GUIDE TO
SOIL SUITABILITY AND SITE SELECTION FOR
BENEFICIAL USE OF
SEWAGE SLUDGE
I
MANUAL 8
MARCH 1990
OREGON STATE UNIVERSITY
EXTENSION SERVICE
-------
If you would like additional copies of Guide to Soil
Suitability and Site Selection for Beneficial Use of
Sewage Sludge, Manual 8, please order by title and
series number from:
Publications Orders
Agricultural Communications
Oregon State University
Administrative Services A422
Corvallis, OR 97331-2119
The first six copies of this publication available at
no charge. Please call (503) 737-2513 for price
quote for orders larger than six copies.
OREGON STATE UNIVERSITY
EXTENSION SGRVKjE
-------
GUIDE TO
SOIL SUITABILITY
AND SITE SELECTION
FOR BENEFICIAL
USE OF
SEWAGE SLUDGE
J. H. Huddleston
Extension Soil Scientist
Oregon State University
M. P. Ronayne
Water Quality Specialist
Oregon Department of Environmental Quality
-------
ACKNOWLEDGMENTS
Funding
The preparation and publication of this guide
were made possible by a grant from the U.S.
Environmental Protection Agency (EPA).
The EPA reference number for this guide is
EPA-910/9-90-008.
Technical Review
Reviewers whose comments were particularly
helpful in developing the guide, include Bob
Bastian, Bob Brobst, and Dick Hetherington,
EPA; Craig Cogger, Washington State University
Cooperative Extension; John Hart and Jim
Moore, Oregon State University Extension
Service; Lee Sommers and Ken Barbarick,
Colorado State University; Steve Wilson and
Stuart Childs from Cascade Earth Sciences; and
Kurt Leininger from CH2M Hill.
Editing and Design
Project Editor:
Evelyn A. Liss,
OSU Department of
Agricultural Communications
Text Editor:
Laura Meek, WordWrite, Albany, Oregon
Designer:
Tom Weeks, OSU Department of
Agricultural Communications
Production:
Myrna Branam, Word Design, Corvallis, Oregon
-------
CONTENTS
Chapter 1. Introduction 1
Framework for Evaluating Sludge Utilization Proposals 1
Unit Conversions 2
Definitions 2
Chapter 2. Sludge Characteristics 3
Types of Sludge 3
Pathogen Reduction 3
Sludge Characterization 4
Chapter 3. Soil Properties 7
The Roles of Soil 7
Morphological Properties 7
Inferred Properties 13
Chapter 4. Site Selection 23
Keys for Rating Soil Suitability 23
How to Use Soil Surveys to Facilitate Site Selection 28
Chapter 5. Crop Management Factors 31
Choice of Crop 31
Nutrient Management 31
Soil Testing 33
Water Management 34
Soil Conservation Practices 34
Monitoring and Record-Keeping 35
Chapter 6. Design Calculations 37
Carryover Nitrogen from Previous Sludge Applications 37
Sample Calculations 38
Chapter 7. Practical Applications 45
Guidelines for Evaluating Project Proposals 45
Worksheet for Evaluating Sludge Data 47
Worksheet for Evaluating Soils Data 49
Worksheet for Evaluating Cropping Systems Information 51
Worksheet for Evaluating Issues and Interactions 53
Sources of Information 56
Appendixes 57
A. Technical Aspects of Soil Morphology 59
B. Estimating AWHC and CEC Values 63
C. Sample Clauses for Use in Permits 65
Glossary 69
-------
Introduction
CHAPTER 1
This guide differs from other publica-
tions on the land application of
sewage sludge because it emphasizes
the soil. Natural processes in the soil
store and release water and nutrients for plant
use, break down organic matter, immobilize
metals and organic contaminants, and reduce the
number of pathogenic organisms. Understanding
these processes requires understanding soil
properties because they control both the natural
processes and the overall suitability of a site for
land application of sewage sludge.
The interactions among sludge, soil, crop,
and farm management are also emphasized in
this guide. Under the right conditions, almost
any arable land can be used for the beneficial use
of sewage sludge. The right conditions, however,
depend on the nature of the sludge, the proper-
ties of the soil, the kind of crop, the cropping
system, and most importantly, the interactions
that ultimately control the decisions regarding
sludge application and site management.
Throughout the guide both principles and
practical applications are stressed. Whether
you're conducting a site evaluation, writing a
permit application, or reviewing a project
proposal, you need a solid base of technical
information and a healthy dose of common
sense. This guide emphasizes principles that will
supplement your technical information and
provides practical, useful information to add to
your storehouse of common sense.
The objective of this guide is to assist
treatment plant operators, permit writers, and
others involved in sludge management in the
development, evaluation, and implementation of
plans for the beneficial use of sewage sludge.
The guide provides needed information, explains
methods of evaluating the adequacy of
information in site studies and project proposals,
and identifies available resources.
The guide cannot, however, provide a
complete prescription for site management.
Because each site represents a unique combina-
tion of sludge, soil, and farming system, a
unique set of procedures must be prepared for
each combination. In developing site-specific
plans, take advantage of local experience and
knowledge available from agricultural Extension
agents, Soil Conservation Service (SCS) district
conservationists, agricultural consultants, and
the farmers who will be using the sludge.
Framework for Evaluating
Sludge Utilization Proposals
Land application of sewage sludge benefits
both agriculture and society. Agriculture benefits
because sludge supplies nutrients for crop
growth and improves the physical condition of
the soil. Society benefits from the "disposal" of
"waste" in a safe and effective manner. All
sludge utilization proposals should clearly
indicate how both sets of benefits can be
achieved, while at the same time maintaining
environmental quality and protecting the public
from health hazards associated with pathogens,
metals, and organic contaminants.
Project evaluation has three components:
1. Evaluating the completeness of the data;
2. Evaluating the accuracy of the data;
3. Evaluating the extent to which the project
accounts for issues and design considera-
tions stemming from sludge, soil, and
cropping system interactions.
Evaluation of data completeness requires
verifying the data on fertilizer recommendations
and cropping practices as well as verifying that
the chemical and physical characterization of
both sludge and soil are sufficient to make all
necessary design calculations. The data also
should be sufficient to ensure compliance with
all pertinent regulatory standards. If any of the
required data are incomplete or missing, you
may need to request additional information
before proceeding with the project review.
Evaluation of data accuracy requires
ensuring that both sludge and soils have been
sampled and analyzed according to approved
procedures and that the data are consistent with
results from similar sludges and soils analyzed
previously. Confidence in data calculations and
interpretations depends on this assurance. If the
data are not valid, it is not reasonable to expect
a county Extension agent or anyone else to make
recommendations for a sludge utilization plan.
Evaluation of issues and design considera-
tions requires verifying that all calculations
account for all relevant interactions, and that
management decisions reflect the principles
discussed in this guide. Some of the more
important issues include:
1. Ensuring that soil properties and soil
surveys, or on-site investigations, have
been used to help select and evaluate
potential sites;
2. Ensuring that the calculated agronomic
loading rate accounts for all nutrient
interactions in sludge and soil, and that it
delivers the right amount of nutrients at
the right time for crop utilization;
Elements of
Project
Evaluation
• Completeness of
data
• Accuracy of data
• Assessment of
interactions
Sources
of Local Help
• County Extension
agents
• Soil Conservation
Service district
conservationists
• Agricultural
consultants
• Farmers and land
managers
-------
3. Ensuring that the timing of sludge
applications accounts for any soil, site, or
climatic limitations, and is coordinated
with crop management plans;
4. Ensuring that appropriate management
practices have been specified to mitigate
limiting soil properties and protect the
public from adverse effects of metals,
organic contaminants, and pathogens.
Unit Conversions
Analytical data on sludge characteristics and
the standards established by regulatory agencies
are usually expressed in metric units. Farmers and
farm advisors, however, usually use English units
for expressing nutrient requirements and the
amounts of sludge to apply. Table 1 is designed
to help you convert from one unit to the other.
The following tips may be useful in
simplifying your calculations and presenting
information in terms understandable to your users:
I. Make all conversions at one time, either
at the beginning or the end. Rounding
errors in metric-English conversions can
be very large, and the number of times
units are converted should be minimized.
2. Milligrams per kilogram (mg/Kg) is the
same as parts per million (ppm). Thus, if
sludge analysis data report 65,000 mg
NH4-N/Kg, that is the same as 65,000 mg
NH4-N per 1,000,000 mg oven-dried
sludge.
3. When the units are the same in the
numerator and the denominator, the same
fraction can be expressed in any unit. In
this example, the equivalent English unit
is 65,000 lbs NH4-N per 1,000,000 lbs
oven-dried sludge. This can then be
expressed as 65 lbs NH4-N per 1,000 lbs
oven-dried sludge, or 130 lbs per ton of
oven-dried sludge.
Definitions
Throughout the text, many soil science
terms such as adsorption, cation exchange
capacity, denitrification, mineralization, soil
structure, and soil texture are used. These and
several other terms are defined in the glossary at
the end of this guide.
Table 1.—Guides for converting between metric and English units
Metric unit'
Conversion factor
English unit2
Centimeter
0.3937
Inch
Meter
3.2808
Foot
Kilometer
0.6214
Mile
Hectare
2.4711
Acre
Cubic meter
35.3147
Cubic foot
0.00081071
Acre-foot
264.25
Gallon
Gram
0.002205
Pound
Kilogram
2.205
Pound
0.0011
Tons
Metric ton
1.10
Tons
Kilograms per hectare
0.000446
Tons per acre
0.892
Pounds per acre
Metric tons per hectare
0.892
Tons per acre
Cubic meters per hectare
0.0001069
Million gallons per acre
Gallon of water
8.34
Pounds of water
Gallon of water
0.1336
Cubic feet of water
1 To convert metric to English, multiply the metric unit by the conversion factor in the middle column.
2 To convert English to metric, divide the English unit by the conversion factor in the middle column.
-------
Sludge Characteristics
CHAPTER 2
The nutrient value of sludge, and the
potential for environmental degrada-
tion from land-applied sludge, depend
on the sludge composition, the
handling and processing of the sludge prior to
land application, and the manner, timing and
location of the sludge application. This chapter
discusses some of those interactions.
Types of Sludge
Sewage sludge consists of water, dissolved
solids, and suspended solids removed from
municipal wastewater during the treatment
process. Sludge solids contain plant nutrients,
trace metals, organic chemicals, and inert solids,
many of which are combined with complex
organic compounds. The percent total solids in
the sludge determines the type of sludge.
Liquid sludge is a very dilute mixture. The
solids content is usually within a range of 2 to
5% but may be as low as 0.5% or as high as
10%.
Dewatered sludge is a more concentrated
mixture produced by mechanically removing
some of the liquid. Many dewatering processes
produce a sludge that contains between 16 and
22% solids, although some dewatered sludges
may have as much as 40% solids.
Dried sludge is an even more concentrated
mixture that results from evaporation through air
drying or heating. The solids content of a well
dried sludge is typically 50% or more.
Composted sludge is produced by combin-
ing dewatered sludge with a bulking agent such
as sawdust and aerating the resulting mixture
under controlled temperatures. Recycled
compost may be used as a bulking agent also.
The solids content of composted sludge is
generally about 40%.
The type of sludge applied to the land has
several implications for a beneficial use program.
First, any process that reduces the volume of
sludge reduces the storage and transportation
needs and costs. Second, the solids content
dictates whether the sludge can be applied
through an irrigation system, a dry cast manure
spreader, a truck driving over the soil, or some
other means. The method of application may
affect the timing of the application. Third,
hydraulic loading of the soil may be a problem
with liquid sludges that have a very low solids
content. Fourth, dewatering, drying, and com-
posting affect the fertilizer value of the sludge.
These changes must be recognized when
planning a land application program.
Pathogen Reduction
Virtually all pathogenic bacteria, viruses,
protozoa, and parasitic helminths must be
eliminated from municipal sewage sludge in
order to prevent contamination of human and
livestock food and water supplies. Management
of a land application program should strive to
provide conditions that eliminate pathogens from
the sludge and prevent their entry into the food
chain. Most of this can be accomplished by
processing the sludge prior to land application.
The remainder can be accomplished by natural
processes in the soil. All processes to treat sludge
solids, whether prior to land application or in the
soil itself, are designed to create environments
unfavorable to pathogen survival.
Pathogens prefer cool, wet, and dark
environments and do not survive in warm, dry
environments that are exposed to sunlight.
Pathogenic organisms become a health hazard if:
1. The numbers of pathogenic organisms in
sludge are not reduced to tolerable limits;
2. Pathogens are leached into groundwater,
carried by runoff into surface water,
ingested with plant or soil materials, or
transported by vectors.
The regulatory strategy for protecting public
health is a 2-step process. The first step involves
maximum treatment at the treatment plant to
reduce pathogen populations. The second step
involves additional treatment in the soil to
decrease the number of pathogens remaining.
The greater the kill at the treatment plant, the
less is required of the land management system.
Current regulations classify treatment
options at the plant into processes that signifi-
cantly reduce pathogens (PSRP) and processes
that further reduce pathogens (PFRP). New regu-
lations, however, may change this classification.
Processes that significantly reduce pathogens
(PSRP) include anaerobic digestion, lime stabili-
zation, air-drying, and composting. Each of these
processes takes advantage of one or more of the
environmental conditions that reduces the number
of pathogens. Specific requirements for pathogen
reduction and vector control are indicated in
Appendix II of 40 CFR part 257.3-6.
Sludge that has been processed to
significantly reduce pathogens is generally safe
to apply to the land, although some pathogens
may remain. Additional soil treatment and man-
agement practices are necessary to maintain high
standards of public health protection.
Surface applications expose sludge to both
drying and sunlight, conditions that facilitate
rapid decline of most remaining pathogens and
parasites. Similarly, sludge applied directly to
growing crops is expected to be pathogen-free in
The Type of
Sludge Influences
• Storage and
transportation
requirements
• Application
methods
• Hydraulic loading
• Fertilizer value
I
-------
Sludge data must
represent the final
processed sludge
that will be
applied to the
land.
a short time, usually a few days. A lag time of a
month between sludge application and grazing of
pastures is usually required to ensure that there
are no harmful effects from pathogens. Mixing
sludge into the soil also provides the right
environment for pathogen treatment as long as
the soil is kept well aerated.
Additional solids treatment processes may be
used to further reduce pathogens. These processes
include high temperature digestion, gamma
irradiation, heat drying, advanced alkaline
stabilization, and certain types of composting.
Sludge that has been treated with one or more of
these processes is considered safe to apply to
land regardless of the type of crop to be grown.
Both PSRP's and PFRP's affect the fertilizer
value of sludge. In particular, the amounts of
readily available nitrogen and the rates of
mineralization of organic nitrogen are affected
by the particular process used to reduce patho-
gens. When determining the amount of the sludge
to be applied, these interactions must be recog-
nized in order to deliver the amount of nitrogen
needed to meet a fertilizer recommendation.
Sludge Characterization
Rules for Sludge
Sampling
1 .Collect samples
that are truly
representative of
the sludge product
2. Store samples in
appropriate glass
or plastic
containers
3. Refrigerate or
freeze samples to
prevent changes
in nitrogen data
I
Planning a program for beneficial use of
sewage sludge requires complete and accurate
data. An important part of project evaluation is
ensuring that the data used to make management
decisions are both complete and accurate. ,
Complete sludge data include the amount of
solids produced and the amounts of plant
nutrients, trace metals, and organic contaminants
contained in the solids. The data should also
indicate the type of sludge that will be applied to
the land and the process or processes used to
reduce pathogens and control vector attraction.
Data accuracy depends on the use of standard
procedures for sampling, handling, and analyzing
samples. The EPA publication, Analytical Meth-
ods for the National Sewage Sludge Survey (see
page 56), is a useful reference for some of these
procedures. This document, and any other
approved procedures, should be available from
the agency that regulates sludge treatment and
disposal in your state.
Some rules of common sense apply to sludge
sampling and sample handling. For example,
samples should not be put in a paper bag and left
in a truck for several days. Samples should be
stored in glass or plastic containers to prevent the
loss of liquids, volatilization, and contamination
with extraneous organic material. Samples must
be representative of the sludge to be applied.
Biological activity does not stop once a
sample has been taken. Organic matter continues
to decompose, and organic nitrogen continues to be
mineralized. To get good nitrogen data, the sample
should be refrigerated or frozen immediately after
sampling and stored in that condition until labo-
ratory analyses can be performed. Otherwise the
nitrogen data may not represent the actual amounts
of each form of nitrogen that are in the sludge.
Laboratory procedures for sludge analyses
are essentially standardized. Nevertheless, there
will be some variation among laboratories in both
the accuracy and the precision of analytical data.
Your state regulatory agency can provide a list of
reputable laboratories whose work is reliable.
Analytical data are expressed in terms of the
dry weight of the sludge. This is the weight of
the residue left after driving off all the water in a
sludge sample by heating in an oven at 105°C.
The dry weight includes all solids suspended in
the original sludge mixture, plus all constituents
dissolved in the liquid portion of the sludge. All
references to dry weight, dry pounds, or dry tons
are for dry weight determined in this way.
Sludge volume
The amount of sludge a wastewater treat-
ment plant produces each year can be expressed
either as dry tons or as gallons. Dry tons is
preferred because it is independent of liquid
content, which varies according to the method of
sludge processing and handling. The number of
dry tons produced determines the total area of
land required for beneficial use applications.
For liquid sludges, knowing the gallons
produced may help determine sludge storage and
transportation requirements as well as calculating
the hydraulic loading rate.
Sludge storage is necessary during the times
when sludge cannot be applied to the land. These
times occur when the ground is frozen, when soil
water tables are high, and when the soil surface
is wet. Depending on specific climatic
conditions, these times may range from a few
days to a few months. Sludge storage may also
be required because certain crops limit sludge
applications to specific times of year. You
probably can't drive over a grain or row crop
after the crop has been planted, and public health
regulations may preclude sludge application for
part or all of the growing season. Applications to
permanent pasture, however, are generally
limited only by the timing of livestock grazing.
Sludge transportation needs depend on the
volume of sludge produced, the percent solids,
and the times of year during which sludge can be
applied. The larger the volume of sludge
produced, the greater the transportation
requirements, and if sludge can only be applied
during a portion of the year, the required hauling
-------
capacity may be even higher. The percent solids
dictates whether tanks, watertight boxes, or
regular boxes are needed for transportation.
Percent total solids
The method of land application depends on
the percent total solids of the processed sludge.
Liquid sludge that contains less than 6% solids
can be surface applied either from tank trucks with
special deflection plates, from irrigation guns, or
by direct injection into the soil. At higher solids
contents, the slurry may be too thick to pump.
Dewatered sludge, dried sludge, and
composted sludge will not flow. They must be
hauled in dump trucks and spread mechanically
with hammer throw or manure spreader devices.
If sludge volume is expressed in terms of
gallons per year, then the percent total solids is
needed to calculate the dry tons total solids
produced. This is done by converting the total
gallons produced to tons (see table 1) and
multiplying by the percent solids.
Nutrients
Sludge is a low-analysis fertilizer. Although
sludge is a valuable source of plant nutrients, the
nutrient concentrations are significantly lower
than most commercial fertilizers. The nutrient
content of a sludge depends on the primary
source of the sludge and the methods of process-
ing, handling, and application of the sludge.
Consequently, the actual fertilizer value of a
sludge, and the determination of appropriate agro-
nomic loading rates, depend on the specific data
reported for that sludge. It is essential, therefore,
that these data represent the final processed sludge,
not an intermediate sludge product.
The most important nutrients in sludge are
nitrogen, phosphorus, and potassium. Other
nutrients that may be present include copper,
calcium, magnesium, and sulfur. Nutrient
contents of sludge are usually expressed either as
percent of dry weight or as mg/kg dry weight.
Most calculations, however, use the equivalent
concentration in lbs per dry ton of sludge. To
convert percent to lbs/ton, multiply by 20. To
convert mg/kg to lbs/ton, multiply by 0.002.
Nitrogen in sludge occurs in both inorganic
and organic forms. Sludge analysis data usually in-
clude the amounts of inorganic ammonia nitrogen
and inorganic nitrate nitrogen, and either the
total Kjeldahl nitrogen or the total nitrogen. To
determine the amount of organic nitrogen, either
subtract the ammonia nitrogen from the total
Kjeldahl nitrogen, or subtract the sum of ammonia
plus nitrate nitrogen from the total nitrogen.
Inorganic forms of nitrogen are dissolved in
the liquid portion of sludge and are readily
available to plants. For this reason, ammonia
nitrogen and nitrate nitrogen in sludge liquids
serve as short-term, or quick-release fertilizers.
Organic nitrogen in sludge is a long-term,
slow-release fertilizer. As organic matter
decomposes in the soil, microorganisms convert
the organic nitrogen to inorganic ammonium
nitrogen. This process is called mineralization.
Other organisms then convert the ammonium to
nitrate. This process is called nitrification. Only
after these conversions is the nitrogen in sludge
organic matter readily available to plants.
The fertilizer value of the sludge is changed
by the concentration of sludge solids and the
processing to reduce pathogens and vector
attraction. Dewatering processes reduce the
fertilizer value because they remove some of the
nutrient-containing liquid. Drying also reduces
the fertilizer value because much of the ammonia
nitrogen is lost by volatilization. As a result, both
dewatered and dried sludges deliver much less
quick-release, readily available nitrogen to the
soil than liquid sludge.
Composting converts most of the inorganic
nitrogen in sludge to organically bound nitrogen,
a process called immobilization. As a soil
amendment, composted sludge supplies little
readily available nitrogen to plants. However, the
nitrogen that is released by further decomposition
in the soil continues to supply plants for a longer
period of time than for other kinds of sludge.
In all sludges much of the fertilizer value
comes from the slow release of organic nitrogen
through mineralization. The rate of this release,
usually between 8 and 30%, depends on many
factors, including the form of sludge applied.
In general, the highest rate of release during
the year following application is obtained with
liquid and dewatered sludges. Dried sludges and
composted sludges have slower rates, but
because these sludges have much higher solids
contents, more nitrogen will be released in the
second and third year after initial application
than from other types of sludge.
Methods of processing also affect the
mineralization rate. For aerobically processed
sludge the mineralization rate may be 30% or
more. Anaerobic processing often results in a
mineralization rate of about 20%. Composted
sludge has even lower mineralization rates
during the year after application.
The fertilizer value of liquid sludge is
affected by the method of land application.
