of society. Consequently,
it shares concerns of others with regard to conservation of energy,
land and water resources as well as the impact of man's activities
on the environment. In addition, it is in agriculture's best interest
to maintain an environment compatible with the needs of others since
such an environment is also essential to the continued production of
food and fiber.
59
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LITERATURE CITED
Barrons, K. C. 1971. Environmental Benefits of Intensive Crop
Production. Down to Earth. 27:18-22.
Delwiche, C. C 1970. The Nitrogen Cycle. Scientific. American
223:137-146.
Rible, J. M., P. A. Nash, P. F. Pratt, and L. J. Lund. 1976.
Sampling the Unsaturated Zone of Irrigated Land for Reliable
Estimates of Nitrate Concentrations. Soil Sci. Soc. Am.
Journal. 40:566-570.
60
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SOURCES OF NITROGEN FOR CROP UTILIZATION
L. S. Murphy—''
ABSTRACT
Nitrogen fertilization of crops is essential for maximum food
production. Nitrogen from organic matter in the soil declines with
continued cultivation to the point that supplemental nitrogen appli-
cations are necessary. Supplemental sources of nitrogen for crop
production include symbiotically fixed nitrogen from legumes, non-
symbiotically fixed nitrogen from free-living organisms in the soil,
inorganic nitrogen from lightning discharges and industrial emissions,
industrially fixed nitrogen and nitrogen from various waste products.
All sources of nitrogen for plants with the exception of sym-
biotically and non-symbiotically fixed nitrogen undergo the same chemi-
stry in the soil from application until plant use. Under aerobic con-
ditions, all nitrogen applied to the soil will eventually reach the
nitrate form. Plants are therefore unable to distinguish the original
source of nitrogen.
Symbiotically fixed nitrogen supplies important amounts of this
element to legumes and crops which follow the legumes. Industrially
fixed nitrogen plays a major role in supplying nitrogen to crops in most
areas of the world. Characteristics of manufactured nitrogen products
differ more than those of plant and animal residues. Those character-
istics and their effects on utilization of these nitrogen sources are
discussed in detail in this review. Also discussed are practices in-
tended to maximize the efficiency of applied nitrogen from any source.
— Great Plains Director, Potash/Phosphate Institute, Manhattan,
Kansas 66502.
61
-------
Use of animal wastes is discussed with emphasis on determining
application rates by nutrient needs.
Recommendations for nitrogen applications in either dryland or
irrigated agriculture are based on crop need as determined by agri-
cultural research. Soil analysis is an important tool in determina-
tion of needs. Use of present and future recommendations will both
maximize production of food and maintain environmental quality.
INTRODUCTION
Most of the nitrogen in the Earth and its atmosphere is the inert
gas N^ which represents about 78% of the atmosphere. In that form,
however, nitrogen cannot be utilized directly by higher plants. This
chapter will serve to outline the processes involving conversion of
elemental nitrogen into fixed forms which can be utilized by plants
but will focus on properties and use of nitrogen sources. Fixation
by both the activities of symbiotic bacteria such as Rhizobia and
also by the techniques which have been devised industrially to convert
elemental nitrogen into fixed forms will be considered.
The characteristics of nitrogen sources including their agronomic
properties will be considered.
FORMS OF SOIL NITROGEN
Most of the nitrogen which exists in the soil is present in
organic matter and occurs in a very large number of compounds in-
cluding many of the nitrogen-containing materials normally found in
plants. If we analyzed the soil for its organic nitrogen compounds,
we would find some proteins, amino acids, amides, amines, aklaloids,
nucleic acids, nucleotides, and others. The fact which we should
keep in mind concerning all these organic compounds, however, is that
62
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the nitrogen contained in them is all essentially the same chemical
form. Nitrogen in these organic compounds exists in the form of
amino (NH^ ) groups which represent the same oxidation state (-3)
as represented by ammonia (NH^) and ammonium (NH^) .
SYMBIOTIC NITROGEN FIXATION
An important contribution to nitrogen nutrition of all plants is
made by certain species, principally the legumes which enter into a
symbiotic relationship with certain soil bacteria. In this relation-
ship, the bacteria inhabit nodules on host plant roots and are pro-
vided with carbohydrates by the host plant. The bacteria subsequently
convert elemental nitrogen into a form usable by the host plant. The
contribution of this form of nitrogen to plant nutrition in general is
substantial. Quantities of nitrogen fixed in any symbiotic relation-
ship between bacteria such as Rhizobia and a legume such as alfalfa
vary with the strain of the organism, the condition of the host plant
and various environmental conditions. Nitrogen fixation by this pro-
cess has been estimated as high as 500 kg nitrogen/ha by a crop of
clover in New Zealand. However, climatic conditions and soil condi-
tions must be optimum for such large amounts. Some indication of the
amount of nitrogen fixed by various legumes is presented in Table 1.
The type of bacteria which produces nitrogen in the nodules on
roots of one crop may not be adapted to a second host plant. For
instance, the inoculum which is normally applied to soybeans prior
to seeding to insure adequate numbers of Rhizobia for nitrogen fix-
ation is not the same strain that would be utilized in inoculating
alfalfa.
Environmental conditions favoring nitrogen fixation symbiotically
include a relatively high soil pH (7.0 or higher), adequate soil mois-
ture, a good supply of available calcium, warm temperatures, and a low
supply of available inorganic soil nitrogen. Symbiotic nitrogen fixing
63
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bacteria have a high requirement for calcium, thus the relationship of
both pH and available calcium to their activity.
Temperature affects the activity of any biological process, and
thus, cooling the soil temperature to near freezing reduces nitrogen
fixation to near zero. The temperature would also affect the host
plant. Inadequate numbers of bacteria in the soil obviously would re-
duce fixation. These organisms are normally added to the seed of
legumes at least when the production of such legumes is initiated in
a given field.
Referring back to inorganic nitrogen supplies, researchers have
reported that applications of available nitrogen prior to legume
seeding reduce the number of nodules and reduce the amount of nitrogen
fixed by the host plant-bacteria relationship. Apparently, the
activity of the organisms is sensitive to the presence of such forms
of nitrogen as ammonium and nitrate. An indication of the effects of
nitrogen on nitrogen fixation under controlled conditions and on number
of nodules on soybean plants is presented in Tables 2 and 3.
Use of symbiotically fixed nitrogen by other than the host plant
involves the residual effects of legume production. Nitrogen contained
in the roots and straw of the preceeding crop becomes available follow-
ing mineralization. Voss (1978) has provided some estimates of the
contribution of a legume to a following crop (Table 4). Note that
the contribution is based on the amount of legume in the stand of
plants preceeding the crop in question. Raney and Sharplaz (1975)
reported yield increases of irrigated corn as high as 3300 kg/ha in
the third crop following two years of alfalfa (Table 5). Legume
sources of nitrogen should be taken into account in the following crop
and proper adjustments made in the nitrogen to be applied to that
crop. This adjustment is important with regard to costs of production,
efficient use of nitrogen and control of leaching of nitrate-nitrogen
toward shallow aquifers. The contribution from a legume to the follow-
ing crops will decline with time. In the Raney and Sharplaz data,
64
-------
Table 1. Estimated fixation of nitrogen by legumes.*
Legume
Nitrogen Fixed
Kg/ha
Legume
Nitrogen Fixed
Kg/ha
Alfalfa
220
Lespedezas (annual)
95
Ladino clover
200
Vetch
90
Sweet clover
130
Peas
80
Red clover
125
Soybeans
110
Kudzu
120
Winter peas
60
White clover
115
Peanuts
47
Cowpeas
100
Edible beans
45
*Compiled from Alexander (1961) and Tisdale .and Nelson (1975).
Table 2. Effect of ammonium on nitrogen fixation in gravel culture.*
Ammonium
nitrogen
added
Sources of Plant Nitrogen
Fertilizer Air
Portion of nitrogen
from air
mg/pot
mg
mg
%
Soybeans
0
0
1639
100
80
68
1692
95
320
252
2243
89
560
464
2185
82
800
648
2423
79
Ladino clover
0
0
188
100
80
63
234
75
320
282
159
35
560
527
98
15
800
609
82
12
*Allos and
Bartholomew (1959).
65
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Table 3.
Effects of nitrogen carriers and time of nitrogen appli-
cation on the nodulation of dryland soybeans. All
carriers were added at a rate of 168 kg nitrogen/ha.
Nitrogen
Carrier
Time of
Application
Nodules/
5 Plants
Mean Values
Nodules/
5 Plants
No nitrogen
445
Ammonia
171
Ammonia
Pre-plant
144
Urea
294
Ammonia
Split
281
Nitrogen
solution
251
Ammonia
Sidedress
391
Calcium
nitrate
294
Urea
Pre-plant
141
SCU-30
226
Urea
Urea
Spl it
Sidedress
253
488
LSD _ r
.05 for
Nitrogen Carrier
NS
Nitrogen
Solution
Pre-plant
125
Pre-plant
134
Nitrogen
Solution
Split
182
Split
251
Nitrogen
Solution
Sidedress
447
Sidedress
416
Calcium
nitrate
Calcium
nitrate
Pre-plant
Split
142
295
LSD.05 for 56
Time of Application
Calcium
nitrate
Sidedress
445
SCU-30^
Pre-plant
118
SCU-30
Split
247
SCU-30
Sidedress
313
LSD.05
Treatment
122
— Meyer et al (1974).
2/
— Sulfur-coated urea.
66
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Table 4. Corn yields from two crop-rotation fertility experi-
ments-Iowa. Data are in kg/ha.
Nitrogen added, kg/ha
Cropping system —g^^^ ^02
Clarion-Webster Research Center, 1972-76
Continuous corn
2885
—
3763
—
6209 7275
Corn after soybeans
5896
—
6586
—
7526 7589
Corn after 1 yr. meadow
8091
—
7777
—
8091 8216
Corn after 2 yr. meadow
8655
—
7840
—
7150 8091
Northwest
Research
Center,
1957-76
Continuous corn
4390
5833
—
6397
6648
Corn after soybeans
5832
6523
—
6836
7025
Corn after 1 yr. meadow
6460
6648
—
6962
—
Corn after 2 yr. meadow
7025
6899
—
6899
—
Estimated nitrogen contribution of the legume crop to the following
corn crop.
First-year corn Kg nitrogen/ha
following: contribution from legume
157
112
22
112
1 Kg nitrogen/60 Kg soybeans
Voss (1978)
50-100% legume meadow
20-50% legume meadow
0-20% legume meadow
Legume green manure
Soybeans for grain
67
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Table 5. Effects of nitrogen, phosphorus, and residual nitrogen
from alfalfa on irrigated corn.*
Nitro-
gen
Phos-
phorus
.05
Corn yield
no
alfalfa
Corn yield
third crop
after 2
years
alfalfa
Yield differences due
to alfalfa effects
1975
-kg/ha-
1974
0
0
2885
3981
1129
2070
90
0
7652
9596
1944
1693
90
49
6460
9784
3324
2822
179
49
10349
10035
-313
1505
267
0
11415
13171
1756
-63
267
49
12293
12544
251
-125
Mean
8530
9847
1317
1254
LSD
1631
2822
*Raney et al (1975)
Table 6. Nonsymbiotic nitrogen fixation under variable conditions
(Delwiche et al, 1956).
Treatment
Soil
Fixed nitrogen present as Total
nitrogen
fixed
Ammonia
Nitrate
10 g. Yolo soil
+ 780 rag. sucrose
Davis
0.055
-meq/100 g soil-
0.028
0.083
Growing lawn,
Davis
—
0.0004
0.0004
photosynthesizing
Berkeley
0.
o
o
0.001
0.005
Inverted lawn,
Davis
0.
.18
0.26
0.44
decaying
Berkeley
0.
.087
0.047
0.134
Soil with grass
Davis
0.
.001
0.007
0.008
removed
Berkeley
0.
.001
0.002
0.003
68
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however, the contribution to the third corn crop was just about the
same as that contributed by the alfalfa to the second corn crop.
NON-SYMBIOTIC NITROGEN FIXATION
Fixation of elemental nitrogen by bacteria other than those
inhabiting the nodules of legumes and other plants has recently come
under scrutiny. Nitrogen fixation by certain organisms which exist
free of any host plant relationship has been indicated by studies in
soil microbiology. The organisms which are involved in such nitrogen
fixation include blue-green algae and certain free living bacteria
such as Azotobacter, Clostridium, and Rhodospirillum. Rhodospirilium
is a photosynthetic organism while Clostridium and Azotobacter are
saprophytic organisms of anaerobic and aerobic characteristics,
respectively.
Photosynthetic organisms, including blue-green algae and Rhodo-
spirillum, are thought to make only minor contributions to the nitro-
gen in upland agricultural soils because of the requirement of sun-
light in the physiological activity of these organisms. They may play
a more major role, however, in soil which is wet for extended periods
such as the culture of rice.
Other organisms obtain their energy from the oxidation of organic
carbon (organic matter) and may be able to contribute more nitrogen
to the environment than the photosynthetic organisms. Microbiologists
have estimated that nitrogen fixed by some of these organisms may
range from 20-30 kg/ha annually but most researchers feel that the
value is much lower, around 7 kg/ha. Some indication of the amount of
nitrogen which has been fixed by free living organisms in a given soil
is presented in Table 6 (Delwiche et al, 1956).
69
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ATMOSPHERIC SOURCES OF FIXED INORGANIC NITROGEN
Lightning results in the conversion of some elemental nitrogen
(N^) to oxides of nitrogen such as nitric oxide (NO) and nitrogen
dioxide (NO^). Lightning produces extremely high air temperatures
which in the presence of oxygen and elemental nitrogen result in
the production of nitric oxide. This gas combined with oxygen to pro-
duce nitrogen dioxide (NO^) which is absorbed in water producing
nitric acid. Nitrogen oxides from industrial sources and automobile
exhaust also provide a source of nitric acid in rainfall.
Another form of nitrogen in the atmosphere which has become in-
creasingly common with industrialization is ammonia gas. This gas is
soluble in water and i.s readily washed out of the atmosphere with
rainfall. It is also absorbed directly from the air by surface water
bodies and by soil colloids. Studies in industrialized areas have
indicated that 60 to 70 kg nitrogen/ha could be supplied as ammonia
from industrial contamination of the atmosphere.
The contribution of fixed atmospheric nitrogen to the soil from
all sources is generally less than the amounts quoted above. Con-
sidering the low industrialization of much of the area of the U.S. and
the world, contributions of these sources to soil nitrogen supplies is
probably less than 10 kg/ha/year on the average.
COMMERCIAL NITROGEN FERTILIZERS
Anhydrous Ammonia Production
The production of most nitrogen fertilizers in the world is
dependent upon the synthesis of anhydrous ammonia (NH^). Commercial
production of this compound is based on the discoveries of two
German chemists, Fritz Haber and Carl Bosch, in 1908. Their work
with equilibrium chemistry and the subsequent development of a suit-
able catalyst for the combination of nitrogen and hydrogen under
70
-------
pressure led to development of the process used throughout the
world. The first small ammonia synthesis unit was placed in operation
in 1911. Properties of anhydrous ammonia are presented in Table 7.
Table 7. Properties of anhydrous ammonia.*
Color
Odor
Molecular weight
Weight per gallon at 60F
Weight per cubic foot at 60F
Boiling point at 760 mm Hg
Freezing point at 760 mm Hg
Heat of fusion
Heat of vaporization at 760 mm Hg
Critical temperature
Critical pressure (absolute)
Critical density
Dielectric constant ag 760 mm Hg
Vapor at OC, 1 x 10 cycles/sec
Liquid at -34C, 4 x 10 cycles/sec
Solid at -90C, 4 x 10 cycles/sec
Electrical conductivity of liquid at
freezing point
Viscosity
Liquid at -33.5C
Vapor at -78.5C
0.0C
20.0C
100.0C
Solubility in water
Solubility in alcohol
Heat of formation gas (26C, 1 atm)
Decomposition temperature
Colorless
Pungent - Sharp
17.03
5.14 lb
38.45 lb
-33.35C (-28F)
-77.7C (-107.9F)
79.4 cal/g (143BTU/lb)
327.4 cal/g (589.3 BTU/lb)
132.4C (270.3F) _
111.5 atm (1,639 lb/in 1
0.235 g/ml (14.67 lb/ft )
1.0072
22.0
44.01
—8
13 x 10 mho/cm
0.266 centipoises
0.00672 centipoises
0.00926 centipoises
0.01080 centipoises
0.01303 centipoises
Very soluble
Soluble
1,160 BTU/lb (644 cal/g)
871.1-872.2C (1,600-1,800F)
*McVickar et al, 1966.
Anhydrous ammonia production in the United States alone totaled
15.36 million metric tons in 1977. In the United States, Australia
and certain parts of western Europe, anhydrous ammonia is utilized for
direct soil application. In most other countries of the world,
anhydrous ammonia is utilized entirely for production of other ferti-
71
-------
lizers and other industrial uses, but little is consumed by direct
application. Direct application of ammonia in the United States
accounts for about 40% of the nitrogen applications.
Direct Application of Anhydrous Ammonia
Application in Irrigation Water. The first use of anhydrous
ammonia for direct application as a fertilizer involved injection of
ammonia into irrigation water in California (Waynick, 1934, Rosenstein,
1936). These early efforts demonstrated that ammonia was a feasible
source of nitrogen but also revealed that problems can occur when
ammonia is injected into water containing large quantities of
dissolved calcium and magnesium salts (Warnock, 1966).
When ammonia is injected into irrigation water, an equilibrium is
set up which generates hydroxyl (OH ) ions along with ammonium
(NH*) ions.
4
The hydroxyl ions increase the pH of the water causing the solubility
of dissolved salts to decline, particularly calcium salts. Sub-
sequently, these salts precipitate, nozzles are clogged in sprinkler
systems, pipe weight increases substantially in the case of surface
systems and water flow is restricted.
Ammonia injection into ditch or siphon tube irrigation systems is
less troublesome. Siphon tubes can become encrusted with the pre-
cipitated calcium salts but are more easily cleaned. If the precipi-
tation occurs in the irrigation ditch, no particular problems are
encountered.
Some relief from problems of ammonia injection into hard water
can be obtained by using an inhibitor such as sodium hexametaphosphate
which tends to sequester the calcium reducing precipitation. Agricul-
nh3 + h2o ?
NH* + OH
4
ammonia
ammonium hydroxyl
72
-------
tural engineers at the University of Nebraska have published guidelines
for precipitation inhibitor use (Mulliner, 1974) based on amount of
ammonia being applied and the quality of the water. That information
is summarized in Table 8.
Table 8. Suggested anhydrous ammonia and calcium precipitation
inhibitor rates (adapted from Mulliner, 1974).
Irrigation water
hardness
Maximum ammonia application
per 4000 lH^O/min
Inhibitor required—''
per 4000 lH^O/min
PPm
kg/hr
g/hr
35-120
18
120
120-170
18
180
170-425
18
300
425-850
13
360
—^Sodium hexametaphosphate, "Calgon".
Another major problem in applying anhydrous ammonia or free ammonia-
containing solutions through sprinkler irrigation systems is the fact
that volatilization losses of ammonia will occur from the time that the
ammonia-water mixture leaves the sprinkler until the ammonia reaches
the soil surface. Admittedly, some of the nitrogen is present as ammo-
nium ions but much more is in the ammonia form. Subsequently, as water
vaporizes, ammonia is also lost to the atmosphere as a gas.
There has been much controversy over the actual amount of ammonia
lost by such a process. Most of the work in that area was carried out
in the mid-1950's. Scott (1956) suggested that the net loss might
range from 5 to 40%. Jackson and Chang (1947) estimated losses to
range as high as 58% while Henderson et al (1955) presented data show-
ing losses as high as 60%. Recently, University of Nebraska research
has corroborated these findings. Conditions governing such losses
73
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include initial pH of the water, temperature at the time of application,
wind velocity, and concentrations of ammonia in the water.
Application of ammonia or any other nitrogen source in irrigation
water (fertigation) is appealing to the Irrigation farmer because of
possible economies in application costs and improved management of the
nutrient supply. Applications close to the time of plant use may avoid
nitrogen losses by leaching and denitrification. Soil compaction may
be reduced by elimination of a trip over the field.
Ammonia or any nitrogen source applied in irrigation water is dis-
tributed in about the same manner as the water. Poor sprinkler patterns
are not too much of a problem with ammonia application because of the
already overwhelming problem of volatilization losses. Application
through gated pipe or siphon tube systems must consider the fact that
more water percolates into the soil at the head of the run than at the
tail. Similarly, more ammonia would be expected to be absorbed by
colloids at the head of the run. To offset this problem, ammonia is
frequently withheld from the water during the first part of the irriga-
tion avoiding part of the accumulation at the head of the run.
Fischbach (1964) recommends that nitrogen injection begin as soon
as the water stream is started down the furrow and continue until the
water and fertilizer reach the end of the run. Fertilizer application
should then be shut off and water application continued until the proper
amount of water has been applied.
Leavitt (1966) commented on his early experience with ammonia
application in irrigation water for contour irrigation of rice and
noted that severe problems were encountered due to absorption of all
of the ammonia by soil and organic matter during the stages of migra-
tion of water through the contour system. All ammonia was absorbed in
the first 2-3 kilometers of travel.
Conventional Soil Application. Development of ammonia applica-
tion equipment began soon after the initial work with ammonia
application in irrigation water. Interestingly, the original designs
74
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are not radically different than the types of "shank" applicators
in use today. Many designs of ammonia applicators exist, consider-
ation of which is far beyond the scope of this paper. Basically,
ammonia equipment consists of a rigid or flexible shank set to pene-
trate the soil to a predetermined depth. A tube is attached to each
shank which opens 2-5-cm above the point of the shank. Ammonia is
delivered to this metal tube by flexible plastic lines from the
device controlling the flow of ammonia from the tank.
Spacings between points of ammonia release in the soil vary
greatly between different cropping systems. Basically, uniform plant
growth is the goal of the equipment design but considerations must be
given to practical aspects of draft and equipment wear. Obviously,
since ammonia has an appreciable vapor pressure, all applicator designs
for both anhydrous ammonia and aqua ammonia must provide for sealing
ammonia in the soil by dragging soil into the knife opening. Improper
sealing, insufficient depth of application, excessively wide spacing
between release points and high rates of ammonia application can all
contribute to volatilization losses.
The soil retention pattern produced by ammonia injection is an
important function of ammonia use. Early efforts in developing com-
mercial ammonia application involved distribution pattern studies
(Leavitt, 1966). The ammonia retention zone was noted to be about
15 cm in diameter with elongation to the top because of the "broken
soil zone produced by passage of the ammonia knife. Continuing re-
search into the 1970's has corroborated those early findings. Distri-
bution patterns vary with soil cation-exchange capacity, moisture
content, magnitude of application and spacing of ammonia knives. That
retention zone should not come into contact with the soil surface
otherwise volatization losses will occur.
New Developments in Ammonia Application. Achron et al (1977)
recently surveyed the industry for new developments and identified a
growing area of interest in ammonia application, namely adaptation of
75
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tillage equipment for ammonia application during tillage operations.
This practice is not totally new but the concept has increased merit
today because it poses the possibility of eliminating or combining
operations with subsequent savings in time and energy. The imagina-
tion of ammonia users has led to development of ammonia application
capabilities for field cultivators, moldboard plows, tandem and off-
set discs, undercutting equipment, rod weeders, bedding equipment and
a host of others.
Recently, a system designed to aid in adaptation of ammonia
application by tillage implements was introduced. This system known
as Cold-Flo ammonia (Achorn, 1977} allows shallower ammonia applica-
tion by reducing the vapor pressure of the ammonia by allowing the
ammonia to boil, act as its own refrigerant thus cooling itself to -33C.
Vapor pressure of the liquid ammonia is essentially zero. Some of the
ammonia mu^t be injected into the soil as a vapor, however. At 25C,
about 15% of the ammonia is converted into vapor in the boiling process
and obviously must not be lost. Still, this vapor is under relatively
low pressure and does not have to be injected so deeply. This process
seems to be particularly well adapted to areas where ammonia applica-
tion under wet soil conditions may be necessary. If the lower soil
profile is wet but the surface is dry enough for equipment operation,
sealing could be a problem. The shallower depth of application of
Cold-Flo ammonia then could allow producers to use the lower cost (per
unit of nitrogen) source.
Time of Ammonia Application. This point alone could be the subject
of an entire chapter. Without all the discussion pro and con of fall
pre-plant ammonia versus spring pre-plant versus sidedress nitrogen for
row crops, let's remember a couple of things about ammonia application.
The closer the application to the time of crop need, the greater the
efficiency possibly may be. Less loss by leaching and/or denltrifica-
tion may be a result of careful selection of application time. On the
other hand, applications very close to the time of seeding (pre-plant)
76
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create a greater chance for germination damage from ammonia.
When ammonia is injected into the soil, it immediately begins to
react with water. Subsequently, the ammonia and any seedlings in the
vicinity compete for available soil moisture. Plants close to the
ammonia retention zone experience high osmotic pressure of the soil
solution in the vicinity of the retention zone and also may be
affected by ammonia toxicity directly. Good rules of thumb to help
avoid the problem of germination damage are to (1) apply the ammonia
as far in advance as possible, (2) to use as narrow release points as
feasible, (3) to apply as deeply as practical, and (4) to apply the
ammonia at an angle to the direction in which the rows will be planted.
Suspected ammonia germination damage can be diagnosed in several ways
including pH of the soil surrounding the damaged seedling (high pH
probably means high ammonium concentration), and ammonium concentration
in the soil samples surrounding the damaged seeding. Colliver and
Welch (1970) studied the effect of rate, depth, and time of ammonia
application before corn planting on the germination and early growth.
Their work clearly indicated that as depth of application increased
from 10 to 17.5-cm, effects on stand were essentially nil at rates of
112 and 224 kg nitrogen/ha. Applications immediately prior to planting
were still acceptable as long as the seed and the ammonia retention
zones were separated.
Agronomic Comparisons of Ammonia and other Nitrogen Sources.
Since the development of ammonia as a nitrogen source for direct
application, comparisons of the performance of this material and other
nitrogen sources have been carried out. Smith (1966) discussed com-
parisons of materials and concluded that ammonia is an effective source
of nitrogen for a variety of crops, including row crops, small grains
and forages. Smith noted in his discussion of ammonia application
that time of application and rate of application, each a factor in
determining final agronomic results, cannot be arbitrarily specified
without consideration of many soil and climatic factors.
77
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Results of comparisons of nitrogen sources for any crop will
vary with years but usual differences between carriers are small
when several years' means are considered. An example of the varia-
tion that can result between such comparisons is indicated in Table 9.
Table 9. Nitrogen sources for irrigated corn (Gallagher et al, 1975).
Nitrogen Carrier Corn Yield, kg/ha
168 kg nitrogen/
ha/yr
1971
1972
1973
1974
1975
5-year
Mean
Control
5268
7275
5519
2571
2007
4516
Ammonia
6836
8718
7903
7526
9031
8078
Urea
7338
7714
8843
7401
6648
7589
Sulfur-coated urea
6836
7526
8906
8028
6335
7526
LSD05
501
1066
752
NS
564
696
Azmonium Nitrate
Ammonium nitrate (NH^NO^) was the first solid nitrogen product
that was produced on a major industrial scale. After World War II,
plants which had been designed to produce ammonium nitrate for muni-
tions were converted to fertilizer production. Plants today involve
a process for conversion of ammonia into nitric acid and then react
the nitric acid with ammonia to produce ammonium nitrate. The
properties of solid ammonium nitrate are presented in Table 10.
Table 10. Properties of ammonium nitrate.
Color White crystalline
Molecular weight 80.04
Density (24C) 1.725
Melting point 169.6C
Boiling points 210C
Solubility 118 g/100 ml at 0C
871 g/100 ml at 100C
78
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Table 10. Continued
Nitrogen, % (pure) 35
Nitrogen, % (fertilizer grade) 33.5-34
The U.S.A. ammonium nitrate production was 4.18 million metric tons
in 1977.
Use of Ammonium Nitrate. Probably fewer agronomic precautions
are necessary with the use of ammonium nitrate than with any of the
other materials discussed. Direct seed placement of ammonium nitrate
in either irrigated or dryland agriculture can pose problems of seed
germination through salt effects. The amount of-ammonium nitrate
which can be placed in direct seed contact varies with crop, soil
type, water supply and the amounts of other nutrients placed in the
same soil zone. As a general rule, greater than 20 kg of nitrogen/ha
in direct seed contact as ammonium nitrate might be expected to pro-
duce problems. These problems relate strictly to water availability
to the seedlings so irrigated conditions might reduce the problem.
It is not peculiar to ammonium nitrate, other soluble salts can create
the same problem.
Soil chemistry of ammonium nitrate is straight forward and has
been covered adequately in another chapter of these proceedings
(Broadbent, 1978). Oxidation of the ammonium portion of ammonium
nitrate by soil bacteria does generate soil acidity but an identical
effect is produced by the oxidation of ammonium ions from any source
including plant residues and animal manures. Tisdale and Nelson (1975)
have summarized the acidity-basicity of nitrogen fertilizers and point
out that nitrification of one kg of ammonium produces sufficient
acidity to require 1.8 kilos of pure calcium carbonate for neutrali-
zation. Half of the nitrogen in ammonium nitrate is already in the
nitrate form and does not undergo nitrification. Liming becomes
increasingly important as ammoniacal fertilizers (those supplying
ammonium nitrogen) are used for extended periods.
79
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The effect of ammonium fertilizer use on soil pH is a function
of the soil's content of free calcium carbonate, the cation-exchange
capacity of the soil and the soil's mineralogy. This discussion is
not intended to focus attention primarily on ammonium nitrate, rather
the problem is a universal one and net effects of continued use of
ammoniacal fertilizers can only be monitored by soil testing. European
farmers and fertilizer manufacturers have attempted to modify this
effect by producing several materials known as ammonium nitrate of
lime (ANL) or calnitro. These materials are merely mixtures of ammonium
nitrate and calcium carbonate. Liming is then occuring when nitrogen
is applied. Depending on the formulation (ranging from 16-20% nitro-
gen) the net effect of continued use may be either to maintain the
soil pH or to increase it slightly. The obvious disadvantage of such
materials is the low nitrogen content. Only very small quantities of
such materials are used in the U.S.
Surface applications of ammonium nitrate have generally not been
as subject to volatilization losses of ammonia as similar applications
of urea. Rolston (1978) has reviewed this subject.
Ammonium nitrate, while having excellent agronomic capabilities
does have some specific characteristics which can make the material
hazardous. Contamination with organic materials such as carbon
black, diesel fuel or ground plant material provides a combination
which is easily oxidized in the presence of ammonium nitrate. Ammonium
nitrate is classified as hazardous product due to the fact that it is
a strong oxidizing agent. The presence of carbon, high temperature
2
and pressures in the vicinity of 36 kg/cm can cause ammonium nitrate
detonation. The strong oxidizing tendency of ammonium nitrate is
reason enough to protect this material in storage from contamination
with carbonaceous material.
For years ammonium nitrate has reigned supreme among nitrogen
carriers in the world for direct application. Over the last several
years, however, a decline in ammonium nitrate usage has been noted
80
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with the rapid rise in production of urea. Ammonium nitrate pro-
duction today is subjected to severe environmental restraints, both
from the release of nitrogen oxides (NO and NO^) to the atmosphere
and also from the dust produced in the prilling and coating process.
New ammonium nitrate plant construction in the United States is rare
and future use of this material for direct application will probably
decline. This should not be interpreted as an indication that ammonium
nitrate will not be manufactured because it is a necessary component
of nitrogen solutions.
Urea
The existence of urea (CCKNI^^) nature has been recognized for
over 200 years but commercial synthesis of this material was not
achieved until the 20th century. Synthesis of urea for both ferti-
lizers and an animal feed additive involves the use of anhydrous
ammonia and carbon dioxide generated in the production of anhydrous
ammonia. Characteristics of solid urea are indicated in Table 11.
Table 11. Properties of urea.
Color
Molecular weight
Density crystalline
Density, fertilizer
Solubility
Melting point
Boiling point
Nitrogen, % (pure)
Nitrogen, % (fertilizer grade)
White
60.06
1.32 g/cc
42 lb/cu. ft.
78 g/100 ml at 5C
132.7C
Decomposes at atmospheric pressure
46.67
45-46
United States urea production in 1977 amounted to 3.09 million metric
tons.
Use of Urea. Urea reactions in the soil have been described
earlier in these proceedings. Some of those soil reactions relate to
the efficiency of urea as a nitrogen source for dryland or irrigated
81
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crops. One of the most persistent problems with use of urea has been
that of ammonia volatilization following hydrolysis. This problem,
has been most troublesome on high pH soils or on soils where a large
amount of crop residue limits urea contact with the soil surface.
Research has clearly demonstrated that incorporation of urea into
the soil will reduce problems of ammonia volatilization to a workable
level (Rolston, 1978).
Surface applications of urea are not always conducive to prob-
lems, however. Dibb and Welch (1974) noted that aerial top dressing
of corn with prilled urea in a wet spring when ground application was
not possible proved feasible with only slight leaf burn from material
trapped in the whorl. Comparisons of urea with many other nitrogen
sources have shown that this material is a very excellent nitrogen
source (Table 18). Engelstad and Hauck (1974) have recently summarized
the productive capabilities of urea. They note, as do others, the
emerging importance of urea as a solid nitrogen source, frequently at
the expense of ammonium nitrate. Solubility of urea makes possible
the use of this material as a foliar nitrogen source for many crops. Less
leaf burn from foliar applications of urea as compared to ammonium
nitrate results from lower ammonium ion concentrations on the leaf
surface. Foliar applications of urea have been used successfully with
many crops particularly high value crops such as citrus where repeated
sprayings are a part of normal production practices. Inclusion of urea
in sprays does allow for close control of nitrogen concentrations in
plant tissue. Late growing season applications of urea solutions have
been used to increase the protein content of wheat (Finney et al, 1957).
Apparently, such applications at flowering are readily absorbed and
translocated into the filling grain as functional protein.
Urea-ammonium nitrate combinations have become increasingly popu-
lar in formulation of nitrogen solutions. These two compounds exert a
synergistic effect on each other's solubility producing much higher
nitrogen concentrations (28-30% nitrogen) than can be achieved by a
82
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urea-only or ammonium nitrate only solutions (approximately 18-20%
nitrogen). Urea-ammonium nitrate (UAN) solutions are not commonly
used for foliar applications because of leaf burn from the ammonium in
the ammonium nitrate.
Seedling Damage. Urea in direct seed contact has caused problems
with seedling survival in several crops, particularly wheat, corn
and grain sorghum. Urea hydrolysis produces ammonium ions or ammonia
molecules depending upon the soil conditions. Characteristics of the
soil zone where urea hydrolyzes include high pH from the hydroxyl ions
(OH ) produced and possibly free ammonia resulting from the high pH.
Free ammonia is toxic, apparently interfering with electron transport
in the plant. Also, ammonia has a high affinity for water and in suffi-
cient concentrations exerts a desiccation effect on seedlings.
Susceptibility to these problems varies with amount of urea applied,
the clay-organic matter content of the soil, pH and variety. Applica-
tions of urea as low as 10 kg nitrogen/ha in direct seed contact have
caused extreme problems in grain sorghum emergence. Like other phenom-
ena, this problem is not totally predictable. However, it is of high
enough probability to suggest that urea should not be used in a starter
fertilizer that will come into direct seed contact. Placement of urea
away from the seed eliminates this problem.
Biuret. Biuret, a condensation product of urea, was identified
many years ago as a toxic agent. Brage (1960) studied the effects
of biuret on winter wheat in South Dakota and noted a substantial re-
duction in stand when biuret was present in urea at a concentration of
2.5%. This effect is well known to manufacturers who closely monitor
the presence of the compound in urea formulations. Today, concentra-
tions are usually well below 1^ biuret in urea and the problem is largely
of academic interest. It should be noted, however, that specifications
for urea for foliar application call for 0.5^ of less biuret.
83
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Nitrogen Solutions
Nitrogen solutions have dramatically increased in popularity in
the last 20 years. Two major classes of nitrogen solutions exist.
This classification is based upon the presence of free ammonia or a
positive vapor pressure of ammonia. Solutions which contain free
ammonia are classified as 'pressure solutions' while those without
free ammonia are 'non-pressure'. Non-pressure solutions are utilized
in largest quantities world-wide. They are comprised primarily of
ammonium nitrate and urea but can contain other compounds such as
ammonium sulfate and calcium nitrate. Pressure solutions must be
applied to the soil or in irrigation water in a manner similar to
anhydrous ammonia due to the presence of ammonia. Pressure solution
components include ammonia, ammonium nitrate, urea, and possibly ammonium
sulfate or calcium nitrate.
Theoretically, higher nitrogen concentrations are associated with
pressure solutions. However, the greatest usage in the United States
of pressure solutions is as aqua ammonia which contains only 20% nitro-
gen. On the other hand, non-pressure solutions in most common use
contain from 28 to 32% nitrogen. The higher nitrogen concentration of
these solutions coupled with their greater adaptability to different
types of application including application in irrigation water, combina-
tions with herbicides, and direct surface applications dictates their
greater popularity.
The production of nitrogen solutions involves no special chemistry
since the three major components have already been discussed. A pecu-
liar chemical quirk does exist in the production of these solutions,
however, particularly non-pressure solutions. Solutions of either
ammonium nitrate alone or urea alone fail to achieve the nitrogen con-
centration that results from a mixture of the two materials (see Table
12). Solubility of both urea and ammonium nitrate is enhanced by the
presence of the other compound. The solubility of ammonium nitrate is
118.3 g/100 ml of water at OC. The solubility of urea, on the other
84
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Table 12. Composition and nomenclature of pressure and non-pressure
nitrogen solutions.
Solution
nomenclature**
Classification
%nk3
Composition
°A by weight
%NH.NO
4 3
% Urea
Salt-out
temp., C*
160(0-46-0)
Non-pressure
0
46
0
-12
200(0-57-0)
Non-pressure
0
57
0
5
200(0-0-44)
Non-pressure
0
0
44
5
230(0-0-50)
Non-pressure
0
0
50
17
245(0-70-0)
Non-pressure
0
70
0
30
280(0-39-31)
Non-pressure
0
39
31
-18
320(0-44-35)
Non-pressure
0
44
35
0
201(24-0-0)
Pressure
24
0
0
-52
410(19-58-11)
Pressure
19
58
11
-14
410(26-56-0)
Pressure
26
56
0
-31
* Salt-out temperature is that temperature where crystal formation
occurs in solution. Note that salt-out temperature increases as
the nitrogen concentration increases (non-pressure solutions).
** The first number outside parenthesis refers to the % total
nitrogen (without decimal; 160 = 16.0% nitrogen). Numbers
inside parentheses refer to components of the solution.
85
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hand, is 78 g/100 ml of water at 5C. In the presence of the
other compound the solubilities of ammonium nitrate and urea have
been changed to 130 g/100 ml and 103 g/100 ml at 0C, respectively.
Use of Nitrogen Solutions. Use of nitrogen solutions is deter-
mined entirely by the type of solution being considered. As noted
earlier, non-pressure solutions are much more flexible in terms of
application techniques than are pressure solutions. Urea-ammonium
nitrate solutions (IJAN) are adapted to a number of application tech-
niques. In irrigated agriculture, water application of UAN solutions
(fertigation) has become increasingly popular as a means of improving
nitrogen use efficiency, and elimination of one or more trips over
the field. Fertigation has developed relatively recently although as
noted in the section on use of ammonia, the process dates back to the
1930's.
The University of Nebraska has the leader in the development and
study of fertigation. Paul Fischbach and colleagues in Agricultural
Engineering at Nebraska began fertigation research in 1960. Their work
has shown that nitrogen use efficiency can be improved by applications
through the water. Losses due to leaching are minimized by split or
multiple nitrogen applications as compared to applications prior to
seeding particularly on sandy soils. Data in Table 13 indicate the
effect that fertigation can have on corn yields (Fischbach, 1964).
Table 13. Corn yields for application of nitrogen by mechanical means
and for application in the irrigation water (Fischbach, 1964).
Method of application Yield kg/ha
Normal mechanical application 7275
Normal (mechanical and irrigation split) 7526
Normal irrigation (all in one irrigation) 7777
Normal irrigation (split into two irrigations) 7840
Normal mechanical plus 20 kg of nitrogen in irrigation
water 8216
3 Normal-recommended amount of nitrogen determined by a soil test
and past cropping history.
86
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Widest adaptation for this technique has occurred in sprinkler irri-
gated sandy soils. Estimates indicate that about 60-70% of the
sprinkler irrigation systems in the Colorado, Kansas, Nebraska, Okla-
homa area involve application of nitrogen and/or other nutrients
through the irrigation water. This technique is also adaptable to
gated pipe or ditch irrigation but development there has not been
quite so widespread as with sprinkler systems.
Fischbach (1970) examined the distribution pattern of nitrogen
solutions applied through sprinkler and gated pipe irrigation systems.
Since nitrogen applications could be affected by improper or incom-
plete mixing in the water, tests were conducted for uniformity of
nitrogen concentrations at various points on a gated pipe system and
at points along a center pivot sprinkler system. Data reported in
Tables 14 and 15 show good uniformity indicating good mixing.
Timing nitrogen solution application in water under furrow
irrigated conditions is important in maintaining a good distribution.
Fischbach (1964) suggested that acceptable nitrogen distribution could
be achieved by using as large an irrigation stream in each furrow as
possible without causing serious erosion on the particular slope
being irrigated (Table 16). He suggested that the stream should reach
the far end of the furrow in a maximum recommended time for various
soil textures (Table 17). If it does not reach the end of the furrow
in the time indicated, the run may be too long, water penetration is
uneven and nitrogen distribution suffers. Fischbach notes that if
the water flows through the field in less than the recommended maxi-
mum time, the distribution will be still better. For efficient use
of fertilizer nitrogen, Nebraska researchers suggest that the nitrogen
solution should be injected into the water as soon as the stream is
started down the furrow and continue until the water and fertilizer
reach the far end of the field in all furrows. The nitrogen should
then be shut off and irrigation continued until water penetrates to
the desired depth.
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Table 14. Concentration of nitrate-nitrogen in the irrigation water
from the 4th, 11th, 23rd and 29th gate openings at various
intervals of time in a gated pipe irrigation system
(Fischbach, 1970).*
Elapsed Time Gate Number
Minutes
5th 11th 23rd 29th
Nitrate-nitrogen, mg/1
10 27.2 27.2 27.2 27.2
20 27.2 28.8 27.2 27.2
30 28.8 28.8 28.2 27.2
40 27.2 28.8 28.2 27.2
* A 4000 liter per minute system, using 20 cm gated pipe
384 m. long. The^water pressure at the beginning of the gated
pipe was 528 g/cm .
Table 15. Concentration of nitrate-nitrogen in irrigation water
caught half-way between towers of a 12 tower center-pivot
sprinkling system (Fischbach, 1970).*
Location of Sample Nitrate-nitrogen
mg/1
Pivot
Point and Tower No. 1
35
Tower
No. 1 and 2
35
Tower
No. 2 and 3
35
Tower
No. 3 and 4
32
Tower
No. 4 and 5
36
Tower
No. 5 and 6
34
Tower
No. 6 and 7
33
Tower
No. 7 and 8
34
Tower
No. 8 and 9
35
Tower
No. 9 and 10
34
Tower
No. 10 and 11
37
Tower
No. 11 and 12
34
* A 3600 I/minute system, 392 m. long using 17-cm diameter OD pipe,
4.22 kg/cm pressure at the pivot. Fertilizer solution (UAN)
injected by positive displacement pump.
88
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Table 16. Maximum furrow stream size for various slopes
(Fischbach, 1970).
Percent slope
Liters per minute per furrow
1.0
40
0.5
80
0.3
120
less than .3
200
Table 17. Approximate maximum length of time for water to
flow to the end of the field on various soil
textures (Fischbach, 1970).
Soil texture
Hours
Loamy sands
2-3
Sandy loams
3-4
Fine sandy loams
4-5
Silt loams
5-6
Silty clay loams
6-7
89
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Use of nitrogen solutions or other nitrogen sources in irriga-
tion water should include a re-use system for tailwater in order
to protect against contamination of surface water with nitrogen
and to make efficient use of both nitrogen and water.
Non-pressure nitrogen solutions are most commonly used for
fertigation because they avoid problems of calcium salt precipita-
tion produced by ammonia and avoid problems of ammonia loss through
sprinklers (see ammonia use section). Pressure solutions would be
most adapted to ditch-siphon tube irrigation systems.
Nitrogen solutions can be used in dual application with herbi-
cides. This process results in elimination of a trip over the field
with savings in time and fuel. Herbicides applied in this manner
have been effective as long as the placement and timing were compat-
ible with recommendations for the herbicide. Some suggestions have
been made that the activity of the herbicide is actually improved
due to the more complete coverage generated by the higher amount of
solution applied.
Comparisons of the agronomic effectiveness of nitrogen solutions
versus ammonia or the various solid materials have indicated good
performance. Nitrogen solution management, like that of urea, can
possibly be improved at times by incorporation, particularly when
applied to high pH soils where ammonia volatilization from urea could
be occurring at the soil surface. When irrigation water is applied
shortly after application, incorporation may be less important. In-
corporation of nitrogen and other elements is always a good idea if
adaptable to the operations.
In summary, recommendations from most sources do not distinguish
between nitrogen fertilizers but leave that decision to the producer
based on applied cost and how the particular type of nitrogen fits
into the operation. Iowa data presented in Table 18 (Voss, 1978)
are an excellent indication of the uniformity which exists between
performance of nitrogen sources.
90
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Table 18. The influence of source, time, and rate of nitrogen
fertilizer application on corn yields (Voss, 1978).*
Nitrogen
Treatment
Corn Yields,
kg/ha
Rate
Time
Source
Nashua
Kanawha
kg/ha
0
4077
4578
90
Early
Am. nitrate
6397
7903
Solution 28
6209
7589
Urea
6334
8028
90
Late
Am. nitrate
6586
7840
Solution 28
6021
7714
Urea
6648
8091
180
Early
Am. nitrate
6711
8655
Solution 28
6586
8718
Urea
6397
8404
180
Late
Am. nitrate
6397
8593
Solution 28
6586
8593
Urea
5958
8781
* From data courtesy of Dr. John Webb, Iowa State University.
Ammonium Sulfate
Ammonium sulfate ((NH.)„SO.) has a long history as a nitrogen
hi 4
source. This material has been manufactured as a by-product from
several industrial processes. At one time, ammonium sulfate rep-
resented the most common source of nitrogen in many sections of
the world and still remains an important source particularly in
rice producing areas. A major disadvantage to its use is its
relatively low nitrogen content (21%) and the acid residue it
leaves in the soil.
Ammonium sulfate is a common by-product of the steel industry
particularly the coking of coal when some ammonia is present in the
coke-oven gas. Sulfuric acid, another common by-product material,
can be marketed by conversion into ammonium sulfate either by reacting
91
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with anhydrous ammonia, aqua ammonia or waste gas streams which may
contain some free ammonia. Waste ammonia is frequently recovered
in this manner from the production of synthetic fibers. Production
chemistry is very simple:
2NH3 + H2S04 >(NH4)2S04.
Whatever the source, ammonium sulfate's physical and chemical
characteristics remain the same. These characteristics are set out
in Table 19.
Table 19. Properties of ammonium sulfate.
Color
Molecular weight
Density
Melting point
Solubility
Nitrogen, % (pure)
Sulfur, % (pure)
Nitrogen, % (fertilizer grade)
Sulfur, % (fertilizer grade)
White crystalline; varies
with presence of contaminants
132.14
1.769
Decomposes at 235C
70.6 g/100 ml at 0C
103.8 g/100 ml at 100C
21.2
24.2
21
24
The color of solid ammonium sulfate varies depending upon its source.
Ammonium sulfate produced from petroleum by-products frequently has
a dark color due to the presence of hydrocarbons or coal tars. United
States consumption of ammonium sulfate was 1.82 million metric tons
in 1977.
Use of Ammonium Sulfate. Ammonium sulfate has proven to be an
excellent nitrogen source. Obviously, an advantage for ammonium
sulfate might exist in its sulfur content if that element were in
short supply. Ammonium sulfate contains about 24% S.
Residual effects of ammonium sulfate on the soil were noted
earlier as being more acidic than residual effects of other nitrogen
92
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sources. Guidelines for the soil acidification produced by ammoni-
acal nitrogen materials are outlined in Table 20.
Table 20. Equivalent acidity or alkalinity of nitrogen fertilizers
expressed in kg pure calcium^arbonate required to
neutralize residual acidity.—
Materials
Nitrogen
%
Pei kg of
ammonium
nitrogen
Pet 20
kg nitrogen
Per 100 kg
Material
Ammonium sulfate
21
5.35
107
110
Monoammonium
phosphate
11
5.00
100
55
Anhydrous ammonia
82
f—1
00
o
36
148
UAN Solution
28
o
00
1—1
27
38
39% ammonium nitrate
31% urea
Presure Solution
41
1.80
27
75
19% NH3
58% ammonium nitrate
11% urea
Ammonium Nitrate
34
o
00
18
15
Urea
46
1.80
36
84
Calcium carbonate equivalent added to the soil
Nitrogen
%
Per kg
nitrogen
Per 20
kg nitrogen
Per 100 kg
Material
Calcium nitrate
15.5
1.35
27
21
Sodium nitrate
16.0
1.80
36
29
—^Adapted from Tisdale and Nelson, 1975.
According to the theory of Pierre, application of a kg or nitrogen
as ammonium sulfate requires approximately 5 kg of effective calcium
93
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carbonate to offset the residual acidity from the nitrification of
the ammonium nitrogen and the sulfuric acid formed. Compare these
values with those for urea, anhydrous ammonia and ammonium nitrate.
Continued use of ammonium sulfate for extended periods of time
without adequate liming can result in drastically reduced plant
growth. The high acidity produced results in the destruction of clay
minerals and subsequently the release of large amounts of aluminum
which is toxic to plants. In addition, phosphorus fixation may be
enhanced due to the presence of large amounts of soluble iron and
aluminum in the soil.
Ammonium sulfate has fallen to a rather insignificant position
in the total supply of nitrogen in the U.S. That is not to say that it
is not used but is frequently used in the manufacture of mixed ferti-
lizers, both solids and liquids. Its primary disadvantage aside from
the acidity mentioned above is its low nitrogen content, only about
20-21%, depending upon the source. Freight rates then tend to in-
crease the cost per unit of nitrogen.
Agronomic comparisons of ammonium sulfate and other nitrogen
sources have usually indicated agronomic equality. Some volatiliza-
tion losses of ammonia can occur under certain conditions. This is
not necessarily a characteristic of ammonium sulfate since factors
other than nitrogen source control the process.
Sodium Nitrate and Calcium Nitrate
Sodium nitrate (NaNO^, 16.5% nitrogen) was the first commercial
source of nitrogen utilized in the United States. This material,
imported from Chile, was the nitrogen source for the cotton industry
in southeastern United States.
Sodium nitrate is produced by processing a natural ore mined
from high, dry Andean valleys. The source of the nitrate in these
deposits is of debatable origin but one of the prominent theories
relates to its derivation from fumaroles associated with volcanic
94
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activity. The nitrogen was eventually nitrified and deposited
beneath detritus washed down from surrounding slopes. The dry
climate of the region protected the highly soluble nitrate salts.
This deposit is unique in the world in terms of its extent and access-
ibility.
Today, sodium nitrate is produced from the Chilean deposits as
well as from other industrial sources. Major disadvantages include
low nitrogen content, high freight costs associated with movement of
the natural material from Chile, and the sodium content. It assumes
a very minor role in terms of nitrogen materials in the United States
today.
Calcium nitrate (Ca(NO^)2» 15.5% nitrogen) was one of the first
commercial fertilizers manufactured in the 20th century from synthetic-
ally fixed nitrogen. The process developed in Norway was dependent
upon the production of nitric acid from electric arc fixation of
elemental nitrogen and is essentially identical to the mechanism
described for nitrogen fixation by lightning.
Today, calcium nitrate is produced in Norway from nitric acid
produced from anhydrous ammonia. The disadvantage of the use of cal-
cium nitrate is the high cost per pound of nitrogen which is largely
due to freight required to move the material from Scandinavia to the
United States. Agronomically, this material is an excellent source
of nitrogen and in addition to providing soluble nitrate also provides
a readily available source of calcium to plants. A characteristic of
both sodium and calcium nitrate is an alkaline residual effect on the
soil. Continued use will tend to raise soil pH.
Slow Release Nitrogen Fertilizers
Low percentage recovery of applied nitrogen has been recognized
foi years. Results of field and lysimeter studies have shown that
70% or less of applied nitrogen is recovered by crops. Reasons for
poor recovery oi highly soluble nitrogen sources include ammonia
95
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volatilization, leaching of nitrate and denitrification. Large amounts
of available nitrogen in the soil at one period in time may also lead
to luxury nitrogen consumption by plants which represents absorption
above that necessary for normal plant growth. Slowing the rate of
release of nitrogen from fertilizers is a conceivable means of avoiding
these problems.
The problem of slowing nitrogen release from fertilizers has been
approached by altering physical characteristics of fertilizers and by
developing compounds which have low water solubility. The release of
nitrogen from soluble materials has been altered by coating water
soluble compounds with less water soluble materials which retard entry
of water into the particle and movement of nitrogen out. Coatings
applied to soluble nitrogen materials generally have been of three types:
1. Coatings with small pores which allow slow entrance of water
and slow passage of solubilized nitrogen compounds out of the
encapsulated area;
2. An impermeable coating that requires breakage by physical
or chemical action before the nutrient is dissolved; and
3. Semi-permeable coatings through which water diffuses and
creates internal pressure sufficient to disrupt the coating.
Sulfur-coated urea (SCU) has been developed in recent years as a
product with the characteristics of slow nitrogen release. This mater-
ial is basically urea with a coating of elemental sulfur including a
binding agent, a sealant and a microbiocide. Elemental sulfur was
chosen as a coating agent because of its relatively low cost and
ease of handling. Nitrogen release rates are varied by controlling
the thickness of the coating. The nitrogen content of SCU ranges from
about 30 to 37% dependent upon the thickness of the sulfur coating.
Nitrogen release is calculated as the amount of the nitrogen that will
solubilize in seven days. Characteristics of SCU which have been
examined by the Tennessee Valley Authority include material ranging
from 10% nitrogen solubilized in the first seven days up to 40% nitrogen
96
-------
solubilized in the first seven days. These forms of SCU are noted
as SCU-10 and SCU-40.
Agronomic evaluations of SCU have indicated some possible improve-
ment in nitrogen efficiency particularly with crops with very large
demands for nitrogen throughout an extended growing season. Rice,
bermudagrass and sugarcane have been crops where positive responses
to SCU versus conventional urea have occurred. (Uchida et al, 1975;
Shelton, 1976; Snyder and Gascho, 1976; Gascho and Snyder, 1976;
Diamond, 1975; Dalai, 1974 and 1975).
Uncoated organic, compounds of low water solubility have been studied
as sources of nutrients for over 50 years. Intensive studies during the
last 25 to 30 years have resulted in the production of certain urea-
derived compounds which have a characteristic of slow nitrogen solu-
bility in water. Typical of these types of compounds are those pro-
duced by the reaction of urea and formaldehyde to produce condensation
products known generally as urea-formaldehyde or urea-form.
Urea reacts with the formaldehyde in the presence of a catalyst to
form a mixture of compounds. These materials are white, odorless
compounds which contain varying amounts of nitrogen but averaging
around 38%. A basic component of urea-form is methylene-urea polymer
varying in chain length and in degree of cross-linking between chains.
Basically, as the compound links are extended, the water solubility of
these materials declines.
Although the urea-forms have been examined agronomically in the
United States, they have not received wide attention except as nitro-
gen sources for very high value crops, turf and ornamentals. More
attention has been given to these compounds in Europe and in Japan.
Isobutylidenediurea (IBDU) is a reaction product of urea and
isobutyraldehyde. The reaction has been utilized in Germany and Japan
to produce a compound containing approximately 32% nitrogen and having
a low water solubility (0.1 -0.01%). This compound also has the
ability to be granulated with soluble nutrient sources of phosphorus
97
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and potassium to supply mixed fertilizer formulations with slow
nitrogen release.
Crotonylidene diurea (CDU) is another urea based material which
is produced in Germany, Japan and France from erotonaldehyde and urea.
The nitrogen content of this material is about 32%.
Other slow release nitrogen compounds have been studied or pro-
duced in small quantities in some countries. Basically, the charac-
teristics of these materials involve high cost per unit of nitrogen.
Use should be considered as being relegated to very high value crops,
ornamentals and turf. Extensive agronomic use has not been demonstrated
to date as being economically feasible in the U.S.
ORGANIC NITROGEN FERTILIZERS
In addition to the manufactured nitrogen sources, organic, nitrogen-
containing compounds available for fertilizer use include bird guano,
poultry and animal manures of all types and municipal sewage sludges
(Table 21). The analyses of these materials vary widely depending upon
their source and the amount of weathering which has occurred prior to
their being collected for fertilizer use. Bird guano has one of the
longest histories of direct use and contains not only nitrogen but
appreciable quantities of phosphorus and other plant nutrients. Supplies
of guano are relatively scarce and are not used extensively today.
Animal wastes as sources of nutrients for crops have long been
recommended because of the efficiency involved in re-cycling nutrients
and the fact that the materials being added to the soils contain not
only nitrogen but carbon and many other plant nutrients. Animal wastes
vary widely in composition depending upon the composition of the rations,
types of animals involved and the storage conditions for manure prior
to its application. Some indication of the range in nitrogen concen-
trations of various animal wastes can be obtained from Table 22.
Studies of the availability of nitrogen from animal waste applica-
98
-------
Table 21. Average composition of some common organic materials.
Source
N
Percent
P
K
Activated sewage sludge
6.0
0.9
-
Blood, dried
13.0
-
-
Bone meal (raw)
3.5
9.5
-
Bone meal (steamed)
2.0
12.2
-
Castor pomace
6.0
0.6
0.4
Cocoa meal
4.0
0.6
2.1
Cocoa shell meal
2.5
0.4
2.5
Cocoa tankage
2.5
0. 6
1.0
Cottonseed meal
6.6
1.1
1.2
Fish scrap (acidulated)
5.7
1.3
-
Fish scrap (dried)
9.5
2.6
-
Garbage tankage
2.5
0.6
0.8
Peanut meal
7.2
0.6
1.0
Peanut hull meal
1.2
0.2
0.7
Peat
2.7
-
-
Peruvian guano
13.0
5.4
2.1
Process tankage
8.2
-
-
Soybean meal
7.0
0.5
1.2
Tankage, animal
7.0
4.3
-
Tobacco stems
1.5
0.2
4.2
Whale guano
8.5
2.6
-
Tisdale and Nelson (1975).
99
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Table 22. Composition of various manures and waste materials.
Dry Matter
%
N
%
K
%
P
PPm
Beef feedlot manure-
High values 38.5
Low values 24.2
Mean values 32.4
2/
Liquid poultry manure- 8.1
Liquid swine manure—
T . .i . 1/
Liquid swine manure-
Liquid dairy manure^
2/
Dairy cattle manure—
2/
Sheep manure—
Q * 2/
Swine manure-
Poultry manure^
(w/o litter)
p i. 3/
Poultry manure-
High values
Low values
Activated sewage^
sludge-
Lagoon sewage^j
sludge—
0.6
2.6
8.6
21
35
25
46
26
28
100
100
1.08
0.95
3,680
0.44
0.58
2,100
0.71
0. 74
2,830
0.16
0.24
180
0.01
0.06
20
0.09
0.07
239
0.24
0.19
210
0.56
0.50
1,000
1.40
1.00
2,100
0.50
0.38
1,400
1.56
0.35
4,000
1.50
0.74
7,100
1.00
0.70
6,800
5.56
0.36
25,860
1.71
0.23
18,750
1/
2/
3/
Murphy, L.S., W. L. Powers and H. L. Manges, unpublished data,
Kansas State University, 1971.
Benne, E.J., C. R. Hoglund, E. D. Longnecker, R. L. Cook,
Michigan Agricultural Experiment Station Circular Bulletin
231, 1961.
Robertson, L.S., Department of Crop and Soil Sciences, Michigan
State University. Manure utilized in fertilization study with
corn, 1 source.
4/
— Knezek, B.D., Department of Crop and Soil Sciences, Michigan
State University.
100
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tions have indicated that not all the nitrogen obtained in the wastes
is available the first year. From nitrogen contents of around 1%
in feedlot wastes, calculations from California, Kansas and Wiscon-
sin, have indicated that about 13 kg of nitrogen will be available
the first year per metric ton of applied animal waste on a dry matter
basis. This will be a function of the climate, the original compo-
sition of the manure and storage. Data from Texas have indicated
generally higher concentrations of nitrogen in beef feedlot wastes if
the material is collected and applied after short storage periods,
Iowa research and extension personnel have calculated nitrogen avail-
ability from several types of animal wastes (Table 23) (Voss, 1978).
Table 23. Nutrient content per metric ton of solid manure from solid
handling systems (Voss, 1978).
Animal
Bedding
or litter
Dry
matter
Available
nitrogen
Total
nitrogen
Phosphorus
Potassium
%
/ m A 4- V" a
ton of manure
-K.g/ metric
Swine
No
18
3
5
2
3.3
Yes
18
2.5
4
1.5
3
Beef
No
15b
1.5
5.5
1.5
4
cattle
Yes
52
3.5
10.5
3
9.5
50
4
10.5
4
10.8
Dairy
No
18
1.5
4.5
1
4
cattle
Yes
21
2.5
4.5
1
4
Sheep
No
28
1
9
2.4
10.8
Yes
28
1
7
2
10.4
Poultry
No
45
13
16.5
10.4
14
Yes
75
18
28
9.8
14
Deep pit 76
22
34
14
00
•^1
Turkey
No
22
8.5
13.5
4.3
7
Yes
29
6.5
10
3.5
5.4
Horse
Yes
46
2
7
1
5.8
Source: Livestock Facilities Handbook, 1975. Iow& State Univ. Coop.
Ext. Serv. MWPS-18. a. Open concrete lot. b. Open dirt lot.
101
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Applications of animal wastes as sources of nitrogen for irri-
gated crops should be dictated by the amounts of nutrients needed
and the amount calculated to become available from a waste source
during the course of the growing season. Guidelines for such
applications have been formulated in several states (Powers et al
1973, 1974). Excessive rates of application of wastes (solids and
liquids) can lead to many problems in crop production including plant
damage from accumulated salts and high accumulations of nitrate
nitrogen in forages and in the soil. Adriano et al (1971) studied
this problem in California. Wallingford et al (1974 and 1975) noted
high accumulations of potassium and ammonium cations in soils receiv-
ing large amounts of animal wastes. Severe yield depressions resulted
from the salt accumulations. Had recommended rates of application
been followed, however, these problems would not have occurred.
Waste disposal and waste utilization management are vastly different
concepts. Soil incorporation of wastes shortly after application will
improve nitrogen use efficiency by avoiding volatilization losses of
ammonia.
Municipal wastes also contain nitrogen and have been utilized
extensively all over the world as nitrogen supplies for crops. Highly
processed and leached sewage sludge (solids) may contain low amounts
of nitrogen, frequently less than 1%. Nitrogen has been lost by
volatilization of ammonia or has been leached out into the liquid
portion of the waste. This is a fate of much of the nitrogen in
animal and human wastes since nitrogen is largely voided in the form
of urea.
Seaweed has been utilized as a source of nitrogen under some
conditions where it has been harvested for extraction of certain chem-
icals. Composition will vary depending upon the source and this
practice is less widespread than the use of animal manures for nitro-
gen. Effective use of any waste material as a fertilizer demands
some familiarity with the composition of the material. Analyses are
102
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urged where possible and. when time permits.
