WATER POLLUTION CONTROL RESEARCH SERIES
16060 DOE 04/72
NITRATE IN THE UNSATURATED ZONE
UNDER AGRICULTURAL LANDS
U.S. ENVIRONMENTAL PROTECTION \GFNCY
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WATER POLLUTION CONTROL RESEARCH SERIES
The Water Pollution Control Research Series describes the
results and progress in the control and abatement of pollution
in our Nation's waters. They provide a central source of
information on the research, development, and demonstration
activities in the water research program of the Environmental
Protection Agency, through inhouse research and grants and
contracts with Federal, State, and local agencies, research
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Inquiries pertaining to Water Pollution Control Research
Reports should be directed to the Chief, Publications Branch
(Water), Research Information Division, R&M, Environmental
Protection Agency, Washington, DC 20460.
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NITRATE IN THE UNSATURATED ZONE UNDER AGRICULTURAL
LANDS
by
P. F. Pratt
Department of Soil Science and Agricultural Engineering
University of California
Riverside, California 92507
for
Research and Monitoring
ENVIRONMENTAL PROTECTION AGENCY
Project No. 16060DOE
April, 1972
For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 - Price 55 cents
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EPA Review Notice
This report has been reviewed by
Protection Agency and approved for publication.
Approval does not. signify that the contents necessarily
reflect the views and policies of the Environmental
Protection Jvgency, nor does mention of trade names or
commercial products constitute endorsement or recommenda-
tion for use.
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ABSTRACT
Six treatments from a fertility trial with citrus, four commercial
citrus groves and nine row-crop fields were studied to assess the
effect of fertilizer N on the concentrations of NO^ in the unsat-
urated zone between the root zone and the saturated zone. Nitrate-N
concentrations of 21 to 70 and 36 to 123 ppm in the water in the
unsaturated zone were found for citrus and row crops respectively.
The main factors were 1) the difference between N input and crop
removal, 2) drainage volume and 3) soil profile characteristics that
apparently control denitrification. In soil profiles having no
layers that restrict water movement and in which the net change in the
amount of N in the organic pool was essentially zero the amount of
N input minus crop removal expressed as N03 in the drainage water
provided a reasonably reliable estimate of the NO-} concentration in
the solution of the unsaturated zone. An estimate of transit time
for drainage water through the unsaturated zone provided a means of
calculating a field N balance.
This report was submitted in fulfillment of Grant No. 16060DOE under
the sponsorship of the Office of Research and Monitoring, Environ-
mental Protection Agency.
111
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CONTENTS
Section Page
I Conclusions 1
II Recommendations 3
III Introduction 5
IV Approach 7
V Methods 11
VI Results 13
VII Discussion 35
VIII Acknowledgments 39
IX References 41
X Publications and Patents 43
XI Glossary 45
v
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FIGURES
PAGE
RELATIONSHIP BETWEEN N03-N CONCENTRATION AND EX- 8
CESS N FOR FOUR DRAINAGE VOLUMES, CALCULATED
USING EQUATION J2]
RELATIONSHIP BETWEEN N0~ CONCENTRATION IN THE 14
SATURATION EXTRACT AND DEPTH (0- TO 3-m) AND
BETWEEN NOo-N CONCENTRATION IN THE SOIL SOLUTION
AND DEPTH (3- TO 30-m) FOR TWO TREATMENTS
RELATIONSHIP BETWEEN Cl~ CONCENTRATION AND DEPTH 15
FOR TWO TREATMENTS. THE TOP PART IS FOR
SATURATION EXTRACTS AND THE LOWER PART IS FOR
SOIL SOLUTIONS
RELATIONSHIPS AMONG EXCESS N IN THE SOIL, 19
NO^-N IN THE SOIL WATER OF THE UNSATURATED
ZONE (3- TO 30-m DEPTH) AND THE LEACHING
VOLUME. THE CURVES WERE CALCULATED FROM
EQUATION [2], THE NUMBERED POINTS ARE FROM
THE FERTILITY TRIAL, AND THE LETTERED
POINTS ARE FROM COMMERCIAL GROVES. FOR
TREATMENT 21 ONLY THE DATA FOR THE 3- TO
20-m DEPTH WERE USED
RELATION BETWEEN MEASURED NO^-N IN THE SOIL 20
AND THE AMOUNT ESTIMATED BY MULTIPLYING THE
AVERAGE EXCESS N PER YEAR TIMES THE TRANSIT
TIME IN YEARS PER 30 m
NITROGEN RECOVERY AND N LOSS BY DENITRI- '22
FICATION IN RELATION TO TOTAL N INPUT
RELATIONSHIP BETWEEN NOo-N CONCENTRATION AND 28
DEPTH FOR THREE SITES IN FIELD USED FOR ROW
CROPS
RELATIONSHIP BETWEEN Cl~ CONCENTRATION AND 29
DEPTH FOR THREE SITES IN FIELDS USED FOR
ROW CROPS
RELATIONSHIP BETWEEN EXCESS N AS CALCULATED FROM 34
EQUATION [2] PER TRANSIT TIME AS CALCULATED FROM
EQUATION [3], AND THE MEASURED NO~-N EXCESS IN THE
SOIL FOR THREE BULK DENSITIES. THE CORRELATION
COEFFICIENT IS FOR A B.D. OF 1.8. THE NUMBER
IDENTIFIES THE SITE
vl
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TABLES
No. Page
1 Nitrogen Fertilizer Treatments on the Six 13
Treatments Selected for Deep Soil Sampling
in the Long-Term Fertilizer Trial
— 2 — —
2 Average Concentration of SO, , Cl, and NO^ 16
in the Saturation Extract or the 0- to 2.4-m
Depth of Soil
3 Average Concentration of NO^-N and Cl in the 17
Soil Solution, (Field Water Content) of the
3- to 30-m Depth of Soil and the Calculated
Leaching Fraction
4 Linear Correlation Coefficients and Regression 17
Equations for the Relationships of SOT^, Cl~,
and NO^ Concentrations in the 0- to 2.4-m Depth
and the NO^-N Content of the Soil Solution of
the 3- to 30-m Depth Versus Leaching Fraction
5 Data for Transit Time for Water and NCC to Move 18
30 m and N Balance for the Indicated Transit
Time
6 Data for Four Commercial Groves Sampled in the 23
Spring of 1970
7 Description of the Row-Crop Sites Studied 24,25
8 Data for NO^, Cl~, EC and Water Content for 27,28
the 0- to 2.4-m and 3.3- to 15-m Depths of
Soils for Nine Row-Crop Sites
9 Estimates for the Transit Time for Water in 30,31
the Unsaturated Zone and N Balance for Nine
Row-Crop Sites
Vll
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SECTION I
CONCLUSIONS
1. Factors that have an effect on the NOo in drainage water under
lands used for citrus and row crops in the Santa Ana River Basin in
Southern California are
1) difference between N input and N removed by harvested crops,
2) drainage volume in which the NO;} is dissolved,
3) mineralization of N from the organic pool, and
4) denitrification.