Methods that leave the liquid sludge on the soil
surface may result in volatilization of up to half
the ammonia. On the other hand, liquid sludge
that is injected or worked into the soil retains
most of its nutrient value.
Surface
spreading of
liquid sludge
leads to
volatilization
of up to one half
of the ammonia
nitrogen applied.
I
-------
Phosphorus and potassium are important
plant nutrients, but in most cases they are needed
in smaller amounts than nitrogen. Their availa-
bility to plants is less dependent on the extent of
sludge processing prior to land application than
nitrogen. As a result, if the sludge application
rate is based on nitrogen, you can use the sludge
analysis data to calculate the amounts of P and K
delivered with it and compare them with crop
requirements. If there is still a deficiency,
supplemental fertilizer can be added.
Excessive amounts of P and K delivered in
sludge have no short-term impacts on crop
production, but monitoring of long term increases
in soil salinity and nutrient balance may be
appropriate. Particular care may be required to
prevent surface runoff and overland transport of
sludge that could lead to an increase in the
phosphorus content of nearby rivers and lakes.
Metals
Trace metals in sludge may include arsenic,
cadmium, chromium, cobalt, copper, mercury,
manganese, molybdenum, nickel, lead, selenium,
tin, and zinc. Sludge analysis data usually report
concentrations of these metals in mg/kg dry weight.
Multiplying these numbers by 0.002 converts the
expression to lbs metal per dry ton of sludge.
Excessive applications of metals are of
concern because:
1. Some may be toxic if ingested at high
levels for long periods;
2. Some are potential carcinogens;
3. Some tend to accumulate in body tissues
and, in large quantities, may impair the
function of vital organs, particularly the
liver and kidneys;
4. Some may enter human food supplies, either
through animals that graze on crops that
take up metals from soils, or through direct
consumption of accumulator crops;
5. Some may be toxic to plants.
To protect public health and help assure
beneficial use of sludge, the Clean Water Act
required EPA to identify any toxic pollutants in
sludge that could affect public health, and to
propose regulations specifying acceptable
management practices for sewage sludge that
contains toxic pollutants.
This mandate resulted in establishment of
standards for allowable cumulative limits of
metals in soils at land application sites.
Cadmium, lead, nickel, zinc, and copper are the
heavy metals currently used to calculate the
allowable accumulation period for land applica-
tion of sludge. Of these, cadmium poses the
greatest long-term threat to human health and is
the metal for which the most stringent standards
for both annual and cumulative loading limits
have been established.
Specific regulations regarding metal loading
rates and cumulative limits undergo periodic
revision. This reflects increasing understanding
of the reactions of these metals in soils, their
tendency to be immobilized in the soil, and their
uptake by specific crops. For these reasons, you
should refer to the applicable federal and state
regulations in force when a particular land
application project is being developed.
Organic contaminants
Many different kinds of organic chemicals
may be found in sewage sludge, depending on
the number and kind of industries discharging
wastes into municipal sewer systems. Solvents,
paints, pesticides, and polychlorinated biphenyls
(PCB's) are some of the classes of organic
chemicals that may occur. In general, trace
organics such as halogenated hydrocarbons,
benzo(a)pyrene, dimethyl nitrosamine, and hexa-
chlorobenzene appear to pose the greatest potential
hazard to human health. Sludge data, therefore,
should at least include the amounts of these
compounds present. The data are usually reported
either as parts per million or as mg/kg dry weight.
Land application of sludge provides several
possible mechanisms for mitigating toxic effects
of organic contaminants. Some organics may be
subject to volatilization or may be decomposed
by sunlight. Some undergo rapid microbial
decomposition; others decompose very slowly.
Some organics may be immobilized by
adsorption on surfaces of clay particles and
organic matter in the soil. Others may be leached
out of the soil system. Leaching losses are not
very likely because the organic matter added to
soil by the sludge itself adsorbs organics and
immobilizes them in soil.
Current knowledge of all of these mecha-
nisms leads to the conclusion that organic
chemicals in land-applied sludge do not pose a
serious threat to plants or animals. The major
concern is human toxicity caused by ingestion of
plant or animal products, the sludge itself, or the
sludge-amended soil.
Because of the extreme diversity in the
kinds of organic contaminants found in a
particular sludge and in the specific interactions
between organic chemicals and soil environ-
ments, prescribing universal guidelines for
managing sludges containing organic chemicals
is difficult. The best common sense advice is to
consult with your state regulatory agencies and
comply with all pertinent federal and state stand-
ards. If this is done, organic chemicals in sludge
should not pose a problem for public health.
-------
Soil Properties
CHAPTER 3
Site evaluation, site selection, and site
management all begin with an assess-
ment of soil properties. These properties
control the biological, physical, and
chemical processes in soils that release plant
nutrients and immobilize toxic chemicals.
This chapter gives you a working knowledge
of the soil properties that influence the beneficial
use of sewage sludge. With this knowledge you
should be able to read and understand technical
soil profile and map unit descriptions in soil
survey reports. You can then retrieve the
maximum amount of information from those
descriptions. You should also be able to evaluate
site feasibility studies and permit applications for
the adequacy of soils data and the appropriate-
ness of proposed management plans.
Morphological properties define the nature
of the soil profile and are determined in the field.
Properties that are based on interpretations of
soil morphology are called inferred properties.
Information about both morphological and
inferred properties can be obtained either from
direct field observations or from published soil
survey reports.
The Roles of Soil
Within a sludge management program, the
three roles of soil are to provide a medium for:
1. Plant root growth;
2. Water entry and transmission;
3. Immobilization of metals and toxic
chemicals.
Soil as a medium for plant roots
An aerobic environment is necessary both
for plant roots and for the soil microbes that
decompose organic residues and destroy patho-
gens. Aerobic environments provide a favorable
balance between air-filled pores and water-filled
pores. Soil management for beneficial utilization
of sewage sludge should strive to maintain
aerobic conditions in the soil.
Aerobic conditions are related to soil
texture, soil structure, and soil water content.
Sandy soils, and loamy soils with good structure,
provide aerobic conditions. Clayey soils, and
soils with poor structure tend to be less well
aerated. Soils that are saturated for long periods
of time are anaerobic and are not favorable for
mineralization. Saturation is more likely in soils
that are clayey, or have impermeable horizons, or
occur in low-lying landscape positions.
Soil as a medium for water entry
and transmission
Rainfall, irrigation water, and sludge liquids
can be transmitted to surface and ground waters
through soil. The rate of transport depends on the
soil properties. Soil management for land
application of sludge must regulate water
movement over and through the soil in order to
prevent contamination of water supplies with
nitrates, phosphates, metals, and organics.
Soils in high rainfall areas and soils that are
irrigated are subject to leaching. Water moving
through the soil transports any nutrients or toxic
chemicals that are in solution. The more perme-
able the soil, and the higher the rainfall or
irrigation, the greater the potential for leaching.
Runoff occurs when the soil cannot absorb
the rainfall or irrigation. Surface runoff increases
the potential for contamination of lakes and
streams with sludge solids transported over the
soil surface by runoff water. The runoff potential
of a soil depends on the soil's slope and wetness,
and whether the soil is frozen. Ground cover,
rainfall intensity, and the efficacy of soil
conservation measures also influence runoff.
Bare soil on steep slopes in an area subject to
high-intensity storms represents an extreme case
of runoff potential. Thick sod cover and conser-
vation practices such as minimum tillage help
reduce runoff.
Soil as a medium for
immobilization of metals
and toxic chemicals
Soil immobilizes many metals and other
toxic chemicals. Soil pH and soil cation ex-
change capacity (CEC) are the primary control-
ling factors. Slightly acid to slightly alkaline
soils (pH 6.1-7.8) are generally preferred for land
application of sludge. The cation exchange
capacity depends on the amount of organic
matter and the amount and type of clay in the
soil. Immobilization increases as the cation
exchange capacity increases.
Morphological Properties
The most important morphological proper-
ties are texture, structure, color, mottles, hori-
zons, and soil depth. The definitions of these
properties, their significance for land application
of sewage sludge, and their evaluation in the
field are described in the following sections.
-------
Texture
Soil Texture
Affects
Porosity
Water movement
Aeration
Water retention
Organic matter
Plant nutrition
Metal adsorption
Soil texture refers to the soil's particle size
distribution. Soil particles are classified by size
into two groups: fine earth (<2 mm) and coarse
fragments (2 mm-10 in). Fine earth is subdivided
into sand (.05-2.0 mm), silt (.002-.05 mm), and
clay (<.002 mm). The sand fraction is further
divided into very coarse, coarse, medium, fine,
and very fine sand. Coarse fragments include
gravel (2 mm-3 in), charmers (2 mm-6 in and
flat), and cobbles (3-10 in).
Sand particles feel gritty and are so large
that each grain is visible. Silt has a smooth
feeling, like flour or corn starch. Neither sand
nor silt contribute much to the chemical behavior
of the soil.
Clay feels sticky and can be molded into
ribbons and wires. The particles are flat and can
be seen only with high-powered microscopes.
Clay has a large amount of surface area per unit
volume and is much more active chemically than
silt or sand. Many aspects of soil behavior
affecting utilization of sewage sludge depend
heavily on the behavior of clays in soils.
Every soil contains a mixture of sand, silt,
and clay. A textural triangle (figure 1) shows all
the possible combinations and is used to form
groups, or classes, of soil texture. Specific
combinations of sand, silt, and clay have names
such as loam, sandy loam, and silly clay loam.
All the names of soil texture classes, their
abbreviations, and their grouping into
generalized classes are shown in table 2.
If rock fragments larger than 2 mm are
present in sufficient quantity, then names such as
gravelly loam or very cobbly clay, are used.
Precise definitions of coarse fragment modifiers
are given in Appendix A.
Figure 1 is a generalized textural triangle, A
soil that is almost all sand would be very close to
the sand corner of the triangle, and the textural
Table 2.—Names of soil texture classes
Abbreviation Textural class Generalized
term
s
Sand
Coarse
Ls
Loamy sand
Coarse
SI
Sandy loam
Moderately
coarse
L
Loam
Medium
Sil
Silt loam
Medium
Sicl
Silty clay loam
Moderately
CI
Clay loam
Moderately
Scl
Sandy clay loam
Moderately
Sc
Sandy clay
Fine
Sic
Silty clay
Fine
C
Clay
Fine
class name would be sand. Similarly, a soil
dominated by clay would be near the clay corner
of the triangle, and the class name would be clay.
Soils that contain a balanced mixture of
sand, silt, and clay are called loams. These soils
are just below the center of the triangle. Loams
require less clay than sand or silt to balance the
mixture because clay has such a pronounced effect
on both the chemical and the physical behavior.
If the balance of a loam is changed by
adding sand, the sand begins to dominate, and
the particle size distribution moves away from
the loam toward the sand corner. The texture
changes from loam to sandy loam, then to loamy
sand, and ultimately to sand.
If clay is added to a loam, the texture moves
toward the top of the triangle. Adding just a little
more clay changes the texture from loam to clay
loam. If the sample contains more than 40%
clay, the textural class name is simply clay.
If both silt and clay are added to a loam at
the expense of sand, the texture moves away
from the sand corner towards a point in between
the silt and clay corners. The name of this
textural class is silty clay loam. Similarly, adding
both sand and clay at the expense of silt becomes
sandy clay loam.
Texture influences soil suitability for sludge
application and utilization in many ways.
Texture is related to the size and shape of soil
pores, which affects water movement into and
within the soil. Texture influences the balance
between water-filled pores and air-filled pores,
creating different soil environments for root
growth and microorganism activity. Texture also
influences the rate of accumulation of organic
matter. Organic matter and clay content together
-------
determine the soil's capacity to immobilize
metals and supply nutrients.
The medium-textured soils (loam, silt loam,
and fine sandy loam) are usually best for land
application of sewage sludge. The range of pore
sizes in these soils allows water to flow through
the smaller pores and exchange air in the larger
pores. Medium-textured soils provide favorable
environments for root growth, store large
amounts of water for plant use, and have good
nutrient-supplying power.
Sandy soils have more large pores and fewer
small pores. Usually they are well aerated, but
they store much less water for plant use. Sandy
soils are droughty soils, and yields of dryland
crops are likely to be lower than on medium-
textured soils. Nutrient requirements are lower,
and the rate of sludge application may need to be
adjusted accordingly.
Many sandy soils are well suited for the
production of irrigated crops. Since water enters
and moves throughout the soil readily, irrigation
can compensate for droughty conditions. In
contrast, rapid water flow through sandy soils
increases the risk of groundwater contamination.
Groundwater quality problems can stem either
from over-irrigation on a site to which
dewatered, dried, or composted sludge has been
applied, or from applications of excessive
amounts of liquid sludge.
Another potential limitation of sandy soils is
low cation exchange capacity. Cation exchange
capacity depends on both the amount of clay and
the amount of organic matter, and both are low in
most sandy soils. Metal immobilization is likely
to be lower, and the allowable accumulation
period may be shorter.
Clayey soils have more very tiny pores and
fewer large pores. Air exchange and water
movement are much slower than in medium-
textured soils. Water applied as rainfall,
irrigation, or liquid sludge is more likely to cause
temporary soil saturation, reducing oxygen
supplies to soil microorganisms.
Water entry into clayey soils may be very
slow, so runoff is a greater risk. Care should be
taken when applying dried or composted sludge
on clayey soils so that runoff doesn't physically
wash sludge from the site.
Clayey soils have higher cation exchange
capacity, a factor that may be favorable for
treatment of sludge containing higher amounts of
heavy metals.
Coarse fragments don't necessarily render a
soil unsuitable for sludge application, but they do
make management more difficult. Coarse frag-
ments reduce the volume of soil through which
water can flow and in which water can be stored.
In soils containing coarse fragments, both
water flow and water retention depend on the
texture of the fine earth. If the fine earth is
medium- textured, then the soil may be suitable
for sludge utilization, but application rates may
have to be reduced and land area requirements
increased. In other soils, coarse fragments exacer-
bate problems associated with sandy or clayey
textures, and management is more difficult.
Structure
Soil structure refers to the aggregation of
individual grains of sand, silt, and clay into larger
units called peds. Plant roots, soil organic matter,
and clay particles provide physical and chemical
binding agents.
Soil structure is important because it modifies
some of the undesirable effects of certain textures
on soil behavior. Structure creates relatively large
pores, which favor movement of air and water into
and through the soil. Even clayey soils can have
good rates of infiltration and permeability if they
have well-developed, stable structure.
Good soil structure also means good aeration
and a favorable balance between pores that contain
water and pores that contain air. Structure creates a
favorable environment for root growth and
microbial activity in all soils, especially in the
finer-textured soils.
Maintaining strong, stable aggregates is an
important management objective in any farming
operation, including those that utilize sludge as a
fertilizer material. Three factors influence the
maintenance of strong, stable aggregates: organic
matter, clay, and heavy equipment.
Organic matter is vital to the formation and
maintenance of good soil structure. Living roots
help surround soil particles and bind them together.
Exudates from roots and other soil biota provide
a kind of "glue" that stabilizes peds. Decomposed
organic matter, or humus, is particularly valuable
in the development of structure.
Sewage sludge is a valuable soil amendment
because it can improve soil structure. Organic
sludge solids mixed into the surface soil help
restore the structure of overworked soils. Land on
which row crops have been grown repeatedly is
particularly prone to structural deterioration, and
sludge application on this kind of land can be very
beneficial.
Clays help aggregate soil particles due to their
chemical activity and their tendency to shrink and
swell. One mechanism of structure formation is the
attraction between negative charges on clay sur-
faces and positive charges on die edges of clay
particles and organic soil constituents.
Soil structure is very sensitive to the weight of
heavy equipment. Driving on wet soil breaks down
Good Soil
Structure
• Promotes
aeration
• Promotes
infiltration
• Improves air-
water balance
I
-------
One of the
reasons sewage
sludge is such a
valuable soil
amendment is its
potential for
improving soil
structure.
inferences
from Soil Color
• Organic matter
content
• Degree of aeration
• Evidence of water
tables
I
soil aggregates and compacts the soil. Surface soil
compaction retards germination and emergence of
plant seedlings, and reduces the infiltration rate
substantially, thus increasing the potential for
surface runoff. To avoid these problems, wait
until the surface soil has dried out before driving
on the soil. Sod crops (hay and pasture) protect
the structure much more than grain or row crops.
Soil structure is characterized by the shape,
size, and grade of the peds. Common shapes are
illustrated in figure 2. Granular peds are common in
surface soils, and plates occur in some soils
just below the surface horizon. Blocks and prisms
are both common in subsoils. Ped size is de-
scribed with terms such as fine, medium, and
coarse. Peds are measured in millimeters, and the
range in values for each term depends on the
shape. The relationships are given in Appendix A.
10
ft
1
¦**
Granular
Platy
Blocky
Prismatic
Massive
Figure 2.—Common shapes of soil structure
Structural grade refers to the degree of
structural development and the strength of the
peds. The structural grade is described as strong,
moderate, weak, and structureless. These terms
also are defined in Appendix A.
Structural grade is important for sludge
utilization because it affects soil porosity and soil
strength. Soils with moderate or strong structures
are ideal because they have good mixtures of
large and small pores and optimum environments
for growing plants. The peds tend to resist
breakdown under the impact of falling water
drops or from normal traffic. Soils with weak
structure tend to have fewer large pores. They are
less permeable and have slower infiltration rates.
Peds are not as stable, and the soil surface is
more likely to form a nearly impermeable crust.
Color
Soil colors provide clues about the nature of
the root zone. Dark colors usually mean favorable
amounts of organic matter. Gray colors often
indicate soils that are poorly aerated due to long
periods of wetness. Yellowish-brown and
reddish-brown colors indicate favorable air-
water relations.
Because different people might perceive and
describe the same color differently, soil scientists
describe soil color quantitatively by matching the
color of a soil clod with a standard color chip in
a special book of soil colors. This method is
described in detail in Appendix A. Nevertheless,
generalization of soil colors into a few broad
groups helps to interpret their significance for
sludge utilization on land.
Dark brown, very dark brown, and black
colors are caused by accumulations of organic
matter in soils. Usually the darker the color, the
more the organic matter in the soil and the more
fertile and productive the soil is. Organic matter
is a major factor in structure development, so the
darker the soil, the better formed and more stable
the soil peds.
Some soils have black colors extending well
down into the subsoil. If the soil beneath is gray,
then the black color in these soils indicates
prolonged wetness in cool to cold climates.
Organic matter in cool, wet soils breaks down
much more slowly than in warm, moist soils.
The extra organic matter that accumulates
darkens the soil to a greater depth.
Brown and yellowish-brown colors indicate
well-aerated soil. As soil microorganisms and
plant roots use up oxygen in soil pores, oxygen
from the air above the soil readily moves in to
replace it. Well-aerated soils are ideal for crop
growth and beneficial use of sewage sludge.
Brown and yellowish-brown colors are caused
-------
by iron oxide coatings on soil particles.
Chemically, these coatings are the same as rust
on a piece of iron. Iron oxide is stable, and the
coatings remain on soil particles as long as there
is plenty of oxygen in soil pores. If oxygen is not
available, as in soils that are saturated for long
periods, the iron oxide coatings are removed, and
the soil turns gray. This is why brown colors
indicate that the soil has good air-water relations
and is never wet for prolonged times.
Red and reddish brown colors are also
caused by iron oxide coatings and indicate well-
aerated soils. The soil is red, rather than brown,
only because the chemical form of the iron oxide
is a little different. These soils are very strongly
weathered and tend to be more leached, more acid,
and less fertile than brown soils. They have a
different group of clay minerals, resulting in a
lower cation exchange capacity. For these reasons,
red soils may be somewhat less effective
than brown soils in immobilizing heavy metals.
Gray colors are the colors of wet soils.
When soil pores are full of water, oxygen can't
get in. This creates a reducing environment, and
the iron oxide coatings begin to change to the
more soluble ferrous form. Gradually the iron
oxide coatings are stripped away and leached out
of the soil. The gray color is the natural color of
the mineral grains of the soil, darkened a little by
organic matter.
Soils that have gray colors near the surface are
poorly suited for sludge application during times
when the water table is high. But if the soil occurs
in a climatic region having a prolonged dry period,
sludge application may be feasible, especially if
dried or composted sludge is being applied.
Many wet soils are permeable enough that
artificial drainage can lower the water table. If the
soil permeability is at least moderately slow, and
tile drainage has been or can be installed to lower
the water table to a depth of 3 feet, then a wet soil
can be used for most types of sludge application.
Before draining a wet soil, check two things.
One, outlets must be available to receive the
drained water. Two, ensure that draining a wet soil
does not violate provisions of the Food Security
Act of 1985, or any other legislation pertaining
to natural wetlands. Check with local offices of
the Extension Service, Soil Conservation
Service, Corps of Engineers, and state and
federal Fish and Wildlife Services for possible
regulations that affect wet soils.
Light gray and white colors in soils of
humid regions are also caused by stripping of
iron oxide coatings, revealing the true color of
mineral grains. These colors don't always
indicate wetness, though. If the soil below the
white zone is brown or red and well-aerated, the
white zone is not limited by wetness. If the
underlying soil is gray, then the white layer is
probably saturated for long periods of time. In
arid regions, white subsoil may be the result of
deposition of calcium carbonate and not an
indicator of wetness. But if there is a surface
crust of white color, this indicates that water is
rising to the surface from a shallow water table,
bringing soluble salts that are left on the surface
as the water evaporates. These soils are not only
periodically wet, they are often very alkaline and
very slowly permeable, and are more limiting for
sludge application.
Mottles
Some soils have spots of one color in a
matrix of a different color. The spots are called
mottles, and the soil is said to be mottled. Some
mottles appear as splotches of reddish brown
color in a gray matrix. Others appear as gray
mottles in a brown matrix. Both kinds are
described in terms of their abundance, size,
contrast, and color. Abundance refers to the
percentage of exposed surface area occupied by
mottles. Size is the approximate diameter of mottles.
Contrast is the relative difference between the
mottle color and the matrix color. These terms
are defined more fully in Appendix A. Mottle
colors are described using the same technical
procedure that is used to describe soil colors.
Mottling is caused by fluctuating water
tables. When the water table is high, the soil is
saturated and iron oxide is reduced. When the water
table drops, oxygen begins to reenter the soil through
root channels and large pores, which drain first. As
oxygen comes into contact with moist soil
containing reduced iron, the iron quickly
oxidizes, forming an insoluble precipitate at the
surface of a soil ped. The result is a yellowish-
brown mottle surrounded by gray soil.