DETERMINING NITROGEN NEEDS
Soil analysis has played an increasingly important role in deter-
mination of nitrogen needs of irrigated crops in the past few years.
In most states in the western half of the U.S., state and commercial
soil testing services are now offering a residual test for nitrate-
nitrogen. Accumulations of nitrate in the soil from whatever source
can be effectively utilized by the following crop, lowering the
amount of additional nitrogen needed for the particular yield goal
and diminishing the amount of nitrogen that may leach through the
soil profile and into shallow aquifers. Nitrogen normally does not
store well in the soil and should be utilized by following crops. Soil
analysis is the only means of determining the magnitude of such accumu-
lations. Analyses for total soil nitrogen do not provide an accurate
measurement of the relatively small quantities of available nitrogen
which have accumulated.
Continued adaptation to good management practices already reco-
mmended should allow maximum production from irrigated areas without
environmental degradation. Agricultural research will continue to
be the mainstay of both production of crops and environmental quality.
103
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LITERATURE CITED
Achorn, F. P., H. L. Kimbrough, and L. Murphy. 1977. Latest techni-
ques for applying anhydrous ammonia. Proc. TVA-TFI Fert. Conf.,
Kansas City, MO., July 1977, pp. 36-45.
Adriano, D. C., P. F. Pratt, and S. E. Bishop. 1971. Fate of in-
organic forms of N and salt from land-disposal of manures from
dairies. Livestock Waste Mgt. and Pollution Abatement, Proc.
Internatl. Symp. on Livestock Wastes, Amer. Soc. Agric. Engr.,
Pub. No. 271, p. 243-246.
Alexander, M. 1961. Introduction to Soil Microbiology, Wiley and Sons.
New York.
Alios, H. F. and W. V. Bartholomew. 1959. Replacement of symbiotic
fixation by available nitrogen. Soil Sci. 87:61-66,
Broadbent, F. E. 1978. Mineralization, immobilization and nitrifica-
tion (of nitrogen). Proc. Natl. Conf. on Mgt. of Nitrogen in Irr.
Agric., Sacramento, California. May 1978.
Brage, B. L., W. R. Zich and L, 0. Fine. 1960. The germination of
small grain and corn as influenced by urea and other nitrogenous
fertilizers. Soil Sci. Soc. Amer. Proc. 24:294-296.
Colliver, G. and L. F. Welch. 1970. Toxicity of preplant anhydrous
ammonia to germination and early growth of corn: I. Field
studies. Agron. J. 62:341-346.
Dalai, R. C. 1974. Comparative efficiency of soluble and controlled-
release sulfur-coated urea nitrogen for corn in the tropics. Soil
Science Soc. Amer. Proc. 38:970-974.
Dalai, R. C. 1975. The use of urea and sulfur-coated urea for corn
production on a tropical soil. Soil Sci. Soc. Amer. Proc. 39:
1004-1005.-
Delwiche, C. C. and J. Wijler. 1956. Non-symbiotic nitrogen fixation
in soil. Plant and Soil 7(2):113-129.
Diamond, R. B. 1975. A new way to minimize nitrogen soil losses.
Rice Farming 9(6):6-8.
Dibb, D. W. and L. F. Welch. 1974. Aerial application of nitrogen
as a stop-gap measure. 111. Fert. Conf. Proc. 1974, Univ. of
111. Urbana.
104
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Engelstad, P. 0. and R. D. Hauck. 1974. Urea-will it become the most
popular nitrogen carrier? Crops and Soils, p. 11, May 1974.
Finney, K. F., J. W. Meyer, H. C. Fryer and F. W. Smith. 1957. Effect
of spraying Pawnee wheat with urea solution on yield, protein
content and protein quality. Agron. J. 49:341-346.
Fischbach, P. E. ,1964. Irrigate, fertilize in one operation. Nebraska
Quarterly, Summer, 1964. p. 15-17.
Fischbach, P. E. 1970. Applying fertilizers uniformly through irriga-
tion systems. Fertilizer Solutions 14(6):92-95.
Gallagher, P. K. et al. 1975. Comparisons of nitrogen carriers and
time of nitrogen application for irrigated corn. Kansas Fert. Res.
Rept. of Progress 255. pp. 96-97, 110.
Gascho, G. J. and G. H. Snyder. 1976. Sulfur-coated fertilizers for
Sugarcane: I. Plant response to sulfur-coated urea. Soil Sci.
Soc. Amer. J. 40:119-122.
Henderson, D. W., W. C. Bianchi and L. D. Doneen. 1955. Ammonia loss
from sprinkler jets. Agr. Eng. 36:398-399.
Jackson, M. L. and S. C. Chang. 1947. Anhydrous ammonia retention by
soils as influnced by depth of application, soil texture, moisture
content, pH value and tilth. Agron. J. 39:623-633.
Leavitt, F. H. 1966. Agricultural ammonia equipment development and
history, fri Agricultural Anhydrous Ammonia Technology and Use.
Ed. by McVickar et al., American Society of Agronomy, Madison,
Wise. p. 125-142.
McVickar, M. H., and W. P. Martin et al. 1966. Agricultural Anhydrous
Ammonia Technology and Use. Amer. Soc. of Agronomy, Madison,
Wise. p. 21.
Meyer, L. J. and P. J. Gallagher et al. 1974. Effects of nitrogen
carriers and time of nitrogen application on the yield of dryland
soybeans. Kans. Fert. Res. Rept. Prog. 224, p. 146-147, 157.
Mullinier, H. R. 1974. Applying anhydrous ammonia in irrigation water.
Univ. of Nebr. Guide G 74-129.
Powers, W. L. and R. L. Herpich et al. 1973. Guidelines for land
disposal of feedlot lagoon water. Kans. Coop. Ext. Ser. Cir.
485.
105
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Powers, W. L. and G. W. Wallingford et al. 1974. Guidelines for
applying beef feedlot manure to fields. Kans. Coop. Ext. Ser.
Cir. 502
Raney, R. J. and J. Sharplaz. 1975. Effects of N, P, and alfalfa
residual nitrogen on irrigated corn yields. Kans. Fert. Res.
Rept. of Prog. 255. p. 101-102, 121.
Rosenstein, L. 1936. Increased yields obtained from Shell agricul-
tural ammonia in irrigated agriculture. Shell Chem. Co. Bull. No. 1.
p. 23.
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Conf. on Mgt. of Nitrogen in Irr. Agric., Sacramento, California.
May 1978.
Scott, V. H. 1956. Sprinkler irrigation. Calif. Agr. Exp. Sta. Ext.
Serv. Circ. 456.
Shelton, J. E. 1976. Evaluation of sulfur-coated urea as a pre-plant
total season nitrogen supply for trellised tomatoes. Soil Sci.
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Smith, F* W. 19.66. Application of ammonia. Ijn Agricultural Anhydrous
Ammonia, Technology and Use, Ed. by McVickar et al, American
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Uchida, R. S., J. B. Thompson, and C. J. Young. 1975. Slow-release
fertilizers-experimental results. Hawaii Sugar Tech. Report No.
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Wallingford, G. W., L. S. Murphy, W. L. Powers and H. L. Manges. 1974.
Effect of beef-feedlot lagoon water on soil chemical properties
and growth and composition of corn forage. J. Envir. Qual. 3:74-78.
Wallingford, G. W., L. S. Murphy, W. L. Powers and H. L. Manges. 1975.
Disposal of beef-feedlot manure: Effects of residual and yearly
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106
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Warnock, R. E. 1966. Ammonia application in irrigation water. In
Agricultural Anhydrous Ammonia Technology and Use. Ed. McVickar
et al. American Society of Agronomy, Madison, Wise., p. 115-124.
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Citrog. 19(11):295.
107
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108
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MINERALIZATION, IMMOBILIZATION AND NITRIFICATION
F. E. Broadbent—
ABSTRACT
The decomposition of organic substances containing nitrogen
through the activities of soil microorganisms and resulting in the
release of some of the nitrogen as ammonia is called nitrogen
mineralization. Net mineralization occurs when the quantity of
nitrogen in the material undergoing decomposition exceeds the needs
of the microbial population for nitrogen to produce new cells. If
the decomposing substances do not contain enough nitrogen to meet
the needs of the microbial population, any inorganic nitrogen present
in the soil will be utilized by the microbes and converted to cell
protein and other nitrogenous compounds. This process is called
immobilization. In a sense the assimilation of inorganic nitrogen by
growing plants is also immobilization, but this discussion considers
only the microbiological process.
The nitrogen supplying capacity of soils depends to a large extent
on rates of mineralization and immobilization. Mineralization must
exceed immobilization in order for nitrogen to be made available for
crop plants. Various procedures have been utilized to obtain esti-
mates of nitrogen mineralization over a growing season, the most
successful of which have been based on incubation of soil samples
under controlled conditions for a few weeks.
The oxidation of ammonium nitrogen by certain species of soil
bacteria is called nitrification. Nitrification is seldom a limiting
— Department of Land, Air and Water Resources, University of
California, Davis 95616.
109
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step with regard to the capacity of a soil to produce available nitro-
gen, but it has an important influence on fertilizer efficiency and
movement when amnionic fertilizers are applied. Nitrification rates
are affected somewhat by environmental parameters such as temperature
and pH.
Desirable management practices should favor not only efficient
utilization of fertilizer nitrogen, but also maximum crop uptake of
mineralized nitrogen. Experiments with isotopioally labeled ferti-
lizers indicate that these two objectives are compatible with each
other and also with the need to minimize leachable nitrate.
INTRODUCTION
The process of mineralization, involving the gradual decomposi-
tion of organic forms of nitrogen with the release of ammonia, largely
determines the nitrogen fertility status of a soil. The reverse
process of immobilization, whereby inorganic forms of nitrogen such
as ammonium and nitrate are converted into organic forms, may pre-
dominate for a short period of time, but the normal situation is that
mineralization is the more rapid of the two. This makes passible a
continued slow release of mineral or available nitrogen which permits
plants to grow. Plant uptake of mineral nitrogen is a form of
immobilization, but will not be discussed in this paper.
Once ammonia is produced, it is readily converted to nitrate by
specialized bacteria which are abundant in most soils. This process
is called nitrification. The term nitrification is sometimes applied
to the entire sequence of events starting with organic nitrogen and
ending with nitrate, but correctly used, it applies only to the oxida-
tion of ammonia to nitrate. Somewhat illogic-ally, the reverse of
nitrification is not called denitrific.ation, but nitrate reduction,
since the term denitrification through common usage has come to mean
110
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a special type of nitrate reduction resulting in evolution of gases.
The subject of denitrification is discussed in detail in another
paper in this volume.
We can depict the transformations under consideration by means
of a simple diagram as follows:
nitrification s.
N0~
3
>nitrate reduction
In the soil system the forward and reverse reactions all proceed
simultaneously, but at different rates. The reaction rates may be
altered considerably by environmental conditions and soil management.
If we let m = mineralization rate, i = immobilization rate, n =
nitrification rate, and r = nitrate reduction rate, the usual situa-
tion is that m>i, n)m, and n))r. The practical consequence of these
relationships in agricultural soils is a continued conversion of
organic nitrogen to nitrate, with little or no accumulation of ammon-
ium. Although this is the usual situation, it is not the invariable
one, or all organic nitrogen would eventually be converted to nitrate
This paper will discuss how these interrelated reactions are affected
by soil conditions and management practices.
MINERALIZATION-IMMOBILIZATION
It is necessary to consider mineralization and immobilization
together, since one does not occur without the other. Mineralization
is the result of microbial decomposition of nitrogen-containing
organic materials such as plant residues and soil humus, which are
used by the microbes as a source of energy. At the same time, the
development of microbial cells requires c.arbon and nitrogen, along
with other essential elements, which most microorganisms obtain from
mineralization x
Soil Organic N NH+
v immobilization ^
111
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the substances undergoing decomposition. If there is more nitrogen
present in these substances, called the substrate of the micro-
organisms, than meets their needs, some is released as a waste product
in the form of ammonia, or more correctly, of ammonium, since in the
presence of water ammonia forms the ammonium ion. In this situation
net mineralization occurs, as is illustrated in Figure 1 (a). On
the other hand, if there is insufficient nitrogen present to satisfy
the needs of the soil organisms, as is often the case when a highly
carbonaceous material is undergoing rapid decomposition, any inorganic
nitrogen present will be assimilated by the organisms as rapidly as
it is produced and net immobilization will occur. Usually the rate
of immobilization will not exceed the rate of mineralization for long
periods of time. Numbers of microorganisms will increase very rapidly
while a source of energy is available, but when the readily available
fraction of the energy source becomes depleted, numbers of micro-
organisms decline rapidly to a much lower level. When this occurs
some of the nitrogen previously locked up in microbial cells becomes
surplus as the cells die off and are themselves decomposed. This
nitrogen is released as ammonia. The rapid turnabout from a period
of net immobilization where m(i to one of net mineralization where m^i
is illustrated in Figure 1 (b). This pattern is typical of that which
occurs when a mature crop residue is turned into the soil. During
the initial period of net immobilization inorganic nitrogen supplied
as fertilizer would be converted to organic nitrogen in the form of
microbial protein, thereby making it inaccessible to crop plants.
However, after the elapse of a few weeks the supply of inorganic
nitrogen may attain a value as high as or higher than before the
addition of the organic residue.
Influence of Environmental Factors on Immobilization and Mineralization
It is important to recognize that some interchange of nitrogen
between the organic and inorganic forms occurs whenever environmental
112
-------
TIME
»
Figure 1. Changes in mineral nitrogen in a situation of continuous
net mineralization (a), and a period of net immobilization
followed by net mineralization, (b),
113
-------
conditions permit microbial activity. Although in general microbial
activity is favored by those conditions which are optimal for crop
growth, microorganisms will usually tolerate a wider range of environ-
mental conditions than will higher plants. The relative rates ol
immobilization and mineralization depend primarily on the kind and
amount of organic energy material or substrate present in the soil,
but are also significantly affected by such factors as temperature,
moisture, aeration and pH.
Temperature. When organic residues of low nitrogen content
undergo decomposition, net immobilization occurs, but the total amount
of nitrogen eventually immobilized is not strongly temperature depend-
ent. The lower the temperature, the longer the perioo of net immobili-
zation (Broadbent, 1968). Under conditions such that net mineraliza-
tion is favored, the rate increases with increasing temperature up to
a maximum at about 40C. The poor nitrogen-supplying power of cold
soils is a reflection of the influence of low temperature in retarding
rates of mineralization.
Moisture and Aeration. Within the range of moisture contents
which provide enough water for microbial activity there is little
influence of moisture on quantity of nitrogen immobilized. The effect
of excess moisture sufficient to restrict aeration is to induce a
qualitative change in the population of soil microorganisms. Whereas
adequate oxygen permits the development of a heterogeneous population
of bacteria, fungi, algae, actinomycetes and protozoa, the exclusion
of oxygen inhibits the development of aerobic forms, particularly the
fungi and actinomycetes, These two groups make up a large part of
the total mass of living cells in soil, or biomass, and their inhibi-
tion results in a substantial reduction in the amount of nitrogen
needed to support the soil population. Another consideration is that
aerobic metabolism in microorganisms is inherently more efficient in
cell synthesis than is anaerobic metabolism. Thus, as the soil
moisture content approaches saturation there is a decrease in the
114
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immobilization rate, while mineralization rate is less affected.
For this reason net mineralization may be greater in wet or flooded
soils than in those same soils at field capacity. This is not to say
that saturated soils have less need for fertilizer nitrogen, as for
example in rice culture, because the mineralized nitrogen is also
more likely to be lost through denitrification or other mechanisms.
Nitrogen mineralization can proceed slowly in soils too dry
for crop growth. Stanford and Epstein (1974) reported that in soils
ranging from sandy to silty clay loams, mineralization of nitrogen
was linearly related to per cent soil water in the range of moisture
contents between field capacity and the wilting percentage.
Birch (1960) reported that when soils are re-wetted after a
period of drying there is an increase in the rate of nitrogen mineral-
ization in comparison with soils which have been maintained in the
moist condition. The magnitude of the increase is related to the
organic matter content of the soil and to the length of the drying
period. He reported increases in mineralization of nitrogen ranging
from 32 kg/ha in soils containing 1% carbon and which had been air-
dried for three weeks, up to 133 kg/ha in soils containing 5.5%
carbon. This phenomenon is probably not of great importance during
growth of crops because the degree of drying required would be
attained in a shallow surface layer at most. However, it may account
for initial flush of available nitrogen when rains or irrigation wet
the soil after a fallow period in dry weather.
pH. The optimum pH for most soil organisms is in the neutral
to slightly alkaline range. Immobilization and mineralization rates
are therefore highest in this range. However, immobilization rates
are affected by the nature of the inorganic nitrogen present and its
relationship to soil pH. Unlike most crop plants, soil microorganisms
utilize ammonium nitrogen preferentially over nitrate, but the quantity
of either ammonium or nitrate immobilized in a given situation is
affected by soil pH (Broadbent and Tyler, 1965) . As the pH increases
115
-------
above 7, the amount of ammonium nitrogen immobilized increases because
the effect of ammonium assimilation is to lower the pH. Conversely,
as the soil becomes more acidic there is a tendency for immobilization
of nitrate to increase because the effect of its utilization is to
increase pH.
Quantities of Nitrogen Immobilized
Immobilization of nitrogen is commonly discussed in terms of
carbon/nitrogen ratios, which gives a quantitative expression of the
relationship between the size of the food supply represented by a
given organic substance and the nitrogen present in it. Since the
carbon content of most organic residues of the kind that find their
way into soils is usually in the range of 40 to 45% there is little
to be gained by using carbon/nitrogen ratios as compared with a simple
expression of nitrogen content. A fairly good rule of thumb is that
mature plant materials containing less than 1.3-1.5% nitrogen on the
dry weight basis can be expected to immobilize additional nitrogen
during the initial stages of decomposition. Residues such as straw
and stubble of small grains will immobilize up to 10 kg nitrogen
per metric ton (mt) of residue, corn stalks about 8 kg/mt, and sawdust
and rice hulls about 5 kg/mt. The latter have very low nitrogen
contents, but immobilize less nitrogen than would be expected because
of their slow rate of decomposition.
In the decomposition of carbonaceous organic materials where
supplemental inorganic nitrogen is not available, the decomposition
rate is temporarily retarded by lack of nitrogen because the soil
population does not attain the size it would have done had sufficient
nitrogen been present. Even in this condition the period of net
immobilization seldom lasts more than a few weeks, and the reestablish-
ment of net mineralization is simply delayed.
A factor which complicates attempts to establish a quantitative
relationship between residue composition and the amount of nitrogen
116
-------
actually immobilized is the utilization of excess nitrogen by micro-
organisms when a large supply of inorganic nitrogen is available.
When varying levels of nitrate were added to a soil along with corn
stover containing 0.83% nitrogen, Munson and Pesek (1958) found that
net immobilization was directly related to the amount of nitrogen
added, but the additional nitrogen taken up through luxury consumption
was rapidly re-mineralized, as illustrated in Figure 2.
Release of Immobilized Nitrogen
Since immobilization of inorganic nitrogen represents a diversion
of this plant nutrient from its intended use, it is a matter of prac-
tical importance to know something about the length of time which
elapses before immobilized nitrogen is again converted to the mineral
form. This information can be obtained conveniently when a labeled
source of inorganic nitrogen is present in the soil during a period
of net immobilization. The re-conversion of labeled organic nitrogen
to the inorganic form appears to be a very slow process, sometimes
requiring years, or even decades (Broadbent and Nakashima, 1967;
Shields et al, 1973). Some of the recently immobilized nitrogen may
be mineralized fairly quickly, but over a period of time the immobilized
nitrogen equilibrates with the nitrogen in soil humus, becoming pro-
gressively more resistant to mineralization in the process. Thus,
although the particular nitrogen molecules which are immobilized at
a given time may remain in the organic form for several years, but
other nitrogen molecules, perhaps immobilized at a much earlier time.
This is illustrated in Figure 3, showing concentrations of labeled
fertilizer nitrogen and unlabeled soil nitrogen in the inorganic
form in a soil during and after a period of rapid immobilization
induced by addition of barley straw to the soil. The recently added
nitrogen, applied as labeled ammonium sulfate in this case, was mostly
immobilized within 15 days, with little change during the subsequent
45 days, whereas mineralization of unlabeled nitrogen proceeded during
117
-------
¦Z.
-------
80
E
Q.
Q.
Fertilizer N
60
£ 40
2
Soil N
20
20 30 40 50 60
DAYS
Figure 3. Illustration of mineralization of soil nitrogen during
a period of immobilization of fertilizer nitrogen
following addition of barley straw.
119
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the time of maximum immobilization and continued at a lower rate
throughout the period of measurement. These data illustrate the
great value of tracer nitrogen In making it possible to measure
opposing reactions proceeding simultaneously.
Immobilization Relative to Nitrate Pollution
Net immobilization has the effect of removing nitrate from the
soil solution, and in this way may reduce the quantity of nitrogen
susceptible to leaching in a situation where inorganic nitrogen
surplus to the needs of crops is present. The difficulty of sustain-
ing conditions conducive to net immobilization for more than a few
weeks limits the practical value of deliberately promoting immobiliza-
tion. An even more important consideration is the incompatibility
of net immobilization with crop production. In the long term, buildup
of nitrogen-containing organic residues in soil may contribute to
nitrate pollution through higher mineralization rates, unless soil
and crop management practices are adjusted to take this into account.
Prediction of Mineralization Rates
Many attempts have been made to devise a test which will accurately
predict the quantity of nitrogen mineralized during a growing season.
Clearly, such a test would be a useful aid to efficient fertilizer
management. The most successful of such tests have been based on
incubation procedures which utilize normal biological processes, for
example, those of Bremner (1965), Sims et al (1967), Stanford and
Smith (1972) and Stanford et al (1974). The increase in mineral
nitrogen is measured over a specified period of time, usually 1-4
weeks, and the value thus obtained under controlled conditions is
correlated with field responses over a growing season. The principal
drawback of incubation procedures is that they are time-consuming
and not well adapted to routine soil testing. Accordingly, consider-
able attention has been given to development of a chemical procedure
120
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which correlates well with mineralization rates estimated by the
more time-consuming incubation techniques. One promising method
is the 16-hour autoclaving procedure of Stanford and Smith (1976),
which was compared with values obtained by an incubation procedure
for 475 soils representing 54 soil types. An overall correlation
coefficient of 0.87 was obtained. It appears that with the exception
of certain kinds of soils, notably highly calcareous ones, this
chemical procedure may be applicable to a rather broad range of soils
in estimating plant-available nitrogen.
A-Value Method
Another approach to estimating the nitrogen supplying capacity of
soils is the A-value concept, defined by Fried and Dean (1952) as the
quantity of a given plant nutrient in soil which is equivalent in
availability to that of the same nutrient added as fertilizer. In
practice the fertilizer is isotopically labeled to permit different-
iation between soil and fertilizer nitrogen in the crop. The amount
of available nitrogen in the soil, designated by the symbol A, is given
by the equation
U
A = T ——
A s T
c
where Tg is the amount of labeled or tagged fertilizer applied, Uc is
the amount of soil-derived or untagged nitrogen in the crop, and T
is the amount of fertilizer-derived or tagged nitrogen in the crop.
The equation simply states that the plant will take up soil and ferti-
lizer nitrogen in the same proportions as they occur in the soil solu-
tion. Theoretically, the A-value is independent of the fertilizer
level, but in practice it is often found to vary with fertilizer rate.
It is also affected by nature of the crop, form and placement of
fertilizer, and other variables (Broadbent, 1970). Although not
121
-------
adapted to routine determination because of the sophisticated and
expensive equipment required for analysis, A-values correlate well
with actual uptake of soil nitrogen, as shown in Figure 4, calculated
from data of Legg and Stanford (1967) involving 12 soils fertilized
at 3 rates. By contrast with the correlation coefficient of 0.97,
total nitrogen in these same soils when plotted against soil nitrogen
uptake had a correlation coefficient of only 0.55. One of the signifi-
15
cant findings of recent field research with N-depleted fertilizer
is that A-values for a variety of soils and crops can be easily and
inexpensively determined.
MINERALIZATION OF NITROGEN IN ORGANIC AMENDMENTS
When animal manures, sludges, or other organic wastes are applied
to soils, particularly where large amounts are used, the rate of
nitrogen mineralization is of importance not only in relation to the
fertility status of the soil, but also in relation to the potential
hazard of nitrate pollution. Because of the great variability in
composition of such materials, it is difficult to make general state-
ments regarding mineralization rates, particularly since such rates
are subject to rapid change over time.
Animal Manures
Estimates of rates of mineralization of organic nitrogen in
animal manures can be obtained from field trials comparing crop
response at various application levels to responses obtained with
different rates of inorganic fertilizers as sources of nitrogen. A
comparison of this kind based on the data of Tyler et al (1964) in-
dicated that about 41% of the nitrogen in dry steer manure was
mineralized the first year and 25% the second year. Herron and
Erhart (1965) compared the nitrogen supplied to sorghum by manure
at three levels compared to that furnished by ammonium nitrate.
122
-------
450 i—
LxJ
3
_J
<
>
i
y = 27.7 + I.54X
r = 0.97
50 100 150 200 250 300
UPTAKE OF SOIL N
Figure 4. Correlation of A-value with uptake of soil nitrogen,
calculated from data of Legg and Stanford (1967).
123
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Gilmour et al (1977) calculated an equation from their data relating
total mineralized nitrogen to elapsed time, showing 34% mineraliza-
tion the first year, an additional 17% the second year, 11% the 3rd
year, and so on. Pratt et al (1973) suggested the determination of
nitrogen mineralization rates based on "decay series" which assume
progressively decreasing decomposition rates. They stressed the need
for long term field trials to estimate nitrogen mineralization rates
from animal manures under a variety of soil and climatic conditions.
Mineralization rates are highly variable, depending on the nature of
the manure, conditions of storage prior to application, climatic
conditions, and the amount of straw, sawdust or other such bedding
material which may be mixed with it. Nitrogen in poultry manure is
more readily mineralized than that from large animals (Macmillan et
al, 1972).
Sewage Sludge
In sewage sludge, nitrogen mineralization is usually somewhat
less rapid than in other organic materials of comparable nitrogen
content. Peterson et al (1973) reported nitrogen contents in sludges
as ranging from 3.5 to 6.4%. As much as half pr more of this nitrogen
may be present as ammonia. Considering the organic nitrogen in
anaerobic digested sludge, Ryan et al (1973) found from 4 to 48% of
the nitrogen converted to nitrate in 16 weeks. This wide range of
values obtained under a variety of conditions illustrates the diffi-
culty of making generalizations.
NITRIFICATION
The sequel to nitrogen mineralization in soils is the process
whereby ammonium is oxidized to nitrate, called nitrification. The
reactions? may be written as follows:
124
-------
NH+ + 1% 02
^N02 + H20 + 2 H
,+
no2 %o2
The first of these reactions is carried out by bacteria of the genera
Nitrosomonas and Nitrosoeoccus, and the second reaction by Nitrobacter
species. Nitrifiers are abundant in most soils except those which are
rather acidic, and are capable of remaining active over a wide range
of temperature and moisture conditions. These bacteria are auto-
trophic; that is, they require no organic matter as a source of food
since they derive their energy from the oxidation of ammonium or
nitrite. They do, however, require several inorganic ions including
micronutrients in order to function properly. It rarely occurs that
nitrification is inhibited in soil systems as a result of nutrient
deficiency.
There are in soils some heterotrophic microorganisms, which require
organic matter as a source of food, capable of converting ammonium to
nitrite, and a few of them can produce nitrate (Alexander, 1965).
Thus far, the evidence indicates that heterotrophic nitrification is
of minor importance in soils.
Although normally nitrate is the principal product of the nitrif-
ication reactions, under certain conditions other compounds may be
produced. One of these of particular interest is nitrous oxide, a
gas which may be evolved from the soil and lost. Nitrous oxide
evolution is commonly associated with denitrification, but as suggested
by Yoshida and Alexander (1970) its presence in the atmosphere may be
partly a result of nitrification. These investigators showed that
high ammonium levels favored the formation of nitrous oxide by
nitrifying bacteria.
Influence of Environmental Factors on Nitrification
Ammonia Concentration. One of the curious things about nitrifying
125
-------
bacteria is that they are quite sensitive to ammonia concentration,
even though ammonia is their source of energy. Thus, high ammonia
concentrations in the vicinity of a fertilizer band may retard nitri-
fication. The nitrite oxidizers are particularly sensitive to free
ammonia. At pH values above 7, where the equilibrium between ammonium
ion and free ammonia is shifted in the direction of producing more
free ammonia, there may be a temporary accumulation of nitrite in
soils treated with amnionic fertilizers or urea owing to inhibition of
Nitrobacter species.
pH. The optimum reaction for nitrification is in the range
7.5-8.5. Under acid conditions, the activity of nitrifying bacteria
falls off sharply, ceasing altogether when the pH drops to about 4.0
(Weber and Gainey, 1962). Because the nitrification process itself
generates acid, very large and rapid changes in soil pH may occur in
poorly buffered soils, especially in zones of active nitrification.
For example, it was shown by Broadbent et al (1957) that the pH of a
sandy soil treated with aqueous ammonia dropped from 9.6 to A.5 over
a six-week period as a result of nitrification. Liming often stimulates
nitrification because it neutralizes the acid produced by conversion
of ammonium to nitrate. Fertilizer materials which are alkaline in
reaction, such as aqueous and anhydrous ammonia, or which become alka-
line after addition to soil, such as urea, tend to be nitrified more
rapidly than are materials such as ammonium sulfate or ammonium
nitrate which are acidic in reaction.
Temperature. Assuming that other conditions are favorable, nitri-
fication is affected by temperature in a way similar to other biologi-
cal processes. The minimum temperature is near the freezing point.
Measurable oxidation of ammonium and nitrite has been reported at 2C
(Frederick, 1956). The optimum tempeiature varies somewhat with
location, since the nitrifying bacteria can adapt somewhat to their
environment. The optimum temperature is higher in warm climates than
in cool, ones (Mahendrappa et al, 1966), but usually falls in the range
126
-------
between 25 and 35C. Above 40C the rate of nitrification falls off
sharply, but as a practical matter, lack of moisture probably in-
hibits nitrification at high temperatures more than does temperature
per se.
Oxygen. The nitrifying bacteria are obligate aerobes, but they
function at oxygen concentrations somewhat lower than that of the
atmosphere (Amer and Bartholomew, 1951). Nitrification is most rapid
near the soil surface, but can occur at considerable depths if sub-
strate is present.
Rates of Conversion of Fertilizer Nitrogen
In agricultural practice most of the nitrogen applied to soil is
either in the amnionic form or one which is rapidly converted to the
amnionic form, as in the case of urea. The availability of this
nitrogen to growing plants and its susceptibility to leaching are
influenced by the nitrification process, which converts the relatively
immobile ammonium ion to the highly mobile nitrate. Nitrification
rates are commonly considered to follow first-order kinetics; that is,
the rate of production of nitrite is proportional to the amount of
ammonium present at a given time, and the rate of nitrate formation
is proportional to the concentration of nitrite. Normally nitrite is
oxidized to nitrate as fast as it is formed, and oxidation of ammonium
to nitrite is the rate-limiting step. Using a value of 25 kg N/ha/day
as a reasonable estimate of maximum nitrification rate, based on rates
reported by Tyler et al (1959) for a variety of soils, it can be cal-
culated that 90% of a fertilizer application in the range of 100 kg/ha
would be nitrified within 8 days at 24C, At 7C, 32 days would be
required for 90% conversion.
Nitrification Inhibitors
Because nitrification converts nitrogen to a form more susceptible
to loss by leaching and by denitrification, scientists have searched
127
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for many years for ways to retard or inhibit the nitrification process.
Two approaches have been followed. The first is to add a toxic sub-
stance to the soil or to the amnionic fertilizer which wil] inhibit
the nitrifying bacteria without adversely affecting plant growth or
the activities of other soil microorganisms. Naturally occurring sub-
stances have been found which have these properties, for example,
exudates from the roots of prairie grasses (Neal, 1969) and tannins
produced from the leaves of oak and oak-pine forests (Rice and
Pancholy, 1973). A few synthetic compounds of greater potency than
the natural ones have been developed, and a few are being marketed
commercially. One of these in particular, nitrapyrin, has been sub-
jected to extensive testing with respect to its agronomic value, but
much less is known about its usefulness in decreasing the leaching
of nitrates. In many instances, improvements in yield or of nitrogen
uptake efficiency have been achieved with the use of nitrification
inhibitors, but as a rule, the improvements have not been dramatic.
In reviewing the subject of nitrification inhibitors, Gasser (1970)
stated, "More potent and more specific inhibitors are required to
secure the large gains in the efficiency of nitrogen fertilizers
that are needed in practical agriculture".
The second approach to control of nitrification rates is to
prevent access of nitrifying bacteria to the fertilizer nitrogen.
This has been done by incorporating the nitrogen into large organic
molecules as with urea-formaldehyde and isobutylidene diurea formula-
tions, thereby retarding nitrogen mineralization, or by means of
coatings. Sulfur coatings are the best known example of the latter.
These simply provide a physical barrier which prevents the fertili-
zer from going rapidly into solution. Since such preparations are
more expensive than conventional fertilizers, their economic use
requires improvements in fertilizer performance commensurate with
their higher cost. With few exceptions, improvements of this magni-
tude have not been realized with any of the so-called slow release
128
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fertilizers. It may be reasoned that the value of pollution control
should also be considered in weighing the evidence for and against
inhibitors, coatings, or other formulations designed to achieve
control of nitrification rates. However, there is as yet insuffi-
cient evidence to demonstrate that these are of real value in mini-
mizing nitrate leaching from soils. In some instances, nitrogen
uptake efficiency has been poorer with coated fertilizers than with
conventional ones, which would have the effect of leaving a larger
quantity of residual nitrogen subject to leaching. Gradual release
of mineral nitrogen during the growth of a crop actively assimilating
nitrogen may well decrease the amount of nitrate subject to leaching
during the growing season, but continued release of nitrogen after
the crop is removed can result in substantial leaching losses overall.
MANAGEMENT PRACTICES
There is a certain degree of inevitability about nitrogen miner-
alization and nitrification in soils. These processes occur in nature
because they yield energy to the microorganisms responsible for them.
Rates of reaction can be altered by management, but only within
limits. At the present time, the best practical means available for
maximizing the utilization of available nitrogen by crops and mini-
mizing the potential for ground water pollution by nitrates produced
in soil is to assure the presence of an actively absorbing root
system during the period of most rapid mineralization, and to apply
fertilizer nitrogen at rates such that crop needs are met but not
exceeded by the supply of soil plus fertilizer nitrogen. This is
illustrated in Figure 5, which presents data from a field trial with
isotopically labeled nitrogen fertilizer, which made it possible to
identify fertilizer-derived nitrogen in the crop and in the soil.
In these experiments 180 kg of nitrogen was sufficient to produce
the maximum yield of corn. Up to that level of fertilizer, there
129
-------
160
80
0
SO
A'
<1^
, NITROGEN IN CROP
— X*,'
s
y
\£.
C7>
JX.
240
160
LEACHABLE NITROGEN
L IN SOIL
Soil N
-• -V
80
0
Fertilizer N ~'
,1—A K
0 90 I80 360
FERTILIZER N, kg/ha
Figure 5. Influence of fertilizer rate on crop nitrogen and
leachable nitrogen in soil following crop harvest.
130
-------
was essentially no contribution of fertilizer nitrogen to the
quantity of leachable nitrogen in the soil remaining after crop
harvest, but above 180 kg the quantity of leachable nitrogen in-
creased sharply. It will be noted that utilization of soil nitrogen
was increased by fertilization up to the 180 kg rate. The data in-
dicate that if mineralized nitrogen is fully exploited, fertiliza-
tion for maximum yield is compatible with minimum pollution hazard.
131
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LITERATURE CITED
Alexander, M. 1965. Nitrification. In Soil Nitrogen. W. V.
Bartholomew and F. E. Clark (ed.), Agronomy 10:307-343.
Amer. Soc. Agron., Madison, Wisconsin.
Amer, F. M., and W. V. Bartholomew. 1951. Influence of oxygen
concentration in soil air on nitrification. Soil Sci. 7]:215—219.
Birch, H. F. 1960. Nitrification in soils after different periods
of dryness. Plant and Soil 12:81-96.
Bremner, J. M. 1965. Nitrogen availability indexes. In Methods of
Soil Analysis. C. A. Black (ed.), Agronomy 9:1324-1345. Amer.
Soc. Agron., Madison, Wisconsin.
Broadbent, F. E. 1968. Turnover of nitrogen in soil organic matter.
Pontificiae Academiae Scientarum Scripta Varia 32:61-88.
Broadbent, F. E. 1970. Variables affecting A-valnes as a measure of
soil nitrogen availability. Soil Sci. 110:19-23.
Broadbent, F. E., and T. Nakashima. 1967. Reversion of fertilizer
nitrogen in soils. Soil Sci. Soc. Amer. Proc. 31:648-652.
Broadbent, F. E., Tyler, K. B., and G. N. Hill. 1957. Nitrification
of ammoniacal fertilizers in some California soils. Hilgardia
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Broadbent, F. E., and K. B. Tyler. 1965. Effect of pH on nitrogen
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133
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-------
REMOVAL OF NITROGEN BY VARIOUS IRRIGATED CROPS
T. C. Tucker and R. D. Hauck-^
ABSTRACT
Harvested agronomic and horticultural plants, excluding rice,
are grown under irrigation on over 16 million hectares of land in
33 states. This paper presents results of a survey of published
and recent unpublished information on removal of soil and fertilizer
nitrogen from irrigated land in the harvested portion of crops. The
amount of nitrogen removed varied considerably among different plant
species and within the same species when grown under widely differ-
ent management and environmental conditions. The amount of fertili-
zer nitrogen apparently removed during harvest was greater for hay,
silage, and pasture crops and least for certain vegetables, fruits,
and nuts. The amounts of nitrogen removed by grain crops generally
was within the average range for all crops, AO to 60% of the nitro-
gen applied.
Fertilizer efficiency is discussed from three main viewpoints:
in terms of (1) the amount of applied nitrogen found in the plant,
(2) the yield of harvested plant parts in relation to amount of fer-
tilizer nitrogen applied, and (3) the cash value of marketable crop
in relation to nitrogen cost. Examples are given, using data for
coastal bermudagrass and wheat, of fertilizer efficiency as calcu-
lated on the basis of these viewpoints.
Methodological problems of collecting data and problems of data
- Department of Soils, Water, and Engineering, University of Arizona,
Tucson, Arizona 85721, and Soils and Fertilizer Research Branch,
National Fertilizer Development Center, Muscle Shoals, Alabama 35660.
135
-------
interpretation are discussed in relation to the several concepts of
fertilizer efficiency, and emphasis is placed on the need to recog-
nize the role of all soil, fertilizer, and crop management factors
in determining the efficiency by which plants use nitrogen.
INTRODUCTION
The amount of nitrogen contained in the harvested crop generally
exceeds the amount of any other nutrient. The few exceptions include
such crops as potatoes, sugar beets, sugarcane, and certain vegetables.
Because crop removal of nitrogen is appreciable, it is important from
the viewpoints of economics, energy conservation, and environmental
concerns.
In this paper, we have searched the published literature for base
information on nitrogen removal by irrigated crops and have surveyed
research and extension workers for recent and unpublished data. Our
intention is not to present a comprehensive review of the hundreds of
citations available, but to indicate the current status of knowledge
as revealed by the relatively recent literature.
LAND AREA IN PRINCIPAL CROPS
In presenting information on the distribution of irrigated agri-
culture in the United States, we found it convenient to use the crop
and state groupings used in the most recent Census of Agriculture
(1974 State Summary Data). For example, all vegetable crops are
grouped together, except for dry beans, peas, and lima beans, which
are placed into a separate group, and potatoes, listed separately.
We have not included data for rice in the figure for land area
under irrigation because rice irrigation practices differ greatly
from those used with all other crops. The total land area planted
136
-------
to rice in 1974 was slightly more than 1 million hectares, most of
which was in four states—Arkansas, Louisiana, Texas, and California.
Irrigation to some extent is practiced in every state in the
United States. However, in four states—Alaska, Rhode Island, Vermont,
and West Virginia—the area under irrigation did not exceed 400 hectares
in 1974 (1974 Census of Agriculture). Thirty-three states each had
10,000 or more hectares under irrigation, totaling over 16 million
hectares. The states with the most land area under irrigation were
California, Texas, Nebraska, Idaho, Colorado, and Kansas ^Figure 1).
The remaining 11 western states, Florida, and Oklahoma each had more
than 200,000 irrigated hectares.
Except for the vast area of dryland wheat in 5 of the 11 western
states in the Mountain and Pacific Divisions, the major portion of
field crops in these divisions are grown under irrigation (Table 1).
The relatively high percentage of total cropland under irrigation in
the West South Central Division is due largely to Texas and, to some
extent, Oklahoma. Most of the irrigated corn and sorghum of the West
North Central Division is grown in Nebraska and Kansas, and all irri-
gated cotton in this division is grown in Missouri. Except for
potatoes and vegetables, only small amounts of other crops are grown
under irrigation in the remaining five divisions. Over one half of
the land nationwide cropped to potatoes and vegetables is irrigated.
The percentage of harvested cropland that was irrigated in 1974
is illustrated by ranges in Figure 2. Of the 11 western states, 7
have more than one half of their cropland under irrigation. Large
land areas of dryland wheat account for the smaller percentages of
irrigated cropland in the remaining four states—Colorado, Oregon,
Washington, and Montana.
The principal field crops (separately or in groups) and fruits
and nuts for states with more than 10,000 hectares of irrigated land
are given in Tables 2 and 3. As mentioned previously, the groupings
have been made according to Census Bureau categories; in sooe cases,
13 7
-------
Thousand Hectares
n <10
l0-4<
E3 4'-
m
Figure 1- Irrigated land area, in the U.S. by states, 1974.
138
-------
fable i. Land area of irrigated crops as a percentage of total for each crop-*.
ve
All
field
corn
All
sorghum
Wheat
Hay
Cotton
Potatoes
Vegetal
U.S.
9
18
5
—% Irrigated
15 30
58
52
2
Divisions
New England (6)
4
23
Middle Atlantic (3)
36
28
last North Central
(5)
50
15
West Rortfa Central
(?)
13
7
1
3
5
11
4
South Atlantic (8)
1
1
1
1
38
52
last South Central
(4)
1
8
10
West South Central
(4)
63
25
10
?
29
?4
65
Mountain (8)
94
46
10
69
100
99
94
Pacific (5)
92
96
16
68
100
98
87
Calculated from 1974 Census of Agriculture state summary data. Values are rounded to nearest
11, Individual states are given on pages 11-33 to 11-44.
^Number of states in each division given in parenthesis.
-------
%
~ <2
m 25
m 6 24
U7X 7 b-49
BS ->0 74
n 7 j-84
H «.o-i oo
figure 2. Percentage of harvested cropland under Irrigation in
the U.S. by states,, 1374.
i m
-------
Table 2, Irrigated land area in selected field crops (ha * 10^) > 1974.
U.S.
CA
IX
m
ID
CO
KS
^ MT
H
n.
OS
m
m
&z
l-T
KM
IV
OK
AR
All Ml
All aorghud hay
Total field Corn except Wheat except
irrigated corn grain syrup grain aorghun Peanuta
«
2,703
2,239
1,073
1,311
3,184
72
2,fc?a
140
77
61
151
485
-
125
Iff
650
357
96
36
1, €03
1,259
1,183
51
15
128
-
1,134
45
13
< i
177
345
-
1,129
294
205
34
51
398
-
813
421
375
134
155
54
-
696
25
2
-
2?
412
-
624
8
7
< I
-
5
< 1
ill
13
2
-
50
244
-
578
31
10
-
4
320
-
518
19
7
-
121
123
426
5
< 1
40
if
74
-
363
28
4
< 1
22
177
-
328
13
15
60
36
80
2
298
1
-
_
11
170
-
206
34
27
36
48
25
20
§§
< 1
< 1
2
< 1
2
-
(Cont inued on nest page)
Dry
beana,
peaa,
Sugar lima
Cotton Potatoes Vegetable* Soybeans Barley beets beans Oats
1,498
311
648
192
543
310
262
71
465
24
298
-
162
95
77
7
733
6
44
33
10
7
3
5
-
4
-
36
1
29
38
4
-
126
15
-
121
36
69
7
-
18
9
-
61
48
30
7
-
-
< I
8
< 1
13
2
< 1
-
2
-
-
54
16
3
10
-
8
75
8
-
-
-
-
22
47
-
24
5
2
9
-
2
-
-
34
21
9
10
»
36
29
-
5
25
18
1
160
4
21
-
20
3
< 1
< 1
-
2
3
-
37
6
-
3
58
1
6
-
6
-
2
1
< 1
3
-
-
4
-
-
< 1
24
< 1
< 1
2
< 1
-
-
< I
33
„
2
54
-
_
< 1
-------
able 2. Continued
Total
Irrigated
All
field
corn
Corn
grain
All
sorghum
except
syrup
Wheat
arain
All
hay
except
sorghum
Peanuts
Cotton
Potatoes
vegetables
Soybeans
Barley
Sugar
beets
Dry
beans
peas
lima
beans
Oats
SD
61
26
18
< 1
2
24
—
—
—
_
1
< 1
-
< 1
1
MO
57
18
17
3
2
1
6
-
-
< 1
24
-
-
-
-
WI
51
10
9
-
-
1
-
-
14
18
-
-
-
< 1
< 1
GA
45
15
13
-
-
< 1
13
< 1
-
3
1
-
-
-
-
Ml
38
9
8
-
-
< 1
-
-
6
12
< 1
-
-
1
-
NJ
36
-
-
-
-
-
-
-
3
23
1
-
-
-
-
M
MM
31
11
10
_
< 1
2
_
_
7
1
2
_
2
S3
ND
28
4
< 1
-
4
11
-
-
< 1
-
-
< 1
4
2
< 1
NY
22
< 1
-
-
-
-
-
-
9
7
-
-
-
-
-
IL
22
9
8
-
-
-
-
-
< 1
4
4
-
-
-
-
LA
20
< 1
-
-
-
< 1
-
5
-
< 1
7
-
-
-
-
NC
20
1
1
-
-
-
-
-
-
< 1
<, 1
-
-
-
-
1A
16
13
12
-
-
-
-
-
-
-
1
-
-
-
-
MS
16
< 1
-
-
-
-
10
-
-
-
2
-
-
-
-
IN
13
5
5
-
-
-
-
-
2
9
1
-
-
-
-
VA
11
< 1
< 1
_
—
-
_
2
2
_
_
_
_
_
-------
3
Table 3. Irrigated land area in selected fruit and nut crops (ha x 10 ), 1974.
Apples Peaches
Pears i
Cherries
Plums
and
Grapes prunes
Valencia
oranges
Navel
oranges
Other
oranges
Grape-
fruit
Lemons
Other
citrus
Stone- Other
fruit Almonds nuts
U.S.
56
41
31
18
255 49
164
50
156
92
32
16
54 111 106
CA
8.9
36.4
15.0
5.3
246 43.7
30.4
41.7
0.8
8.5
21.5
2.0
51.4 111 72.9
TX
-
-
-
-
-
4.9
0.8
6.5
18.2
-
-
18.2
ID
2.0
0.4
0.4
-
0.8
-
-
-
-
-
-
-
CO
2.4
0.8
-
-
-
-
-
-
-
-
-
-
FL
-
-
-
-
-
123.5
5.3
147.4
60.7
2.8
12.6
-
OR
2.4
0.8
8.1
6.9
3.2
-
-
-
-
-
-
8.9
WA
38.5
1.2
7.7
4.5
7.3 0.8
-
-
-
-
-
-
_
AZ
-
-
-
-
1.6 -
5.7
2.0
0.8
4.0
7.7
1.6
5.7
UT
1.2
0.8
-
1.2
-
-
-
-
-
-
-
-
NM
0.8
-
-
-
-
-
-
-
-
-
-
-
-------
small acreages of crops were combined. States are listed in descending
order based on total irrigated land area. Slight differences may be
noted between the sum of land areas for a particular crop and the
total for the United States because data for some very small areas
of irrigated land were omitted and individual values were rounded
to the nearest thousand hectares.
Only a few states are involved in the production of certain irri-
gated field crops such as peanuts, dry peas, lima beans, tobacco,
fruits, and nuts. Most of the irrigated land (> 75% of the United
States total) in corn, sorghum, wheat, cotton, potatoes, vegetables,
soybeans, barley, sugar beets, and dry beans is located in only
six states. A major position of the total irrigated acreages of a
particular crop may be grown in a single state (e.g., Nebraska, corn;
Texas, sorghum; and Idaho, potatoes).
Land areas of crops under irrigation have remained constant or
declined slightly in some states because of limiting water supplies.
In other states irrigation of cropland has increased from a small to
a marked amount since the 1974 census (e.g., Nebraska has more than
doubled its land area under irrigation since 1974).
Census data show that irrigated acreages are increasing in many
states in regions which previously had little or no irrigated agri-
culture. Although as yet confined to a small percentage of cropland
in these states, irrigation is being practiced to provide supplemental
water. This indicates that management practices common to irrigated
agriculture are becoming more important nationwide. The increase in
yields obtained from irrigation during otherwise moisture-deficient
periods in turn leads to increased removal of nitrogen in the harvested
crop. Adoption of irrigation practices requires changes in fertilizer
and other management practices, and it is obvious that high yield
potentials are achieved only through efficient management of both
fertilizer nitrogen and water.
Data on average crop yields, nitrogen concentration, and irri-
144
-------
gated acreage permit one to calculate nitrogen removal in the harvested
portion of each crop by state or region. Garman and White (1964)
reported such information for several crops. They used values for
percent nitrogen in various crops which are similar to those being
reported currently for the same crops grown under irrigated con-
ditions, but their yields and, consequently, calculations of total
nitrogen removed are somewhat lower than those reported elsewhere
for most irrigated crops. Nevertheless, calculations such as those
reported by Garman and White can be used in part to estimate future
nitrogen needs based on total acreages of each crop.
SOIL-FERTILIZER NITROGEN AND CROP REMOVAL
For a particular plant species, a relative index of the effective-
ness by which plants use nitrogen can be obtained from noting the
amount of nitrogen removed in the harvested portion of the crop in
relation to the amount of fertilizer nitrogen applied. This relative
index can be used in comparing nitrogen removal by the same crop at
different locations or under different crop and fertilizer management
systems. However, the relative index should be used with caution to
compare nitrogen removal among different plant species because the
total nitrogen taken up may be distributed between the harvested and
nonharvested portions of the crop in an entirely different way in
one plant species as compared to another. For most crops of concern
here, a large proportion of the total nitrogen in the plant is removed
from the land during harvest, but the total amount of nitrogen removed
may vary considerably within the same plant species (depending on site
and management factors) and among different species. For example,
wheat grown at either of two locations was found to contain about 75%
of its total nitrogen in the grain. However, because of differences
in grain yields, the amount of nitrogen removed from each site was
different. For cotton, only about 50% of the nitrogen taken up is
145
-------
removed in the harvest portions of the plant, and the total amount
removed may be very different from that removed by wheat.
Cotton and wheat remove different amounts of nitrogen from the
land because they differ in their dry matter yields, the nitrogen
concentrations in their various plant parts, and in the proportion of
total plant nitrogen which is removed at harvest.
In the discussion which follows, let us assume that the nitrogen
removed from the land at harvest is derived solely from soil and
fertilizer. For nonleguminous plants, we include in soil nitrogen the
relatively small amount of nitrogen added during the season through
nonsymbiotic biological nitrogen fixation.
Nonleguminous plants take up nitrogen almost entirely in the
ammonium and/or nitrate form. Under aerobic soil conditions and with
other factors favorable, soil- and/or fertilizer-derived ammonium at
low concentrations in soil is rapidly changed to nitrate (via the
biological nitrification reaction). Undex these conditions, plants
normally take up most of their nitrogen as nitrate. However, in
concentrated bands of ammonium or ammonium-producing fertilizers
(e.g., urea), nitrification proceeds more slowly and, consequently,
appreciable amounts of ammonium may be taken up by the plant. Whether
nitrogen in the soil-plant system is in the ammonium or nitrate form
may determine the amount of nitrogen in the plant and the manner in
which it is used. The plant physiological aspects of ammonium versus
nitrate nutrition will not be discussed here, but we will be con-
cerned with the form of nitrogen as it affects nitrogen supply to
the plant. Nitrification converts a relatively immobile form of
nitrogen, ammonium, to a relatively mobile form, nitrate, and
produces oxidized forms of nitrogen, nitrite and nitrate, which are
subject to loss through leaching and denitrification.
Ideally, we would like to relate crop removal of nitrogen from
the land to the overall nitrogen balance of the soil-fertilizer-plant
system. For a first approximation of a working model of such a
146
-------
system, let us assume that the crop can remove all (100%) of the
available forms of nitrogen added, already present, and to be formed
in the root system sorption zone (Bray, 1954). Let us also assume no
nitrogen losses from the system but a soil nitrogen supply during the
growth of the crop that is equal only to that which is returned to
the land as crop residue. Obviously, the soil nitrogen supply is
deficient and optimum yields cannot be obtained. In this highly
stylized and simple model, the nitrogen fertilizer requirement becomes
equal to the amount of nitrogen removed in the harvest portion of the
plants. It follows that the soil nitrogen requirement (SNR) is equal
to the amount of nitrogen removed by the crop plus the amount returned
to the land. The fertilizer nitrogen requirement (FNR) is equal to
the soil nitrogen requirement minus the amount (or level) of nitrogen
in the soil (SNL), that is, FNR = SNR - SNL. The amount of available
nitrogen either present, added, or formed in the root zone of the soil
must be at least equal to the total crop content of nitrogen at the
yield level acceptable for that crop. Realizing the tremendous over-
simplification of this model, we present it nevertheless to serve as
a starting point for modeling a more realistic system, and to dramatize
the importance of corn idering the amounts of nitrogen removed in the
harvested parts of crop plants.
In practice, the values for SNR and FNR must be multipled by a
factor which considers nitrogen losses by leaching and volatilization,
nitrogen removal from the plant-available form by immobilization
reactions in soil, movement of nitrogen from the plant root system,
and incomplete uptake and use of nitrogen because of environmental
stresses. Gains of nitrogen from rainfall and biological nitrogen
fixation also must be considered. Obviously, accurately determining
this factor presents many problems.
147
-------
NITROGEN CONTENT OF CROPS
Total Uptake
The total amount of nitrogen in the plant is equal to the amount
of total dry matter times its average nitrogen concentration (percent-
age). As discussed in the preceding section, this value represents the
minimum soil nitrogen requirement, that is, the supply of available
nitrogen from the soil plus fertilizer nitrogen must equal the total
amount in the plant, assuming no nitrogen losses and complete recovery
of available nitrogen by the plant. Because losses of plant-available
nitrogen occur and the plant does not take up all available nitrogen,
nitrogen supplies greater than the minimum SNR must be present in order
to obtain optimum yields; how much greater depends on the crop. For
example, for lettuce, carrots, and hay crops, almost all of the nitrogen
taken up is removed from the land. Crop removal of nitrogen is almost
equal to the minimum SNR. On the other hand, crops grown for grain
return appreciable amounts of nutrients to the soil and removal of
nitrogen in the harvested portion of the crop is less than the minimum
SNR. In either case, the amount of soil and fertilizer nitrogen needed
is determined both by the total amount of nitrogen removed and the crop
and soil factors which determine how much nitrogen is taken up by the
plant.
Crop Removal
Our survey of data from research and extension workers showed a
very great variation in yields and nitrogen contents among different
plant species as well as within the same species. Rather than present
average or mean values, the range of values obtained is given in Table
4, except for reports of single values. Usually, single values for yield
and nitrogen content were taken from a California study (Rauschkolb
and Mikkelsen, 1978). In many instances, average farm applications of
nitrogen exceeded the rates usually applied at experimental fields at a
148
-------
Table 4. Summary of survey data for yield, nitrogen removal, and fertilizer application
under irrigation.
Total nitrogen content of: Fertilizer Average
Harvested Total nitrogen farm
Crop Yield portion crop applied application
Agronomic crops
kg/ha
Cotton
900- 1,700
63-109
125-230
100-134
90-109
Corn
7,200-10,000
110-180
165-280
160-280
160-280
Barley
3,760- 4,500
72- 85
85-125
140
90
Oats
3,000- 3,600
63- 67
94-100
72-140
72- 90
Silage, Corn
6,000-29,000
111-361
-
150
240
Sorghum
4,500- 7,800
84-157
137-235
123-224
110-224
Wheat
2,700- 7,700
56-172
72-262
56-224
45-224
Beans, dry
3,000- 5,800
90-116
168-184
56
56
Sugar beets
56,000-86,300
112-167
168-253
224-336
250-440
Hay, all non-legume
4,500-18,000
94-438
-
70-700
120-250
Safflower
3,000
92
120
-
-
Vegetables
Asparagus
3,800
17
-
111-209
159
Beans, lima
3,360
84
-
168
67
snap
12,000
52
-
60-125
86
Broccoli
11,530
60
-
-
280
Cabbage
30,000
67
-
106-194
147
Cantaloupe
22,400
36
-
77-171
106
Carrots, roots
42,000
76
-
87-164
134
tops
20,000
98
-
-
-
Cauliflower
15,000
56
-
142-263
205
Celery
75,000
135
-
222-458
321
Cucumbers
30,000
30
-
102-194
136
Lettuce
30,000
48
-
130-228
178
(Continued on next page)
149
-------
Table 4. Continued
Crop
Yield
Total nitrogen content of: Fertilizer
Harvested Total nitrogen
portion crop applied
Average
farm
application
Vegetables (continued)
Onions 36,000
Peas, green 3,600
Peppers, green 17,233
Potatoes 34-45,000
Spinach 22,400
Sweet corn 12,000
Tomatoes 56,000
Fruits and nuts*
Apples
Apricots
Avocados
Cherries
Citrus:
Oranges
Grapefruit
Lemons
Tangerines
Figs
Grapes
Olives
Peaches
Pears
Prunes and plums
Strawberries
Nuts:
Almonds
Pecans
Walnuts
36,000
18,000
5,800
9,000
18,000-55,000
24,000-48,000
30,000-73,750
12,000
6,500
22,400
4,600
36,000
34,000
18,000
44,000
2,000
2,800
2,200
86
36
113
227-229
114
77
101
29
34
20
19
24-90
38-78
57-92
16
12
22
9
50
25
54
66
60
67
52
68-120
281-317
94
114
82
125-224
48- 97
143-256
276-356
142-264
116-222
103-212
41-134
67-148
92-235
40- 99
95-217
124-280
132-270
118-243
54-122
32-111
46-150
87-198
74-194
52-156
110-246
81-208
74-211
163
59
252
400-500
196
162
159
92
96
140
73
138
172
186
159
92
63
92
144
133
115
178
142
134
*50-80 kg/ha in leaves and wood recycled in fruits and nuts.
150
-------
particular locality. This means that on farms where application rates
were higher and yields comparable to or lower than those of experiment
fields, recovery of applied nitrogen by the crop was less on farms
than on experiment fields. From the data given in constructing Table
4, we calculated the percent of applied fertilizer recovered in the
harvested portion of the crop, based on the lower range of values
for nitrogen application rates and yields. Apparent recovery values
ranged from 48 to 88% for agronomic crops.
The data in Table 5 were adapted from Miller and Smith (1976).
Their values for apparent removal of applied nitrogen agree fairly
well with our survey data (Table 4) for a number of crops but differ
considerably for others. For example, their value for the percent
of applied fertilizer nitrogen removed in corn grain (36%) is con-
siderably lower than values obtained in our survey (average of 65%), and
is in between the range of 0 to 58%, depending on rate and time of
nitrogen fertilizer application, reported by Perry and Olson (1975).
Similar differences in nitrogen recovery values occur for cotton.
Miller and Smith (1976) reported 43%, Halvey (1976) reported 98 to
109%, and our survey showed 63 to 81%.
Two possible explanations for the above discrepancies come to
mind. First, where low recoveries of applied nitrogen are obtained,
the plant may be taking up a considerable amount of soil nitrogen
that.is assumed to be fertilizer nitrogen. (The measurement of soil
versus fertilizer nitrogen is discussed later). This is especially
apparent where the harvested portion of the crop contains as much
or more nitrogen as was applied. Second, percent recovery of applied
nitrogen is affected by level of soil nitrogen and rate of fertilizer
application, among other factors. Corn and sorghum data of Perry and
Olson (1975) and wheat data of Gardner and Jackson (1978) show that
percent recovery of applied nitrogen decreases with increase in nitro-
gen application rate. Further discussion of nitrogen recovery will
be given later in the section dealing with nitrogen efficiency.
151
-------
Table 5. Estimated crop yields, nitrogen fertilizer application and
utilization in the San Joaquin Valley of California, 1971*.
Nitrogen
applied
kg/ha
Yield
mt/ha**
%
Removal
of applied
Cereal hay
72
7.6
97
Sugar beets
127
42.5
86
Safflower
103
3.2
85
Silage
174
44.2
76
Irrigated pasture
80
9.0
68
Rice
90
4.9
68
Grain sorghum
117
4.2
64
Wheat
120
3.4
60
Barley
114
3.3
53
Cotton
115
2.1
43
Corn
242
6.3
36
Tomatoes
138
50.8
66
Carrots
133
41.7
56
Potatoes
250
39.5
53
Onions
222
51.78
50
Lettuce
222
24.1
21
Cantaloupes
134
16.3
13
Watermelons
178
28.7
12
Olives
28
5.6
50
Oranges
111
12.1
23
Peaches
154
20.2
22
Nectarines
147
15.6
18
Plums
125
11.3
7
Walnuts
141
1.6
12
Almonds
151
1.0
10
Grapes
49
14.6
37
*Adapted from Miller and Smith (1976).
**Metric tons/ha.
152
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NUTRIENT UPTAKE AND DRY MATTER PRODUCTION
Data for coastal bermudagrass adapted from Fisher and Caldwell
(1958) and presented in Table 6 show increasing dry matter production
with increasing nitrogen application to 1,344 kg nitrogen/ha, after
which yield declined.
Table 6. Coastal bermudagrass yield, nitrogen percentage, and nitrogen
fertilizer efficiency calculated on the basis of three
concepts*.
Incremental
Applied
nitrogen
Yield
Nitrogen
in hay
Removal of
fertilizer
nitrogen
Dry
matter
increase
Economic
return**
Accumulative
fertilizer
removal
kg/ha
metric
tons/
ha
%
%
kg/kg
nitrogen
$ output/
$ nitrogen
cost
X
0
5.98
1.28
-
-
-
-
112
9.81
1.46
60
34.2
4.37
60
224
13.28
1.68
71
31.0
3.97
65
448
19.24
1.87
61
26.6
3.40
63
672
23.86
1.98
50
20.6
2.63
59
896
27.10
2.06
3 b
14.5
1.85
54
1,120
29.01
2.11
24
8.5
1.09
48
1,344
29.57
2.19
16
2.5
0.32
42
1,568
28.74
2.26
1
-3.7
-0.47
37
1,792
26.59
2.35
-
-9.5
-1.22
31
2,016
23.09
2.51
-
-15.6
-2.00
25
*Calculations from data of Fisher and Caldwell (1958).
**Based on hay at $0.055/kg and nitrogen at $0.43/kg.
Nitrogen concentration, on the other hand, increased from 1.28 to
2.51% at application rates of 0 to 2,016 kg/ha. Maximum nitrogen
153
-------
removal occurred at an application rate of 1,568 kg nitrogen/ha.
At higher rates of application, increasing nitrogen concentration
did not compensate for declining yield; thus, total crop removal of
nitrogen also declined.