2. The N input minus N removed in harvested crops provides a
reasonable estimate of the N0~ that will leach to the ground water
from soils that have open-porous profiles with no layers that restrict
water movement and in which there is no net change in the N in the
organic pool.
3. Knowledge of the drainage volume, N inputs, crop removal of N and
transit time for drainage water to move through the unsaturated zone
provides a means of estimating denitrification in the field in soils
where the organic N content is constant over the period of time that
is considered.
4. The general correlation between soil profile characteristics and
denitrification losses, as shown in this report, suggests that a
rating of soils in terms of probable denitrification might be useful
in the assessment of the effects of agriculture on ground water
quality.
5. The transit times measured for drainage water to move to the
ground water suggests that the effects of agricultural land use in
the 1920 to 1940 period are showing in wells in 1971 and that the
effects of present practices will not show until about 1990 or 2000.
6. Recommended practices for fertilizer N for citrus are such that
when combined with adequate leaching and good yields will put no
more than about 20 ppm NO^-N into drainage effluents, assuming no
denitrification. Very careful management could reduce this estimate
to near 10 ppm.
7. Recommended practices for N fertilizer in row crops would put the
N03~N concentration between about 20 to 50 ppm, assuming no
denitrification.
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SECTION II
RECOMMENDATIONS
An intensive educational program needs to be developed to show farm
managers the effects of the use of excess N on NO^ in ground waters,
to encourage them to use no more N than is recommended by the
Agricultural Experiment Stations and the Agricultural Extension
Services and to use diagnostic techniques not only to avoid defi-
ciencies of N in crops but also to avoid excesses of NO^ in drainage
waters.
Research along the following lines should be encouraged:
1. The development of denitrification ratings for soil
series to help delineate where NO I in drainage waters
is a serious problem and where it might be ignored
because of effective denitrification.
2. Development and assessment of costs of fertilizer
materials, and methods of application that increase
the efficiency of fertilizer N use.
3. Feasibility studies on the interception of drainage
waters before they become mixed with ground waters
and the recycling and disposal of such drainage
waters to suitable sumps.
4. Socio-economic studies aimed at the conflict between
fertilization for maximum yield of crops and restriction
on fertilizer use for maximum quality of drainage
waters.
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SECTION III
INTRODUCTION
Until recently the main reason for measuring the concentration of
nitrate (NOo) in the water in the soil material below the effective
rooting depth of crop plants was to evaluate the loss of NOo to the
crop and estimate the loss of production or a need for more fertilizer
N to adjust for the losses. In the last few years, with a tremendous
increase in the concern for NO^ in surface and ground waters, the
emphasis has changed to the study of N07 leaching because of the
potential pollution of waters.
A great deal of information is available on nitrogen (N) behavior in
soils and the NOg leaching from the plant-soil system in relation to
climate and the nature of the soil and crop. Thomas (12) and
Stewart (10) have reviewed some of the more important factors. In
both of these reviews, the authors strongly emphasized the importance
of the rainfall or quantity of drainage water as a factor in leaching
of N03. Nelson (7) discussed the research techniques and approaches
needed to resolve some of the unanswered questions on nutrient losses
from the soil-plant system. He strongly emphasized the need for
assessing NO^ losses to ground waters using small confined watersheds,
but he also stated that deep soil profile samples can lead to useful
information. Stout and Burau (11) have demonstrated the usefulness
of deep soil profile samples to determine the NO^ concentration in
the unsaturated zone.
In irrigated areas of Southern California, with its short rainy season
which comes during the winter months and a long dry season requiring
intensive irrigation for successful crop production, a number of
studies of NO^ losses by leaching have been conducted. Pratt and
Chapman (8) found small leaching losses of NO^ in a 20-year lysimeter
study in which leaching was accomplished only during the years of
above average winter rainfall. Bingham, et_ a^. , (2) found consi-
derable NO^ removed in drainage waters of a small watershed in which
the leaching fraction was approximately 50 per cent of the irrigation
water used.
The present investigation was undertaken to determine the potential
contribution of fertilizer N use on citrus and row crops to the NOg
in ground waters in the Santa Ana River Basin of Southern California
where a deep unsaturated zone exists between the soil profile or
crop-root zone and the water table or saturated zone. Specifically,
the objectives were 1) to relate NO^ concentrations in the unsat-
urated zone to N inputs, N removal by harvested crops and the volume
of drainage water, 2) to obtain some estimates of transit times for
drainage water through the unsaturated zone, 3) to estimate denitri-
fication losses on a field basis and 4) to assess the probable effects
of present fertilizer N recommendations on NOg in ground waters.
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The main field used for this study was a long-term fertilizer trial
with Washington navel oranges. The trees were planted in 1917, but
differential treatments were started in 1927 when 43 treatments were
imposed. The main treatments consisted of various rates and sources
of N fertilizers. The treatments were discontinued in 1962 after
which uniform N treatments were used on all plots. Six treatments
that were thought to have had a long-term effect on leaching fractions
were sampled to the 30-m depth in June, 1969. In addition, four
commercial citrus groves were sampled to the 15-m depth or to the top
of the water table in June of 1970 and nine row-crop fields were
sampled to the 15-m depth in the Winter of 1971.
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SECTION IV
APPROACH
In the analysis of the data the assumptions were that 1) the pool of
organic N in the soil was constant from year to year so that the net
mineralization or immobilization was zero; 2) the rate of movement
of NO^, below the zone of root influence, was the same as the movement
of water; and 3) the amount of N denitrified was equal to the total
N input minus the sum of removal in the fruit plus that found in the
soil. All three assumptions seem to be justified. In the climate of
Riverside, moderate additions of organic materials add little to the
permanent or semi-permanent organic pool because of rapid minerali-
zation. The second assumption is justified for steady-state or near
steady-state conditions but would not be justified in cases of large
and abrupt changes in the NO^ concentration of the water leaving the
root zone.
The volume of the drainage water was calculated from the equation
D =
1-LF
which is derived from the definition that the leaching fraction is
equal to the drainage volume divided by the sum of the evapotrans-
piration plus the drainage volume, where D is the volume of drainage
expressed in surface cm/year, LF is the leaching fraction, and ET is
the evapo transpiration in surface cm/year. In all cases with citrus,
the average ET demand during the irrigation season was assumed to be
60 surface cm/year. In row-crop fields, D was calculated as the
product of the amount of water used and the leaching fraction.
The amount of excess N available for leaching was calculated from the
equation
DNOn
Excess N = - - [2]
10
where excess N is expressed in kg/ha per year, D is the drainage
volume in surface cm/year, and the NO^ is ppm NO^-N in the soil
water below the zone of root influence. The units for the constant
are mg cm ha kg~l liter . The relationship between NOg-N concen-
tration and excess N available for leaching as NO^ for four drainage
volumes is presented in Fig. 1.