By understanding these processes, we can
use observations of soil colors and soil mottles to
make inferences about the height and duration of
water tables in soils, even though the soil may be
quite dry when we look at it. Soil colors and
mottles are used to define classes of internal soil
drainage. These are discussed in the section on
inferred properties.
There are three situations, however, in
which mottles do not indicate wetness. These are
the chemical weathering of rocks, relict mottling,
and coatings on soil peds.
Rocks are the parent materials of soils and
are composed of a variety of different minerals.
Each mineral reacts differently to the processes
of chemical weathering. Some turn yellow, some
red, some gray, and some are destroyed
completely. The result of rock weathering can be
a mixture of colors that may look like drainage
mottles, even though the soil is well drained.
Before draining a
wet soil, check
with state or local
officials for
possible
regulations that
affect the use of
wet soils.
Mottles are spots
of gray or brown
color that indicate
the height of
fluctuating water
tables.
11
I
-------
Any horizon that
differs markedly
in texture,
structure, or
density from the
one above or
below affects
water movement.
Kinds of
Restrictive Layers:
• Claypans—
some Bt horizons
• Frag i pans—
Bx horizons
• Duripans—
Bkqm horizons
• Petrocalcic
layers—
Bkm horizons
• Dense till—
Cd horizons
• Weathered
bedrock—
Cr horizons
• Hard bedrock—
R horizons
I
12
This situation i s often encountered in the lower part
of a soil as it grades into weathered bedrock. It
may also be encountered in some glacial till soils
in which a wide variety of rocks and minerals
has been mixed together in the parent material.
The key to avoiding a false interpretation of
rock weathering is to study climate, soil, and
landscape factors carefully. In humid regions,
soils that are gray and mottled and occur in
concave depressions, low-lying areas, or on broad,
flat terraces, are probably wet soils. Soils that
occur on rounded hilltops, sloping hillsides, and
narrow ridges, as well as soils in arid regions, are
likely to be well drained soils with brown colors.
Color variations in these soils are more likely to
be associated with rock weathering than wetness.
Relict mottles are mottles that formed when
the soil environment was wetter than it is now.
Once formed, mottles are a relatively permanent
feature of the soil, even if the climate changes.
Thus, mottles in soils on convex uplands for
which there is no other evidence of periodic
water tables are probably relict mottles and do
not indicate wetness.
Some peds have coatings of substances
other than iron oxide. Organic matter, clay, and
even moisture films can create colors that differ
from the matrix color. These coatings should not
be confused with mottles caused by reduction
and oxidation of iron, and they should not be
interpreted as indicating wetness. The best way
to avoid this mistake is to break open soil peds
and evaluate the color from a freshly exposed
interior surface.
Soil horizons
A soil horizon is a layer of soil parallel to
the earth's surface. Each horizon is defined and
described in terms of its morphological properties:
texture, structure, color, etc. Together, all of the
horizons in a soil constitute the soil profile. A
soil profile description is a complete set of
horizon descriptions for all the horizons that
occur in a soil.
Soil horizons are named using combinations
of letters and numbers. Six general kinds of
horizons can occur in soil profiles (see figure 3):
O, A, E, B, C, and R. These are called waster
horizons. Gradual changes from one master
horizon to another give rise to transition horizons.
These are named with two letters, for example,
AB, B A, and BC. Special kinds of master
horizons are recognized by adding lower case
letters, as in Ap, Bt, Bk, and Cr. Master horizons,
transition horizons, and special kinds of master
horizons all are described more completely in
Appendix A.
. o
Litter layer
A
Mineral surface horizon,
n
dark colored, granular structure
isss
E
Strongly leached horizon,
light-colored, platy structure
Subsoil horizon of maximum
B
development,
"brown", blocky structure
Wml
Wfi
r;
Weathered 'parent material",
V
"brown", massive structure
Is
¦PI Jr *\
T Ji
R
Hard bedrock
m
Figure 3.—Generalized soil profile
A single soil profile never has all possible
horizons. Most soils have an A horizon, one or
two specific types of B horizons, a C horizon,
and one or two transition horizons. Some soils
have only an A horizon and one or more C
horizons. Others have bedrock (R horizon) at
shallow depths. Some soils have an A-E-B-C
horizon sequence, or even an O-E-B-C profile.
Soil horizons are important for sludge utiliza-
tion because of their effect on transmission of air
and water through soil. Any horizon that differs
markedly in texture, structure, or density from
the one above or below affects water movement.
Horizons that are very clayey and have a
weak or massive structure, and horizons that
have very high densities, are called restrictive
layers. Examples of restrictive horizons include
claypans (Bt, or argillic, horizons that are clayey
and massive), fragipans (Bx horizons that are
silty and very dense), and Cd horizons (very dense
glacial till). Water cannot move into and through
these layers as fast as it moves down through the
soil above them. During periods of excess
rainfall, irrigation, or liquid sludge application,
water perches on top of the restrictive layer,
saturating the soil and creating shallow, but
temporary, water tables.
Restrictive layers thus limit the times during
which a soil can be used for land application of
sludge. Liquid sludge can be applied only during
dry seasons, when there is no perched water
table. Dried sludge can be applied to the surface
at other times, as long as the water table is not at
the surface and the,soil is capable of supporting
the weight of application equipment.
-------
Another hazard associated with restrictive
layers is increased potential for surface runoff.
As soon as the soil above the restrictive layer
fills with water, any additional increments of
water must run off. The shallower the depth to a
restrictive layer, the sooner this occurs, and the
more likely sludge solids lying on the surface
will be carried into surface waters. This hazard is
particularly serious on soils that are sloping, in
addition to having a restrictive layer.
Gray horizons that are saturated for long
periods of time are said to be gleyed. They are
called Bg horizons. The limitations they present
are similar to those caused by restrictive horizons.
Gleyed soils can be used for land application of
sludge as long as the soil is dry enough to
support the weight of application vehicles and as
long as sludge liquids are prevented from
entering horizons of saturated soil.
Rapidly draining horizons have sandy
textures and are often gravelly or cobbly. They
have the potential to transmit sludge liquids into
groundwater aquifers before soil treatment is
complete. The risk is not very great with surface
applications, however. Most sludges, even liquid
sludges, do not carry enough water to wet the soil
above a rapidly draining layer enough to cause
rapid transmission of incompletely treated waste.
There is some potential, though, that heavy rain
or irrigation right after a sludge application could
leach the sludge and the soil, moving some liquid
directly into the rapidly draining layer.
Soil profile descriptions are an excellent
source of information about the kinds of horizons
that are present in a soil. By reading a profile
description, you can find out if abrupt textural
changes occur, if restrictive horizons, gleyed
horizons, or rapidly draining horizons are
present, and if they are, at what depth they occur.
Soil depth
The terms shallow, moderately deep, and
deep have very specific meanings in soil science.
They apply when the soil profile contains
bedrock (R horizon), weathered bedrock (Cr
horizon), or a cemented horizon (Bkqm or Bkm).
Shallow always means that one of these
horizons occurs at a depth between 0 and 20 inches.
Moderately deep means that one occurs at a depth
between 20 and 40 inches. Deep means that none
of them occur within a depth of 40 inches.
These terms do not apply to restrictive
layers, gleyed horizons, or rapidly draining
materials. Descriptions of soils that contain these
horizons usually indicate that the soil is deep,
even though the restrictive layer may occur at a
depth less than 20 inches.
Study the profile descriptions and map unit
descriptions in a soil survey report carefully to
determine if a particular soil has a restrictive
layer, and if so, the depth at which it occurs.
Inferred Properties
Several aspects of soil behavior are difficult
to measure directly in the field, but inferences can
be made about these properties on the basis of
primary morphological properties. Inferred
properties that are particularly important for land
application of sewage sludge include permeabil-
ity, infiltration, internal drainage class, available
water holding capacity, leaching potential,
shrink-swell potential, trafficability, pH, nutrient
availability, and heavy metal immobilization.
Permeability
Soil permeability is the rate that water
moves through the soil. Permeability depends on
the amount, size, shape, and arrangement of soil
pores, and on the degree of homogeneity of the
pore structure from horizon to horizon.
Water moves through soil pores in response
to two general kinds of forces. One is gravity,
which pulls on water all the time. The other is an
attraction between water molecules and the
surfaces of soil particles. Very thin films of
water are bound very tightly to soil particles. The
thicker the water film, the lower the attractive
force at the outer edge of the film. As a result,
water moves along an energy gradient from
moist soil, where the attractive forces are
relatively weak, toward dry soil, where the
attractive forces are relatively strong.
In saturated soils, water moves through large
pores because the gravitational attraction is much
greater than the water-soil attraction. Water
moving in this way is called gravitational water.
As long as the soil remains saturated, we refer to
this water movement as saturated flow.
If a saturated soil is allowed to drain under the
influence of gravity with no further additions of
water, then gravitational water is gone after a few
days. All the water remaining in the soil is held
against the force of gravity by the attractive force
between water and soil particles. We refer to the
water content at this point as field capacity.
Any further water movement occurs as
unsaturated flow. Water moves around the soil
particles from thick films toward thin films, i.e
from lower attractive forces toward higher forces,
or from moister soil toward drier soil. The rate of
unsaturated flow is variable, depending on the
pore structure and the moisture content of the
Soil profile
descriptions in
soil survey reports
are an excellent
source of
information about
soil horizons
• Abrupt textural
changes
• Gleyed horizons
• Mottled horizons
• Rapidly draining
horizons
• Restrictive layers
13
I
-------
Soil Properties
that Influence
Permeability
• Texture
• Coarse
fragments
• Structure
• Organic matter
• Restrictive layers
soil. The maximum rate occurs when the soil is at
field capacity. As plants remove water and the soil
dries, all of the moisture films become thinner,
and the rate of flow decreases substantially.
Because of the complexity of the soil pore
system, permeability in the field is difficult to
measure. A soil's hydraulic conductivity is easier
to determine by measuring the rate of saturated
flow in a vertical direction through a sample of
soil in the laboratory. By relating the lab data to a
soil's texture, structure, and horizons, then the soil
permeability can be estimated by observing soil
properties. The specific relationships between
soil morphology and the classes of hydraulic
conductivity are summarized in table 3.
Several properties influence permeability.
Coarse-textured soils, for example, have larger
pores and more rapid permeability than fine-
textured soils. Coarse fragments can't conduct
any water; their effect is to reduce the volume of
soil available for movement and retention of
water. Good soil structure enhances permeability
by providing stable aggregates that have small
pores within peds and large pores between them.
Organic matter enhances permeability through its
effect on forming and stabilizing soil structure.
Whenever the pore structure changes drastically
and abruptly from one horizon to another, there
is a major impact on permeability. But if the
texture and structure change gradually from one
horizon to the next, the rate of water movement
is relatively unaffected, and water continues to
move down through the profile.
Soil scientists use the permeability of the
least permeable horizon in a soil to characterize
the permeability of the whole soil. Because of
the effect of soil layering, however, each
horizon's permeability should be evaluated
separately. This is the only way to determine if
restrictive layers or layers of coarse grained
materials are present, the depth at which they
occur, and whether there is enough soil above
these layers to provide adequate protection for
groundwater.
Table 3.—Relationships between hydraulic conductivity, permeability class, and
soil morphology
Soils that have
moderate or
moderately slow
permeability are
well suited for
land application
of sewage
sludge.
I
Hydraulic
conductivity
(in./hr.)
Permeability
class
Morphological
characteristics
<0.06
0.06-0.20
0.20-0.60
Very slow
Slow
Moderately slow
0.60-2.0
2.0-6.0
6.0-20.0
>20.0
Moderate
Moderately rapid
Rapid
Very rapid
14
Massive, clayey (>35% clay) horizons with
few or no roots
Continuous strongly cemented horizons with
few or no roots
Clayey (>35%) horizons with either weak
structure, platy structure, or slickensides
Continuous moderate or weak cementation
Clayey (>35%) horizons with moderate
structure but no slickensides
Medium-textured soils (18-35% clay) with
weak structure
Sandy soils that are cemented
Soils with very few medium or larger
continuous vertical pores
Medium-textured soils (18-35% clay) with
moderate structure
Fine sandy loams
Soils with a few medium or larger continuous
vertical pores
Medium-textured soils (18-35% clay) with
strong structure
Sandy loams and loamy fine sands
Soils with common medium or larger
continuous vertical pores
Coarse sandy loams and fine sands
Soils with many medium or larger continuous
vertical pores
Sands and coarse sands that contain more
than 15% cparse fragments
-------
Table 4.—Sample data included in soil survey reports
Soil name
and Depth
map symbol (in.)
Clay
(%)
Moist Available
bulk Water Soil
density Permeability Capacity reaction
(gm/cc) (in/hr) (in/in) (pH)
Shrink-
swell
potential
Erosion
factors
K T
Organic
matter
(%)
52B, 52D
0-11
27-40
1.20-1.40
0.6-2.0
0.16-0.18
5.6-6.5
Mod
0.32 2
2-4
Hazelair
11-15
35-50
1.05-1.20
0.2-0.6
0.13-0.19
5.1-6.5
High
0.28
15-36
>36
60-70
1.00-1.20
<0.06
0.09-0.12
5.1-6.5
High
0.24
53
0-5
3-10
1.20-1.40
6.0-20
0.05-0.07
5.6-6.5
Low
0.10 5
2-4
Heceta
5-60
3-15
1.30-1.60
6.0-20
0.05-0.07
5.6-6.5
Low
0.10
54D,54G
0-12
18-27
0.90-1.00
0.6-2.0
0.19-0.21
4.5-5.5
Low
0.32 3
4-8
Hembre
12-44
25-32
1.00-1.15
0.6-2.0
0.16-0.20
4.5-5.5
Low
0.28
Tabular data in soil survey reports (see table 4)
give numbers for the permeability of each different
layer in the soil. For this purpose, horizons that
have similar properties are grouped together, and
only the major differences are shown. The numbers
given in these tables represent the value of the
laboratory test of hydraulic conductivity. Since
the sample is very small and measures only
vertical flow, this test does not reflect actual field
behavior because it does not consider
unsaturated flow. Nevertheless, the test provides
a form of soil characterization that allows
comparison of different soils.
In general, soils that have moderate or
moderately slow permeability (see table 3) are well
suited for application of all types of sludge. Soils
that have slow permeability throughout and do
not have a water table problem are suitable for
application of dewatered, dried, and composted
sludges. Liquid sludges also can be applied as
long as care is taken not to saturate the soil.
Slowly permeable soils that do have a water
table problem are usually somewhat poorly
drained or poorly drained (see page 17). As long
as the permeability is uniform throughout,
lowering the water table with tile drains may be
possible. Where this is done, sludge can be
applied safely.
Soils that have a slowly or very slowly
permeable restrictive layer at relatively shallow
depth (for example, 12 to 20 inches) have serious
limitations for applications of liquid sludge. Other
sludges, however, can still be applied as long as
the soil above the restrictive layer is not saturated.
Soils that are very slowly permeable
throughout are poorly suited for land application
of liquid sludge. The rate of water movement is
too slow to provide both adequate treatment and
sufficient oxygen supplies. Correction of this
limitation is very difficult and costly without any
guarantee of complete success. Even very slowly
permeable soils, however, may be used for dried
or composted sludge during seasons when the
soil is dry enough to support heavy equipment.
Soils with moderately rapid or rapid
permeability are useable for sludge application,
but leaching of nitrates and other chemicals in
solution could be a problem in areas where a lot
of water is added to the soil. The risk can be
reduced by applying only dried or composted
sludges and avoiding times of high rainfall or
excessive irrigation.
Infiltration
Infiltration is the rate that water enters the
soil. Infiltration depends primarily on the pore
geometry at the soil surface, and pore geometry
depends on the texture and structure of the
surface soil. The relationship between texture
and infiltration is illustrated by the data shown in
table 5. Clearly, coarse-textured soils have much
faster infiltration rates than fine-textured soils.
Note that even small increases in clay content, as
in the change from sand to loamy sand, can have
marked effects on infiltration.
Table 5.—Typical infiltration rates of soil
texture classes
Soil texture
Infiltration rate
inches per hour
Sand
2.0 - 5.0
Loamy sand
1.0-1.5
Loam
0.5 - 0.75
Silt loam
0.2 - 0.3
Clay loam
0.15-0.3
Silty clay loam
CM
©
i
O
Clay
0.05-0.15
Soils that have a
slowly or very
slowly permeable
restrictive layer at
shallow depth
have serious
limitations for
application of
liquid sludge.
Soil Properties that
Affect Infiltration
• Texture
• Aggregate
stability
• Organic matter
• Antecedent
moisture
• Subsoil
permeability
• Plant cover
15
I
-------
Techniques for
Maintaining
Optimum
Infiltration Rates
• Don't drive on
wet soil
• Keep organic
matter levels high
• Grow sod crops
wherever possible
Classes of
Internal Drainage
• Excessively
drained
• Somewhat
excessively
drained
• Well drained
• Moderately well
drained
• Somewhat poorly
drained
• Poorly drained
• Very poorly
drained
I
Aggregate stability is the most important
effect of soil structure on infiltration. Strong, stable
peds at the soil surface create and maintain
relatively large pores. Organic matter is particu-
larly important in this regard. Soils that are well
supplied with organic matter are more likely to
have moderate infiltration rates than soils that are
deficient in organic matter. If the organic matter
content is low, and soil peds are not very stable,
then the impact of falling water drops breaks the
peds apart. Soil grains wash into the larger pores,
clogging them and sealing the soil with a crust of
very low porosity. This process of ped break-
down is called slaking, and the formation of a
crust that seals the soil is called puddling.
Infiltration also depends on the moisture
content and the permeability of the soil beneath
the surface. The higher the moisture content, the
slower the infiltration rate. This means that soils
that are nearly saturated are not going to accept
sludge liquids readily. These soils are too wet to
drive on, so the only situation in which a
problem might occur is with application of liquid
sludges through irrigation cannons.
Permeability affects infiltration because water
has to move away from the surface before
additional water can enter the soil. Obviously if
water is applied continuously at rates greater than
soil permeability, then saturated conditions form
and additional water can't enter the soil. A likely
consequence is surface runoff accompanied by
surface water contamination.
Infiltration is important for land application
of sewage sludge because of its relationships to
water quality. By itself, rapid infiltration is a
desirable trait, but if coupled with rapid
permeability throughout the soil, then there is a
greater risk of groundwater contamination either
by applications of dilute liquid sludge, or
applications of dewatered or dried sludge
followed by heavy doses of rain or irrigation water.
Slow infiltration is a more common problem.
This increases the potential for surface runoff
and subsequent contamination of surface waters.
The problem is exacerbated by the tendency for
soil structure to deteriorate in the surface soil,
further lowering the infiltration rate.
Since infiltration is tied to water quality,
maintaining optimum infiltration rates is an
important objective of soil management for sludge
application. These rates can be maintained by:
1. Not driving on wet soil to avoid compac-
tion and structural breakdown;
2. Keeping organic matter levels high by
incorporating sludge and other organic
residues into the soil;
3. Using sod crops in the rotation as much
as possible.
16
Internal drainage class
Internal drainage refers to the ability of free
water to escape from a soil. Internal drainage is
not the same thing as permeability because
permeability indicates only the rate that water
moves //'it has some place to go. Classes of
internal drainage are based on the height that a
water table rises in the soil and the length of time
that the soil remains saturated.
Rapidly permeable soils that are never
saturated are called excessively drained. Soils
that are rarely saturated above 3 or 4 feet are
called well drained. Soils that are periodically
saturated in the lower part of the soil profile are
called either moderately well drained or
somewhat poorly drained, depending on the
depth to the water table and the duration of
saturated conditions. Soils that are thoroughly
saturated for long periods of time are called
poorly drained, or even very poorly drained.
Internal drainage is important because it
affects both the oxygen supply and the tempera-
ture of the environment in which plant roots and
soil microorganisms live. Ideally, about half the
pores in the soil should contain water. The other
half should be filled with air. Well drained soils
can provide this condition, but poorly drained
soils cannot.
Drainage affects temperature because wet
soils are cold soils. Biological processes,
especially those that decompose organic residues
and release nitrogen for plant use, do not operate
as fast in cold soils. As a consequence, if sludge
is applied to a cool season grass or pasture crop,
nitrogen may not be released until later in the year,
after the soil has warmed up and after the crop's
peak demand for nitrogen has passed. The excess
nitrogen that is released later in the year may be
lost either into the groundwater by leaching or
into the atmosphere by denitrification.
When oxygen is limiting, denitrification oc-
curs in wet soils because some of the inorganic
nitrogen is converted to nitrogen gas and escapes
to the atmosphere. Denitrification losses usually
are not large, but if you do not account for them in
planning your application rate to meet crop needs,
the crop may suffer from nitrogen deficiency.
Internal drainage is also important because it
indicates the volume of soil available for plant
root development and uptake of soil nitrogen.
Because these processes occur mainly in aerobic
environments, only the soil above a water table is
available for sludge utilization. The more poorly
drained the soil, the more restrictive it is for both
crop growth and beneficial utilization of sludge.
Soils of any drainage class, however, can be used
for land application of sludge, provided that
shallow water tables are neither present when the
sludge is applied nor for a period of time thereafter.
-------
The extent to which drainage is limiting
depends on the solids content of the sludge, the
permeability of the soil, and the climate. Poorly
and somewhat poorly drained soils are most
restrictive for application of liquid sludges. If
they have uniformly slow or moderately slow
permeability, however, they may be artificially
drained, provided suitable outlets are available.
These soils are less limiting than soils that have
very slow permeability or shallow restrictive
layers that preclude effective drainage.
Climate dictates the amount and frequency
of rainfall, hence the frequency with which water
tables are high. In humid continental climates,
where summer storms occur periodically, there
may be only a few weeks when water tables in
somewhat poorly and poorly drained soils are
low enough to apply sludge safely. In arid
regions and in marine climates, water tables may
be low enough during dry periods to allow land
application of sludge for several months.
Water table fluctuations in soils are rarely
observed directly. Most of the time soils are
studied in the field during dry seasons, and the
internal drainage class is determined by inference
from the evidence in the soil's morphology.
Color, mottling, permeability, restrictive
horizons, pH, and landscape position are the
most important indicators of soil drainage class.
Unfortunately, soil variability across the
U.S. precludes the use of a single, standard
definition of soil drainage classes. Interactions
between rainfall, temperature, organic matter,
biological activity, and parent materials are so
complex that interpretations of internal drainage
must be tailored to regional conditions.