Macy (1936) hypothesized that each plant species must maintain at
least a minimum nutrient (nitrogen) concentration in order to sur-
vive, this concentration being different for different species. He
further stated that when the supply of nitrogen sufficient to maintain
this low concentration within the plant is absorbed by and translocated
from the roots in small increments, the intial effect will be to in-
crease growth (dry matter production) without change in the plant's
average nitrogen concentration. When the supply of nitrogen is
greater than that needed to maintain the minimum concentration, the
growth rate increases with increasing nitrogen supply and the nitrogen
concentration in the plant also will increase. When maximum growth
has been obtained, increasing the nitrogen supply to the roots serves
only to increase the nitrogen concentration within the plant (luxury
consumption). The concentration will continue to increase beyond the
point where further increases in soil supply of nitrogen result in
yield depression. Finally, when yield is severely depressed, nitro-
gen concentration decreases. Thus, at maximum yield, the nitrogen
concentration within the plant is not at maximum. It reaches maximum
only when yield is severely suppressed because of adverse effects on
growth from oversupply of nitrogen.
The above sequence of changes cannot be illustrated by data
from most experiments reported in the literature because of the
relatively narrow range of nitrogen supply encountered in these ex-
periments. It is seldom possible to show the low level effect on
concentration because most cultivated soils will provide more than
enough nitrogen for the nitrogen concentration in the plant to be at
the survival level. In practice, a maximum concentration may be
observed where fertilizer nitrogen added in excess of need for
154
-------
maximum production produced some yield depression (e.g., see data
for coastal bermudagrass, Table 6, which shows a continuing increase
in concentration with increasing nitrogen supply including the rate
where yield is depressed).
FERTILIZER EFFICIENCY
Fertilizer efficiency can be defined in various ways. It can be
expressed in terms of the amount of nutrient taken up by the plant
and removed at harvest, the amount of nutrient used by the plant to
increase growth, the economic return for amount of nutrient applied,
or variations of the above. Most of the data in the preceding
sections are presented in terms of nutrient uptake and/or yield.
In this section we discuss fertilizer efficiency from several view-
points.
Fertilizer Uptake Versus Use
Probably the most common expression of nitrogen fertilizer effi-
ciency relates the amount of total nitrogen in the plant derived
from fertilizer to the amount of fertilizer nitrogen applied, that
is, percent recovery of applied nitrogen. Assume for the moment that
the amount of fertilizer-derived nitrogen in the plant can readily be
measured. Consider two nitrogen fertilizers, A and B. At equal rates
of nitrogen application, if more nitrogen from A is found in the plant
than from B, then A is said to be more efficient than B, using the
above definition of fertilizer efficiency. However, suppose that
crop yields from A and B are the same, such that even though less
nitrogen was taken up from B than from A, the nitrogen from B was used
more efficiently by the plant; that is, less nitrogen was needed to
produce the same yield. From a plant physiological point of view,
fertilizer B could be judged more efficient. It is not uncommon for
nitrogen to be taken up by a plant in amounts exceeding those needed
155
-------
at certain growth stages, which results in some luxury consumption
of nitrogen; that is, some nitrogen is not used in the production
of plant cells or cell constituents.
Seldom is the nitrogen in the total plant determined. Most
commonly it is the nitrogen in the above-ground portions of the
plant. The agricultural economist is concerned with those parts of
the plant which have marketable value. Except where nitrogen content
affects crop quality, and only where crop quality has marketable
value, the economist is not concerned with the amount of nitrogen in
the plant derived from fertilizer, but is concerned with the yield
of commodity for each increment of fertilizer applied. The economist
views fertilizer efficiency in terms of the cash value of product
in relation to the cost of the nitrogen input, or profit per incre-
ment of applied nitrogen.
Short-term Versus Loug-terin Efficiency
From the above, it can be seen that nitrogen fertilizer efficiency
can be expressed in terms of: (1) the amount or percentage of applied
nitrogen found in the entire plant or in the above-ground parts of
the plant, (2) the yield of harvested plant parts in relation to the
amount of nitrogen applied, and (3) the value of the harvested plant
parts in relation to the cost of the nitrogen fertilizer application.
These expressions of efficiency usually are used in relation to nitro-
gen uptake and/or yield during the season of fertilizer application.
A longer-term viewpoint of fertilizer efficiency considers also
the residual value of the fertilizer. Commonly 20 to 45% of applied
fertilizer nitrogen remains in the soil after the first growth season,
and some part of this residual nitrogen is recovered by plants in
succeeding years. Very little information is available on the long-
term efficiency of nitrogen fertilizers because such information is
difficult to obtain, requiring expensive, long-term nitrogen tracer
studies in order to obtain accurate data. Whether fertilizer A is
156
-------
more efficient overall than B is of concern more to those interested
in the total environmental impact of nitrogen fertilizers than to
those who are concerned mainly with the crop production value of the
fertilizers.
Measurement of Efficiency
Regardless of how one views nitrogen fertilizer efficiency,
certain measurements must be made which provide the data needed to
calculate efficiency. The collection of accurate yield data is
essential for all calculations of efficiency. Further, it is neces-
sary to estimate the increase in yield from fertilizer above that
which would have been obtained in the absence of fertilizer. It should
be noted that the yield increase is the cumulative effect of the fer-
tilizer application on growth, including the effect of the fertilizer
nitrogen on the possible increased supply of soil nitrogen to the
plant and the increased ability of the plant to obtain more nitrogen
and use it efficiently.
Without use of tracer nitrogen—to be discussed later—two experi-
mental plots are needed to obtain data which will permit one to
estimate fertilizer efficiency in relation to yield. However, it is
desirable to have at least three rates of nitrogen application in
addition to the untreated control plots, plus adequate replication,
depending on site variability. One can expect yield data from a
properly executed field experiment to vary by no more than 10%.
Accurately determining the amount of fertilizer-derived nitrogen
in the whole plant or plant parts is more difficult than determining
yield alone. The total amount of soil plus fertilizer nitrogen taken
up is equal to the yield multipled by the average total nitrogen
concentration in the plant(s). Since the analytical error for
determining total nitrogen in plant tissue is relatively low (about
2 to 6%), the accuracy of the yield data determines the accuracy by
which the total nitrogen content of the crop can be determined.
157
-------
However, there is an additional source of error in the determination
of the proportion of the total nitrogen in the plant which is derived
from fertilizer. We refer to error in data interpretation as follows:
The difference method commonly is used in determining the amount
of nitrogen in the plant derived from fertilizer (except where nitro-
gen tracers are used). That is, the amount of fertilizer nitrogen
taken up by the plant is calculated as the difference between total
nitrogen uptake from fertilized and unfertilized plots. Use of this
method for calculating fertilizer recovery requires one to assume that
all of the nitrogen transformations which affect the amount of soil
nitrogen which is taken up by the plant occur in the same manner in
both fertilized and unfertilized plots. Very often this is not a
valid assumption; the nitrogen fertilizer may produce effects in the
soil or on the plant's ability to obtain soil nitrogen that does not
occur in the unfertilized plot. For example, the fertilizer may stim-
ulate root growth which results in greater root exploration for soil
nitrogen. Usually, the difference method gives higher values for
percent recovery by plants of applied nitrogen than methods which use
nitrogen tracers.
In the usual tracer method, fertilizer labeled with the stable
14 15
(nonradioactive) isotopes of nitrogen, N and N, are applied to
the soil. No control plots are necessary in the usual experiment,
although some researchers have included control plots to enable them
to compare the direct tracer method with the indirect nontracer
method. At harvest, the tracer content of the crop is determined
and the amount of fertilizer versus soil nitrogen taken up can be
calculated. Because the tracer method is not dependent upon use of
data obtained from a control plot, and because measurements are made
directly on the nitrogen in the plant which is derived from the
fertilizer, the tracer method is considered by most researchers to
give a more accurate measure of fertilizer efficiency, where efficiency
is defined in terms of amount of fertilizer-derived nitrogen in the
158
-------
plant. However, the most obvious limitation of this method is its
expense and requirement for sophisticated analytical facilities.
Less obvious limitations also may be inherent in the method. We
refer to possible error in the interpretation of tracer data.
Whether this error is significant is still open to question, and its
discussion lies beyond the scope of this paper. For a more detailed
analysis of tracer versus nontracer methods for measuring plant
recovery of applied nitrogen, see Hauck and Bremner (1976) and
Hauck (1978).
Linear regression methods have been applied to tracer and nontracer
data in order to improve the statistical basis for the relationship
between amount of nitrogen taken up to that applied. The method is
discussed by Terman and Brown (1968); an example using tracer and
nontracer data is giver, by Westerman and Kurtz (1974).
The direct tracer and indirect nontracer methods of determining
plant recovery of applied nitrogen attempt to distinguish between the
fertilizer- and soil-derived nitrogen in the plant. Another method
of estimating fertilizer nitrogen recovery which does not distinguish
between soil and fertilizer nitrogen uses the concept of "apparent
recovery". The calculation is made by expressing the amount of nitrogen
in the harvested portion of the crop as a percentage of the nitrogen
fertilizer applied. Although this is a gross rather than net measure
of nitrogen fertilizer efficiency, it provides a useful value when
one is concerned mainly with measuring the total removal of nitrogen
from the soil by the harvested portion of the crop.
Use of Efficiency Data
It is not our purpose here to select the concept of nitrogen
fertilizer efficiency which we consider to be the most useful. However,
we will illustrate how the several concepts can be used.
Table 6 presents calculations of nitrogen fertilizer efficiency
based on three concepts of efficiency—percent removal by the crop
159
-------
of applied nitrogen, yield (dry matter) increase per increment of
fertilizer nitrogen, and dollar return per dollar invested for fer-
tilizer nitrogen. The calculations use information on yield of
and nitrogen percentage in a coastal bermudagrass crop at 10 nonzero
rates of nitrogen application. Yields of: dry matter increased with
increasing rates of nitrogen application up to about 1,350 kg nitro-
gen/ha. The percentage of total nitrogen in the plant increased
with rate of nitrogen application, the percentage at the highest
rate (2,016 kg nitrogen/ha) being twice that of the unfertilized
crop. The nitrogen application rate of 224 kg/ha was the one most
efficiently removed by the crop (65% of the total nitrogen applied)
but did not produce the largest average yield increase per kilogram
of nitrogen applied, nor the highest dollar return for increment of
fertilizer dollar invested. An application of 112 kg nitrogen/ha
gave the highest dry matter increase, hence dollar return per incre-
ment of nitrogen applied but not the highest profit. (Note that the
economic analysis is based only on the market value of the crop and
the cost of the fertilizer nitrogen based on May 1978 figures. The
analysis does not consider the cost of fertilizer application or the
slightly higher cost of harvesting the higher yields).
Percent recovery of the applied fertilizer nitrogen was calcu-
lated both on an incremental and cumulative basis. On an incremental
basis, the percent recovery is expressed in terms of the amount of
nitrogen in the plant that apparently was taken up for each increment
of nitrogen applied. The yield and nitrogen concentration data in
Table 6 indicate that increasing the nitrogen application rate from
1,344 to 1,568 kg nitrogen/ha had very little effect on the total
nitrogen content of the harvested crop; less than 1% ot the additional
224 kg of nitrogen was taken up. None of the additional nitrogen at
the higher application rates apparently was recovered in the crop.
On the contrary, less total fertilizer nitrogen was taken up because
of adverse effects on plant growth.
160
-------
Percent recovery on a cumulative basis is calculated in terms of
the total fertilizer nitrogen in the harvested crop in relation to
the total nitrogen applied. Notice the qualitative similarity of
percent recovery values calculated in this manner as compared to the
incremental method. However, quantitatively the values grow progress-
ively dissimilar with increasing rate of nitrogen application. Most
economists base their economic analysis of nitrogen fertilizer use on
incremental values because this method of calculation makes obvious
the dollar value of each fertilizer increment.
On the basis of percent plant recovery of applied nitrogen (incre-
mental or cumulative), one could judge that maximum fertilizer effi-
ciency was obtained at the 224 kg nitrogen/ha rate. From a plant physio-
logical viewpoint, a greater percentage of the applied nitrogen was
used in dry matter production at the 112 kg nitrogen/ha rate. The
economist might regard the 1,120 kg/ha application rate to be of
economic value, being concerned not with the highest return for
dollar invested for nitrogen (incremental basis), but with the nitrogen
application rate which gives maximum profit. Perhaps the economist
would consider that an application of about 1,000 kg nitrogen/ha would
provide a safer margin for experimental error, ensuring maximum profit.
At application rates greater than 1,344 kg nitrogen/ha in this study,
profits would have declined because yields were decreased and nitrogen
was wasted. Engelstad (1963) concluded from an analysis of corn and
cotton yields and profits that a rather wide range in nitrogen fer-
tilizer rates is possible without seriously affecting either yield or
profit. Those concerned mainly with the amount of fertilizer nitrogen
left residual in the soil which is subject to leaching might judge
the 224 kg nitrogen/ha rate to be the most efficient in terms of per-
centage removal of applied nitrogen, but might recommend the 112 kg
nitrogen/ha rate because it left the least amount of nitrogen in the
soil after harvest. This viewpoint would ignore the fact that the
dollar return at the latter application rate would be less than one-
161
-------
fourth of that (about $162 versus $789) which would be obtained from
the rate giving maximum profit.
In a second comparison of fertilizer efficiency concepts, we make
use of data for small grains grown under irrigation presented in Table
7, which represents the results of multiple rate experiments at two
locations in Montana and one in Arizona. At site 1, the 100 kg
nitrogen/ha application rate could be judged most efficient from
several viewpoints. This rate produced the highest grain yields; the
crop contained more fertilizer nitrogen in the grain or whole plant
and percent recoveries of applied nitrogen in the grain or whole plant
were highest from this rate, regardless of whether they were calcu-
lated on an incremental or cumulative basis. However, in terms of
grain increase per amount of nitrogen applied, or dollar return for
fertilizer dollar invested, the 50 kg nitrogen/ha rate could be con-
sidered more efficient. From the viewpoint of profitability, one
might choose the 100 kg nitrogen/ha rate. The next higher increment
clearly was wasteful and inefficient, from any viewpoint.
The data from site 2 indicate that the highest application rate
(150 kg nitrogen/ha) was the most efficient in terms of grain yield
and nitrogen uptake in grain or whole plant. The data also strongly
indicate that additional nitrogen would have produced higher grain
yields and more profit, albeit at the 150 kg nitrogen/ha rate, about
half of the applied nitrogen was not taken up. It is interesting to
note that at both sites, half of the nitrogen applied at the rate
considered to be most "efficient" remained in the soil or was not
accounted for.
Data for site 3 are from a southwestern location with a higher
yield potential for wheat than sites 1 and 2. Yet, the first incre-
ment of applied nitrogen (112 kg/ha) resulted in the highest recovery
of nitrogen in the grain and the greatest economic return. The 224
kg nitrogen/ha application produced slightly more grain containing
more nitrogen, but higher application rates up to 560 kg/ha caused
162
-------
Table 7. Wheat grain yield and nitrogen fertilizer efficiency calculated on the basis of
three concepts.
Incremental Accumulative
Removal of
Total
Removal of
Total
Nitrogen
fertilizer
recovery
fertilizer
recovery
Applied
removed
nitrogen
Grain
Economic
grain &
nitrogen
grain &
nitrogen
Yield
in grain
in grain
increase
return*
straw
in grain
straw
kg/ha
kg/ha
kg/ha
%
kg/kg N
$ output/
%
%
%
$ N cost
Spring wheat, site 1**
0
3,293
61
-
-
-
-
-
-
50
4,455
78
34
23.2
5.40
40
34
40
100
4,946
102
48
9.8
2.28
66
41
53
150
4,287
95
-
-13.2
-3.06
-
23
34
Site 2**
0
1,095
23
-
-
-
-
-
-
50
2,305
35
25
24.2
5.63
28
25
28
100
4,005
60
50
34.0
7.91
60
37
44
150
4,838
84
48
16.7
3.88
58
41
49
Site 3***
0
7,090
129
-
-
-
-
-
-
112
8,254
179
45
10.4
2.42
-
45
-
224
8,333
200
19
.7
.16
-
32
-
336
7,868
196
-
-
- .96
-
20
-
448
7,941
198
-
-
-
-
15
-
560
7,795
196
-
-
-
-
12
-
*Based on nitrogen at $0.43/kg and wheat at $0.10/kg.
**Calculated from data supplied by Dr. Neil Christensen, Montana State University, Bozeman,
Montana.
***Calculated from data of Gardner and Jackson (1976).
163
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yields to decline while the nitrogen content in the grain remained
constant.
Comparing the data from sites 1, 2, and 3, one can note a marked
difference in yields without added nitrogen which can be attributed to
a large degree to differences in soil nitrogen supply. Even though
the soil nitrogen level at site 3 was high, its greater yield poten-
tial compared to those of sites 1 and 2 permitted a marked response
to fertilizer nitrogen such that at maximum yield, recovery of fer-
tilizer in the grain approached 50% of the nitrogen applied. Thus,
not only level of soil nitrogen, but all factors which determine yield
potential determine the response of plants to applied nitrogen; that
is, they determine the efficiency by which plants use fertilizer
nitrogen.
Fertilizer Efficiency Versus Management Efficiency
The preceding discussion of the several concepts of fertilizer
efficiency presupposes that by one method or another we measure the
efficiency of applied nitrogen. When comparing different nitrogen
fertilizers, one may be justified in comparing the relative efficiency
of one fertilizer to that of another. But with a single nitrogen
fertilizer, can one justifiably state that the fertilizer per se is
more or less efficient, depending on how much fertilizer is applied,
on how it is applied, or on other crop management factors? How can
one state the efficiency of a single fertilizer in absolute terms
when one knows that relative "efficiency" values are influenced by
the crop and soil management system? With irrigated agriculture in
particular, there is a close relationship between water use and plant
use of applied nitrogen. All data relating to nitrogen fertilizer
uptake and use should be evaluated on the basis of an intimate know-
ledge of the soil-fertilizer-crop management system. Therefore, it is
not the efficiency of the fertilizer which is being measured, but
the efficiency or effectiveness of the fertilizer management system.
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The term, fertilizer efficiency, no doubt will continue to be
used in many ways. It is our intention to draw attention to its
ambiguous nature, and to suggest that users of this term clearly
define the manner of its use. We agree with Terman (1976) that
effective use of fertilizer nitrogen produces the "highest possible
yields consistent with fertilizer supply, profit, and environmental
effects".
ACKNOWLEDGEMENT
The uncited data presented in this paper was obtained through
the cooperation of 25 extension and research workers who graciously
responded to our request for information.
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LITERATURE CITED
Bray, R. H. 1954. A nutrient mobility concept of soil-plant
relationships. Soil Sci. 78:923.
Census of Agriculture for 1974. 1974. U.S. Dept. of Commerce,
Bureau of the Census. Washington, D.C.
Engelstad, 0. P. 1963. Effect of variation in fertilizer rates and
ratios on yield and profit surfaces. Agron. J. 55:263-265.
Fisher, F. L. and A. G. Caldwell. 1958. Coastal bermudagrass as an
irrigated hay crop. Progress Report 2035. Texas Agri. Expt. St
Gardner, B. R. and E. B. Jackson. 1976. Fertilization, nutrient com
position, and yield relationships in irrigated spring wheat.
Agron. J. 68:75-78.
Garman, W. H. and W. C. White. 1964. Crop removal of plant nutrient
balance sheet. Plant Food Rev. 10:14-16.
Halevy, J. 1976. Growth rate and nutrient uptake of two cotton
cultivars grown under irrigation. Agron. J. 68:701-705.
Hauck, R. D. and J. M. Bremner. 1976. Use of tracers for soil and
fertilizer nitrogen research. Advances in Agron. 28:219-266.
Hauck, R. D. 1978. Comments on field trials with isotopically label
fertilizers. In D. R. Nielsen and J. G. McDonald (Ed.).
Nitrogen in the Environment. Academic Press, Inc. N.Y.
Macy, P. 1936. The quantitative mineral nutrient requirements of
plants. Plant Physiol. 11:749-764.
Miller, Robert J. and Richard B. Smith. 1976. Nitrogen balance in
the Southern San Joaquin Valley. J. Environ. Qual. 5:274-278.
Perry, L. J., Jr. and R. A. Olson. 1975. Yield and quality of corn
and grain sorghum grain and residues as influenced by N
fertilization. Agron. J. 67:816-818.
Rauschkolb, R. S. and D. S. Mikkelsen. 1978. Survey of fertilizer
use in California, 1973. Div. Agri. Sci., University of Califor
nia. Bui. 1887.
Terman, G. L. and M. A. Brown. 1968. Crop recovery of applied fer-
tilizer nitrogen. Plant and Soil 29:48-65.
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Terman, G. L. 1976. Efficient use of fertilizers. Crops and Soils
29(3):5-6.
Westerman, R. L. and L. T. Kurtz. 1974. Isotopic and nonisotopic
estimations of fertilizer nitrogen uptake by Sudangrass in
field experiments. Soil Sci. Soc. Amer. Proc. 38:107-109.
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VOLATILE LOSSES OF NITROGEN FROM SOIL
D. E. Rolston—¦
ABSTRACT
Nitrogen may be lost from soil in the gaseous form by two major
mechanisms, ammonia volatilization and denitrification. Ammonia gas
may be lost to the atmosphere whenever ammonium compounds are applied
to the soil surface. The greatest ammonia losses occur from cal-
careous soils at high soil pH. Fertilizers such as urea and ammonium
sulfate result in greater ammonia loss than that from ammonium
nitrate when applied to a moist soil surface. More than 50% of the
applied fertilizer may be lost by ammonia volatilization if precautions
are not taken. The best solution for minimizing ammonia loss is to
incorporate or place ammonium compounds approximately 10 cm below
relatively dry surface soil. The volatile products of denitrifica-
tion, nitrous oxide and nitrogen gas, may be lost from the soil when-
ever the soil becomes wet enough that oxygen becomes depleted and
sufficient carbon is available from organic materials to support
microbial activity. Denitrification occurs significantly only over
a very narrow soil-water content range near saturation and in those
portions of the soil profile with fairly high organic material. Con-
sequently, denitrification will generally only occur in the surface
60 cm of most soils of arid regions unless perched water tables
exist at a buried surface horizon. Management practices to either
minimize or maximize denitrification should be directed at controlling
nitrate position and water content in the surface soil.
— Department of Land, Air and Water Resources, University of
California, Davis, California 95616.
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INTRODUCTION
Nitrogen may be lost from soil in the gaseous form by two major
mechanisms, ammonia volatilization and denitrification. Ammonia gas
may volatilize to the atmosphere whenever ammonium containing mater-
ials are applied to soil. The volatile products of denitrification,
primarily nitrous oxide and nitrogen gas, may be produced whenever
the soil becomes so wet that oxygen becomes depleted. Ammonia vola-
tilization is a chemical reaction whereas denitrification is a biologi-
cal process of soil microorganisms. Both processes result in an economic
loss to a grower.
Another mechanism which may result in volatile losses of nitrogen
from soil is the production of nitrous oxide during the nitrification
of ammonium compounds to nitrate (Bremner and Blackmer, 1977) . This
process has only recently been elucidated, and the magnitude of losses
are not yet established.
AMMONIA VOLATILIZATION LOSS
Ammonia may volatilize to the atmosphere whenever ammonium or
ammonium-forming inorganic or organic nitrogen materials are applied
to soil. The ammonium may be derived from fertilizer or it may be
derived from manures or organic wastes. Organic nitrogen must be
mineralized to the ammonium form before ammonia volatilization can
occur. For instance, manures placed on the soil surface would result
in ammonium being mineralized from the organic form and be available
for volatilization. Ammonium compounds can also reach the soil from
rainwater or irrigation water. Besides an economic loss to a grower
applying fertilizer or manure for crop production, the loss of
ammonia to the atmosphere also results in a pollution problem inasmuch
as the ammonia may eventually be absorbed by surface waters and con-
tribute to the growth of algae in lakes and streams. Thus, it is
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desirable from both an agricultural and an environmental viewpoint,
that ammonia volatilization losses be kept to the very minimum.
Measurement Methods
The primary method of measuring ammonia volatilization loss from
soils is to place a cover or chamber over the soil surface, pass air
through the chamber, and trap any ammonia gas released from the soil
by bubbling the air stream through an acid solution (boric acid is
commonly used). An experimental apparatus used extensively for re-
search on ammonia volatilization loss from laboratory experiments is
described by Fenn and Kissel (1973). Nearly all measurements of
ammonia volatilization loss have been conducted in the laboratory
where the soil is placed within an airtight chamber so that air from
outside of the immediate soil cannot move into the system. The extra-
polation of this procedure to the field is a more difficult task in-
asmuch as the process of using pressure or vacuum in moving air through
the chamber over the soil surface may either underestimate or overestimate
the actual ammonia loss, respectively.
An alternative approach (Denmead et al, 1974, 1976) for measuring the
ammonia volatilization loss from soil surfaces is to measure the con-
centration of ammonia in the atmosphere above the soil or crop canopy.
Concentration measurements in conjunction with meteorological measure-
ments such as air temperature and wind speed at several heights above
the soil are used to calculate the amount of ammonia gas escaping from
the soil surface. The advantage of this method is that there is no
disturbance to the soil environment and, thus, it gives an undisturbed
value. The amount of loss would be representative of a much larger
area than could be measured by placing covers over the soil surface.
The ammonia volatilization loss of an entire field or basin is potent-
ially within the realm of such a procedure.
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Factors Affecting Loss
When ammonium compounds which have been applied either as ferti-
lizer or have been mineralized from organic material such as manure
or crop residues reach the surface of calcareous soil, it is likely
that some of the compounds will react with calcium carbonate (lime)
in the soil to form calcium precipitates. The product, ammonium
carbonate, is unstable and subsequently decomposes to ammonia gas,
water, and carbon dioxide. The ammonia gas given off may form ammonium
hydroxide with the amount of ammonium hydroxide formed dependent upon
the solubility of the calcium precipitate and the rate of formation.
The formation of ammonium hydroxide in the vicinity of the ammonium
fertilizer granules or solution results in an increase in the soil
pH (becomes more basic) which tends to further increase ammonia gas
evolution.
A complete description of the reactions resulting in ammonia gas
production from ammonium compounds is very well described by Fenn and
Kissel (1973). Fenn and Kissel (1973, 1974, 1976) have studied many
of the factors affecting ammonia volatilization loss from soil. The
two major factors affecting ammonia loss are soil pH and the source
of the ammonium compound. The ammonia volatilization loss increases
as the soil pH increases. It is expected that very little volatili-
zation loss would occur from surface applications of ammonium compounds
to acid soils with pH values less than 6. However, ammonia loss can
occur in acid soils, as has been demonstrated by Du Plessis and
Kroontje (1964). Considerable loss of ammonia can occur when ammonium
compounds are applied to the surface of basic soil with pH above 8.
However, soil pH is not the only important factor in determining
loss. Another important factor associated with absolute values of
pH is the buffering capacity of the soil. The buffering capacity is
the ability of the soil to withstand changes in pH when either acidic
or alkaline compounds are added to the soil. The ability to withstand
changes in pH is frequently associated with the amount of lime
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(calcium carbonate) in the soil. Sandy soils generally have a low
buffering capacity, whereas loam or clay soils have considerable
buffering capacity. Another important factor associated with pH
is not the initial pH or the buffering capacity of the soil, but
the upward pH shifts which occur near the band or granule of ferti-
lizer resulting in increasing ammonia volatilization loss. Various
ammonium compounds react differently in soil in terms of the pH
shift near the fertilizer particle. Figure 1 shows the relative
ammonia volatilization loss at two times after application from three
ammonium compounds used as fertilizers. The compounds for which the
calcium reaction products have low solubilities result in large
ammonia losses whereas the calcium compounds formed that have rela-
tively high solubilities result in low amounts of ammonia volatiliza-
tion. This is well demonstrated in Figure 1 for two common ferti-
lizer materials, ammonium sulfate and ammonium nitrate. Calcium
sulfate, which is the reaction product from ammonium sulfate, has a
very low solubility and precipitates, thus driving the reaction pro-
ducing ammonium hydroxide and eventually ammonia gas. However, cal-
cium nitrate which is the reaction product of ammonium nitrate is very
soluble and decreases the reaction to ammonia resulting in low ammonia
losses. Thus, the solubility of the calcium reaction products has a
strong effect on the pH shift near the fertilizer material and greatly
affects the volatilization loss. Ammonia volatilization from urea is
very much dependent upon the soil surface conditions at time of appli-
cation since urea will not convert to ammonium in a relatively dry
soil. If urea is placed on the surface of a wet soil, however, con-
version to ammonium will occur with a subsequent rapid ammonia vola-
tilization. Ammonia volatilization from urea is directly related to
initial soil moisture content due to the effect of water content on
the duration of drying and the time for hydrolysis (Ernst and Massey,
1960). Figure 2 demonstrates the influence of water content on ammonia
loss from surface-applied urea. There is generally some time lag in
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9- 50
9 40
O 20
Ammonium
Sulfate
Ammonium
Phosphate
Ammonium
Nitrate
Figure 1. Total ammonia loss at the end of 24 and 100 hours at 22C
for three ammonium sources. Ammonium was applied on the
soil surface at 550 kg nitrogen/ha. (Redrawn from Fenn
and Kissel, 1973).
174
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25
20
Q.
Cl
O
<
O
IE
<
Air dry
DAYS AFTER UREA APPLICATION
Figure 2. Total ammonia loss of nitrogen added as urea (109 kg
nitrogen/ha) to a silt loam soil (pH 6.5, 24C) of differ-
ing initial water content as a function of time after urea
application. (Redrawn from Ernst and Massey, 1960)
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the hydrolysis of urea to ammonium. However, after hydrolysis occurs,
urea has a greater ammonia volatilization loss in moist soil than
that of the other compounds inasmuch as the calcium reaction product,
calcium carbonate (lime), is less soluble than the other calcium
reaction products and results in large ammonia losses.
Ammonia volatilization loss tends to increase as temperature of
the soil surface increases, although not as drastically as that which
occurs with changes in soil pH or ammonium source. A major point in
terms of temperature effects on ammonia volatilization is that con-
siderable volatilization loss may occur at temperatures as low as 12C.
The rate that ammonium fertilizer is applied to the soil has an
effect on the ammonia loss for some of the compounds, but has very
little or no effect on the loss from other compounds (Fenn and Kissel,
1974). For instance, an increase in ammonium application rate from
approximately 50 to 500 kg of nitrogen/ha resulted in an increase of
from 20% loss to approximately 45% loss in 100 hours for ammonium
sulfate and ammonium phosphate. However, the percentage loss of
ammonia from ammonium nitrate remained at approximately 20% regardless
of application rate.
The time that the material is on the soil surface also has a
slight effect on the ammonia volatilization (Figure 1). The greatest
loss, however, occurs within 24 hours after application of the ammonium
compound to the soil surface. Of the common fertilizer materials,
ammonium sulfate and ammonium phosphate resulted in very little in-
creased ammonia loss after 24 hours. However, for ammonium nitrate,
the ammonia loss approximately doubled from 24 to 100 hours after
application to the soil surface. As discussed before, the exception
to this very rapid loss may be the loss associated with application
of urea to soils in which there is a delay in the ammonia volatiliza-
tion due to the necessity of converting urea to ammonium. This delay
in volatilization from urea may be approximately two days.
Since considerable ammonia loss can occur when ammonium compounds
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are applied to the surface of soils, another important factor affect-
ing volatilization loss is the depth of placement of ammonium com-
pounds below the soil surface or the depth of incorporation into the
soil. Smaller losses of ammonia occur if the ammonium compounds are
placed below the soil surface than if the compounds are simply mixed
uniformly in the surface soil as would occur as a result of harrowing
or discing. This is due to the fact that some ammonium remains close
to the surface and may result in ammonia loss. The moisture content
and texture of the soil over the fertilizer material has an effect on
ammonia volatilization. Dry soil over the fertilizer material is
generally more effective in reducing ammonia losses than is a very moist
soil above the zone of placement. This is primarily due to the fact
that water, moving to the soils surface by evaporation, also carries
along ammonium and results in ammonia volatilization, whereas very
little or no movement of water to the soil surface occurs through
dry soil. A clay or loam soil over the ammonium compound decreases
the ammonia loss much more than does that of sand. This is due to
the greater capacity of a clay soil for adsorbing ammonium than that
of a sand. The ideal depth which generally insures that very little
or no ammonia loss will occur if fairly dry soil is placed over the
ammonium compounds is approximately 7- to 10-cm. Thus, placement
below a relatively dry soil surface is the primary management factor
for the minimal ammonia loss.
Rates and Total Amounts of Ammonia Loss
Rates and total ammonia volatilization losses vary greatly depend-
ing primarily upon the source of the ammonium compound, the initial pH
of the soil, and the placement or incorporation of the material below
the soil surface. Other factors causing lesser effects are the time
after fertilizer application, the temperature, and the rate of ferti-
lizer application. Diagrams showing the general effects of these
factors on ammonia volatilization are given by Figure 3. Figure 3
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T3
-------
shows the effects of soil pH, placement, soil types, temperature,
and rate of application on ammonia loss from ammonium sulfate, a
common material resulting in large losses. Ammonium nitrate gives
smaller absolute amounts of loss than does ammonium sulfate and is
affected by time and rate of application in a somewhat different
manner. For example, percentage loss of ammonia from ammonium
nitrate does not change as the rate of ammonium nitrate application
increases. Another exception for ammonium nitrate is that substantial
loss continues after 24 hours as compared to only small loss after
24 hours for ammonium sulfate. An important fact is that the absolute
amount of ammonia loss can vary from 0 to 70% of the. total ammonium
fertilizer applied.
Practices to Prevent Loss
The major practice to prevent loss of ammonia by volatilization
from ammonium compounds is to place or incorporate the materials approx-
imately 10 cm below the soil surface. Placement of the fertilizer 10 cm
below the soil surface is more effective in preventing loss than is
incorporation uniformly within the top 10 cm of soil. Incorporation
in the soil should take place immediately after broadcasting or placing
the material on the soil surface, inasmuch as the maximum rates of
ammonia loss occur very quickly after application. If the soil surface
is very dry and the fertilizer materials are applied dry, the material
can be left on the soil surface for a few hours until incorporation can
be accomplished. However, a rain or nightly dew could solubilize the
materials and begin ammonia volatilization. In general, ammonium com-
pounds should not be applied to the surface of a wet soil, since
ammonia volatilization loss begins immediately after application.
Even if the material is placed below the soil surface, a wet surface
soil may cause some loss by movement of ammonia to the surface with
the evaporating water.
If circumstances prohibit placement below the soil surface or in-
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corporation of the ammonium compounds in the soil, other practices
can be used to minimize the ammonia volatilization loss. Using
ammonium nitrate, for example, on the soil surface, would result
in much less volatilization loss than if urea, ammonium sulfate,
or ammonium phosphate were broadcast on the soil surface. Another
means of minimizing loss for those circumstances where the material
cannot be placed or incorporated, is applying the material to the soil
surface and immediately irrigating in order to move the material
slightly below the soil surface. This approach is not an entirely
effective means of minimizing loss, inasmuch as the ammonium com-
pounds will adsorb on the soil and move only a very small distance
into the soil profile (a matter of one or two centimeters). The
exception to this is urea. Urea must be hydrolized to ammonium before
the strong adsorption processes will occur. Thus, if urea is applied
to the soil surface and the soil immediately irrigated, urea would
move far enough into the soil profile to minimize the loss.
A relatively common practice for applying nitrogen in irrigated
agriculture is to apply the nitrogen materials along with the irriga-
tion water. This practice may result in considerable ammonia volatili-
zation if the nitrogen is applied as ammonium. Ammonia loss is great-
est if ammonium compounds are applied through a sprinkler irrigation
system due to evaporation than that which occurs from furrow and drip
irrigation systems. Unpublished data of Rolston indicate that very
little loss of ammonia occurred when ammonium sulfate was applied
through a drip irrigation system, due primarily to the fact that the
ammonium was applied at a point and most of the material moved below
the soil surface. Ammonium nitrate is less susceptible to loss than
is ammonium sulfate or ammonium phosphate and would result in smaller
losses if applied with irrigation water. Urea is the most desirable
compound to apply through irrigation systems, inasmuch as it must be
hydrolized to ammonium before ammonia can be volatilized.
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DENITRIFICATION LOSS
Denitrification is the biological reduction of nitrate and nitrite
to several volatile gases. These gases may be nitric oxide, nitrogen
dioxide, nitrous oxide, and molecular nitrogen. However, in most field
situations where oxygen is present at least in some portion of the soil
profile, the two gases released are nitrous oxide and molecular
nitrogen. Denitrification is accomplished by bacteria capable of us-
ing nitrate in place of oxygen. Thus, whenever the soil becomes wet
enough that oxygen is excluded from part of the soil profile, the
potential for denitrification exists. Other necessary conditions for
denitrification are the presence of nitrate and soluble carbon in the
zone which has been depleted of oxygen. Carbon is derived from soil
organic matter, manures, or plant residues. The gases released from
denitrification are harmless to human health at the concentrations that
would be emitted into the atmosphere. However, nitrous oxide gas may
contribute to the destruction of the ozone layer of the lower strato-
sphere. The destruction or decrease in the amount of ozone in the
stratosphere would result in increased incidents of skin cancer and
other biological effects due to increased ultraviolet radiation reach-
ing the earth. It has been postulated that increased use of nitrogen
fertilizers will contribute to the depletion of the ozone layer. How-
ever, that has not been substantiated inasmuch as the magnitude of the
total earth's sources and sinks for nitrous oxide has not been eluci-
dated. Denitrification would constitute an economic loss to a grower
fertilizing with either commercial fertilizers or manure but would be
advantageous for decreasing possible nitrate pollution to the ground-
water from heavy applications of sewage effluents, sewage sludge, or
manure to soil.
Measurement Methods
The amount of denitrification is generally the unknown in attempts
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to evaluate the fate of nitrogen fertilizers or wastes applied to
soils. Denitrification is usually calculated by difference from
measurements of the other components of the nitrogen cycle, such
as fertilizer addition, plant uptake, leaching, and residual soil
nitrogen. Determination of denitrification by difference is commonly
used inasmuch as the various components of the nitrogen cycle, such
as fertilizer addition and plant uptake, have been extensively and
relatively easily measured. The techniques for measurement of these
various quantities is relatively well known and familiar to people.
However, determining denitrification by difference is at best no
better than the reliability of the other measurements with all errors
accumulating in the difference value. The measurement of such com-
ponents as the residual soil nitrogen remaining in the soil after
fertilization and the amount of nitrogen which has leached from the
soil profile is very difficult to accurately measure, especially over
relatively large land areas. Rolston and Broadbent (1977) and Biggar
and Nielsen (1976) have shown that the leaching component, even from
very small field plots, is very difficult to measure accurately. In
addition, determining denitrification by difference generally does
not allow evaluation of the rates or the dynamic nature of the denitri-
fication process. It also does not allow for determination of that
proportion of denitrification consisting of nitrous oxide, a potential
environmental concern.
Field methods for measuring denitrification other than by differ-
ence have required placing a sealed compartment over the soil surface
and either trapping or sampling the gases evolved. This method can
measure nitrous oxide evolution reasonably well. However, it is
difficult to determine how much nitrogen gas has evolved because
small increases above the ambient atmospheric concentration of 78%
nitrogen cannot be measured. Direct methods of measuring the volatile
gases from denitrification have been used by Rolston et al (1976)
and Rolston and Broadbent (1977) by using large quantities of iso-
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topically tagged nitrogen in very small field plots. The use of
the stable isotope, "'""'n, to tag the fertilizer nitrogen allows the
nitrogen gas to be measured in the soil atmosphere and within
compartments placed over the soil surface. Since nitrous oxide has
a low concentration (0.3 ppm) in the atmosphere, it can be easily
measured within the soil and in a chamber over the soil. Measuring
denitrification directly has the disadvantages of a very expensive
means of determining denitrification and, thus, is limited to rela-
tively small field plots or laboratory experiments. Another dis-
advantage of the direct method using tagged nitrogen is that the
method requires fairly complicated measurement techniques, exper-
ienced technicians, and special instrumentation. The advantages of
the direct method are that both nitrous oxide and molecular nitrogen
from denitrification can be measured as well as total denitrifica-
tion. Thus, the time period over which denitrification occurs can
be elucidated in order to develop management decisions to control
denitrification.
Factors Affecting Loss
As with all biological and physical processes in the field, several
soil parameters affect denitrification. The most important factors
are the water content of the soil, the development of pockets or zones
where oxygen is excluded, the amount and availability of carbon,
soil temperature, and soil pH. Each of these factors will be considered
in turn.
Water content influences denitrification by providing a suitable
environment for microbes and through blockage of the soil pores re-
ducing the ability of soil to transmit oxygen to the zones of high
microbial activity and respiring plant roots. As the soil pores become
nearly filled with water, oxygen can reach the sites of high oxygen
consumption only by diffusing through the water, which is a very slow
process, or by diffusing through a few large pores. The greatest
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potential for denitrification would be for completely water saturated
soil. In the field, soil is saturated for only short periods during
an irrigation or rainfall, or the soil may become saturated directly
above clay pans or impeding layers in the soil profile. In many
irrigated soils of semiarid regions, only the upper few centimeters
(0-10-cm depth) are truly saturated during an irrigation or rainfall.
All depths below the surface few centimeters will be slightly unsatu-
rated with some pores filled with air through which oxygen may diffuse
rather rapidly. Denitrification occurs over a very narrow range of
soil-water contents or soil-water tensions. No denitrification appar-
ently occurs when soils become drier than field capacity (approximately
30 centibars tension) as discussed by Focht and Verstraete (1977).
Rolston et al (1976) and Rolston and Broadbent (1977) have shown
for an alluvial loam soil that very little denitrification occurred
at soil-water tensions greater than 10 centibars (field capacity is
generally considered to be approximately 30 centibars and the wilting
point is 1500 centibars). For a clay soil, the upper limit of tension
for which denitrification may occur would be expected to be approximately
15 centibars, whereas in a sandy soil the upper limit may be approxi-
mately 5 centibars.
The development of soil zones with all oxygen excluded is another
important factor affecting denitrification loss. Not only is the
total volume of soil without oxygen important, but also the position
of that zone within the soil profile. In deep uniform soils with a
water table greater than 10 or 20 meters, it would be expected that
the zone of lowest oxygen levels would occur near the soil surface.
This has been shown by Rolston et al (1976) and Rolston and Broadbent
(1977) for a deep loam soil with the water table at approximately 25
meters. Oxygen levels were smallest within the top 15-30 cm of soil
than anywhere else in the soil profile. This is due primarily to the
fact that the upper portion of the soil profile becomes wettest due
to generally higher bulk densities and an impermeable layer for water
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infiltration at the surface and to high oxygen use by microbes and
roots in the surface. Other positions which may become excluded of
oxygen are above slowly permeable layers, such as hard pans, or clay
pans within the soil profile. In many soils, however, it may be un-
likely that soils above clay pans become anaerobic due to insufficient
carbon or microbial activity at the depth of the perched water table.
Denitrification will not occur unless sufficient available carbon
is present regardless of the amount of nitrate present or lack of
oxygen in the soil. Thus, the amount of carbon, its position in the
profile, and its position in relation to the nitrate of the soil, will
have a large influence on the rates and amounts of denitrification.
Percentages of carbon are highest at the soil surface and decrease
rather rapidly with depth in most soils of semiarid regions. Excep-
tions are peat or high organic matter soils, such as the Sacramento-
San Joaquin Delta of California. Thus, as nitrate is moved by irriga-
tion water or rainwater to deeper depths within the soil profile the
chances of any substantial denitrification decrease. Rolston (1976)
measured no denitrification in a Yolo loam soil kept near saturation
at the soil surface after the nitrate had leached from the upper 60-cm
of soil. Pratt et al (1976) determined that denitrification was
limited mostly to the top 60-cm of three soil profiles (sandy loam,
sandy loam with clay pan at 60-cm, and silty clay soils) to which
manure had been applied for four years. An exception to the greatest
denitrification occurring in the upper few cm of soil would be those
cases where a buried profile containing considerable organic matter
lies somewhere within the soil profile and at a position such that a
perched water table would occur.
The increase in denitrification from the addition of carbon is
well established (Bremner and Shaw, 1958) . The effect of plant
roots in accelerating denitrification is also well known (Stefanson,
1972; Volz, 1976; Woldendorp, 1962). Rolston and Broadbent (1977)
also found that very little denitrification occurred in a loam soil
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maintained nearly water saturated and uncropped. The presence of a
crop not only contributes organic material from dead roots but also
decreases oxygen concentrations through root respiration. Thus, the
presence of a crop growing on the soil at the time fertilizer is
applied can be expected to increase the potential for denitrification.
The length of time that plants have been growing on the soil also
greatly increases denitrification. Rolston and Broadbent (1977) showed
much smaller rates of denitrification from a plot to which grass had
been planted approximately three months prior to the experiment than
did those from experiments (Rolston et al, 1976) where grass had been
grown in the plot for approximately two years. With grass on the plot
for a long time period, the amount of organic material added to the
soil from the root system was apparently large. The addition of
other organic sources to the soil will also increase the potential for
denitrification. For instance, incorporating manure into the soil
would increase carbon levels and potentially could result in increased
denitrification. However, the manure would most likely have to be
left in the soil long enough for sufficient carbon to become available
to induce denitrification. As discussed previously, the nitrate must
be at the same position in the profile as the carbon in order for
denitrification to occur. Denitrification increases as the soil tem-
perature increases due to increased biological activity. Rolston and
Broadbent (1977) determined that denitrification was still measure-
able in plots to which manure had been added at a soil temperature of
8-IOC. Thus, denitrification may occur in the winter months if other
conditions are optimum.
Soil pH influences the rate of denitrification and the proportion
of the different gases released (Focht and Verstraete, 1977). Denitri-
fication is small at very low pH and increases as pH increases
(Broadbent and Clark, 1965). However, in the pH range between 6 and
8, total denitrification is only slightly affected by changes in soil
pH.
186
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Rates and Total Amounts
The rates and total amounts of denitrification are dependent upon
each of the factors described in the previous section. The total
denitrification occurring from a nitrate application of 300 kg nitro-
gen/ha to a Yolo loam field soil as affectecj by carbon level, water
content, and temperature are given by Figure 4 (Rolston and Broadbent,
1977). The rates measured by Rolston and Broadbent (1977) vary from
0 to 70 kg of nitrogen/ha/day. The denitrification rate of 70 kg
nitrogen/ha was for a plot maintained nearly saturated and into which
34 metric tons of manure/ha had been incorporated in the upper 10 cm
of soil approximately three weeks before the nitrate fertilizer was
applied to the wet soil surface. The total amount of denitrification
for this manure treatment, representing nearly maximal denitrification
potential, was approximately 200 kg of nitrogen out of 300 kg applied.
Most of the denitrification occurred in the first five days after the
fertilizer was applied. The soil was kept continuously wet (near
saturation) during this time period. No denitrification could be
detected after approximately twenty days. This time period corresponds
approximately to the time that the nitrate would have been leached from
the top 30-45 cm of soil. The maximum rate measured in a cropped
field plot (perennial rye grass) by Rolston et al (1976) was approxi-
mately 10 kg of nitrogen/ha/day. Again, most of the denitrification
occurred over a 30-day period with approximately 150 kg of nitrogen
denitrified out of a total of 300 kg applied. For uncropped plots
with no carbon addition and maintained close to saturation, the maxi-
mum denitrification rate measured was approximately 2.5 kg of nitro-
gen/ha/day with the total loss occurring over a 15-day period of
only 8 kg of nitrogen out of a total of 300 kg applied. Maintaining
the soil constantly wet without allowing for redistribution, evapora-
tion, or plant uptake represents the optimum conditions for denitri-
fication in terms of water content and anaerobic development. The
denitrification occurring during and after normal irrigation or
187
-------
220 r
o
2 1 0
xz
V
c
m
cn
o
200 -
2
CP
#•>
2
50 -
O
h-
<
O
40 -
U_
oc
H
30 -
2
tu
Q
20 -
<
H
O
H
10 -
300 kg Nitrogen/ha applied
SC 27C
7/Ss
90% Water
Saturation
1.5 Centibars
80 % Water
Saturation
7 Centibars
2^
MANURE CROPPED UNCROPPED
Figure 4. Total denltrification from cropped, uncropped, and
manured field plots (Yolo loam) at two soil tempera-
tures arid two water concents.
188
-------
rainfall events should be considerably smaller than these values
inasmuch as tlx :><»i 1 profile would be less wet and less anaerobic
over shorter time periods. An estimate of the expected denitrification
that might occur over an entire season may be calculated, from the
rate experiments of Rolston and Broadbent (1977). The assumptions
used in. the calculation are as follows; 1) The denitrification
flux during wet periods is 5 kg of nitrogen/ha/day as measured for
the nearly saturated, cropped plots of Rolston and Broadbent (19 7?);
2) The above flux occurs for 12 hours during an Irrigation cycle or
heavy rainfall event; 3) The above flux occurs for the first four
Irrigations of a season and for one rainfall event, from these
assumptions, denitrifIcation was calculated to be approximately 13
kg of nitrogen/ha. If 150 kg of nitrogen/ha was applied as fertilizer,
denitrification loss would be about 31. Although, little evidence
exists for the above assumptions, the percent denitrifIcation is
the same order of magnitude as values determined from nitrogen bal-
ance experiments. Broadbent and Carlton (1976) found that denitrl-
fication from a three-year nitrogen balance field study was between
5 and 8% per year on a sandy loam soil cropped with maize.
Management Practices
Management of fertilizer nitrogen or nitrogen from wastes
applied to soils may be directed in either of two directions. One
goal would fee to minimize denitrification losses, such as would be
the objective' of a grower applying fertilizer and wanting to maximize
the use of that fertilizer. The other management "b Arrive might be
to increase denitrif ication to the maximum for applications of large
amounts of sewage effluent, sludge, or manure to soil in order to
minimize fin" amount uf nitrate leaching b^low disposal sites. In
order to »f ttfroizr deiiitriffrat ion, the major Cat-tors affecting denitri-
fication should not occur .tc the saint5 point In the soil or at the
same time. Fur example, high concentrat ions of nitrate should not
189
-------
be allowed to occur at a position in the profile with high concen-
trations of carbon and high water contents. The absence of either
nitrate or carbon or the; presence of oxygen would result in very
little or no den.itrificat.ion. Thus, management practices should be
directed at managing the water, the carbon, and the fertilizer
such that they do not occur simultaneously in the soil profile. A
proper management decision would be that fertilizer should not be
applied to the surface soil (high carbon) at times when water
contents are likely to be very close to saturation. Thus, one would
not want to apply fertilizer to the surface of soils in winter or
fall due to the likely chance that water contents would become large
along with high amounts of carbon in the surface portion of the soil
profile. Another management decision to decrease or eliminate
denitrification would be to insure that: fertilizer was placed at
such a depth where plant roots could still obtain the needed nitrogen
yet not be in a zone of high carbon or potentially high water contents.
Application of manure along with nitrogen fertilizer would be an un-
realistic practice if one wanted to minimize denitrification. if
water contents or tensions could not be maintained outside the range
where denitrification occurs, it may be possible to time fertilizer
application so that nitrate will not occur in the zone containing high
carbon at a time when water contents would, be close to saturation,
for instance, it may be possible to apply ammonium fertilizer imme-
diately before a large irrigation for wetting the profile at the
beginning of the season. The next irrigation would be tilted and
managed such that the nitrate would, be moved out of the zone where
most of the carbon occurs yet remain within the zone where roots could
obtain the nitrogen.
Opposite kinds of management decisions would be made to maximize
denitrification. One would want to have high water contents and thus
low oxygen and high carbon levels in the zone containing nitrate.
This could be accomplished by keeping the soil very wet by w,jt «•»
190
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management and adding carbon materials, such as manure, sludge or
crop residues to the same zone where the high water contents and
high nitrate concentrations would occur. One would also have to
consider that for disposal of wastes, the nitrogen is generally in
the ammonium form. Thus, one would have to allow a cyclic period of
aerobic conditions for ammonium to transform to nitrate. Such a
system to maximize denitrification becomes fairly complex when the
nitrogen is applied as ammonium.
191
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LITERATURE CITED
Brenner, J. M. and K. Shaw, 1958. Denitrification in soil, II.
Factors affecting denitrif ication, J. Agrie. Seil, 51:40,
Brenner, J. M, and A. M, Blackmer. 197). Nitrous oxide; Emission
from soils during nitrification of fertilizer nitrogen. Science
139:295-296,
Broadbent, F, E. and F. Clark. 1965. "Denitrification" in soil
nitrogen. ASA Monograph No. 1.0, Madison, Wisconsin. 615 pp»
Broadbent, F, E, and A. B. Carlton. 1976, Field trials with isotopes,
Kearney site. Annual Report to the 'National Science Found.it icm-
Nitrates in Effluents from Irrigated Lands - University of Califor-
nia. p. 31-34.
Denmeaci, 0. T.» J, 1. Simpson, and J. 1, Preney, 1974. Ammonia flux
into the atmosphere from a grazed pasture. Science. 185:603-610.
Denmead, O, T.» J. R. Preney, and J. R. cu#i»son. 1976, A closed
ammonia cycle within a plant canopy. Soil Biol. Biochem. K: I f« I -
164,
Du Plessis, If, C. F» and W. Kroontje. 1964, The relationship between
pB and anmonia equilibrium in soil. Soil Sci. Soc. An. Proc.
28:751-754.
Ernst, J. W. and 1. F, Massey. 1960. The . t H-cts of several factors
on volatilization of anwmla formed from urea in the soil. Soil
Sci. Soc. Am. Proc. 24:87-90.
Fenn, L. B. and D. 1. Kissel. 1973. Ammonia vulai iIization from
surface applications of ammonium compounds on calcareous soils:
I. General theory. Soil Sci. Soc. Am. Proc, 37:855-859.
Fenn, i, B. and D. E. Kissel. 1974. Ammonia volatilization from
surface applications of ammonium compounds on calcareous soils;
IT, Effects of temperature and rate of ammonia nitrogen appli-
cation. Soil Sci. Soc, Am. Proc. JH;h06—610,
Fenn, L. 1. and B, E. Kissel. 1916. The influence of cation exchange
capacity and depth of incorporation on ammonia volatilization
from ammonium compounds applied to calcareous soils. Soil Sci.
Soc. Am. J. 40:394-398.
192
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Foeht, D. D. artel W. Verstraete. 19??, Biochemical ecology of nitri-
fication and denitrification In Advances in Microbial Ecology,
M. Alexander (ed,) 1:135-214,
Pratt, P. F,, L. J, Lund, and J. E. Warneke. 1976. Nitrogen losses
in relation to soil profile characteristics. Annual Report to
National Science Foundation-Hitrates in Effluents From Irrigated
Lands - University of California, pp. 141-168,
Rolston, D. E,, M. Fried, and D. A. Goldhamer. 1976, Denitrification
measured directly from nitrogen and nitrous oxide gas fluxes,
Soil Sc±. Soc. Am. J. 40:259-266.
Rolston, D. E. and F. E, Broadbent. 19??, Field measurement of
denitrlfication. EPA-600/2-77-23. U.S. E.P.A., Ada, Oklahoma,
pp. 75,
Stefanson, i, C, 1372. Soil denitrlfication in sealed soil-plant
systems. I. Effect of plants, soil water content and soil
organic matter content, Plant Soil 33;113-127.
Volz, M. 6.» M. S, Ardakani, R. 1, S'hulz, L, i, Stolzy, and k, D» McLaren.
1976, Soil nitrate loss during irrigation: Enhancement by plant
roots, Agron. J» 68:621-627,
Woldendorp, J. W. 1362, The quantitative influence of the rhIzosphere
on denitrlfication. Plant Soil 17:267-270,
193
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LEACHING OF NITRATE FROM SOILS
B. L. Mcieal and P, F. Ppwit
ABSTRACT
Nitrate leaching from irrigated croplands reflects the super-
imposing of water management on the nitrification, crop removal,
and denitrification processes. Rapid movement of the nitrate anion
through most irrigated soils results from its repulsion or "negative
adsorption" by negatively-charged soil particles. Under selected
conditions (e.g., in high-clay soils), nitrate may appear to move
even more rapidly than does the water in which it was present
initially. It is only in a relatively few highly-acidic or highly-
weathered soils that nitrate may actually be adsorbed by positively-
charged sites.
Calculation of soil solution nitrate concentrations, and of mass
emissions of nitrate, are illustrated. Estimation of average water
flux from nitrate/chloride ratios, and fro* drain field outflows,
are discussed. The high potential for nitrate leaching from irrigated
animal and municipal waste disposal sites Is mentioned, with stress
being placed on the high denitrification potential at many such sites,
because of high soluble carbon loadings, high soil microbe levels, and
rapid oxygen depletion,
loot-zone and subsoil nitrate-nitrogen values are provided for
typical crop lands of southern Gal I fornla arid central Washington.
1/ ' — ¦
— Department' of Autonomy and S«»f f «*, W;mhing(«n Staff University, Pull—
furtn, Washington 99 J hi; and Depart mail of Soil nt in, Riverside, t..-i j i I urn i a l)c> 'J 1 .
!
-------
Such values are related to irrigation management and to downward
water flux. Crop differences in ability to utilize all soluble
nitrate from the soli solution are stressed, and some 'management
alternatives for preventing deep percolation losses of nitrate
are detailed.
Predictions of nitrate leaching losses from data for nitrogen
application rate and drainage water volume are discussed. Typical
predictions are presented and are compared to experiineB.tally~ii)easured
values, Overprediction of nitrate leaching estimates for some furrow-
Irrigated tracts in the Pacific Northwest is demonstrated.
INTRODUCTION
Nitrate leaching represents the superimposing of water manage-
ment on ttje nitrification, crop removal, and denitrification
processes discussed in other chapters of this volume. Except in
cases where fertilizer nitrogen has already been provided In the
nitrate for®, production of nitrate represents the end step during
the transformations of nitrogen in well-aerated soils, whether the
original nitrogen, came from inorganic fertilizers, from crop residues
or organic wastes, or from soil organic natter. It mist be recognized
from the outset that substantial soil solution, nitrate concentrations
are the rule, rather than the exception, during the growing season
for well-managed, "fertile" soils. Soil nitrate, once produced,
can be removed from the crop root zone either by plant uptake, by
gaseous denitrification losses from poorly-aerated soil zones, or
by nitrate leaching whenever excessive Irrigation water is applied.
NITRATE MOVEMENT
The vast majority of irrigated soils are negatively charged.
196
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Hence, the nitrate anion is repelled front, soil colloid surfaces,
and can be readily leached by percolating waters. The process
whereby anions are repelled by charged particle surfaces is termed
anion repulsion or "negative adsorption". Such repulsion Is greater
for more highly-charged soils (e.g. those having higher cation-exchange
capacities), and usually Increases with Increasing soil pH. A few
anions, such as phosphate and arsenate, move only slowly through soils,
for they are either rapidly adsorbed on soil particle surfaces or are
readily precipitated as slightly-soluble compounds. Anions such as
nitrate and chloride, however, tend to be repelled by all but the
least highly-charged or most acidic (e.g., most highly-weathered)
soils.
In cases where highly-weathered or highly-acidic soils are being
irrigated, one should be aware that nitrate may move less readily.
Such soils would generally have pB values less than 5,5 to 6
(Espinoza et al, 1975), as well as the dark brown or deep red colors
indicative of highly-weathered soils. Jfit adsorption is also
high in typical soils having high contents of amorphous minerals of
volcanic origin (Kinjo anil Pratt, 1971).
That nitrate will normally be repelled from particle surfaces,
however, has been demonstrated by numerous studies. For example.
Smith and Davies (1974) found only one Alabama subsoil (of pi 5,1)
from among a group of 16 U.S. surface soils and subsoils that ex-
hibited anion retention, whereas the other 15 samples (of pH 6.2 to
8,7) all evidenced anion repulsion to varying degrees. Because of
this repulsion, they found that nitrate and bromide anions moved 1.1
to 1.6 times more rapidly through the samples than did the water in
which the anions were present initially.
Two explanations can be offered for such enhanced mobility of
anions. Because of anion repulsion,, a lower concentration of
nitrate exists in the water immediately adjacent to charged soil
surfaces, and the anion must move through only a portion of the water
i'JZ
-------
in each pore sequence as it traverses the pore, it is also in the
central part of the pore that the pore water velocity is greatest,
so each anion is moving, on the. average, more rapidly than is an
average water molecule during bulk flow of water and solutes through
a soil.
Because of anion repulsion, experiments in which a given amount
of nitrate or chloride is Introduced to a col mm of soil simultan-
eously with a given amount of "tagged" water commonly result in the
nitrate or chloride anions appearing In the leachate before the
corresponding increment of water appears. Such behavior is demon-
strated In Figure 1. In either case, whether the anion was Intro-
duced as a single pulse of material or as a continuous source, the
anion emerged from the soil column more rapidly than did the "tagged"
water In which the anion was present Initially. Such behavior la
of the greatest concern in highly-charged, high-clay soils, such
as the "black clays' of southeastern Texas (Thomas and Swobode,
1970) , Water movement through such soils is generally through fine
pores, where the influence of the charged soil surfaces la particu-
larly evident. The effect of anion repulsion, on relative movement
of water and nitrate through raoderate- to coarse-textured soils is
often negligible, for the nitrate ant! the water move essentially in
tandem through relatively large water-filled pores. The key factor
is the extent to which the pore transmitting most of the water are
large relative to the thickness of the layer froto which anions have
been repelled near soil particle surfaces.
Anions such, as chloride or bromide are used commonly as "tracers'
of nitrate movement through soils, because they enter into relatively
few biological reactions. Actual movement of "pulses" of nitrate
through soils may be retarded, more diffuse, and of lower peak con-
centration than for these more inert analogues, however. This Is
due to the highly reactive nature of nitrate in actual biological
systems* such as soils (e.g., Smith and Bavies, 1974),
198
-------
o
o
x
u
1.0
0.8
0,6
0.4
o
o J*
u .•» >lt ptfr ot ^ *'® in
soae field {t foiu vim
-------
NITRATE CONCENTRATIONS AW) FLUX
Results of soil analyses may be reported in a variety of ways.
Some of these may be misleading, -end may create the impression that
a relatively minor nitrate pollution potential exists, when In
reality the potential for nitrate leaching is substantial, It aust
be kept in aind, for example, that it is the soil solution, rather
than the soil itself, which is displaced during leaching. Hence,
information cm residual nitrate levels in a soil before, during
and following the cropping season must be converted to a solution
basis prior to comparisons between studies, For example, a typical
value for residual soil nitrate-nitrogen following a crop growing
season may be on the order of 3 to 6 parts per million parts of
dry soil. As a 30-c.m depth of "average" soil weighs roughly four
million kilograms per hectare, this means that 10 to 25 kilograms
of soluble niirate-nitrogen are present in each 30-cm depth of
soil and might be leached whenever excess Irrigation water or rain-
water is supplied. Furthermore, the average water content of field
soils is commonly on the order of only 15 to 301 (expressed on a
dry-soil basis). This means that the nitrate concentration of the
soil solution is 3 to ? times higher than the level of residual soil
nitrate (assuming that the latter was also expressed on a dry-soil
basis). Hence, our 3 to 6 parts per willion nitrate-nitrogen In
the dry soil becomes 10 to 40 parts per million nitrate-nitrogen
in the soil solution. Any displacement of this solution deeper
into the soil profile can lead to appreciable environmental
hazards.
Substantial concentrations of nitrate-nitrogen persist In the
plant root zone of most "fertile" soils. Such concentration® need
not constitute an environmental hazard, however, if no soil solu-
tion Is displaced from the root zone. It is only the necessity of
providing excess water for salinity control» and the inefficiency
200
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of water application in the 1which leads to the nitrate leach-
ing associated with many irrigated croplands.