The transit time for water in the unsaturated zone below the root
zone was calculated as
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LEACHING VOLUME IN
SURFACE cm
(SURFACE inches)
' A 15 (6)
B 30 (12)
C 45 (18)
L D 60 (24)
0
50 100
EXCESS
200
per ha
300
Figure 1. Relationship between NOo-N concentration and excess N
for four drainage volumes, calculated using equation [2]
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T = S£ [3]
D
where T is transit time in years, S is the soil depth in cm, 9 is
the volumetric water content, and D is the volume of drainage water
in surface cm/year. The volumetric water content was calculated
as the gravimetric water content times the bulk density, which was
1.8 in the long-term fertilizer trial and was assumed to be the
same in the four commercial groves. Equation [3J assumes a complete
displacement of water in the soil profile.
The leaching volume, defined as the water that moved past the
effective rooting zone of the crops and expressed as a fraction of
the irrigation water used, was calculated as a ratio of the Cl~
concentration in the irrigation water to that in the drainage water -
i.e., the solution in the unsaturated zone - and is equal to the
volume ratio of drainage water to irrigation water. This equality
comes from a consideration of simple concentration effects assuming
no precipitation or dissolution of Cl~ salts, negligible Cl~ removal
in harvested crops and no Cl~ additions in fertilizers. The soils
studied have no known mechanism for precipitation or dissolution of
Cl~ salts, and most crops remove only relatively small amounts of Cl
The mineral fertilizers used contained no important amounts of Cl~
salts, and the Cl~ added in animal manures was relatively small
compared to the amount in the irrigation water.
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SECTION V
METHODS
Soil samples were taken with a power-driven bucket auger at 30-cm
intervals to the 3-m depth inclusive and at 1.5-m intervals below the
3-m depth. Samples were taken to the drying shed in air-tight
containers where they were weighed, air dried and weighed again. The
loss in weight was used to calculate the water content of the field
sample. Air-dried samples were made to a saturated paste by adding
distilled water, and saturation extracts were obtained by suction.
Extracts were analyzed for NO^ and Cl~ and in some cases for SO^. In
the long-term fertility trial, three holes were drilled per plot and
two plots were sampled per treatment. In the commercial citrus groves
and in the row-crop fields three holes were drilled per site.
The data reported for the 0- to 3-m depths are in meq/liter in the
saturation extracts. The variations in field water content within
this soil depth were considered to be so great, because of cyclic
changes with irrigation, that conversion to the soil solution basis
was considered meaningless. Below the 3-m depth the data were con-
verted from concentrations in the saturation extract to concen-
trations in the soil solution. The cyclic changes in water content
with irrigation and removal by evapotranspiration are limited to the
soil volume above the 3-m depth, so that below this the water content
is considered to be that at which water is moving to the water table.
11
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SECTION VI
RESULTS
Citrus
A description of the treatments selected from the long-term fertility
trial with citrus is presented in Table 1. The soil is a Greenfield
Table 1 - Nitrogen fertilizer treatments' on the six
treatments selected for deep soil sampling
in the long-term fertilizer trial
Fertilizer nitrogen treatments
Treatment 1963 to 1969 inclusive Prior to 1963
No. Annual rate Source* Annual rate Source*
kg/ha kg/ha
1 168 NH,NOo 56
6 168 NH4N03 56
21
23
26
30
168 NH4N03
168 NH,NOo
168 NH4N03
168 NH^NO-j
392
616
392
220
Ca(N03)2
Ca(N03)2
NaN03
Steer
Manure
*The average amount of N added as N03~N in the irrigation water (56
kg/ha/yr) is included in these rates.
sandy loam which has no layers that restrict water movement within
the 0- to 3-m depth. Below the 3-m depth the soil material consists
of interlayers of fine to coarse sands with sandy loams to loams.
The only layer with an appreciable organic matter content is the
surface 30 cm.
Figures 2 and 3 present concentration-depth relationships for N03 and
Cl~ respectively (see pages 14 and 15).
Data for the 0- to 3-m depth are given for saturation extracts because
of the transitory nature of the field water content of this depth;
whereas, for the 3- to 30-m depth the water content was considered
sufficiently constant that the conversion of the data to the field
13
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0_
LU
Q
NITRATE , MEQ PER LITER
2 4 6 8 10
•-
TREATMENT
I
I
I
TREATMENT 23
NITRATE-N, ppm
20 40 60 ' 80 100
30
Figure 2. Relationship between NO concentration in the saturation
extract and depth (0- t8 3-m) and between NOI-N concen-
tration in the soil solution and depth (3- to 30-m) for
two treatments.
14
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CHLORIDE, MEQ PER LITER
.0 12345
E
x
H
Q.
UJ
Q
O TREATMENT I
• TREATMENT 26
Figure 3,
Relationship between Cl concentration and depth for two
treatments. The top part is for saturation extracts and
the lower part is for soil solutions.
15
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water content was justified. The data in these figures are typical
of the variabilities found in all treatments. The data represent
the averages of six sampling sites for a stated depth of soil.
2— — —
Table 2 presents data on the average SO, , Cl and N03 concentrations
in the saturation extracts of the 0- to 2.4-m depth of soil for the
long-term fertility trial.
2-
Table 2 - Average concentration of SO, , Cl , and
NO-} in the saturation extract of the 0-
to 2.4-m depth of soil
Treatment Average Concentration
No. Sulfate Chloride Nitrate
1
6
21
23
26
30
3.4
4.2
3.9
8.8
11.4
6.3
— meq pel j_j_ue;L-
1.4
1.8
1.5
2.1
2.5
1.8
2.0
3.3
2.9
7.1
8.1
3.0
Table 3 presents data on the average NO--N and Cl concentrations in
the soil water, i.e., soil solution for the 3- to 30-m depth of
soil and the leaching fraction. Linear correlation coefficients
and regression equations for NOo and SO, ~ in the saturation extract
of the 0- to 2.4-m depth, and NO^-N content of the soil solution in
the 3- to 30-m depth versus leaching fraction are presented in
Table 4.
16
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Table 3 - Average concentration of NO -N and Cl in the soil
solution (field water content) of the 3- to 30-m
depth of soil and the calculated leaching fraction
Treatment
No.
Leaching
Nitrate N Chloride Fraction
Water
Content
1
6
21
23
26
30
men/ •L-L Lt:i "~
1.50
1.57
1.50(3.21)*
3.36
4.57
1.79
1.62
1.81
1.58
1.91
2.51
1.81
0.40
0.36
0.41
0.34
0.26
0.36
/o uy wco-giiL.
9.0
9.4
9.0
7.9
9.1
8.9
Average was 21 ppm to the 20-m depth inclusive and 45 ppm from the
21- to 30-m depth.