In general, a soil that is sandy or gravelly
and has rapid or very rapid permeability is
excessively drained. A soil that is brownish or
reddish, has moderate to slow permeability, and is
not mottled in the upper meter or so, is well drained.
The lower part of the soil occasionally may be
saturated for a day or so, but neither the
frequency nor the duration of water tables is enough
to adversely impact a sludge utilization program.
A moderately well drained soil is brown or
red in the upper part and has a few gray or
yellowish brown mottles in the lower part. Water
tables periodically rise into the lower part of the
soil, saturating it for short periods of time. When
the water table retreats, the large pores drain
easily, but small pores within soil peds stay wet
long enough for some iron to be reduced. The effect
is not large but it is enough to create either some
small gray mottles within peds or some small
yellowish brown mottles on the surfaces of peds.
A somewhat poorly drained soil is mottled
higher in the profile, although the surface
horizon is usually not mottled, and there may be
a second horizon of well-oxidized, unmottled
soil beneath the surface soil. Water tables rise
higher and persist longer than in moderately well
drained soils.
Somewhat poorly drained soils often have
slow or very slow permeability. If the duration of
saturated conditions is relatively short, mottles
may appear as spots of bright color in a matrix that
still has a brown color. As the duration increases,
however, the soil gradually becomes grayer and
grayer, until the horizon is characterized by a
dark gray matrix containing many prominent
yellowish brown or reddish brown mottles.
Another manifestation of somewhat poor
drainage is high pH and high sodium content.
This occurs in arid regions, where locally high
water tables cause water to move upward through
the soil, carrying salts and sodium with it, and
depositing those salts at the surface as the water
evaporates. These soils have limited agricultural
value and are difficult to reclaim. Sludge appli-
cation, however, may be particularly beneficial
in improving the physical condition of the soil.
A poorly drained soil usually is either
mottled in the surface horizon or is gray and
unmottled throughout the profile. The soil is wet
for such long periods of time that most of the
iron has been reduced and leached away, leaving
only the gray color of uncoated mineral grains.
Some poorly drained soils can be improved
with artificial drainage if the permeability is at
least slow. Often, the permeability is very slow,
there is a shallow restrictive layer, or the land-
scape position is so low that outlets are not
available. These soils are suited for land applica-
tion only if the water table retreats naturally for a
period of several weeks during the growing season.
Available water holding capacity
(AWHC)
Available water holding capacity refers to
the amount of water that soils can store for plants
to use. Like permeability and infiltration, the
water holding capacity depends on the amount
and size distribution of soil pores. Not all of the
pores can retain water for plant use. Conse-
quently, only a portion of the total amount of
water in a soil is available.
Gravitational water is not available because
it drains out of the soil as soon as the water table
drops. When the soil is at field capacity, plants
can remove water easily, but each increment of
water removed makes it increasingly difficult for
plants to withdraw the next increment. When a
soil is so dry that a plant cannot remove any
more water, the soil is said to be at the wilting
point. This marks the low moisture end of the
available water supply. Water in the soil between
Internal drainage
does not prevent
land application
of sludge
provided that
shallow water
tables are not
present when the
sludge is applied.
Indicators of
Restricted
Drainage
• Gray colors
• Mottles
• Very slow
permeability
• Shallow
restrictive layers
• Very high pH
• Low-lying
landscape
positions
17
-------
The higher the
water holding
capacity, the
more productive
the soil, and the
better it is for
beneficial use of
sewage sludge.
Factors
Conducive to
High Leaching
Potential
• Rapid infiltration
• Rapid
permeability
• High rainfall
• Heavy irrigation
• Heavy
applications of
liquid sludge
I
field capacity and the wilting point is called
available water. Water that remains in the soil at
the wilting point is called unavailable water. It is
held so tightly in very small pores, or in very thin
moisture films, that most plants cannot extract it.
Some soils, especially the clays, contain large
amounts of unavailable water.
AWHC is important for land application of
sludge for two reasons. First, it is a measure of
the ability of the soil to sustain vigorous growth
and high yields of common agronomic crops. In
general, the more productive the soil, the better it
is for beneficial utilization of sewage sludge.
Second, it is a measure of the soil's capacity to
store water applied to the soil as rainfall,
irrigation, or liquid sludge. The higher the
AWHC, the more suitable the soil, particularly
for application of liquid sludges.
AWHC, like permeability, is difficult to
measure in the field. AWHC can be measured in
the laboratory, but this test has the same kinds of
limitations as the hydraulic conductivity test. The
usual practice is to estimate AWHC using the
key morphological properties of texture, coarse
fragments, and depth of rooting. Structure and
organic matter increase the volume of water-
storing pores, especially in the A horizon.
AWHC is expressed as the number of inches
of water stored in the entire depth of soil. If the
available water from a column of soil wet to field
capacity could be drained into a pan with an area
identical to that of the soil column, then the
depth of water in the pan would be a measure of
the AWHC of the soil.
A deep (>40 inches), medium-textured soil
with no coarse fragments and no restrictive
horizons can store 12 inches or more of available
water. This is an excellent soil for crop growth
and for land application of sewage sludge.
Soils that are shallow to bedrock or to
restrictive layers, and soils that have a large
volume of coarse fragments, may have AWHC's
as low as 1 or 2 inches. These are very poor soils
for growing most common crops, and for most
sludge utilization programs.
Soil AWHC also can be expressed as the depth
of available water per unit depth of soil . This is
particularly useful in non-uniform soils, because
you can calculate the amount of available water
in each horizon separately, then sum the total of
all horizons within the depth of rooting.
Each class of soil texture has a characteristic
AWHC, expressed commonly as inches of
available water per inch of soil depth (Appendix
B). If you're working in the metric system, the
same numerical value can be used as cm of water
per cm of soil.
To determine the total AWHC for a given
horizon, multiply the inches per inch of available
water by the thickness of the horizon. If coarse
fragments are present, multiply again by the
proportion of the soil that is fine earth. See
Appendix B for an example of this procedure.
Information on water holding capacity is
given in two places in soil survey reports. The first
is the map unit description, which reports the
total soil AWHC as a range (for example, 10 to
12 inches or 1 to 3 inches). The second is the
tabular data (see table 4) in which the AWHC is
given for each major layer in the soil. As with
permeability data, horizons with similar proper-
ties are grouped together. The AWHC data,
however, represent water storage capacities that
already take into account the presence of coarse
fragments. Simply multiply a representative
value times the thickness of the layer and sum all
horizons within the effective depth of rooting.
Leaching potential
Leaching refers to the removal of materials
in solution by water passing through the soil.
Leaching potential is a composite interpretation
developed from information on a soil's infiltra-
tion, permeability, water holding capacity, and
hydraulic loading. It is one component in the soil
water budget, which balances all water inputs
against all water losses.
Whether from rainfall, irrigation, or liquid
sludge, water added to the soil follows several
possible pathways. Some may be taken up by
plants and transpired into the atmosphere. A
small amount may be lost directly from the soil
by evaporation. These two are often combined
into a single factor called evapotranspiration.
Some water may be lost by runoff from the soil
surface, and some may be stored in the soil, if
the available water holding capacity is not full.
Water not accounted for by any of the above
processes passes through the soil. This water
creates the leaching potential. Whether this water
moves slowly by unsaturated flow or rapidly by
saturated flow, any soluble nutrients, metals, or
organics move through the soil with it.
A high leaching potential would occur if
liquid sludge were added to a rapidly permeable
soil already wet to field capacity. A very low
leaching potential would occur if dried sludge
were added to a soil that has moderately slow
permeability and is dry to the wilting point.
Combinations of permeability, water holding
capacity, climate, and type of sludge intermedi-
ate between these two extremes would represent
intermediate leaching potentials.
18
-------
Sludge alone rarely creates enough hydraulic
loading to increase the leaching potential signifi-
cantly. For example, a liquid sludge containing
2% solids and 25 lbs of available nitrogen per
dry ton, applied in sufficient amount to deliver
150 lbs N per acre, adds about 2.5 inches of water
to the soil. Even this level of hydraulic loading
should not cause a problem on deep well drained
soils that are not already wet to field capacity.
Recognizing the leaching potential that
exists under natural soil and rainfall conditions,
or under irrigation, is important. If the leaching
potential is naturally high, then even dried sludge
applied to the soil is subject to leaching. In such
cases, site management should time the applica-
tion of sludge to coincide with periods of low
hydraulic loading. The objective is to provide
sufficient time for immobilization of metals and
organics before the next pulse of added water
passes through the soil.
Trafficability
Trafficability refers to the soil's ability to
support the weight of farm equipment, heavy
trucks, or irrigation equipment with a minimum
of compaction or structural deterioration.
Trafficability is important because:
1. Compaction and rutting of the soil reduce
infiltration and permeability;
2. Loss of traction can delay and increase
the cost of the sludge application;
3. Crops don't grow as well in compacted
and rutted soil, and the potential for
surface runoff is greater.
Trafficability depends mainly on three
things: texture, moisture content, and plant
cover. Soil moisture is the most important factor.
All soils support weight when they're dry and
lose strength when they're wet. The most
important rule is: Avoid driving on any soil when
there's free water (gravitational water) in the top
18 inches. Wait until the soil has dried out at
least to field capacity before driving over it. This
is as true for normal agricultural operations as it
is for land application of sludge.
Sandy and gravelly soils provide the best
support for vehicular traffic. Unless they are
completely saturated you should be able to drive
on them soon after a rain. A sandy soil can be
compacted, however, so wait until the soil is
dry enough.
Loams and sandy loams should be able to
support most traffic if they are at least as dry as
field capacity. The higher the content of silt and
clay, however, the less stable the soil. Again, if
you're not sure if the soil is dry enough, be safe
and wait.
Silt loams, silty clay loams, clay loams, and
clays have the lowest strength and are the most
susceptible to compaction. Even at field capacity
these soils contain a lot of water. The weight and
vibrations from heavy vehicles are likely to
break down soil aggregates, compact the soil,
and seal the surface. For these soils, wait until
the soil is considerably drier than field capacity,
even to the point where the surface few inches is So// ffagf j§ {qq
practically at the wilting point sticky to work With
There is a simple field test to determine if the J,.
soil is above or below field capacity. Take a your lingers IS tOO
sample with a shovel or an auger, grab a handful, W&t to drive OP.
and work it in your hand, squeezing it between
your thumb and fingers. If it sticks to your fingers,
the moisture content is above field capacity. If
you can work it easily but it doesn't stick to your
hand, it's approximately at field capacity. If it
won't stick together in a single, cohesive mass,
it's considerably drier than field capacity.
Soils with a thick, continuous sod crop provide
better vehicle support than bare soils. Plants
remove some of the water, speeding up the drying
of the soil, and the sod acts as a cushion that
prevents breaking through to the mineral soil.
If a soil is wetter than field capacity,
especially if it's a silt loam or silty clay loam, even
a sod cover may not be sufficient to support
traffic. Attempting to drive on such soils may
result in the same loss of traction as in a bare soil.
Shrink-swell potential
To a greater or lesser degree, clays tend to
expand when they wet up and shrink when they
dry. Modest shrink-swell activity is beneficial in
forming a well-developed, blocky structure
common to medium-textured soils.
Certain clays undergo large volume changes
upon wetting and drying. These clays are
commonly called montmorillonites, or smectites.
When they dry, they shrink so much that deep,
wide cracks form in the soil. The masses of soil
themselves are so hard and have such tiny pores
that neither roots nor water can penetrate. Upon
wetting, these clays expand so much that all of
the cracks are tightly closed, and the soil is one
large, structureless mass of clay. Both infiltration
and permeability are slow, and the soil provides
a hostile environment for biological activity.
Most soil survey reports contain information
on the shrink-swell potential of the soils in that
area (see table 4). Any soil, all or part of which is
rated "high" for shrink-swell behavior, requires
careful management for sludge utilization. One
of the best ways to overcome the limitations of
these soils is to continually add and incorporate
organic matter into the surface soil.
Any soil, all or
part of which is
rated "high" for
shrink-swell
behavior,
requires careful
management for
sludge utilization.
19
I
-------
Soil pH
Accurate
Determination of
Soil pH Requires
• Proper soil
sampling
• Standard
laboratory
procedures
Factors that Affect
Cation Exchange
Capacity
• Amount of
organic matter
• Soil pH
• Amount of clay
• Kinds of clay
minerals
Soil pH is a measure of the degree of acidity
or alkalinity of the soil. Technically pH is a
measure of the concentration of hydrogen ions in
the soil solution. Numbers on the pH scale run
from 0 to 14. A neutral soil has a pH of 7.0.
Lower numbers indicate more acid soils and
higher numbers indicate more alkaline soils.
Each whole number unit on the scale represents
a 10-fold increase or decrease in hydrogen ion
concentration.
Soil pH can be measured accurately in the
laboratory using a pH meter. The pH can be
measured in the field with colored indicator
solutions. The field method is faster but not as
accurate.
Information on soil pH can be obtained from
soil survey reports. In some soil surveys, pH data
are tabulated under the heading of soil reaction
(see table 4). The data are given as expected ranges
of pH for a given layer and are particularly
useful for making comparisons among soils that
may be used for land application of sewage
sludge. Some soil surveys and soil profile
descriptions report the pH as a reaction class,
using terms such as slightly alkaline, or strongly
acid. Each of these terms refers to a standard
range in pH values. These are defined in table 6.
Ultimately you'll need to determine the
exact pH of the specific soil at your site. First
sample the soil correctly, using procedures
discussed in the "Soil Testing" section on
page 33. Then prepare and analyze the sample
according to standard laboratory procedures.
Because soil pH values vary, both naturally and
as a result of past management practices, careful
determination of soil pH at the site is important.
On-site sampling and analysis is the only way to
Table 6.—Reaction classes for pH values
PH
Reaction class
<4.5
Extremely acid
4.5 - 5.0
Very strongly acid
5.1 -5.5
Strongly acid
5.6 - 6.0
Moderately acid
6.1 -6.5
Slightly acid
6.6 - 7.3
Neutral
7.4 - 7.8
Mildly alkaline
7.9 - 8.4
Moderately alkaline
8.5-9.0
Strongly alkaline
>9.0
Very strongly alkaline
20
determine the actual value of an important
chemical property that affects both nutrient
availability and heavy metal immobilization.
Nutrient availability
The best way to determine nutrient avail-
ability is to take a representative sample and have
it tested. This is discussed in more detail in
Chapter 5. You can, however, make some pre-
liminary inferences about nutrient availability
from some key soil properties. The most
significant of these are texture, color, and soil pH.
Plants extract nutrients from the soil only if
the nutrients are dissolved in the soil solution.
Dissolved nutrients are in an electrically charged,
or ionic form. Many nutrients, such as calcium,
potassium, magnesium, and iron carry a positive
charge. These are called cations. Others, such as
phosphorus and nitrate nitrogen, carry a negative
charge and are called anions.
Cations—calcium, magnesium, potassium.
The availability of nutrient cations depends on
the soil's Cation Exchange Capacity (CEC). The
atomic structure of tiny, flat clay particles
generates a small amount of negative electrical
charge within each particle. To balance this
charge, cations in the soil solution are attracted
to and held near the surfaces of the clay particles.
This process, called adsorption, retains cations in
the soil so they are not readily lost by leaching.
Organic matter in soils also provides sites of
negative charge for cation adsorption. In soils
that are nearly neutral, i.e. that have a pH around
7.0, some of the hydrogen ions in organic soil
compounds dissociate, creating additional sites
of negative charge. Cations in the soil solution
can be adsorbed at these sites as well.
If the soil becomes more acid, the concentra-
tion of hydrogen ions in the soil solution increases,
and the hydrogen reclaims its place on the
negatively charged site. This blocks the site from
exchange with other cations in the soil solution,
and the soil CEC decreases. For this reason, the
exchange capacity associated with organic matter
is referred to as pH-dependent CEC.
The numerical value for soil CEC is a
measure of the total amount of negative charge
available to adsorb cations. The CEC value is
expressed in terms of milliequivalents per 100
grams of soil. A typical CEC value is 17 meq/
100 grams soil. The actual value for a specific
soil's CEC depends on four things: amount of
clay, amount of organic matter, pH of the soil,
and kind of clay minerals. The CEC increases as
the amounts of clay and organic matter increase,
and it is higher in neutral soils than in acid soils.
-------
The kind of clay minerals affects CEC
because different clays have different atomic
structures, and therefore different CEC values.
Clays that are very sticky and have high shrink-
swell potentials have relatively high CEC's.
Soils that contain these clays may have CEC
values of 40 or 50 meq/100 gm.
Clays in brown soils of cool climates
typically have CEC's in the upper teens to mid
20's. Clays in the red soils that are common in
the southeastern U.S. have relatively low CEC
values, typically in the low teens.
CEC data are not always available in
published soil survey reports. For precise site
planning, you should have data from soil
samples taken at the site. For preliminary site
evaluation, however, you can estimate CEC from
the amount and kind of clay and the amount of
organic matter. The procedure is explained in
Appendix B.
In review, the availability of the nutrient
cations is directly related to the soil's cation
exchange capacity. The higher the CEC, the
more these cations are retained in the soil, where
they are available for plant uptake. The lower the
CEC, the greater the need for supplemental
sources of nutrients for optimum plant growth,
and the greater the potential for loss of nutrients
by leaching into the groundwater.
CEC is not subject to large changes through
soil management. The most important practice is
to maintain high levels of soil organic matter.
Additions of sewage sludge can be beneficial for
this purpose. Encouraging optimum plant growth
and returning crop residues also help maintain
high organic matter levels. Liming the soil to
raise soil pH helps release some of the pH-
dependent CEC. Preventing soil erosion not only
protects environmental quality but also keeps the
organic matter-rich topsoil in place.
Anions—nitrate, phosphate. The availabil-
ity of nutrient anions depends mainly on their
solubility in water and the rate of water move-
ment in soil. Anion exchange capacity does exist,
but it is very small relative to the cation exchange
capacity and has little effect on the retention of
nitrate and phosphorus in the soil.
Nitrate is very soluble in water and moves
with the flow of soil water. When plants are
actively growing, their uptake of soil water
creates a hydraulic gradient toward plant roots,
and the nitrate goes with the soil water into the
plant. Conversely, when there is a hydraulic
gradient downward, as there is following a rain
or an irrigation, water in the soil moves toward
the groundwater, and nitrate in solution goes in
that direction.
Because of nitrate solubility, nitrate manage-
ment depends heavily on the rate and timing of
nitrate applications. Nitrate should not be added
to the soil, either as sludge or as ordinary
fertilizer, when plants don't need or can't use it,
or at times of high leaching potential.
Nitrate management also depends on good
management of the organic nitrogen reservoir.
The objective is to encourage mineralization to
coincide with times when plants are growing
vigorously and can use the nitrogen released.
Land application of sewage sludge should
therefore be timed to match conditions that favor
either slow mineralization or uptake of mineral-
ized nitrogen. Should mineralization occur when
plants are not present, or at times when plants
can't use all the nitrate produced, leaching into
groundwater may occur.
Phosphorus availability follows another set of
rules. As with all other plant nutrients, phosphorus
is available only when it is in ionic form in the soil
solution. There are several forms of available
phosphorus, all of which are some variety of
phosphate and are negatively charged anions.
Phosphate ions are not very soluble in most
soils, and leaching losses are rare. A greater
problem exists in encouraging phosphorus to go
into solution and supplementing with more readily
available forms of phosphorus fertilizer when
natural soil processes provide insufficient amounts.
The key soil properties for judging phospho-
rus availability are soil color and soil pH. Soil color
indicates the kind of clay minerals that are likely
to be present and the relative amount of organic
matter. Soil pH controls phosphate solubility.
Much of the phosphorus in soil occurs as
organic phosphorus. Decomposition of organic
matter slowly releases this phosphorus so that it
can enter the soil solution. However, several
things happen to phosphate ions in solution to
render them unavailable.
In acid soils, phosphate forms complex,
insoluble precipitates with aluminum ions. In
alkaline soils, insoluble calcium phosphates form.
The best remedy for both situations is to maintain
the soil pH as nearly neutral as possible. Values
of soil pH between 6.0 and 7.0 are generally ac-
ceptable for maintaining phosphate availability.
Very old, highly weathered soils contain
relatively high amounts of iron and aluminum
oxides in the clay fraction. These clays readily
react with phosphate ions to form complex
precipitates that are insoluble. These soils
usually have strong red or reddish brown colors
and have limited phosphorus availability.
Sources of CEC
Information
• Data in some
soil surveys
• Analysis of
samples taken
on-site
• Estimates based
on clay and
organic matter
Nitrate should not
be added to soil
when plants don't
need or can't use
it, or at times of
high leaching
potential.
Plan sludge
applications
so that
mineralization of
organic nitrogen
coincides with
times of vigorous
plant growth.
21
I
-------
Metal immobilization
Phosphate is
sparingly soluble
in most soils.
Leaching losses
are rarely a
problem.
Most metals of concern in sewage sludge are
cations. One might expect that metals would be
immobilized in the soil by adsorption onto the
exchange complex. Research has shown,
however, that this is not always the case. Metals
are immobilized by forming complex substances
with clay, organic matter, and iron and aluminum
oxides. The higher the amount of clay and
organic matter in the soil, the lower the
probability that metals are taken up by plants in
amounts large enough to affect either crop yields
or animal health.
Current regulations use CEC as a general
indicator of soil suitability for immobilizing
metals. Soils with a CEC less than 5 meq/100 gm
are generally sandy and gravelly soils that have
low clay and organic matter contents. These soils
immobilize the least amounts of metals.
Most sandy loams and some of the coarser
loams have CEC's in the range of 5-15 meq/100
gm. These soils have higher quantities of clay
and organic matter and are better able to
immobilize heavy metals.
Most soils that are finer-textured than a
sandy loam have CEC's greater than 15 meq/100
gm. These soils are generally capable of
immobilizing the greatest amounts of heavy
metals. Exceptions include soils of arid areas, in
which organic matter contents may be very low,
and some of the red soils, whose clay materials
have very low CEC's, even though the amount of
clay present is relatively high.
Soil pH also affects heavy metal availability.
The lower the pH, the more soluble the metal
complexes in the soil. That's why EPA currently
prefers a soil pH above 6.5.
Many soils are more acid than pH 6.5.
Raising the pH by liming may be feasible if the
soil pH is not much below 6.5, such as 6.0 or 6.2.
Many soils are more acid in the surface horizon
than in subsoil horizons and in these cases liming
may be feasible too.