The amount of excess water passing through the crop root zone
varies substantially from one area and front one type of irrigation,
system to another. For well-managed sprinkler Irrigation systems,
where minimal leaching for salinity control is being practiced,
leaching fractions of only 10 to 20% (based upon the amount
of water applied) are not uncommon. With typical crop water needs
throughout the irrigated West averaging 60 to 90 surface centimeters
per year (with the exception of crops having unusually high demands
for water, such as alfalfa), this means that only 6 to 20 surface
centimeters of water need percolate through a well-managed, sprinkler-
irrigated soil. For furrow-irrigated fields, on the other hand,
variations in amounts of water applied from point to point on the
landscape are considerably more pronounced. For some of the older
irrigation projects of the Pacific Northwest, for example, it is not
uncommon to have annual application rates (after conveyance losses
have been deducted) of 120 to 180 cm of water. Hence, after
crop water needs have been subtracted, one fourth to one half or more
of the applied water is available for leaching of solutes (including
nitrate) from the field. Fortunately, however, much of this water
passes through previously-leached soil beneath the irrigation furrow,
and I:» rcl.u fvt ly Ineffective in removing additional nitrogen from
the crop toot zone, la calculating the concentrations and total
quantities (mass emissions) of nitrate from irrigated fields, it. is
necessary to have estimates of both the amounts of water being leached
from a given field and of the corresponding concentrations of nitrate-
nitrogen In that water. It is then a straightforward matter to
multiply the two terms together in order to obtain an estimate of
the quantity of nitrogen I< aving the field,
In cases where a r h !
-------
Letey et al» 1977, found it virtually impossible In some settings),
and to periodically monitor the nitrate concentration of the drainage
waters (which normally changes only slowly with time), in order to
obtain a measure of the mass emission of nitrate. One must be careful
In such situations, however, that large quantities of water are not
entering the drainage field from surrounding areas, This Is particu-
larly likely for undulating lands or where tiles drain relatively-
level fields near the foot of surrounding slopes. it is also likely in
cases where water Is seeping from unlinec! canals in the vicinity. Such
water coassonly is quite low in nitrates, and can considerably dilute
the concentrations of nitrogen actually leaving the crop root zone of
the field in question.
In cases where a field is not tile-drained» the estimation of
downward flux of water across the landscape becomes considerably more
difficult. One cannot simply rely on the; periodic monitoring of a
few points in a field, for the points chosen may not adequately repre-
sent average water-flow characteristics of the particular soil, crop,
and irrigation-system conditions which exist at the site. In addition,
"pulses" of nitrate-nitrogen become less concentrated and more broadened
as they move downward through the soil, Examples are provided in
Figure 2. Such variations in pulse height and shape make intermittent
sampling as hazardous with respect to time trends as with respect to
positional trends across the landscape.
Crude estimates of the amount of water leaving the crop root zone
can be obtained by measurement of the amount of water applied, coupled
with eatioation of the amount of water being used by the plant; through
micrometeorological measurements, evaporation pan estimates, or other
types of crop consumptive use calculations, Estimates tan also be
obtained from lysimeter installations, where large, soil-filled
cylinders are plated in the middle of a cropped field and either
weighed continuously with an hydraulic device or evacuated periodi-
cally, with the amount of water leaving the cylinder then being trapped
202
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COLUMN 8
COLUMN ?
* DAY I 2
• DAY ! 9
¦ DAY 26
0 100 200 500 400 0 100 200 500 400
FERTILIZER NITRATE CONCENTRATION (mg N/liter)
500
Figure.2, Conffnt oition of fertiliser tut rate as.a funot :•«»
soil Utjvth for three times m tr-t application «u nnr,ite
puise to two calumiis of Yolo low topsoil. ±3 id met
broken curves we re calculated appropriate >¦* ot'ull t I-
fication rate constants (from Kei st on and iariius
203
-------
and measured» However, it is difficult in many cases to exactly
simulate crop growth and soil profile water-transmission properties
with a lysineter installation. For research purposes, estimates
of downward water movement can be obtained from heavily-Instrumented
sites, Tensiometers, for example, can be placed at several depths
in the soil profile in order to estimate water contents and 'hydraulic
gradients in the soil, once curves of water content versus soil
tensions have been obtained for the soils at the site. Similarly,
neutron probe measurements can be used to monitor soil water contents,
This information,, coupled with a knowledge of the water-holding
capacity of the soil, can be used to estimate the downward flux of
water under prescribed field conditions. Such installations are
extremely expensive, however, and are not practical for routine
usage.
Several workers have attempted to monitor downward water move-
ment In freely-draining fields through the use of nitrate/chloride
ratios. The rationale for the approach, is an assumption that the
growing plant and soil microbes cause continued changes in soil solu-
tion nitrate concentrations, whereas chloride concentrations in many
settings remain relatively unaffected by biological processes other
than plant water uptake, Hence, if chloride is not being taken up
by the plant, is not being produced by the dissolution of soluble
chloride minerals in the soil, and is relative!y-uniformly applied
if any chloride-containing fertilizers are in use, changes in
chloride concentrations should reflect only the removal of water by
the transpiring plant, Pratt et ai (1972) and Adrians efc al (1972a,
1972b), for example, used relative chloride concentrations in the
irrigation water and in the water of the unsaturated zone below
the root zone to approximate the leaching fraction for fields they
were monitoring. The approach worked particularly well, for citrus,
which almost quantitively excludes chloride. Changes in the
nitrate/chloride ratio then reflect additional removal of nitrate
204
-------
from, the system, as during crop nitrogen uptake or during losses of
gaseous denitrification products from poorly-aerated soils. Correc-
tions are required for crops like asparagus, which remove appreciable
amounts of chloride from the soil-water system and adjustments must
be made for the chloride added in fertilizers and in manures,
Early comparisons of relative nitrate and chloride movement,
as by Wetselaar (1962), Involved the monitoring of different salts
in different soil profiles or at different points on the landscape.
More recent work has centered around the use of ratios ui di'Tiere;nt
solutes for the same soil samples, however. For example, Kimble
et al (1972) have also used chloride/nitrate ratios to delineate the
amounts of denitrification occurring where manure has been applied
as a nitrogen source. Once the amounts of water applied annually,
and the leaching fractions, have been calculated, the downward water
flux and the transit time through the unsaturated zone to the water
table beneath an area can be estimated. Adriano et al (1972a)
calculated transit times through the unsaturated zone beneath irrigated
fields from the relation
SCI
transit time in ye.tr - | (1)
where S = soil profile depth, 9 = volumetric soil water content, and
D = drainage water volume in surface cm/year. Using this relation,
Pratt et al (1972) calculated transit times of 12 to 47 years for
nitrate to move through a 30-meter unsaturated zone beneath southern
California citrus groves, for an annual leaching fraction of approx-
imately 0.4
In a similar vein, Pratt and Adriano (i 9 7.S) related nitrate-
nitrogen (NO^-N) concentrations in the unsaturated zone beneath
irrigated fields to (the excess nitrogen, defined as nitrogen
inputs (I ) minus nitrogen uptake (N ) by the crop) and D (defined
above) through the relation
205
-------
NCL-N (ppm) = 10 N /B
j e
(2)
They reported the relation to be accurate to within + 20% for fields
where appreciable amounts of cienitrifi.cation or of mineralization of
organic nitrogen were not occurring. Unfortunately, the latter
"exceptions" constituted 16 of the 25 settings reported in their
paper.
In cases where the percolating waters are entering a relatively
static and relatively deep groundwater body, the concentrations of
nitrate-nitrogen in the percolating waters will be reflected by the
concentrations of nitrate-nitrogen in the groundwater, or at least
in the upper portions of the groundwater, as long as mixing remains
incomplete. Where sprinkler irrigation Is being employed~ so that
concentrations of nitrogen leaving the bottom of the root zone are
¦at least hypothetically uniform across the field, the concentrations
of nitrogen. In the water leaving the bottom of the root zone will
correspond roughly to the concentrations of nitrogen in the upper
portions of the underlying groundwater body. In cases where a field
is being furrow-Irrigated, however, increments of water containing
rather high concentrations of nitrogen will be diluted by increments
of water that flow through the well leached portion of the soil under
the furrow, and the groundwater body will In this case reflect the
weighted average of the respective nitrate-nitrogen concentrations.
Pratt et al (1.972) have repotted, in general, an inverse relation
between the leaching fraction and nitrate-nitrogen concentrations of
the unsaturated zone, although Pratt et al (1976) found this to be
true only at higher rates of manure application for a set of field
aamnre application plots, and Letey et al (197?) found no apparent
relation between volume of tile drain outflow and drainage water
nitrate concentration. In cases where the economies of nitrate
loss are trader consideration, where.the leached nitrogen is being
returned to surface streams or to rapidly-moving groundwater aquifers.
206
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or where, numerous sources are contributing to the nitrogen levels
of a groundwater body, the total quantity of nitrogen being emitted
is of considerably greater importance than the actual concentrations
of nitrate involved. This point has been stressed strongly in the
conclusions of Letey et al (19??), who point out that the situation
is not unlike that of leaching for salinity control, where the
smallest total quantities of salt are returned to receiving bodies
whenever the salt is concentrated into as small a volume of water as
possible. The same trends have been repotted for nitrates (Devitt
et al, 1976; Pratt et: al, 1576). Whether one is concerned with the
concentrations or with the total quantity (mass emissions) of nitrate
leaving a given irrigated tract, largely depends on the ultimate fate
of the nitrogen, on the purposes for which the monitoring program
is being conducted, and on the extent to which single vs. multiple
sources of nitrogen for the receiving body need be considered.
IRRIGATED CROPLAND
According to Stanford (1973), corn grown under good management
commonly recovers 50 to 701 of the nitrogen applied, with the test
being largt, 1 v iimnobilized during residue decomposition, and hence
unavailable for nitrate leaching. He acknowledged that ouch nitro-
gen leaches or denitrifies at higher application rates, however.
Data such as those provided by Kelson and MacGregor (1973) and
MacGregor et al (1974) confirm such predictions, with little residual
soil nitrate accumulating in long-term corn plots until nitrogen
application, rates reached nearly 200 leg nitrogen/ha/vr, at which
rate residual soil nitrate levels increased on the order of three-
to five-fold. Similar results were reported by Schuraan et al (1975)
as long as nitrogen fertilization rates for corn tt-inaiitec! less than
110 kg nitrogen/ha/yr, though Liutwirk et 1 (. 19/M rc-jior {<_>.! residual
soil nitrate-nitrogen levels to isi.-frase lino-it-1 v with nitrogen
207
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fertilization rate at rates ranging from 70 to 270 kg nitrogen/ha/yr.
In the latter studies, it was concluded from data for 270 irrigated
farms in central and eastern Colorado that residual nitrate can accu-
mulate in irrigated profiles even if nitrogen has been applied at
rates less than required for optimal crop production,
Pratt et al (1972) monitored long-term research plots and comer-
cial citrus groves in southern California, and concluded that
nitrate-nitrogen concentrations in the unsaturated zone beneath
well-managed groves were being kept below acceptable levels if the
fields were managed for maximal crop production, and if a reasonably
high leaching fraction was being maintained for salinity control,
If nitrogen application rates were not excessive, and if the soils
were porous and had no water-impeding textural discontinuities,
nitrate-nitrogen concentrations in the unsaturated zone could he
predicted adequately from equation 2. Where impeding layers occurred
in the top 60 to 90 cm of soil at two sites, however» nitrogen losses
(most probably due: to denitrification) averaged 501 of nitrogen in-
puts.
For a nearby watershed from which about half of the applied water
emerged as drainage water, Bingham et al (1971) found nearly half of
the fertilizer nitrogen to be leached each year,
Results of nitrate leaching studies reported By Adriano et al
(1972a), Adrian© et a I (1972b), and Pratt and Adrian© (1973) are
suamiarized in Table 1, As annual nitrogen inputs were increased,
the most consistent change was in terms of the decreased percentage
of nitrogen reaching the crop, along with, consistent (though not
proportional) increases in nitrate-nitrogen levels of the unsaturated
zone "beneath the fields. An exception to the latter trend was for
citrus, where growers seemed to be managing their nitrogen more con-
sistently than for any of the other crop groups. Leaching losses
of nitrogen commonly averaged 25 to 501 of the nitrogen applied in
most cropping situation. In cases where large percentages of nitrogen
.208
-------
were being leached, grower inputs such as improved water and fer-
tilizer management, as well as possible changes in water applica-
tion method, could reduce losses of fertilizer nitrogen (and hence
nitrogen requirements) substantially.
Table. 1. Data for nitrogen inputs, nitrogen balance, and measured
nitrate-nitrogen concentrations in waters of the unsaturated
zone beneath some typical southern C'.-iiii'nrnia croplands
(recalculated from Pratt and Adriatic, 1971),
Crop
No,
Nitrogen
Nitrogen
litrogen
Nitrogen
Nitrate-
Sites
Inputs
in Crop
Leached
Losses
nitrogen
(kg/ha/
Concen-
yr)
%
tration
(nty/l)
Citrus
7
111-194
21-34
22-91
-17 to 56
21-70
Citrus
2
256-330
11-16
43-48
36-47
29-64
Citrus
1
414
10
36
54
47
Asparagus
2
130-144
62-69
19-29
9-12
1.2-14
Asparagus
2
478-492
27-31
23-28
45-47
29-90
Celery
1
385
32
58
9
35
Celery
1
1663
14
29
58
78
Misc. Row
Crops
2
210-215
70-71
40-102
-10 to -73
1 56-60
Misc.
Crops
5
360-530
15-44
13-71
1-55
36-123
Misc.
Crops
1
740
31
76
-6
85
Misc,
Crops
1
1525
25
60
15
i?n
In cases where substantial nitrogen losses (e.g., through denitrifica-
tion) were recorded, factors such as water-impeding layers within the
crop root zone may well have been, beyond grower control. Cases where
209
-------
substantial negative "losses" were recorded generally corresponded to
sites where nitrogen, was being mineralized from former manure appli-
cations or from residual soil nitrogen on the site. Aelrlano et al
(1572a) concluded that growers in the Santa Ana River Basin were us-
ing about 135 kg nitrogen/ha/yr above that removed by the harvested
crop at the time of these studies. In cases where no denitrification
losses occurred (e.g., where no restrictive layer impeded water move-
ment. in the. surface 2 meters of depth)» this would lead, to nitrate-
nitrogen levels in the unsaturated zone of 20 to 40 ppm far the 30 to
60 cm leaching volumes common to this area. Adriano et al (1972b)
concluded that crops consistently remove less than 50% of the nitrogen
applied throughout the southern California axea. For asparagus, they
reported 350 to 500 kg nitrogen/ha/yr being applied routinely to crops
needing only 200 to 250 kg nitrogen/ha/yr» and that levels as high as
1000 to 2000 kg/ha have accumulated in 15 meter profiles below the most :
heavily-fertilized fields. A 36 cm. leaching volume was reported as
common for this crop, for celery, it was reported as common to apply
450 to 500 kg nitrogen/ha/yr to crops requiring only 300 to 350 kg nitrogen/
ha/yr. Highest denitrification and immobilization losses In this case
(up to 681 of the nitrogen applied) were fram plots to which part of
the nitrogen had been supplied in the form of chicken manure. On the.
other end of the spectrum, Letey et al (1977) reported extremely low
losses of nitrate-nitrogen from alfalfa fields receiving virtually no
nitrogen fertilizer applications (with crop nitrogen needs being supplied
by nitrogen fixed from the atmosphere by this leguminous crop),
Relation of unsaturated-zone nitrate-nitrogen levels to the texture ¦
of the hydraulic control section of the overlying soil profile has been '
demonstrated by Lund et al (1974) and Devitt et al (1976). Lund et
al showed that 86% of the variability in nitrate-nitrogen levels of
the lower profile could he related to texture of the hydraulic control
section, with lower nitrate-nitrogen concentrations evident wherever
there was some accompanying restriction to water movement through the
210
-------
profile. Their results are illustrated in Figure 3, where lower-
profile nitrate-nitrogen levels are seen to be well correlated with
clay content of the hydraulic control section. In a similar vein,
Devitt et al (1976) found very low denitrification potentials in
coarse-textured profiles, with chloride/nitrate ratios indicating
appreciable denitrification only in cases where high-clay subsurface
layers occurred. Their only reported exception to a high-clay, low
nitrate-nitrogen trend occurred for one profile which had high clay
content, and also high initial residual nitrogen levels, throughout.
Similar relation of denitrification losses to soil profile clay contents
were reported by Letey et al (1577),
Many workers regard nitrate-nitrogen which has leached beyond the
top 2 meters of soil as beyond the zone of significant denitrification,
although Meek et al (1970) suggested substantial reductions in nitrate-
nitrogen levels of drain-tile effluents if the tile lines could be
submerged in order to promote denitri: h-
-------
20
15
10
Y=I5.2-37X
R2 *.68
F=2760**
5 10 15 20 25
CLAY CONTENT OF CONTROL SECTION (%)
30
Figure 3. Relationship between average
(1,8 to 8 ib soil depths) and
control section of some soil
subsoil nitrate concentrations
average clay content of the
profiles (fram Lund et al» 1974).
212
-------
Table 2, Nitrate-nitrogen concentrations
various irrigation systems.
Experiment Soil Type Depth
(cm)
Ind—of-Season Values
Irrigation, rate study Sand 0-120
120+
Furrow irrigation Silt 0-60
loam 60+
Within-Season Values
Sprinkler irrigation Silt 0-60
loam 60+
Grower's field Silt 0-60
center-pivot loam 60+
Grower's field Sand 0-60
center-pivot 60+
Grower's field Sandy 0-60
alternate-furrow loan 60+
Grower's field Silt 0-60
solid-set sprinkler loam 60+
Ln and beneath potato root zones in the Columbia Basin for
Nitrate-nitrogen Concentration
(rag/1)
Sprinicler Rate
lead of Field
Low late
Biett Rate
38
13
30
6
Furrow Rate
Low High
26 2
12 3
Tail of Field
High late
Low Rate
104
93
44
31
EarJy Season
Late Season
Low Rate
436
68
612
71.
193
52
461
48
250 kg N/ha 520 kg N/ha
17? 204
56 68
110 kg 1/faa 430 kg N/ha
10? 248
10 22
155 367
95 121
Low Rate
88
102
100 kg 1/faa
121
39
48
61
53
7
43
69
High late
24
61
178
68
520 kg N/ha
154
127
110 kg N/ha 430 kg M/ha
130
11
101
111
-------
tlve drainage volumes of sprinkler-irrigated and furrow-irrigated
fields. There would also be considerably more low-nitrate water
being leached under furrow irrigation, as water passed through
previously-leached soil immediately beneath the irrigation furrow.
One of the factors accounting for high nitrate leaching potentials
for potato fields is the fact that the potato is a crop having
atypically-high water requirement. If the amount of water available
to potato roots decreases appreciably, older leaves will seo.esce and
the amount of photosynthetic surface will be decreased. Hence, high
soil water contents must: be maintained if one is to maximize crop
production potential. In addition, potatoes commonly are grown on
rather sandy soils, so the combination of high water application rates
and low soil water-holding capacities leads to considerable potential
for leaching. In addition, the potato does not seem to be able to
utilize soil-solution nitrogen nearly as efficiently as does a crop
such as wheat. In the wheat-growing region of eastern Washington, for
example*, it is common to find virtually no nitrate-nitrogen remaining
in the surface 180 cm of soil by the end of the growing season, if
sufficient water has been present to realize maximal crop growth
potential. Under controlled growth conditions, Warneke and Barber
(1974) reported that sorghum, soybeans and broute-grass reduced root-
zone nitrate-nitrogen levels to the range 0.02- to 0,04 ng/1. In a
we 1.1-managed potato field, on the other hand, it is common to find 20
to 40 mg/1 of nitrate-nitrogen remaining in the soil solution at the
end of the growing season (McNeal and Carlile, 1.976) , Kirkham et al
(1974) reported a situation where 50 mg/1 nitrate nitrogen was insuffi-
cient for continued potato growth on a rather sanely soil. Hence, crop
utilization efficiency must be considered when evaluating pollution
potential or ability of selected management practices to reduce
nitrate-nitrogen losses from croplands. In cases where crops are to
he grown which are inefficient nitrogen utilizers, it may be necessary
to put almost complete stress on proper water management, and on sub-
214
-------
sequent growth of deep-rooted crops to "scavenge" residual nitrate-
nitrogen from the soil profile (Mathers et al, 1975), if pollution
from nitrate leaching is to be minimized.
As is evident from Table 3, the concentrations of dissolved
nitrogen in the root zone vary markedly from early season to late
season. Hence, the passage of excess water through the soil profile
in early season can be particularly hazardous, as in cases where the
water application rate is improperly adjusted to plant growth needs
or where additional water is applied for wind-erosion control on
sandy soils. It should also be noted from the table that concentra-
tions of nitrate-nitrogen beneath the crop root, zone in a furrow-
irrigated, sandy loam soil were quite low throughout the growing
season. This is probably because nitrate was continually being leached
by the excessive amounts of water passing through the soil beneath the
furrows under these conditions. The alternate-furrow irrigation approach,
as used at this location and as commonly employed throughout the Columbia
Basin, tends to "trap" nitrogen in the crop root zone, for the nitrogen
Is moved back and forth between, furrows, rather than being leached from
soil beneath the furrows, during irrigation.. A coio.parl.soii of the
alternate-furrow data of Table 3 for the sandy loam site to that for
the silt loam site shows that soil texture has a narked effect on the
amounts of nitrogen persisting in the crop root zone and in the under-
lying subsoil. In the latter"case, because of smaller amounts of water
moving downward during irrigation, the amounts of nitrogen remaining
even in the soil beneath the cxop root zone were substantial through-
out the growing season.
A common problem, associated, with tho growth of potatoes in the
Columbia Basia is the fact that there appear to be excessive amounts
of nitrogen recommended for growth of the potato crop. In some cases,
nitrogen recommendations were developed originally for lands which
had been only recently brought into product ion. Such recosmendatioiis
have been maintained, even though potato yields commonly decrease on
215
-------
Table 3, Nitrate-nitrogen concentrations within and beneath potato root zones in the Columbia Basin for
various fertilizer treatments (all values in msr/1).
Experiment
Soil Type
Depth
(cm)
Nitrate-nitrogen Concentration
(nig/1)
End-of-Season Vaiues
Broadcast
Banded
110 kg l/faa
670 kg N/ha
110 kg N/ha
670 kg N/ha
Suspension Fertilizers
Silt
0-60
37
89
38
54
(1972)
loam
60+
21
43
23
42
Following-Season Values
Spring
of 1972
Fall of
1972
110 kg I/'faa
670 kg N/ha
110 kg N/ha
670 kg N/ha
Suspension Fertilizers
Silt
0-60
80
174
15
27
(1971 Expt. Sampled in 1372)
loam
60+
32
105
39
67
Within-Season Values
Early
Season
Late Season
Traditional
Slow 1
Traditional
Slow N
Grower's center-pivot
Sand
0-60
469
352
150
226
(Slow-release I)
60+
83
51
105
64
Grower's alternate-furrow
Sandy
0-60
2 56
128
95
94
(Slow-release I)
loam
60+
18
9
14
12
Grower's solid-set sprinkler
Silt
0-60
296
162
85
146
(Slow-release 1)
loam
60+
84
50
71
57
Alternate-furrow
Silt
0-60
354
265
236
154
(Slow-release 1)
loam
60+
31
23
100
32
340 kg N/ha
670 kg N/ha
340 kg N/ha
670 kg N/ha
"Nitrogation"
Silt
0-60
102
33
76
89
(Sprinkler-applied N)
loam
60+
71
17
40
22
-------
recropped potato lands because of plant disease problems. Because
of the decreased crop yields on recropped lands, and also because of
the substantial amounts of nitrogen released from potato crop resi-
dues during the following season on such lands, the amounts of nitro-
gen required are considerably less under recropped conditions.
Information has also been obtained from the Columbia Basin
studies suggesting that the concentrations of nitrogen recommended
to be in plant tissues throughout the growing season may In some cases
be excessive. It might be desirable to retest some of the plant '
tissue needs against crop yields periodically, or whenever fertilizer
management changes markedly. With respect to nitrogen, for example,
the traditional approach to nitrogen management was to put on
virtually all of the required nitrogen at the start of the growing
season, so that high tissue nitrogen levels were common in early
season, with low levels being attained by mid- to late-season, as the
nitrogen supply became exhausted. Recommended tissue nitrogen levels
were largely developed from this particular approach to nitrogen
fertilizer management. In recent years, however, more and more of
the nitrogen has "been applied in the irrigation waters during the
growing season, so it may no longer he necessary to have the high
early-season nitrogen levels which were once thought necessary.
Attempts to 'maintain such, levels may be contributing somewhat to
an over-supply of nitrogen to the potato crop in this area, and
hence to a nitrate leaching problem.
Unfortunately, the determination of actual crop nitrogen needs
under constantly-changing growing conditions is difficult for most
growers and even for many farm management services. These people,
faced with the need to make management decisions throughout the
growing season, prefer to keep the crop both heavily watered and
heavily fertilized, in an attempt to maximize economic production.
In relatively few cases are adequate field plot data available as
new management techniques become popular for a given area. These
217
-------
field plots are essential if one is to demonstrate' whether the
amounts of nitrogen that were required formerly will still be needed
under the new set of conditions or If new criteria are needed.
Some management alternatives for preventing deep percolation
losses of excess nitrogen are listed in Table 4. Possible con-
straints to the adoption of each alternative are also listed In the
table.
Table 4» Some management alternatives for preventing leaching losses
of excess nitrogen.
Management Alternatives
1. Use recommended nitrogen fertili-
zation rates.
Possible Constraints
May be leaching wore nitro-
gen than normal, or having
substantial denitrification.
Soil solution nitrate levels
may not be reduced in pro-
portion to decreased ferti-
lizer usage.
Avoid heavy early-season irriga-
tions., unless ammonium nitrogen is
present or split nitrogen appli-
cations are used.
Increased wind erosion, isore
expense if several light
irrigations are substituted.
3. Avoid single large nitrogen appli-
cations, while using more split
nitrogen applications, or apply-
ing more nitrogen via the irriga-
tion water,
4. Use slow-release nitrogen sources
May be more expensive for
multiple nitrogen applica-
tions, or for metered nitro-
gen sources,
increased fertilizer costs.
218
-------
Table 4. Continued
Management Alternatives
Possible Constraints
or nitrification inhibitors.
Properly sample and allow for
residual, crop residue* and mineral-
izable soil nitrogen,
Increased sampling and analysis
costs, uncertainty of nitrogen
release rates.
Convert from furrow- to sprinkler-
irrigation systems.
Increased capital outlay, in-
creased energy demands.
7. Use light, frequent irrigations
to keep nitrogen in the crop root
zone.
Increased labor costs, loss of
grower freedom, for other oper-
ations, less reserve in case
of marked changes in crop water
demand.
8. Irrigate at a rate approximating
crop evapotranspirative needs.
More rigorous demands on the
irrigator, insufficient water
where sprinkler patterns are
wind-distorted or near the tail
of furrow-irrigated fields.
9. Strive for increased water-applica-
tion uniformity, and adequate
levels of other nutrients, in order
to maximize crop yields and crop
nitrogen uptake.
Increased irrigation-system
costs, or increased demands on
the irrigator.
10. Minimize the amount of excess
Increased costs of improved
? 19
-------
Table 4. Continued
Management Alternatives Possible Constraints
water applied, consistent: with water management, less
requirements for salinity uniform crop appearance
control. and yields because of lion-
uniformities in water
application.
11. Promote development of deeper
crop root zones, or periodically
plant more deeply-rooted crops
in the rotation.
12. Avoid excessive nitrogen leaching
at the end of the cropping season,
or plant winter cover crops which
will use water and nitrogen.
13. Permit less thorough aquatic weed
control in return-flow ditches.
14. Use more alternate-furrow irriga-
tion in gravity-irrigated areas.
15. Refine crop nitrogen recommenda-
tions as changes occur in crop
management practices or average
crop yields for the area,
16. More carefully monitor and regu-
late amounts of water applied.
Sufficiently deep-rooted crops
not always av-i f I -ib I , and de-
creased flexibility or de-
creased economic returns may
result from the crop rotation.
Increased costs of additional
monitoring or irrigation manage-
ment , iriereasi'ct difficulty of
planting in cover-crop residues.
Increased water usage for the
system.
Slightly increased irrigation
management costs.
Necessary correlative work is
both time-consuming and costly.
Both time consuming and labor-
intensive .
220
-------
Table 4. Continued
Management Alternatives
Possible Constraints
17. Try to grow crops that are rela- Limited crop selection, possible
If high nitrate concentrations arise or persist?
1, Limit domestic use of such waters.
2, Provide alternate water supplies for susceptible persons.
3, Adequate publicity arid/or warnings in more seriously-
affected areas,
4, Dilute or mix high-nitrate waters with low-nitrate waters,
5, Use adequate sampling programs when trying to isolate
particularly-severe nitrate sources.
Animal and municipal waste disposal sites often offer almost
singular potential for nitrate leaching, because of the high nitrogen
loading rates which are commonly employed. The amounts of nitrogen
contained in wastes, though variable with storage and pretreatment}
is generally appreciable, so that annual waste loadings at a particu-
lar site are often baaed upon the amount of nitrogen that will be
utilized by the crop growing on the site, plus the amount of nitrogen
that can be salestimated to be denitrified at the site during the
waste disposal season. Fortunately» the amounts of denitrification
at waste disposal sites are generally substantial, because of the
soluble carbon loadings and the rapid oxygen depletion that is common
as the active microbial population at the site oxidizes organic com-
ponents from the wastes. A combination of high soluble carbon and
nitrate loadings, high microbial activity, and high water contents or
rapid oxygen depletion due to microbial activity present near-optimal
tively efficient utilizers of
nitrogen, whenever possible.
economic losses when using more
nitrogen-efficient crops.
IRRIGATED WASTE DISPOSAL SITES
221
-------
conditions for the denitrifi.cat.ion process.
For example, Kimble et al (1972) found more nitrate leached from
ammonium nitrate fertilizer than from dairy manure of approximately
equivalent nitrogen content, and King and Morris (1972) found that
more than 50% of the nitrogen applied as sewage sludge stayed in a
surface crust of sludge, and much of the remainder was denitrifled,
so that little nitrate pollution arose even from sludge application
rates of 1000-2000 kg nitrogen/ha/yr. At the same site, nitrate
pollution from commercial fertilizers began at 360-490 kg nitrogen./
ha/yr, Sird I'irly, Smith (1976) determined that only about 10 kg
nitrogen/ha/vr entered a water table at the 150-cm soil depth beneath
plots to which over 1000 kg nitrogen/ha/yr had been applied as food
processing wastes, in a setting where .360 kg nitrogen/ha/yr was removed
by a grass crop. In each of the last two cases, denitrification losses
were 2 to 4 times crop removal losses under well-managed waste dis-
posal conditions.
Meek, et al (1974) must have achieved some type of a record with
the application of 180-360 tons of manure/ha to Holtville clay under
irrigated conditions without appreciable increase in subsoil nitrate-
nitrogen concentrations at the 140-cm soil depth, although soluble
carbon moved to at least 80 2.111 in one set of plots, A more realistic
manure loading rate was probably that of Mathers and Stewart (1974),
who found that feedlot manure could be applied to corn at rates of up
to 22 tons/ha (equivalent to 300 kg nitrogen/ha/yr) without appreci-
able. nitrate-nitrogen leaching, although manure loading rates of 112
to 240 tons/ha/yr gave nitrate-nitrogen levels of over 1000 kg/ha to
the 180-ctn soil depth. Pratt ft 1 I ( l'>/M found appreciable nitrate-
nitrogen leaching at 21 tons/ha/yr for a liquid manure averaging 4,51
nitrogen, and at 40 tons/ha/yr for a dry manure averaging 1.6%
nitrogen. Mathers et al (1975) also demonstrated the effectiveness
of alfalfa in removing water and «irr.i t«»- nitrogen from manure-contami-
nated plots to a depth of 3,6 meters in 2 seasons, at a seasonal
222
-------
nitrate-nitrogen removal rate of 300 kg nitrogen/ha. Adriano et al
(1971) demonstrated nitrate-nitrogen flux to the groundwater which was
approximately 3 to 3.5 times higher for corral sites than for pasture
or cropland, but they also provided perspective with the statement
that the ratio of cropland to corral areas in their study locale was
nearly 13 to 1,
From the above evidence we can conclude that it is not unusual
for den.itrificat.ion from waste disposal sites to reach levels of 50
to 75% of the applied nitrogen, It should be kept in tnind that the
denitrification potential of a site may not be fully realized, however,
if the site is maintained relatively dry (hence decreasing both
microbial activity and the tendency toward anaerobic conditions), is
relatively cold (because of decreased microbial activity), or is used
for wastes which contain sources of soluble carbon that ate only
slowly utilized as an energy source by soil micro-organisms. Mineral-
ization of nitrogen from wastes applied in former years must also be
considered in deciding on proper waste loading rates for a given site,
For example, Pratt et al (1976) found 45% and /'jf of t ho nitrogen in
a dry dairy manure and a liquid feed lot manure, respectively, to be
available the first year after application, with decay functions
approximating 10-151 release in the second year after appliest ton and
52 in the third and fourth years. In some respects the manager of a waste
disposal site may be regarded as a "conductor", as he attempts to
orchestrate conditious at the site which will lead to as much denitri-
fication as possible, so that, the amounts of nitrate leaching from the
site will be minimized.
PREDICTION OF NITRATE LEACHING LOSSES
N-fmte 1 earhlng loss values from apvptiI of the southern Califor-
nia '.fii'-li.-K h.«vo been described by the ompif it.a I <*quat fori
I, - 0,20 m (3)
L i
223
-------
where N is nitrogen leached (kg/ha/yr), is nitrogen inputs
(kg/ha/yr), and D is drainage water volume (surface cm/ha/yr).
Although using a different descriptive equation, Letey et al (1977)
also found the total quantity of nitrogen leached from tile-drained
fields to be well correlated with total water discharged and with
fertilizer nitrogen application rates. As Is evident from Figure 4,
the relationship adequately described a wide range of data for
cropped fields and for -11131111x6 field plots In the Santa Aria River Basin
and in the coastal and interior valleys of California, The correla-
tion coefficient relating predicted and measured leaching losses was
0.95, with a correlation coefficient of 1.0 indicating perfect
correlation between sets of values. Because of the marked effect of
the largest numbers on a regression such as this, however, the
correlation coefficient would have been considerably lower had the
data been restricted to the 0-200 or 0-300 kg nitrogen/ha range.
Pratt et al (1977) have subsequently shown that predictions of
nitrate leaching can be Improved significantly by Incorporating the
saturated hydraulic conductivity of undisturbed cores of surface
soli into the predictive equation. Such Incorporation accounts for
variations in denitrification as drainage becomes impeded at higher
soil clay contents. -Their best predictive equation had the fonn
IT » 22.3 + 0,195 Mill + 0.0337D2 {4)
L
where H is saturated soil hydraulic conductivity (on/hr) and all
other terms are as described previously. Because of the strong
positive Influence! of soil hydraulic conductivity on nitrate leaching
from croplands, Pratt has even suggested that surface soil hydraulic?
conductivity might be lowered in some cases to promote* denitrif Ication
and decrease nitrate leaching losses,
When the approach of equation 3 was applied to data fro® furrow-
irrigated portions of the Pacific Northwest, the equation appeared to
significantly over-estimate the amounts of nitrogen being leached.
224
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Y • 0 97 X
r = 0.353
SANTA ANA RIVER BASIN
MANURE FIELD PLOTS
HANFORD
RAMONA
DOMINO
COASTAL. AND
INTERIOR VALLEYS
0 100 200 300 400 500 600 TOO 800 900 1000
ESTIMATED NlTRATE-N LEACHED, kg/ho/yr
Figure 4, Relationship 'between measured and estimated nitrate
leaching, The estimated values are from the equation
N - 0.20(NiD)°-712.
Li
225
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This comparison is provided in Table 5.
Table 5. Field-testing of nitrate leaching predictions for tw z
Northwest irrigated tracts.
Irrigation Measured Predicted Predicted/
Tract, Workers Losses Losses* Measured
—kg nitrogen/ha/yr—-
Wapato, Nelson 31.8 55.2 3,0
Washington and
Wea¥er (1971)
Twin falls, Carter et 33.0 70,6 2.1
Idaho
* Based on equation 3, using crop acreage and average leaching
volume figures provided by the authors, and crop fertilisation
rates conforming to current fertilize! for the i«* »«>*<. t~
ive areas. Carter' » !".ure of subsurf.-i. < i•turn-flows u»~
Ing 501 of water diverted, and Twin Fails tract estimates of
301 cumulative project-wide and em-farm conveyance losses
(13, L. Carter, personal, coiiiiiuiiication), which would constitute
part of the subsurface return flows, used for both tracts.
As Is evident fro® the table, the amount of nitrogen predicted to
be lost frost these irrigated areas through use of equation 3 exceeded
actual losses by a factor of "2- to 3-fold. Although part of the
discrepancy could be due to enhanced di-n S i * J f i< n ton under Pacific*
Northwest conditions, a niore prob.thh r t-,)son for this discrepancy
is the fact that furrow-irrigated fields have considerably larger
amounts of water passing rather harmlessly through the root zone
once the soil beneath the irrigated furrow has been, leached. In
other words, incorporation of the entire volume of drainage water
weights the values too heavily in relation to the amounts oI
which would actually be leached per unit of water transmitted through
the soil.
226
-------
Attempts are being made at present to improve the leaching pre-
diction by Incorporating coefficients of uniformity for various
types of irrigation systems. Hence, furr©w-irrigated systems, with
their lower coefficients of uniformity (in cither words with a greater
amount of lieterogenity of water application), should be expected to
have lower leaching estimates from the equation than sprinkler-
irrigated fields, with considerably higher uniformity coefficients,
it should be kept in mind in this regard that extreme inhomogenities
in water movement have been reported even for sprinkler-irrigated
California fields, and have been verified under Columbia Basin conditions
(McNeal and Carllle, 1976), Hence the variations for furrow-irrigated
fields are truly enormous.
227
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LITERATURE CITED
Adriano, D. C., P. F. Pratt, and S. E. Bishop, 1971. Nitrate and
salt in soils and ground waters from land disposal <<(" dtit ,
manure. Soil Sci. Soc, Araer. Proc. 35:759-762,
Adriano, D. C,„ P. F. Pratt, and P. B. Takatori. 1972a. Iit.rate in
unsaturated zone of an alluvial soil In relation to fertilizer
nitrogen rate and Irrigation level. J. Env. Qual. 1:418-422.
Adriano, D. C., F. B. Takatori, P. F. Pratt, and 0. A. Lorenz. 1372b,
Soil nitrogen balan.ce in selected row-crop sites in southern
California, J. Env. Qual. 1:279-283.
Bingham, P. T., S. Davis, and E. Shade. 1971. later relations* salt
balance, and nitrate leaching losses of a 960-acre citrus water-
shed. Soil Sci. 112:410-418.'
Carter, D. L.» J. A, Bondurant, and C. W, Bobbins. 1971. Water-soluble
MO -nitrogen, PO,-phosphorus, and total salt balances on a large
irrigation tract. Soil Sci. Soc. Airier, Proc. 35:331-335.
Devitt, Dale, J. Letey, L. J. Lund, and J. W. Blair. 1976. Nitrate-
nitrogen movement through soil as affected by soil profile
characteristics, J. Inv. Qual. 5:283-288.
Espinoza, W., 1. G. Gast, and 1. S. Adams, Jr. 1975. Charge character-
istics and nitrate retention, by two andepts from south-central
Chile. Soil Sci. Soc. Amer. Proc. 39:842-846.
Kimble, J. M.» R. J. Bartlett, J. L. Mcintosh, and 1. E. Varney. J972.
Fate of nitrate from manure and inorganic nitrogen in a clay
soil cropped to continuous corn. J. Env. Qual. 1:413-415.
King, L. D. and 1, D. Morris. JVJ'J. L:ind disposal of liquid sewage
sludge. III. The effect on soil nitrate. J. inv. Qual. 1:442-
446.
Kinjo, T. and P. F. Pratt. 1971. Nitrate adsorption. 1. In some
acid soils of Mexico and South America. 11, In comp*i iiion with
chloride, sulfate, and phosphate. Soil Sci, Soc. Amer. Proc. 35:
722-728.
Kirkham, M. B., D. 1, Keeney, and W. R. Gardner. 1974. Uptake of
water and labelled nitrate at different depths in the root zone
of potato plants grown on a sandy soil. Agro-Ecosystem Is31-44.
228
-------
Letey, J,, J. W. Blair, D. Devitt, L, J. Lund, and P» Nash, 1977,
Nitrate-nitrogen, in effluent from agricultural tile drains
in California. Hilgardia 45:289-313.
Ludwick, A, I., J, 0, Reuss, and E, J, Langin. 1976. Soil nitrates
following four years continuous corn and as surveyed in Irrigated
farm fields of central and eastern Colorado. J, Env. Qua!, 5:
82-86.
Lund, L, J., D. C. Adria.no, and P. P. Pratt. 1974. Nitrate concentra-
tions in deep soil cores as related to soil profile character-
istics. J. Env. Qual. 3:78-82.
MacGregor, J. M., G. R. Blake, and S. D. Ivans. 1974. Mineral nitrogen
movement into .-subsoils following continued annual fertilization
for corn. Soil Sci, Soc. Amer. Proc. 38:110-113.
Mathers, A. C. and B. A. Stewart, 1974, Corn silage yield and soil
chemical properties as affected by cattle feedlot manure, J,
Env, Qual. 3:143-147.
Mathers, A, C., B. A. Stewart, and Betty Blair. 1975. Nitrate-nitrogen
removal from soil, profiles by alfalfa. J. Env, Qual. 4:403-405,
McNeal, B, L. and. B, L. Carlile. 19/6, Nitrogen and irrigation manage-
ment to reduce return-flow pollution in the Columbia. Basin. Final
report to the Environmental Protection Agency. Environmental
Protection Technology Series, EPA-600/2-76-158. 141 pp.
Meek, B. D,» L. B. Grass* L. S. Willardson, and A. J» Mackenzie. |<>/0.
Nitrate transformations in a column with a controlled water
table. Soil Sci, Soc. Amer. Proc, 34:235-239.
Meek, B, D., A. J, Mackenzie, I. J. Donovan, and W. F. Spencer. 1974.
The effect of large applications of manure on movement of nitrate
and carbon in an irrigated desert soil. J. Env. Qual. 3:253-258.
Nelson, C. E. and W. H. leaver, 1971. Salt 'balance for the Wapato
Project for 1970-71 compared with the salt balance for 1941-42,
Washington Agric. Expt, Sta, Bull. 74',!.
Nelson, W. W. and J. M MacGregor. 1973. Twelve years of continuous
corn fertilization with ammonium nitrate or urea nitrogen. Soil
Sci. Soc, Amer, Proc. 37:583-586,
Pratt, P, F, and D. 0, Adri.ino. Nitrate concentrations in the
it m '.."i t urated zone ben<-aili irrl>»uUd fields in southern California.
So i I Sci. Soc, Amer. Prix*, 17 ; 1.'I-322.
229
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Pratt, P. F,, 3, Davis, and R. G» Sharpless* 1976. A four-year
field trial with animal manures. I, Nitrogen balances and
yields?, II. Mineralization of nitrogen. Hilgardia 44:99-125,
Pratt, P. P., W. W. Jones, and ¥, E. Hunsaker. 1972, filtrate in
deep soil profiles in relation to fertilizer rates and leaching
volume. J, Env. Qual. 1:97-102.
Pratt, P. P.» J. M. lible, and L. J. Lund. 1977. Leaching of nitrate
from freely-drained fields. Annual Report to National Science
Foundation-Nitrates in Effluents from Irrigated Lands, Univer-
sity of California, pp. 12-21,
Rolston, D. E, and M. A. Marino, 1976, Simultaneous transport of
nitrate arid gaseous devitrification products in soil. Soil Sci.
Soc. Amer. J. 40:860-865,
Sefaiiman, G. E,» T. M. MeCalla, 1. E, Sax ton, and H. T. Knox. 1975,
Nitrate movement and its distribution in the soil profile of
differentially fertilized corn watersheds. Soil Sci, Soc, Amer.
Proc. 39:1132-1197.
Smith, J» H. 1978. Treatment of potato processing waste %,,ta on
agricultural land. J, Env. Qual, 5:113-116,
Smith, S. J, and R, J, Davis. 1974. Relative movement of bromide
and nil. .*» through soils. J, Env. Qual, 3; 152-155,
Stanford, George, 1973. Rationale for optimum nitrogen fertiliza-
tion in corn production, J. Env. Qual, 2:159-166.
Thomas, G. W. and A. R. Swoboda. 19/0. Anion exclusion effects on
chloride movement in soils. Soil Sci. 110:163-166,
van de Pol, R. M., P. J» Wierenga, arid D, 1, Nielsen, 1977, Solute
movement in a field soil. Soil Sci. Soc, Amer. J. 41:10-13.
Warncke, D. D. and S. A, Barber, 1974. Nitrate uptat'o .iii-it iveness
of four plant species. J, Env. Qual, 3:28-30.
Wetselaar, R. 1962. Nitrate distribution in tropical soils,
Downward movement and accumui if ion of nitrate in the subsoil.
Plant Soil 16:19-31.
230
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EFFECT OF WATER MANAGEMENT ON NITRATE LEACHING
J. Letey, J. V. Biggar, L, H. Stolzy and I. S. Ayers—'
ABSTRACT
Nitrates which are leached (transported below the root zone)
represent a resource loss and a potential contribution to water
pollution. The amounts of leached nitrates for a given time period
were determined at various coantereial fanning sites in California
and in a carefully controlled experimental plot receiving various
water and fertilizer application treatments. Some of the agricul-
tural sites had tile drainage systems and others had "free drainage"
to the groundwater. Linear regression analyses were conducted on
the data. Similar results were observed for the tile and free
drainage systems. The highest correlation coefficient was achieved
for the amount of leached nitrates versus the product of the drainage
volume and fertilizer nitrogen application. The nest highest correla-
tion coefficient was for amount leached "versus drainage volume followed
by amount leached versus fertilizer nitrogen application, In isost
cases there was no significant correlation between nitrate concen-
tration in the water below the root zone and drainage volume or
fertilizer nitrogen application, A significant linear relationship
between, amount of leached nitrate and drainage vo1ume was also ob-
tained at the experimental plot.
If
— Letey and Stoliy are m ih<* Department uf Soil ar.d Enviun>m»nt.i'.
.Sciences, University of t';< I i forniu, R1 vi-rntde, <*/f 1 i 1 nnu<-; I .
Biggar and Ayers are in the bepar uf Land, air ,-md Water
Resources*'University of C,i ! i f oro ki , I'wais, C.al I n»r >i1 a "«i 6 »
231
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INTRODUCTION
There are two concerns with nitrate leaching (the term "nitrate
leaching" will be used to represent the process of transportation of
nitrates below the root zone). First, nitrate leaching represents a
resource loss. Nitrogen is an essential element for plant growth,
and crop product ion is usually lncrea.se.cl by nitrogen addition to the
root environment. Leaching of nitrogen beyond the root zone, there-
fore, represents the waste of an Important resource. Secondly,
nitrate can be considered, a water pollutant under certain circum-
stances, Nitrogen in water can contribute to eutrophlcatlon and
stimulate -vigorous biological activity when the activity is undesir-
able. High nitrate concentration in water ts .1 Ho considered to he
a health hazard when given to infants. Maters that leach nitrates
may become surface waters through natural or artificial drainage
systems or move to the ground water which will be later puinped to
the surface for various uses.
Minimizing nitrate leaching is a worthy goal because it: conserves
a valuable resource and minimizes the hazard of water pollution.
The purpose of this report is to evaluate the relationships between
water management in irrigated agricultural systems arid nitrate
leaching.
The term "water management" encompasses many facets. It can
include consideration of timing and amount of water application, It
also includes consideration of application methods such as furrow,
sprinkler, border or drip irrigation. Each of these water iianagesierit
features conceivably affects nitrate leaching. The paper by Mcleal
and Pratt in this volume considered some aspects of this subject. This
report will be restricted to presenting data obtained by the Univer-
sity of California on a project supported by the Mil Dive, it hi of the
National Science Foundation and the University of California Kearney
Foundation of Soil Science, We do not presently have data which
232
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would allow us to evaluate the effects of irrigation application tech-
niques such as furrow or sprinkler sv-ftms on nitrate leaching. In-
deed, the present data is restricted to considering the amount of
water application or, more specifically, the amount of water leached
beyond the root zone.
It is necessary to have a quantitative expression of nitrate
leaching to report research findings. Two parameters may be used.
The nitrate-nitrogen concentration in the water at a depth below the
root zone may be used as one index of nitrate-nitrogen leaching, A
second parameter is the quantity of nitrate-nitrogen which has moved
beyond the root zone in a given time period. The latter parameter
is typically referred to as mass emission. The question immediately
arises as to which is the preferable parameter to properly characterize
nitrate leaching.
Mass emission is clearly the more significant parameter when
resource loss is of prime consideration. An increase in mass emission
represents an increased loss of nitrogen resource as a fertilizer
element. The appropriate parameter is not so obvious in considering
environmental pollution, Biological systems usually respond to con-
centrations of nitrate-nitrogen. For example, the present U.S. Public
Health standard is 10 ppm nitrate-nitrogen, in drinking waters. Thus,
nitrate-nitrogen concentration is of Importance from an environmental
point of view. On the other hand, when a potential pollutant is dis-
charged Into the environment, its negative effect depends on the
assimilative capacity of the environment for that constituent. The
environment is negatively impacted when the amount discharged exceeds
the assimilative capacity. In this respect, mass emission is a
better parameter than concentration, Clearly increasing the mass
emissions increases the probability that the assimilative capacity
of the environment will be exceeded,
A very important point to recognize is that mass emission and
concentration are not generally proportional in agricultural systems.
233
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Indeed, it is not unusual for mass emission and concentration to "be
inversely related. This is in contrast to the case for water dis-
charge from-municipalities or industries. For municipalities or
industries, concentration is directly proportional to mass emission
because the volume of water to be discharged is relatively constant.
Decreasing the concentration, of a constituent in. a. fixed volume of
water by 50% results in a 50% decrease in mass emission. Thus,
regardless of whether concentration, or mass emission is the better
criteria for controlling pollution of the environment., the same con-
clusions are drawn for the industries or municipalities. We will
report most of our nitrate leaching both in terms of mass emission
and concentration.
EXPERIMENTAL APPROACH
Three research activites are reviewed. One is referred to as the
"free drainage" investigations which were, carried out under the
direction of Drs, P. P, 'Pratt and J. Rible. Another research activ-
ity is referred to as "tile drainage", studies carried out under the
direction of Dr. Latey, The third activity is referred to as the
"Davis plots". Only a small part of the research carried out on ex-
perimental plots located near the Davis campus will be given in this
report, The data presented in this report were provided by Drs. J, W.
Biggar, D. 1, Nielsen, and S» Simmons.
The "free drainage" studies involved drilling on Specifically
chosen agricultural sites and analysing the soil removed at various
depths. The depth of drilling was 50 or more feet except in cases
where Mechanical difficulties with sand or rocks prevented further
digging or a saturated zone was reached.
The concentration values reported, are averages from all samples
collected from the 15-foot depth to the maximum depth of sampling at
a given site. In order to calculate mass emission a value for the
234
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amount of water moving beyond the root zone in a given time is also
required. The following procedure was used to estimate the drainage
volume. The leaching fraction was estimated from the ratio of the
chloride concentration in the irrigation water to the average chloride
concentration in the soil water below the root zone. This procedure
assumes that the only chloride input was that in the irrigation water
and that no significant amount of chloride was removed by the crop.
Thus the increase in chloride concentration, observed in the soil water
below the root zone was a result of water lost from evapotranspiration.
Multiplication of the leaching fraction by the amount of water applied
produced a value for the drainage volume. The calculated drainage
volume was then multiplied by the nitrate-nitrogen concentration in
the water beyond the root zone to estimate mass emission.
Tile drainage effluents were collected from commercial farming
operations at various areas in California where tile drainage systems
are common. file effluent samples were usually collected weekly and
analyzed for nitrate-nitrogen concentration. The tile flow rate was
measured when possible by collecting water In a bucket for a given
period of time and measuring the collected volume. Tile flow rate
measurements were not possible at all sites because of inaccessibility
of the tile line, extremely large flow rates, or both. Mass emission
of nitrate-nitrogen was calculated for systems in which both concen-
tration and effluent discharge rate were measured. The measured con-
centration and discharge rate at a given date were assumed to have
remained constant for a period half-way to the previous and following
monitoring dates. The calculated amounts for each monitoring period
were then totaled to cover one year.
Data on I c-r t i I i/,er and wa 1o r appl f rat ions wet t* rai lifted front growers
on sites itumitotPcj ut the frre dralna^*; and lilt* drainage investiRations,
The research conduct **4 at the fJnviM pitits enni r.istK the free drain-
age and tit o Urn inap.e luvest lg;it Icjiik, In the- Jat tor two rases, rot*a»ure-
stents were made on co«;um*re lal operat ions under a variety <>f condit ions.
lMri
-------
There were no experimentally controlled management variables. On the
other hand, the Davis plot Investigation represents carefully managed,
highly instrumented and replicated experimental plots. Three irri-
gation treatments were Imposed during the growing season of a corn
crop. These, were application of water to replace 1/3, 3/3, and 5/3 of
the water lost through evapotranspiration. These three levels of
irrigation corresponded to approximately 20, 60, and 100 surface
centimeters of water during the growing season. In addition to the
irrigation treatments, there were four fertilizer treatments of 0,
90, 180, and 360 kg nitrogen/ha applied at the time of seeding.
Estimates of nitrogen and water movement past the 300—cm soil depth
during the 1974 and 1973 growing seasons are reported here, (The
experiment and the analysis of the data have utilized more sophisticated
methods of data collection and evaluation, of mean values than most
field experiments, particularly where rate processes are involved.
Spatial variations of both the water flux and nitrate have been con-
sidered, A detailed description and analysis of the data will appear
in other publications),
RESULTS AND DISCUSSION
As previously stated, the only data we presently have available
on water management is the amount of water leached beyond the root
zone for a given period of time. Therefore, we will be analyzing the
relationship between nitrate-nitrogen leached and the drainage
volume. Inasmuch as the nitrate available for leaching is expected
to be related to fertilizer nitrogen application, we will also evaluate
the relationship between nitrate leaching m«d fertilizer nitrogen in-
put . Both mass emission and concentration of the leachate will be
used as parameters for nitrate leaching, Thi* following symbols will
be used hereafter: C represents the nitrate-nitrogen concentration
in iag/1; M is mass emission in kg/ha/yr; W represents the drainage
236
-------
volume under free drainage systems and the effluent volume in
tile drainage systems and is expressed as ha cm/ha; ant! i is the
fertilizer nitrogen applied in kg/ha/vr.
The amount of nitrate-nitrogen leached is plotted as a function
of drainage volume in Figure 1 for sites studied under the free drain-
age investigation, Linear regression, analysis on those data results
in the equation M = 11.7 + 3.05 W with r = .766*** (*, **, and ***
will be used to denote statistical significance at the 5, 1, and
0,1% levels, respectively). The data plotted in Figure 2 show that
there is no significant relationship between, the nitrate-nitrogen con-
centration and drainage volume. The resultant equations and correla-
tion coefficients obtained from linear regression analysis between
various factors (for free drainage sites) are presented in Table 1.
Mass emission had correlation coefficients significant at the 0.1%
level, when related to drainage volume, fertilizer input, or the product
of the two variables. (Mote that the highest correlation coefficient
was achieved for mass emissions versus the product of drainage volume
and fertilizer nitrogen application).
Table 1, Equations and correlation coefficients for nitrate-nitrogen
leached and various factors for sites analyzed in the free
drainage investigation.
Equation
Correlation, coefficient
M - 11.7 + 3,05 W
M - 13.0 + 0.469 N
M - 54,5 + 0.0067 Iftf
C « 36,5 - .162 W
C - 24.9 + .0242 i
,766***
.683 ***
.794 ***
-.220 IS
.216 MS
79.4 ***
26.3 ***
51.1 ***
1.5 MS
1.4 IS
There was no significant correlation between the leacliarte concentra-
tion and either drainage volume or fertilizer input.
J A /
-------
500
= 117+5 05*
r = 0.766
400
or
Q 300
uj
-1 200
• •
100
• •
• •
0
20
60
80
40
100
120
DRAIN AG f„ VOLUME, crr.ha/ho/yr
Figure 1. Corral at Ion between the nitrate-nltr^'P:! leached and
dr.-iIvolume for 58 sites from t and interior
valiti>& ijf California,
238
-------
70
E
CL 60
50
40 ¦
30 •
• •
UJ 20
— 10
i I
t f
10 20 SO 40 50 SO TO 80 90
DRAINAGE VOLUME hocm/ha/yr
Figure 2. K<- f.'tt amFh I f> bcfcw.'it rofuv-itLr-'tt" t'»n <»f n 11 rati -nit cnyy'
fn t!n» dri i rripc. vutfC and J raiuHw vo I umo lor free
d r.-i f ruiK<' i> 11 c-h .
-------
The amount of nitrate-nitrogen discharged during the given year
from various tile drain systems is plotted in Figure 3 as a function
of the amount of water discharged. Sites located on the Oxalis and
Kettleman soil series on the west side of the San Joaquin Valley are
suspected of having considerable amounts of native nitrogen in the
soil profile. Data from those sites are identified in Figure 3 by open
circles. Mote that in general a higher mass emission of nitrate-
nitrogen occurred for a given tile effluent volume for those systems
as compared to other sites analyzed. Those sites which had alfalfa
as a crop and no fertilizer input for several years are also separately-
identified in Figure 3 with open triangles. Note that as expected
there was less mass emission per unit of tile effluent volume as com-
pared to systems receiving fertilizer input, A linear regression
analysis shown in Figure 3 is for all sites exclusive of the sites
on the west side of the San Joaquin Valley arid those having alfalfa
as a crop. Note that there was an increase in, mass emission with
increased tile effluent volume.
The equations and correlation coefficients obtained from linear
regression analysis for data accumulated in the tile drainage investi-
gation are presented in Table 2, The statistical analyses were con-
ducted using data from ail sites and also for data accumulated from
sites exclusive of those suspected of having high native nitrogen or
very low nitrogen because of the alfalfa crop.
The highest correlation coefficient was obtained, between mass
emission and the product of effluent volume and fertilizer nitrogen
application. Correlation, coefficients significant at the 0.1% level
were also obtained between mass amission and tile effluent volume for
both sets of data. There was a reasonably high correlation coefficient
between mass esissioti and fertilizer nitrogen application particularly
when the sites suspected of having high native nitrogen and/or zero
nitrogen input were excluded from the analysis. There was either no
significant correlation or poor correlation between tile effluent
240
-------
400
a*
-* 300
yj
M = -4.52 +2.66 W
f-55.0*"r-,888"
00
«
20
40 60 80 (00 120
TILE EFFLUENT AMOUNT ho cm/ha
160
140
80
Figure 3« Relations!:ip between mass eroissioft *«! nitrnle*nitregen
and ef fi uenl vjOohm.' fox- tilt* >lov rcprestat s>iu*:» t h.it were
cropped wiHi 1}.1!.fa ii no nitregera, fertilizer-
241
-------
Table 2. Equations and correlation coefficients for nitrate-
nitrogen leached and various factors for sites analyzed
in. the tile drainage investigation.
Equation
Correlation coefficient
Sites exclusive of alfalfa and high native nitrogen
M = -4.52 + 2,66 W
M = -48.9 + 3.82 N
M = 16.4 + .0042 NW
C = 7.92 + 0.36 N
C = 17.6 + .119 W
.886 ***
,715 **
.920 ***
.532 *
.290 IS
55,0 ***
18.8 **
82.9 ***
5,9 *
1,4 IS
All sites
M = 7.72 + 2,76 W
M = 66,1 + .4 N
C « 44.9 - .182 W
C = 29.4 - .0007 N
.840 ***
.46 *
.14 NS
,006 MS
47.8 ***
5.2 *
0.4 MS
.001 NS
242
-------
concentration and fertilizer input 01 lile «"'t fluent volume.
Significantly, results from the free drainage and tile drainage
investigations produced similar types of results. In both cases,
there was essentially no correlation between nitrate-nitrogen concen-
tration and fertilizer input or drainage volume. The best correlation
was found between mass emission and the product of fertilizer input
and the drainage volume. The second best correlation was between
mass emission and drainage volume and this was followed in significance
by the relationship between mass emission and fertilizer input.
Clearly, both fertilizer input arid drainage volume significantly
affected the amount of nitrate leached below the root zone, A. com-
bination of both factors provided the highest correlation coefficient.
Considering the factors independently» the amount of nitrate leached
was more closely related to the amount of drainage than to the
fertilizer input.
Me previously discussed whether mass emission or concentration
is the more appropriate parameter to represent nitrate leaching.
We concluded that mass emission is the better parameter because it
clearly represents the amount of a resource lost ant! serves as an
indicator on the magnitude of the nitrate assault on the environment.
Our data suggest that mass emissions can be reduced by reducing the
drainage volume, fertilizer input, or both. Theoretically, at least,
mass emissions can be managed. On the other hand, nitrate-nitrogen
concentrations were not correlated with management variables. It
must be recognized, however, that the correlations were carried out
between two variables with all other variables being uncontrolled.
For example, one would theoretically expect a higher concentration
in the leachate with increased fertilizer nitrogen input if all other
variables were constant. The lack of correlation between concen-
tration and nitrogen input *,Impfy iHf.ms that in the systems investi-
gated other variables are acting on the system to remove the
significant correlation. One would also expect lower nitrate-nitrogen
243
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concentrations with increased drainage volumes simply because of
the dilution factor, The fact that this was not observed again
indicates that there were other interacting variables in the system.
Mater management resulting in high leaching volumes removes
considerable nitrogen fro® the root zone, A grower may learn from
experience that 'he needs to apply higher amounts of nitrogen (to
compensate, for the leached nitrogen) under hie management system
to have oaxlatum crop production. Inasmuch as growers are not
generally aware of the amount of their drainage volume or nitrogen
leaching, their fertilizer practices would have been developed by
experience. Our data allowed us to determine if there wag a signifi-
cant relationship between the drainage volume and fertilizer appli-
cation, A linear regression analysis resulted in the equation
M » 78.8 + -4.07 W, with r =* 0.618 *** for the free drainage sites
and the equation N « 275 + 2,85 W, with r » 0.524 * for the tile
drainage sites. Therefore, we observed that fertilizer nitrogen
application, in general, is Indeed higher under those systems with
higher drainage volumes. These results clearly demonstrate that
fertilizer management and water management are inter-linked and one
cannot be evaluated separately fro®, the other.
Me will now consider results fro* the Davis plots, These data
were collected under carefully controlled conditions with detailed
analytical analysis, The experimental approach on the Davis plots
is ia extreme contrast to the free drainage and tile drainage Investi-
gations. Gross analysis on systems with many uncontrolled and un-
measured variables were carried out on the latter two systems whereas
sa.fficie.nt precision to allow analysis of spatial variability in a
"generally recognized uniform soil" was carried out in the former.