Table 4 - Linear correlation coefficients and regression
equations for the relationships of SOT, Cl , and
NOo concentrations in the 0- to 2.4-m depth and
the NO~-N content of the soil solution of the 3-
to 30-m depth versus leaching fraction
Variable
Correlation
Coefficient
Regression
Equation
Saturation
Extract
Sulfate
Chloride
Nitrate
-0.93**
-0.97**
-0.88*
Y = 26.0 - 55.4x
Y = 4.46 - 7.35x
Y = 19.1 - 41.5x
Soil Solution
t
Nitrate - N
-0.92**
Y = 143.4 - 310.Ix
*Signifleant at the 0.05 probability level
**Significant at the 0.01 probability level
t In treatment 21 the NO^-N concentration in the 3- to 20-m depth
was used.
17
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The relationships among NO»-N in the water of the 3- to 30-m depth,
excess N in the soil, and the volume of the drainage water are
presented in Figure 4. Excluding the lower depths of treatment 21,
calculated excess N varied from 39 to 145 kg/ha per year. From this
figure one can see that without data for the drainage volume the
NOT concentration in the soil water does not provide an index to the
amount of N07 that was available for leaching. Evidently treatments
23 and 26 reflect the higher rates of N application used prior to
1963 more than treatments 1, 6, 30 and 21.
For some unexplained reason, treatment 21 reflects the change in
treatment of 1963, with the lower depth representing the 392 kg/ha
rate and the upper depth representing the 168 kg/ha rate. In all
other treatments the NCK-N concentration showed no changes with
depth that would represent the change in fertilizer rates in 1963.
The lower depth in treatment 21 showed an excess N of 187 kg/ha per
year whereas the upper layer showed an excess N of 87 kg/ha per year.
Comparisons against N added (Table 1) and removal in fruit (Table 5)
show denitrification rates of 34 and 158 kg/ha per year, respec-
tively for input rates of 168 and 392 kg/ha per year.
Table 5 - Data for transit time for water and N0~ to move 30 m
and N balance for the indicated transit time
Nitrogen, kg/ha for transit time
Transit
Time for
Treatment 30 m
No.
1
6
21
23
26
30
Years
12.0
14.2
11.5
15.5
22.7
14.2
In soil
0- to 30-m
Added
1,460
1,580
2,940
6,410
7,330
2,760
Depth*
1,220
1,390
1,670
2,990
3,890
1,510
Loss from
Remove d System**
in
Fruit
470
540
480
640
790
650
Entire
Period
-230
-350
790
2,780
2,650
600
Yearly
-19.0
-24.6
68.7
179.3
116.7
42.2
%
Loss
-15.8
-22.2
26.9
43.4
36.2
21.7
Measured at the end of the period.
If value is negative, it is an unaccounted-for increase in input,
if positive it is an assumed-denitrification loss.
The soils in the commercial groves were mapped as Arlington loam for
sites A and C, Greenfield sandy loam for site B, and Hanford sandy
loam for site D. A more detailed look at the soil profiles on the
sampling sites showed that sites B and C had sandy loam materials to
the 3-m depth with no textural discontinuities and no pans of any
kind. Sites A and D both had clayey horizon at the 45- to 100-cm
depth with coarse to fine sands at the 100-cm depth. This textural
18
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100
0 10 20 30 40
LEACHING VOLUME , surface cm
Figure 4.
water of the unsatu
toS3 -i ?;
(3 ^0 p ^
" fro, the f«ttllt, trl.!. and
used .
19
-------
discontinuity is sufficient to create water-saturated soil during
irrigation and would be conducive to denitrification; whereas, in the
other two soils, a saturated soil during irrigation would not be
expected. Because the rates of N applications varied only slightly
(154 to 188 kg/ha per year); the possible profile effect on denitri-
fication would appear to explain the low NOo-N concentration in sites
A and D in comparison to those in sites B and C.
The relation between measured NOn-N in the soil and the amount estimated
by multiplying the estimated excess N per year times the transit time
in years per 30 m is presented in Fig. 5. Because the excess N per
year is based on the average (for treatment 21 the weighted average)
N03 in the 3- to 30-m depth and the transit time is for the 0- to
30-m depth, we would expect the estimated value to be greater than the
measured value in the 3- to 30-m depth (line C). However, when the
NO^-N in the 3- to 30-m depth is projected to 30 m, a near perfect
agreement between measured and estimated was obtained (line B). The
difference between lines A and B represents the accumulation of the
NO^ in the surface 3 m in comparison to the concentration below 3 m.
4000
3000
- 2000
Q
UJ
cc
r>
<
LJ
1000
A=0- to 30-m depth
B = 0- to 30-m depth
projected from
3-to 30-m depth
_C = 3- to 30-m depth
1000 2000 3000
ESTIMATED N, kg per ha
4OOO
Figure 5. Relation between measured N03~N in the soil and the amount
estimated by multiplying the average excess N per year
times the transit time in years per 30 m.
20
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This accumulation of NO., in the zone of influence of the root system
of the trees is thought to be the result of a combination of factors
including 1) recycling of N by the trees, 2) the heterogenous nature
of the salt distribution with the use of permanent furrows for
irrigation, and 3) the continuous movement of water down with each
irrigation and up during periods of extraction by trees between
irrigations. During irrigation, water moves down rapidly through large
pores with poor efficiency of salt movement, but the water that
moves back up to the root zone moves through smaller pores with a
larger efficiency of salt transport. Also, irrigation with permanent
furrows moves salt into furrow ridges and this salt is moved out slowly
by infrequent winter rains. This combination of salt cycling within
the zone of root influence and the salt entrapment in permanent furrows
is thought to be the mechanism by which the N03 that had accumulated in
soils prior to 1963 was carried over to the 1969 sampling date in treat-
ments 23, 26 and 30.
Data for the calculated transit times and for N balances for the
calculated transit times for the treatments in the long-term fertilizer
trial are presented in Table 5, and data for calculated denitrification
are presented graphically in Fig. 6. Treatments 1 and 6 showed small
gains which could be within errors of estimate. In treatments 21
through 30 fairly large amounts of N were lost by denitrification.
The high correlation between recovery in soil and fruit and the total
N input would suggest that the losses are real and that up to 43%
of the total N input was denitrified.
21
-------
8000
° 6000
-------
merits 32, 26 and 30 is not readily explained by any data or manage-
ment history.
Data for the four commercial groves sampled in the spring of 1970 are
presented in Table 6. The N balance calculated for a 10-year period
is a projection of the data to put all four sites on the same basis.
The N balance data also must be considered as rough estimates pri-
marily because the removal by fruit is an assumed value of 40 kg/ha
per year. However, in this N balance data the two soil profiles that
had clayey horizons overlying the sands had an average of 52 per cent
loss by denitrification, and the two soil profiles with no textural
discontinuities or pans showed an average net unaccounted-for gain
of 5 per cent.