If the pH is lower than 5.5, adding enough
lime to increase the pH to 6.5 may not be
economical. Utilization of such soils for land
application of sewage sludge may require
working with the appropriate officials to seek
waivers of existing standards, especially if the
metal content is low, or if light annual and
cumulative applications are planned.
l 22
-------
Site Selection
CHAPTER 4
Any site on which a commercial crop
can be produced using normal farming
practices holds some potential for
beneficial use of sewage sludge.
Distinguishing the better sites from the poorer
sites is the focus of this chapter. The best sites
can accept sludge in any form and without
restrictions on the timing of the application,
other than those imposed by the crop itself.
Poor sites may restrict the type of sludge
applied, the method of application, and the timing
of the application. Poor sites also are likely to be
more expensive to manage because additional
sludge processing may be necessary, sludge
storage may be needed during times that are
unfavorable for application, or special practices
are needed to mitigate problems caused by high
water tables, restrictive layers, or steep slopes.
Several keys are used to facilitate the
determination of soil-site suitability ratings for
land application of sewage sludge. These suita-
bility ratings, when used in conjunction with the
maps in soil survey reports, provide powerful tools
for making preliminary evaluations of proposed
sites for beneficial use of sewage sludge.
Keys for Rating Soil Suitability
For all soil-site evaluations, the frame of
reference is the set of properties of an "ideal"
soil. Departures from the ideal point to specific
limitations that lower the suitability rating for
land application of sludge. Rating keys,
therefore, are based on departures from the ideal
soil. Once the limitations have been identified,
management practices for dealing with them can
be specified.
The ideal soil
The ideal soil is deep, well drained, and
medium-textured (silt loam, loam, or very fine
sandy loam). It has a black to very dark brown
surface and a brown or yellowish-brown subsoil.
It is neither red nor gray nor mottled.
The subsoil has no restrictive layers (claypan,
fragipan, or dense glacial till) within 40 inches. No
tillage pan or traffic pan has formed beneath the
Ap horizon.
The ideal soil should have more than 3%
organic matter, a cation exchange capacity in
excess of 15 meq/100 gm soil, and a pH between
6.5 and 8.2. The available water holding capacity
should be 12 inches or more.
The texture and organic matter together give
the ideal soil moderate or strong grades of
structure in all horizons. But the structure must
be stable, and the soils must have a low shrink
swell potential.
The ideal soil must allow water to enter and
pass through easily, but not too fast. The
infiltration rate should be moderate to rapid, and
the permeability should be moderately slow to
moderately rapid throughout.
The ideal setting for the ideal soil is a nearly
level to very gently rolling surface having slopes
between 0 and 3%. The site must not be on an
active floodplain.
Departures from the ideal soil
Very few soils qualify as ideal. Most depart
in at least a small way, for at least one of the
critical properties. Those soils that have only a
few small departures are still suitable for land
application of sewage sludge; their limitations
can be overcome easily with a minimum of
special management practices.
The greater the number of properties that
depart from ideal and the greater the degree of
departure, the more severely limited is the soil.
Many of these soils can still be used for sludge
application, but very careful management is
required, for these sites are much less forgiving
than sites with more suitable soils.
The number and degree of departures from
ideal form the basis for rating soil suitability for
sludge application. The keys in tables 7 to 11
show how soil properties are used to rate a soil.
In these keys, the ideal soil is rated excellent.
Soils with a few, easily managed departures are
rated good. Fair suitability and poor suitability
represent increasing degrees of the severity of
limitations that must be overcome with careful
management.
These suitability ratings are not absolute,
quantitative predictors of soil behavior for
beneficial use of sludge. They are guides to the
relative suitability of a soil and facilitate
comparison among soils of alternative sites.
Fair or poor ratings do not mean that the site
cannot be used in a sludge application system.
They indicate that there are more problems to
manage and that it will probably cost more to
overcome the limitations. Many such sites,
however, can and do play an important role in an
overall sludge management operation.
The suitability of a soil depends as much on
interactions among several properties as it does
on each property individually. These interactions
are expressed by combining information from
two, three or four major properties in each key.
For example, in the depth-texture key (see
table 7), texture, coarse fragments, and depth to
bedrock all interact to express the nature of the
23
-------
physical environment for root growth and
biological activity.
In the infiltration key (see table 8), texture,
structure, organic matter, and shrink-swell
potential interact to control the rate of entry of
rainfall, irrigation water, and sludge liquids.
The drainage-permeability key (see table 9)
shows how the effect of soil drainage class depends
on the permeability of the soil, whether a restrictive
layer is present, and if so, at what depth.
Nutrient availability and metal immobiliza-
tion (see table 10) depend on interactions among
CEC, pH, and organic matter.
The utility of sloping sites (see table 11) for
land application depends not only on the
steepness of the slope but also on the infiltration
rate, the depth to bedrock or a restrictive layer,
and the type and density of plant cover.
The procedure for using these five keys
begins by assembling all the data required for
each key. Morphological data (texture, structure,
coarse fragments, depths to bedrock or restrictive
layers) may be taken either from soil profile
descriptions in soil survey reports or from soil
profile descriptions made by professional soil
scientists in pits dug at a proposed site.
Data on inferred properties (drainage,
permeability, and shrink-swell behavior) may be
obtained from map unit descriptions and tables in
soil survey reports. These inferences can also be
drawn from the morphological properties of the
soils described at the site.
Chemical data (pH, CEC, and organic
matter) may be available in some soil surveys or
from laboratory analysis of properly collected
samples. For preliminary evaluation, field tests
of pH and field estimates of CEC and organic
matter may suffice.
Site data (% slope and type of plant cover)
may be taken either from map unit descriptions
or from on-site observations.
The next step in the procedure is to use the
data assembled to enter each key and determine
the suitability rating for that particular interac-
tion. Some of the keys give dual ratings, one for
liquid sludge and one for dewatered or dried
sludge. This recognizes that soils and sites are
more sensitive to liquid sludge applications, and
that the impacts of unfavorable permeability or
water table conditions may be less severe where
dewatered or dried sludge is applied.
The final step in the evaluation process is to
determine the overall suitability of the site. This
is simply the lowest of the five separate ratings
obtained from the keys.
Several examples for using the keys are
shown in table 12. The soils included are
representative soils from widely separated
Table 7.—Depth/texture key for rating soil suitability for land application of sludge
Subsoil
Coarse
Depth to bedrock (in.)
texture1
fragments2
>40
20-40
<20
Sand
None
G3
G
F
Loamy sand
Gravelly, Cobbly
F
F
P
Very grav., very cob.
P
P
P
Extremely grav., cob.
P
P
P
Sandy loam
None
E
G
F
Loam
Gravelly, Cobbly
G
G
F
Silt loam
Very grav., very cob.
F
F
P
Extremely grav., cob.
P
P
P
Sandy clay loam
None
G
G
F
Clay loam
Gravelly, Cobbly
G
G
F
Silty clay loam
Very grav., very cob.
F
F
P
Extremely grav., cob.
P
P
P
Sandy clay
None
G
F
P
Silty clay
Gravelly, Cobbly
F
F
P
Clay
Very grav., very cob.
P
P
P
Extremely grav., cob.
P
P
P
1 Use the texture of the subsoil horizon within 40 inches that has the highest clay content.
See page 8 for definitions of soil textures.
2 Refer to Appendix A for definitions of coarse fragment classes.
3E = Excellent; G = Good; F = Fair; P = Poor.
-------
Table 8.—Infiltration key for rating soil suitability for land application of sewage sludge
Use data from the surface horizon only
Sandy loam
Sandy clay loam
Sandy clay
Sand
Loam
Clay loam
Silty clay
Loamy sand
Silt loam
Silty clay loam
Clay
Grade of
Organic
Shrink-swell Potential2
structure1
matter (%)
Low-Med
High
Weak
0-1
G/E3
F/G
F/G
P/F
P/F
1-3
G/E
G/E
F/G
P/F
P/F
>3
G/E
G/E
G/E
F/G
P/F
Moderate
0-1
G/E
G/E
G/E
P/F
P/F
1-3
G/E
G/E
G/E
F/G
P/F
> 3
G/E
E/E
E/E
F/G
P/F
Strong
0-1
G/E
G/E
G/E
F/G
P/F
1-3
G/E
E/E
E/E
F/G
P/F
>3
G/E
E/E
E/E
G/E
P/F
Massive
0-1
G/E
P/F
P/F
P/F
P/F
1-3
G/E
F/G
P/F
P/F
P/F
>3
G/E
F/G
F/G
P/F
P/F
Single grain
—
G/E
—
—
—
—
1 Refer to page 10 and Appendix A for definitions of structural grades.
2 Refer to page 19 for definition of shrink-swell potential.
3 Entries to the left of the slash are for liquid sludge. Entries to the right are for dewatered,
dried, and composted sludge. E = Excellent; G = Good; F = Fair; P = Poor.
Table 9.—Drainage/permeability key for rating soil suitability for land application of sludge
— Drainage class1 —
ED & PD &
SWED WD MWD SWPD VPD
A. Soils with uniform permeability2
(same class or adjacent classes)
Very rapid
P/F3
P/F
P/F
P/F
P/F
Rapid & Moderately rapid
F/G
G/E
G/E
F/G
P/F
Moderate & Moderately slow
G/E
E/E
E/E
G/E
F/G
Slow
—
G/E
G/E
F/G
F/G
Very slow
—
F/G
F/G
P/F
P/F
Soils with slowly or very slowly permeable restrictive
i layers4
Depth to restrictive layer
< 20 inches
P/F
F/G
P/F
P/F
P/F
20-40 inches
F/G
G/E
F/G
F/G
P/F
> 40 inches
G/E
E/E
G/E
F/G
P/F
Soils with rapidly draining horizons5
Depth to rapidly draining horizon
< 20 inches
P/F
F/G
F/G
P/F
P/F
20-40 inches
F/G
G/E
G/E
F/G
P/F
> 40 inches
G/E
E/E
E/E
G/E
F/G
1 ED = Excessively drained; SWED = Somewhat excessively drained; WD = Well drained;
MWD = Moderately well drained; SWPD = Somewhat poorly drained; PD = Poorly drained;
VPD = Very poorly drained.
2 Refer to page 14 for definitions.
3 Entries to the left of the slash are for liquid sludge. Entries to the right are for dewatered, dried, and
composted sludge. E = Excellent; G = Good; F = Fair; P = Poor.
4 Refer to page 12 for definitions.
5 Refer to page 13 for definitions.
-------
Table 10.—CEC/pH key for rating soil suitability for land application of sewage sludge
Use data from the surface horizon only
CEC, meq/100 gm1 Organic Soil pH2
(texture) matter (%) <4.5 4.5-5.5 5.5-6.5 6.5-8.2 8.2-9.0 >9.0
<5
0-1 P3
P P
F
F
P
(Sand, Loamy sand)
1-3 P
P F
F
F
P
>3 P
P F
G
G
P
5-15
0-1 P
P F
F
F
P
(Sandy loam, Loam,
1-3 P
F F
G
G
P
Sandy clay loam,
>3 F
F G
E
G
F
some Clays)
> 15
0-1 P
P F
G
F
P
(Loam, Silt loam,
1-3 P
F G
E
G
F
Silty clay loam,
>3 F
F G
E
G
F
Clay loam, most Clays)
1 See page 20 for definition.
2 See page 20 for definition.
3 Ratings apply equally to all sludges. E = Excellent; G = Good; F = Fair; P = Poor.
Table 11.—Slope effect key for rating soil suitability for land application of sludge
Depth to bedrock or to
Infiltration rating (from Table 8)
Slope (%) restrictive layer (In.)
E G F
P
<20
G1'2 G F
P
0-3
20-40
E E G
G
>40
E E
: G
G
<20
F F
: P
P
3-7 or 3-8
20-40
G G F
P
>40
EGG
F
<20
F F
: P
P
7-12 or 8-15
20-40
F F
: F
P
>40
G F
: F
P
<20
P F
' P
P
2-20 or 15-30
20-40
F F
: P
P
>40
F F
: P
P
<20
P F
5 P
P
> 20 or > 30
20-40
P F
5 P
P
>40
F F
' P
P
1 Ratings apply equally to all sludges. E = Excellent; G = Good; F = Fair; P = Poor.
2 Increase the rating one class for applications on forested soils that have organic surface horizons.
26
-------
geographic areas of the United States. Two
examples are discussed here.
The Woodburn soil is a deep, moderately well
drained soil on nearly level lacustrine terraces in
the Willamette Valley of western Oregon. Textures
are silt loam in the surface horizon and silty clay
loam in the subsoil. There are no restrictive layers
and no tillage pans. The surface horizon has weak
structure, but it contains over 3% organic matter.
The permeability is moderate above 32 inches and
slow below 32 inches. The pH is 5.6-6.5 through-
out, and the CEC is greater than 15 meq/100 gm.
The first key, depth and texture, gives this
soil a good rating because the subsoil texture is a
little heavier than ideal. The second key,
infiltration, gives the soil a good rating for liquid
sludge and an excellent rating for dried sludge.
Weak structure is the limiting factor.
The third key, drainage and permeability,
also gives the soil a good rating for liquid sludge
and an excellent rating for dried sludge. The only
limitation is a temporary water table between 24
and 40 inches.
The fourth key, CEC/pH, gives the soil a
good rating for all types of sludge. The soil pH
is a little lower than ideal, but high levels of
organic matter and CEC partially compensate.
The fifth key, slope, gives the soil an excellent
rating for both liquid and dried sludge applica-
tions.
Overall, the Woodburn soil has one excellent
and four good ratings for liquid sludge, and three
excellent and two good ratings for dewatered or
dried sludge. The suitability is considered
"good" for either type of sludge application.
The Volusia soil is a deep, somewhat poorly
drained soil formed in dense glacial till on low,
rolling uplands in the southern tier of New York
State. The textures are channery silt loam
throughout the profile. The surface horizon has
weak structure and contains more than 3%
organic matter. The CEC is about 12 meq/100
gm, and the pH is between 5.1 and 5.5.
Volusia has a dense, slowly permeable
fragipan at 17 inches that restricts movement of
both water and plant roots. Both the colors and
Table 12.—Suitability ratings for five representative soils in the United States
—; Soil series (state)
Cecil (SC) Clarion (IA) Plalnfleld (Wl) Volusia (NY) Woodburn (OR)
Soil Properties
Texture of subsoil
Clay
Loam
Sand
Silt loam
Silty clay loam
Coarse fragments
None
None
None
Channery
None
Depth (in)
>40
>40
>40
>40
>40
Texture of surface
Sandy loam
Loam
Loamy fine sand
Silt loam
Silt loam
Structure grade
Weak
Weak
Weak
Weak
Weak
Organic matter, %
1-3
>3
1-3
>3
>3
Shrink-swell potential
Low
Low
Low
Low
Low
Drainage class
WD
WD
ED
SWPD
MWD
Permeability
Moderate
Moderate
Rapid
Very slow
Slow
Depth to restrictive
layer (in)
>40
>40
>40
17
>40
Depth to rapidly
draining layer (in)
>40
>40
>40
>40
>40
CEC (meq/100 gm)
6
28
3
12
27
pH of surface soil
5.1-5.5
6.1-6.5
6.6-7.3
5.1-5.5
5.5-6.5
Slope (%)
6-10
2-5
2-6
8-15
0-3
Suitability ratings (Tables
7-11)
Depth/Texture key
G/G
E/E
G/G
G/G
G/G
Infiltration key
G/E
G/E
G/E
G/E
G/E
Drainage/Permeability key
E/E
E/E
F/G
P/F
G/E
CEC/pH key
F/F
G/G
F/F
F/F
G/G
Slope key
F/G
G/E
G/E
F/F
E/E
Overall rating
Liquid sludge
Fair
Good
Fair
Poor
Good
Dewatered/Dried sludge
Fair
Good
Fair
Fair
Good
-------
Overcoming soil
limitations is
largely a matter
of applying good
common sense in
conjunction with
a good
understanding of
soil, sludge, crop,
and climate.
Kinds of Soil
Survey Information
Maps
• Delineations
• Symbols and legends
Text
Map unit descriptions
Soil profile
descriptions
Tables
Soil and water
features
Physical and
chemical properties
Engineering
properties
Estimated yields
I
28
the presence of mottles indicate that perched
water tables stand above the fragipan for
significant periods of time.
Despite the fragipan, this is considered to be
a deep soil, and the textures by themselves are
favorable for beneficial use of sewage sludge.
Only the coarse fragments cause the depth/
texture rating to drop from excellent to good.
The infiltration key rates Volusia as good for
liquid sludge and excellent for dried sludge, the
only limitation being weak structure.
Interactions between drainage, permeability,
and the restrictive layer, however, cause the soil
to have a poor drainage/permeability rating for
liquid sludge and a fair rating for dried sludge.
The CEC/pH key gives the soil only a fair
rating because both the CEC and the pH are
considerably lower than ideal. The slope rating is
fair for all kinds of sludge applications because of
the relatively high runoff potential on 8-15% slopes
of soils that have a shallow restrictive layer.
Overall, Volusia earns two good, two fair, and
one poor for liquid sludge, and one excellent, one
good, and three fair for dried sludge. The
suitability for liquid sludge is rated as poor, but for
dewatered or dried sludge, the suitability is fair.
How to deal with limiting properties
Overcoming limitations is largely a matter
of applying common sense in conjunction with
knowledge of soil, sludge, crop, and climate.
Most of the possibilities have already been
discussed in Chapter 3.
Since the intrinsic texture or depth of the
soil is difficult to modify, manage such limitations
by applying only dried or dewatered sludge and
timing applications to coincide with dry seasons.
The best way to manage rapid infiltration in
coarse-grained soils is to use dried sludge having
a relatively high percent solids. Do not apply
sludge during rainy seasons when the leaching
potential is high.
The best cure for low infiltration rates is to
add organic matter. Sludge is an excellent
amendment because it provides a source of
organic matter. Mixing the sludge into the
surface soil by disking is preferable to leaving it
on top of the soil surface. If infiltration has been
reduced by formation of a tillage or compaction
pan just below the surface horizon, shattering the
pan by ripping the soil when it is dry may be
very helpful.
Drainage problems in some soils of uniform
permeability may be corrected with artificial
drainage. This may not be a cost-effective solution,
however, and there are federal and state regulations
that preclude drainage of some wetlands.
If you can't drain the soil, then minimize the
problem by using dewatered or dried sludge and
plan on applying the sludge only during dry
seasons after water tables have receded. This
may be the only remedy for soils that have
temporary, perched water tables above slowly
permeable restrictive layers.
The CEC of the soil is difficult to change,
although adding organic matter helps. If pH is
limiting, you may be able to solve the problem
by liming. The feasibility of liming depends on
the economics of the farming operation. If the
pH is not much below 6.5, for example, only 2 to
4 tons of lime may be required, and if a high
value crop is being grown, then liming to
mitigate acid soils is feasible. Another possibility
for mitigating low pH is to either grow crops that
do not accumulate heavy metals, or crops, such
as ornamentals, that are not part of the food
chain.
If neither liming nor alternative crops are
feasible, and if existing standards for metal
loadings in soils cannot be modified or waived,
then strongly acid soils may be unsuitable for
beneficial use of sludge.
Steep slopes need to be managed to encour-
age infiltration and minimize surface runoff.
Appropriate ways to deal with steep slopes in a
sludge utilization program are using high-solids
sludges, applying only on pasture or hay fields,
and practicing soil conservation with cross-slope
farming, reduced tillage, and diversion terraces.
How to Use Soil Surveys to
Facilitate Site Selection
Soil surveys are inventories of the soil
resources of an area. The information in a soil
survey is useful in finding possible sludge
utilization sites and assessing the relative
suitabilities of the alternative sites under
consideration.
Components of soil surveys
Published soil survey reports contain three
interrelated parts: maps, text, and tables. Soil maps
show the spatial distribution of the different kinds
of soils that occur in an area. The text describes
the properties of each of the kinds of soils shown
on the maps. The tables provide additional data
about soil properties and interpretations of those
properties for a variety of land uses.
Maps. Lines drawn on soil maps surround
areas called delineations. Each delineation
represents the size, shapfe, and location of a
specific body of soil. Each delineation contains a
-------
symbol that identifies the map unit of that
delineation. Each map unit has two components:
the dominant kind or kinds of soil, and other
soils that are known to occur but are too small to
be shown separately at this scale of the map.
These other soils are called inclusions.
On most soil maps the smallest area that can
reasonably be shown is 3 to 4 acres. Any body of
soil smaller than that is an inclusion in a larger
delineation of a different kind of soil. Inclusions
also occur because the boundaries between
different soils are not sharp lines but gradual
zones of transition. The lines on a soil map are
placed as nearly as possible at the center of that
transition zone. It is unrealistic to expect natural
soil boundaries to change abruptly within a few
feet, as the map might suggest.
Inclusions are important because they may
affect the suitability of a site. That's why soil
maps, though very helpful in preliminary site
analyses, must be supplemented with on-site
evaluations before final decisions are made.
Text. Soil survey text contains both map
unit descriptions and soil profile descriptions.
Map unit descriptions give the name of the
dominant soil, the texture of the surface horizon,
and the range of slopes on which the soil occurs.
Information on depth, drainage, permeability,
AWHC, and landscape of the dominant soil is
also presented. Map unit descriptions identify the
other kinds of soils that are most likely to be
found as inclusions and indicate how much of
the area the inclusions are likely to occupy.
Soil profile descriptions give detailed,
technical information on the properties of each
major kind of soil, or soil series, found in the soil
survey area. Each series is described in terms of the
horizons that are present and the texture, color,
structure, pH, and other properties of each horizon.
Tables. Tabular information summarizes
key properties and interpretations for each map
unit. Of all the tables, four are particularly
relevant to land application of sewage sludge.
The table on Physical and Chemical
Properties provides information on clay content,
permeability, AWHC, pH, shrink-swell potential,
and in some cases, organic matter content.
The table on Soil and Water Features gives
information on runoff potential (hydrologic group),
flooding, water tables, and depth to bedrock.
The table on Engineering Properties contains
additional data on texture and coarse fragments.
The table on Yields Per Acre of Crops and
Pasture shows the kinds of crops that are
commonly grown and the relative yields that can
be obtained under good management.
Using soil survey information to
rate soil suitability
The objective here is to extract from soil
surveys all the information needed to make a
preliminary evaluation of site suitability by
rating each of the soils present according to the
criteria in tables 7-11. Soil surveys can be used
either to evaluate sites that have already been
identified or to help locate potential sites for
future consideration.