Even though a given -set of plots was irrigated the same, the cal-
culated amount- of-water passing the 300-ca layer during the growing
period was analyzed on different plots.. For plots which, were con-
sidered to be uniform, .receiving the saise treatment,, each had unique
244
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drainage volumes and mass emissions, A. linear regression analysis was
conducted between mass emission and drainage volume from data collected
in each plot receiving a given fertilizer treatment, The linear
regression curves with their equations and correlation coefficients
are illustrated in Figure 4, Note that r is significant to at least
the II level for ail fertilizer treatments. The highest value of r
resulted from plots receiving 360 kg/ha of nitrogen, An intermediate
value of r occurred for the 180 kg/ha fertilizer application and the
lowest r values were obtained for the 0 and 90 fertilizer treatments.
The slopes of the curves also appear to differ for different fertilizer
treatments. The result of a statistical comparison of these slopes is
presented in Table 3.
Table 3, The F values from the statistical comparison of the slopes
of mass emission versus drainage volume curves aad adjusted
means for different fertilizer treatments at Davis plots.
Fertilizer application F f
comparison slope Adjusted Mean
0 vs 90 .36 IS 2.72 IS
0 vs 180 1.51 IS 8,41 **
0 vs 360 18.71 *** 16.10 ***
90 vs 180 3.81 KS 2,91 NS
90 vs 360 25.06 *** 11,02 **
180 vs 360 10.70 ** 5,4? *
A comparison between the adjusted means for different fertilizer treat-
ments is also presented in fable 3, 'Hie 360 treatment is significantly
different froai all other trea t titen t, b^th with rfujuu-t l?> slope and
adjusted nw»an. The nlopt>* <>L the uther Cert 1J i scf-r I rf-atur.nf s are not
significantly dif fc-reat from ?acb «>rh*»r, The ad r*otn between 0
and 90 and also 90 .ind j80 arc- not signiftrantiy dil fer.»nt but the
:?4C»
-------
200
1380)
M =-6.78 ~ 2.81 W
o
a
x,
o»
JC
Q
yj
x
Q
<
UJ
X (180)
M' 19.5* I.38W
r a ,748***
-i too
z
UJ
£
£t
I—
z
M> 7.9? + 0,92 W
r = .597**
80
70
60
50
40
30
DRAINAGE VOLUME ha cm/ha
Figure 4» Relationships between nitrate-nitrogen leached and drainage
volume for four different rates of fertilizer nitruf,i-n
application. The 0, 90, 180, and 380 numbers in p.irMillieses
refer to rates of nitrogen in kg/ha/yr.
246
-------
adjusted means between. 0 and 180 are significantly different at the
1% level. In making these comparisons, it should be recognized
that the inherent fertility of all plots was high at the beginning
of the experiment and that differences between fertilizer treatments
would be expected to be magnified in successive years.
The results from the Davis plots provide significant evidence
that mass emission is related to drainage volume. As might be
expected, the effect of increasing drainage volume on mass emissions
was higher at the highest fertilizer input.
Data, from these three investigations suggest that mass emission
is more strongly related to drainage volume than to fertilizer
application. Indeed, we can note from Figure 4 that there was
relatively little difference in the amount of fertilizer leached
for the different application rates when the drainage volume was
low. It was only at the high drainage volumes that the. effects of
different fertilizer treatments became evident. Conceivably, high
fertilizer applications can be made without excessive pollution
potential if the leaching volume is kept very low. On the other
hand, if the drainage volume is high, considerable nitrate leaching
can occur, even with low fertilizer application. Note in Figure 3
that increased drainage volume produced increased mass emission,
even under systems receiving no nitrogen fertilizer in. which alfalfa
was the crop. Alfalfa is recognized as being a good nitrogen
scavenger,
A comparison, between aass emission and drainage volume for the
free drainage, tile drainage, and the 360 kg/ha fertilizer treatment
at Davis plots is made in Figure 5, The slopes of all 3 curves are
very similar. Indeed, the tile drainage and the Davis plot curves
are almost identical. These results are probably coincidental,.bat
nevertheless they could lead to Interesting sj>rcul-itions.
In conclusionj the results of all three studies clearly indicate
that nitrate leaching is significantly affected by water management.
24?
-------
200
FREE / / /
DRAINAGE-y DAVIS //
// PLOTS//'
// <360jZ *%-0RAINAGE:
Q
LU
I
o
<
LU
_j 100
z
txi
<
C£
!_
2
80
TO
60
40
50
30
10
20
0
DRAINAGE VOLUME ha cm/ha
Figure 5. Rrl ationship between nu.v, emission and drainage volume
for free drainage and tile drainage sites and for the
160 kg/ha nitrogen rate on the Davis plots.
248
-------
Management which results in very low drainage volumes contribute
relatively low mass emissions of nitrate. On the other hand, water
management resulting in high leaching 'volumes will cause considerable
leaching of nitrates. Water management appears to have very little
effect on the concentration of nitrate-nitrogen beyond the root zone,
presumably because fertilizer management has been, adjusted to compen-
sate for the higher leaching,
A strategy for reducing nitrate-nitrogen leaching would be to
reduce the drainage volume. This alternative, however, may require
investment in irrigation systems which allow more precJ.se control of
water application. Furthermore, added instrunientatlor may be required
to guide irrigation practices to produce minimum leaching, In some
cases, there may be a trade-off between lew nitrate-nitrogen leaching
with increased capital and energy investment,
M5
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250
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MONITORING WATER FOR NITROGEN LOSSES FROM CROPLANDS
Kenneth K. Tanji—'
ABSTRACT
This paper reviews monitoring for nitrogen in surface and sub-
surface return flows from irrigated lands.
Presented are the elements of a monitoring program: objectives
of monitoring, parameters to be measured, sampling programs that
include site selection, sampling frequency and sampling nethod,
laboratory methods for nitrogen determinations, requirements for
resources and facilities, evaluations of collected data and other
supporting information and data, and dissemination of monitored
results,
Each of these elements are appraised and a conclusion is drawn
on monitoring waters for nitrogen losses from croplands.
•^Professor of Water Science, Department of Land, Air-and Mater
Resources, University of California, Davis, California 95616,
251
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INTRODUCTION
This paper focuses on the questions of why, what and how to
monitor nitrogen emissions in surface and subsurface? return Flows
from Irrigated agriculture. How much nitrogen Is likely to be leached
from soils, lost to the atmosphere in gaseous forms, and taken up by
cropi plants are addressed, by other papers in this volume, This report
compliments other conference papers dealing with sources and forms of
nitrogen In soils and waters, as well as basin-wide field investiga-
tions and modeling.
It should be mentioned that the State of California is implement-
ing a statewide water quality surveillance and monitoring program as
mandated by the California Porter-Cologne Water Quality Control Act;
of 1970 and to satisfy the: requirements of the Federal Water Pollution
Control Act Amendments of 1972 (PL 92-500). For Che National, Pollutant
Discharge Elimination System (UPDES), permittees from irrigated agri-
culture are required to report the water flow, electrical conduct-
ivity, and suspended solids in their supply and discharge waters.
The State Mater Resources Control Board (SHRCI) does not have a format
statewide monitoring requirement for nitrogen emissions fro® croplands.
However, where nitrogen is a problem, it is being monitored in the:
receiving water and return flows by federal, state, or local entitles.
It is not certain what impact(s), if any, the recent federal Water
Pollution Control Act Amendments of 1978 (PL 95-21?) will have on
irrigated agriculture other than that collected irrigation return,
flows are now reclassified as non-point sources,
Recognizing that nitrogen in return flows from irrigated agri-
culture is monitored only in localized problem areas in California,
an attempt is made herein to describe the elements of a possible
monitoring program. Presented are some details on the objectives of
monitoring, identification of what needs to be measured and how to
measure them, and what to do with this monitored data.
252
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ELEMENTS OF A MONITORING PROGRAM
A monitoring program should include the following elements:
1. Objectives of monitoring,
2. What parameters to measure.
3. How and where to sa the system.
4. Which laboratory analytical method to use.
5. "Requirements for resources and facilities.
6. Evaluation of monitored data,
?. Dissemination of monitored results to information users.
In the following sections, each of the above elements will be
discussed relative to monitoring nitrogen emissions from croplands in
surface and subsurface return flows,
OBJECTIVES OF MONITORING
The aim of monitoring nitrogen losses from croplands. Ideally,
is to satisfy a number of informatIon and data needs. The objectives
may Include:
1. Obtain, baseline data to define existing nitrogen status.
2. Detect developing problems at the earliest stages.
3. Evaluate nitrogen fertilizer arid water management
practices,
4. Appraise land disposal of nitrogen-containing wastewaters
and wastes.
5. Evaluate potential reuse of return flow waters and/or
Impacts on the beneficial uses of revolving waters.
6. Acquire Input data as well as test t1.itn for models
that examine cause and effect relations, and management
alternatives and ant i< 1 fMt od changes.
7. Provide guidelines <<>r y.th'i qiwtftv management plans,
i.e., 208 Planning.
-------
As noted above, the purposes of monitoring could be diverse
and there is some question on whether any one selected monitoring
system would be able to satisfy all or most of these objectives,
IDENTIFICATION OF MEASUREMENT PARAMETERS
Nitrogen
Thf various forms of dissolved and particulate (suspended) nitro-
gen that, raay be: present in surface and subsurface irrigation return
flows are:
1. Dissolved nitrogen gases (dinitrogen, ammonia, etc.)
2. Soluble inorganic nitrogen (aowoniu®, nitrate and nitrite)
3. Soluble organic nitrogen (amino acids, sugars, etc.)
h. Particulate organic nitrogen (suspend eel matter consist-
ing of plant and animal origins).
5. Sorted inorganic nitrogen (exchangeable and fixed
ammonium in sediments, etc.).
The discharge of the above forms of nitrogen varies with site-
specific conditions. Items 1-5 may he present in surface runoffs and
items 2-3 in percolating and subsurface drainage waters. The nitrogen
species that are most frequently Measured in supply and return flows
are nitrate or nitrate plus nitrite, less frequently ammonium and
ammonia, and infrequently dissolved and suspended organic nitrogen.
Aii}isonioia» and ammonia in high pB waters, should be »ore frequently
analyzed because it is an indicator of pollution, and of inefficient
use of ammonia/ammonium fertilizers, and is highly toxic to aquatic
organisms. Nitrate is of importance because it plays a role In
eotrophication of surface waters, in high concentrations may cause
methemoglobinemia, may he indicative of excessive leaching, and may
be detrimental to certain crops during the maturation stage. Signifi-
cant concentrations of nitrite nay occur only under unusual anoxic
{reduced) conditions. Organic nitrogen present as nitrogenous fraction
254
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of biochemical oxygen demand (BOD), when BOD determinations are
made for more than 5 days of incubation, is also indicative of pollu-
tion, and may cause a lowering in dissolved oxygen (DO) In the receiv-
ing waters as well as:? contributing mineralized forms of nitrogen to
waters,
In surface runoff from close-growing crops, such as pasture and
flooded rice fields, organic nitrogen usually dominates ewer the
mineralized forms while in widely-spaced crops, such as furrow irri-
gated field crops, ammonium and nitrate may be more prominent than
organic nitrogen.
Other _S_igni_f leant Parameters
Along with the measurement of nitrogen species in surface and
subsurface return flows, water flow should be estimated so that both
the concentration and mass emission (concentration times volune) of
nitrogen can be considered.
Since the concentration and mass of nitrogen in return flows
are affected by site-specific conditions and local management prac-
tices, it is necessary to obtain additional information and data to
more fully evaluate nitrogen losses from croplands. The following
are some of the principal factors that influence the quantity and
quality of irrigation return flows:
Quantity - availability and cost of supply water, irrigation
application methods and attainable efficiencies, special
cultural practices, extent of water reuse at the on-farm,
district and basin levels, constraints on water reuse due
to the presence of excess salts, boron, sodium or chloride,
and the response or lag-tine in water flows.
Quality - supply water, presence of nitrogen native to the soils,
nitrogen fertilizer practices and management, amount of
iiittmH-u t,1 fup by crops, discharges into irrigation
drains oy otner sectors of society and land uses, and
255
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disposal of animal wastes and wastewaters in croplands.
SAMPLING PROGRAM
How and where to sample for nitrogen in surface1 and subsurface
return flows from croplands are difficult to answer. Ideally, the
sampling should represent the system being monitored In both time and
space. It begs the questions of sampling site selection, sampling
method, and sampling frequency.
Sit8 Selection
The sain consideration for where to sample is the identification of
the principal water flow pathways and related physical and operational
characteristics of the system being monitored. In the case of irri-
gated croplands, the main components of return flows are surface run-
off (irrigation tailwater, precipitation runoff, operational spills
from distr iliutic s), collected subsurface? drainage (effluents
from tile drainage? systems and drainage wells, subsurface , ium-i-
cepted by natural, and man-made open channels), and percolating drain-
age water. With some exceptions, the surface runoffs and collected
subsurface drainage waters are not handled separately in the drainage
network, and the conkined flows from these two major components are
referred » , .t<. .<.(!>. ted surface irrigation return flows.
At the systems level (for instance, irrigation district) of
monitoring, sampling stations are usually located at the outermost
boundaries where surface return flows from the given system discharge
into a contiguous drainage area or receiving water body, in addition,
for such large tracts of croplands, it is d*»« i«> »">i i-r < , At Ut<* 1 sn ,s,i si tS.-M U-yi-1 ..t
monitoring, it %.,t / '•>«' '!«• .f »/iMf to t> 11 ~, ih«» v.ir t»»n .
components of . ut tace <1 r:» i n «}',<* .tml t mn»f f wyrr-fj.
256
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Those components of return flows that are collected and/or
controlled are more amendable for estimating water flows and sampling
for nitrogen than those that are uncollected (diffuse) and/or un-
controlled, In some instances, however, estlasting surface water
flows can become quite complex when the direction of flow of water
changes. For uncollected, uncontrolled flows such as water percolat-
ing beyond the crop root zone, it is extremely difficult to measure
them directly, and perhaps should be obtained by difference.
Other criteria for site selection may be monitoring objective,
ease of accessibility to the sampling site and available resources.
Sampling Frequency
The frequency of sampling return flows should be dictated by the
system behavior and the nature of the data, characteristics represent-
ing this behavior. A grab sample gives instantaneous characteristics
for a given time and a given place, A composite sample over a finite
time interval gives the average characteristic for that time interval,
Sequential and continuous samplings usually offer a better opportunity
of obtaining pait ^rns of fluctuations and trends.
When large changes in water flow or nitrogen concentrations are
expected over a short tine period, the frequency of sampling should be
increased, and, conversely, when these changes are small. the fre-
quency of sampling may be lengthened. At any rate, it is desirable
to have both time-integrated measurements over « iinfd point in
space as well as space-integrated measurements over a fixed time.
Sampling Method
Although continuous water sampling and water flow measurements
may be desirable, this is frequently not possible for all the com-
ponents of return flows because of costs and/or lack of suitable
sampling and gaging efuipaent.
Continuous surface water flow oeasureaieiits can be obtained by a
257
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variety of methods, but most frequently it is measured with a stilling
well and waterstage recorder for open channels and a volumetric water
meter for a pipeline. Instantaneous sun » . rater flows are »•••! i\,;.rad
by measuring head on a weir or by a curr. eloclty meter If .ire
no structures. For tile drainage effluents discharged by gr-e 11 ,•
into an open channel, instantaneous flow estimates may be obtained by
the trajectory method or the bucket and stop watch method. Tile
drainage effluents collected in a tile sump arid pumped Into an ooen
drain are more amendable to flow measurements by knowing the < ».» trical
charges and pump efficiency. This method of flow estimate is also
applicable to drainage wells. As mentioned previously, direct measure-
ments of flow of percolating waters are usually not possible, at least
on a routine basis.
As for sampling water for nitrogen, automated portable sequential
and composite samplers are available. A composite saio " -ollects
a single; water sample by setting a fixed time interval o<-t-roen sampling
while a sequential sampler collects separate water sampi.- iccordlng to
a preset time interval and sample volume. Some automated samplers are
equipped with flow meters so that: they can operate on intervals of
flow poises. Since nitrogen species are subject to biological trans-
formations provisions for chilling with ice or refrigeration must be
made during the collection, transportation, and other handling stages
up to the point of chemical analysis.
Water samples are also obtained by taking periodic grab samples.
If particulate forms of nitrogen are being analyzed, the usual precautions
and techniques In sampling that apply to sediment sampling should be
followed, especially in fast-flowing open channels. Water may be also
obtained from observation (piezometric) wells to sample the water
table and fro® wells to obtain the "pumped" groundwater.
Nitrogen in the soil is generally sampled in two ways. One method
samples the soil profile with hand- or machine-operated soil augers
followed by measurement of the water-soluble nitrogen in the soil at
258
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some reference laboratory soil water content. The other method in-
volves placement of a porous ceramic cup (suction probe) in the soil
profile and extraction by vacuum. Soil solutions, however, can be
obtained only up to a vacuum of about 20 inches of mercury, i.e.,
only when the soil is moist.
LABORATORY ANALYTICAL METHODS
Approved analytical methods for various nitrogen species are
found in a number of references such as Standard Methods for the
fix am ination of Water and Wastewaters (APIA, AWA, WPCF), American
Society for Testing and Materials (ASTM), Methods for Chemical
Analyses of Waters and Wastes (EPA), Methods for Collection and
Analysis of Water Samples for Dissolved Minerals and Gases (USGS),
and Methods of Soil Analysis (ASA).
Although other analytical methods and procedures not approved by
the organizations above are available, their accuracy and precision
may be of some concern. Even with the use of approved methods, some
quality control should be exercised because of potential variabilities
in the use of differing chemical reagents and instruments» performance
and capabilities of the analysts and sample presentation, preparation
and handling.
RESOURCES AID FACILITIES
The extensive and intensive aspects of the monitoring program is
influenced oiiongly by available resources and facilities, which
inelocl.es wages and salaries, transportation and travel expenses,
analytical costs, purchases and maintenance of equipment and
machinery, etc. Trade-offs are frequently made between the
scientific-engineering requirements for monitoring and what is
economically feasible. The surveillance type of monitoring is
259
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generally less costly than those which attempt to identify and
control the sources of nitrogen discharged from croplands.
The bottom line, for the frequency of sampling, number of
sampling sites, and the methods of sampling are the associated costs,
In spite of all the desirable sampling features and requirements that
were pointed out, Monitoring nitrogen in return flows usually reduces
to a limited number of sampling stations, a limited sampling frequency,
and a limited number of nitrogen species analyzed.
DATA EVALUATION
The roost difficult and perhaps the most important task in the
monitoring program is adequate evaluation of monitored information
and data.
The monitored data is usually characterized by tilninram and isaxi-
Bjuin values as well as arithmetic means. The arithmetic wean concen-
tration of nitrogen in waters should preferably be the flow-weighted
average rather than a simple average of the concentrations obtained.
In addition, seasonal trends such as Irrigation versus nonirrigation
seasons and other data fluctuations are reported in tabular or
graphical forms as a function of tliae and/or space (distance).
Since errors may be introduced in the sampling program as well
as the analytical determination for nitrogen* the monitored data should
be subjected to further statistical analyses such as standard deviation
and standard error. The latter error can be controlled to acme extent
'by quality control but the former is more difficult to appraise and
control. The question of whether the sanpllng program is a valid
sampling of the population is of concern here.
Some parameters exhibit normal (Gaussian) distribution while
others aay be log-normal^ bl-modal, or have so«e other distribution
characteristics, 'If the population is not normally distributed, the
statistical analyses become more complex and sampling must he mm
2-60
-------
extensive, Currently, the spatial variability of the leaching charac-
teristics of field soils is receiving much attention where spatial
variability is pronounced, substantial errors can be made in point-type
measurements if the mass of nitrogen is taken as the product of average
nitrogen concentration and average water flow. To minimise this kind
of error, the number of samplings over 'both time arid space must be
significantly increased. In other space-Integrated or tints-integrated
types of measurements* this problem nay be minimized. For instance, a
tile sump collecting tile drainage effluents from a field usually
would yield better measurements for the concentrations of nitrogen
percolating to the water table than the use of suction probes which
is a point sampling method-
Besides the nitrogen and water data obtained in the sampling pro-
gran, other supporting information arid data base are required to more
fully appraise the results of the monitoring program. These support-
ing information and data are the previously mentioned factors in-
fluencing the quantity and quality of irrigation return flows. Such
information and data would increase the utility of the monitored data
and may help in ascertaining cause and effect relationships.
DISSEMINATION OF 'MONITORED RESULTS
Too often the Information and data collected .are used solely by
the party responsible for the monitoring program. Although many do
publish water quality and water flow data or submit such data into
data storage and retrieval banks like the STGRET System, this does
not folly satisfy the objectives of monitoring nitrogen in irrigation
return, flows. Some exceptions are the detailed reports that contain
not only the monitored data hut also supporting Information and data
mentioned previously along with sone Interpretation.
261
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CONCLUSIONS
Monitoring for nitrogen in surface and subsurface return flow
waters from croplands Is easier said than done. Although the goals
of monitoring nitrogen emissions may be definitive, the design,
implementation, operation and data evaluation of present day
monitoring efforts generally fail far short: of these goals. As
Professor I). R. Nielsen of the University of California at Davis has
stated: "We are still learning how to monitor the out-of-doors".
That is not to say that we should cease to monitor for nitrogen from
croplands, but to be cognizant of the deficiencies and strive to
make some improvements.
From a water quality and pollution control point of view, as
well as from a point of view of agricultural, production, this author
suggests that emphasis be placed upon, developing and applying best
management practices along with some limited effluent monitoring
dictated by the degree of actual or potential problem.
ACKNOWLEDGEMENT
This work was supported in part by NSF Grant No. AEN 74-11136 A0.1,
EPA Grant No. 1 803603, and University of California Experiment Station
Project Ho. Ca-D*-WSE-3098-H.
262
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SELECTED REFERENCES
Biggar, J. W. and D. R. Nielsen. 1976. Spatial variability of the
leaching characteristics of a field soil. Water Resources
Research 12(1):78-84.
California Department of Water Resources. 1971, Nutrients from Tile
Drainage Systems. Bio-engineering Aspects of Agricultural.
Drainage, San Joaquin Valley, California. EPA 1.3030 ELY 5/71-3.
90 p.
California State Water Resources Control Board. 1975. Program for
Water Quality Surveillance and Monitoring in California. 138 p.
California Department of Water Resources. 1977. San Joaquin Valley
Drainage Monitoring Program. Summary Report:1975. 47 p.
National Academy of Sciences and National Academy of Engineering. 1973,
Water Quality Criteria 1972. EPA-R3-73-033. 594 p.
The National Research Council. 1977. Analytical Studies for the U.S.
Environmental Protection Agency. Vol. IV. Environmental
Monitoring. 181 p.
Tanji, K. K. and J. W. Biggar, et al. 1977. Irrigation Tailwater
Management. 1976-77 Annual Report to U.S. EPA on EPA Grant No.
RS03603-02. University of California Water Science and Engineer-
ing Paper No. 4014. 245 p.
Tanji, K. K., M. M. Iqbal, and A. F. Quek, et al. 1977. Surface
irrigation return flows vary. California Agriculture 31(5):30-31.
U.S. Geological Survey. 197 7. National Handbook of Recommended
Methods for Water-Data Acquisition, Chapter 5. Chemical and
Physical Quality of Water and Sediment,
Viets, F. G., Jr. and R. H. Hageman. 1971. Factors Affecting the
Accumulation of Nitrate in Soil, Water, and Plants. USDA
Agricultural Handbook No. 413. 63 p.
263
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264
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ESTIMATING THE INFLUENCE OF SOIL RESIDENCE TIME ON
EFFLUENT WATER QUALITY
W. A. Jury--^
ABSTRACT
Models are proposed to calculate the time required for dissolved
chemicals to move from the soil surface to either underlying ground-
water in the case of free drainage or to tile drain outlets in the
case of artificial drainage. The models assume that dissolved sub-
stances are transported primarily by moving soil solution, which dis-
places soil water initially present in the wetted pore space (piston
flow approximation). For free drainage,, this results in a single
travel time equation which is a function of soil water content and
drainage volume, for tile drainage, the travel time depends also on
the surface entry point, which is illustrated with a graph showing
residence time as a function of place of origin. This graph nay be
used, for all tile drain systems.
Calculations ate presented to show the Influence of travel time
on drainage concentrations, illustrating also how tile drain concen-
trations are a mixture of contributions from different parts of the
field arriving at different: times.
Field studies of salt movement are analyzed and compared to the
model predictions, with rt»ff« rences explained on the basis of soil
variability or stagnant water in part of the wetted pore space.
of \ t -Hid 1.11 v I rt.niMt ni tl sciences, tr n I .t iL' > ' 1,
265
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INTRODUCTION
Concern has risen In recent years over the contribution of agri-
culture to rising nitrate concentrations in streams and groundwater
{Ayers and Branson, 1973; Bingham et al, 1971). As a result, a
number of investigators have been measuring nitrate concent rat ions
in drainage water below crop root zones and at tile drain outlets.
As numerous researchers have pointed out, however, nitrate concentra-
tions below the root zone alone will not give a good estimate of the
pollution potential of a field, but; must be coupled with an estimate
of the drainage volume in order to calculate the amount of nitrate
moving downward (tetej et al» 1977). For artificially drained systems,
the amount of nitrate leaving a field may be calculated from a simul-
taneous measurement of concentration and tile discharge, but: for free
drainage systems water flow below the root zone is more; difficult to
measure.
to important characteristic of nitrate movement in soil is the
time required to reach the groundwater or tile drain fro® the soil
surface. Knowledge of this travel time is Important for determining
the impact: of a change in surface practices on groundwater concentra-
tions and for identifying the source of a pollutant detected in the
soil,
Determination of travel time is difficult In the field for a
number of reasons. In free drainage, water flow rates bellow the root
zone are hard to measure or even to estimate, and vary from point to
point: under the field (Biggar and Nielsen, 1976), In tile drained
systems„ the water flow path to the drain depends on where the water
first entered the soil, so that chemicals introduced at the surface
and moving with the soil water will require different: lengths of
time to reach the drain. As a result, the tile outlet concentration
is a complicated space and time average of soil water concent < >« ,< ..3
within the
266
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This paper will discuss ways of calculating and measuring travel
time for nitrates; moving downward to the groundwater (free drainage)
or moving to a drain tile or ditch (artificial drainage)» Research
studies on this topic will be reviewed and summarized. Problems en-
countered due to variability in soil physical characteristics in the
field will be discussed.
REVIEW OF RELATED RESEARCH
Free DraInage
In the absence of nitrate reduction by denitrification or plant
uptake, nitrate should move through the soil in a manner similar to
the chloride ion. Conclusions about ion transport based on chloride
studies, therefore, should be applicable to nitrate also.
Warrick et al (1371) studied the movement of chloride through a
clay loam during infiltration and found that a chloride pulse injected
with water at. the soil surface moved somewhat faster than predicted
by a model which assumed that the infiltrating solution moved through
all the wetted soil pore space. They also observed that the chloride
pulse spread out rapidly due to dispersion while moving through the
top 150 era. This finding was similar to that of Miller et al (19f3.5)
who concluded that ponding water on the soil surface was le...» < f Ki t~
ive in moving salt, from the root zone than the sane volume of water
applied intermittently because the leaching solution tended to move
preferentially through, the largest ltd pore space,
Endelman .died leaching of solid fertilizer nitrate
and chloride in Plainfield sand under dally water applications of 2,5
cm. Each addition caused nitrate and. chloride movement of .15 to 20
cm/day, slightly less than would, be predicted from a perfect displace-
ment (piston flow) model, Shuford et al (1977), in a similar experi-
ment in silt loam, found that a nitrate and chloride pulse applied
in solution mowed faster through the soil than piston flow would
26?
-------
predict. However, when this model was applied to an identical experi-
ment on a Hawaiian soil, nitrate movement was overpredicted by a
factor of three (Balasubramanian et al, 1973).
Other workers have observed that water moved preferentially
through cracks and channels in. the soil, shortening the travel time for
surface applied chemicals and decreasing leaching efficiency for chemi-
eals in soil solution (Kissel et al» 197 5; Tyler and Thomas, 1977).
Movement of nitrate varies significantly with water input at the
soil surface. Saffigna et al (197?) found that nitrate had moved 400
cm to the water table within 11 months after application to an irrigated
potato field. A lysimeter recorded 80 em drainage past the 150-cm
depth during this time. However, Somraerfeldt et al (1965) found, that
nitrate applied to dryland grass had moved only 180 cm in 8 years where,
the; annual average precipitation was 41 cm,
A few investigators have measured nitrate movement at great
depths, Pratt et al (1972) and Adriano *' 11 (l<*/ 1973) measured
nitrate concentrations to 15 or 30 in below irrigated fields and
estimated travel time by a piston flow -model. They calc allies
from 7 to 50 years to travel 30 meters» depending on the .surface
management and soil character istics.
iecent investigations have observed great: variability in solute
movement at different locations within a field. liggar and Nielsen
{\ T)7t>) estimated that 100 observations would be required on a 1.50 ha
field to e fce the mean water velocity within -501 of its true
value. Jury et al (1976) found nonuniform infiltration of water and
nitrate under sprinkler irrigated potatoes with nitrate penetrating
much faster tinder the furrows than under the shoulders of the hills
where the potato tubers were located.
1 i If j»r i I!!
Travel time for the drained fields is more difficult to estimate
because of the uneven contributions froo, .Hit. i ent parts of the
268
-------
field. Much of the work on nitrate movement has consequently focused
on relating input of nitrate Co output concentrations or nitrate
accumulation at the tile line.
Glandon (1971) and Letey et al (197?) monitored numerous tile
outlets in California for nitrate concentration and accumulation and
found that nitrate concentration was not wej1 correlated with water
input, nitrogen input, or nitrate output flow. Nitrate flux through
the tile was correlated with water flux through the tile in both
invest igations»
The ratio of nitrogen drained to nitrogen applied during the same
year is frequently quoted by investigators monitoring tile drains,
but this index may be Misleading without some knowledge of the travel
time from surface to drain (Jury, 1975). Johnson et al (1965)
monitored drainage from four cropped fields for one year and found a
range from .09 to ,7 in this ratio, Meek et al (1969) found only ,05
for the ratio of (nitrate out/aitrate in) during 5 months of monitor-
ing an irrigated field, Calvert and Phmig (1973) observed a ratio of
0.35 after one year for a field in Florida*
Several repotted studies looked at leaching efficiencies for
various surface water applications. Sadler et al. (1365) leached &
small experimental plot of chloride "by continuous ponding and dropped
the chloride concentration to .1 of the initial value after 145 hours,
Talma {1967) leached field plots of chloride with continuous and
intermittent ponding and found the latter more effective in removing
salt located far frost a drain.
Jury (1975) proposed a method for calculating the travel time
for tile drained fields and showed that the predictions of his model
were consistent with the published observations of tile drain research
studies, but often lent a different physical interpretation to the
data than the one offered "by the authors. This model will be die-
cussed in greater detail helow.
569
-------
MODELS FOR CALCULATING TRAVEL TIME
N ivement in_ Soi 1
Nitrate ions are transported through the soil by three mecha-
nisms: i) bulk flow (convection) within the moving soil solution,
ii) self-diffusion by molecular collision with other ions, hydro-
dynamic dispersion by mixing processes caused by local, variations in
water flow velocity and water flow direction due to the presence of
the porous medium. Transport by diffusion is usually small compared
to convection, and hydrodynamic dispersion is important o * * ises
of high solution velocity or large inhowogeneities (i.e. e. i
the soil profile. In all applications, the mechanisms
and dispersion are used to describe spreading and mixing of solute
pulses, and need not be Included in an approximate calculation of
travel times for surface-applied nitrate pulses.
Using the approximation that nitrate transport is due to soiling
solution alone, we may write
JN ' Vh <>)
2
where J is nitrate flux (gm/cm /day)
1 2
Jy Is solution flux (cm soln/esn /day)
%
C8 Is nitrate solution concentration (gin/cia'' solo).
PISTON FLOW MODEL
The piston flow model is used by many Individuals to approxi-
mately describe how water and dissolved chemicals move through the
soil. This model assumes that solution applied to soil will replace
the soil water previously in the port , >•! iactively pushing i» t >.,r. u>»h
the soil like a piston. Figure 1 will help to illustrate the
270
-------
APPLY W cm3 SOLUT ION
SOLUTION
MOVES TO
DEPTH
L =¦ W/A8
BY REPLACING
SOIL WATER
AREA A cm5
POROUS
MEDIUM
&.
SOIL VOLUME
1 cmJ \
AIR
WATER
SOLID
AiR
, /'VOLUME 0-8
PORE VOLUME o
volume e
[SOLID VOLUME (-a cm3
PISTON FLOW MODEL
figure 1. Illustration of the piston flow model.
271
-------
¦)
A long column of soil has a cross-sectional area A cm". Each volume
element of soil contains a fraction 1-a of solid soil particles and
a fraction a of water and air. The quantity a la called the fractional
pore space or porosity. The fraction 0 of the- total volume filled
1 "I
with water is called the volumetric water content (cin" water/cm* soil).
'I
If a volume W cm" of solution with concentrat ion Co is fed into
the top of this column (which we assume contains pure water C • 0),
the solution will move to a depth L. The piston flow mo-del assumes
3
that the input solution pushed out W cm of water initially in the
column, which means that the new solution has moved to a depth L »
W/Aft (cm),
If we apply the solution at a rate Q (ciB*Vday)» this solution
will move through the soil at a rate Q/A9 (cm/day) because It can
only cove through the area AO which contains water, The quantity
Q/A0 is called the solution velocity V^,
Fill DRAINAGE TRAVEL TIME
The time required for a pulse of nitrate to move a distance
L (cut) through the soil in, the piston flow approximation is siaply
t - L/Vk (2)
where Is the solution velocity (an/day), provided is
constant.
Movement Below Root Zone
Below the root gone, «ay be approximated by
Vw - »/§ (3)
*i 2
where 1 is average drainage flux (era" /era '/day) and § is
3 3
average volumetric water content (cm" /cm'"} „ If the »ter
212
-------
content In the root zone is changing then
R I + P - ET
¦where I Is the irrigation, P is precipitation applied at
the soil surface and ET is evapotransplration (consumptive
use). Combining Eq (2) and (3) gives, for the time required
to travel a distance L below the root zone
t - L6 (4)
E "
The travel time for free drainage is thus seen to depend on
surface management (I)P external weather conditions (P, ET), and
soil type (0),
Movement Within Root Zone
The travel time for movement through the root zone is more com-
plex, because the soil water flux is not constant, but decreases
from a value X + P at the surface to 1 at the bottom of the root
zone. Furthermore, I + P is not constant with time and single irri-
gations or storms may leach nitrate through the entire root zone.
Only in cases of frequent irrigation stay we use a steady state model
for water flow near the surface. Even in that case, however, Eq (2)
must he replaced by a more complicated expression. Since is not
constant, the solution velocity « J^/9 is not constant either.
One way of averaging Vy is to set Jw * (I + R)/2 so that the time
required to travel through the root zone is
t - 210/ (I + t) (5)
where 1 is the depth of the root zone and 9 is the average
water content. Equation (5) will usually underestimate"t.
Some simple examples wil.1 help to illustrate the use of Eq (A)
and (5),
213
-------
EXAMPLE 1
A quantity of nitrate near a sand)? soil surface is moved down-
ward by a series of rain storms totalling 20 cm. Assuming a water
3 '}
content of 0.15 (cm' /cm") and negligible evaporation, calculate 'how
deep the nitrate will be moved.
Since only 15% of the soil area is wetted, 20 e» tain will dis-
place soil solution to 20/. 1.5 ¦ 133 rm.
EXAMPLE 2
A field under continuous cropping has an annual water application
(irrigation + precipitation) of 140 (cm/year). The crop water use
and soil evaporation ate 80 (cm/year) and the depth of rooting is
1 (m), if the average water content is 0.25, calculate how long it
will take nitrate to travel 25 (m) front the bottom of the root zone
to the groundwater.
Drainage E is 140-80 = 60 cm/year; solution velocity is
60/,25 = 240 cm/year; travel time is t • !/v » 2500/240 « 10,4
years. The approximate time to move to the Bottom of the root zone
by Ecj (5) is 2 x 100 x .25/(60 + 140) = ,25 year.
ARTIFICIAL DRAINAGE TRAVEL TIME
Determination of travel tines tot nitrates moving through i
or tile-drained fields is wore difficult than the free drainage
calculation because the water flow path is curved from the surface
to the tile or ditch line. For this reason, locations on the
surface near the tile line will have; much shorter flow paths and
hence shorter travel, tiroes than locations near the midpoint between
lines. To illustrate the procedure for determining travel time
for artificially-drained fields, the following situation will, be
considered.
1) Uniformly spaced tile lines 2 S apart at depth d below surface
2) Steady water flow with uniform net water entry rate I
274
-------
3) Impermeable barrier at depth d + D below surface
4) Isotropic soil properties
5) No crop in unsaturated zone
With this ideal system, shown, in figure 2, the movement of
nitrate initially on the surface may be divided into two stages
1) vertical flow to the water table, il) curved flow (in the satu-
rated region) converging to the drain line. The first stage is
identical to free drainage and has a travel time
t± = <10/K (7)
where © is the average water content between the surface and
the water table.
The flow lines for the saturated region may he calculated from
a mathematical model (Kirkham, 1958). A plot of these lines, called
streamlines, is shown, in Figure 3 for the region below the tiles in
figure 2.
The curved lines, representing the flow path for ions moving
toward the drain, may be used to calculate the travel time by the
following procedure. Two adjacent stream lines form a stream tube,
shown in Figure 3. Since these lines are along the flow path, fluid
will enter or leave the tube only at the ends, and the amount of solu-
tion required to move an ion through the tube is equal to the amount
of fluid in the tube. The time required for this transit is the
fluid volume in the tube divided by the entry flow rate.
Our model considers the tile lines to be very long and parallel,
so that no changes in flow characteristics take place in a direction
parallel to the drain. Out analysis will assume a unit thickness in
this direction so that we will be working with area units rather than
2
volume units. If the area of the stream tube in Figure 3 is a cnt ,
then the amount of fluid inside is 8„a where Q„ is saturated volu-
b h
metric water content. The solution flow rate into the tube is
275
-------
SURFACE FLUX RATE R
J ! I I I J I J
WATER TABLE
REFERENCE LEVEL
DRAIN TILE
UNIFORM CONDUCTIVITY K
IMPERMEABLE BARRIER
Figure 2, Meal tile drained field with steady water uptake,
-------
OtMENSlONLt V; DIO f ANCE FROM TILE LINE X=x/8
/TILE o.
STREAM TUBE
Figure 3. Streamlines for tile drained field.
-------
R&x where fix (cm) is the width of the tube at the entry point
(wafer table) and 1 is the net drainage (cm/day). Thus, the
travel time to go from the water table to the drain is
t = ©sa(x> {8)
RAx
where a(x) the tube area depends on the entry
Solutions to Eq (8) could be plotted for each
drainage rate and water content but a more compact
able if we change to dimensionless variables,
X = x/8 dimensionless distance between drains
Z » z/S diniensioTiless depth
2
A(X) • a/5 diiiiensionless area
T = Rt/9^5 dimensionless time
"*1 * S/l) drain spacing to depth ratio (9)
With these substitutions Eq (8) becomes
T = A(X)
AX (j o)
The advantage to Eq (10) is that T(X) is the same for all.
systems with the • »> ratio of drain spacing to harrier depth *) •
S/D. Figure 4 is a plot of dimension!ess travel time T as a function,
of point of entry X from X -- 0 (over the drain) to X <• 1 (midway
between drains). This result gives the travel time through the satu-
rated zone. The total travel time. should Include the time to reach
the water table
-------
CO
CD
X
oE
ii
h-
t±j
m
m
yj
o
w
z
UJ
t, = S/D
0 0,2 0.4 0,6 0.8 1.0
DIMENSiONLESS DISTANCE FROM TILE X = x/S
figure? 4. Dimensionless travel time T = Rt/9S vs lateral point of
entry X/S,
279
-------
-j i ,.-31.
cat* ). Saturated water content 0 = 0,35 (cm /cm"). Salt is leached
" 3 2
from the s jrizon by sprinkler irrigation at 1 » 5 cm' /cm /
day. Calculate the time required for salt to reach the drain.
The travel time through the* unsaturated zone (Eq (?)) Is 10
days. In the saturated zone; we consider entry points 250 em, ! * ,1
and 2250 cm, X = .9 from the drain. Using Figure 4, rj = 10, we find
T(. 1) = 0.008, T( ,9) = 0.15. To find the time t we use Eq (9) t: «
(0 S/'R) I - (175 days)! so t(250 cm) = 1.1 clay; t(2250) - 26.3 days.
Thus, salt leached from the surface near the tile (X ¦ 250 cm)
will reach the drain in 11.1 days, whereas salt originating t,»r ! r<««
the drain will require 36,3 days.
EXAMPLE 2
Same tile geometry Cd = 200 cm, S ¦ 2500 cm, Fj • 250 cm) with
deep rooted crop grown in the unsaturated zone 8 » ,35 of a fine
textured soil 0 = .50. Consumptive use (EI) is 100 cm/year, and
yearly irrigation is 150 c® (I), Rainfall Is negligible. Calculate
travel time for nitrate added with the irrigation water. The yearly
drainage is equal to the irrigation minus consumptive use or 1 *
50 cm/year.
Using the approximate relation (Eq (5)) for root zone travel
time gives t = ,70 year. In the saturated zone t • (8„S/R) I = (25 yr)
T. Me again use the | « 10 curve to get X(.l) • ,006, X(.9) - .15
so that t(25 cm) = .15 year; t(2250 cm) « 3,75 year.
Thus, nitrates leached from is • 250 cm will reach the tile in
0,85 year, whereas the arrival t jm«. fioin x » 2250 ria Is 4.45 year.
These two examples show the versatility of dimensional ar»-i I v. I:.,
One figure may be used for virtually all ralculas ions, with soil,
management, and drain geometry factors telescoped into the relation
t = (®gS/R) T (Eq (9)) between real and dimensionless time.
Equation (9) suggests that the product (S-S/B) with climrnKinnti
of time, may be used along with Figure 4 to efiarai i t*r i il»,. t iuw
response of o« .a m» «.»* ^ system, with snail valuer
-------
small travel times (fast response) and vice versa. Table 1, taken
from Jury (1975) gives response time parameters (Q^S/R) for a number
of tile drain systems published in the literature.
References 5 and 7 are California tile lines with 8 S/R from 20
b
to 1000 years, so that travel times will be extremely long for such
systems. The last column gives the percent of salt initially in
the soil that would be leached in one year, ranging from 4 to 100%.
Tile Drain Concentrations
Because of the variable distance and travel time to the drain,
effluent concentrations represent a complicated average over the
field. Some simple calculations will be used to illustrate this
point.
EXAMPLE 1
Sudden change in salinity of the irrigation water from C = 0 to
C = Co.
The effluent concentration will remain at C = 0 until the salt
applied directly over the drain arrives at the tile. Calling this
T = 0, we find that for T>0 the effluent concentrations is a
mixture of solution Co from those partsof the field with travel times
less than T , and of pure water from the rest of the field. Thus
Cout (T) = CoX(T) (1.1)
A plot of Eq (11) is given in Figure 5. For a California field
with (QCS/R) = 100 years and ^)= 10, the concentration will rise to
0.6 Co after 10 years.
EXAMPLE 2
Single pulse of concentration Co applied at the surface between
T = T. and T = T„. (T -T. = &T).
A D D A
If we again call T = 0 the time when the pulse first reaches
the tile then the output concentration for times T^) 0 is made up
of solution concentration Co from that part of the field discharging
281
-------
Table 1. Published tile-drainage studies and estimate travel time parameters (9gS/R).
Source
Soil
Type
Tile
Spacing
Barrier
Depth
Depth
to Tile
Tile
Discharge
V
R
7
/o
Leached
m/yr
yr
1st yr
1.
Bolton et al
(1970)
Clay
76.0
?
0.7
0.16
122
12
2.
Calvert and
Phung(l97l)
Fine
Sand
18.3
?
?
1.01
3.6
74
3.
Erickson and
El 1 is(1971)
Clay
Loam
20.0
?
1.0
0.03
157
8
4.
Glandon
(1971)
Several
60.0-80.0
1
1.5-3.0
<0.47>*
> 32
i 28
5.
Letey et al
(1977)
Several
>35.0
?
2.0
varied
20-100
14-40
6.
Meek et al
(1969)
Silty
Clay
Loam
60.0
1
1.8
0.48
31
30
7.
Johnson et
al (1965)
Loam
60.0-200.
0 ?
2.0
<..18>
32-1000
4-30
8.
Sadler et
al (1965)
Silt
Loam
6.1
0.2
0.9
43.8
.025
100
9.
Talsma
(1967)
Sand
20.1
2.0
1.5
1.8
2.3
85
10.
Talsma
(1967)
Loam
26.8
2.0
1.5
5.3
1.0
98
*<)denotes average for all fields studied.
-------
o
(_>
\
(J
I-
<
QC
h-
2
UJ
<_>
2:
o
o
t-
2
UJ
3 0.2
17=10
Li-
Li.
LlI
1.0
0.001
0.01
0.!
DIMENSIONLESS TIME T=Rt/0S
Figure 5. Output concentration vs dimensionless time T - Rt/9S
for system leached with pure water.
283
-------
the pulse and C = 0 from the rest of the field.
C (T1) = Co (XCTj) -X (T] -AT)) (12)
A plot of Eq (12) is shown in Figure 6 for the case AT = .01.
The pulse appears very spread out at the outlet, requiring a long
time to move completely through the system. For example if (9 S/R)
s
= 100 years (California field) then Figure 6 represents At = 1 year
of sending in solution of concentration C = Co at the surface. This
might represent a single year of intensive fertilization of a field
naturally low in nitrogen. Nine years later (T = .1) only 60% of
the pulse has flowed through the tile, and residual nitrate from the
single fertilization is still appearing in the drain concentration.
Other Artificial Drainage Systems
1. Ponded surface conditions
The main difference between this case and the one discussed is
that the rate of water entry is not the same at different points
along the surface. The result is that even longer travel times are
required to enter the drain from large distances away from the tile
(Jury, 1975).
2. Layered soil profile
For this system the streamlines will have different shapes
than Figure 2, but they may still be calculated (Kirkham et al, 1974)
and the rest of the procedure applied as above.
LIMITATIONS OF THE MODEL
Free Drainage
There are two major problems with the application of the piston
flow model Eq (4). First, water does not move through the entire
pore volume 9, particularly at high water contents. For this reason,
284
-------
0
0
s
0.15
0
Z
O
t-
<
cr
h-
0.10
z
11 l
LU
0
z
0
0
h-
z
0.05
LU
Z)
_J
1.
LI_
Lu
RESPONSE TO PULSED INPUT FOR 77 = 10"
AT = 0.01
0.8 -
0.6
0.4 <
O
0.2
0.02 0.04 0.06 0.08 0.10
DIMENSIONLESS TIME T = Rt/6S
0.12
Figure 6. Output concentration
Rt/0S for step input
interval AT = 0.01.
C/Co vs dimensionless time T =
of solution C = Co during a time
285
-------
the model often underestimates the movement of solution introduced
at the soil surface, as observed by Warrick et al (1971) and
Endelman et al (1974). For the same reason, solutes initially in
the soil are not leached as rapidly by water introduced at the sur-
face as Eq (5) predicts, because part of the salt js present in stag-
nant regions of the wetted pore space (Miller et al, 1965-
Balasubramanian et al, 1973). Furthermore, certain soils, particularly
clays, have preferential channels for water flow, so that travel times
are much shorter than predicted by Eq (4) (Kissel et al, 1975; Tyler
and Thomas, 1977). Finally, some soils show uneven infiltration
patterns due to surface irregularities or instabilities (Jury et al
1976).
The second problem in using Eq (4) is in estimating drainage
flux. Although Pratt et al (1972) assumed that the ratio of chloride
concentration at the surface to chloride concentration In the drainage
equalled the ratio of the drainage flux to the irrigation flux
chloride values at a given depth are quite variable and lead to error
in this prediction (Biggar and Nielsen, 1973). An alternative is to
measure evapotranspiration directly, but this requires considerable
equipment.
Artificial Drainage
Problems in the piston flow model for movement to drains are the
failure of water to move through the entire pore space, nonuniform
infiltration through cracks, channels, or backfill material, and
neglecting diffusion and dispersion of salt within the soil solution.
No studies have been made of the magnitude of these effects.
SUMMARY AND CONCLUSIONS
Use of the piston flow approximation for water and salt movement
through soil leads to simple equations to estimate the soil residence
286
-------
time for nitrate between surface and groundwater or tile drain.
These equations require knowledge of drainage or discharge rate,
soil water content, location of tile drains, water table, and depth
to layer of reduced permeability. For some systems, these equations
will be inaccurate because of nonuniformities in water flow or sub-
stantial stagnant space in the wetted soil volume, but in many cases
they will give a reasonable estimate of the phase lag between the
application of a chemical at the soil surface and its subsequent
appearance in a drain outlet or in underlying groundwater.
287
-------
LITERATURE CITED
Adriano, D. C., P. F. Pratt; arid F. H. T-akatori. 1972a. Nitrate in
unsaturated zone of an alluvial soil in relation to nitrogen
rate and irrigation level. J. Environ. Qual. 1:A18—422.
Adriano, D. C., F. H. Takatori, P. F. Pratt, and 0. A. Lorenz. 1972b.
Soil nitrogen balance in selected row crop sites in Southern
California. J. Environ. Qual. 1:279-283.
Ayers, R. S. and R. L. Branson. 1973. Nitrates in the upper Santa
Ana River basin in relation to groundwater pollution. Calif.
Agr. Expt. Station Bull. 861.
Baker, J. L., K. L. Campbell, H. P. Johnsoi\ and J. J. Hanway. 1975.
Nitrate, phosphorous, and sulfate in subsurface drainage water.
J. Environ. Qual. 4:406-412.
Balasubramanian, V., Y. Kanehiro, P. S. C. Rao, and R. E. Green.
1973. Field study of solute movement in a highly aggregated
oxisol with intermittent flooding. I. Nitrate. J. Environ.
Qual. 2:359-362.
Biggar, J. W. and D. R. Nielsen. 1976. Spatial variability of the
leaching characteristics of a field soil. Water Resources Res.
12:78-84.
Bingham, F. T., S. Davis, and E. Shade. 1971. Water relations, salt
balance, and nitrate leaching losses of a 960-acre citrus
watershed. Soil Sci. 112:410-417.
Bolton, E. E., J. S. Aylesworth, and F. R. Hore. 1970. Nutrient
losses through tile drains under three cropping systems and
two fertility levels on a Brookston clay soil. Can. J. Soil
Sci. 50:275-279.
Calvert, D. V. 1975. Nitrate, phosphorous, and potassium movement
into drainage lines under three soil management systems. J.
Environ. Qual. 4:183-186.
Calvert, D. V. and H. T. Phung. 1971. Nitrate-nitrogen movement into
drainage lines under different soil management systems. Soil
Sci. Soc, Amer. Proc. 31:229-232.
Endelman, F. J., D. R. Keeney, J. T. Gilmour, and P. G. Saffigna.
1974. Nitrate and chloride movement in the Plainfield loamy sand
under intensive irrigation. J. Environ. Qual. 3:295-298.
288
-------
Glandon, L. R. 1971. Nutrients from tile drainage systems. Water
Poll. Control Res. Ser. 13030 ELY5/71-73.
Johnson, W. R., F. I. Ittihadieh, R. M. Daum, and A. F. Pillsbury.
1965. Nitrogen and phosphorous in tile drainage effluent.
Soil Sci. Soc. Amer. Proc. 29:287-289.
Jury, W. A. 1975. Solute travel time estimates for tile-drained
fields. I. Theory. II. Application to experimental studies.
Soil Sci. Soc. Amer. Proc. 39:1020-1028.
Jury, W. A., W. R. Gardner, P. G. Saffigna, and C. B. Tanner. 1976.
Model for predicting simultaneous movement of nitrate and water
through a loamy sand. Soil Sci. 122:36-43.
Kirkham, Don. 1949. Flow of ponded water into drain tubes in soil
overlying an impervious layer. Trans. Amer. Geophys. Un.
30:369-385.
Kirkham, Don. 1958. Seepage of steady rainfall through soil into
drains. Trans. Amer. Geophys. Un. 39:892-908.
Kirkham, Don, S. Toksoz, and R. R. van der Ploeg. 1974. Steady flow
to drains and wells. Agronomy 17:203-243.
Kissel, D. E., J. T. Ritchiei, and Earl Burnett. 1974. Nitrate and
chloride leaching in a swelling clay soil. J. Environ. Qual.
3:401-404.
Letey, J., J. W. Blair, Dale Devitt., L. J. Lund, and P. Nash. 1977 .
Nitrate-nitrogen in effluent from agricultural tile drains in
California. Hilgardia 45:289-319.
Meek, B. D. , L. B. Grass, and A. J. Mac Kenzie. 1969. Applied nitrogen
losses in relation to oxygen status of soils. Soil Sci. Soc.
Amer. Proc. 33:575-577.
Miller, R. J., J. W. Biggar, and D. R. Nielsen. 1965. Chloride dis-
placement in Panoche clay loam in relation to water movement
and distribution. Water Resources Res. 1:63-73.
Pratt, P. F., W. W. Jones, and V. E.Hunsaker. 1972. Nitrate in deep
soil profiles in relation to fertilizer rates and leaching
volume. J. Environ. Qual. 1:97-101.
Sadler, L. D. M., S. A. Taylor, L. S. Willardson, and J. Keller. 1965.
Miscible displacement of soluble salts in reclaiming a salted
soil. Soil Sci. 100:348-355.
289
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Saffigna, P. G., D. R. Kenney, and C. B. Tanner. 1977. Nitrate,
chloride, and water balance with irrigated Russet Burbank
potatoes in a sandy soil. Agron. J. 69:251-257.
Shuford, J. W., D. D. Fritton, and D. E. Baker. 1977. Nitrate-
nitrogen and chloride movement through undisturbed field soil,
J. Environ. Qual, 6:255-259.
Sommerfeldt, T. G. and A. D. Smith. 1973. Movement of nitrate
nitrogen in some grassland soils of southern Alberta. J.
Environ. Qual. 2:112-115.
Talsma, T. 1967. Leaching of tile-drained saline soils. Aust. J.
Soil Res. 5:37-46.
Tyler, D. D. and G. W. Thomas. 1977. Lysimeter measurements of
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290
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USE OF MATHEMATICAL RELATIONSHIPS TO DESCRIBE THE
BEHAVIOR OF NITROGEN IN THE CROP ROOT ZONE
J. M. Davidson and P. S. C. Rao—
ABSTRACT
A procedure to estimate the movement of water-soluble nitrogen
species (NO.^ and NH^) was developed by assuming that (i) the soil-
water residing in all pore-sequences participates in the transport
process, and that (ii) the soil-water initially present in the soil
profile was completely displaced ahead of the water entering at the
soil surface. Field-capacity and initial soil-water content distri-
bution in addition to total water inputs were necessary parameters to
estimate solute transport in the root zone. First-order kinetics were
assumed to describe the nitrogen transformations (mineralization,
immobilization, nitrification, and denitrification). These transforma-
tion processes were considered to occur under ideal conditions. Plant
uptake of water and nitrogen (nitrate and ammonium) was estimated,
respectively, from potential evapotranspiration and nitrogen uptake
rate under ideal environmental conditions for a given crop. Actual
plant uptake of water and nitrogen was dependent upon the available
soil water and total mineral nitrogen within the crop root zone. These
mathematical relationships could be solved using a programmable desk-top
calculator; however, a larger computer was needed when more complex
sub-models were employed to describe soil-water uptake. The proposed
mathematical relationships can provide field managers and regulatory
personnel with an integrated description of the behavior of nitrogen in
the root zone during a crop growing season.
-^Soil Science Department, University of Florida, Gainesville, Florida
32611.
291
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INTRODUCTION
Nitrogen is an essential element in all biological systems. In
an undisturbed soil environment, the nitrogen cycle is closed and the
total nitrogen content of the system (soil plus plants) does not
change with time. This delicate balance is disturbed, however, when
man uses the soil for food production and separates the site of nitro-
gen consumption (metropolitan areas) from where it is assimilated
(plants and animals). Because of this, it has been necessary to supple-
ment most agricultural soils in recent years with nitrogen fertilizer.
The inorganic nitrogen added in this manner represents only a small
part of the total amount of nitrogen contained in a soil since most
soil nitrogen is organically bound. However, it is the inorganic nitro-
gen which is the most dynamic and of interest when evaluating environ-
mental contamination and agricultural production potential.
The fate of various nitrogen forms at and below the soil surface
is governed by a variety of interrelated and complex processes. These
processes involve inorganic (NH^, NO^, NO2, N^O, and ^) and organic
nitrogen forms which exist simultaneously in the soil. These and other
nitrogen substrates undergo continuous reversible and/or irreversible
transformations owing to chemical and microbiological processes. The
water-soluble nitrogen species (NH^, NO^, and NO^) are also translocated
through the soil by molecular diffusion and more importantly by soil
water. Ammonium (NH^) and nitrate (NO^) concentration in the soil is
complexed further by absorption by plant roots. The extent of water
and nitrogen uptake by plants is determined, in part, by the transpira-
tion demand, which in turn is dependent upon plant species, growth
stage, and meteorological conditions.
The complexity of the soil-water-plant system is enhanced further
by the fact that all of the above processes are transient in nature
and occur simultaneously. Therefore, a prerequisite to describing the
fate of inorganic nitrogen forms in soil-water-plant systems is a
292
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complete understanding of the nitrogen transformation and transport
processes. A considerable amount of information is available regard-
ing soil nitrogen and its behavior under laboratory and field con-
ditions (Bartholomew and Clark, 1965). However, due to the nature
and conditions under which much of the research was conducted, it
is difficult to combine the results from these individual experi-
ments into a form that can be used to develop relationships for simu-
lation and/or prediction purposes.
Mathematical relationships to describe physical, chemical, and
biological processes are generally of three types. Stochastic relation-
ships assume the processes being described obey or follow the laws of
probability. Empirical relationships are developed from experience
and observations and regression equations are used to correlate known
inputs (temperature, rainfall, nitrogen, etc.) with measured outputs
(yield, nitrogen leached, etc.). Mechanistic relationships are based
upon established physical, chemical and biological laws that describe
specific processes. Mechanistic relationships are versatile in that
extensive historical records are not necessary for their application
at a given site; however, these relationships require a complete under-
standing of processes. The general limitation of mechanistic relation-
ships, therefore, is an inadequate understanding of the system.
Empirical relationships are of assistance in such situations because
major variables have been identified as well as their influence on the
system.
The mathematical relationships used in this manuscript to describe
the movement, transformations, and plant uptake of various nitrogen
forms in an unsaturated soil profile are mechanistic in concept, but
empirical in development and presentation. A minimum amount of soil
input information is needed for these relationships to obtain an in-
tegrated description of the various nitrogen forms in a plant root
zone. The benefits of having a procedure for estimating the behavior
of various soil inorganic and organic nitrogen forms with time are
293
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thought to outweigh the loss in accuracy resulting from this
simplified approach.
SOIL-WATER AND NITROGEN MOVEMENT
As the soil solution moves through the interconnecting pore sequen-
ces of a soil, nitrate and ammonium ions as well as other soluble
substances are carried with it. To describe the actual distribution
of these solutes in a soil profile requires complex mathematical
equations and analytical procedures. However, computational procedures
to estimate only the position of a nitrate or ammonium front entering
the soil at the soil surface and moving vertically down through the
profile in response to different water inputs are easier and may be of
equal benefit. This type of computation requires the assumptions that
(i) the soil solution residing in all pore sequences participates in the
transport process, and (ii) the soil solution in the profile is dis-
placed ahead ("pushed ahead") of the applied water. These assumptions
were also used by Frere (1975) to predict water and sediment transport
from watersheds. Exceptions to the assumption that, all pore-sequences
participate in the transport process are discussed by Rao et al (1974)
and Quisenberry and Phillips (1976); however, for many cases the assump-
tion appears reasonable as demonstrated by Rao et al (1976b).
Water ponded on a soil surface fills all the pores as it moves
down through the soil profile. The location of the wetting front (sharp
boundary between the "wet" and "dry" soil) below the soil surface is
related directly to the amount of water that has entered the soil. If
a fertilizer containing nitrate is broadcast over the soil surface
prior to applying the water, nitrate will enter the soil during infil-
tration and move with the water. Thus, the applied water and nitrate
move together because nitrate is an anion and is not significantly
influenced by the positively-charged clay particles in most soils.
To estimate the location of the wetting front for various amounts
294
-------
of infiltrated water, consider the following mathematical relation-
ship:
dwf = 9 - 0.
w 1
where I is the amount of infiltrated water (centimeters or inches),
3 3
9 the average volumetric soil-water fraction (cm /cm ) in the wetted
zone behind the wetting front, and 9. the volumetric soil-water
3 3 1
fraction (cm /cm ) in the soil prior to adding the water and d the
distance from the soil surface to the wetting front.
Using equation 1 to calculate the position of the wetting front
after 5 and 10-cm of water have entered a dry soil (9^ = 0) places the
front at 14.3 and 28.6-cm if the soil-water content behind the wetting
3 3
front is taken as 0.35 cm /cm (Figure 1). However, if the initial
3 3
soil-water content (9^) were 0.1 cm /cm , the wetting front would be
located at 20 and 40-cm below the soil surface (Figure 2). The differ-
ence in wetting front position for the same input arises from the fact
that the initial water in the previously wet soil was displaced ahead
(assumption (ii) in the previous section) of the infiltrating water.
Thus, the soil water between the 14.3 and 20-cm soil depth (Figure 2)
is water that was in the soil prior to the 5-cm water application.
From this it is concluded that the 5-cm of water added to the wet soil
moved to approximately the same depth as that added to the dry soil
(0_^ = 0). This phenomenon can be observed where discharge from drain
tiles frequently begins shortly after the initiation of water applica-
tion to the soil surface. The water in the tiles is that displaced
from the soil by the infiltrating water. This concept is important when
determining the depth to which nitrate and ammonium will move in the
soil.
The initial soil-water content of a field soil is generally not
uniform, but dry near the surface and wet deeper in the profile. Al-
though more complex, this case does not alter the previous discussion
or conclusions. Using a programmable desk-top calculator, the amount
295
-------
Soil-Water Content
cm
water
Q 30
10cm
water
O
10 40
40 80
Concentration (^^cc)
Figure 1. Distribution of soil-water (solid lines) and nitrate
(dashed lines) after 5 and 10-cm of water had infiltrated
into an initially dry (0 =0.0) soil profile (from Rao et
al, 1976b).
296
-------
Soil - Water Content
0.2
0.4
P.o
r"
20
5cm
water
Q 30
CO 40
10cm water
50
60
0 40 80
Concentration
Figure 2. Distribution of soil-water (solid lines) and nitrate
(dashed lines) after 5 and 10-rm of water had infiltrated
into a soil profile at a uniform initial soi^-wa^er content
(9. shown as vertical dashed line) of 0.1 cm /cm (from
Rao, 1976b).
297
-------
of water pushed ahead of the infiltrating water for this case can be
calculated. For example, if the water content behind the wetting
3 3
front was 0.35 cm /cm following the application of 5-cm of water
to the initially nonuniform soil-water case, the infiltrating water
would still be located between the 0 and 14.3-cm soil depth with the
wetting front below 14.3-cm.
The position of the nitrate entering the soil with the infil-
trating water can be calculated from the following mathematical re-
lationship:
dnf = Q~ (2)
w
where d _ is the distance from the soil surface to the nitrate front
nr
and the other symbols are as described earlier. Note that the position
of the nitrate front depends only upon the amount of water that has
infiltrated the soil and the average volumetric soil-water content be-
hind the wetting front. This means that the soil water containing
the nitrate solution displaces the soil water initially present in the
soil profile. This is illustrated in Figures 1 and 2 where the nitrate
pulse from the fertilizer is indicated by a dashed line. Note that
in both cases (Figures 1 and 2), the nitrate pulse front is located at
14.3 and 28.6-cra after 5 and lQ-cm of water had entered the soil. The
nitrate front lags behind the wetting front for the previously wet
soil case; thus, 'the wetting and nitrate front are not analogous.