Table 6 - Data for four commercial groves sampled in the
spring of 1970
Commercial groves sites
Data description ABC D
Leaching fraction
Leaching volume, surface cm
Water content, % by weight
Water content, % by volume
Sampling depth, m
Transit time, years
Transit time, years per 30 m
0.15
10.6
9.70
17.5
13.5
22.0
49
0.25
20.0
7.36
13.2
15.0
10.0
20
0.31
27.0
9.61
17.3
7.5
4.8
19
0.24
18.9
21.4
31.1
13.5
22.0
49
Nitrogen Balance for 1960-1970 Period
Added, kg/ha
Fruit removal,
Excess in soil
Loss, kg/ha*
Loss, %*
kg /ha
, kg /ha
1,540
400
390
750
49
1,540
400
1,420
-280
-18
1,670
400
1,160
110
7
1,880
400
430
1,050
56
If value is negative, it is an unaccounted-for increase in input,
if positive, it is assumed denitrification loss.
Row Crops
The soil type and subgroup, a brief profile description, the rate and
sources of fertilizer N and the cropping history for each of nine sites
are presented in Table 7. All sites were irrigated by furrow irri-
gation except winter barley crops which were irrigated with sprinkler
systems. The NOo added in the irrigation water was not included in
the data in Table 7 but was included for the N input data in Table 9.
23
-------
Table 7 - Description of the row-crop sites studied
Site Soil
No. Description
Profile
Description*
N rate (kg/ha/yr)
and source
Croppings
San Emigdio sandy Open-porous
loam (pH 8.0); well drained.
Typic Xerorthent
350 as (NH4)2S04 for 5 yrs.
Sorrento clay
loam (pH 7.5);
Calcic Haploxeroll
Buchenau loam
(pH 7.9); Typic
Durixeralf
Arlington sandy
loam (pH 7.8);
Haplic Durixeralf
Stratified with
silts & sandy
loams.
Stratified,
presence of
caliche at the
1.5-m depth.t
Hardpan about
40 cm thick
at the 0.8-m
depth.
1,350 as chicken manure
(625 kg N per year),
(NH4)2S04, NH4N03, and
cyanamid for 5 years.
470 as barnyard and
chicken manures (1960-
68), Ca(N03)2 solutions,
and mixed fertilizers
for 11 years .
163 mostly as chicken
manure and a small
amount of mixed ferti-
lizers applied during
seeding for 6 years.
Strawberries & barley on
alternate years. The
strawberry crop was
mulched with a plastic
film each season to in-
crease soil temperature
and conserve soil
moisture.
Celery during winter &
sweet corn during
summer.
Cabbage, green onions,
celery, and romaine
lettuce in rotation.
Sugar beets and grain
crops in rotation.
-------
Table 7 - Continued
Site Soil
No. Description
Profile
Description*
N rate (kg/ha/yr)
and source
Croppings
Hanford loamy
sand (pH 7.7);
Typic Xerorthent
San Emigdio fine
loam (pH 7.3);
Typic Xerorthent
Hanford sandy
loam (pH 7.3);
Typic Xerorthent
Grangeville fine
sandy loam (pH
7.7); Aquic
Haploxeroll
Open-porous
well-drained.
Stratified; pre-
sence of claypan
about 30 cm thick
at the 0.9-m depth
Hardpan about 45
cm thick at the
0.8- to 1.0-m
depth.
Stratified; slight
mottling at about
the 1-m depth.
Same as site 4.
370 mostly as chicken
manure & a small amount
of mixed fertilizers for
4 years.
224 as chicken manure,
aqua NH3, and
for 11 years.
480 as barnyard manure
(1959-63), (NH4)2S04,
and mixed fertilizers
for 11 years.
Delhi loamy fine Open-porous, well 320 mostly as barnyard
sand (pH 7.4); Typic drained manure & a small amount of
Xeropsamment (Taxadjunct) (NH4)?SOA for 11 years.
Grain crops with sugar
beets in 1969-70 only.
One year of watermelons
& three consecutive
years of carrots.
Potatoes for one year
followed by three con-
secutive years of
cereal crops.
Potatoes in 1964,'65
and '69 & sweet corn.
Eight consecutive years
of alfalfa & later 3
years of potato-barley.
*Surface 2 m with emphasis on the presence of restricting layer.
+Annual average for the indicated period.
tFor sites 3, 6 and 8 the profiles were stratified with silts, very-fine sands, and sandy loams.
The caliche in site 3 had not accumulated in a solid layer, was about 1 to 2 cm in diameter, and
scattered.
-------
Data for NOo and Cl concentrations, electrical conductivity, and
water content for two depths are presented in Table 8. The values are
means of three holes for each site. The NO^ and Cl~ are expressed as
ppm and meq/liter, respectively, in the saturation extracts for the
0- to 2.4-m depth since at this depth water contents fluctuate with
irrigation practice and evapotranspiration by the crop and in the soil
solution for the 3.3- to 15-m depths where the water is considered
unavailable to crops and would be leached to the ground water. The
average N07-N content for the 3.3- to 15-m depths ranged from 36.0 to
122.6 ppm. There was a positive correlation (r = 0.70, n = 9),
significant at the 5 per cent level, between the N input per year
and the average concentration of NOo-N in the soil solution at the
3.3- to 15-m depths. The sites having the highest NO^-N concen-
trations of 84.9, 119.2, and 122.6 ppm had average application rates
of 470, 1,350, and 480 kg N/ha per year, respectively - whereas the
sites having the lowest NC^-N concentrations of 36.0, 53.4, and 56.2
ppm had application rates of 224, 350, and 163 kg N/ha per year
respectively.
The ground water at the 12-m depth (top of the saturated zone) in
site 3 had a NOo-N concentration of 140 ppm and that for site 8 at
the 14-m depth had 93 ppm NOo-N. In contrast, the irrigation waters
pumped from wells approximately 26 m deep for site 3 and 45 m deep
for site 8 had only 19 and 1 ppm NOo-N respectively. This pattern
of wide divergence in NOo content at different aquifer depths was
similar to that reported for dairy farming areas, where NOo concen-
trations in the ground waters at shallow depths were considerably
higher than in waters pumped from deeper depths (1). This distri-
bution of NO-j concentrations as a function of depth within the
aquifer (saturated zone) indicates that the recycling of high NOo
ground water from shallow depths as a source of N for plants might
be feasible. The limitation, however, might be the salt levels that
are positively correlated with the NO^ concentrations.
Data for the NO-j and Cl concentrations of three selected sites are
presented graphically in Figures 1 and 8, respectively. Site 1
represented an area which received a medium rate of fertilizer N but
had a relatively high drainage volume and, thus, should have
relatively low NOo and Cl~ concentrations. Site 2, which had high N
input rates, had high NOo concentrations in spite of a high drainage
volume. Site 1 had low rates of N and also a low drainage volume.