Evaluation of known sites. The first step is
to locate the area in question on the Index to
Map Sheets. This index identifies the specific
map sheet or sheets that include the area. Turn to
those specific map sheets and locate the area.
Make a list of all map unit symbols for the
delineations that cover the potential land
application area. Use the legend to determine the
specific soil name that corresponds to each
symbol.
The next step is to read about the soils in
both the map unit descriptions and in the soil
profile descriptions. Much of the information
needed is in these narratives. Finally, consult the
relevant tables to complete your data collection
process.
You should now have enough information to
rate the dominant soil of a map unit using the
five keys in tables 7-11. Then write the suitabil-
ity for each delineation on the map or color each
delineation according to its suitability class.
Two things should now be apparent: the
amount of land in each suitability class, and the
pattern of admixture of soils in different classes.
The pattern is much more significant. Areas of
uniform suitability, even if they are uniformly
fair or even poor, are easier to manage than areas
of non-uniform suitability. Such an area is
illustrated in figure 4.
limiting for land application of sludge.
29
Using Soil
Surveys for
Preliminary Site
Evaluation
• Locate the site
on the Index to
Map Sheets
• Locate the site
on the
corresponding
detailed map
sheet
• List all map unit
symbols shown
• Identify symbols
in the legend
• Read map unit
descriptions
• List inclusions
likely to be
present
• Read
descriptions of
dominant soils
and inclusions
• Consult tables
for relevant data
-------
Using Soil Surveys
for Locating
Potential Sites
• Consult the
General Soil Map
• Select a soil
association
dominated by
suitable soils
• Locate a large
area of that
association on the
General Soil Map
• Locate the same
area on the
Index to Map
Sheets
• Turn to the
corresponding
detailed maps
and study the
soil patterns
• Consult text and
tables as
necessary
When small areas of fair or poor soils are
distributed entirely throughout larger areas of
good or excellent soils, the pattern may limit the
management of the entire site to that of the least
suitable soil. Figure 5 shows a pattern of an area
of dominantly well drained and moderately well
drained soils dissected by thin strips of poorly
drained soil.
I
30
Figure 5.—Soil map showing a limiting pattern for
land application of sludge
One problem with limiting patterns is that
suitable soil areas may be too small to manage
independently, yet managing the whole area as if
it were suitable increases the potential for
contamination of surface or groundwater in the
poorly suited areas.
Another problem is that access to the more
suitable areas may be limited to times when the
trafficability of the poorly suited soils is
acceptable. The only viable solution to
management of these kinds of soil patterns is to
manage the entire area as though it all consisted
of the more limiting soils in the pattern.
The last step in using soil survey information
is to consider possible effects of the inclusions
within each map unit delineation. The
descriptions of each map unit state explicitly
what kind of included soils are most likely to
occur. Read the map unit and soil profile
descriptions of the inclusions to determine their
properties. If necessary, determine the suitability
rating for each of the inclusions.
Locating potential sites. The other use of
soil surveys is to help find some possible sites
for sludge application. Start with the General
Soil Map, a small scale map of the entire soil
survey area. Map units for the General Soil Map
are Soil Associations. Each soil association
consists of two or three major soils that occur in
a regular, predictable pattern on the landscapes
represented by the corresponding areas shown on
the General Map. The legend for the General
Map is printed on the same page as the map.
Each association is described in more detail in
the text, usually near the beginning of the report.
Use these two sources to identify associations
dominated by deep, well and moderately well
drained soils on nearly level landscapes.
Associations dominated by poorly drained soils,
soils on flood plains, or soils on steep slopes are
not as likely to have large areas of suitable soils.
Return to the General Soil Map and locate
the areas where the more suitable associations
occur. Locate the corresponding areas on the
Index to Map Sheets and select two or three map
sheets. Turn to these sheets and find the areas
dominated by map units of suitable soils. There
may not be any good areas for land application
on a particular map sheet, but you can use this
process for some initial screening to identify
areas that warrant further investigation.
Limitations to the use of soil maps
Published soil surveys are excellent tools for
generalized site evaluation and preliminary site
screening. Soil surveys cannot be used for
detailed site evaluations because of the scale of
the map. This is not the fault of the soil survey,
nor does it reflect on the accuracy with which the
map was made. It means that standard soil
survey maps cannot resolve soil differences that
are smaller than four or five acres in size. To
assume otherwise is to use the information
provided incorrectly.
The only solution to this problem is to make
an on-site investigation, and, if necessary, have a
more detailed soil map made by a professional
soil scientist. This investigation will reveal
exactly what kinds of inclusions are present,
their location, and the extent to which they, or
the pattern of admixture of other soils, may limit
the use of the site.
Detailed on-site studies are particularly
necessary for major projects and projects in
which liquid sludges are going to be applied.
Where only dried sludges will be applied, or
where sludge will be applied only once or at
infrequent intervals, the information obtained
from the soil survey may be adequate to assess
site suitability.
-------
Crop Management Factors
CHAPTER 5
Designing, implementing, or evaluating
a plan for beneficial use of sewage
sludge requires working within the
farmer's or site operator's existing
management system. Sludge utilization is not
likely to alter decisions on the crops to grow, the
crop rotations to use, and whether to drain,
irrigate, or lime the soil. The crop management
system dictates when a field is accessible, the
frequency of sludge applications, the expected
amount of nutrients the sludge must deliver, and
the application methods.
Crop management factors that bear directly on
the design of a sludge utilization program include
crop choice, nutrient management, water manage-
ment, and conservation practices. Application of
sewage sludge to farmland also requires the
management practice of long term monitoring.
Choice of Crop
Sludge can be applied to row, grain, pasture,
horticultural, and tree crops. Some crops, such as
leafy green vegetables and root crops, accumulate
heavy metals, and should not be grown on soils
to which sludge has been applied. The crops
most likely to be used in a sludge management
program are pasture and forage, grain and grass
seed, and row crops. Row crops include food crops
(crops grown for direct human consumption or
animal feeds) and non-food crops such as cotton,
Christmas trees, and ornamentals.
Pasture and forage crops offer the greatest
flexibility for application of sewage sludge
because access is not limited by the crop's
growth stage. Sludge may be applied whenever
climatic and soil moisture conditions are
favorable. The sod created by pasture and forage
crops also promotes infiltration, controls erosion,
and enhances site trafficability.
One disadvantage of pasture sites is that
sludge cannot be easily worked into the soil. With
surface applications, up to 50% of the NH4-N in
the sludge may be lost by volatilization. This must
be considered when calculating the amount of
sludge needed to meet nutrient requirements. Fur-
thermore, some of the benefits of sludge as a
physical soil conditioner cannot be realized with
surface applications. Another disadvantage is that
a waiting period after application, usually 30 days,
is necessary before animal grazing can resume.
Grain and grass seed crops are well suited for
sludge utilization, although sludge application
may be limited to a single annual application
approximately a month prior to planting. At that
time, you can drive over the soil and apply sludge
in any form and work it into the soil. This preserves
both the ammonia and the physical benefits.
For fall-seeded crops, sludge can be applied
in August or September. Usually there are
enough times when the soil is dry enough to
apply sludge without undue risk of runoff,
leaching, or soil compaction.
In some climatic regions, fall sludge
applications to warm, moist soil may create nearly
optimum conditions for mineralization of organic
nitrogen. As a result, the production of nitrate is
out of phase with crop needs. The excess nitrate
is subject to leaching as soil moisture increases
through the following winter and spring.
For spring-seeded grains, the window of
opportunity for applying sludge is much
narrower. In many areas the soil does not dry out
enough to support traffic until planting time. In
this situation, it may be necessary to apply the
sludge, plow and/or disk it into the soil, and plant
the grain immediately.
Low amounts of readily available N in most
sludges indicate that supplemental fertilizers may
be needed to start spring seeded crops. Subsequent
mineralization, however, is more likely to match
plant needs throughout the remainder of the
growing season. These relationships must be
considered when determining the total amount of
sludge to apply to the field.
Row crops, especially annual ones, are
generally planted in the spring, and the same
principles apply as for spring grains. A single
application prior to planting is the most common
procedure. Access is good, but trafficability may
be limiting, and starter fertilizers may be
necessary.
Farmers need to be aware that unprocessed
fruit and vegetable crops cannot be planted on
sludge-amended soil for 18 months. Processed
fruits and vegetables can be grown on sludge-
amended soils, but some processors may not
accept such crops simply to avoid possible
repercussions from consumers.
Row crops grown for animal feed, such as
corn and soybeans, and row crops that are not
eaten, such as cotton, are good choices for a
sludge utilization program.
Work closely with
farmers and their
Extension agents
or crop
consultants to
make sure that
sludge can be
properly fitted into
the overall crop
management
plan.
Nutrient Management
Sewage sludge is a fertilizer and is therefore
an integral part of the nutrient management
program. The crop type, yield, rotation, and soil
test data are all used to design an overall soil
fertility management program.
Fertilizer recommendations are based on a
combination of farmer experience and long-term
research to correlate soil test data with crop
response to added fertilizer. These research
results are available in many states as fertilizer
31
-------
The amount of
fertilizer nutrients
that sludge must
deliver depends on:
• Kind of crop
• Expected yield
• Amounts of
residual nutrients
• Amounts of other
commercial
fertilizers used
I
32
guides published by the Extension Service.
County Extension agents and agronomic
consultants use fertilizer guides to assist farm
managers develop nutrient management programs.
Fertilizer nutrients are expressed in terms of
pounds N, pounds P2Oy and pounds K,0. For
example, a bag of fertilizer labeled 16-8-8 delivers
16% N, 8% P205, and 8% K,0 by weight.
Sludge data, by contrast, are expressed in
terms of the elemental concentrations of N, P,
and K, not their oxide equivalents. Although
sludge data and fertilizer conventions are the
same for nitrogen, sludge P must be converted to
P20,, and sludge K to K20, in order to accurately
assess the nutrient value of sludge.
To convert P to P2Os, multiply by 2.27, or to
convert P,05 to P, multiply by 0.44. Thus, adding
100 pounds of sludge P to soil is the same as adding
227 pounds of P,05. Conversely, if a fertilizer
recommendation calls for 100 lbs of P205, that
would require only 44 pounds of sludge P.
To convert K to K,0, multiply by 1.20, or to
convert K20 to K, multiply by 0.83. Adding 50
pounds of sludge K to a soil adds the equivalent
of 60 pounds of K20, whereas a fertilizer
recommendation calling for 50 pounds of K.,0
would require only 42 pounds of sludge K.
The amount of fertilizer nutrients that sludge
must deliver depends on the kind of crop,
expected yield of the crop, amount of residual
nutrients in the soil, and use of commercial
fertilizers or lime.
The kind of crop and the expected yield of
the crop determine the total nutrient requirement
for the crop. Grass pastures and hay crops, for
example, may require up to 250 pounds per acre
of fertilizer nitrogen during the growing season.
If the grass contains a legume, only 70-80
pounds of nitrogen should be applied. Spring
grains and grass seed crops need about 150
pounds N per acre, whereas row crops require
approximately 250 pounds N per acre.
Higher yields mean greater nutrient uptake,
and this implies a greater need for nutrients to be
supplied from fertilizers or sludges. Crop yields
depend on the weather and the farmer's
management program. Historical records of crop
yields, and the knowledge of local county
Extension agents and crop consultants may be
your best guides to estimating crop yields.
Fertilizer recommendations must account for
residual nutrients from all sources. Prior crops,
residue management, and prior sludge applications
affect the amount of residual nutrients in the soil.
If sludge is applied to a field where spring
grain will be grown and the previous crop was
grass hay or pasture, then there is essentially no
residual nitrogen. All of the crop's needs must
come from supplemental sources. If the previous
hay crop contained a legume, then as much as 75
lbs N/A may remain in the soil. If the previous
crop was a row crop, as much as 50 lbs/A of
residual N may remain. If the previous crop was a
grain crop, there may be only 25 lbs/A residual N.
Like sludge, crop residue is a valuable soil
amendment and should be returned to the soil.
Sludge and crop residue add plant nutrients, help
maintain soil organic matter levels, and improve
the physical condition of the soil.
Harvesting pasture, hay, and silage crops leaves
little residue to return to the soil. Residual nutrients
from these crops are correspondingly low. Field
corn, grain, and grass seed crops leave relatively
large amounts of residue. Working these residues
into the soil returns larger amounts of nutrients.
Removing these residues, either by burning or by
baling, lowers the residual nutrient supply.
Prior applications of sludge may result in
significant increases in nutrient pools. The rule
of thumb for nitrogen is: the mineralization rate
of organic nitrogen in subsequent years is about
half of the previous year's rate. Calculations of
residual nitrogen from prior sludge applications
are illustrated in Chapter 6.
Some sewage sludges deliver more potas-
sium and phosphorus than a crop needs in a
single year. These excesses add to the residual
nutrient supply, especially when sludge is
applied to the same field several years in a row.
The amount of sludge to apply depends on
whether the fertility plan intends to meet all, or only
a portion, of a crop's needs with land-applied
sludge. Many row crops, for example, require more
nitrogen at the beginning of the growing season
than can be supplied by slow mineralization of
organic nitrogen. Farmers can meet this need by
placing fertilizer in a band near the seed when
planting. This reduces the amount of sludge
required to provide the remaining nitrogen needed.
For fall-planted grains, nutrient management
may include a broadcast application of fertilizer
in the spring. This would substantially reduce the
amount of sludge applied and worked into the
soil prior to planting. This spring fertilizer need
might be met with sludge, if a liquid sludge,
which could be applied with an irrigation gun,
were used.
Another nutrient management factor
influencing sludge utilization is liming a field to
raise the pH. Soil pH affects metal immobiliza-
tion, but farmers usually apply lime only if there
will be an economic return through increased
crop yields. Metal loading rates are subject to
regulatory standards; the combination of crop
and soil pH may restrict the rate of sludge
application and the length of the allowable
accumulation period.
-------
Soil Testing
Regular soil testing is essential in evaluating
residual nutrient supplies and formulating
fertilizer recommendations. Soil testing is also
essential in planning and designing good crop
and sludge management programs.
Getting a good sample is vital in getting
good soil test information. Neither the sludge
generator nor the sludge regulator should be
expected to sample or test the soil. For sludge
utilization programs, the farmer, site operator,
county Extension agent, or crop consultant
should sample the soil. Soil samples should truly
represent the field on which sludge will be
applied. Most state agricultural Extension
Services can provide a list of acceptable soil
testing laboratories and have publications on
obtaining a good sample (see figure 6).
If a field contains two or more distinctly
different kinds of soils, separate samples of each
soil should be taken. For each kind of soil,
several subsamples should be taken from all over
the field, mixed together in a bucket, and a small
portion withdrawn for analysis. The soil should
not be contaminated with manure, sludge, lime,
fertilizer, or other substance.
Soil test data on available potassium and
phosphorus are the basis for recommendations
on supplemental potassium and phosphorus
amounts. The soil pH and the lime requirement
are the basis for the amount of lime to apply.
Nitrogen values may fluctuate widely as
environmental conditions and biological activity
in the soil change. Therefore, soil tests for nitrate
are not often used to measure residual N or as the
basis for crop nitrogen requirements.
Instead, research results on crop responses
under controlled conditions are used to forecast
nitrogen needs. If soil test nitrate values are
reported, check that the samples were properly
handled prior to analysis. Proper handling means
minimizing the opportunity for mineralization
between sampling and analysis. This can be done
by refrigerating or freezing the sample. If the
sample is dry to the wilting point, little minerali-
zation will occur, as long as the sample is
maintained in the dry state.
Soil testing is
essential in
order to plan
good crop
and sludge
management
programs. The
most important
part of a good
soil testing
program is getting
a good sample
of soil to test.
-.npling Soils for Testing
E.E. Schutte, L.Q. Bundy and J.B. Peters
Importance of Taking
Good Soli SamplM
A soil test is the only practical way
of Ming whether lime and fertilizer
are needed. However, if a eoil sam-
ple does not represent the general
soli conditions of the field, the
recommendations baaed on this
sample will be useless, or worse,
misleading. An acre of soli to the
depth of plowing weighs about
1,000 tons. Lett than one ounce of
sou la used for each test in the
laboratory. Therefore, It la very im-
portant that the soli sample is
characteristic of the field. The
following directions will help you
take good soli sampiee.
Whan to Take Soil SamplM
Ikka loll samples at any oorwa-
nlant time. However, to receive your
mcommendatlona early enough to
enable you to gat tha lima and fer-
tilizer needed, K la beet to aampla
In tha tat. Anothar advantaga of
fall sampling la that fertilizer die-
oounta at* utually offend than.
tfeyMda can be aamplad aftar any
cutting.
Winter aampllng, or aampllng
whan tha aoll la fratan, la partnM-
Ma only whan It la poaalbla to take
a uniform boring or cora of aoll to
ptcw depth. Normally, thla requlree
ualng a portable power boring tool.
Uaing a piok or epade to ramme a
law chunfca of fnsan aoll from tha
aurfaea la net aatlafaototy.
Whan to tWnSoN SamplM
If tha Md la qulta uniform, ona
compoM sampla from vary flva
acres la aufdolant. A oompoalla
MMfMtf «f Wring eampoeas aaatpfea item (our, Rw*on aeeffene of a M-eoi»
ffeM. *1 otmpotht aampha afwufc bt a eamUMfwi
-------
Wet soils that are
artificially drained
require more
careful
management
than naturally
well drained soils.
Runoff and
erosion control
are absolutely
essential to sound
management of a
sludge application
program. The
best way to
promote
infiltration and
reduce erosion
is to keep the
soil under a
permanent sod
crop.
I
34
Water Management
Water management deals with deficiencies
and excesses of water in the soil. Deficiencies
are managed with irrigation, and excesses are
managed with drainage.
Irrigation affects sludge management in two
ways. First, crop yields are higher on irrigated land
than on dryland, which increases the amount of
nutrients that the sludge must deliver. Second,
irrigation increases the hydraulic loading of the
soil, which increases the potential for both
leaching and runoff. The risk is not large, however,
as long as the irrigation system is well designed
and the irrigation program is well managed.
Good sludge management, with respect to
irrigation, includes not irrigating immediately
after the sludge application, avoiding over-
irrigation, and refraining from applying sludge
on flood or furrow irrigated fields. County
Extension agents and crop consultants can provide
advice on irrigation practices and scheduling.
Drainage of wet soils affects crop manage-
ment and land application of sludge. Farmers
and site operators are unlikely to drain soils only
to accommodate a sludge application program.
Soils that are already drained present a wider
choice of suitable crops. Higher yields, higher
nutrient requirements, and higher sludge
application rates are more likely on drained soils.
Artificially drained soils require more
careful management than naturally well drained
soils. Drainage with surface ditches may lower
the water table, but because the ditches drain
into natural streams, care is required to avoid
direct input of sludge into surface drainage ditches.
Limiting sludge applications to the dry times of
year when water tables are naturally low and
ditches are empty is a good management practice.
Drainage through subsoil tile lines
effectively lowers the water table, but tile drains
ultimately empty into surface waters. Leaching
of excess nitrogen applied either as sludge or as
commercial fertilizer can and does occur.
Planning sludge applications so that the release
of nitrogen coincides as nearly as possible with
plant uptake of nitrogen is a good management
practice for this situation.
Soil Conservation Practices
Runoff and erosion control are essential to
sound management of land application of sewage
sludge. Overland flow increases the potential for
contamination of surface waters with sludge
solids. Erosion decreases soil productivity,
increases sediment loads in streams, and carries
sludge solids into surface waters.
Soil conservation practices are designed to
promote infiltration and slow down the velocity
of water that flows over the surface. The best
way to promote infiltration and reduce erosion is
to keep the soil under permanent sod crop plant
cover, such as pasture and hay crops. Some
perennial grass seed crops are also very effective
in reducing runoff and erosion.
For cultivated crops, particularly grain
crops, reduced tillage can be an effective erosion
control measure. Reduced tillage, as opposed to
conventional tillage, does not turn the soil over
with a moldboard plow. Instead the soil is disked
or mixed slightly with sweep plows in such a
way as to partially incorporate crop residues,
loosen the soil, break up compacted layers, and
leave a rough soil surface.
Some studies have shown that additions of
sewage sludge enhance the erosion control
effectiveness of reduced tillage. The organic
matter in sludge augments the organic matter
in crop residues and enhances the formation of
stable soil aggregates that increase the porosity
and infiltration rate of the surface soil.
Widely spaced row crops provide little
protection for the soil surface, particularly in
early stages of growth. For these crops, reduced
tillage is better than conventional tillage, and
injecting or working sludge into the soil is better
than surface applications.
As soil slope increases, the potential for
runoff and erosion increases dramatically.
Permanent cover sod crops are particularly
valuable for controlling erosion on steep soils.
Residue incorporation and reduced tillage methods
are particularly important for grain and row crops.
Effective erosion control on steep slopes may
require additional conservation practices. Cross-
slope farming, i.e., planting crop rows on
the contour instead of up and down hills, is one
practice.
Diversion terraces are low ridges con-
structed at intervals across a slope. These
terraces interrupt the flow of water down slope
so that high velocities of flow cannot occur.
Water caught behind a terrace has more time to
soak into the soil, and excess water can be
diverted across the slope to a grassed waterway,
where it can be conducted safely downslope
without causing erosion.
Sometimes runoff is inevitable, even from
pastures and well-protected crop fields. This is
especially true during high-intensity storms and
when the soil is frozen. Regardless of other
conservation practices that may be in place, sludge
should not be put on the soil at these times.
-------
Monitoring
and Record-Keeping
If sludge is going to be applied to a farm
field over a number of years, the soil should be
sampled and tested regularly to monitor residual
nutrient supplies, accumulations of heavy metals
and organic contaminants, and increases in soil
salinity from year to year. For row crops, grains,
and other cultivated crops, annual sampling is a
good idea. For pastures that are managed at a
low level of intensity, sampling does not need to
be done as frequently.
Some sewage sludges may be quite high in
soluble salts. Long-term, heavy applications may
cause an increase in soil salinity, even to the
point where salt-sensitive crops are affected.
Nitrogen in soil generally does not accumu-
late to levels dangerous for plant survival. The
only problems with excess nitrogen are excessive
vegetative growth that leads to a plant condition
called lodging, and in some cases, a delay in
flowering and fruit development.