Following the cessation of infiltration the soil-water content
behind the wetting front decreases owing to drainage or redistribution.
This process results in a deeper penetration of the water and nitrate
front into the soil profile. Following two to three days of drainage,
the change in soil-water content with time behind the wetting front
become small and a soil-water content commonly referred to as "field
capacity" (9^c) is said to have been reached. The nitrate movement
that occurs because of redistribution can be estimated from the amount
of "drainable" water (9 - ) between the soil surface and the
w fc
298
-------
nitrate front (d f)• Fine textured soils generally have a field
capacity soil-water content two to three times larger than that of
coarse textured soils; thus, they have a smaller amount of drainable
water. The distance the nitrate front will move owing to redistribu-
tion is considerably less for fine compared to coarse textured soils.
The position of the nitrate front following infiltration and
redistribution of the soil-water to field capacity can be calculated
from the following mathematical relationship:
d f = 7T— (3)
nf 9r
f c
The use of equation (3) is limited to cases where the nitrate front
originates at the soil surface. If the nitrate front is located at
some depth in the soil prior to the application of water, then the
amount of drainable water above the nitrate front after infiltration
must be determined. This can be done by knowing the initial soil-
water content distribution above d r, 9 , and 9_ . These calcula-
nr w f c
tions can be made on most programmable desk-top calculators.
The retardation of the ammonium front due to ion-exchange with
the soil can be calculated from a knowledge of the adsorption character-
istics for ammonia with a given soil. The retardation would be greater
for fine than for coarse textured soils (Rao and Davidson, 1978). The
retardation coefficient (R) is proportional to the soil bulk density
(p), magnitude of adsorption (K), and inversely proportional to
volumetric soil-water content (9). This relationship may be expressed
as:
R = (1 + £§) (4)
Note that, for a given K and p, the retardation coefficient increases
with decreasing 9. Thus, retardation of ammonium movement is greater
in a drier soil than in a wet soil. The ammonium front location is
calculated by dividing the nitrate front location (see Eq. 3) by the
retardation coefficient (R).
299
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NITROGEN TRANSFORMATIONS
As stated earlier, nitrogen exists simultaneously in various
inorganic and organic forms in the soil. These nitrogen forms
undergo continual transformations from one form to the other. Both
microbial and chemical processes are responsible for these transfor-
mations. Some of the important nitrogen transformation processes in
soils are: mineralization, immobilization, nitrification, and vola-
tilization. In cases where urea fertilizers are added to the soil,
hydrolysis of urea to ammonium must also be considered. The inter-
relationships between these nitrogen transformation processes in soils
are schematically illustrated in Figure 3.
Mineralization process is the microbiological conversion of
organic nitrogen to ammonium. This process is also referred to as
ammonification. Immobilization is the opposite to mineralization,
since this process involves microbial assimilation of inorganic nitro-
gen into organic forms. Thus, when both these processes are occurring
simultaneously in a soil, only the net result may be observed. A net
increase in the inorganic nitrogen content is evident when mineraliza-
tion prevails, while a net decrease in the inorganic nitrogen content
results when immobilization is predominant. Although large supplies
of organic nitrogen exist in soils, only a small fraction of it is
available to the micro-organisms as an energy source. This fraction
is referred to as the potentially mineralizable organic-nitrogen and
constitutes about 10% of the total soil organic-nitrogen (Stanford
and Smith, 1972). On the other hand, potentially mineralizable
fraction of the organic-nitrogen in many common organic wastes (e.g.,
animal manure, liquid dairy waste, etc.) has been estimated to range
from 50 to 90% (Reddy et al, 1977). It should be recognized that min-
eralization is the predominant soil process responsible for the net
production of ammonium from organic forms.
Nitrification is the microbiological oxidation of ammonium to
300
-------
o
nitrification
hydrolysis
NH
Figure 3. Schematic representation of the inter-relationships among
the major nitrogen transformation processes in soils.
301
-------
nitrate, with the formation of nitrite as an unstable intermediate
product; under field conditions, nitrite is rapidly oxidized to
nitrate. Hence, nitrification may be considered as a single-step
process with the ammonium to nitrite conversion step controlling the
rate of nitrification. Nitrogen is lost from soils under certain con-
ditions in gaseous forms due to volatilization of ammonium and de-
nitrification of nitrate. It is known that ammonia volatilization
losses increase with increasing soil pH (alkaline conditions). Since
microorganisms responsible for denitrification are anaerobic in
nature, gaseous losses of nitrogen by denitrification increase with
increasing soil-water content. Volatilization and denitrification
processes are discussed in detail by RoLston (1978), whereas nitri-
fication, mineralization, and immobilization are discussed by Broad-
bent (1978) .
Most of the nitrogen transformation processes are mediated by soil
microorganisms. Hence, the rate at which a given transformation process
proceeds is determined by the size of the population and/or the activity
of specific soil microorganisms. Several soil environmental factors
(pH, soil-water content, temperature, and aeration) are known to
significantly influence nitrogen transformations. Soil temperatures
in the range of 30-35C and a neutral soil pH appear to be optimal for
most microbial populations controlling nitrogen transformations in
soils.
Several researchers have reported that nitrogen transformations
were first-order processes, i.e., the reaction rate was directly pro-
portional to the concentration of the nitrogen species undergoing the
transformation. These proportionality constants are referred to as the
rate coefficients and are symbolized as k. The values of k depend
upon several soil factors. It should be recognized that large values
of k suggest rapid transformations. The time required for 50% of
the initial amount of a given nitrogen species to be transformed is
defined as the half-life (11) . Assuming a first-order process, the
'S
302
-------
values of t, can be calculated given a k value. Estimates of tj for
various nitrogen transformations in soils, based on published data,
are listed in Table 1. On this basis, hydrolysis of urea appears to
be the fastest reaction, while mineralization-immobilization processes
are the slowest.
Table 1. Values of half-lives for selected nitrogen transforma-
tion processes in soils,2 estimated on the basis of published
data.
Transformation Process
Half-life, tj (days)
¦h.
Mineralization * (org N -*NH^)
100-200
Nitrification (NH. N0„)
4 3
10-20
Volatilization (NH. NH_)
4 3
20-50
Denitrification (N0^ -»N^ + N^O)
7-15
Urea Hydrolysis (Urea ->NH^)
3-7
* Net Mineralization
Mathematical relationships for describing various nitrogen trans-
formations in soils have been proposed by several investigators (Mehran
and Tanji, 1974; Endelman et al, 1973; Rao et al, 1976a; Selim et al,
1978). These relationships are complex and sophisticated computers are
required to solve them. However, simpler equations to calculate the
net rates of production or loss may be developed when only the total
amounts of each nitrogen species in the profile is considered. These
rate equations and their analytical solutions, adaptable for use with
programmable desk-top calculators, were described in detail by Davidson
et al (1978), and will be used in this manuscript. Principal limita-
tions of this approach are: (i) only total amounts of each nitrogen
species were considered and not their concentration distributions in
the soil profile, and (ii) the transformation rate coefficients were
303
-------
assumed to not be influenced by soil environmental factors. Further-
more, these relationships are applicable only when denitrification,
volatilization, and nitrate immobilization (see Figure 3) are insignifi-
cant. Since the mathematical relationships considered in this manu-
script are designed to provide an integrated description of nitrogen
transformations, the above stated limitations are not expected to be
too severe, especially for well-drained soil.
PLANT UPTAKE OF WATER AND NITROGEN
The inherent complexity of the crop root zone and the dynamic nature
of the plant's uptake of water and nutrients defies an exact mathematical
description. Furthermore, an understanding of the physiological charac-
teristics of crops and their needs for water and nutrients is at best
incomplete. This necessitates a gross simplication of the actual pro-
cesses involved in plant uptake when developing empirical relationships
for predictive purposes.
In a macroscopic approach, the entire root system may be considered
as a sink for nitrogen and water. The crop demand during the growing
season then represents the sink strength, while the distribution of this
sink in the soil profile is represented by the root distribution. In
this approach, the absorption capacity of the roots for water and nitro-
gen is assumed uniform over the entire root system. Thus, the rate of
crop uptake for water (C^) and nitrogen (QN> may be expressed, respect-
ively, as cm water/cm root/day and jug nitrogen/cm root/day.
The rate of water uptake by plant roots is governed by (i) the
ability of the soil to conduct water to the roots, and (ii) the plant
water demand rate as determined by the transpiration rate. The
hydraulic conductivity (K(6)) of the soil is dependent on the soil-
water content and in most soils exponentially decreases as the soil-
water content decreases. Therefore, plants growing in a dry soil may
undergo wilting if the water transport rate to the roots is less than
304
-------
the transpiration demand. Molz and Remson (1970) have proposed an
empirical relationship to describe the dynamics of water extraction
by plant roots. This approach takes into account the factors dis-
cussed above. Using their relationship, the transpiration demand
for water is distributed over the entire root system and is propor-
tional to the product of K(9) and R(z,t), where R(z,t) is the root
length distribution.
Nitrogen uptake by plant roots involves the movement of water-soluble
nitrogen species (ammonium and nitrate) to roots followed by their
absorption across the root surfaces. Convective flow of soil-water
towards roots in response to transpiration results in the mass-flow of
ammonium and nitrate along with the soil water. The concentration of
these ions at or near the roots decreases when the rate of uptake
exceeds the rate of their supply by mass flow. Diffusion of nitrate
and ammonium towards roots in response to this concentration can then
take place.
Although arguments abound in the literature as to the relative
importance of these two processes, it is generally agreed that when
mass-flow is restricted (such as due to moisture stress) , nitrogen
uptake due to diffusion can be significant. Furthermore, the relative
importance of mass-flow and diffusion depends upon the geometry of
the root system. Higher root densities result in shorter distances
over which the ions must be transported; hence, diffusive supply of
nutrients to roots can be substantial (Barley, 1970; van Keulen et
al, 1975). Russel and Shone (1972) have demonstrated that when part
of the intact root system of barley was exposed to solutions of
higher nitrogen concentrations than the remainder, root proliferation
was limited exclusively to that zone in which more favorable conditions
existed. Thus, higher root densities would result in a greater nutrient
uptake from the favorable zone in order to compensate for the remainder
of the root system, Jungk and Barber (1974, 1975) have also arrived
at similar conclusions based on their experiments. On the other hand.
305
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Brower and deWit (1969) have observed that root densities increased
when nutrienC supply was limited.
It is apparent from the foregoing discussion that our present
understanding of the physiology of the plant root system and its
ability for compensatory growth and uptake under conditions of nutrient
and/or water stress is limited. However, the principal factors that
govern nitrogen uptake (and that of other nutrients) by plants appear
to be: (i) concentration of the nutrient in soil-water, and (ii) the
root density (number, length, area, etc.) at a given depth in the soil
profile. Several researchers have proposed empirical relationships,
where the rate of nitrogen uptake (Q^) remained essentially constant
over a broad concentration value and exponentially decreased at low
concentrations. Such relationships are based on analogies to enzyme
kinetics and are generally referred to as Michae1is-Menton relation-
ships. A graphical representation of the dependence of on the
total amount of nitrogen is shown in Figure 4. Based upon the work
of Dibb and Welch (1975), it will be assumed here that both ammonium
and nitrate are taken up by plants. Data are presently unavailable
to determine the fractional uptake of ammonium and nitrate when both
species are present in the root zone. Hence, it will be assumed that,
the rate of uptake of each species is proportional to its fraction of
total inorganic-nitrogen. Based upon these assumptions, the following
relationships may be stated:
= Q1 + Q2 = Qmax <*)
+ T
N
Q, = Qn (_*J—) (6)
\ + V
Q2 = ( )
kn + V
where, Q is rate of plant uptake, T is total amount of a given nitrogen
306
-------
25
0.8
50
I
X
o 0.6
E
75
O
O" 0.4
max
0.2
0
100
200
300
Tn
Figure 4. Graphical representation of the dependence of the rate
of nitrogen uptake (Q ) on the total amount of mineral
nitrogen (T ) in the crop root zone.
307
-------
species, and subscripts 1, 2, and N refer to, respectively, ammonium,
nitrate, and total inorganic-nitrogen (nitrate + amnioniurn) . Also,
K is an empirical constant that determines the rate at which Q%
N N
diminishes with decreasing T (See Figure 4), and Q is the crop
N max
uptake demand for nitrogen under ideal growth conditions. Note
that Eqs. (5)-(7) allow for simulation of reduced uptake under nitrogen
"stress" conditions.
SIMULATION OF FIELD EXPERIMENTS
Case 1
NaNagara et al (1976) performed field experiments to measure
nitrogen uptake by corn during a growing season in Kentucky. In
addition to measuring nitrogen accumulation in corn plants, these
authors also collected data on root length distributions in the soil
profile, evapotranspiration water losses, rainfall/irrigation water
inputs, and nitrogen fertilizer additions to the^ crop throughout the
season. These experimental data were utilized to evaluate the
empirical relationships referred to in the previous sections of this
manusc ript.
Calculated and measured cumulative nitrogen uptake by corn during
the growing season are compared in Table 2. Reasonable agreement between
calculated and measured uptake, especially for the total during the
34-97 day period, is encouraging; the differences are small consider-
ing the gross simplifications used in performing the calculations.
The total amounts of ammonium, nitrate, and organic-nitrogen remaining
in the corn root zone, as a fraction of that present initially, during
the growing season are shown in Figure 5. These curves were calculated
using the mathematical relationships for describing the nitrogen move-
ment, transformations, and uptake. It is apparent that net mineral-
ization of organic-nitrogen was small, while most of the ammonium was
rapidly oxidized to nitrate. The total amount of nitrate in the crop
308
-------
root zone increased during the first 20 days (Figure 5) in spite of
plant uptake, suggesting that the rate of nitrification exceeded
that of nitrate uptake by the plant. However, beyond 30 days, the
amount of nitrate decreased rapidly as nitrate became the major
source for plant nitrogen due to depletion of ammonium. A total of
18.7-cm of water was received as rainfall during the corn growing
season, whereas the accumulated transpiration losses were 23.9-cm.
Thus, leaching losses of nitrogen were insignificant.
Table 2. Comparison of calculated and measured nitrogen uptake by
corn grown under field conditions (Adapted from
Davidson et al, 1977).
Growth Period
(Days)
Nitrogen uptake
(mg nitrogen/plant)
% Error
Measured*
Calculated
34-49
1435
1928
+34%
49-76
1101
1948
+772
76-97
1496
683
-54%
TOTAL
4002
4559
+14%
(34-97)
* Measured data was taken from NaNagara et al (1976).
Case 2
Watts and Hanks (1978) conducted field experiments to design
efficient management practices for irrigation and nitrogen fertili-
zation of irrigated corn on sandy soils of Nebraska. As a part of
these studies, cumulative nitrogen uptake by corn and cumulative
leaching losses of nitrogen beyond the root zone were measured
throughout the season. Three levels of irrigation were considered:
0,8 evapotranspiration (ET) (low irrigation), 1.15 ET (medium irri-
gation) and 1.5 ET (high irrigation). All treatments received 250 kg ni-
trogen/ha as ammonium nitrate (broadcast) at planting. Other treat-
309
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1.2
Management Model
1.0
Org-N
z 0.8
NH.-N
(* 0.6
i= 0.4
0.2
80
120
40
TIME,DAYS AFTER PLANTING
Figure 5. Fraction of applied nitrogen remaining in the plant root
zone of Maury soil, simulation by the mathematical
relationships, during the corn growing season (from
Davidson et al, 1978).
310
-------
merits were initiated by Watts and Hanks (1978), but only the above
will be considered in this manuscript. Nitrogen balance sheet for
the above cases for the end of the corn growing season, prepared from
measured data and simulations is compared in Table 3. The calculated
values were obtained using the previously described procedures and
processes and input data provided by Watts and Hanks (1978). The
unaccounted for nitrogen (measured data) is that necessary to obtain
a mass balance from the field measurements. Factors such as nitrogen
losses due to denitrification nnd volatilization in addition to
experimental errors associated with sampling are probably responsible
for the unaccounted-for nitrogen. Agreement between measured and
calculated values is reasonable. The mathematical relationships used
to obtain the calculated nitrogen data in Table 3 can also be used to
calculate a nitrogen balance for other times during the growing
season.
Although the procedure described and used in this manuscript to
calculate nitrogen balances during the growing season requires several
simplifying assumptions, it appears to give a reasonable first-order
approximation. The procedure requires further testing and evaluation,
but may be of some immediate benefit to field personnel not having
another alternative. An example of such use is described in the
next section. Most desk-top programmable calculators could be pro-
grammed to make the necessary calculations.
SIMULATION OF THE EFFICIENCY OF MANAGEMENT PRACTICES
The empirical relationships developed in this manuscript were also
used to simulate three irrigation application schemes in order to
examine their relative efficiencies in maximizing plant uptake and
thereby reducing leaching losses beyond the crop root zone. The soil
parameters were chosen to represent a well-drained homogeneous, and
deep sandy soil profile, while the crop parameters were for corn.
311
-------
Table 3. Comparison of nitrogen balance sheet prepared from measured data and that calculated using
the mathematical relationships described in this manuscript. Measured data are for corn
grown under three irrigation management schemes (Data of Watts and Hanks, 1978).
Low Irrigation (0.8 ET) Medium Irrigation (1.15 ET) High Irrigation (1.5 ET)
Measured
Calculated Measured
Calculated Measured
Calculated
(kg nitrogen/ha)
Initial Mineral-
nitrogen 58
Fertilizer-
nitrogen added 250
Mineralized from
Organic-nitrogen 84
Plant uptake 178
Mineral-nitrogen
in Profile at
Harvest 38
Leaching Loss 60
Unaccounted 116
58
250
86
200
132
62
(kg nitrogen/ha)
55
250
84
160
44
73
112
55
250
77
206
39
136
(kg nitrogen/ha)
47
250
84
153
26
120
83
47
250
76
186
6
182
-------
Two water management schemes simulated were: (i) natural rainfall
with no supplemental irrigation, and (ii) controlled amounts of
irrigation under no rainfall conditions. The amount of irrigation
applied was equal to or 1.5 times greater than the transpiration
demand. Irrigation was allowed only when plant available water in
the root zone was less than 60% of the total available water. The
rainfall data used as input in scheme (i) was obtained from weather
records (for May-Aug., 1974) maintained at the Agronomy Research
Farm, University of Florida, Gainesville. A single broadcast applica-
tion of ammonium nitrate fertilizer at 300 kg nitrogen/ha was applied
at planting.
The position of the nitrate pulse in the soil profile during the
growing season, simulated for three water application schemes, is
presented in Figure 6. Also shown is the progression of maximum
depth in the soil profile to which plant roots had grown. The
nitrate pulse resides well within the crop root zone during the
entire season for the case in which the amount of irrigation water
was equal to that depleted by the crop (curve labeled 1.0 ET in
Figure 6). For the case in which the amount of irrigation water
applied was 1.5 times that required by the crop (curve labeled 1.5 ET
in Figure 6), the nitrate pulse was leached beyond the root zone
after about 65 days. The intensity and frequency of the rainfall events
chosen here were such that the nitrate pulse was leached rapidly out
of the root zone very early in the season (only five days after
planting) as indicated by the curve labeled "rainfall" in Figure
6. Such observations are not uncommon in field studies involving sandy
soils in Florida. A major rainfall event of approximately 5-c.m can
move the fertilizer nitrogen essentially out of the plant root zone.
The effect of simulated water application management schemes
on the cumulative nitrogen uptake by corn is illustrated in Figure 7.
The "ideal" uptake demand for nitrogen was met under the 1.0 ET
treatments at all times. This was possible since the nitrate pulse
313
-------
120
Rainfall
80
1.0 ET
—I 40
SIMULATED IRRIGATION SCHEMES
90
120
60
30
TIME,DAYS AFTER PLANTING
Figure 6. The predicted depth of nitrate front under three water
application schemes during the growing season in a sandy
soil planted to corn. Increase in the maximum depth of
rooting (L) with time is shown as the dashed line (from
Davidson et al, 1978).
314
-------
2.4
rvj
E
u
o>
E
LlI
<
I—
Q_
Z>
z 1.6
lu
CD
O
m
^ 0.8
>
Z>
2
Z>
O
1 1 1
1 1
-
1.0 ET"~7
Management Model
ET
IDEAL
DEMAND /
-
1
RAINFALL*^
l I
40 80
TIME, DAYS AFTER PLANTING
120
Figure 7. Cumulative nitrogen uptake by corn grown in a sandy so
under three water application schemes, as simulated by
the mathematical relationships described in this
manuscript (from Davidson et al, 1978).
315
-------
resided within the root zone and was available to roots for absorp-
tion in sufficient quantities. The "ideal" demand, however, was not
satisfied for the 1.5 ET treatment; the time at which this curve
deviated from the "ideal" (Figure 7) corresponds to the time when
the nitrate pulse was leached out of the root zone (Figure 6). The
cumulative nitrogen uptake curve for the rainfall treatment deviates
significantly from the "ideal" curve, since the nitrate pulse was
beyond the root zone at all times.
316
-------
LITERATURE CITED
Barley, K. P. 1970. The configuration of the root system in relation
to nutrient uptake. Adv. in Agron. 32:159-201
Bartholomew, V. W. and F. E. Clark, Eds. / 1965. Soil Nitrogen.
Agronomy Monograph No. 10, Amer. Soc. Agron., Inc., Madison,
Wisconsin. 615 p.
Broadbent, F. E. 1970. Mineralization, immobilization, and nitrifi-
cation. National Conference on Management of Nitrogen in Irrigated
Agriculture. (This Proceedings).
Brower, R. and C. T. deVJit. 1969. A simulation model of plant growth
with special attention to root growth and its consequences. In
Root Growth. W. J. Whittington, ed. p. 224-242.
Davidson, J. M., P. S. C. Rao, and H. M. Selim. .1977. Simulation of
nitrogen movement, transformations, and plant uptake in the root
zone. Proceedings of the National Conference on Irrigation Return
Flow Quality Management, Fort Collins, Colorado, May 16-19, 1977.
p. 9-18.
Davidson, J. M., D. A. Graetz, P. S. C. Rao, and H. M. Selim. 1978.
Simulation of nitrogen movement, transformation, and uptake in the
plant root zone. Environmental Protection Technology Series.
(In Press).
Dibb, D. W. and L. F. Welch. 1976. Corn growth as affected by
ammonium vs nitrate absorbed from soil. Agron. Jour. 68:89-94.
Endelman, F. J., M. L. Northrup, R. R. Hughes, D. R. Keeney, and J. R.
Boyle. 1973. Mathematical modeling of soil nitrogen transformations.
A. I. Ch. E. Symp. Series. 70(136):83-90.
Frere, M. H. 1975. Integrating chemical factors with water and sediment
transport from a watershed. Jour. Environ. Qual. 4:12-17.
Jungk, A. and S. A. Barber. 1974. Phosphate uptake rate of corn roots
as related to the portion of roots exposed to phosphate. Agron.
Jour. 66:554-557.
Jungk, A. and S. A. Barber. 1975. Plant age and the phosphorus uptake
characteristics of trimmed and untrimmed corn root systems.
Plant and Soil 42:227-239.
Mehran, J. and K. K. Tanji. 1974. Computer modeling of nitrogen trans-
formations In soils. Jour. Environ. Qual. 3:391-395.
317
-------
Molz, F. J. and I. Remson. 1970. Extraction-term models of soil
moisture use by transpiring plants. Water Rosour. Res. 6:1346-
1356.
NaNagara, T., R. E. Phillips, and J. E. Leggett. J976. Diffusion and
mass-flow of nitrate-nitrogen into corn grown under field
conditions. Agron. Jour. 68:67-72.
Quisenberry, V. L. and R. E. Phillips. 1976. Percolation of surface-
applied water in the field. Soil Sci. Soc. Amer. Jour. AO:
484-489.
Rao, P. S. C., R. E. Green, V. Balasubramanian, and Y. Kanehiro. 1974.
Field study of solute movement in a highly aggregated Oxisol
with intermittent flooding: II. Picloram. Jour. Environ.
Qual. 3:197-202.
Rao, P. S. C., H. M. Selim, J. M. Davidson, and D. A. Graetz. 1976a.
Simulation of transformations, ion-exchange, and transport of
selected nitrogen species in soils. Soil Crop Sci. Soc. of
Florida Proc. 35:161-164.
Rao, P. S. C. and J. M. Davidson. 1978. Adsorption and movement of
selected pesticides at high concentrations in soils. Water Res.
(Submitted).
Rao, P. S. C., J. M. Davidson, and L. C. Hammond. 1976b. Estimation
of non-reactive and reactive solute front locations in soils.
Residual Management by Land Disposal. Proc. Hazardous Waste
Research Symp. Tucson, Arizona. EPA-600/9-76-015. p. 235-242.
Reddy, K. R., R. Khaleel, M. R. Overcash, and P. W. Westerman. 1977.
Conceptual modeling of non-point source pollution from land
areas receiving animal wastes: I. Nitrogen formations. A
paper presented at the 1977 summer meetings of the Amer. Soc. Agr.
Eng. at Raleigh, N. C., June 21.-29, 1977 .
Rolston, D. E. 1978. Volatile losses of nitrogen from soil.
National Conference on Management of Nitrogen in Irrigated Agri-
culture. (This Proceedings).
Russell, R. S. and M. G. T. Shone. 1972. Root function and the
soil. Proc. 11th British Weed Control Conf. p. 1109-1191.
Selim, H. M., J. M. Davidson, P. S. C. Rao, and D. A. Graetz. 19 78.
Nitrogen transformations and transport during transient un-
saturated water flow in soils. Water Resour. Res. (Submitted).
318
-------
Stanford, G. and S. J. Smith. 1972. Nitrogen mineralization poten-
tials of soils. Soil Sci. Soc. Amer. Proc. 36:465-472.
van Keulen, H., N. G. Seligman, and J. Goudriaan. 1975. Availabili
of anions in the growth medium to roots of an actively growing
plant. Neth. Jour. Agric. Sci. 23:131-138.
Watts, D. G. and R. J. Hanks. 1978. A soil-water-nitrogen model fo
irrigated corn on sandy soils. Soil Sci. Soc. Amer. Jour.
(In Press).
319
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320
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DIAGNOSTIC TECHNIQUES USED TO IDENTIFY OPTIMUM
LEVELS OF NITROGEN FERTILIZATION FOR
IRRIGATED CROPS-
2/
T. L. Jackson—
ABSTRACT
The nitrogen available for plant growth comes primarily from
1) the nitrogen released from soil humus and crop residues, 2)
nitrogen added as commercial fertilizers and 3) residual inorganic
nitrogen from previous growing seasons or previous crops.
Irrigation insures adequate moisture and relatively uniform yields
from year to year. Irrigation also makes feasible the application
of fertilizer nitrogen during the growth of the crop and this
provides the opportunity to use soil and plant analysis early in the
crop season to assess nitrogen needs.
Examples of the use of diagnostic techniques for estimating
the fertilizer nitrogen required to supplement residual nitrogen
levels and the capacity of the soil to release nitrogen are presented.
-Technical Paper No. 4818 of the Oregon Agricultural Experiment
Station, Corvallis, Oregon 97331.
2/
-Professor, Department of Soil Science, Oregon State University,
Corvallis, Oregon 97331.
321
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INTRODUCTION
The nitrogen available for plant growth comes from three major
sources. Nitrogen is released from soil humus during the growing
season. Soil humus, which contains about 5% nitrogen is the major
storehouse for nitrogen in the soil. On the average, about 2.5% of
this nitrogen is mineralized and released for growth of the crop each
year. To estimate the amount of nitrogen that might be mineralized
from soil humus we assume that a ha of land to a depth of 30 cm has
3,850,000 kg of soil. If the humus contains 5% of nitrogen the
quantity of nitrogen for each 1% humus is 1925 kg. With 2.5% of the
nitrogen released by mineralization, the crop would be supplied with
about 48 kg/ha/yr for each 1% humus in the surface foot (30 cm of
soil).
Nitrogen can be released in the decomposition of crop residues.
This release can be significant if these residues contain relatively
high nitrogen contents such as might be found when leguminous crop
residues are incorporated into the soil.
Second, there can be residual nitrogen present in the soil that
might have accumulated from 1) mineralization of humus, 2) decomposi-
tion of crop residues, and 3) applied as commercial fertilizer to a
previous crop. Nitrogen is also supplied to crops as commercial ferti-
lizer that might be applied before the crop is planted and/or during
the early to the middle of the growing season. In some areas the
nitrogen in animal manures and in irrigation waters can be important
sources.
The residual soil nitrogen is really a combination of all in-
organic sources that might have entered the soil or mineralized from
the soil. It is residual in the sense that it is there at the end
of the previous season or at the beginning of a new season. However,
this is the nitrogen we measure in most soil testing programs. It
is the "left over" nitrogen that has a potential for contaminating
322
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groundwater, and is the "soil nitrogen" that should be considered in
modifying our fertilizer recommendations and applications from year
to year.
Most soils in the irrigated areas of the western United States
were developed under arid or semi-arid conditions. With limited
vegetative growth and high summer temperatures to speed up decomposi-
tion of crop residues, the levels of soil humus are low when compared
with soils in the corn belt and other areas of the United States. The
major exception to this would be the irrigated soils west of the
Cascades in Oregon and Washington. These low levels of soil organic
matter mean that limited amounts of nitrogen are released from humus
mineralization during the growing season.
Soils where legumes have been grown in association with an
efficient strain of nitrogen fixing rhizobium would be expected to
have a relatively high nitrogen release rate but this nitrogen released
would come largely from crop residues or decaying nodules rather than
from decomposition of humus. In some situations a cover crop is
planted in the fall on land that has been heavily fertilized for in-
tensive row crop production. The cover crop helps to control winter
erosion and takes up nutrients left from fertilization of the previous
crop. If this cover crop is plowed down or incorporated into the
soil in an immature stage of growth, decomposition is rapid with re-
lease of nitrogen from the incorporated material.
This release of nitrogen from leguminous crop residues,
and from succulent (immature) cover crops, is difficult to measure.
Dahnke and Vasey (1973) reviewed procedures that require incubating a
soil sample at specified temperature and moisture conditions for a two
or three week period. Although these procedures provide a measurement
of nitrogen released, the time required to ship the sample to a labora-
tory, complete the incubation procedure, measure the released nitrogen,
and send the results back to the grower, has limited the use of this
technique as a basis for modifying application rates for nitrogen
323
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fertilizer.
Yields are high with irrigation and are generally quite consistent
from year to year. This consistency with irrigation helps to establish
optimum levels of nitrogen fertilization for a crop for different
areas of the western United States. Since many crops, especially
cereals and grasses, are fairly efficient in using nitrogen it is
possible to combine uptake data with field experiments to arrive at
reasonably accurate estimates of the nitrogen required for optimum
yield.
Irrigated agriculture offers maximum flexibility in timing of
nitrogen applications. This flexibility can be used to achieve an
optimum level of nitrogen fertilization. Irrigation provides water
in the surface of the soil to help move nitrogen into the root zone
and insures that both water and nutrients are available. Sprinkler
irrigation offers the alternative of applying nitrogen through the
water with the possibility of some uptake through the foliage,
especially with urea nitrogen, and maximum movement of nitrogen into
the root zone.
These alternatives make possible the use of a combination of
both plant and soil analyses during the growing season to measure
the supply of nitrogen available to the crop. Fertilization schedules
can be developed to supplement preplant or planting time applications
plus the soils supply of available nitrogen with applications of nitrogen
during the growing season. The level of nitrogen in the plant is then
a result of residual nitrogen, fertilizer application, and nitrogen
released from crop residues or organic matter.
APPLICATIONS OF DIAGNOSTIC TECHNIQUES
Examples are presented of the use of nitrate nitrogen analyses
of soils and plant samples (generally petioles) to determine the status
of nitrogen in the crop and thus to estimate the specific amount of
324
-------
nitrogen to add to achieve optimum yield. Applying a significant,
amount of the crop's nitrogen requirements during the growing season
reduces the possibility of leaching losses that can occur early in
the season before the root system becomes established.
Sugar Beets
Any discussion of using nitrate nitrogen analyses of petiole
samples during the growing season to adjust applications of nitrogen
should probably start with the early outstanding work of IJlrich
(1948, 1952) with sugar beets. Ulrich and Hills reviewed (.1973) in-
formation on use of plant analyses as an aid in fertilizing sugar
crops. The critical nitrate nitrogen curve taken from Ulrich's work
illustrates two important points: 1) the nitrate nitrogen analysis
of sugar beet petioles can be related to an optimum supply of
nitrogen for this crop, and 2) there is a marked decline in the
critical level of nitrate nitrogen as the crop progresses towards
maturity. These effects are more important for sugar beets than for
many crops because an excess amount of nitrogen late in the season
will stimulate vegetative growth at the expense of sugar production.
Carter, Westermann and others (Carter et al, 1971; Carter et al,
1974; Carter et al, .1975; Carter et al, 1976; Stanford et al, 1977)
refined the information for sugar beets for use in southern Idaho by
establishing information on the rate at which petiole nitrate levels
declines during the crop season. A moderate level of nitrate in the
petiole that is being supplied with subsoil nitrogen or nitrogen
being released from organic sources has the potential of maintaining
excess available nitrogen through to maturity of the crop. Roots of
sugar beets reaching subsoil nitrogen could account for the increase
in petiole nitrate nitrogen in late summer as shown in Ulrich's
work.
James (1970) and James and Johnson (1973) identified a range of
nitrate nitrogen levels in the soil profile before planting that
325
-------
could be related to the quantity of nitrogen fertilizer required to
produce optimum yield and sugar production in central Washington
and Utah. A number of fields were found that had more nitrogen in
the profile at planting than was needed for optimum sugar production;
added nitrogen on these fields decreased sugar yields.
County agents and other agronomists (personal communications)
have found 450 to 680 kg residual inorganic nitrogen/ha in the
0- to 75—era depth of soil in a number of luxuriantly fertilized fields
in central Washington and the Treasure Valley area of southern Idaho
and eastern Oregon.
Another complicating factor has been identified by Nielsen and
Banks (1960) in Utah where most of the sugar beets are grown under
rill or furrow irrigation. Water applied to a furrow which is sometimes
30 cm lower than the top of the adjacent mid-row where the beets are
growing, moved by capillary action into the middle and top of the
row. This water carries nitrate nitrogen with it and often results
in a crust of nitrate nitrogen and other salts at the highest point
of the rill ridge by mid August. Late summer thunder storms or
rains can move this nitrogen down to the root zone where it acts as
a late summer application of nitrogen that is difficult to control.
Obviously this would not be a problem with sprinkler irrigation or in
areas where rains are more frequent throughout the growing season.
Winter Wheat
The use of nitrate nitrogen in stem tissue of small grains to
predict needs for added fertilizer has been investigated by Gardner
et al (1976) and Gardner and Jackson (1976) at Yuma for barley and
by Brown and Jones (1977) for winter wheat in southern Idaho. In
these examples, the nitrate nitrogen concentration in the second
and third internodes at tillering or an earlier stage of growth
can be used as a basis for predicting the nitrogen needed by the
crop. This is a very early stage of growth to evaluate the potential
326
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supply of nitrogen for the season, especially in the work by Brown
and Jones in Idaho (1977). Three important factors contributed to
this.
1. The root system of winter wheat develops in the fall and con-
tinues to grow through part of the winter. By the time spring growth
starts, a significant amount of soil depth has been exploited by the
root system since many of these soils are relatively shallow (less
than 3 feet or 1 meter).
2. Most of the crop available nitrogen in these soils at the end of
a growing season will be in the nitrate form that is readily taken
up by the crop since soil organic matter levels are relatively low.
Thus, residual nitrogen left from potatoes, onions, sugar beets, sweet
corn and other intensive row crops will influence the nitrogen content
of the following crops at an early stage of growth.
3. Nitrogen fertilizer can be added to a cereal crop any time before
jointing, if surface moisture is available to move the nitrogen into
the root zone, without reducing the efficiency of use or the yield
increase from the applied nitrogen.
Potatoes
Potatoes have somewhat different nitrogen requirements than many
row crops since they have an indeterminate habit of growth and can be
harvested as an early crop 90 or 100 days after emergence or after an
extended growing season of 6 months. Lorenz et al (1964) and Geraldson
et al (1973) developed a relationship between nitrate nitrogen of
petiole samples and optimum yield of potatoes where harvest followed
a 3 to 4 month growing season.
Potatoes in the Columbia Basin of Oregon and Washington have a
wide range of growing seasons. Planting starts the last week of
February on many years in the lower elevations around Hermiston,
Oregon and Pasco, Washington. Russet Burbank potatoes planted as
early as March 1 are sometimes maintained in vegetative growth through
327
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October. High rates of nitrogen, 340 to 450 kg/ha, are required to
maintain a crop in a vegetative state of growth for this period of
time and to produce the yields (80 to 90 metric tons of potatoes/ha) that
have been achieved with center pivot Irrigation on sandy soils. The
shallow root system of potatoes makes it advantageous to use growing
season applications of nitrogen on the sandy soils in the Columbia
Basin, Pasco-Hermiston areas, where these practices are followed.
Consulting agronomists in this area emphasize maintaining a
specified level of nitrate nitrogen in petiole samples using appli-
cations of soluble nitrogen fertilizer through sprinkler irrigation
systems. Optimum levels of nitrate nitrogen in petiole samples for
specified dates before harvest have not been established with reliable
field research in this area. It is generally accepted that more than
adequate levels of fertilizers are being applied for production of
this crop. The nitrogen required to produce a crop of wheat is
frequently supplied as residual nitrogen after the potato crop is
harvested. This level of fertilization used undoubtedly results in
nitrogen being leached below the rooting depth of potatoes on sandy
soils.
Tree Crops
Tree fruits present a distinctly different problem in terras of
nitrogen management (Kenworthy, 1973). Trees are a perennial crop
with the same plant occupying an area for many years. This offers
the opportunity to evaluate the previous year's program and make
adjustments on the following year's nitrogen application. Also,
excessive nitrogen can delay ripening and development of color and
sugar for many fruit crops so that crop quality can be reduced with
excessive nitrogen fertilization.
Field experiments have shown that many growers were using excessive
rates of nitrogen for citrus production in southern California and
that these high rates of nitrogen reduced the quality of fruit
328
-------
produced (Embleton et al, 1974; Jones et al, 1968; Jones et al, .1970;
Jones and Embleton, 1967). Leaf analyses from these experiments have
identified critical levels that can be used to monitor nitrogen ferti-
lizer programs to achieve optimum yield and quality (Embleton et al,
1975). Excessive use of nitrogen resulted in leaching losses and
nitrate nitrogen in the drainage water flowing from some of these
orchards (Bingham et al, 1971; Embleton et al, 1974; Embleton and Jones,
1974). The results from this research have been used to modify ferti-
lizer programs to reduce fertilizer costs, improve quality of fruit
produced, and limit movement of nitrogen below the root zone of the
crop.
I have discussed four examples where plant and/or soil analyses
for nitrate nitrogen have been used to predict the optimum rates of
nitrogen fertilizers to be applied for crop production under irriga-
tion. Many other examples could be given.
Farm advisors, county agents and other field men are familiar
with specific calibration data that can be used to modify nitrogen
fertilizer application in local areas throughout the west.
A summary of this information available for California has been
published in Extension Bulletin 1979 (Reisenauer, 1976).
329
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LITERATURE CITED
Bingham, F. T., S. Davis, and E. Shade. 1971. Water relations, salt
balance, and nitrate leaching losses of a 960-acre citrus
watershed. Soil Sci. 112:410-418.
Brown, B. D. and J. P. Jones. 1977. Wheat stem nitrate concentra-
tions as related to the response of nugaines wheat to nitrogen
fertilization. Proceedings—Twenty-Eighth Annual Northwest Ferti-
lizer Conference. Twin Falls, Idaho. July 12-14, 1977. p. 63.
Carter, J. N., M. E. Jensen, and S. M. Bosma. 1971. Interpreting the
rate of change in nitrate-nitrogen in sugarbeet petioles. Agron.
J. 63:669-674.
Carter, J. N., M. E. Jensen, and S. M. Bosma. 1974. Determining
nitrogen fertilizer needs for sugarbeets from residual soil
nitrate and mineralizable nitrogen. Agron. J. 66:319-323,
Carter, J. N., D. T. Westermann, M. E. Jensen, and S. M. Bosma. 1975.
J. Amer. Soc. of Sugar Beet Technol. 18(3):232-244.
Carter, J. N., D. T. Westermann, and M. E. Jensen. 1976. Sugarbeet
yield and quality as affected by nitrogen level. Agron. J.
68:49-55.
Dahnke, W, C. and E. H. Vasey. 1973. Testing soils for nitrogen.
In R. C. Dinauer (ed.) Soil Testing and Plant Analysis. Soil
Sci. Soc. Amer., Madison, Wisconsin. p. 97.
Embleton, T. W., W. W. Jones, and R. L. Branson. 1974. Leaf analysis
proves useful in increasing efficiency of nitrogen fertilization
of oranges and reducing nitrate pollution potential. Comm. In
Soil Sci. and PI. Anal. 5(5):437~442.
Embleton, T. W. and W. W. Jones. 1974. Foliar-applied nitrogen for
citrus fertilization. J. Environ. Qual. 3(4):388-391.
Embleton, T. W., W. W. Jones, and R. G. Piatt. 1975. Plant nutrition
and citrus fruit crop quality and yield. Hort. Sci. 10(1)48-50.
Gardner, B. R., M. D. Openshaw, and T. C. Tucker. 1976. Wheat
fertilizer recommendations. Agricultural Engineering and Soil
Science Mimeograph Series 76-3. Arizona State University,
Tucson, Arizona.
330
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Gardner, B. R. and E. B. Jackson. 1976. Fertilization, nutrient
composition, and yield relationships in irrigated spring wheat.
Agron. J. 68:75-78.
Geraldson, C. M., G. R. Klacan, and 0. A. Lorenz. 1973. Plant
analysis as an aid in fertilizing vegetable crops. In R. C.
Dinauer (ed.) Soil Testing and Plant Analysis. Soil Sci. Soc.
Amer., Madison, Wisconsin. p. 365.
James, D. W. 1970. Soil testing for residual nitrate-nitrogen as a
guide for the production of high sugar yielding sugar beet crops.
Proceedings—Twenty-First Annual Pacific Northwest Fertilizer
Conference. Salt Lake City, Utah. July 14-16, 1970. p. 75.
James, D. W. and R. C. Johnson. 1973. Utah sugarbeets: Yield and
quality as related to soil tests and fertilization. Proceedings—
Twenty-Fourth Annual Pacific Northwest Fertilizer Conference.
Pendleton, Oregon. July 10-12, 1973. p. 27.
Jones, W. W., T. W. Embleton,and R. G. Piatt. 1968. Leaf analysis and
nitrogen fertilization of oranges. Citrograph 53(10):367 and 376.
Jones, W. W., T. W. Embleton, S. B. Boswell, G. E. Goodall, and E. L.
Barnhart. 1970, Nitrogen rate effects on lemon production, quality
and leaf nitrogen. J. Amer. Soc. Hort. Sci. 95(1):46—49.
Jones, W. W. and T. W. Embleton. 1967. Yield and fruit quality of
'Washington' navel orange trees as related to leaf nitrogen and
nitrogen fertilization. Amer. Soc. Hort. Sci. 91:138-142.
Kenworthy, A. L. 1973. Leaf analysis as an aid in fertilizing orchards.
In R. C. Dinauer (ed.) Soil Testing and Plant Analysis. Soil Sci.
Soc. Amer., Madison, Wisconsin, p. 381.
Lorenz, 0. A., K. B. Tyler, and F. S. Fullmer. 1964. Plant analyses
for determining the nutritional status of potatoes. In C. Bould
(ed.) Plant Analysis and Fertilizer Problems. Amer. Soc. Hort.
Sci. p. 226-240.
Neilson, R. F. and L. A. Banks. 1960. A new look at nitrate movement
in soils. Utah Farm and Home Science 21(1):2-3. Mar. 19.
Reisenauer, H. M. (Editor). Soil and plant-tissue testing in Califor-
nia. Bulletin 1879. Division of Agricultural Sciences, Univ.
of California.
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Stanford, G., J. N. Carter, D. T. Westermann, and J. J. Meisinger.
1977. Residual nitrate and mineralizable soil nitrogen in
relation to nitrogen uptake by irrigated sugarbeets. Agron.
J. 69:303-308.
Ulrich, A. 1948b. Plant analysis as a guide to the nutrition of
sugar beets in California. Proc. Amer. Soc. Sugar Beet
Technol. 5:364-377.
Ulrich, A. 1952. Physiological bases for assessing the nutritional
requirements of plants. Ann. Rev. Plant Physiol. 3:207-228.
Ulrich, A. and F. J. Hills. 1973. Plant analysis as an aid in ferti-
lizing sugar crops: Part 1. Sugar beets. In R. C, Dinauer (ed.)
Soil Testing and Plant Analysis. Soil Soi. Soc. Amer., Madison,
Wisconsin, p. 271
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ECONOMIC CONCEPTS AND POLICIES RELATED TO CONTROLLING
NON-POINT SOURCE POLLUTION STEMMING FROM AGRICULTURE
Norman K. Whittlesey and Paul W. Barkley—
ABSTRACT
This paper describes the role of economics in solving non-point
source pollution problems in terms that are understandable for the
noneconomist. An economist attempts to watch over the process of
converting natural resources into marketable commodities to assure
that the process achieves the greatest possible good for members of
society. Economic tools are well equipped to handle this process
for goods and resources known as private property, those items used
exclusively by the individual owners. Unfortunately, most pollution
problems occur through the use and misuse of public property, those
items owned and used jointly by all members of society. Air and water
are the most common examples of public property.
Economic theory of a firm or factory assumes that society uses
well defined signals to tell the producer how much of any product
to produce. Market prices are the signals sent from the consumer to
the producer. The system is imperfect, however. There are residuals
of nitrates, smoke, dust, and noise which are external results or by-
products of activities carried out in normal production processes.
Tl^ese residuals or "externalities" are not reflected in the prices
derived by our market system. The result is often to over-produce
both consumer goods and pollution. The paper describes economic con-
cepts and public policies that are useful for internalizing the costs
—^Professors in the Department of Agricultural Economics, Washington
State University, Pullman, Washington 99163.
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caused by pollution and abating or solving pollution caused problems.
The concepts of opportunity cost and income distribution and their
role in pollution abatement are discussed. Methods of choosing levels
of abatement include the extremes of zero pollution and that which can
be achieved by best management practices. The economic and social
optimum probably lies between these extremes. Arbitrary standards
are sometimes imposed as a compromise between the extremes of zero
pollution and no abatement in an effort to approximate optimal levels
of abatement.
INTRODUCTION
Economic activity is the process of turning nature into products
that satisfy human needs and wants. Sometimes this is done directly.
Wind is harnessed to pump water for an isolated farmstand. Other
times, this is done in a roundabout way. Ore is mined, smelted, and
cast into pigs. The pigs are shipped, melted, stamped, rolled, shaped,
and eventually become an automobile. Although the trip from the
Mesabi range to Detroit and then to the local dealer is very complex,
each step requires that at least some of nature become a part of man's
activities. It becomes part of the production, exchange, and consumption
that characterizes a modern industrial society.
This would be only mildly curious except for two things. First,
nature is finite, but man's wants seem not to be, Man makes a claim on
nature. This claim enables water or rock or timber to be turned into
economic goods, but in most cases the transformation of nature into
economic goods is an irreversible, one time process. Once the first
transformation has been made, the total available stock of natural
endowments has been reduced. One man's incursion on natural resources
limits the activities of all other persons because resources are
limited in supply. A second problem arises in connection with those
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few natural endowments that appear to be free. Air, sunlight,
moving water, and some public lands are free to all who want to use
them. They acquire this cost-free characteristic because they are
ridiculously plentiful or because it would require unmanageable
expense to harness these resources and make the users pay for them.
The fact that they are free has made these resources subject to
tremendous abuse. Smoke is emitted into (free) air; sewage is
dumped into (free) rivers and lakes; hot dog wrappers are thrown on
the (free) grass in the public park.
Smoke, sewage and hot dog wrappers went largely unnoticed until
fairly recent times. True, in the late 14th century, King Edward III
prohibited dumping garbage into the Thames because those wastes pre-
vented ships from coming up the river to the city of London. This was,
however, an unusual event and Edward III stood alone among monarchs.
The removal of smoke, sewage, and hot dog wrappers is a very recent
phenomenon. Until (perhaps) 1960, man lived so close to nature that
to try to clean up the mess was a luxury he could ill afford. Since
then, the view has changed. Economic activity always impinges upon
and often impairs the natural world. If society is to protect the
natural world, it must actively intercede on behalf of the unutilized,
underutilized, and endangered resources.
Agriculture is a smug but difficult industry when cast in this
milieu. It is smug because it knows that its prime task is to turn
nature into meat, vegetables, and fruit. It uses resources but is
generally proud of its record of "wise use" and its infrequent incur-
sions into the domain of others. Feedlots have smelled and have had
their wrists slapped, but the individual farmer has not worried
about polluting anything or bothering anybody.
All this is changing. It is now known that many fertilizers and
pesticides and other items applied to agricultural land can slip into
surface or underground watercourses and cause damage to the water
and its uses. It is known that small - even miniscule - amounts of
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soil erosion can bring sediment damage, allergies, or flooding hundreds
of miles away. Unlike industrial enterprises which can be pointed to
and told to mend their ways, agriculture produces unidentifiable
sources of pollutants that have come to be called non-source point
pollution. Developing the means to deal with any type of pollution
is difficult; dealing with the non-point variety is almost impossible.
This paper is designed to show why this is so. After a brief opening
comment regarding the task of the economist, the paper turns to prop-
erty rights, some lessons in economics, some hypothesized methods for
pollution abatement, and a plan to use in attacking non-point source
problems.
THE ROLE OF THE ECONOMIST
The primary concern of the economist is the allocation of scarce
resources among competing uses. An economist attempts to watch over
the process of converting nature into commodities and to make sure
that the process achieves the greatest possible good. This "greatest
good" is related to efficiency and to the distribution of economic
rewards among members of the community. The tools of economic analysis
are especially well equipped to study and make policies related to the
class of goods or resources known as private property. Private prop-
erty - goods or resources - can be traded and take on value (prices)
that reflect their relative worth in creating welfare. There is,
however, a large class of resources that are not susceptible to restrict-
ive property rules and are known as common property resources, or,
simply, "community property". Common property includes those resources
that are not exclusively controlled or owned by an individual and,
hence, can be used by everyone.
It is often remarked that "everyone's property is no one's prop-
erty" and this phrase illustrates the economist's concern when con-
fronted with the many problems of the environment. If common property
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is present, everyone will use it; no one will pay for it, and no one
will take the trouble to care for it. Environmental and ecological
problems arise in such a setting. One individual, in striving to
use common property to achieve his objectives, may cause detrimental
effects to be imposed on others. Such a phenomenon is known as an
external diseconomy. While society is generally well equipped to
handle the private uses of private property, it has not developed
adequate rules for using common property or for handling the external
diseconomies that arise from such use. Understanding this problem re-
quires a thorough investigation of property rights.
PROPERTY RIGHTS
Property rights are the rules that say who can do what with land,
capital fixtures on the land, and nearly all classes of natural
2/
resources— . These rights are established by society and at least
partially determine the extent of environmental problems that will
surround a particular class of resources. They also provide many clues
about the means of solving problems of environmental degradation.
Unfortunately, the rules by which we operate today were designed
in a period in which there was an abundance of common property and
very little concern about how one person's use of a resource would
affect the value of that resource to others. As time has passed, pressure
on resources has increased and the earlier laws have become obsolete.
This is especially true in cases in which the distinction between
common property and private property has become blurred. Actually,
these are merely the extremes on a continuum ranging from the most
mobile, exclusive private property item (such as a toothbrush) to those
resources held in common by all people in an economic society - air,
- - -
— Property rights also apply to non-land items such as automobiles,
furniture, and the right to work. The discussion here is limited
to that class of resources that is found in nature,
33 7
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radio signals, and Yellowstone National Park.
-rivate Property
The private property rights conferred upon the owner of a tooth-
brush provide him with the privilege of exclusive use and the right to
transfer the use of the toothbrush to anyone else whenever the transfer
becomes advantageous. In other words, an individual's right to an
item can be traded to someone else for an agreed upon value. Generally,
the value of the privately held item is determined in the market place.
Society may impose rules that restrict the use of the item, but the
individual who holds title to the item is the only one who can use
the item even within the bounds of the law. For example, a shotgun
is private property and (usually) one person has exclusive rights to
its use. Society has, however, imposed many rules that restrict use
of that gun. In general, the restrictions are designed to ensure the
safety of those who are near the gun owner. Since safety has differ-
ent meanings in different places, the rules for the use of this
private property item may be changed over time or space by the will
of society.
Economic theory is extremely useful for explaining the values and
the methods of exchange for private property items. If all activities
of all people were carried on exclusively through private property,
the number and magnitude of environmental problems would be greatly
diminished. The market would measure the positive value (utility)
derived from the use of that good and it would reflect the damage to
others caused by one person's use of the same good. Unfortunately,
the market is not up to this task and a great deal of disutility (loss
of value) is created by the use of private property items such as
automobiles when they emit exhaust fumes or fertilizer when residues
from the fertilizer are carried into someone else's property by
return flows.
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Common Property
Common property rights appear at the opposite end of the spectrum
from private property rights. They are not owned exclusively by one
individual and are therefore nontransferrable. Truly common rights
are vested equally in each citizen, and allocation of common property
resources among individuals becomes problematic. Many common property
rights are allocated on a simple availability or first come-first
served basis. Two general types of services flow from common property
rights. One set is subject to limited individual choice, while the
remaining set of services cannot be selected or controlled by the
individual. An example of the former would be sailing, water skiing,
or fishing services available from a publicly owned lake. Individuals
may choose to utilize the wind for sailing or the water for fishing or
skiing. If they are rational, the choosers will participate in the
activity up to the point where their marginal gain in satisfactions
equals the marginal costs of participating in the activity. Climatic
conditions are an example of common property resources which are not
subject to individual choice. When it snows on me, it snows on my
neighbor, too. Or, to use a technologically induced example, both
increased rain and increased sunshine may occur as a result of seeding
clouds. But no person within the affected area can choose how much of
each he will take. He must accept the same rainfall and sunshine as
everyone else.
There is no market where users of common property resources can
interact and negotiate to their mutual advantage. The relative good-
ness or badness of this resource use manifests itself in ways other than
money prices. If one person's use of a common property resource serves
to damage the incentive or reduce the profits of a second person, the
second person has absorbed an external diseconomy and this will be
reflected in the value of his other resources. The familiar kinds of
pollution yield this effect. If one person's use of a common property
resource serves to increase the incentive or income of a second, the
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second person will have absorbed an external economy. An external
economy occurs when one farmer's drain system also lowers the water
table on his neighbor's land.
If the rights to common property resources were rationed or sold
in orderly markets, the affected parties would bargain with one
another over how much of each should be used and what kinds of activi-
ties they should conduct. Since no such market exists, external effects
abound and add to the problem of environmental degradation. Environ-
mental degradation, or pollution, occurs when one user of a common
property resource neglects to consider the fact that others may also wish
to use the resource. Since there is no price for the user to pay, he
uses as much as he pleases without regard for others. The market fails
and the economist as well as the planner is left with few rules on
what can or should be done. The whole concept of resource allocation
loses its meaning and no one can say unequivocally that too much or
too little is being used or that too much or too little is being pro-
duced .
ECONOMIC CONCEPTS
Economists bring a whole set of tools into the study of the above
problems. The classical theory of the firm is probably the most
prominant. The theory of the firm assumes that society uses well
defined signals to tell the producer how much of any product should
be produced. Prices serve to guide resources into their most benefi-
cial uses and the prices of factors or production and goods accurately
represent the contribution each makes to social welfare. If knowledge
is perfect, even the level of pollution will not exceed society's
desires. Total welfare is optimized when each producer's profits and
each individual's satisfactions are maximized.
This hypothetical model presumes that either all inputs are
fully converted into outputs with no residuals created or that the
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environment's capacity to assimilate such residuals is unlimited.
Alternatively, the model assumes that every price automatically takes
into account the effects of residuals or pollutants. The homemaker
who buys beef in Fresno is assumed to be paying for the damage caused
by odors near the meat packing plant in Omaha. Obviously, those
hypothetical conditions do not hold in the real world. Residuals are
not completely absorbed and homemakers are not fully compensated.
Externalities
An economist describes these imperfections as "externalities".
Residuals, pollution, smoke, and noise are external results or by-
products of activities carried out by an individual or firm in the
process of maximizing his welfare or its profit. A farmer will strive
to maximize profits by combining land, water, fertilizer, labor and
sunshine to produce crops in an optimal and efficient way. In the
process the farmer may create externalities such as air pollution
(dust or odors) or water pollution (sediment and nutrients) which can
harm or reduce the welfare of others, but the farmer's instinct tell
him to maximize his own profits without regard for the welfare of his
neighbors. He presumes that maximizing his own welfare will help to
maximize the welfare of society and since all the neighbors harbor
similar thoughts, each attempts to maximize profits and each disregards
the damage he, himself, is imposing on others.
Externalities or pollutants are not all alike. They may be classi-
fied as either pecuniary (priced or having market defined value) or
technical (nonpriced). Pecuniary externalities are those that occur
when the actions of some individuals or firms result in price changes
for others. For example, when Eugene, Oregon, drops raw sewage into
the Willamette River, it imposes pecuniary externality on landowners
who live downstream. Technical externalities are of a different kind.
They affect the welfare of others, but not in an economically measur-
able sense. Developing a very noisy airport may preclude the installa-
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tion of a school or a hospital on .surrounding territory. The airport
creates a technical barrier but its effects are not measurable in
economic terms.
Opportunity Cost
Because many externalities are not easily measurable, it is neces-
sary to devise elaborate schemes to determine their impacts. The
economist relies heavily or. the concept of opportunity cost for this
purpose. Briefly defined, the opportunity cost of a resource is its
value in its best alternative use. Water used for irrigation may
yield a profit of $15 per acre foot. Tf the same water were diverted
to generate power, it might have a value of $10 per acre foot. This
$10 becomes the opportunity cost of water used for agricultual pro-
duct ion.
This concept can be used to assess the impact of pollution that
reduces the recreational value of a Lake. In this case, the value of
the lost recreation is measured by the costs required to develop alter-
native means of recreation. Construction and maintenance of swimming
pools, travel costs to more distant lakes for boating, fishing, or
u - .
water skiing may be involved-- .
The opportunity costs of resource uses affected through external-
ities can be used to measure pollution costs and to help determine
whether or not pollution abatement programs should be undertaken.
Although the opportunity cost is the key to damage assessment, the
distribution of externality damages among members of society is also
useful in rendering opinions about allocating the cost of pollution
abatement among society's members. In a sense, the size and distri-
bution of opportunity costs become substitutes for the true economic
3_
— Values may also accrue to the lost recreational opportunities that
are irreplaceable or unaffordable through conventional alternate
means. The sum of these values becomes a part of the opportunity
cost of the lost recreation, but they defy actual measurement at
this time.
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values of pollution costs and they become a standard against which
the benefits of pollution abatement can be judged.
Costs can be described with respect to their magnitude, their
distribution, and who bears them. The total costs of pollution can
be high or low, but the real test comes in deciding who is responsible
for them. If my chickens get out and ruin your garden, the question
of distribution of costs is easily settled. If the return flow from
irrigation in the Upper Colorado River drainage causes damages through-
out the desert southwest, the matter becomes more complex.
Income Distribution
In general, costs occurring because of my chickens and your garden
can be measured as the change in income of welfare that you and I re-
ceive. Your income is reduced; mine is increased (because I do not
have to buy feed for the chickens). In some cases, it is possible to
add the changes in income of individuals to derive an estimate of
total costs and benefits that stem from a particular activity in-
cluding such activities as polluting or pollution abatement. In other
cases, the costs may be so dispersed that no private entities are
susceptible to significant change. In those cases we must introduce
some arm of government and ask it to become involved in setting and
enforcing standards, collecting taxes, redistributing wealth; and
carrying out other activities that are beyond the scope or management
of individuals.
We are not here to discuss my chickens in your garden, but, rather,
the more complicated case occurring when many pollutors each contribute
an unknown amount of pollutants that eventually affect an unspecified
number of people. This is not an easy task, but one thing is certain:
No private entity has enough knowledge, enough power, or enough incen-
tive to undertake the task. If the problem of non-point source pollu-
tion is to be solved, the solution must come through group activity
sponsored by the government. The example of farmers in the Upper
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Colorado River Basin provides a vehicle for discussing several aspects
of the problem.
Assume for a moment that farmers in a river basin are polluting the
river because their irrigation return flows carry residuals from fer-
tilizers that have been applied to their lands. The government may seek
to reduce pollution by imposing a tax on either the fertilizer, the
water, or both. As a result of the effective increase in the price of
these inputs, farmers will use less of each and in so doing will reduce
the volume of pollutants reaching the river. This reduction In
pollutants will help meet our objective since now the river Is "cleaner".
The cleaner river does not, however, come without cost. Several things
follows from the individual farmer's decision. Reducing the use of
inputs will reduce the amount of product and this in turn will reduce
farmers' net incomes. The aggregate reduction of net income for all
farmers in the basin becomes one measure of the costs of pollution
abatement.
In the process of reducing pollution, however, society has collected
a new tax from the farmers who continue to apply fertilizer and water.
Revenues from this tax can be redistributed to other members of society
to help compensate for the reduction in water quality. Thus the net
(or total) effect of a taxing program becomes the sum of lost net farm
income minus the new tax revenues. A problem still remains, however,
because no one can be sure just how the tax rates should be set or just
how the tax revenues should be distributed.
The government can take a second approach to the problem. It can
choose to pay (bribe) farmers not to pollute the river. The bribe
induces the farmer to reduce the use of pollution inputs, but it maintains
farm incomes by transferring general tax revenue to the farmers. The
direction of income transfers in this case is opposite from the flow
in the case of a tax program. Which is better? No one can say for
sure.
A third method of control calls for the government to impose a
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legal limit on the quantity of offending materials that can be used.
This limit would be enforced through threat of fines or jail sentences,
and there would be no tax or transfer of payments involved. Since
farmers would still have to comply with the rules, they would exper-
ience a reduction in net farm income. The river would be clean, but
one group would be made economically worse off, while some elements of
society would benefit from a cleaner environment. Unfortunately, in
none of these cases can we be sure that benefits exceed costs.
It now becomes clear that the means of controlling pollution -
especially the capricious non-point source pollution - should be chosen
with some care. The choice should be tempered by some careful reflection
regarding the distribution of the costs and benefits accruing to each of
the alternative plans. The effectiveness of a pollution control program
may appear vastly different depending on the vantage point. A tax
penalizes the pollutor and provides revenue. A bribe asks society to
buy out the offender. A law reduces the income of the offender and
the amount of product available to society. It is not clear which means
of control is most desirable in non-point source cases.
DETERMINING THE "CORRECT AMOUNT" OF POLLUTION CONTROL
Most U.S. citizens will agree that the territory that composes the
United States is now much too heavily polluted. Most would similarly
agree that something should be done about the problem. There is little
agreement, however, on how much should be done because no one knows the
magnitude or distribution of costs and benefits associated with each
level of pollution or each level of control. One thing is certain:
Each of the several methods of control will, if left to itself, yield
a different amount of control. A tax may be 60% effective in removing
pollutants (and 100% effective in reducing the pollutor's incomes.').