Data for transit time for water in the unsaturated zone and for
calculated N balance are shown in Table 9. The N balance data are
for periods when records were available for fertilization, irrigation,
and crop yields for each site studied. The data cannot be extended
to cover a period equal to the transit time for water for a given
depth or to compare all the sites on the same time basis because
records were not available for all sites for these periods. Thus,
26
-------
Table 8 - Data for N03, Cl , EC and water content for the 0- to 2.4-m and 3.3-
to 15-m depths of soils for nine row-crop sites.
Site
No.
1
2
3
4
5
6
7
8
9
NO~-N
0-
to
2.4 m
15.5bt
52. 5b
24. 9b
27. 9b
18. 9b
48. 5b
21. 9b
122. 7b
25. 7b
,ppm*
3.3-
to
15 m
53.4bc
119. 2a
84. 9b
60.0bc
56.2bc
61.7bc
36.0°
122. 6a
76. 5b
Cl~
0-
to
2.4 m
0.59d
2.86cd
4.52C
10.33b
16.72a
5.92°
5.98C
0.97d
2.69Cd
,meq/liter*
3.3-
to
15 m
1.37C
5.18C
9.15bc
36.08a
10.96bc
21.24b
12.56bc
2.16°
1.53C
EC,
0-
to
2.4 m
0.47d
1.40bcd
1.20cd
1.70abc
2.45a
2.13ab
2.28ab
1.27bcd
0.98cd
imnho / cm
3.3-
to
15 m
0.53°
1.37ab
1.27bc
2.01a
0.83bc
1.17bc
0.87bc
1.03bc
0.58C
0-
to
2.4 m
8.5
17.9
16.4
7.8
6.8
11.4
7.1
12.6
10.2
Soil water
% ny wt
3.3-
to
15 m
9.9
16.9
18.0
9.7
15.5
9.0
10.2
22.8
12.3
*Nitrate and Cl concentrations in saturation extracts at 0- to 2.4-m depths and in soil water at
3.3- to 15-m depths.
+In saturation extracts at 25 C.
tAt the 5% level, values sharing the same letter are statistically alike.
csi
-------
. 3.0
Q.
8
O
V)
12.0-
15.0
2 4
NO§-N, meq/liter
40 80 120 160 200 240
Figure 7- Relationship between NO -N concentration and depth for
three sites in field used for row crops.
28
-------
1.5
3.0
-*-
i-
•fr
10
Cl~, meq /liter
a.
UJ
o
o
CO
12.0-
i5.a
12
CI", meq/liter
16
20
Figure 8. Relationship between Cl concentration and depth for
three sites in fields used for row crops.
29
-------
Table 9 - Estimates for the transit time for water in the unsaturated zone and N balance
for nine row-crop sites.
Description
1
Leaching 0.65
fraction
Leaching volume 58
surface cm
Water content 17.1*
% by volume
Sampling depth, 15
m
Transit time for 4.4*
15 m, years
Period, years 5
— — olLc 1NO . — — —
23456789
0.42 0.46 0.15 0.46 0.16 0.17 0.38 0.29
76 66 14 39 12 13 26 31
31.0 31.3 16.4 23.2 17.6 16.7 35.3 21.1
15 12 15 15 15 15 14 15
6.1 7.1 17.6 8.9 22.0 19.3 20.5 10.2
Nitrogen Balance for the Indicated Period
5 11 6 6 4 11 11 11
Input, kg/ha** 2185 7625 8140 1260 1290 1920 3960 5830 4785
Crop removal, 335 1925 2485 890 910 580 1330 1205 2080
kg/ha
-------
Table 9 - Continued
Period, years 5 5 11 6 6 4 11 11 11
Excess in soil,
kg/ha+
Loss, kg/ha
Loss, %t
1550
300
13.7
4350
1350
17.7
5324
331
4.1
510
-140
-11.1
1320
-940
-72.9
265
1075
56.0
572
2058
52.0
3487
1138
19.5
2610
95
2.0
Values for water content and transit time are for a bulk density of 1.8.
**Input data include N added in irrigation waters.
+Calculated at the end of the period from equation 2.
tNegative values were unaccounted-for increases in input and positive values were presumably
caused by denitrification.
-------
the N balance data for site 3 are for transit time for a 30-m depth,
whereas the data for site 6 were based on a transit time for a 3-m
depth. However, for sites 1, 2, 5 and 9, the indicated periods were
close to the values for transit time for 15 m. The values for the N
loss were calculated as equivalent to the total N input minus the sum
of N removed by crops and excess N in the soil. The amount of N
removed by crops was calculated as the product of harvested crops for
the indicated period and the N concentration in the crops. The
values for NOo used for the calculation of excess N (equation 2) were
the average in the soil solution at the 3.3- to 15-m depths. The
amount of N loss was assumed to be a result of denitrification
although some of this apparent N loss could have resulted from N
incorporation into the organic N pool in the soil.
The apparent N losses ranged from -72.9 to 56.0 per cent of the total
N input during the test periods. However, the sites fit into
essentially four categories: sites 3 and 9 with near zero loss;
sites 1, 2 and 8 with losses of 13.7 to 19.5 per cent; sites 6 and 7
with 56.0 and 52.0 per cent losses, respectively; and sites 4 and 5
with unaccounted-for inputs. Site 3 had some degree of stratification
but had no restricting layer in the 0- to 2-m depth. Site 9 had an
open-porous soil profile, and like site 3, had minimum possibilities
for denitrification to occur. Sites 1, 2 and 8 also had reasonably
open-porous profiles, except that sites 2 and 8 had relatively more
stratification, and evidently some denitrification may have occurred.
Actually, in these types of field studies, 13 to 20 per cent
unaccounted-for losses are not high and one might consider the use
of equation [2], in which excess N is equal to inputs minus crop
removal, to give a valid estimate for NOo concentrations in the
drainage water in these three sites. Sites 6 and 7 had profiles
conducive to denitrification. In both cases, soil layers that would
restrict the rate of movement of water existed within the usual soil
profile (0- to 2-m depth). In sites 4 and 5, which had been used
for disposal of steer manure prior to the start of row cropping in
1964, mineralization from the organic N pool apparently occurred.
Site 4 (Arlington) had the profile most likely to have produced deni-
trif ication, whereas site 5 (Hanford) had an open-porous profile.
Thus, if both sites had about the same mineralization rates, the
much lower apparent unaccounted-for inputs in site 4 could have been
caused by a much greater loss by denitrification.
Calculations for site 9, which had 8 consecutive years of alfalfa and
a N balance of 2 per cent, did not include an estimate of N fixation
by symbiosis.
If some N were taken from the air by this process, it was balanced by
losses or by errors of estimate in the factors that went into the
calculation of the N balance. Actually, the inefficiency of N
fixation by symbiosis in alfalfa when the soil is well supplied with
32
-------
available N is well known. In this case with an average input of 435
kg N per ha per year, the fixation of N by symbiosis is likely to have
been negligible.