The major risk from long term applications
of nitrogenous materials is the possible increase
of nitrate nitrogen in groundwater sources used
for drinking water. This occurs only when nitrate
nitrogen leaches through the soil. The relevant
parameters are the amount of sludge applied, soil
permeability, timing of mineralization in relation
to crop uptake, and interactions with rainfall or
irrigation water.
Phosphorus does not accumulate to toxic
levels in the soil. However excessive amounts of
available phosphorus may lead to decreased crop
vigor through nutrient imbalances. For this
reason, long-term phosphorus accumulations
should be monitored.
Phosphorus is not very mobile in the soil
and is not subject to large leaching losses. Heavy
applications over a long period of time, however,
will move some phosphorus through the soil. The
rate and distance of movement may be of concern
near lakes whose eutrophication rate could in-
crease rapidly with small additions of phosphorus,
The greatest concern about excessive
phosphorus loading is overland flow. Either the
physical transport of sludge solids or the erosion
of soil enriched with phosphorus could cause
significant deterioration of surface water quality.
Monitoring programs should watch for signs that
excessive additions of phosphorus to lakes and
streams could occur.
Excess potassium is not a serious problem if
sludge is applied only once or at intervals
separated by several years. It is possible, though,
that regular additions of excess potassium could
elevate soil salinity to harmful levels or could
cause potassium to accumulate to the point that
potassium interferes with magnesium nutrition of
plants. One result of this is a condition known as
grass tetany, which afflicts animals that graze on
Mg-deficient forages. For this reason, regular
soil-test monitoring of potassium levels in
sludge-amended soils is a good idea.
Similarly, regular monitoring of heavy
metals and organic chemicals in sludge-amended
soils is important to ensure that accumulated
amounts do not exceed cumulative limits set by
regulatory agencies.
A good monitoring program also means a
good record-keeping program. Careful, complete
records should be kept of the amounts of dry
sludge, metals, organics, and nutrients applied to
each field each year. These records are necessary
to evaluate the significance of data collected in
the monitoring program and to document
compliance with all pertinent regulatory standards.
Avoid sludge
application during
times of high-
intensity storms
and times when
the soil is frozen.
35
-------
I 36
-------
Design Calculations
CHAPTER 6
T
he general procedure for designing a
sludge application system is as follows:
1. Assemble data on sludge, soil,
cropping system, and fertilizer
recommendations.
2. Calculate amounts of nutrients the sludge
must deliver. Subtract from fertilizer
recommendations the amounts supplied
by residual nutrients and the amounts
supplied by other commercial fertilizers.
3. Calculate the amount of available N per
dry ton of sludge. Add the fractions of
NH4-N and NO,-N recovered to the
amount of organic N mineralized.
4. Calculate the Agronomic Loading Rate.
Divide lb sludge N required by lb/ton
available N in sludge.
5. Calculate the Maximum Annual Loading
Rate. Divide the maximum annual
Cadmium application permitted by the lb
Cd per ton of sludge.
6. Calculate the amounts of P and K
delivered in the agronomic loading rate.
Compare with fertilizer recommendations
for P20, and K20.
7. Calculate the application area required.
Divide the total amount of sludge
produced each year by the amount
applied per acre.
8. Calculate the Hydraulic Loading Rate.
Convert gallons per acre to inches of
water applied.
9. Calculate the Allowable Accumulation
Period. Multiply lb per dry ton of each
metal by the Agronomic Loading Rate,
then divide each metal's annual loading
rate into the cumulative limit set by
regulatory standards.
Every location, every site is unique. Sewage
sludges are extremely variable in the amounts of
total N, organic N, phosphorus, potassium,
metals, and organics they contain. The numbers
used in this guide represent a single, specific
example and are intended only to illustrate the
principles involved in calculating nutrient
requirements and loading rates.
Before completing calculations for a
particular sludge, soil, and crop, you should
consult with agriculture professionals, soil
scientists, and wastewater treatment agencies in
your area to make sure you have the right
numbers for that situation.
Here's a list of some of the specific numbers
needed:
1. Crop requirements and fertilizer recom-
mendations for N, P, and K.
2. Amounts of residual N supplied by
previous crops, crop residues, and prior
sludge applications.
3. Amounts of available P and K in the soils
at your application sites.
4. Amounts of N, P, and K that may be
supplied with other commercial fertilizers.
5. Amounts of nitrate nitrogen and ammonia
nitrogen actually delivered to and
recovered from the soil.
6. Annual mineralization rates for the type
of sludge, climate, and time of year applied.
Carryover Nitrogen from
Previous Sludge Applications
Organic nitrogen applied in sludge continues
to decompose and release mineral nitrogen over
a period of several years. Nitrogen carryover from
prior sludge applications is an important issue that
may need careful checking in your calculations.
The critical point is whether or not the amount
of fertilizer nitrogen recommended for a crop has
already accounted for nitrogen mineralized from
prior sludge applications.
Extension agents are accustomed to
developing fertilizer recommendations based on
residual nitrogen from both the previous crop and
crop residues. They may not be as accustomed to
calculating the amount of carryover nitrogen
from previous sludge applications before making
recommendations for additional fertilizer.
The sample calculations that follow in this
guide assume that the recommended fertilizer
nitrogen has accounted for all residual nitrogen
sources, including prior sludge applications. If
this were not true, then you may need to
calculate the carryover and reduce the sludge
loading rate accordingly.
Here's how the procedure works.
First, check with professional agronomists in
your area to determine specific rates of minerali-
zation of organic nitrogen in years subsequent to
the initial application. A common approximation
is to use a rate that is one half of the previous
year's rate.
Second, calculate the amount of nitrogen
mineralized in each year for 2 or 3 years after the
initial application. Suppose, for example, an
anaerobically processed sludge delivers 100 lb
per ton of organic nitrogen when it is first
applied. If the first year's mineralization rate is
Make sure that
fertilizer
recommendations
account for all
sources of
residual nutrients
in the soil:
• the previous crop
• returned crop
residues
• prior sludge
applications
37
I
-------
20%, then each dry ton of sludge will have 20 lb
of nitrogen mineralized and 80 lb of organic
nitrogen remaining in the soil to start year 2.
If the mineralization rate for year 2 is 10%,
then 8 lb (0.1 x 80 lb) of organic N per ton of
sludge will be mineralized, and 72 lb will
remain. In year 3, the mineralization rate might
be 5%, and 5% of 72 lb yields 3.6 lb per ton of
mineralized nitrogen for year 3.
Third, calculate the cumulative amount of
carryover N. If the same kind of sludge were to
be applied at the same rate for three years
consecutively, then at the beginning of the third
year the amount of carryover nitrogen from
previous sludge applications would be 3.6 lb from
the first year's application plus 8 lb from the second
year's application, for a total of 11.6 lb.
This is the amount that should be subtracted
from the fertilizer recommendation. If the
fertilizer recommendation did not allow for this
residual nitrogen, then the amount of nitrogen
delivered by the current year's sludge application
should be reduced by that amount.
Sample Calculations
The steps necessary to calculate agronomic
loading rates, sizes of application areas, and al-
lowable accumulation periods are detailed below.
They are intended to be logical, orderly, consis-
tent, and simple. English units of measurement
(gallons, pounds, tons, and acres) are preferred
because they are more familiar to most operators,
Extension agents, and farmers.
Current regulatory standards for cumulative
metal loadings are still assumed to be valid.
These standards are being reviewed, and should
they change, you may need to adjust your
calculations of allowable accumulation period
accordingly.
Each step in the calculations is illustrated
using the actual data for a sludge generated by a
city of about 10,000 people. The sludge is an
anaerobically processed liquid sludge, and the
example assumes that it will be applied to a row
crop.
38
/
-------
Step 1.—Assemble relevant data
Kinds of Data
Example
Sludge
Type
Liquid Anaerobically Processed
Volume
Gallons produced yearly
2,673,550 gal
Percent solids
1.93%
Dry tons produced yearly
215.17 tons
Drv tons = Gallons x 8.34 x % Solids
2,000 100
Nutrients (% x 20 = lb/ton)
Percent
lb/ton
Total Kjeldahl N
10.7
214.0
nh4-n
5.45
109.0
NO,-N
0.015
0.3
Organic N (TKN - NH4-N)
5.25
105.0
Phosphorus
0.78
15.6
Potassium
0.015
0.3
Metals (mg/kg x .002 = lb/ton)
mg/kg
lb/ton
Lead
442.5
0.89
Zinc
1060.5
2.12
Copper
627
1.25
Nickel
72.35
0.14
Cadmium
24
0.05
Application method
Once per year, spring
Disked into soil
Soil
Data from soil testing laboratory
pH
6.0
CEC
21 meq/lOOgm
Soil test P (Bray PI)
10 ppm
Soil test K (NH4OAc extractable)
120 ppm
Estimated residual N
35 lb/acre
(includes prior sludge applications)
Crop
Information from farmer, farm advisor,
fertilizer guides
Type
Field corn
Expected yield
170 bushels
Rotation
Follows grain
Total fertilizer N recommendation
(accounts for all residual N)
265 lb/acre
Supplemental fertilizer N
30 lb/acre, banded
Fertilizer P requirement
75 lb PjOj/acre
Fertilizer K requirement
50 lb K20/acre
-------
Step 2.—Determine the amount of available N the sludge must provide
Procedure
Example
Sludge N needed
= Total Fertilizer N recommended -
Supplemental Fertilizer N
Sludge N needed = 265 lb/acre - 30 lb/acre
Sludge N needed = 235 lb/acre available N
Step 3.—Calculate the amount of available nitrogen per dry ton of sludge
Procedure
Example
Use the lb/ton data for NH4-N, NO,-N, and
organic N in Step 1.
Use sludge nitrogen data from Step 1. For liquid,
anaerobically processed sludge that is worked into
the soil, the recovery factors are 0.85 for
ammonium and 1.0 for nitrate. A reasonable
mineralization rate is 20%.
A. Available NH4-N
= lb/ton NH4-N x Fraction Recovered
Assuming the sludge analysis data are for
the processed sludge that will be applied to
the land, the fraction recovered can be taken
as 0.85 for sludge that is worked into the soil
and 0.5 for sludge that is left on the soil
surface.
Available NH4-N =109 lb/ton x0.85 = 92.6 lb/ton
B. Available N03-N
= lb/ton NO,-N x Fraction Recovered
Assuming the sludge analysis data are for
the processed sludge that will be applied to
the land, the fraction recovered can be taken
as 1.0.
Available NO,-N =0.3 lb/ton x 1.0 = 0.3 lb/ton
C. Mineralized Organic N
= lb/ton Organic N x Mineralization Rate
The mineralization rate for the year immedi-
ately following land application usually
varies from about 8 to 30%, depending on
the type of sludge, the type of processing,
the method of application, the time of
application, the cropping system, and the
local climate. Consult local agronomists and
soil scientists to determine the best number
for your land application program.
Available Org-N =105.0 lb/ton x0.2 = 21.0 lb/ton
Total Available N in sludge
= A + B + C above.
Total Available N in sludge = 113.9 lb/ton
/
-------
Step 4.—Calculate the Agronomic Loading Rate (ALR)
Procedure
Example
ALR (tons/acre) =
sludge N needed for crop (lb/acre)
available N in sludge (lb/ton)
From Step 2:
the amount of sludge N needed is 235 lb/acre.
From Step 3:
the available N in the sludge is 113.9 lb/ton.
ALR - 235 lb/acre _ 2 j tons per acre
113.9 lb/ton
Step 5.—Calculate the Maximum Annual Loading Rate (MLR)
Procedure
The maximum annual loading rate is the
amount of sludge that can be applied without
exceeding the maximum amount of Cadmium
that can be applied in a year. The annual limit for
Cadmium applications is 0.45 lb/acre for all
crops and soils.
MLR = 0.45 lb Cd/acre
lb Cd/ton sludge
Example
From the data in Step 1, the Cd content of the
sludge is .05 lb/ton.
MLR - 0.45 lb Cd/acre =9.0 tons per acre per year
0.05 lb Cd/ton
As long as the Agronomic Loading Rate,
2.1 tons per acre in this example, is less than the
Maximum Annual Loading Rate (9.0 tons /acre),
it is safe to apply sludge to the land at the
agronomic loading rate calculated.
Step 6.—Determine the fertilizer P and K value of the sludge
Procedure
A. Calculate the amounts of P and K delivered
annually
P (lb/acre)
= sludge P (lb/ton) x ALR (tons/acre/year)
K (lb/acre)
= sludge K (lb/ton) x ALR (tons/acre/year)
B. Convert P to P2Os and K to K20
(see page 32).
P x 2.27 = P2Os
K x 1.20 = KjO
C. Compare nutrients delivered in sludge
with fertilizer recommendations for
P and K.
Example
Using the data in Step 1 and the ALR from Step 4,
P = 15.6 lb/ton x 2.1 tons/acre = 32.8 lb P per acre
K = 0.3 lb/ton x 2.1 tons/acre = 0.6 lb K per acre
32.8 lb P/acre x 2.27 = 74.5 lb P205 per acre.
0.6 lb K/acre x 1.20 = 0.72 lb K20 per acre.
Phosphorus added to the soil very nearly
equals the recommended fertilizer rate of 75 lb
P2Os per acre. For cool season, spring crops,
however, the farmer may wish to band-place 20
lb or so P20, per acre just to make sure there is
enough P available to meet initial crop demands.
In this case the small excess P delivered by the
sludge should not create a problem, but careful
monitoring of available P in the soil is a good idea.
This sludge has virtually no fertilizer
potassium value. Supplemental fertilizer will be
needed to provide the 50 lb K20 per acre recom-
mended (see Step 1).
-------
Step 7.—Calculate the area of land required
Procedure
Acres land =
tons dry sludge produced annually
Agronomic Loading Rate
Example
From Step I:
The amount of dry sludge produced is 215.17
tons per year.
From Step 4:
The Agronomic Loading Rate is 2.1 tons per acre.
Acres land =
215.17 tons sludge/year = 103 acres/year
2.1 tons/acre
Step 8.—Calculate the Hydraulic Loading Rate (HLR)
Procedure
HLR (inches) =
Gallons sludge produced x 12 inches/foot
Acres land x 43560 sq. ft/acre x 7.48 gal/cu. ft
Example
From Step I, the gallons produced is 2,673,550.
From Step 7, the acres of land needed is 103.
HLR =
2,673,550 gal. x 12 inches/foot = 0.96 in
103 acres x 43560 sq. ft/acre x 7.48 gal/cu. ft
This application rate poses little threat of
contaminating ground water resources. Many
soils could accommodate this much hydraulic
loading in the surface foot as long as the soil
moisture content was a little below the maximum
water holding capacity.
Hydraulic loading could be a problem if the
sludge is applied to a soil that is wet to, or
slightly above, field capacity, or has a high water
table. These situations should be avoided.
If sludge were to be applied by irrigation,
then the rate of application should be kept below
the soil's infiltration rate to avoid runoff.
Infiltration rates vary, but 1/4 inch per hour is a
good working number for medium-textured soils.
/
-------
Step 9.—Calculate the allowable accumulation period
Procedure
A. Calculate the amount of each metal applied
per acre per year.
Sludge metal content (lb/ton) x ALR =
Amount metal (Ib/acre/year)
B. Calculate the number of years for each
metal to reach its cumulative limit, as
defined by regulatory standards
(see table 13).
Years for a metal =
cumulative limit (lb/acre)
annual metal applied (lb/acre/year)
C. The Allowable Accumulation Period is the
minimum number of years to reach any
one metal's cumulative limit.
Example
Use metal data from Step 1 and the ALR from
Step 4.
Lead: 0.89 lb/ton x 2.1 ton/acre/year =
1.87 lb/acre/year
Zinc: 2.12 lb/ton x 2.1 ton/acre/year =
4.45 lb/acre/year
Copper: 1.25 lb/ton x 2.1 ton/acre/year =
2.63 lb/acre/year
Nickel: 0.14 lb/ton x 2.1 ton/acre/year =
0.29 Ib/acre/year
Cadmium: 0.05 lb/ton x 2.1 ton/acre/year =
0.11 Ib/acre/year
Use data from Table 13 and Step 9A above. The
soil we're using has a pH of 6.0 and a CEC of 21
meq/100 gm (from Step 1). Use the third column
of data in Table 13, except for Cadmium, as noted.
Lead: 2,000 lb/acre / 1.87 lb/acre/year =
1,070 years
Zinc: 1,000 lb/acre / 4.45 lb/acre/year =
225 years
Copper: 500 lb/acre / 2.63 Ib/acre/year =
190 years
Nickel: 200 lb/acre / 0.29 lb/acre/year =
670 years
Cadmium: 5.0 lb/acre / 0.11 lb/acre/year =
45 years
In this example, the allowable accumulation
period is 45 years, the minimum for Cadmium.
Table 13.—Cumulative limits of metal loadings
Metal
Soil cation exchange capacity, meq/100 gm
<5 5-15 >15
5-15
¦ Pounds per Acre •
Lead (Pb)
Zinc (Zn)
Copper (Cu)
Nickel (Ni)
Cadmium (Cd)
500
250
125
50
5.0
1,000
500
250
100
10*
2,000
1,000
500
200
20*
* If the soil pH is less than 6.5, use 5.0 lb/acre for Cd regardless of CEC.
-------
| 44
-------
Practical Applications
CHAPTER 7
One of the objectives of this guide was
to assist those responsible for
evaluating project proposals. This
chapter presents detailed checklists for
that purpose. Project developers and permit
writers can use the same checklists as a guide to
the kinds of information that should be included.
Another objective was to identify sources of
information pertaining to soils, crops, and land
application of sewage sludge. Several of those
sources are listed in this chapter.
Guidelines for Evaluating
Project Proposals
The framework for project evaluation
discussed in Chapter 1 identified data
completeness, data accuracy, and issues and
interactions as the three components of the
evaluation process. The worksheets that follow
are designed to guide your review with respect to
each of these three components.
The first step in evaluating project proposals
is to assess the completeness of the data. The
more complete the data, the better the proposal is
likely to be. If data are missing or incomplete, it
may be necessary to request additional informa-
tion before proceeding with the review.
The next step in project evaluation is to check
the accuracy of the data. This is crucial, for no
matter how well planned the project might be, if
the raw data are bad, the calculations and designs
based on the data are likely to be bad as well.
One way to judge data accuracy is to see if
sludge and soil samples have been collected,
handled, stored, and analyzed according to
standard, approved procedures. For sludge,
standard procedures should be available from the
agency that manages the sludge utilization
program in your state. That agency may also be
able to provide a list of analytical laboratories
whose work is known to be reliable. For soils,
standard sampling and analytical procedures are
available from your county Extension agent.
That person can also provide a list of reliable soil
testing laboratories.
Another way to judge data accuracy is to
compare the data with results for similar sludges
and soils analyzed previously. If the data are
consistent, then you can be reasonably comfortable
with their accuracy. If there are significant
deviations, then you should at least expect a
good explanation of the reasons for the
differences, and you may wish to request a
re-analysis for the anomalous data.
Should there be any doubt about the
accuracy of the data, get a second opinion.
Agricultural consultants, agricultural Extension
agents, and waste management specialists in
state environmental regulatory agencies all may
be able to provide assistance.
The final step in project evaluation is to
assess the adequacy with which interactions
among sludge, soil, and cropping system have been
addressed. This is the most difficult step, but it's
critical because the amount of sludge to apply,
the timing of the application, the prevention of
disease and other public health problems, and the
protection of environmental quality all depend
on these interactions.
The most important criterion for evaluating
interactions is to judge them against the principles
and the common sense discussed in this Guide. If
the proposal demonstrates a clear understanding
of the principles that control these interactions,
and if the calculations and management plans
account for the effects of the interactions, then
the project is probably a good one.
Conversely, if the proposed project violates
one or more of the principles of sludge behavior
in soil, or if the interactions are disregarded in
developing the proposal, then the proposal
should either be rejected outright or returned for
further planning and development.
One other criterion for evaluating interactions
is to check their consistency with other success-
ful land application projects. If the management
plan is patterned after one or more previous
projects that are known to function properly, and
if all limiting factors have been accounted for,
then the proposal is probably adequate.
To help you conduct your evaluation of
interactions, the worksheet for interactions is
designed with questions that require a yes or no
answer. Any question answered "no" should be
taken as a warning that the report may be
inadequate or incomplete. At the very least,
further information may be required, and it may
be necessary to disapprove of the entire project.
Project proposals
that are based on
complete and
accurate data,
that are well
thought out, and
that address all
the pertinent
interactions
among sludge,
soil, and cropping
system are
suitable for
approval.
-------
In summary, proposals that are based on
complete and accurate data, are well planned, and
address all the pertinent interactions, are suitable
for approval. Proposals that are incomplete, fail
to substantiate data accuracy, and fail to account
for important principles and interactions, should
be disapproved.
Proposals between these two extremes need
careful evaluation. Some proposals may need to
be returned for more data, better data, or
clarification of explanations regarding data
anomalies, mitigation of soil limitations, or effects
of interactions on system design. Other proposals
may be approved subject to specific conditions.
Some examples of restrictive clauses that may be
included in permits are given in Appendix C.
Proposals based on questionable or incorrect
data should be disapproved. If the proposals
don't make sense or fail to account for soil
conditions and farm management plans, they
should not be approved.
Finally, remember to seek help from local
experts for difficult situations that require
professional judgment. With their help, and with
your own understanding of the principles and
interactions involved in the sludge-soil-crop
system, you should soon become sufficiently
experienced to make many of these judgment calls.
-------
Worksheet for Evaluating Sludge Data
I. Completeness (see pages 4-6)
A. Volume
Gallons produced annually
Dry weight produced annually (Kg or Tons)
Percent solids
B. Type
Liquid
Dewatered
Dried
Composted
C. Nutrients (% or mg/kg)
Total Kjeldahl N
nh4-n
no3-n
Organic N
Total P
Total K
D. Metals (mg/kg)
Cadmium
Copper
Lead
Nickel
Zinc
Arsenic
Chromium
Mercury
Molybdenum
Selenium
E. Organic contaminants (mg/kg)
Aldrin/dieldrin
Benzo(a)pyrene
Chlordane
DDT, DDE, DDD
Dimethyl nitrosamine
Heptachlor
Hexachlorobenzene
Hexachlorobutadiene
Lindane
PCB's
-------
Toxaphene
Trichloroethvlene
F.
Pathogen reduction processes (PSRP's, PFRP's)
Aerobic digestion
Anaerobic digestion
Lime stabilization
Air-dried
Composted
Heat-dried
High temperature digestion
Gamma irradiation
G.