A bribe may be 40% effective and a law can be as effective as the
administrative unit that makes the law and the policy unit that enforces
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it. Interested and affected parties in a given non-point source
pollution case will probably choose different criteria depending on
whether they are on the side of the polluters or on the side of those
who are injured by the pollution. The decision is thus arbitrary.
This section will describe some of the more evident means of setting
abatement levels or environmental standards for agricultural pollution
k,
problems—.
Zero Pollution
Zero pollution is not a means, but an end. Zero pollution is men-
tioned here because of the large outpourings of interest surrounding
the nation's capacity to rid itself of all forms of pollution. Zero
pollution is an extreme position advocated hy a small number of
zealous individuals and a smaller number of environmental groups. Tt
is questionable whether zero pollution - as conventionally defined -
can be met in nature let alone under the conditions imposed by an
industrial society. Indeed, air quality standards for western Colorado
were recently set so high that they were occasionally violated by
natural conditions. Rules insisting on zero pollution are not generally
taken seriously because most people realize that there is little room
for economic activity in such cases. We know of no serious conversations
that could lead to a zero pollution standard for that segment of agri-
culture that produces non-point source pollution.
The Economic Optimum
Common sense indicates that there can be too little or too much
pollution. If these two extremes can be identified, there must also
be some amount that is "just right" and that amount is the economic
4/
— The standards to be met by non-point source polluters in agriculture
have not yet been determined. Because of this, one can find a wide
range of views regarding the "appropriate" standards which should be
set. At this point, no particular view can be judged to be either
right or wrong.
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optimum. In a perfect world where all effects and all costs are
measurable, the optimum is defined by extending pollution control
activities to the point where the added dollars of expenditures or
control are just offset by the added dollars in benefits. This is a
familiar rule in all of economics and it has a common sense interpre-
tation: Keep doing something as long as it pays.
One can imagine a large industrial plant dumping waste into a
river. The water is subsequently taken from the river to be used for
a municipal water supply. Pollution abatement procedures could be
initiated to force the plant to clean up its own mess. The plant would
pay and the downstream city would benefit. As long as increased costs
of abatement were covered by dollar savings to the city, society could
recommend that more and more pollution abatement capacity be added to
the plant - marginal costs would be exceeded by marginal benefits.
The method of approach, however, breaks down in a non-point source
pollution case. Although it is known that an optimum exists, the physi-
cal and economic data appropriate to that optimum are largely unknown.
It is not known whether one of the hundreds of polluting farmers could
solve the whole problem if he were to discontinue all activities that
contribute to the problem. Nor is it known whether or not a 10%
reduction in all polluting activities would reduce pollution by 10%.
Because of this lack of knowledge, the economic optimum cannot be pre-
cisely determined. All that is known is that it is more than zero;
less than present levels. Policy makers are forced to choose one of two
approaches to the problem. They can recommend that all farmers use
the best possible management practices to achieve an acceptable level
of pollution control or they can set arbitrary standards and attempt
to enforce these by law, taxes, bribes, or combinations of these.
Best Management Practices
Those working closely with agriculture often claim that the only
realistic pollution standards for the agricultural industry are those
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that can be met by applying the best available management practices to
each farming enterprise. This is a peculiar position to take. It
requires that farmers utilize the best method of combining inputs to
yield a profit maximizing output. It says nothing whatsoever about
the relationship between economic activity and pollution. This is a
"leave-things-alone" position that suffers from over-simplification.
There are some problems with this kind of reasoning. Recommending best
management practices (BMPs) always begs a question: Best with respect
to what? There are BMP schemes that relate to water, to land, to the
farmer's labor, or to the farmer's credit rating. They are not neces-
sarily the same and, since they are not the same, each best practice
will yield a different pollution loading for a stream or for the atmos-
phere .
It is likely that in most cases the application of BMPs would improve
water quality or environmental conditions affected by agriculture. In
the ferti1izer/return flow case, BMPs might require the farmer to adopt
a method of water application that resulted in lower return flows.
These lower return flows would carry smaller amounts of fertilizer
residuals back to the stream so the stream would be cleaner; the increased
parity coming as a by-product of improved resource management. Although
BMPs can be recommended on efficiency grounds, there is no guarantee
that these practices would produce a socially desirable amount of pollu-
tion control. There also remains the problem of how to get farmers to
adopt a specific set of management practices, particularly if the
practices increase production costs or reduce Income. Otherwise, one must
assume that the "good" farmers are not non-polluting and that others
should merely follow in their footsteps, a position that is difficult
to justify.
Arbitrary Standards
Although the word "arbitrary" has a terribly unscientific and non-
technical ring, it is often the only way to approach pollution abatement.
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It will remain so until data systems are improved. But "arbitrary"
is indeed tempered by reason so arbitrary rules are not often to be
confused with ridiculous rules. Agencies charged with the responsi-
bility for improving the environment are frequently left with few
rational criteria for setting standards. They roust strike a balance
between affected parties demanding complete abatement and polluters
who provide convincing arguments for continuing current activities.
The result is often a bounded set of arbitrary standards that may be
designed to prevent further degradation of a local environment or designed
to meet certain health related criteria for water or air quality. The
final set of standards will probably depend upon some combination of
the attitude of the enforcement agency, the relative bargaining power
of affected parties, the apparent seriousness of the pollution prob-
lem, and the knowledge and tools available for alleviating the problem.
Using arbitrary standards brings some disadvantages, but has some
merits, too. Since conditions are likely to vary considerably from
one geographic area to another, a general, or nationwide, standard would
not solve many problems. It would be too strict for some areas; too
lenient for others. This is, of course, a disadvantage since each area
must develop its own standard and that requires time, effort, and
money. It also carries the distinct advantage of requiring each
affected area to study carefully its own problems and this necessary
element of local citizen input is quite likely to make the eventual
resolution more palatable.
Even using arbitrary standards for pollution abatement, there is
still the problem of how best to achieve the chosen standard. The
choice of the abatement enforcement tool will ultimately determine
the costs of abatement and the distribution of those costs among
society's members.
349
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DISCUSSION
The problem of pollution has become an important fact of the U.S.
political economy. It is a serious technical problem and a serious
economic problem. It has become a political problem and an emotional
problem for some individuals and some groups. There is little doubt
that this industrial society has the technical means to create a
pollution-free environment or, at least, a tolerable environment.
The question remains: Do we want to? That question is not easily
answered.
Most pollution problems arise because of ambiguous rights in
property or market failures. An industrial plant dumps smoke into
the atmosphere because no one has "rights" to the air so no one can
order the plant to quit using that resource. A homeowner can irk
his neighbors by mowing his lawn at seven o'clock on Sunday morning.
He can continue to do this because there is no organized market in
which the neighbor can buy silence - the market for this service has
failed to develop.
An industrial polluter or a residential lawn owner can be controlled
through fairly straightforward means. Zoning regulations, pollution
control devices, laws, economic incentives, and peer pressure. The
problem becomes extremely difficult when the source of the pollution
is not easily recognizable and when those who must endure the lower
quality environment are a public group who may have nothing in common
except the fact that they are imposed upon by the same pollutants
coming from the same source. Although traffic noise and auto emissions
in a metropolitan area have some characteristics of non-point pollution,
attention is now focussing on agriculture.
U.S. agriculture is a highly intensive industry that uses complex
sets of inputs to produce huge volumes of foodstuffs. The industry,
especially as it is organized in the West, has been held up as a
picture of efficiency and a model for other parts of the nation and
350
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the world. Only recently have environmentalists begun to point to
agriculture to indicate that some environmental problems may arise
in that sector of the economy. Insecticides and herbicides may be
applied in such a way as to produce wind-born residuals that affect
cities many miles away. Fertilizers and other inputs applied to
irrigated land may be leached into streams and eventually pose a
threat to aquatic life or produce new forms of equatic life that
interfere with the traditional uses of the river or stream. This
type problem is insidious and, although it is justifiably being
noticed, the solutions to the problem are often equally insidious.
This non-point source pollution can be ended by stopping agricultural
production or at least reducing output to pitiably small levels.
It may be stopped by generating a new class of inputs that leave
fewer residuals or are less amenable to transport into other areas or
into common property resources. In any event, the structure of the
industry will change in response to rules about input use. The new
rules will also change the output mix of agriculture, the size and
distribution of income within the industry, and the relationship be-
tween agriculture and the local and national economies.
Many people view these kinds of problems as technical. That may
be true, but the economist has a stake in these things, too. The
economist's specialities include studies of cost, efficiency, demand,
and income distribution. An economist can make refined recommendations
about how much fertilizer should be applied, how much hay should be
fed, and whether or not the cows should be milked by hand or by
machine. This is so because each of these activities centers on the
use of exclusive, private property and the technical behavior as well
as the market behavior of each of these inputs is well known.
The economist is not so well equipped when it comes to studies of
non-point source pollution. By definition, fingers cannot be pointed
and blame cannot be assigned. The production of the pollution is not
well understood and its effects can be only estimated. Nonetheless, the
351
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economist should be invited to work with both the technical expert
and the policy maker. The economist can tell the technical expert
something about the costs of each activity and lie can make judgments
regarding how these costs might be distributed among suspected
polluters or beneficiaries of a clean-up program.
The best work of the economist will come from his alliance with
the policy maker. Previous sections of this paper indicate that
there is a certain amount of arbitrariness associated with pollution
control policy. Regardless of this arbitrary characteristic, the
policy will always divide society into gainers and losers. A tax will
impose costs on one set of peopLe; a bribe will cause others to bear
the cost. An arbitrary allowable level of pollution will cause still
another arrangement of distribution of gains and losses. The econo-
mist is trained to study these and his expertise should not be over-
looked .
In sum, pollution, degradation, and despoliation are problems
stemming from technical activities. It is reasonable that most
solutions to the problems be sought by investigating technical rela-
tionships. Thus, the engineers, the 1imnologists and ecologists, and
the foresters must play important roles in assisting to solve the
problem. But one major—perhaps the major—consequence of a despoiled
environment is a rearrangement of social relationships existing among
individuals and groups. It seems only reasonable to expect the econo-
mist to become involved in these problems and their solution.
352
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SELECTED REFERENCES
Baumol, W. J. 1972. On taxation and control of externalities.
Am. Econ. Rev. 62:307-322.
Baumol, W. J. and W. E. Oats. 1971. The use of standards and prices
for protection of the environment. Swedish J. Econ. 73:42-54.
Coase, R. H. 1960. The problem of social cost. J. Law and Econ. 3(0ct)
1-44.
Dick, Daniel T. 1976. The voluntary approach to externality problems:
A survey of the critico. J. Environ. Econ. and Mgmt. 2:185-195.
Freeman, A. M., R. H. Haveman, and A. V. Kneese. 1973. In The Economics
of Environmental Policy, John Wiley and Sons, Inc. New York.
Gossett, D. L. 1975. The economics of changing the water quality of
irrigation return flow from farms in central Washington. Unpub-
lished Master's thesis, Dept. of Agr. Econ., Washington State
University.
Horner, G. L. 1975. Internalizing agriculture nitrogen pollution
externalities: A case study. Am. J. Agr. Econ. 57:33-39.
Kraft, D. F. 1975. Economics of agricultural adjustments to water
quality standards in an irrigated river basin. Unpublished Ph.D.
thesis, Dept. of Agr. Econ., Washington State University.
Langhan, M. R. 1972. Theory of the firm and the management of residuals.
Am. J. Agr. Econ. 54:315-322.
Miller, W. L. and J. H. Gill. 1976. Equity considerations in controlling
non-point pollution from agricultural sources. Water Res. Bull.
12:253-261.
Pfeiffer, G. H. 1976. Economic impacts of controlling water quality in
an irrigated river basin. Unpublished Ph.D. thesis, Dept. of Agr.
Econ., Washington State University.
Randall, Alan. 1972. Market solutions to externality problems: Theory
and practice. Am. J. Agr. Econ. 54:175-183.
Rose, Marshall. 1973. Market problems in the distribution of emission
rights. Water Resources Res. 9:1132—1144.
Wenders, J. T. 1975. Methods of pollution control and the rate of
change in pollution abatement technology. Water Resources Res. 11:
393-396.
353
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T)4
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A CASE STUDY-
NITRATES IN THE UPPER SANTA ANA RIVER BASIN
IN RELATION TO GROUNDWATER POLLUTION
R. S. Ayers—''
ABSTRACT
In response to a request from the Santa Ana Watershed Planning
Agency (a regional planning agency) the Kearney Foundation of Soil
Science of the University of California conducted an interdisciplinary
3-month study of the nitrate problem in the Basin. The study was
limited to the upper part of the basin where nitrate degradation of
waters was more serious.
The study included 1) a review of available data to identify
existing areas of high nitrate concentrations in underground waters,
2) a review of past land use, water and fertilizer use and waste dis-
posal practices, and 3) estimates of the impact of irrigation, fertili-
zation and use of animal wastes on leaching of nitrate from root zones.
Guidelines for the use of water, fertilizers and manures were developed.
—^Extension Soil and Water Specialist, Department of Land, Air and
Water Resources, University of California, Davis, California 95616.
355
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INTRODUCTION
Nitrogen is present in a number of areas in the underground waters
of California. In several instances it exceeds E.P.A. and U.S. Public
Health drinking water standards. Two previous reports had discussed
the problem of nitrate in groundwaters in specific nreas to a certain
extent (Stout et al, 1965; California State Department of Water
Resources, 1968). Considerable publicity was generated in 1966-67 when
two of the nine city wells at Delano, Kern County, California were re-
ported to have nitrate concentrations well in excess of drinking water
standards. Following these and other similar reports from scattered
areas of the state, interest in nitrates in groundwaters in California
increased. From published well water analyses (DWR Bulletin 130-series)
a statewide distribution map of nitrates was prepared for use in Coop-
erative Extension to alert our extension audience to the problem of
nitrates in the state's underground waters.
Since agriculture is a large user of nitrogen and nitrogen was
known to be present in the groundwater, it seemed to much of the public
that this was a clear cut case of agricultural pollution and should be
stopped. Clearly those of us working closely with agriculture needed
answers. We needed immediate answers to questions that had not prev-
iously been asked - at least not in the very loud, demanding voice of
1970-71.
THE SANTA ANA PROJECT
One such area with apparent nitrogen problems in the ungerground
waters was the upper Santa Ana River Basin of Southern California. It
had a concerned planning agency - the Santa Ana Watershed Planning
Agency (SAWPA) - which was charged with planning for complete water
resource management. This agency had identified nitrogen as a
problem and wanted some sort of evaluation as to its extent, probable
356
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cause, and some ideas as to how to correct or adequately manage
the nitrates to prevent further degradation. The underground waters
are the primary source for water users in the upper basin and waters
that pass from the upper basin through the Prado dam become recharge
water to the underground, the primary water supply for Orange County
and its nearly two million people.
The agency asked for help from the University of California (UC) -
Cooperative Extension and Experiment Station - through personal contact
with the UC Riverside Extension Soil and Water Specialist. He, in
turn, enlisted the help of others and together with SAWPA informally
worked out the details of an agreement as to what IJC could do to help
SAWPA. A formal request was submitted by SAWPA asking the Kearney
Foundation of Soil Science (a statewide organization within UC) to
make a study on "Nitrates in the Upper Santa Ana River Basin in Relation
to Groundwater Pollution".
The Kearney Foundation agreed and a group of interested Extension
and Experiment Station staff were invited to participate. No reimburse-
ment from SAWPA was involved except that some unstated "Grant-in-Aid"
might be forthcoming at the conclusion of the project. The Kearney
Foundation was to sponsor and fund the project, which it agreed to do.
The effort was considered to be an interdisciplinary learning process
in which UC would bring its resources to bear on a problem for a short
time to seek answers based primarily on existing data with only a very
limited amount of development of new data.
After an initial meeting the group met about every two weeks over
a three-month period to report on progress, ask questions, and assign
tasks. The study group included a truly interdisciplinary group. In-
cluded were the following from Cooperative Extension: 2 Soil and Water
Specialists, 1 Irrigation Specialist, 1 Agricultural Engineer, 1
Agricultural Economist, and 1 Staff Research Associate. From the
Experiment Station: 2 Hydrologists, 4 Soil Scientists, 1 Systems
Analyst, 1 Sanitary Engineer, 1 Sub-tropical Horticulturist, 2 Oleri-
357
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culturists, 1 Soil Microbiologist, and 1 Water Scientist.
What Did We DoJ?
1. Nitrogen sinks and sources were? identified.
Various large scale world wide nitrogen sinks and sources were
quantified. Then these were narrowed down to the Upper Santa Ana
basin and schematica]ly presented as nitrogen pools and fluxes,
figure 1.
2. Locations of high nitrate concentrations in groundwater were identified,
Figure 2, in which the concentration contours are in mg nitrate/1.
Water analyses covering the basin were available from the
California Department of Water Resources dating back as far as
1919. These were entered on base maps to identify nitrate "hot
spots". Changes in nitrate concentrations in both underground and
surface streams also were evaluated.
3. History of land and water use and fertilizer rates and waste dis-
posal practices surrounding each of the "hot spot" areas were
investigated.
Crop acreages in some instances had changed over the years
but in general land use stayed about the same. Fertilizer rates,
however, had changed. Rates for citrus had been greatly reduced -
attributed primarily to the adoption of plant tissue analysis as
a management tool to improve efficiency. In contrast, fertiliza-
tion rates for vegetables had increased dramatically.
Sewage effluent, sludge, and even primary sewage had been
disposed of in the immediate vicinity of the two primary nitrate
"hot spots". Since this study was to evaluate agricultural in-
puts, we did not attempt a more detailed study of the effects of
sewage disposal.
One great change in the basin nitrogen loading had been the
influx of dairies and dairy animals into the Chino Basin. At the
time of the study 130,000 dairy animals were confined within an
J 58
-------
C Losses
t.2
32
ATM N POOL
1.25 x 109 tons
4.0
C 6ains ^
Interbasin Transfers
\&ecip. 21
ir
5.0
7Y
ISmjlE
Waste,N Fart.
17.4
LAND
SURFACE
N POOL
29 x 10 tons
t* J
Combustion
Gas Losses
Return Flow
SURFACE
WATER
N POOL
5 "
5.9X10 to
Plant Uptake
SOIL
N POOL
Gas Losses
2.1x10 tons
Infiltration
Subsurface Flow
SUBSTRATA
N POOL
. _ 40x10 tons
—
Water Table
GROUNDWATER
P»mf» N POOL
Total = 58xio*tons.
5.0 Satd. Zone =mio3, *
Unsaid. Zone=8.7xio3«
Groundwater Flow
NITROGEN POOLS AND FLUXES
Figure 1. Nitrogen pools and fluxes in the 356,000-acre upper Santa
Ana Basin based upon the 1960 level of development. The
mass of nitrogen in the pools is in tons of nitrogen;
Tiuntbers near the arrows represent the flux of nitrogen
between pools in thousands of tons of nitrogen/year.
359
-------
(
J
w4 aAakij
( C
C-- , / •
pOtfOH* ^ J
s-< -
S/1
\*rcy<
( ' ' ' I ' ' i
f _./l\ {
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r
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. 1
NITRATE STUDY Ail as
Figure 2. Mao off the Santa Ana Basin showing locations of areas of
MrIi filtrate concentration. (Courtesy Mater Resources
Engl iimts, Inc.)
360
-------
area of roughly 12,500 acres which included dairies and surround-
ing farm land available for disposal of dairy manure and bairn
washings. This equates to about: 10 cows/acre.
Groundwater hydrology was studied in relation to nitrates.
Data from existing reports showed that the groundwaters were,
for the most part, unconfined and in 'hydraulic contact with the
ground surface. Horizontal water velocities were slow lit the areas
of high nitrate (1 mile in 10 years to 1 mile in 300 years), and
since the rate of vertical movement of nitrates through the non-
saturated soil before it reached the water table was also slow,
this suggested that the high nitrates found may have reflected
changes in surface nitrogen loading that occurred over a span of
tens or perhaps hundred of years,
Three soil maps were prepared showing general soil associations anil
water transmission classes and their potential for deoitrifieatioa,
in general, nitrate concentrations were higher in groundwaters
located below coarse textured good agricultural soils (loamy sands,
aaridy loams) under intensive agricultural, use. But, not ail such
areas bad high nitrates.
It was suggested that soils of fine texture (clay loam and clay)
may offer a useful potential for denitriflcation.
Water requirements and. relative efficiency of use for several Im-
portant crops were studied.
For citrus, water use efficiency for surface irrigation varied
from about 44% to 60% (LP « 481 to 251) and efficiency did not appear
to be appreciably better tinier spr i <»k U.rs.
Nitrate concentrations in soil f»r *H J« - wi«t u studied-in relation
to agricultural practices. This wr; new data.
Fertilization of citrus at a rate of 1 '.() pounds ait ro^en/-irre/
year applied annually resulted jm -t nil ».»t» nfs ro^-is cAr«t»ntrat w<«
of 19 stg/i in soil watrr b«s'..vv.» t he r«»(n .< i .h <• of <£>u f>. .t..od? u i t *
gen/acre/year re'5«lt**d In a . r.ti inn -m njv/l. '"ftj/ih at
>61
-------
sampling was 50 to 100 ft or to depth of water table. In general,
the higher nitrate concentrations were associated with lower
leaching fractions.
On vegetable crops, annual nitrogen rates varied from 100 to
1740 pounds nitrogen/acre. Nitrate concentration below the crop
tended to increase as nitrogen rates increased, but factors such
as variations in water application, denitrification, and crop
removal as well as other factors often caused Jess nitrate to
leach than expected. Because of these other factors nitrogen
losses under vegetables were not as consistent or predictable as
nitrogen losses under citrus. For example, for vegetables, the
nitrate nitrogen concentration in soil water at the 11- to 50-foot
depth ranged from 8 to 121 mg/1, whereas application rates varied
from 100 to 1740 pounds/acre/year.
8. Nitrates in soil water and groundwater below the dairy farm area
of the Chino Basin were measured by deep soil borings. Again,
this was new data.
The amount of nitrate varied widely among sites. On the
average, however, total nitrogen/acre for profiles through the
19-foot depth showed 1938 pounds of soluble nitrate nitrogen under
the corral, 670 pounds under pasture, and 727 pounds under crop
land fertilized with 20 to 45 tons of manure/acre/year. Nitrate
nitrogen concentrations in the soil water of these same areas
averaged 91, 74, and 65 mg/1. In contrast the water at top of
the water table averaged 56, 74, and 45 mg/1 and deep well waters
in the same vicinity averaged 6 mg/1.
This data suggested that the dairy wastes had not yet had
full impact on the quality of water in the saturated zone (water
table).
A typical balance sheet for estimating the average nitrate nitrogen
concentration of the water leaving the root zone under typical double-
cropped manure disposal areas was prepared based on the number of
362
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cows/acre. In the calculations we assumed that 1) the nitrogen defecated
was 0.4 pounds/cow/day, 2) 50% of the defecated nitrogen was lost to
the atmosphere by volatilization of ammonia, 3) the nitrogen uptake
by crops followed the usual pattern, and 4) that the drainage or per-
colating water was 15 inches/year (See Table 1).
Table 1 shows a nitrogen balance sheet for estimating the average
nitrate-nitrogen concentration in soil water leaving the root zones
under pastures and croplands. One-half of the nitrogen excreted is
assumed to volatilize as ammonia, and the other half is incorporated
into the soil. (Our analysis showed about 38% total nitrogen loss
from corral manures, but more losses can be expected after land appli-
cation) . The excess in the soil is the amount incorporated into the
soil minus removal by crops. This assumes that an amount of nitrogen
equivalent to the amount of manure nitrogen incorporated into the soil
each year was mineralized each year. This assumption is valid only
after several decades of applications of manures and was probably
justified only to obtain a rough idea of the amounts of nitrogen
going into various sinks in relation to cow density. The rough estimates
are the volume of drainage water, and the extent of denitrification and
mineralization. The drainage volume used was 15 inches, which corres-
ponds to an average leaching fraction (LF) of about 0.30, which is a
common value for successful irrigation management. (Leaching fraction
is the volume of water that moves past crop-root systems, expressed as
a fraction of the total irrigation water used). The values for the
25% loss column were calculated on the assumption that one-fourth of the
excess nitrogen is nitrified and then denitrified. Compared with some
of the published data this assumption seems reasonable (Tisdale and
Nelson, 1966).
Average nitrate-nitrogen concentrations in soil water at depths
of 10 to 19 feet under the pasture and cropland sites were respectively
74 and 65 ppm. The concentrations, however, ranged from 24 to 196 ppm
in pastures and from 15 to 210 ppm in croplands. Only 3 of the 18 sites
363
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Table 1.
Total nitrogen input
concentration in the
excess nitrogen.
in soils, removal
soil water in the
by crops, excess in the soil, and calculated nitrate
unsaturated zone, assuming two levels of loss of
Number of cows
Total
Amount of
Amount of
Excess
Nitrate-nitrogen in unsaturated
per disposal
nitrogen
nitrogen
nitrogen
nitrogen
zone
assuming*:
area
excreted
(all cows)+
incorporated
in soil
removed
by crops+
in soil
0% loss of
excess
nitrogen
25% loss of
excess
nitrogen
-pounds/acre/year mg/1
3
438
219
190
29
8.4
6.3
4
584
292
240
52
15.1
11.5
5
730
365
270
95
27.8
21
6
876
438
290
148
43
32
8
1,168
584
320
264
77
58
10**. . . .
1,460
730
350
380
110
84
12
1,752
876
370
506
148
111
14
2,044
1,022
380
642
187
140
** The values for the nitrogen rate equivalent to the 10-cow level were based on 200 pounds nitrogen
removal by 30 tons/acre of corn silage plus 150 pounds nitrogen removal by 4 to 5 tons/acre of
green crops. In all cases, double cropping was assumed.
+ Assuming that each cow defecates 0.40 pounds nitrogen/day.
+ Figures based on published data and field data of Extension Specialists in the area.
* In soil water in aerated zone below the root system, assuming the drainage volume to be 15 surface
inches/year.
-------
had nitrate-nitrogen concentrations of 20 mg/1 or less in soil water at this
depth. These variations were obviously due to differences in dis-
posal rates, crop removal, leaching losses, and denitrification.
Comparing these data with data in Table 1 we can see that some of the
rates apparently exceeded the equivalent of 14 cows/acre.
Guidelines Suggested to Prevent Further Degradation
Rates of Fertilization. Because nitrogen is necessary for
acceptable crop yields, the use of nitrogen fertilizers should be con-
tinued. For citrus the nitrogen should be applied in a single applica-
tion using leaf analysis as a guide to the quantity to apply. Foliar
feeding for this crop holds some promise for reduced nitrate leaching.
For vegetable crops maximum yields require from 120 to 300 pounds of
nitrogen/acre depending on the crop. Split nitrogen applications are
recommended.
Use of Manures. Manures should be applied well ahead of planting
and disced or otherwise incorporated into the soil. With small seeded
crops an irrigation should be scheduled between incorporation of the
manure and planting of the seed. For a number of crops a starter appli-
cation of nitrogen fertilizer should be used. Manure should not be
applied to growing crops. Washwater and liquid wastes should be applied
at rates and at a time consistent with crop demands for water and
nutrients.
Water Management. Excessive leaching losses can be reduced by re-
ducing the amount of either the quantity of nitrogen applied or the
volume of water applied. Overirrigation tends to reduce the concen-
tration of nitrate in the drainage water but it decreases the effi-
ciency of both nitrogen and water use. Nitrogen rates needed for
adequate yields can be reduced (nitrogen use efficiency increased) by
improving the efficiency of irrigation. Crop needs for water can be
calculated or measured in various ways. Evapotranspiration can be
used as a guide to irrigation, to estimate use efficiency and to
365
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estimate probable quantities of drainage water. Chloride balances
can also be used to estimate the leaching fraction and the drainage
volume.
Agricultural Waste Management. Wastes from agricultural opera-
tions (mainly dairies) should not be allowed to run-off into surface
water. They should be used on cropland at rates consistent with re-
cycling of nutrients and the excess should be exported from the basin.
At the time of the study waste disposal practices were not
adequate. Specific proposals and suggestions were 1) the wastes from
no more than three dairy cows should be used/acre to reduce nitrate
leaching to a reasonable level, 2) restrict disposal of manure on
lands of low underground pollution potential, 3) holding ponds to
contain storm run-off should be sufficiently large to hold the run-off
from a 10-year frequency storm, 4) each producer shoilld be required to
prepare a satisfactory plan far use or disposal of wastes, 5) zone the
watershed for pollution potential based on soil characteristics,
6) relocate nitrogenous waste producers to areas with minimal under-
ground pollution potential, 7) recharge underground with high quality
water, 8) develop shallow wells for agriculture and deep wells for
municipal supplies, 9) consider zoning a portion of the basin as a
waste acceptor and recover water and nitrate for use in agriculture
and 10) as a last resort, construct a "brine line" for collection,
treatment and export of wastes from the basin.
366
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LITERATURE CITED
Ayers, R. S. and R. L. Branson (editors). 1973. Nitrates in the Upper
Santa Ana River Basin in Relation to Groundwater Pollution.
CaJif. Agr. Exp. Sta. Bui. 861. 59 pages.
California State Department of Water Resources. 1968. Delano Nitrate
Investigation. DWR Bui. No. 143-6. 42 pages.
Stout, P. R., R. G. Burau, and W. R. Allardic.e. 1965. A Study of the
Vertical Movement of Nitrogenous Matter from the Ground Surface
to the Water Table in the Vicinity of Grover City and Arroyo
Grande, San Luis Obispo County. Research Report of Univ. of
Calif. Dept. Soils and Plant Nutr., Davis, Calif, to Central
Coastal Reg. Water Qual. Control Bd. 51 pages.
Tisdale, S. L. and W. L. Nelson. 1966. Soil Fertility and Fertilizers.
New York. MacMillan Co.
367
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368
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AN ECONOMIC METHODOLOGY FOR EVALUATING
"BEST MANAGEMENT PRACTICES" IN THE
SAN JOAQUIN VALLEY OF CALIFORNIA
Gerald L. Horner, Daniel J. Dudek, and Robert B. McKusick—
ABSTRACT
Section 208 of Public Law 92-500 (The Federal Water Pollution
Control Act Amendments of 1972) requires the preparation of areawide
waste treatment management plans. Agricultural related pollutants
such as subsurface drainage water and irrigation tailwater contain-
ing nutrients, sediment and pesticides must be identified and
procedures and methods to control such discharges specified. Under
Section 208, the reduction of pollutants may be achieved by adopting
a set of "best management practices" that could include varying
economic incentives, establishing resource use controls and suggest-
ing public resource investment. To determine the economic and
environmental impact of "best management practices", a methodology
must be derived that specifies the relationship of agricultural
production practices to water and land quality and the economic cost
and benefits of changing those practices. Such a methodology is
being developed to systematically collect and organize physical and
economic data, specify the physical and economic relationships and
estimate the changes in agricultural production, income, employment
and resource demands from proposed "best management practices".
— Agricultural Economists, Natural Resource Economics Division,
Economics, Statistics, and Cooperatives Service, U.S. Department
of Agriculture.
369
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INTRODUCTION
The purpose of this paper is to: (1) establish the need for a
comprehensive analytical system to evaluate water quality policies
affecting irrigated agriculture; (2) delineate alternative "best
2/
management practices" (BMP)— for irrigated agriculture; (3) discuss
economic variables and relationships needed to evaluate Areawide
Waste Treatment Management (208) plans; and (4) suggest a methodology
and analytical system to evaluate BMPs and 208 plans for irrigated
agriculture.
The Federal Water Pollution Control Act Amendments of 1972
(PL 92-500) and the Clean Water Act of 1977 suggest that water resource
development, land use planning and environmental policies be coordi-
nated, integrated and updated in a continuing planning process. This
process is required by each state under two types of area specifica-
tions, designated and nondesignated. Designated areas are urban-
industrial communities and the principle focus of the planning process
is to coordinate non-point and point source waste disposal controls.
Local planning agencies, and associated governments are responsible
for developing designated area 208 plans.
Nondesignated areas require the identification of "agriculturally
and silviculturally related non-point sources of pollution including
runoff from manure disposal areas and from areas used for livestock
and crop production" and the specification of "procedures and methods
to control, to the extent feasible, such sources" (Section 208 (b)
(2) (F)). The formulation of nondesignated area plans is the direct
2 /
— "Best Management Practices", are specified in PL 92-500, Section
208 as methods of achieving the comprehensive planning objectives
in non-designated areas. BMPs are defined in the more limited
sense to mean physical adjustments in the productive process or
resource capacity to change the environmental condition.
370
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responsibility of the state government. The Federal Government has
allocated $600 million dollars in the next two years through the
Culver Amendment to cost share applications of BMPs in those areas
that will have the most significant effect on water quality.
Section 303 of PL 92-500 requires that the state adopt a contin-
uing planning process that is consistent with all provisions of the
Act. This process is designed to insure that the initial plan formu-
lated under Section 208 remains effective under changing environ-
mental conditions. In addition to its planning responsibility, each
state must prepare an estimate of: (1) the environmental impact;
(2) the economic and social costs necessary to achieve the objectives
of this act; (3) the economic and social benefits; and (4) the date of
achievement (Section 305 (b) (1) (D)).
Section 304(k) (1977 Clean Water Act) authorizes the Administrator
of EPA to enter into agreements with other federal agencies to utilize
their authorities and programs to support the development and imple-
mentation of state and local water quality management plans. This
section recognizes that implementation of many state and local pro-
grams is dependent upon the expertise and support of other federal
agencies. Traditionally, most agricultural programs have been conser-
vation or production oriented. The extent that past conservation and
production practices will become "best management practices" is a
subject of current debate.
At the present time, a clear set of alternative "best management
practices", implementation procedures and evaluation criteria do not
exist. To systematically evaluate 208 plans and BMPs, definite alter-
native strategies and evaluation criteria must be specified prior to the
formulation of an analytical system. The list of alternative BMPs
and economic parameters specified in this paper is not intended to be
complete but it does include some practices and economic implications
not currently being considered by 208 planners.
The BMPs implementation incentives and the institutional arrange-
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ment for 208 plan implementation are interdependent. The selection
of a particular control strategy must be a combination of practices
and incentives tailored to the geographical setting, the pollution
problem and the responsible institution (Bower et al, 1977).
ALTERNATIVE BEST MANAGEMENT PRACTICES
Best Management Practices can be specified for agriculture,
silviculture, industrial or municipal operations to decrease the im-
pact of non-point sources of pollution on land and water resources.
The BMPs discussed here are limited to irrigated agriculture and the
impact of irrigation return flows on agricultural land and ground and
surface water supplies. The BMPs for irrigated agriculture can be
broadly classified into four categories: (1) reducing the quantity or
improving the quality of irrigation return flows; (2) treating or re-
using irrigation return flows; (3) improving irrigation return flow
disposal practices to minimize the environmental impact; and (4)
increasing the capacity of the receiving resources to assimilate the
irrigation return flows.
REDUCING IRRIGATION RETURN FLOWS
Reduction of the quantity or improving the quality of irrigation
return flows by modifying cultural practices has received most of
the attention of point and non-point pollution control institutions.
One control practice is to change the amount of inputs used in the
agricultural production process. Reducing the. amount and increasing
the quality of irrigation water, fertilizer and chemicals are prime
examples of procedures to reduce the toxic chemicals, nutrient content,
sediment content, salinity content, and volume of irrigation return
flows. These general relationships are fairly well established as
pollution control practices (USEPA, 1973 and USDA-USEPA, 1975).
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Another method of improving return flow quality is to change the
method and timing of water, fertilizer and chemical applications to
either maintain the same production with less inputs or improve pro-
duction with the same amount of inputs. Both procedures will generally
reduce the environmental impact of irrigation return flows.
Changes in the cropping pattern or type of agricultural use can
drastically affect the amount of polluted water entering a watercourse.
In areas possessing high soil erosion potentials, row crops and
orchards could be replaced with crops that provide better surface pro-
tection. Replacing row crops with irrigated or non-irrigated pasture
would be an extreme but perhaps necessary land use change to reduce
erosion to an acceptable level.
TREATING OR REUSING IRRIGATION RETURN FLOWS
Once irrigation return flows are produced, they can be reused on
the field where they were generated, reused on the farm, reused within
the irrigation district or region, treated to remove contaminants or
disposed of in the watercourse. The reuse of surface irrigation return
flows is currently practiced in most areas in the west where it is con-
venient to do so. Many economic and institutional questions regarding
the rights of irrigators currently relying on irrigation return flows
will need resolution before extensive recycle programs can evolve as a
workable solution. Reusing irrigation return flows to cool electric
generation plants and/or to maintain wildlife habitats is currently
occurring in areas of the Central Valley of California. The possibility
of expanding these uses exists.
The amount of irrigated land requiring subsurface drainage is
projected to increase over the next fifty years (U.S. Department of
Interior, 1969). The quality of irrigation return flows over time
will depend on the proportion of subsurface to surface drainage water.
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In the early 1970's, the California Department of Water Resources,
the U.S. Bureau of Reclamation and the Environmental Protection
Agency explored the physical and economic possibility of removing
nitrogen and salts from subsurface irrigation return flows. Results
from that study indicate that removing most of the nitrogen from
return flows cost about $44.00 per acre foot and removing salts would
cost about $100.00 per acre foot (1971 Dollars) (California Department
of Water Resources, 1971).
IMPROVING RETURN FLOW DISPOSAL PRACTICES
Watercourses receiving irrigation return flows have a natural
capacity to assimilate nutrients, chemicals and sediment without
impairing subsequent use or enjoyment of the water. However, when
the natural assimilative capacity is exceeded, conflicts among the
firms or persons affected by the decrease in quality usually occur.
Changing the spatial or temporal distribution of return flow dis-
posals will take advantage of the assimilative capacity of the environ-
ment without affecting agricultural production. This change in dis-
tribution can be accomplished in areas such as the San Joaquin Valley
by collecting return flows with high pollution concentrations and
storing them for release in high natural flow periods or transporting
them to areas that have sufficient assimilative capacity.
INCREASING THE ASSIMILATIVE CAPACITY OF THE RECEIVINC WATERCOURSE
A good example of increasing the assimilative capacity of the
environment was demonstrated last year when releases from the State
Water Project were made to decrease the salinity concentration of
the Delta. Releases of stored water in normal rainfall years to
achieve minimum water quality standards is one alternative to
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accomodate increased irrigation return flows.
INCENTIVES FOR IMPLEMENTING BEST MANAGEMENT PRACTICES
The states face a formidable problem in implementing control
{strategies for non-point pollution. Most methods to control point
sources consist of "end-of-pipe" treatments. Most non-point pollution
problems are not amenable to treatment and those that are cannot be
associated with any one emitter. Consequently, responsibility for the
cost of treatment is difficult to assign. There are many alternative
implementation incentives and they vary in effectiveness, costs and
institutional and political requirements. Incentives include both
positive and negative values to producers and resource owners. The
proper set of incentives and control methods is dependent on the type
of agriculture and pollutant. Five classes of implementation incentives
relating to irrigated agriculture are delineated by Bower (1977) .
They are regulatory, economic, administrative, judicial and educational.
INCENTIVE BY REGULATION
Many point source pollution programs employ regulation to control
one or more aspects of the productive or waste emission process.
Regulations are usually effective in achieving point source pollution
standards but the same success may not be realized in non-point control.
The relationship between changes in the productive process and water
quality is usually lagged by a substantial period of time and is diffi-
cult to measure because of numerous uncontrolled factors.
The specification of cultural practices in irrigated agriculture
to reduce return flows would consist of limiting the amount or type
of productive inputs, limiting the timing and method of polluting
activities and limiting crops that generate unusually large amounts
of pollutants. Other specifications might include land use controls,
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collection and separation of subsurface drainage water, implementa-
tion of structures to contain or reduce the pollution potential of
return flows and limiting the quality of irrigation return flows for
disposal. These specifications are well known to farmers and environ-
mental planners. However, some of the institutional and economic
considerations of regulation may not be so well known. These will be
elaborated in the discussion on evaluation criteria.
ECONOMIC INCENTIVES
In contrast to regulating agricultural activities, financial aids
and penalties which apply to emission of pollutants can potentially
achieve an environmental standard at a lower cost than regulations
because the farmer has a choice of actions. A charge on pollutants
discharged to a watercourse allows the discharger to reduce emissions
if the cost of doing so is less than the charge. However, the
effluent charge system is difficult to apply to non-point polluters
as emissions from each farmer have to be measured. Even in point
source control programs, charges on pollution emissions have never
received the political support required for implementation in the
United States.
Financial incentives can also be placed on the use of specific
inputs. Reducing the nitrogen content of irrigation return flows may
be accomplished by taxing fertilizer. Farmers, if faced with a sub-
stantial increase in fertilizer costs, could employ more efficient
application and timing techniques to reduce fertilizer use and
maintain production.
Increasing the efficiency of irrigation water conveyances,
delivery and application systems will generally reduce irrigation
return flows and mass emissions of nutrients, sediments and salts.
High water use efficiency is usually observed where water prices
are high or water allocations closely approximate crop requirements.
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In recognition of this fact, the Governor has established a commission
to review the Water Rights Laws of California. The purpose of the
commission is to suggest any needed legislation which would improve
the water allocation system. Lee (1977) has explored the possibility
of allowing water rights to be transferred by a competitive market.
The effect of such a scheme would be a general increase in the value
of water in those areas that are currently experiencing low prices.
Gardner and Fullerton (1968) concluded that allowing water transfers
reduced the risk of insufficient supplies, increased effective water
supplies and resulted in a more efficient use of water resources.
Recent research suggests that increasing the price of water will
have a dramatic impact on reducing the quantity and improving the
quality of irrigation return flows. In a study conducted in the Yakima
Basin of Washington, Pfieffer (1976) concluded that increasing the
price of Irrigation water was the least costly method of achieving
temperature, sediment and nutrient standards on the Yakima River.
Horner and English (1976) concluded that increasing the price of sur-
face irrigation water would reduce the total salt loading and quantity
of return flows in the San Luis Drain.
A bill recently introduced in Congress reflects this same theme
(Miller et al, 1977). The bill proposes that a base volume of water
be established for areas using Federal water based on crop require-
ments. Additional water supplies in excess of the base volume would
be available at increased block rates.
Land use directly affects the demand for water resources and the
quality of water within a region. One example of using economic in-
centives to achieve a desired land use is a preferential property tax
system. The California Land Conservation Act (Williamson Act) employs
a use-value assessment mechanism to base taxes upon income rather
than market value. The nature of non-point pollution from an agri-
cultural area differs from that from the urban setting and different
controls need to be considered in the planning process.
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PROVIDING INCENTIVES THROUGH EDUCATION
Programs to extend knowledge about reducing pollution can be an
effective method of reducing irrigation return flows if the proposed
actions are in the best interest of the irrigator. Convincing a
person that his actions will benefit society and yet cost him money,
is usually a difficult task for the best extension person. However,
a substantial portion of the pollutants currently found in irrigation
return flows result from irrigators not managing their systems as
efficiently as possible. These systems could be improved at low costs.
JUDICIAL INCENTIVES
Many of the present public policies and the administration of these
policies regulating the environment are a result of previous judicial
decisions. Recently a court decided that an environmental impact state-
ment will be required on the proposed rules to enforce the 1902 Reclama-
tion Act. In other cases, "cease and desist" orders against polluters
have been granted to injured parties. Although the courts do provide
an alternative implementation incentive, many disadvantages exist.
First, courts are usually slow in rendering decisions and given the
appeals process, damages from emissions may not be stopped soon enough.
Second, the persons being damaged by pollution are not usually suffi-
ciently organized to bring court action.
CRITERIA FOR EVALUATING BEST MANAGEMENT PRACTICES
The approval of the Section 208 plan and the related BMPs will
depend on several criteria. The approval mechanism provides organized
groups and individuals with the opportunity to participate in the final
review of the plan. That review process is expected to require diverse
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sets of information. The following criteria were suggested by Bower
et al (1977) specifically for evaluating alternative environmental
strategies.
ENVIRONMENTAL OR PHYSICAL EFFECTS
The degree to which an environmental strategy can improve the
overall quality of resources is the ultimate criteria. The quality
of resource will affect other physical conditions such as improved
wildlife habitats, outdoor recreation opportunities or decreased
water treatment costs. Irrigation return flows either terminate in
surface watercourses or underground aquifers. Measuring the quality
and quantity of return flows prior to being incorporated with the re-
ceiving waters would be an appropriate measure of control method
effectiveness but it would not determine the degree of improvements in
the quality of the watercourse. The relationship between changes in
irrigated agricultural practices and the quantity and quality of
irrigation return flows is not completely known for a very broad set
of physical conditions. Therefore, a certain amount of guesswork
will be involved in determining appropriate BMPs. Even less is known
about the relationship between quantities and qualities or return
flows, overall water quality, and damages to fisheries, wildlife or
persons using the watercourse. These relationships need to be measured
before control strategies can be correctly evaluated.
ECONOMIC EFFECTS
Direct Benefits and Costs
Direct benefits from improved water quality can be measured in
reduced water treatment costs, increased value of a fishery, reduced
medical costs, and reduced damages to property. Direct costs are
those incurred by farmers as a result of the control method. These
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include the costs of capital, labor, land, management, water and re-
duced production should it result from the controL method. Other
direct costs include the cost to the public agency of providing
facilities to reduce return flows or to improve the assimilative
capacity of the environment.
Indirect Benefits and Costs
The modern industrial economy is highly interdependent. Reductions
or expansions in a given industry will result in changes in the amount
of resources purchased from other industries and supplied to other
industries. Depending on the degree of interdependence, the indirect
effects can exceed the direct cost and benefit to a single industry
in terms of income, employment and resource use.
Administrative Costs
Implementing control strategies usually requires additional account-
ing, monitoring, reporting, supervision, enforcement and management.
These additional costs vary substantially according to the type of
control method selected and they are usually borne by public agencies and
irrigators alike. The importance of administrative costs is usually
not considered in deciding on a course of public, action. In some cases,
the public and private administrative costs of a program could exceed
the public benefits derived from the program (Seckler, 1965).
The Distribution of Benefits and Costs
Persons or firms benefiting or bearing the costs of a pollution
program should be identified by geographical area or socioeconomic
group. Horner and Dudek (1977) concluded that increasing the cost of
irrigation water to reduce irrigation return flows reduced the income
to small farmers proportionately more than large farmers. Bower et
al (1977) characterize the distribution of benefits and costs as the
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most important physical and economic effect of pollution control.
However, measurement of the distributional effects of public policies
is difficult due to the confidential nature of personal income and
asset data.
Ease of Administration
An effective control strategy must be flexible enough to accomo-
date changing economic conditions. Included in this criteria is the
ability of the method to self adjust to changing conditions. The
ability of one control method to be applied to various pollution para-
meters and activities is a desirable characteristic. The procedural
ease of adjustment and application is also important. For example, a
permit system applied to irrigation return flows requires considerable
effort and time to file applications and establish monitoring positions.
Political Considerations
The 208 plans require approval from a diverse set of interest
groups. Each interest group representative will probably weigh the effects
of each BMP on the goals of the interest group. This approval process
suggests that compromise BMPs will probably result. The politics of
governments may also be important. The responsibility for the
Sacramento-San Joaquin River Delta water quality is a case in point.
Although the Bureau of Reclamation and the State Water Project use the
Delta to transport water to the San Joaquin Valley and Southern Califor-
nia, the Bureau does not accept responsibility for maintaining the level
of quality as the State does. In addition, the institutional arrange-
ment and legal framework may not exist to allow some control strategies
to operate. The establishment of new institutional structures represents
an important political decision requiring general support of the public.
The last, but certainly not the least, political consideration is the
acceptance of the program by the general public.
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Interdependence of Land and Water Quality
The National Water Commission's call for closer ties between
water resources planning and land use planning and their subsequent
integration with water quality policy is explicitly recognized by
PL 92-500. Currently, opportunities exist for the integration and
simultaneous completion of several water resource planning activities.
Water and related land resource planning is required under the Water
Resources Planning Act (PL 89-80), the Watershed and Flood Prevention
Act (PL 83-566), the Colorado River Basin Salinity Control Act (PL 93-320),
and the Federal Water Pollution Control Act Amendments of 1972
(PL 92-500). In particular, PL 92-500 requires planning by states
(Sections 303 and 305), river basins (Section 209) , and areas within
states (Section 208). These individual planning requirements are not
currently integrated or coordinated. Section 208 of PL 92-500 pro-
vides an opportunity to integrate water quality planning with water
resource development and land use planning.
Current and future conflicts in resource policies must be iden-
tified. One potential policy conflict is between the development of
agricultural preserves of prime agricultural land and non-point source
controls. Agricultural preserves could concentrate production and
thus compound non-point source pollution output. However, the environ-
mental impact of the point source discharges from urban development
may be greater. There could also be a conflict between maintaining
agricultural lands for food production and improving the economic
viability of agricultural sector (through tax advantages), open space,
and non-point source pollution. These issues would involve a conflict
between national and regional economic objectives and environmental
quality objectives.
Other questions relate to the relationship between productive
capacity and projected demands facing the agricultural sector. One
report concludes that the "consideration of adverse environmental
impacts from agricultural production activities may ultimately prove
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to be the major constraint on resource development in the next 24
years" (USDA-Economic Research Service, 1976). Crosson (1975)
amplifies these concerns in a consideration of the kinds of technology
that may be used to generate increased agricultural production. He
classifies the technological alternatives as either land-using or
land-conserving. Land-using technologies result in loss of wildlife
habitat and soil loss due to erosion with resulting increases in turbid-
ity and phosphate fertilizer transport. The environmental damages
associated with land-conserving technologies are increased loadings of
fertilizers and pesticides.
THE METHODOLOGY OF EVALUATING BEST MANAGEMENT PRACTICES
Resource and environmental planning cannot be separated. Planning
for resource use without recognition of the environmental goals or
objectives of society may result in resource allocations which are socially
suboptimal. Similarly, planning for environmental quality without
assessing the suitability, availability and productivity of the resource
base may impair economic efficiency and distribution of output.
The economic information required for resource planning and evalu-
ation can be discussed under four interrelated headings; commodity
demand, commodity supply, resource demand and resource supply. Given
the planning setting, the necessary set of information and the analy-
tical models to be employed, it is useful to see how the basic economic
concepts of commodity demand and supply and resource demand and supply
will be operationalized. These concepts can readily be identified as
flows of information between different components of the analytical
system as indicated in Figure 1. This analytical system is being
developed for the San Joaquin Valley research project funded by the
Environmental Protection Agency and the Economics, Statistics, and
Cooperatives Service, USDA.
3 83
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PROJECTIONS
MODEL
RESOURCE DEMAND
LAND
USE
MODEL
REGIONAL
PRODUCTION
MODEL
RESOURCE SUPPLY
WATER
QUALITY
MODEL
Figure 1. Interactions among components of the irrigated agri-
cultural analytical system.
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The demand for regional commodity production is projected from
estimates of changes in population, income and U.S. trade policies.
The production model determines the amount of land and productive
inputs to achieve the projected level of commodity demand which in
turn is effected by land use policy decisions. The specific location
of commodity production and resource use is determined by the water
quality model and projections of the amount and quality of irrigation
return flows result. Alternative BMPs can be simulated in the appro-
priate model to determine the effect on the criteria just discussed.
Each model will be explained below in more detail.
COMMODITY DEMAND
The process begins with the specification of commodity demand
from the projections model. In order to accurately portray changes
in the irrigated crop economy of the valley between periods, pro-
jections of changes in technological coefficients, yields and the
demands for crop commodities facing the region are needed. Particularly
crucial to the determination of resource problem effects is the pro-
jection of commodity demands. While there is a wide variety of pro-
jection approaches varying with the quantitative techniques employed,
the area of interest and their ultimate application, the basic approach
employed is that reported by King et al (1977).
Essentially, projections of national demand are disaggregated to
the state or regional level through the projection of share of national
production as in Dean (1970). The principal shifters of the domestic,
component of national demand are population and per capita income
(Dean and King, 1970), but trends in per capita consumption, presumed
to capture changes in tastes and preferences, may also be included.
The addition of projected export-import balances produces estimates
of national demand requirements, i.e., given favorable economic con-
ditions, no radical changes in price-cost relationships and the
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assumption of perfectly elastic supply, these are the commodity
quantities which would have to be produced in order to satisfy
the domestic and export demands. Thus, these projections are con-
ditioned upon the levels specified for key parameters — popula-
tion, per capita consumption, per capita income, exports and imports.
COMMODITY SUPPLY
Commodity supply response is estimated by the Regional Production
Model using the San Joaquin Valley Basin regional linear programming
model. This model has been developed by the USDA California River
Basin Planning Staff as part of an ongoing analysis of the water
and related land resource problems of the area (McKusick, 1973 and
McKusick, 1974). The linear programming model was developed to analyze
the impact of deteriorating drainage conditions on production patterns
in the study area and the evaluation of program, policy or project
measures designed to remedy the problem. The model is static, i.e.,
designed to produce solutions for a single specified period. The
objective function maximizes net agricultural crop returns in the
basin subject to the availability of land resources by soil group,
drainage condition and location, the availability of surface and
groundwater for irrigation, labor, crop production (or crop acreage)
restrictions and the underlying production technology.
The San Joaquin Valley is divided into two component subbasins,
each separately modeled. Each subbasin is subdivided into an east
and west side on the basis of differing salinity and drainage charac-
teristics. For 30 principal crops, production activities differ by
soil group (of which there are 18), drainage condition (three discrete
categories) and location. Technological coefficients on an annual
per acre basis are specified for yields, applied water requirements
(which include estimates of the leaching requirement and embody an
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implied irrigation efficiency), harvest and nonharvest labor, ferti-
lizer (in nitrogen equivalent terms) and gas and diesel fuels.
RESOURCE DEMAND
The regional linear program will be used to generate the demands
for those land and water resources. These resource demands are the
basis of land use valuationswithin the control model. The optimal
control model will be used to simulate the dynamics of changes in
land uses over time for the valley in reaction to changes in agri-
cultural commodity demands, population, resource productivity and
availability and environmental and land use policies. The control
model will generate optimal land uses over time, the social values
resulting from these allocations and the opportunity costs of changing
those resource uses.
RESOURCE SUPPLY
The control model, in turn, predicts the availability of land
resources for agricultural production which are required in the
regional linear program. Projections of commodity demands from the
system previously described are introduced into the linear program
and a new cycle of model interactions is initiated. In this manner,
the process is repeated over the planning horizon with the result
being a close approximation of the optimal patterns of land uses
over time in the Valley.
Resource and commodity supplies are also utilized by the water
quality model. This analytical subsystem consists of two specific
models sequentially linked to simulate agricultural production and
environmental adjustments that occur as a result of an environmental
policy (Horner and English, 1976), The first is a linear programming
model that derives optimal cropping pattern, water application tech-
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nologies, water use and fertilizer in 300 subregions of the valley.
The production patterns derived in the linear programming model
serve as data inputs to the physical model. The physical model
estimates the subsurface vertical and horizontal water movements and
surface runoff created by the irrigation activity. The costs for
collection and disposal of return flows, the costs for installing tile
drainage to relieve high water tables, and yield reductions from high
water tables are also calculated by the physical model. The change in
production costs for subsequent periods are then adjusted in the linear
programming model. Solutions from the models are derived annually
and are iterated a sufficient number of times to simulate the environ-
mental adjustments from a change in water and land use as a result of
alternative control policies.
INFORMATION AND DATA REQUIREMENTS
The analytical system requires information about the economic
and physical systems of the region. Table 1 relates specific data
requirements with each analytical system. The physical data is used
extensively by the water quality model and to a lesser degree by the
land use and regional production models. The bulk of the data has
been collected from the U.S. Geological Survey, California Department
of Water Resources, U.S. Bureau of Reclamation, U.S. Soil Conservation
Service, University of California and other published sources. The
data have been computerized to accomodate the common data requirements
of these models.
The economic data are used principally to project the regional
demand for agricultural production. Resource and commodity price
data are used by all models. Most of the economic data was collected
from publications of USDA, Bureau of Census, California Department of
Food and Agriculture and California Counties located in the study
area.
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Table 1. Data requirements for the analytical systems.
Projec- Water
Land
Regional
tions Quality
Use
Produc-
Data Category
Model Model
Model
tion
Model
Physical Data
Unconfined Aquifer
Depth to
X
X
Quality
X
X
Amount Pumped
X
X
Confined Aquifer
Depth to
X
X
Quality
X
X
Amount Pumped
X
X
Surface Water
Flow or Runoff
X
Quality
X
Amount Diverted
X
X
X
Precipitation
X
X
Soil Characteristics
Acreage
X
X
X
Location
X
X
X
Productivity
X
X
X
Resource Use by Crop
X
X
Porosity
X
Permeability
X
Specific Retention
X
Geological Formations
X
X
Land Use by Location
X
X
X
Economic Data
Commodity
United States
Acreage
X
Production
X
Exports
X
Imports
X
California
Acreage
X
Production
X
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Table 1. Continued
County
Acreage
Production
x
x
x
x
Prices
Input Prices
Population
Tax Rates
Ag Land Preservation Programs
Drainage Installations
x
x
x
x
x
x
x
x
x
X
X
SUMMARY AND CONCLUSIONS
Why should a regional comprehensive analytical system be developed
to evaluate water quality policies affecting irrigated agriculture,
in particular 208 plans and BMPs? As Bower et al (1977) point out,
the degree to which an environmental strategy can improve the overall
environmental quality is the ultimate policy criteria. The physical,
environmental and economic impacts of implementing BMPs and 208 plans
are not known but society has to anticipate these impacts in order
to develop workable national and local water quality policy.
By 1979, 208 plans are to be developed and implementation started.
By 1983 the nations waters are to be fishable and swimmable and by 1985
there is to be zero discharge of pollutants to these waters. The
potential physical, environmental, and economic impacts (both on-site
and off-site) of implementing BMPs and 208 plans have generally been
ignored by resource planners. Information on these impacts is needed
to determine if BMPs will be adopted in irrigable areas, the impact
on the quality and quantity of irrigation return flows and the
feasibility of the 1979, 1983, and 1985 goals.
The paper did not present empirical results. An anlytical system
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to quantify impacts of BMPs for irrigated agriculture, on resource
use and irrigation return flow has been presented. It recognizes
that successful implementation of BMPs and 208 plans cannot exclude
other conservation and production practices. The scope of analysis
involves not only the structure and operation of the individual
farmer but the agricultural economy of the watershed and/or river
basin where BMPs are implemented. Economic impacts of proposed
environmental strategies should recognize the direct and indirect
benefits and costs of improved water quality, program administration
and other institutional costs, the distribution of costs and benefits,
and the implications of their distribution for cost-sharing and
penalties.
Water quality policy should not be developed in isolation of
land, air, conservation, production and other water policies. A
model or mechanism to analyze all of these policies simultaneously
does not exist and it is questionable if such a system will ever
exist. The comprehensive irrigated agriculture analytical system
presented in this paper is a step in the right direction because we
allow the interaction of national commodity demands with regional
production, land use and water quality. The model does require a
significant amount of data, both in quantity and detail. In the
future the sensitivity of various environmental strategies to model
specification and data aggregation will be tested. The sensitivity
analysis will allow for a more generalizable model and data set that
could be applied to other areas of the West.
In the future, it might be possible to address such irrigated
agriculture policy issues as: the trade offs between BMPs, and con-
servation and production practices; the conjunctive management of
surface, ground, return flow and recycled water; the economic
definition of prime agricultural lands; the productive supply poten-
tial of the San Joaquin Valley given resource suitability, avail-
ability and quality; and the evaluation of proposed changes in water
rights.
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LITERATURE CITEH
Bower, Blair T., Charles N. Ehler and Allen V. Kneese. 1977.
"Incentives for Managing the Environment". Environmental
Science and Technology. 11:3, March. pp. 250-254.
California Department of Water Resources. 1971. Removal of
Nitrate By An Algal System. DWR No. 174-10.
Crosson, Pierre R. 1975. "Environmental Considerations in
Expanding Agricultural Production", Journal of Soil and Water
Conservation. Jan-Feb. pp. 23-28.
Dean, Gerald W. and Gordon A. King. 1970. Projections of California
Agriculture to 1980 and 2000: Potential Impact of San Joaquin
Valley West Side Development. Giannini Foundation Research
Report No. 312. California Agricultural Experiment Station.
, G. A. King, H. 0. Carter and C. R. Shumway. 1970.
Projections of California Agriculture to 1980 and 2000.
Bulletin 847. California Agricultural Experiment Station.
September.
Gardner, B. Delworth and Herbert H. Fulleton. 1968. "Transfer
Restrictions and Misallocation of Irrigation Water". Am.
Journal of Ag. Econ. 50:3. pp. 556-57.1
Horner, G. L. and Daniel J. Dudek. 1977. "The Distributional
Aspects of Water Quality Policy". Paper forthcoming.
, and Marshall English. 1976. "Can Water Pricing
Solve the Water Quality Problem"? Paper presented to the
Western Ag. Econ. Assoc.
King, Gordon A., Harold 0. Carter, and Daniel J. Dudek. 1977.
Projections of California Crop and Livestock Production to 1985.
Giannini Foundation Information Series No. 77-3. California
Agricultural Experiment Station. May.
Kneese, Allen V., Robert U. Ayers, and Ralph C. D'Arge. 1970.
Economics and the Environment: A Materials Balance Approach.
John Hopkins Press, Baltimore.
Lee, Clifford T. 1977. "The Transfer of Water Rights in California,
Background and Issues". A staff paper prepared for the Governor's
Commission to Review California Water Rights Laws.
392
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McKusick, Robert B. 1974. "Multiple Objective Planning and Evalua-
tion as Related to Drainage Problems: A Linear Programming
Model for Agriculture". Paper presented at the 12th annual
meeting of the California Irrigation Institute. February 19-20.
Fresno, California.
McKusick, R. B., I. M. Kress, P. G. Ashton, and W. A. Bunter, Jr. 1973.
"The Development of a Plan of Study: An Interagency Approach
to Multiobjective Planning and Evaluation of Water and Land
Resource Use". Water Resources Bulletin. Vol. 9, No. 3. June,
pp. 467-484.
Miller, Bedell, Downey, McCloskey, Vento, and Weaver. 1977. "Water
Resources Management and Pricing Reform Act of 1977". October
17. H. R. 9592 95th Congress, First Session.
Pfeiffer, George H. and Norman K. Whittlesey. 1977. "Economics of
Water Quality Improvement in an Irrigated River Basin".
Western Journal of Ag. Econ. June. pp. 264-267.
Seckler, David. 1965. Discussion of paper by Dr. Water Chryst.
"Theoretical and Practical Issues in Quality Management of Water
and Land". Conference Proceedings Committee on the Economics of
Water Resource Development of the Western Ag. Econ. Research
Council. San Francisco. December 15-16.
USDA, Agricultural Research Service and USEPA. 1975. Control of
Water Pollution from Cropland. Volume I and Manual for Guideline
Development.
. 1976. "U.S. Land and Water: Assessment Through
2000 A.D.". Farm Index. September. pp. 4-7.
U.S. Department of the Interior. 1969. Federal Water Pollution Control
Administration. "Effects of the San Joaquin Master Drain on
Water Quality of the San Francisco Bay and Delta". San Fran-
c isco.
U.S. Environmental Protection Agency. 1973. Methods and Practices
for Controlling Water Pollution from Agricultural Non-point Sources.
EPA-430/9-73-015. October. ~
393
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394
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NITROGEN BALANCES FOR THE SANTA MARIA VALLEY
L. J. Lund, J. C. Ryden, R, J. Miller, A. E. Laag and W. E. Bendixen—
ABSTRACT
A simplified steady-state model has been used to develop nitrogen
balances in the Santa Maria Valley, California. Balances have been
developed for selected management units and for the valley as a
whole. The balance developed for a field cropped with vegetables
for the past 12 years showed that 30% of the applied nitrogen was
removed in harvested crop, 37% was leached below the root zone and 33%
was unaccounted-for which was attributed to gaseous losses of nitro-
gen as products of denitrification. Direct field measurement of
denitrification over an eight-month period at one site in the same
field found a 29% loss of the applied nitrogen. The first estimation
of the nitrogen balance in the valley attributed 24% of the applied
nitrogen to removal in harvested crops, 39% to leaching anc) 37% to
denitrification.