Figure 9 presents the relationship between excess N as measured and
as calculated for the transit time for 15 m or the unsaturated zone.
The measured excess N was the measured NOo-N expressed as ppm in the
dry soil times the total weight of soil per 15-m depth or the depth
to the ground water. Three bulk densities were used to calculate this
total weight, and these three bulk densities were also used in
calculating the volumetric water content, 9, that was used to
calculate transit time T. Thus, for each bulk density a different
transit time was involved. The calculated excess N was obtained
from equation 12] using the average NOo-N concentration in the 3.3-
to 15-m depth or to the ground water and D from Table 3. In each
case, the basic data are the NOZ-N concentrations, but two different
routes are followed to arrive at the excess NOo-N, in kg/ha per year,
available for leaching by drainage waters. These relatively high
correlations and the near 1.0 slope between these two methods of
estimating excess N suggest that the data for transit times, leaching
fractions, drainage volumes, and water intake volumes are reasonable
and that the N balance data are realistic.
33
-------
6
IO
O
I 3h
-------
SECTION VII
DISCUSSION
Citrus
Fruit removal by citrus varies from about 20 to about 60 kg/ha per
year, respectively, for mediocre yields and high yields; maximum
yields would remove about 100 kg (9). At a rate of N inputs of 150
kg/ha per year these differences in yield could result in substantial
differences in the NO^-N concentrations in the water in the unsat-
urated zone at any given value of 'the drainage volume. Thus, high
production is an important factor in reducing the movement of NO^ to
the ground water.
Judging from the data presented for N balances, the estimates of
leaching fractions, volumes of drainage water and transit times for
water and NOo are realistic.
Fertilizer practice for citrus in Southern California is based on
leaf analysis standards such that the N input is adjusted to main-
tain 2.4 to 2.6 per cent in leaves from nonfruiting terminal growth,
using samples taken in August or September (3,4). The usual rates
of N vary from 100 to 150 Ib/acre per year or 112 to 168 kg/ha per
year. These rates combined with high production and leaching fractions
of 0.35 or more could lead to a drainage water having considerably
less than 20 ppm NOo-N. Treatments 1 and 6 with rates of fertilizer
N of 168 kg/ha per year during the past 7 years showed 21 ppm of
NO^-N in the soil solution at leaching fractions of 0.36 and 0.40,
respectively.
Thus, the citrus industry in Southern California seems to have
adjusted its fertilizer input to a level that, when combined with
good water management for maintenance of low salt levels in soils and
with high yielding trees, does not leave a high NOo load in the
drainage water. However, some doubt exists that the drainage water
from lands in Southern California that are used for citrus production
will have less than 10 ppm NO^-N. Thus, to meet the U. S. Public
Health Service standards for NO^ the drainage will need to be diluted
with low NO^ waters from other sources.
Row Crops
The high leaching losses of N associated with intensive irrigation of
crops with shallow root systems may be one of the reasons for applying
high rates of N on some vegetables. Most vegetables are grown on
sandy and sandy loam soils in which conditions are not favorable for
denitrification. However, for some crops like celery, potatoes, and
strawberries which are irrigated frequently (sometimes every two days
35
-------
with potatoes) sizable denitrification could occur even in profiles
without significant textural discontinuity. Under these conditions,
drainage water could have low NOo levels.
The fertilization rates for row crops, particularly the vegetables,
have increased considerably during the last four decades, with the
peak in the early 1960s. For example, in the Santa Ana Drainage Basin
which includes parts of Riverside, San Bernardino, Orange, and Los
Angeles counties, the amount of N used per unit area in vegetables
has increased more than five times since 1930, but, the removal of N
per unit area of land by the harvested portions increased less than
three times. During this same period, the area in the basin used for
vegetable production decreased about 30 per cent. Thus, while this
practice increased the yield, it also increased the potential for
excess N in the soil which becomes available for leaching to the
ground water. However, there was a declining trend in N use during
the last ten years. The current recommended rate for N for most
vegetable crops supplies about 135 kg N/ha above that removed in the
harvested portions. Thus, four to twenty years hence, if recommended
rates are used, the concentration of NO^-N in the solution at the 15-m
depth in the basin would be about 22 ppm for a leaching volume of 60
surface cm/year or 44 ppm for a leaching volume of 30 cm/year. These
NO-^ levels, however, could be reduced by denitrif ication. Obviously,
in all but one of the fields studied for this report, the combination
of excess N, leaching volume and net mineralization or denitrification
gave NOT concentrations higher than these expected values.
Thus, the current fertilization practices on the row crops, especially
for vegetables, seem to require some measures to reduce the NOo load
in the leaching water. This could include a) the use of recommended
rates, sources, and time of application of N; b) soil fertility
evaluation before each cropping season/ and c) the use of proper
amounts of water for maximum yields and additional water needed to
maintain a suitable salt balance in the surface soil. The
recommended rates of N application to most row crops and the amounts
of N that can be recycled by optimum yields have been published
earlier (5,6).
Further investigation on specific crops in many production areas will
be required to determine the pollution potential of the fertilizer
practices used in each growing area.
General
Apparently, the three equations in the approach section can be used
to make estimates of N0« concentrations in the unsaturated zone
between the zone of root influence and the saturated zone. The
equations work best in open-porous soil profiles where water saturation
is unlikely and actually difficult to attain and where the organic N
36
-------
pool has reached a constant level, i.e., no net mineralization or
immobilization of N. The important factors in this approach are
1) the volume of the drainage water and 2) the yearly excess of N input
over removal in harvested crops.
The approach used here can also provide estimates of denitrification
in the field. However, the uncertainty here is the constancy of the
organic N pool. If this pool gains or loses N during the experi-
mental period, the estimates of denitrification will be in error by
these gains or losses.
Transit times through the unsaturated zone suggest that the excess N
in a surface soil in any one year might not be a contributor to the
NO^ in well waters until many years in the future. If the transit
distance or depth to .the water table is 30 m, a time lag of from
about 8 to 50 years is likely in the alluvial materials of the Santa
Ana River Basin in which the studies herein reported were made.
The data presented here suggest that a useful general relationship
exists between soil profile characteristics and denitrification.
Water management should enter as an important factor in denitrifi-
cation, but it does not show clearly in this work primarily because
of a lack of a definition or of a meaningful description of this
factor. In any event, the work herein reported suggests that research
aimed at developing numerical values for the probable denitrification
in soil profiles would be desirable.
37
-------
SECTION VIII
ACKNOWLEDGMENTS
The support of co-investigators W. W. Jones, Professor of Horticulture,
V. A. Hunsaker, and D. C. Adriano, Postdoctoral Fellows in Soil
Science, F. H. Takatori, Research Specialist in Plant Sciences and
0. A. Lorenz, Professor of Vegetable Crops, is gratefully acknowledged.
The interest of W. A. Cawley, Deputy Director, Program Management
Division, and J. W. Keeley, Chief, National Ground Water Research
Program, of the Office of Research and Monitoring of EPA is acknowl-
edged with sincere thanks.