— Lund, Ryden and Laag are in the Department of Soil and Environmental
Sciences, University of California, Riverside, California 92521.
Miller is in the Department of Land, Air and Water Resources, Univer-
sity of California, Davis, California 95616. Bendixen is a Cooperative
Extension Farm Advisor, Santa Barbara County. P. 0. Box 697, Santa
Maria, California 93454.
395
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INTRODUCTION
A decision was made in 1975 to include a basin study in the
last two years of a project entitled "Nitrates in effluents from
irrigated lands" financed in part by NSF. This basin would serve
as a focal point for findings from earlier research on this project
as well as contribute to a better understanding of nitrogen sources
and sinks within agricultural production systems. After discussions
by project personnel, the Santa Maria Valley was selected for study.
Some of the reasons for selecting this valley were that it is a well
defined land area, a variety of field and vegetable crops are grown
there, some information about nitrates leaching in the valley was
available, and cooperation of farmers and extension service personnel
was assured.
The Santa Maria Valley is a coastal valley located along the
Santa Maria River at the boundary of Santa Barbara and San Luis
Obispo Counties in California. The Santa Maria Drainage Basin en-
compasses approximately 67,000 ha (Hughes, 1975) of which 18,600 ha
are intensively farmed with field and vegetable crops. The flat
bottomlands and adjacent terraces are used for irrigated crops whereas
the surrounding foothills are used for rangelands and some dryland
farming.
One of the objectives of the "Santa Maria Project" was to develop
data for nitrogen balances for the valley. These balances were to
be on two levels, one for the valley as a whole and the other for a
number of individual management units (fields) within the valley.
The techniques used to accomplish this objective will be discussed
in this paper along with data from one management unit to serve as
an example of results from the project (eight units were included
in the study). The nitrogen balance for the entire valley is still
not complete and thus only the first approximation of the balance
will be discussed.
396
-------
The second objective of the project was to develop techniques
to determine denitrification directly in the field. Results of this
research are presented elsewhere (Ryden et al, 1978a, 1978b), how-
ever, some reference will be made to results obtained for one manage-
ment unit.
APPROACH
A steady-state conceptual model that has been described by
numerous authors, including Fried et al (1976), served as the basis
for the studies carried out in the Santa Maria Valley. Basically
this is an input-output model with the underlying assumption that
at steady state the net change in the organic nitrogen pool over
the period of study is zero. The greater the length of a study and
the more uniform the management variables over time for an area, the
greater the probability that this assumption will be fulfilled. For
the management unit considered here, 12 years of cropping history
were available. With certain assumptions, this information was inte-
grated with the nitrogen input and output data collected over the two-
year study period to develop a nitrogen balance for the 12 years.
The nitrogen inputs considered in this model were nitrogen
added as fertilizer and in irrigation waters. The sinks for nitro-
gen considered in the model were removal in harvested crops, leaching
and denitrification. Other inputs (e.g. nitrogen in rainfall, dust,
etc.) and outputs (e.g. erosion, runoff, etc.) were considered to
contribute little to the overall balance and the assumption was made
that they cancelled each other out.
For individual management units, the information on fertilizer
nitrogen inputs was obtained from records of management history that
were supplied by farmer-cooperators. Nitrogen inputs in irrigation
waters were based on measured nitrate-nitrogen concentrations in the
waters and records of water application in the various fields.
397
-------
During 1976-1977, the yields and nitrogen contents were determined
for a variety of crops in the valley to serve as a basis for deter-
mining crop removals of nitrogen (Lund and Pratt, 1977). Average
nitrate-nitrogen concentrations in soil solutions leaching below
root zones of typical fields in Santa Maria were determined by soil
sampling. These were used in conjunction with leaching volumes as
obtained from a chloride balance model to determine nitrogen leached
(Pratt et al , 1978). The soil sampling program was also designed to
examine the spatial and temporal variation in nitrate-nitrogen leach-
ing for the selected management units (Lund et al, 1978). One
estimation of the denitrification-volati1izntion sink was obtained
by balancing inputs with crop removals and leaching with the "unaccounted-
for" nitrogen attributed to den itrification-vn1 ati1ization. Denitri-
fication was also measured in the field as reported by Ryden et al (1978b)
and the data compared Co those obtained by the balance approach.
BALANCE FOR SIMAS FIELD
As an example of a nitrogen balance for a defined management
unit, data obtained for the Simas study area will be discussed in
this section. This field was selected for study because it was
typical of the heavier-textured, high organic matter soils in the
western part of the Santa Maria Valley where vegetable crops dominate
the irrigated agriculture. The Simas field had been planted to
vegetable crops for at least the past 12 years. Data for the Simas
field relative to crop type, fertilizer and irrigation water applied
and yield as supplied by the farmer-cooperator are given in Table
1. The field was divided into three blocks, each 4 to 5 ha in size.
A number of important points can be made relative to the data in
Table 1. Typical of vegetable crop production in California, large
amounts of water and fertilizer nitrogen were applied. Also more
than one crop was typically grown each year on lands used for
398
-------
Table I. Cropping history of Simas field. Yields reported here
are based on material leaving the field. For lettuce,
field weight is equivalent to market yield; whereas, for
cauliflower marketable yield is approximately 34% of field
weight, and for broccoli and clery it is 80% of field weight,
Year
1976
Crop
Broceo 1 i
Let t nee
Block A
Wa t e r
ha-m
.91
.73
Fertilizer
nitr ogen__
kg /ha
232
125
Yield
YTha"
14.3
51.7
1975
Broccoli
Broccoli
.85
.82
242
197
15.7
17.9
1974
Can 1i f1ower
Lettuce
.91
.67
416
218
40. 2
44.7
1973 Seeded celery
1972 Broccoli
Seeded celery
1 . 10
.67
1 . 10
469
239
348
84.2
14.7
92.2
19 71 Broccoli
Seeded celery
Broccol i
.85
1.07
.76
194
432
171
14.2
92.4
14.7
1970 Lettuce
1969 Celery
1968 Cauliflower
Ce1ery
.73
.79
.91
.82
3 70
671
298
651
35.6
93.0
12.4
69.4
1967
Broccoli
Ce1ery
. 76
.76
153
608
18.6
68.6
1966
Broccoli
Let tnee
.70
.73
176
183
17.2
48.1
1965
Celery
.79
659
100.1
399
-------
Table 1. Continued
_____ __ __ „JU°''k B
Year Crop Water Fertilizer Yield
„ . J'i1 r( >p, e n
ha-m kg/ha T/ha
1976 Broccoli .91 232 14.3
Lettuce .73 116 62.8
197 5 Broccoli .85 236 15.7
Seeded celery 1.04 283 72.3
1974 Cauliflower .91 290 41.2
Lettuce .67 220 45.7
1973 Seeded celery 1.10 393 105.7
1972 Broccoli .67 176 18.6
Lettuce .73 175 39.2
Broccoli .76 227 12.7
1971 Seeded celery 1.07 351 81.6
1970 Broccoli .73 227 15.8
Lettuce .73 357 35.6
1969 Celery -79 448 93.0
1968 Cauliflower .91 310 12.4
Celery -82 596 69.4
1967 Broccoli .76 153 18.6
Celery -76 557 68.6
1966 Broccoli .70 194 17.2
Lettuce .73 200 53.9
1965 Celery .79 627 87.5
400
-------
Table 1. Continued
Year Crop
197 6 Brocco1 I
Lett uce
197 5 Broccoli
Seeded celery
1974 C.'ml if lower
Let tuce
197 3 Seeded celery
1972 Broccoli
Let tuc e
Brocco1i
1971 Seeded celery
1970 Broccoli
Let tuce
1969 Celery
1968 Cauliflower
Ce1ery
1967 Broccoli
Ce lery
1966 Broccoli
Lettuce
_ Bl°ck C
Water
Fertilizer
nitrogen
Yield
hn-m
.91
.73
.85
1 .04
.91
.67
1. 10
.67
.73
.76
1.07
.73
.73
.79
.91
.82
.76
.76
.70
. 73
kg/ha
232
127
236
289
290
243
399
176
164
227
530
171
307
562
299
630
148
485
207
160
T/ha
14.3
44.1
15.7
72.3
41.2
42.5
84.2
18.7
38.9
12.7
64.3
14.7
35.6
80.6
12.4
69.4
18.6
68.6
17.2
49.6
1965
Celery
. 79
577
83.3
401
-------
vegetable crops. For this field 21 crops were grown in 12 years.
These data show that over the past 12 years, 7053 kg fertilizer
nitrogen/ha had been applied to Block A, 6368 kg to Block B, and
6459 kg to Block C.
Amounts of water applied and nitrogen concentration of that
water were needed to calculate the amounts of nitrogen applied in
the irrigation water. For the Simas field, 17.43 ha-m/ha of irri-
gation water had been applied to Block A over the past 12 years
with 17.16 ha-m/ha being applied to Blocks B and C. All three blocks
were irrigated from the same well; however, no data were available on
the nitrate-nitrogen concentrations of the water over the past 12
years. The only alternative was to use data obtained during the
study period and assume that it represented the earlier periods.
Twelve water samples were collected from the Simas well during the
study period and were found to have a mean nitrate-nitrogen concen-
tration of 12.5 mg/1 with a range of 11.4 to 14.7 mg/1. Using a
mean concentration of 12.5 mg/1, the amount of nitrogen added in the
irrigation water over 12 years was calculated at 1530 kg/ha for Block
A and 1500 kg/ha for Blocks B and C. These calculations were based
on a 30% runoff which was measured for this furrow irrigated field
in another study. Total nitrogen inputs are given in Table 5.
The assumption also had to be made that nitrogen removed by
crops and leaching characteristics of the field had not changed
significantly over the past 12 years in order to calculate crop
removal of nitrogen and nitrogen leached. The amount of nitrogen removed
by a broccoli crop in Simas field in 1977 is given in Table 2. The
yield was determined for nine study sites within the field. These
sites (10 x 10 m plots) were the same as those used for soil sampling
as discussed later. Broccoli samples were taken from each plot for
nitrogen analyses. Nitrogen removals could then be calculated on
the basis of yield and nitrogen content, which average 5.2% for the
field. The Simas data show that yield and nitrogen removals differ
402
-------
significantly between sites within the field. Thus, in conducting
nitrogen studies one should recognize that nitrogen removals are
not uniform within a management unit.
Table 2. Yield and nitrogen removal for nine sites in the Siraas
field.*
Site
YieJd+
Nitrogent
Content
Nitrogen
Removed
T/ha
%
kg/ha
390
9.55 yz
4.88 z
49.86 z
391
11.03 xyz
5.18 yz
56.59 yz
392
11.42 xyz
5.17 yz
64.04 yz
393
8.45 z
5.69 y
49.23 z
394
12.06 xyz
5.28 yz
65.31 yz
395
12.57 xyz
5.12 yz
63.63 yz
396
14.97 x
5.05 yz
77.03 y
39 7
12.79 xy
5.32 yz
63.28 yz
398
11.00 xyz
4.91 z
56.26 yz
Values followed by the same letter are not significantly
different at the 1% level.
+ Determined from three replicates per plot.
t Determined from nine replicates per plot and expressed on a
dry weight basis.
As no information was available on nitrogen removal by past crops
at Simas, nitrogen removals were calculated using nitrogen contents
for the same crops determined during studies at other locations. The
nitrogen values used were 5.2% for broccoli, 3.6% for celery and
lettuce and 4.8% for cauliflower (expressed on a dry weight basis).
These calculations resulted in crop removal values of 2545 kg
nitrogen/ha for Block A, 2380 kg nitrogen/ha for Block B and 2310
403
-------
kg nitrogen/ha for Block C.
As mentioned previously the nitrogen leached was determined
by soil sampling and analyses. Within the Simas field nine sites
were sampLed. Three holes were drilled at each site and soils were
collected in 0.30-m increments from the 1.0-m depth to the water
table, generally 4.0 m. These samples resulted in an estimate of
the spatial variability of the nitrogen leaching within the field.
The temporal variability was studied by collecting samples at three
times during 1976-1977. A summary of the nitrate-nitrogen and
chloride data for soil solutions below 1.0 mat Simas field is given
in Table 3. The analyses of variance of the nitrate data show signif-
icant difference between sites and times. Mean nitrate-nitrogen
concentrations for the nine sites ranged from 30.6 to 81.2 ppm for
a field mean of 41.1 if one assumes that each site represents an
equal area of the field.
Nitrate-nitrogen concentrations over the field as a whole decreased
significantly for the third sampling period. A possible explanation
for this was a reduced fertilizer application during the preceding
crop. From Table 1 it can be seen that fertilizer nitrogen was
applied at a rate of approximately 125 kg/ha for the 1976 lettuce
crop which was about one-half or less of the rates used for previous
crops. Chloride concentrations in the soil solutions were not signif-
icantly different over the same time period (7.3 vs 7.4 meq/1) which
would rule out an increased leaching volume to dilute the nitrate-
nitrogen .
The data given in Table 3 were used to calculate the nitrogen
leached at each site in the Simas field (Table 4). The mean chloride
concentrations in the soil solution below 1.0 m were used to calcu-
late a leaching fraction at each site (Pratt et al, 1978) using the
equation
. , . c . . chloride (irrigation-corrected)
leaching fraction = ——¦— ¦ mr ~rr ¦; : —
chloride (soil solution)
404
-------
Table 3. Analysis of variance for nitrate-nitrogen and chloride data for three sampling times for
the Simas field. Values are mean concentrations in soil solution (ug/ml) below the 1.0-m
depth.
Site
Field
as
390
391
392
393
394
395
396
397
398
Whole
Nitrate-nitrogen
Significance+
N.S.
***
***
•k
N.S.
**
***
*
**
***
Time 1
78.7z
58. 7x
45. 8y
46. 7y
43.5z
44 .3y
33. 6y
36. ly
47. Oy
48.3y
Time 2
89. Oz
40. 6y
54.2y
41.9yz
40.5z
40.5y
43.5y
34.9y
31.2z
46. 2y
Time 3
76.2z
17.5z
20.5z
32.5z
33.2z
22. lz
17.6z
20.9z
19.lz
28.9z
Meant
81. 2w
38.9xy
40. 2x
40. 4x
39.lxy
Chloride
35.8xyz
31.6yz
30. 6z
32. 4xyz
Significance
***
***
*
***
**
*
***
N.S.
N.S.
N.S.
Time 1
7. lz
10.3y
6. 2z
7. 8z
7 „ lz
5.9z
5.9y
6.9z
8. lz
7. 3z
Time 2
9.8y
6. 8z
7.9y
9. 3y
6. 5z
7. 2yz
3. 8z
6. 9z
7. 9z
7 .3z
Time 3
10.3y
5. 8z
6.6z
7. 2z
9. ly
7 • 7y
6.7x
7. Oz
6.6z
7. 4z
Mean
9. lw
7. 6xy
6.9y
8. lx
7. 6xy
6.9y
5. 5z
6.9y
7 .5xy
+ Significance of differences between mean concentrations with time at each site and for the field
as a whole are indicated here. Values within each site followed by the same letter are not
significantly different at the level indicated.
$ Site means followed by the same letter are not significantly different at the 5% level.
-------
Again, data for the chloride concentrations of the irrigation water over
the 12-year period were lacking. Therefore, the assumption was made
that chloride concentrations in the Irrigation water had not changed
significantly over the 12 years. During 1976-77 twelve irrigation
water samples were collected at Simas and analyzed for chloride.
The concentration mean was found to be 4.8 meq/1 with a range of
4.6 to 5.0 meq/1. This input chloride concentration was then corrected
for crop removal using chloride contents of 0.66% for broccoli, 0.34%
for cauliflower, 2.48% for lettuce and 3.20% for celery (dry weight
basis) .
Table 4. Nitrogen leached at each site in Simas field.
Site Block Chloride* Chloride Leaching Nitrogen
(irrigation- Fraction Leached
corrected)
—meq/1
kg/ha
390
A
9.1
4.44
.49
4834
391
B
7.6
4.46
.59
2742
392
C
6.9
4.54
. 66
3177
393
A
8.1
4.44
. 55
2702
394
C
7.6
4.54
.60
2806
395
C
6.9
4.54
.66
2829
396
C
5.5
4.54
.83
3133
397
B
6.9
4.46
.65
2376
398
A
7.5
4.44
.59
2340
* In the soil solution below the root zone.
Leaching fractions for the different sampling sites ranged from
.49 to .83 with a field mean of .62 based on the assumption that each
site represented an equal area in the field. While at first this
406
-------
leaching fraction appears to be quite large, a companion study con-
ducted at the Simas field to evaluate the irrigation system found
deep percolation accounted for more than 50% of the applied water.
The nitrogen leached was then calculated on a product of the leach-
ing fraction, water applied and mean nitrate-nitrogen concentrations
in the soil solution below 1.0 m (Table 4).
A summary of the measured and calculated nitrogen inputs and out-
puts at Simas field is given in Table 5 for each site and the field
as a whole.
Table 5. Summary of nitrogen inputs and outputs at each site in the
Simas field for 12 years.
Inputs Outputs*
Site Fertilizer Water Crop Leached Unaccounted-for
kg/ha
390
7052
1525
2545
(30)
4834
(56)
1198
(14)
391
6368
1500
2380
(30)
2742
(35)
2746
(35)
392
6459
1500
2310
(29)
3177
(40)
2472
(31)
393
7052
1525
2545
(30)
2702
(31)
3330
(39)
394
6459
1500
2310
(29)
2806
(35)
2843
(36)
395
6459
1500
2310
(29)
2829
(36)
2820
(35)
396
6459
1500
2310
(29)
3133
(39)
2516
(32)
397
6368
1500
2380
(30)
2376
(30)
3112
(40)
398
7052
1525
2545
(30)
2340
(27)
3692
(43)
Field
6636
1508
2404
(30)
2993
(37)
2747
(33)
* Values in ( ) are outputs as percent of nitrogen input.
For the field as a whole, crop removal amounted to 30% of the applied
nitrogen and 37% of that applied was leached. The unaccounted-for
nitrogen which in nitrogen balances is attributed to devitrification
407
-------
ranged from 14 to 44% of the applied nitrogen for a f j l> ] ii average of
34%. When interpreting these figures one must keep in mind that all
errors in measurement of other inputs and outputs are combined into
the unaccounted-for category.
Direct field measurements of donLtrif teat ion were made at site
396 during .June to October, 197 7 when a celery crop was growing in
the field. For 335 kg nitrogen applied/ha, measured losses of nitro-
gen by denitrification amounted to 51 kg/ha or 15.2% of the applied
nitrogen. Evidence was also obtained (Kyden et al, 1978b) that
denitrification losses that occur during wet periods (winter rains)
increase the proportion of applied nitrogen lost as a result of
denitrification. At the same site in the Si mas field, the denitri-
fication loss between June, 1977 and February, 1978 was approximately
135 kg/ha or 29Z of the nitrogen applied during that period. Other
data obtained in these studies indicate that for similar irrigation
management on similar soils, the amount of nitrogen lost by denitri-
ficat.ion is fairly similar within quite a wide range of fertilizer
inputs. Thus, if 50 kg were denitrified with an application rate
of 200 kg (a rate more typical of lettuce and broccoli) the percent
loss would be 25%. These observations suggest that t tit? value of
34% obtained for unaccounted-for nitrogen in the nitrogen balance for
the Simas field as a whole is not at all unreasonable.
The analyses of the data obtained for Simas field lead to the
following conelusions:
(1) Crop removal oF nitrogen varies within a management unit.
Therefore, sampling programs must be designed to account
for the variability when developing a nitrogen balance
for the unit.
(2) Spatial and temporal variability of nitrate concentrations
in soil solutions leaching below the root zone were ob-
served and have implications relative to a steady-state
model. By sampling at various points in time the relia-
408
-------
bility of nitrate data obtained is increased. However,
it must be kept in mind that this has implications
relative to costs of the studies.
(3) A mean leaching fraction for the field obtained by chloride
balances agreed with that obtained by an evaluation of
the irrigation system efficiency. Within the field the
leaching fractions were quite variable and were generally
related to field position, i.e. greater at the head end
of field.
(4) The field nitrogen balance attributed 30% of the nitro-
gen output to crop removal, 36% to leaching and 34% to
denitrification. Direct field measurements of denitrifi-
cation found that 29% of the applied nitrogen was lost at
one site in the Simas field over an eight-month period.
BALANCE FOR THE VALLEY
Data obtained for the Simas field, data for seven other similar
intensive study areas and data from some supplemental sampling have
been combined with other existing data to develop a nitrogen balance
for the entire drainage basin. As this balance is not yet complete
only some of the data relative to the irrigated lands will be pre-
sented.
A breakdown of the areas in major crops in the valley for 1.976
is given in Table 6. Broccoli, cauliflower and head lettuce account
for approximately 80% of the area in vegetable crops. Seed beans,
lima beans and irrigated pasture were the main field crops grown.
Of the fruit crops, grapes made up most of the area.
The yields for the crops listed in Table 6 were taken from the
Agricultural Commissioners1 reports for Santa Barbara and San Luis
Obispo Counties for 1976 and are averages for the entire basin.
Average fertilizer nitrogen application rates as given in Table 6
409
-------
Table 6. Cropping summary of major crops lor Santa Maria Valley in
1976.
Crop
Hec ta res
Yield
Pert i 1 i zer
n i t rogen
a pp1 ied*
N i t rogen
kemova1
by Crops
me trie
ton/ha**
kg/ha
kg/ha
Vegetable Crops
Artichokes
1 78
11.4
1 79
48
Brorco 1 i
5,663
11.6
280
58
Cabbage
27 3
52. 7
280
1 23
Carrots
475
40. H
1 34
78
Cau1 if lower
1,623
31 .4
392
128
Celery
524
7 5.6
33 6
136
Lettuce, leaf
241
35.9
168
52
Lettuce, head
4,867
33. 6
224
29
Peppers, dry
294
17.2
252
1 1 5
Potatoes
7 5 3
35.0
280
113
Spinach
184
20.2
224
91
Tomatoes
108
38. 6
157
7 3
Field Crops
Beans, seed
1 ,221
2. 1
67
78
Beans, small, white 81
1 .9
67
7 3
Beans, lima
1,666
2.4
67
91
Hay, alfalfa
202
15.7
0
4 53
Pasture, irri.
2,222
2.2
80
36
Sugar beets
405
64. 5
280
109
Fruit Crops
Lemon s
81
24.2
224
12
Strawberr ies
299
47 . 1
3 36
52
Grapes
1.739
3.5
67
14
* Does not include
nitrogen from
i rr igat ion
water, rainf
all, etc-.
**Yields are based
on material 1e;
410
iving field
-------
were developed from published data and information supplied by farmers
and extension personnel. Many areas of the valley are used to pro-
duce more than one crop/yr. Thus, the data on fertilizer applied are
not- necessarily the amount applied/ha of land/yr. Some estimates
indicate 1.3 crops/yr on the average for the valley.
During the 2 years of the Santa Maria project, plant samples of
all major crops were collected and analyzed. These data were then
used to calculate nitrogen removal from fields as given in Table 6.
The average nitrogen removed by all crops was 31% of the fertilizer
input. If the nitrogen applied in the irrigation water was included
the crop removal value for the valley would be reduced to close to
24%. From the data in Table 6, the fertilizer nitrogen applied in
1976 was 4770 metric tons. Based on an average water application
of 75 cm/ha, 80% application efficiency and a nitrate-nitrogen content
of 10 mg/1, the nitrogen applied in the irrigation water was calcu-
lated as 1390 metric tons. Thus, the total nitrogen input to the irri-
gated lands of the valley was 6160 metric tons.
At the present time nitrogen losses from the irrigated lands in
the Santa Maria Valley appear to breakdown as follows. From the data
in Table 6, the crop removal of nitrogen was calculated as 1500 metric
tons. Based on nitrate-nitrogen concentrations under a number of
study areas in the valley and a leaching fraction of .25 as calculated
from chloride concentrations in leaching solutions, the amount of
nitrogen leaching below the root zones of irrigated lands in the
valley was calculated as 2380 metric tons. The combined figures for
crop removal and leaching was 3880 metric tons leaving 2280 metric
tons in the unaccounted-for category. These resulting figures are
24% of the applied nitrogen removed in harvested crops, 39% leached
below root zones and 37% unaccounted-for, which can be attributed to
denitrification.
The first approximation of a nitrogen balance for the Santa
Maria Valley has been presented here. The amounts of nitrogen attributed
411
-------
to the various sinks for the valley .is ;i whole were not greatly
different from those of the Simas field. This is not too surprising
as the Simas Field is very typical of the vegetable crop production
area and the crops grown in the rotation (broccoli, celery, lettuce,
and cauliflower) at Simas account for some 55% of the crop production
in the valley. Thus, these crops dominate any balance for the valley.
Additional approximations for the nitrogen balance for the valley will
be made as more data on individual study areas are obtained and will
be reported in the final report of the NSF project "Nitrates in
effluents from irrigated lands"
ACKNOWLEDGEMENTS
Financial support of the National Science Foundation through
Grant ENV76-10283A01 is gratefully acknowledged. Gratitude is expressed
to Richard Elliott, Stephen Whaley, Fain Sutherland, Edward Betty and
Cuauhtemoc Pallares for their assistance on this project.
412
-------
LITERATURE CITED
Fried, M., K. K. Tanji, and R. M. Van De Pol. 1976. Simplified
long term concept for evaluating leaching of nitrogen from
agricultural land. J. Environ. Qual. 5:197-200.
Hughes, J. L. 1975. Evaluation of ground-water quality in the
Santa Maria Valley, California. U.S.G.S. Water Resources
Investigations 76-128.
Lund, L. J., C. Pallares, S. Whaley, and R. A. Elliott. 1978.
Spatial and temporal variation in nitrate leaching for selected
sites in Santa Maria Valley, California. Final Report. Nitrates
in effluents from irrigated lands. (In preparation).
Lund, L. J. and P. F. Pratt. 1977. Variability of nitrate leaching
within defined management units, Iri J. P. Law and G. V.
Skagerboe (eds.). Proceedings of National Conference on
Irrigation Return Flow Quality Management. Colorado State
University.
Pratt, P. F., L. J. Lund, and J. M. Rible. 1978. An approach to
measuring leaching of nitrate from freely drained irrigated
fields. In D. R. Nielson and J. G. MacDonald (eds.). Nitrogen
in the Environment. Academic Press, Vol. 1:223-256.
Ryden, J. C., L. J. Lund, and D. D. Focht. 1978a. Direct in-field
measurement of nitrous oxide flux from soils. Soil Sci. Soc.
Am. J. (In review).
Ryden, J. C., L. J. Lund, J. Letey, and D. D. Focht. 1978b. Direct
measurement of denitrification loss from soils: II. Development
and application of field methods. Soil Sci. Soc. Am. J. (In
review),
413
-------
ECONOMIC IMPACTS OF CONTROLLING NITROGEN CONCENTRATION
AND OTHER WATER QUALITY DETERMINANTS IN THE
YAKIMA RIVER BASIN
George H. Pfeiffer and Norman K. Whittlesey—^
ABSTRACT
The nonpoint source nature of water quality degradation caused
by irrigation return flows makes control nearly impossible with
traditionally effective measures, such as limitations or taxes on
effluents. As a consequence, control or taxation of those inputs
to production which are involved in effluent input-output relation-
ships is a possible alternative. In the Yakima River Basin, three
determinants of river water quality were identified: nitrogen con-
centration, river water temperature, and soil erosion by irrigation
water. Among the pollution control policies considered for effect-
iveness, producer cost, and social cost were taxation of nitrogen
fertilizer, increasing the charge for irrigation water, reduction
of irrigation water rights, and restrictions on the types of irrigation
systems used.
Results showed that policies which affect the level of one pollu-
tant may or may not significantly affect the levels of others. Further-
more, policies taxing or charging for inputs, such as fertilizer and
irrigation water, cause a substantial reduction in producer income if
acceptable water quality is to be attained. Therefore, water quality
policy formulation and evaluation should consider the interactions
1/
Research Associate in the Department of Agricultural Economics,
North Dakota State University, Fargo, North Dakota 58102 and
Professor of Agricultural Economics at Washington State University,
Pullman, Washington 99163.
415
-------
which exist among water quality determinants
both the expected benefits of improved water
and distribution of improvement costs.
and considerations of
qua 1ity and the level
INTRODUCTION
It has been traditionally assumed that the social benefits of
irrigated agriculture far outweigh the social cost imposed by its
contribution to environmental degradation. However, this assumption
is now being questioned on many fronts. This paper relies on a study
designed to estimate the agricultural contribution to water quality
problems in the Yakima Basin of eastern Washington, to evaluate the
efficiency and the impact on agriculture of alternative water quality
improvement policies in the area, and to recommend policies which
could improve water quality in this and similar irrigated river basins.
Water quality is a concept which transcends any single pollutant.
In fact, many individual pollutants are interrelated chemically and
biologically in the determination of water quality. Thus, policies
designed to affect only one component of water quality may increase,
decrease, or have no impact on other components of water quality.
Three components of water quality were identified for evaluation in
this study: river nitrogen concentration, river water temperature,
and suspended solids resulting from soil erosion caused by irrigation.
As a practical matter, irrigation related water quality degrada-
tion has been a difficult problem. The amendments to the Federal
Water Pollution Control Act of 1972 (PL 92-500) are an important but
perhaps unworkable attempt to address irrigation related pollution.
The root of the problem is the nonpoint nature of such contaminants
as nitrogen and sediment in return flow waters. As a result, it is
difficult if not impossible to restrict, tax, or regulate such pollu-
tants because the exact sources and extent of the losses cannot be
416
-------
identified. Hence, losses can only be controlled by secondary means;
that is, by controlling input use or production practices related to
the losses.
THE STUDY AREA
The Yakima River Basin is an intensively cultivated area of
some 450,000 irrigated acres located in eastern Washington (Figure 1).
Tree fruits and grapes comprise nearly 20% of the irrigated acreage;
sugarbeets, hops, and mint comprise 15%; and potatoes and vegetables
comprise 11%. The remaining 54% is divided among small grains, hay
crops, and irrigated pasture. Abundant and inexpensive irrigation
water has stimulated relatively inefficient water use. Approximately
5.3 acre feet of irrigation water are diverted from the Yakima River/
irrigated acre (Copp and Higgins, 1974). A relatively small proportion
of the canals and ditches in the area are lined, and most of the irri-
gation is by flood or furrow methods, with water diverted from the river
in canals, and the part not consumed by plants returning through surface
and subsurface return flow channels.
In the irrigation process, more water is applied than is consumpt-
ively used by growing plants. As a result, water not used passes through
the soil below the root zone or runs off the land on its surface. Water
passing through the soil picks up soluble compounds, some of which are
nitrogen compounds applied as fertilizers. Runoff water may pick up
some soluble compounds and may acquire suspended solids through erosion.
Because 80 to 90% of the water in the lower reaches of the Yakima
River is irrigation return flow water during the late summer, the
quality of the river water is a direct consequence of the quality of
the return flow. Furthermore, the low summer flow volume in the river
permits the water to warm substantially, diminishing its usefulness
for recreation, fisheries, and promoting the growth of water-borne
organisms. The specific determinants of water quality used in this
417
-------
Kochest
Lota
Eilensburj
O Producing Region
Q R#och Terminu*
Rimrock
COLUMBIA
RIVER
RTOflG
Figure 1. The Yakina liver Basin.
418
-------
study were August mean nitrate nitrogen concentration in the Yakima
River, August mean maximum daily water temperature in the river,
and annual soil losses from farmland. For the purpose of analysis,
the river was divided into seven reaches with corresponding land
areas as shown in Figure 1, Each land area {Regions 1 through 7)
drains principally to its corresponding reach (Reaches 1 through ?).
ECONOMIC THEORY
Economic efficiency is generally maximized when pollution is
directly controlled through taxation, restrictions, or "bribes" for
abatement. However, agricultural effluents are not. subject to the
same types of constraints that are applicable to smokestack and
sewage discharges because it Is difficult, if not impossible, to
identify the? source of the discharges. Consequently, agricultural
pollution abatement must be controlled through policies affecting
the use of inputs causing the externality rather than policies
directly affecting the externality,
Langham (1372) has shown that when pollution is a function of
the use of one Input, taxation, or restriction on the use of that in-
put is equivalent to controlling the pollution output itself fro* an
efficiency standpoint. It can "be shown that when pollution is a
function of more than one input» the efficiency criterion can still
be satisfied by appropriately taxing or restricting the use of ail
those inputs (Baumol and Oat.es, 1975; Pfeiffer, 1976), Income or
cost distribution between the public and private sectors, however,
depends on whether input use is controlled by taxation, restriction,
or bribery, Also, some difficulty nay be encountered in determining
the appropriate set. of taxes or restrictions on individual inputs
which causes environmental standards to be met at minimum cost.
Further, while it Is possible to 'd#*t«»rai»e the of Lr, Jto«« cost
means of achieving any given environment a l quality st-uirfwcd, tittle
419
-------
light is shed on the appropriate standard to be set. While theoretical
economics has approached this question, applied economics has not yet
reached entirely satisfactory methods of application.
Finally, even if the physical and biological relationships and
the benefits to society of differing levels of water quality are known,
the optimal level of water quality and the impacts on agriculture which
are required to attain it are predicted on predetermined prices for
agricultural inputs and outputs. If the prices of factors of produc-
tion or agricultural commodities change, either the optimal means of
achieving a standard will change, the optimal standard itself will
change, or both.
WATER QUALITY COMPONENTS
2
In the Yakima River, field losses of nitrate nitrogen and eroded
soil are two major factors affecting water quality. Both nitrogen
and soil losses are relatively simple to measure in a laboratory
situation, but quite difficult to predict under the variations of
weather, cultural practices, and environmental conditions found in
field situations.
Gossett and Whittlesey (1975) used data from Pratt and Adriano
(1973) to estimate a nitrate nitrogen percolation regression equation
for the Yakima Valley. The equation developed was:
712
N, = .2 (N Q )
L ad
Where:
N = pounds of nitrogen leached/acre/year
Lj
N = pounds of nitrogen applied/acre/year
a
2
Nitrogen percolation losses are based on the weight of nitrogen
lost, although it is assumed that all percolation losses are in
the nitrate form.
420
-------
Q, = drainage volume in acres inches/acre/year
d
The major criticism of this function is that it is convex from
above with respect to nitrogen application. To connect this intui-
tively illogical characteristic, the nitrogen loss estimation equation
used in this study was:
N = .029 (N )1"05(Q ,)'7
Li 3d
So, for example, on a crop to which 200 pounds of nitrogen is applied/
acre/year and for which deep percolation is 12 acre inches/acre/year,
estimated nitrogen loss is 51 pounds/acre/year.
Soil losses were determined using a procedure based on the
Universal Soil Loss Equation developed by Wischmeier and Smith (1965).
As suggested by Gossett and Whittlesey (1975) soil losses were esti-
mated as linear functions of surface water runoff volume for each soil
type and slope gradient in the Valley. Corrugated crops (alfalfa,
pasture, orchard crops, etc.) were assumed to lose soil at a rate one-
tenth as great as furrow crops (corn, vegetables, hops, etc.). Erosion
from land irrigated by sprinkler or tailwater reuse systems was assumed
to be negligible. Soil losses as high as 10 tons/acre were estimated
on surface irrigated potatoes.
River water temperature was the third water quality parameter
estimated. Briefly, changes in the temperature of a body of water
are a function of the net change in stored energy in the water (from
the sun), the volume and exposed surface area of the body, and the
volume and temperature of inflowing water. Yakima River water temper-
ature for August was estimated using equations developed by Raphael
(1962).
ANALYTICAL CONSTRUCT
The analytical construct included a linear programming model
of the agricultural sector of the Basin and the hydrology of the
421
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Yakima River fortran-linked to a simulation model which determined
values for the environmental quality parameters. While it is
theoretically possible to include determination of environmental
quality parameters within the linear programming model ([see, for
example, Kraft (1975)J using separable techniques, it was determined
that such an inclusion would expand the model to a size too expensive
to run repeatedly.
Within the seven agricultural regions in the Basin, constraints
limiting irrigation water diversions, crop acreage and rotation,
and irrigation system use were developed from historical data for
each region. A solution to the linear programming model gave the
profit maximizing combination of cropping activities, irrigation
system use, and nitrogen fertilizer application subject to crop
acreage, water rights, labor availability, and other constraints in
each of the seven production regions within the Basin. Three irri-
gation systems—surface or rill, tailwater reuse, and sprinkler—
and four levels of nitrogen fertilizer application were permitted
for most crops. No nitrogen was applied on alfalfa. River water
flow was estimated within the programming model at each reach
terminus based on water usage in the upstream reaches.
Annual nitrogen lost to return flows in each region was esti-
mated using the nonlinear exponential function discussed earlier.
Soil losses were also estimated as noted. Data regarding annual
and August water flows at the end of each reach, diversion and
return flow volumes leaving and entering each reach, and annual sedi-
ment and nitrogen losses from each region were used as input to the
water quality simulation model. Nitrogen concentration at each reach
terminus was then estimated within the simulation model as a function
of nitrogen entering the reach from the previous reach, nitrogen
entering the reach with return flows, biological uptake of nitrogen,
and river volume. Water quality as determined by sediment was
estimated simply as annual sediment loss per irrigable acre.
422
-------
August water temperature at each reach terminus was estimated as a
function of river water temperature and volume entering the reach,
reach length, river surface area, flow speed, August diversions,
return flow volume and temperature, and net radiant energy.
POLICY OPTIONS
Policies for water pollution abatement were selected for their
operational and administrative feasibility in the control of non-
point source discharges and are designated Solutions 1 through 5.
1. Solution 1 represents existing agricultural production
and environmental quality in the Yakima Basin.
2. Solution 2 imposed a tax on nitrogen fertilizer to control
the level of its use for pollution abatement.
3. Solution 3 imposed a per acre foot charge for irrigation
water delivered to the farm to reduce water use.
4. Solution 4 reduced water rights by a uniform percentage in
all regions of the Basin.
5. Solution 5 combined a nitrogen fertilizer tax with a
charge for irrigation water.
The stringency of each policy was increased incrementally until
the specified level of environmental quality was reached. Specified
environmental quality was a maximum August nitrogen concentration of
0.30 mg/1, maximum sediment loss of one ton/irrigable acre, and
maximum water temperature of 70F. The nitrogen concentration goal is
the level below which algae blooms will not occur (U.S. EPA, 1975).
The water temperature goal satisfies the Washington State Department
of Ecology requirement for a Class A stream. No standards exist
for soil losses specifically, so a standard of approximately half
the existing level was arbitrarily selected. Two measures of abate-
ment cost were calculated: producer abatement cost and social abate-
ment cost. Producer abatement cost was the reduction in farm income
423
-------
caused by a policy. Social abatement cost was the reduction of
income to society calculated as producer abatement cost minus charges
and taxes collected for abatement purposes.
Obviously, producer and social abatement costs are only measures
of the direct impact on the agricultural sector or society of improved
water quality. Secondary impacts, such as reduced employment, re-
duced gross business volume and personal income, and population
changes could be estimated using an input-output technique and demo-
graphic models. Moreover, as noted earlier, the costs estimated in
this study are based on a specific set of input and output prices.
Changing these prices changes the results, but the general conclusions
regarding policy effectiveness and relative abatement costs remain
valid .
RESULTS
Solution 1, the benchmark or existing conditions solution is
shown on each tabular summary of Solutions 2 through 5. Existing
conditions result in a net crop income of $107 million on approxi-
mately 453 thousand acres in the Basin. Approximately 2.4 million
acre feet of irrigation water are diverted each year, with a maximum
August river nitrogen concentration of 0.87 mg/1, 1.74 tons of sedi-
ment lost/acre, and a maximum river temperature of 75.5F.
The nitrogen tax imposed in Solution 2 was believed likely to
be effective particularly in the reduction of nitrogen concentration
because nitrogen losses are in part a function of nitrogen fertili-
zer application. With a sufficiently high tax, it was believed like-
ly that other environmental parameters would improve as a secondary
impact of the reduction in irrigated acreage. A tax of $.60/pound
resulted in the satisfaction of nitrogen concentration and water
temperature goals as shown in Table 1. The sediment loss goal was
not met even at a tax of $.70/pound because the adoption of improved
424
-------
Table 1. Income, abatement cost, resource use and water quality determined with nitrogen taxes.
, a
Solution 2 - Nitrogen tax ($/pound)
Item Unit Solution 1 .10 .20 .30 .40 .50 .60 .70
Net crop
income $1,000 106,910 97,495 88,723 80,860 74,028 68,262 62,972 58,240
Irrigated 1,000
acreage acres 453 453 453 414 436 430 420 408
Producer abate-
ment cost $1,000 9,415 18,187 26,050 32,882 38,648 43,938 48,669
Social abate-
ment cost $1,000 70 1,615 3,770 8,651 10,311 13,164 18,827
N applied/
acre pounds 209 206 183 180 139 132 122 105
Water 1,000
diverted acre feet 2,393 2,400 2,393 2,244 2,057 1,995 1,916 1,849
Mile 30 flow, 1,000
August acre feet 100 100 100 130 158 164 176 186
N concentra-
tion, August mg/1 0.87 0.86 0.76 0.56 0.38 0.35 0.28 0.22
Sediment lost/
irrigable acre tons 1.74 1.72 1.68 1.62 1.60 1.61 1.21 1.26
Maximum
temperature F 75.5 75.4 75.4 74.0 70.9 70.4 69.8 69.3
Benchmark solution,
k Maximum concentration.
-------
irrigation systems is not induced by a nitrogen tax. The nitrogen
concentration goal was met by a combination of reduced nitrogen
losses caused by lower fertilizer application rates and increased
river water flow resulting from reduced irrigated acreage. Reduction
of irrigated pasture crops constituted most of the lost acreage.
Water temperature reduction was accomplished by increased river flow
as documented by flow data at river mile 30 (the main downstream gaug-
ing station) shown in Table 1. Satisfying nitrogen concentration and
water temperature goals was relatively expensive in Solution 2: a
41% reduction of producer income and a social abatement cost of
$13.2 million.
The level of social abatement cost determined in Solution 2
resulting from progressively improving nitrogen concentration, water
temperature, and sediment losses are shown in Figures 2, 3, and 4,
respectively. As expected, the curves are generally convex to the
origin, suggesting that social abatement costs increase at an in-
creasing rate as water quality is improved. Caution should be used
in the interpretation of Figures 2, 3, and 4. The social abatement
cost depicted in each is the cost when the total social abatement
cost is attributed to a single component of water. Therefore, it
is not legitimate to add the abatement cost of reaching a specific
nitrogen concentration goal to the cost of attaining a particular
water temperature goal as that involves double counting of costs.
The fact that policies designed to affect primarily one water quality
component also affect the others makes evaluating the cost of chang-
ing only one impossible.
Solution 3 shown in Table 2 imposed a charge for irrigation
water. Nitrogen concentration reduction occurred for two reasons.
First, nitrogen loss was reduced by improved irrigation systems
which lower deep percolation drainage volume. Second, the increased
river flow caused by higher water prices diluted the nitrogen enter-
ing the river with return flows. Sediment losses were reduced by
426
-------
SOLUTION 2
SOLUTION 3
SOLUTION 5 1
SOLUTION 5 2
N CONCENTRATION (mg/1)
Figure 2. Social abatement costs for reducing nitrogen concentration.
427
-------
20
18
16
SOLUTION 2
SOLUTION 3
SOLUTION 5-1
SOLUTION 5 2
14
12
10
\
\\
\ \
\ \
\ \
\\
\\
\N..
0W
4__
68
69 70 71 72 73 74 75
WATER TEMPERATURE (°F)
Figure 3. Social abatement costs for reducing water temperature.
428
-------
20
SOLUTION 2
SOLUTION 3
SOLUTION 5-1
SOLUTION 5*2
(/)
z
o
-J
_J
h
(O
o
o
H
Z
UJ
s
UJ
h-
<
CD
<
<
O
8
1.6 1.8
1.2 1.4
0.2 0.4
1.0
0.6 0.8
SEDIMENT LOST (TONS/ACRE)
Figure 4. Social abatement costs for reducing sediment losses.
429
-------
Table 2. Income, abatement cost, resource use and water quality determined with water charges.
Solution 3 - Water Charge ($/acre foot)
Item Unit Solution l3 5 10 15 20
Net crop income
$1,000
106,910
98,043
90,064
83,963
78,959
Irrigated acreage
1,000 acres
453
453
395
319
261
Producer cost
abatement
$1,000
8,867
16,845
22,946
27,951
Social abatement cost
$1,000
655
3,369
7,123
11,894
N applied/acre
pounds
209
206
213
226
269
Water diverted
1,000
acre feet
2,393
2,160
1,791
1,415
1,069
Mile 30 flow,
August
1,000
acre feet
100
107
166
211
251
N concentration,
August
mg/1
0.87
0.79
0.45
0.33
0.28
Sediment lost/
irrigable acre
tons
1.74
1.31
0.88
0.64
0.20
Maximum temperature
F
75.5
74.7
70.6
68.8
67.6
a
b
Benchmark solution.
Maximum concentration.
-------
the inducement of improved irrigation systems and practices. Water
temperature was reduced because a water charge promotes water con-
servation and thus increases river water flow. A water charge of
$20/acre foot was sufficient to satisfy all three environmental
goals, though the sediment goal was met with a $10/acre foot charge
and the temperature goal with a $15/acre foot charge. Irrigated
acreage, mostly irrigated pasture, was reduced by 43% with a $20
charge while producer income was reduced 26%. Social abatement, cost
was $11.9 million or 10% lower than Solution 2 with a $.60 nitrogen
tax of $.60/pound, indicating that the water charge was a more
efficient policy tool. Social abatement cost is shown graphically
in Figures 2, 3, and 4.
Solution 4 reduced water rights by a uniform percentage in all
regions of the Basin. Results of Solution 4 are summarized in Table
3. Water quality goals were met in Solution 4 for the same reasons
they were met in Solution 3: reduced irrigation water use. A 50%
reduction of water rights was required to satisfy all environmental
goals, although the sediment loss goal was met by a 30% reduction
and the temperature goal by a 40% reduction. The 50% reduction of
water rights entailed a 36% reduction in irrigated acreage and a 16%
reduction in farm income. The producer abatement cost was lower
with a water rights reduction than with a water charge because re-
striction of rights does not involve the collection of taxes as a
water use limiting mechanism. However, social abatement cost (the
same as producer abatement cost in this case) was higher than either
the fertilizer tax or the water charge. Uniformly reducing water
rights in all regions caused an inefficient allocation of irrigation
water among regions. Those regions where the marginal value of
water was high, such as tree fruit regions, were required to reduce
usage by the same proportion as those regions where its marginal
value was relatively low. The inefficiency was reflected by a rela-
tively high abatement cost.
431
-------
Table 3. Income, abatement cost, resource use and water quality determined with water rights reductions.
Solution 4 - Water Rights Reduction (%)
Item
Unit Solution 1
10
20
30
40
50
Net crop income
Irrigated acreage
Producer abatement
cost
Social abatement
cost
N applied/acre
Water diverted
Mile 30 flow,
August
N concentration,
August
Sediment lost/
irrigable acre
Maximum temperature
$1,000 106,910 106,224 104,649 101,713 96,124 90,317
1,000
acres
$1,000
$1,000
pounds
1,000
acre feet
1,000
acre feet
mg/1
tons
F
453
209
2,393
100
0.87
1.74
75.5
452
686
686
206
2,195
111
0.76
1.12
74.0
417
2,261
2,261
211
1,951
140
0.56
1.05
71.6
381
5,197
216
1,706
167
0.44
0.61
70.1
336
197
0.36
0.38
68.9
288
5,197 10,785 16,593
10,785 16,593
222 237
1,464 1,219
228
0.30
0.23
67.8
a
b
Benchmark solution.
Maximum concentration.
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Solution 5 combined a nitrogen fertilizer tax with a charge for
irrigation water for pollution abatement purposes. Results are
shown in Table 4. Economic theory shows that limiting the use of
all inputs functionally related to an externality can lead to a more
efficient solution than that attained by limiting only a subset of
those inputs. River nitrogen concentration is a function of both
the use of nitrogen fertilizer and irrigation water, albeit indirect.
Sediment loss and water temperature are primarily functions of
irrigation water use. Thus, the inclusion of a nitrogen tax with a
water limiting policy would theoretically lead to less efficient
control of these parameters than a water limiting policy alone.
A nitrogen tax of $.20 in conjunction with a $10/acre foot
water charge (.20-10) or a nitrogen tax of $.40 in conjunction with
a $5/acre foot water charge (.40-5) were sufficient to meet the
environmental standards. The .20-10 policy met standards at a lower
producer and social abatement cost. Producer abatement cost was
relatively high, a 30% reduction in farm income as a result of taxing
both fertilizer and irrigation water use. Social abatement cost,
however, was lowest of all policies considered, $9.7 million.
Figures 2, 3, and 4 show social abatement costs for varying levels
of environmental quality determined with Solution 5. Figure 3 shows
that within the relevant ranges, taxing both nitrogen and water
(Solutions 5-1 and 5-2) resulted in more efficient control of nitro-
gen concentration than taxing either alone. Solutions 5-1 and 5-2
represent Solution 5 with nitrogen taxes of $.20 and $.40/pound,
respectively, and with water charges variable. Figures 3 and 4 show
that the combination policies less efficiently control temperature
and sediment loss than a water charge alone, but were more efficient
than a nitrogen tax alone. These results are consistent with the
functional relationships between inputs and environmental quality
and the theoretical discussion presented earlier.
433
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Table 4. Income, abatement cost, resource use and water quality determined with combination policies.
Solution 5 - Nitrogen tax ($/pound)—Water Charge ($/acre foot)
Item Unit Solution l3 .20-5 .20-10 .20-15 .20-20 .40-5 .40-10 .40-15
Net crop
income $1,000 106,910 80,539 75,233 70,276 65,548 68,350 63,529 59,103
Irrigated 1,000
acreage acres 453 386 296 296 237 295 276 259
Producer abate-
ment cost $1,000 26,371 31,676 36,621 41,349 38,560 43,380 47,807
Social abate-
ment cost $1,000 5,061 9,664 9,921 15,654 10,149 12,535 15,021
N applied/
acre pounds 206 190 203 203 23« 198 197 195
Water 1,000
diverted acre feet 2,293 1,749 1,344 1,317 983 1,356 1,229 1,133
Mile 30 flow, 1,000
August acre feet 100 176 219 219 263 220 233 244
N concentra-
tion, August rog/1 0.87 0.37 0.27 0.27 0.21 0.26 0.22 0.20
Sediment lost/
irrigable acre tons 1.74 0.69 0.50 0.16 0.06 0.60 0.44 0.03
Max imum
temperature F 75.5 69.9 69.5 68.5 67.4 68.5 68.1 67.8
3
Benchmark solution,
k Maximum concentration.
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CONCLUSIONS
This research has shown that it is possible to substantially
improve water quality in the Yakima River by controlling agricultural
inputs or activities. The environmental improvement was accomplished
with a reduction of farm income ranging from 16 to 41%, depending on
the policy employed. In addition to reducing farm income, these
policies would also impose a burden on the agricultural sector in the
form of reduced land values, and decreased activity in agricultural
input supply and agricultural processing firms. In all cases, the
primary crops affected were low value forage crops. Consequently,
the livestock sector would be affected most. The impact on agriculture
would also be felt in nonfarm sectors of the economy because reductions
of income in one sector indirectly reduce income in all sectors.
Net social cost ranged from $9.7 to $16.6 million depending on
the policy used. A trade-off existed between economic efficiency
and producer cost. For example, a combination nitrogen tax and water
charge had the lowest net social cost but was relatively expensive to
farmers. Reducing water rights uniformly had the highest social cost
of policies evaluated but was least costly to farmers. This policy
would minimize adverse agricultural impacts, but would result in less
efficient organization of agricultural production.
The results show that considerable improvement in water quality,
say 50% of the desired water quality standards, can be achieved with
relatively minor costs to agriculture or the public. For example,
from Figures 2, 3, and 4, it appears to be possible to attain 50% of
the desired standards for a social cost of approximately $2.5 million.
In contrast, complete attainment, or getting the next 50%, costs a
total of $9.7 million or $7.2 million more. The fact that environ-
mental improvement can be attained only at a cost is significant.
More significant from a policy standpoint however, is the fact that
costs increase at an increasing rate. Thus, while it may be
435
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relatively inexpensive to make substantial water quality improve-
ments, reaching high levels of water quality may require sacrifices
far out of proportion to the costs of initial improvements.
The policy which most efficiently reduced nitrogen concentration
in the river was one which simultaneously imposed a tax on nitrogen
fertilizer and a charge for irrigation water. This result is con-
sistent with economic theory since the nitrogen concentration of the
river is a function of both fertilizer application and water use.
A water charge alone was less efficient than the combination policy,
but more efficient than the other policies because nitrogen concen-
tration is strongly affected by water use. When used exclusively,
reducing water rights was more efficient than taxing nitrogen for
low levels of abatement, but the nitrogen tax more efficiently met
the 0.30 mg/1 water quality standard.
Water temperature was largely a function of river flow volume,
so policies that reduced irrigation diversions and thus water use
reduced Yakima River water temperature most efficiently. A water
charge was most efficient at temperatures below 71F because of the
economic inefficiencies fostered by a uniform reduction of water
rights. Policies that combined a water charge with a nitrogen tax
approached the efficiency of policies dealing exclusively with water
use when the water charge was sufficient to induce significant water
conservation. The nitrogen tax policy less efficiently reduced water
temperature since its major impact is on the use of nitrogen ferti-
lizer, rather than the use of water.
Policy choices greatly affect reductions in agricultural income,
and would surely affect the economy of the Basin differently. Poli-
cies involving taxes or charges for inputs more severely reduced pro-
ducer income than policies which involved restrictions on input use.
These distributional and secondary impacts may indeed be more im-
portant than strict economic efficiency in the final choice of the
most appropriate policy for water quality improvement.
436
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The choice of policies involves the evaluation of economic
efficiency and income distribution. This analysis has generally
shown that policies with a low social cost may have a high producer
cost, while those with a low producer cost may have a higher social
cost. For example, the uniform reduction of water rights had the
lowest producer cost but the higher social cost for reducing nitro-
gen concentration. It is theoretically possible to design policies
such as allocation of water rights where irrigation return flows are
highest and such as monitoring and restricting the quality of return
flow water leaving each farm. Such policies are both economically
efficient and possess a low producer cost, but the transactions costs
of administration often prohibit their use.
While several policies which could be implemented in the Basin
have been analyzed, there exist other combinations and variations
of these policies which might better combine the beneficial aspects
of the policies while minimizing the detrimental aspects. For
example, the improvement of irrigation efficiency alone was not suffi-
cent to meet proposed environmental standards. No attempt was made
to combine improved efficiency with other policies, though it is
likely that the proposed standards could be met at a considerably
lower cost by combining improved irrigation efficiency with input tax-
ation or restriction. In fact, policies taxing or restricting water
and nitrogen use may be more effective than indicated because no
provision was made to include the improvements in water and fertilizer
management that would likely occur except through the use of more
efficient irrigation systems.
The social cost of the water rights reduction policy could be
substantially lowered by reducing rights among regions in a fashion
that would maintain equivalence of water value among regions. Such
a procedure would involve greater than proportional decreases in
water rights in parts of the Basin where the marginal product of
water is low and smaller than proportional decreases of water rights
437
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in areas where the marginal product of water is high. This policy
would approach the efficiency of a water charge without the adverse
effects of a water charge on producer income. Distribution of in-
come among regions would be less equitable than that caused by a
uniform reduction of water rights, however.
This research demonstrates that the marginal value of water in
the Yakima Basin approaches zero, probably because the marginal cost
of water approaches zero in most cases. More importantly, the
marginal value of water is low over a relatively large segment of
its use. Nearly one-third of the irrigated land in the Basin pro-
duces pasture and grass crops returning a very low net income. Policy
makers should remember that substantial improvements in water quality
are possible by eliminating the low value uses of water with relative-
ly minor reduction in crop sales and net income, though again, the
distribution of reductions among individuals will not be uniform.
Economic theory prescribes that, In order to fully internalize
a pollution problem, abatement activity should continue to the point
that the marginal abatement cost equals the marginal benefit of the
reduced external effect. This study estimated only direct social
and producer costs of environmental improvement in the Yakima Basin.
It is certain, however, that the costs estimated understate the true
social and regional cost because secondary economic impacts were ig-
nored. The Yakima Basin is a farm based economy, and the substantial
reductions in agricultural activity would negatively affect farmland
value, and the farm input supply sector, agricultural processing,
and activities related to agriculture. These costs might be estimated
by an input-output analysis¦ The benefits which would accrue to the
public resulting from improved water quality in the Yakima River are
less easily estimated, however. The state of economic science does
not provide good measures of such benefits. In the case of the
Yakima Basin, it is possible that benefits of environmental improve-
ment would be relatively small because the river already meets the
438
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proposed standards above the City of Yakima.
Little has been said regarding the promise of water quality
improvement through public investment in such facilities as water
treatment plants or increasing upstream water storage facilities
for water quality improvement purposes. The cost and effectiveness
of irrigation return flow water treatment facilities has yet to be
convincingly demonstrated on a large scale basis, so their use remains
an engineering, rather than an economic problem. Some cost estimates
have indicated that increased water flow to dilute existing effluent
discharges might be feasible through enlargement of existing storage
reservoirs. However, the propensity to utilize free (or govern-
mentally subsidized) water resources for irrigation purposes and the
likelihood that construction costs are underestimated cast doubts on
the promise of such a solution.
Little also has been said here regarding the promise of other
kinds of public or private investment in the form of research and
development. There can be little question that technological develop-
ments can alter the inputs-outputs-effluents production relationship.
A recent example is the development of soil nitrogen stabilizing com-
pounds. The development of technologies which reduce externalities
without the private and social sacrifices involved with reducing
input use must be pursued.
Finally, examination of abatement costs shows that social cost
is more a function of the acceptable level of environmental quality
chosen than the method of abatement. While efficient abatement is
desirable, other goals, such as producer income or agricultural output,
may outweight the efficiency criterion, dictating both different
levels of environmental quality and a less efficient method of pollu-
tion abatement.
439
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RECOMMENDATIONS
Those unfamiliar with Pacific Northwest irrigated agriculture
might find water delivery and application efficiency in the Yakima
Basin and other areas appalling. Changes caused by the drought of
1976 demonstrate that crops in the Yakima Basin can be grown with
substantial reductions in water diverted. Agencies involved in
water management could encourage and promote irrigation district
improvements, water conservation, and the development and use of
more efficient and water conserving irrigation systems and practices.
To the extent that such educational and promotional campaigns are
effective, they should be pursued. However, in recognition of the
fact that inputs with zero marginal cost will generally be used to
provide their maximum total physical product, irrigation districts
should be encouraged to charge customers on the basis of water
delivered rather than acreage irrigated. The imposition of such a
charge and the need for water flow gauging would increase awareness
of water conservation from an economic and public relations stand-
point. The improved water use efficiency that would surely result
from even a nominal charge/acre foot of water would reduce losses
of nitrogen and sediment, and increase river flow, allowing greater
effluent dilution and lower water temperature.
It is also suggested that desired water quality standards be
defined in terms of the gain in social welfare associated with irri-
gation return flow pollution abatement. The high cost to agriculture
of meeting proposed standards makes the implementation of policies
to meet such goals politically impossible and morally questionable
unless concrete public benefits can be identified and at least
partially evaluated in economic terms. While the deficiencies of
this and related studies leave room for additional improvement in
the determination of water pollution abatement costs, it is doubtful
that further refinement of costs can further Illuminate the desir-
440
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ability of environmental improvement in the absence of benefit
estimation.
Finally, it is hoped that the more complete discussion and
understanding of problems regarding agriculture and our environ-
ment, that this study has, albeit small, attempted to foster, will
encourage the examination of issues from a variety of perspectives:
economic, technological, social, and environmental. Only through
the rational process of weighing alternatives can judgments be made
for the betterment of mankind.
441
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LITERATURE CITED
Baumol, W. J. and W. E. Oates. 1975. The Theory of Environmental
Policy. Prentice-Hall, Inc., Englewood Cliffs. 272 pp.
Copp, H. D. and D. T. Higgins. 1974. Hydraulics of surface water
runoff, In Model Development and Systems Analysis of the
Yakima River Basin. Washington State University Water Research
Center Report No. 17B. 150 pp.
Gossett, D. L. and N. K. Whittlesey. 1976. Cost of Reducing Sedi-
ment and Nitrogen Outflows from Irrigated Farms in Central
Washington. Washington Agricultural Experiment Station
Bulletin 824. 55 pp.
Kraft, D. F. 1975. Economics of Agricultural Adjustments to Water
Quality Standards in an Irrigated River Basin. Ph.D. Thesis,
Washington State University. 202 pp.
Langham, M. 1972. The theory of the firm and the management of
residuals. American Journal of Agricultural Economics. 54:315-
322.
Pfeiffer, G. H. 1976. Economic Impacts of Controlling Water Quality
in an Irrigated River Basin. Ph.D. Thesis, Washington State
University. 273 pp.
Pratt, P. F. and D. C. Adriano. 1973. Nitrate concentrations in
the unsaturated zone beneath fields in southern California.
Soil Sci. Soc. Amer. Proc. 37:35-36.
Raphael, J. M. 1962. Prediction of temperature in rivers and reser-
voirs. Journal of the Power Division. Proc. of the American
Society of Civil Engineers. 88:157-181.
Wisc.hmeier, W. H. and D. D. Smith. 1965. Predicting Rainfall Losses
from Cropland East of the Rocky Mountains. USDA Agricultural
Handbook No. 282. 47 pp.
442
-------
LITERATURE CITED
Baumol, W. J. arid W. E. Oates. 1975. The Theory of Environmental
Policy. Prentice-Hall, Inc., Englewood Cliffs. 272 pp.
Copp, H. D. and D. T. Higgins. 1974. Hydraulics of surface water
runoff, In Model Development and Systems Analysis of the
Yakima River Basin. Washington State University Water Research
Center Report No. 17B. 150 pp.
Gossett, D. L. and N. K. Whittlesey. 1976. Cost of Reducing Sedi-
ment and Nitrogen Outflows from Irrigated Farms in Central
Washington. Washington Agricultural Experiment Station
Bulletin 824. 55 pp.
Kraft, D. F. 1975. Economics of Agricultural Adjustments to Water
Quality Standards in an Irrigated River Basin. Ph.D. Thesis,
Washington State University. 202 pp.
Langham, M. 1972. The theory of the firm and the management of
residuals. American Journal of Agricultural Economics. 54:315-
322.
Pfeiffer, G. H. 1976. Economic Impacts of Controlling Water Quality
in an Irrigated River Basin. Ph.D. Thesis, Washington State
University. 273 pp.
Pratt, P. F. and D. C. Adriano. 1973. Nitrate concentrations in
the unsaturated zone beneath fields in southern California.
Soil Sci. Soc. Amer. Proc. 37:35-36.
Raphael, J. M. 1962. Prediction of temperature in rivers and reser-
voirs. Journal of the Power Division. Proc. of the American
Society of Civil Engineers. 88:157-181.
Wisc.hmeier, W. H. and D. D. Smith. 1965. Predicting Rainfall Losses
from Cropland East of the Rocky Mountains. USDA Agricultural
Handbook No. 282. 47 pp.
442
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