39
-------
SECTION IX
REFERENCES
1. Adriano, D. C., Pratt, P. F., and Bishop, S. E., "Nitrate and
Salt in Soils and Ground Waters from Land Disposal of Dairy
Manure," Soil Science Society of America Proceedings, 35,
No. 5, pp 759-762 (1971).
2. Bingham, F. T., Davis, S., and Shade, E., "Water Relations, Salt
Balance, and Nitrate Leaching Losses of an 840-acre Citrus
Watershed," Soil Science 112, No. 6, pp 410-418 (1971).
3. Embleton, T. W., and Jones, W. W., "Leaf Analysis - A Tool for
Determining the Nutrient Status of Citrus," Proceedings Soil
Plant Diagnosticians, l:pp 160-169 (1964).
4. Jones, W. W., and Embleton, T. W., "Development and Current
Status of Citrus Leaf Analysis as a Guide to Fertilization in
California," Proceedings First International Citrus Symposium 3,
pp 1669-1671 (1969).
5. Lorenz, 0. A., "Nutrient Uptake by Vegetable Crops," In
Proceedings Sixteenth Annual Fertilizer Conference of the
Pacific Northwest, Salt Lake City, Utah, pp 19-25 (1965).
6. Lorenz, 0. A., and Bartz, J. F., "Fertilization for High Yields
and Quality of Vegetable Crops," In L. B. Nelson (ed.)
Changing Patterns in Fertilizer Use, Soil Science Society of
America, Madison, Wis., pp 327-352 (1968).
7. Nelson, L. B., "Research Needed to Resolve the Plant Nutrient-
Water Quality Issue," In 0. P. Engelstad (ed.) Nutrient Mobility
in Soils: Accumulation and Losses. Soil Science Society of
America Publication No. 4, Soil Science Society of America,
Madison, Wis., pp 75-81 (1970).
8. Pratt, P. F., and Chapman, H. D., "Gains and Losses of Mineral
Elements in an Irrigated Soil during a 20-Year Lysimeter
Investigation," Hilgardia 30, No. 16, pp 445-467 (1961).
9. Smith, P. F., "Citrus Nutrition," In Norman F. Childers (ed.)
Fruit Nutrition, Somerset Press Inc., Somerville, N. J. (1966).
10. Stewart, B. A., "A Look at Agricultural Practices in Relation
to Nitrate Accumulation," _In 0. P. Engelstad (ed.) Nutrient
Mobility in Soils: Accumulation and Losses. Soil Science
Society of America Special Publication No. 4, Soil Science
Society of America, Madison, Wis., pp 47-60 (1970).
41
-------
11. Stout, P. R., and Burau, R. G., "The Extent and Significance
of Fertilizer Buildup in Soils as Revealed by Vertical
Distributions of Nitrogenous Matter Between Soil and Underlying
Water Reservoirs," In N. C. Brady (ed.) Agriculture and the
Quality of our Environment American Association for the
Advancement of Science, Publication 85, Washington, D. C.,
pp 283-310 (1967).
12. Thomas, G. A., "Soil and Climatic Factors which Affect Nutrient
Mobility," In 0. P. Engelstad (ed.) Nutrient Mobility in Soils:
Accumulation and Losses. Soil Science Society of America,
Special Publication No. 4, Soil Science Society of America,
Madison, Wis., pp 1-20 (1970).
42
-------
SECTION X
PUBLICATIONS AND PATENTS
Patents
None
Publications
Pratt, P. F., Jones, W. W., and Hunsaker, V- E., "Nitrate in Deep
Soil Profiles in Relation to Fertilizer Rates and Leaching Volume,'
Journal of Environmental Quality, 1, No. 3, pp 97-102 (1972).
43
-------
SECTION XI
GLOSSARY
Denitrification - The conversion by microbiological reduction of N0~
to N2 or oxides of N that escape to the atmosphere.
Drainage Volume - The amount of water that moves from the root zone of
plants. It is usually expressed as surface cm.
Evapotranspiration - The sum of the water used by plants and that
evaporated from the soil surface.
Immobilization - The incorporation of N into organic forms from the
inorganic forms in soils.
Leaching Fraction - The volume of drainage water expressed as a
fraction of the volume of the irrigation water that infiltrated
into the soil.
Mineralization - The conversion of N from organic forms to the NH^
or NO^ forms.
Organic N Pool - The organically bound N in the soil profile.
Root Zone - The depth of soil that is permeated with plant roots,
or the depth of soil that is influenced by the root system of plants.
Soil Profile - The sequence of layers or horizons or lack thereof
within the 2-m depth of surface of the earth.
Transit Time - The time that drainage water takes to move a given
distance in the unsaturated zone.
Unsaturated Zone - The zone between the zone of rooting of crop
plants and the saturated zone at the top of the water table. Water
movement in the unsaturated zone is by unsaturated flow.
45
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SELECTED WATER
RESOURCES ABSTRACTS
INPUT TRANSACTION FORM
W
Nitrate in the Unsaturated Zone under Agricultural
Land
—Ipnl 1972™
P. F. Pratt
Department of Soil Science and Agricultural
Engineering
University of California
Riverside, Calif. 92502
Environmental Protection Agency
16060 DOE
16060 DOE
Six treatments from a fertility trial with citrus , four commercial citrus groves
and nine row-crop fields were studied to assess the effect of fertilizer N on the
concentrations of NOrj in the unsaturated zone between the root zone and the
saturated zone. Nitrate-N concentrations of 21 to 70 and 36 to 123 ppm in
the water in the unsaturated zone were found for citrus and row crops respectively,
The main factors were 1) the difference between N input and crop removal, 2)
drainage volume and 3) soil profile characteristics that apparently control
denitrification. In soil profiles having no layers that restrict water
movement and in which the net change in the amount of N in the organic pool
was essentially zero the amount of N input minus crop removal expressed as
NOo in the drainage water provided a reasonably reliable estimate of the N0~
concentration in the solution of the unsaturated zone. An estimate of transit
time for drainage water through the unsaturated zone provided a means of
calculating a field N balance.
This report was submitted in fulfillment of Grant No. 16060DOE under the
sponsorship of the Office of Research and Monitoring, Environmental Protection
Agency.
crops, denitrification, drainage water, evaporation, fertilizers, leaching,
nitrates, nitrogen cycle, soil profiles, soil water movement, unsaturated flow,
zone of aeration
V. B,Group 16
F.Prattr
Send To:
WATER RESOURCES SCIENTIFIC INFORMATION CENTER
U S. DEPARTMENT OF THE INTERIOR
WASHINGTON D C. 2O24O
Ttyof CahtoT
U.S. GOVERNMENT PRINTING OFFICE : 1 972 — Wt-l(87 (295)
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