Method of land application
Spread on surface
Spreader plate
Portable sludge cannon
Dry manure spreader
Incorporated into the soil
Liquid injection
Surface applied and disked
II.
Accuracy (see page 4)
A.
When was sludge sampled?
Just prior to land application?
Before processing and treatment?
B.
Were sludge sampling and analytical procedures identified?
Yes No
C.
Was sludge sampled according to approved procedures?
Yes No
D.
How were samples handled prior to analysis?
Stored in proper container to avoid contamination?
Length of time between sampling and analysis?
Refrigerated or frozen if necessary?
E.
How were samples analyzed?
By an approved laboratory? Yes
Using standard procedures? Yes
No
No
F.
Are the results consistent with data from other sludges of this type?
Yes
No
-------
Worksheet for Evaluating Soils Data
I. Completeness
A. Soil types present, list (see pages 28-29)
(e.g. Windthorst fine sandy loam, 3-5% slopes)
B. Data needed to rate suitability for land application (see pages 8-21,
23-26, tables 7-11)
Texture of surface soil
Texture of subsoil
Depth to bedrock or cemented pan
Coarse fragments in surface soil
Coarse fragments in subsoil
Structure grade of surface soil
Organic matter content
Shrink-swell potential
Drainage class
Permeability of surface soil
Permeability of subsoil
Kind of restrictive layer
Claypan
Fragipan
Duripan
Petrocalcic
Weathered bedrock (Cr)
Solid bedrock (R)
None
Depth to restrictive layer
Depth to coarse-grained layer
CEC of surface soil
pH of surface soil
Slope of soil surface
C. Water table Information (see pages 16-17)
Are somewhat poorly drained or
poorly drained soils present? Yes No
Position of water table (in. below surface)
Duration of water table (days or months)
Times when water table is high (months)
-------
I 50
D. Analytical data (see pages 20-21)
PH
CEC (meq/100 gm)
Available P (Bray #1)
Available K (NH4OAc extract)
E. Site data
Size (acres)
Slope (%)
Amount of rainfall (inches)
Distribution of rainfall (months)
Times when soil is frozen (months)
II. Accuracy (see pages 28-30)
A. Sources
Modern soil survey report
Other SCS information
On-site investigations
By a certified soil scientist
By someone other than a soil scientist
B. Sampling (see page 33, figure 6)
According to approved procedures?
Yes
No
Steps taken to avoid contamination?
Yes
No
C. Analysis (see page 33)
Done by an approved laboratory?
Yes
No
Done according to standard procedures?
Yes
No
D. Consistency
Are data consistent with results from other soils similar to these?
Yes
No
/
-------
Worksheet for Evaluating
Cropping Systems Information
1. Completeness
A. Crop (see page 31)
Kind of crop
Metal accumulator?
Yes
No
Expected crop yield
Anticipated planting date
Anticipated harvest date
Can/will sludge be applied while the crop is growing?
Yes
No
B. Fertility Management (see pages 31-32)
Previous crop
Crop residue management
Estimated residual nitrogen
Fertilizer N requirement
Commercial N fertilizer used
Pre-piant
Mid-season
N required from sludge
Fertilizer P requirement
(lbs P205 per acre)
Fertilizer K requirement
(lbs KjO per acre)
C. Water Management (see page 34)
Are wet soils artificially drained?
Yes
No
Surface ditches?
Yes
No
Subsurface tiles?
Yes
No
Will the crop be irrigated?
Yes
No
Flood?
Yes
No
Furrow?
Yes
No
Sprinkler?
Yes
No
Big gun?
Yes
No
0. Runoff and Erosion Control (see page 34)
Perennial sod crop
Yes
No
Reduced tillage
Yes
No
Contour farming
Yes
No
Terraces and diversions
Yes
No
Grassed waterways
Yes
No
Cross-slope drains
Yes
No
Other
Yes
No
I
-------
II. Accuracy (see pages 31, 35)
A. Sources of data
Land Grant University fertilizer guides Yes No
Consultation with farmers Yes No
County Extension agent recommendations Yes No
Crop consultant recommendations Yes No
Other Yes No
B. Consistency of data
Are crop yields, fertilizer recommendations, and management practices
typical for this area? Yes No
-------
Worksheet for Evaluating Issues and Interactions
1. Soil Surveys and Site Investigations (see pages 28-30)
A. Soil identification
Were soil surveys used in the planning stage to make
preliminary determinations of the soils present?
Yes
No
Were the data in the tables and map unit descriptions
of soil surveys used as the basis for evaluating
soil suitability for land application of sludge?
Yes
No
Were soil identifications made from soil maps
verified by field investigations at the site?
Yes
No
Were additional soils data obtained from soil pits dug
at the site?
Yes
No
B. Soil Patterns
Were the inclusions in soil map units (page 29) identified,
their properties listed, and their limitations for land
application accounted for?
Yes
No
Were patterns of admixture of soils of different
suitability classes recognized?
Yes
No
Were plans made for managing sites in which
more limiting soils are disbursed among more
suitable soils?
Yes
No
II. Nitrogen Issues and Interactions
A. Mineralization rate (see pages 5,7,31-32)
Was the determination of the mineralization rate made
by a qualified professional?
Yes
No
Did the determination of the mineralization rate account for effects of:
method(s) of sludge processing prior
to application?
Yes
No
method of application?
Yes
No
timing of application?
Yes
No
climatic conditions?
Yes
No
B. Agronomic Loading Rate (see pages 5-6,16,31-32,37-41)
Did the calculation of the amount of sludge to apply account for these losses:
volatilization during processing?
Yes
No
volatilization during application?
Yes
No
denitrification in the soil?
Yes
No
Did the calculation of the amount of sludge to apply account for these fertilization factors?
the type of crop?
Yes
No
the expected yield of the crop?
Yes
No
the farmer's usual management practices?
Yes
No
the timing of sludge application in relation to
mineralization rates and crop demands for N?
Yes
No
-------
the amount of fertilizer recommended? Yes
No
the estimated residual N in the soil?
from the previous crop? Yes
No
from crop residues? Yes
No
from prior sludge applications? Yes
No
the amount of pre-plant commercial
fertilizer used? Yes
No
the amount of mid-season commercial
fertilizer used? Yes
No
III. Protection of Environmental Quality
A. Leaching to Groundwater (see pages 4, 7, 9,13-21, 34)
Have plans been made to minimize the potential for transport of mobile constituents
through the soil due to any of the following?
application on wet soils with high water tables Yes
No
application during periods of high rainfall Yes
No
heaw irrigation after sludge application Yes
No
application on sandy or gravelly soils that
have rapid or very rapid permeability.
especially if irrieated Yes
No
B. Runoff and Erosion (see pages 7, 9,15-16, 34)
Have plans been made to minimize the potential for surface runoff that could transport
sludge to streams and lakes with respect to:
weather conditions in the area? Yes
No
applications on sloping soils during rainy seasons
or after irrigation? Yes
No
applications on frozen soils? Yes
No
conservation practices on sloping soils? Yes
No
non-application buffer areas around
upland waterways? Yes
No
IV. Mitigation of Limiting Soil Conditions (see pages 11,23-28)
A. Have limiting properties of soils rated fair or poor been identified?
Yes
No
B. Have methods for dealing with soil limitations been specified?
Proper timing on soils with high water tables? Yes
No
Proper timing and application rates on soils with
slowlv permeable restrictive layers? Yes
No
Proper timing and application rates on soils with
rapid or very rapid permeabilitv? Yes '
No
Proper timing and conservation practices on sloping soils?
Yes
No
-------
V. Protecting Public Health
A. Metals and Organic Contaminants (see pages 6, 22, 43)
Have all loading rates for metals and organics been
calculated correctly?
Yes
No
Has the allowable accumulation period been based
on the most limiting loading rate?
Yes
No
Have appropriate measures been taken to compensate
for low pH or low CEC?
Yes
No
B. Pathogens (see pages 3-4, 31)
Have all PSRP's and PFRP's and their effects on the
pathogen content of the sludge been identified?
Yes
No
Have lag times between sludge application and crop
grazing or harvest taken into account the type of
crop and its position in the food chain?
Yes
No
Have steps been taken to restrict public access to
the application site?
Yes
No
C. Drinking Water
Is it absolutely clear that the project will not
contaminate drinking water supplies with nitrates,
metals, organics, or pathogens?
Yes
No
VI. Monitoring and Compliance
A. Monitoring (see page 35)
Are there plans for monitoring:
Nitrates, metals, organics in ground water?
Yes
No
Nitrogen, phosphorus, and pathogens in
surface water?
Yes
No
Accumulations of soil P and K?
Yes
No
Have plans been made for careful, thorough
record-keeping?
Yes
No
B. Compliance (see pages 6,11,22)
Does the plan conflict in any way with regulations
affecting natural or man-made wetlands?
Yes
No
Does the plan comply with all pertinent local, state,
and federal regulations?
Yes
No
55
I
-------
Sources of Information
The following is a short list of available
sources of helpful information on land applica-
tion of sewage sludge. These sources can direct
you to more detailed sources if necessary.
Contacts
• Soil scientists, crop scientists and agronomists
in Agricultural Experiment Stations and Exten-
sion Services at state Land Grant universities
• County Extension agents
• Soil Conservation Service district
conservationists
• Private crop, soil or agronomic consultants
• Field representatives of farm service
companies
• Representatives of state environmental
management agencies
Publications
Soils
County soil survey reports
Donahue, R L„ R.W. Miller, and J.C. Shickluna,
Soils: An Introduction to Soils and Plant
Growth, Prentice-Hall, Inc., Englewood
Cliffs, NJ, 1983.
Harpstead, M.I. and F.D. Hole, Soil Science
Simplified, Iowa State University Press,
1980.
Singer, M.J. and D.N. Munns, Soils: An
Introduction, MacMillan Publishing Co.,
NY, 1987.
Stevenson, F.J., Cycles of Soil: Carbon, Nitro-
gen, Phosphorus, Sulfur, Micronutrients,
John Wiley and Sons, New York, 1986.
"Controlling Pathogens in Municipal Waste-
water Sludge for Land Application, Third
Draft," Prepared by Eastern Research Group,
Inc., Arlington, MA, for EPA Pathogen
Equivalency Committee, November 1988.
Elliott, L.F. and F.J. Stevenson, eds., "Soils for
Management of Organic Wastes and Waste-
waters," American Society of Agronomy,
Madison, WI, 1977.
"Guidance for Writing Case-by-Case Permit
Requirements for Municipal Sludge," EPA
Office of Water Enforcement and Permits,
September 1988.
Journal of Environmental Quality, American
Society of Agronomy, Madison, WI.
Logan, T.L and R.L.Cheney, "Utilization of
Municipal Wastewater and Sludge on
Land—Metals," in Page, A.L. ed., Utilization
of Municipal Wastewater and Sludge on
Land, Univ. California, Riverside, 1983.
"Process Design Manual for Land Application
of Municipal Sludge," EPA-625/i-83-016,
. October 1983.
Runge, E.C A., K.W. Brown, B.L. Carlile, R.H.
Miller, and E.M. Rutledge, eds., "Utilization,
Treatment, and Disposal of Waste on Land,"
Soil Science Society of America, Madison,
WI, 1985.
Sommers, L.E. and K.A. Barbarick, "Con-
straints to Land Application of Sewage
Sludge," in Runge, E.C.A. et al, eds.,
Utilization, Treatment, and Disposal of
Waste on Land, Soil Science Society of
America, Madison, WI. 1985.
State Extension and Agricultural Experiment
Station publications on land application of
sewage sludge.
Crop Management
State fertilizer guides
State soil sampling and testing brochures
Tisdale, S.L., W.L. Nelson, and J.D. Beaton,
Soil Fertility and Fertilizers, MacMillan
Publishing Co., NY, 1985.
Land Application of Sewage Sludge
"Analytical Methods for the National Sewage
Sludge Survey," EPA Office of Water
Regulations and Standards WH-522, Indus-
trial Technology Division, August 1988.
/
56
-------
Appendixes
I
-------
I 58
-------
Technical Aspects of Soil Morphology
APPENDIX A
Field Determination of
Soil Texture
Moisten the soil and knead it between your
thumb and fingers. If the sample sticks to your
fingers, it's too wet. Continue kneading it, or add
a little dry soil, just until it is no longer sticky.
Estimate the amount of sand the soil contains
by the grittiness of the sample. If there is more
than 50% sand, the soil is coarse-textured (sandy
loam, loamy sand, or sand). If there is between
20 and 50% sand, you'll notice the sand, but
there's enough silt and clay present to give the
sample good body. Loam and clay loam are the
most likely textures. If there is less than 20%
sand, you'll have difficulty feeling it, and the
texture will be silt loam, silty clay loam, or clay.
Push the sample upward between your
thumb and finger to form a ribbon. The longer
the ribbon is, the more the clay. If the sample
contains less than 27% clay, you can form only a
short, broken ribbon. The textural name contains
the word loam, but not clay. If the sample has 27
to 40% clay, you should get a ribbon I to 2 1/2
inches long. Textural names contain both the
words clay and loam. If the sample has more
than 40% clay, you should get a long ribbon. The
texture is either clay or silty clay.
If the soil contains coarse fragments, you
may need to add the appropriate modifier to the
textural class name. Estimate the volume of
coarse fragments present by looking at the
vertical face of exposed soil. The proportion of
the area occupied by coarse fragments equates to
the percent of coarse fragments in the soil. Select
the correct modifier from Key 1 on page 60.
Field Evaluation of
Soil Structure
Soil structure is described in terms of the
grade, size, and shape of soil peds. Criteria for
evaluating size and grade are given in Keys 2
and 3 on page 60. Shapes of soil peds are illus-
trated in figure 3, page 10.
Soil structure is determined in the field by
observing both an undisturbed vertical cut and
the way the soil breaks out into your hand. The
easier it breaks out, the stronger the grade. Weak
and moderate structures will not break out of the
cut face into individual peds. You'll have a large
mass of soil in your hand, and you need to gently
break this mass apart by applying gentle pres-
sure. If the soil breaks easily along a natural
Plane of weakness, that plane separates structural
units. If the soil merely fractures, you don't have
structure.
Continue breaking the soil apart until you
can't subdivide it any further without fracturing
it. Then observe the size and shape of the peds in
your hand. The grade is determined according to
the way the soil breaks out of the face, the ease
with which it separates under gentle pressure,
and the amount of unaggregated material left in
your hand. Record your observations, such as
moderate fine granular or weak medium suban-
gularblocky.
Technical Method for
Determining Soil Color
Technical descriptions of soil profiles use
Munsell color notations to describe soil colors.
Each color is characterized by its hue, value, and
chroma. A symbol, such as 10YR 4/3, is used to
record these color characteristics.
Hue represents the spectral wavelength of
the color. A hue of 10R represents a pure red
color. A hue of 10Y has a pure yellow color. In
soils, a very common hue is 10YR, which
represents a color exactly half way between pure
red and pure yellow. Other common soil hues are
5YR (3 parts red and 1 part yellow) and 7.5YR
(5 parts red and 3 parts yellow).
Value represents the amount of light
reflected back to the eye. Value is measured on a
scale of 0 to 10, from no reflection to complete
reflection. Low numbers represent dark soil
colors, as most of the incident light is absorbed.
High numbers represent light colors, as most of
the light is reflected. Common values for soil
colors are 3 and 4, representing 30% and 40% of
the light reflected. Value is shown in the color
symbol as the numerator of the fraction that
follows the hue.
Chroma represents the amount of dilution
with white light. On a scale of 0 to 20,20
represents the pure color, and 0 represents
infinite dilution with white light. Chromas of soil
colors range between 0 and 8 and are commonly
between 1 and 4. The lower chromas are black or
gray colors, whereas the higher chromas are the
bright yellowish or reddish colors. Chroma is
shown as the denominator of the fraction in the
color symbol.
The Munsell Color Company makes small
color chips for each combination of hue, value,
and chroma. Chips of those colors that are most
frequently found in soils are arranged in special
books of soil color charts. To determine soil
color in the field, match the color of a soil
aggregate with a chip of the same color. Then
record the corresponding symbol for that chip's
hue, value, and chroma.
59
-------
Key 1.—Coarse fragment modifiers of
textural class names
Key 2.—Size ranges for soil peds (all sizes
are in millimeters)
% by Gravel Cobbles Channers
vol. 2mm-3 in. 3-10 in. 2mm-6in.
Granular, Prismatic,
Platy Blocky Columnar
<15
-no modifier-
Very fine
0- 1
0- 5
0- 10
15-35
Gravelly
Cobbly
Channery
Fine
1 - 2
5-10
O
ro
o
35-60
Very grav.
Very cob.
Very chan.
Medium
2- 5
10-20
20- 50
>60
Extremely
grav.
Extremely
cob.
Extremely
chan.
Coarse
Very coarse
5-10
>10
20-50
>50
50-100
>100
Key 3.—Grades of soil structure
Strong The soil mass is well divided into distinct, easily recognizable peds that are obvious
on the face of a soil pit. When dug out of the pit face, the soil falls into your hands as
distinct, stable peds that resist further breakdown. Little or no soil remains as
unaggregated loose grains.
Moderate Peds can be seen in a pit face, and they are easily detected when you gently break
apart a mass of soil held in your hands. Grains of soil that are not part of any
aggregate are apparent. Peds are stable against weak forces, but may break down
under stronger pressure.
Weak Peds are difficult to detect, even when you break soil apart in your hands. Many
grains are not part of any aggregate. Peds easily break down when small forces
are applied.
Structureless The grade applied to massive and single grain soils.
-------
Criteria for
Describing Soil Mottles
Soil mottles are described in terms of their
abundance, size, contrast, and color. These terms
are defined below.
Abundance—the percentage of exposed surface
area occupied by mottles:
Few—less than 2%
Common—2-20%
Many—more than 20%
Size—the approximate diameter of individual
mottles:
Fine—< 5 mm
Medium—5-15 mm
Coarse—> 15 mm
Contrast—the relative difference between the
mottle color and the matrix color:
Faint—Mottles are evident only upon close
scrutiny. Mottle color and matrix color are
nearly the same.
Distinct —Mottles are readily seen though
not striking. Mottle color and matrix color
are different, though not widely so.
Prominent—Mottles are so conspicuous that
they are the outstanding visible feature of the
horizon. Mottle color and matrix color are
widely different.
Color—The hue, value, and chroma of the
mottles, as described using a Munsell book
of soil colors.
Horizon Definitions
Master Horizons. The six master horizons (O,
A, E, B, C, R) are defined below.
O Horizon—The O stands for organic. O
horizons don't have to be 100% organic
material, but most are nearly so. Wet soils in
bogs and swamps often have O horizons of
peat and muck. Forest soils usually have thin,
surficial O horizons that consist of leaves and
twigs in various stages of decay.
A Horizon—This is the surface horizon of a
mineral soil. Its unique characteristic is a
dark color formed by the accumulation of
organic matter. Granular or fine blocky
structures are typical.
E horizon—E horizons have light gray or white
colors. Where present, they usually occur
immediately beneath an O or an A horizon.
They are common in sandy soils that formed
under coniferous forests and in medium to fine
textured soils that formed under deciduous
forests. They also occur immediately above
very slowly permeable claypans or fragipans
in situations that indicate wetness.
B horizon —B horizons are subsoil horizons of
maximum alteration due to soil-forming
processes. In some soils, the B has the brightest
yellowish-brown or reddish brown color. In
others, it has the best developed blocky or
prismatic structure. Many B horizons are
distinguished by accumulations of clay, iron
and aluminum oxides, or carbonates.
C Horizon—The C horizon is weathered
geologic material below the A or B horizon.
Anything you can dig with a spade but that
has not been changed very much by soil
forming processes is considered a C horizon.
R Horizon—R stands for rock. It refers to hard
bedrock that you cannot dig with a spade.
Depending on the depth to bedrock, it may
occur directly beneath any of the other
master horizons.
Transition Horizons. Master horizons rarely
change abruptly from one to another. In some
cases, we can describe the nature of the
transition as a characteristic of the boundary
between two horizons. But if the transition
zone is more than 5 inches thick, we usually
describe it as a transition horizon.
AB Horizon —This horizon occurs between the
A and the B. It's dominated by properties of
the A, but some properties of the B are
evident. Dark colors associated with organic
matter are fading as organic matter decreases,
arid the structure often changes from granular
to blocky.
BA Horizon—This horizon also occurs between
the A and the B, but it's more like the B than
the A. Structure is the same shape as in the
B, but the grade is a little weaker. The color
may be a little darker than the B, or the clay
content may be a little less than the maxi-
mum in lower horizons.
BC Horizon—This is a transition from B to C.
Properties of the B are dominant, but some
influence of the C is evident. Often the clay
content will be less than the maximum in the
B, or the color will be fading. If the C is
massive the BC has structure, but the peds
are larger and the grade is weaker than in the B.
-------
Special Kinds of A, B, and C Horizons. Many
horizons are the result of unique processes
that leave a distinct mark on the horizon.
These horizons are identified with a tower
case letter immediately following the master
horizon symbol. There are over 25 such
horizons. Only the more common ones are
defined below.
Ap Horizon—This is the plow layer of the soil.
Cultivation thoroughly mixes the upper 8 to 12
inches of soil and destroys any natural horizons
that may have been present. Even if all of the
original A horizon has been lost by erosion,
plowing the exposed B or C horizon automati-
cally makes the surface horizon an Ap.
Bt Horizon—This is a textural B horizon, or
argillic horizon, formed by accumulation of
silicate clays. Some of the clay comes from
weathering of minerals within the Bt. Most
of it comes from translocation of clay from
horizons higher in the profile. Deposition of
clay platelets on the surfaces of peds in the
Bt creates waxy coatings called clay skins. Bt
horizons are quite common, and they usually
have moderate or strong blocky and pris-
matic structures.
Bg Horizon —This horizon is gleyed. The soil
is so wet for so long that most of the iron is
reduced and leached away. Gleyed horizons
are gray, and they may or may not be
mottled. Gleying is not restricted to the Bg.
Other gleyed horizons include Ag, BAg,
BCg, Cg.
Bk Horizon—This horizon is enriched with
calcium carbonate leached from horizons
above. It is common in soils of dry regions
that receive limited rainfall. Usually you can
see white streaks or nodules of lime, and the
soil effervesces strongly when a drop of HC1
is placed on it. Bk horizons contain more
calcium carbonate than the C horizons
beneath them.
Bkm Horizon—This horizon is called a
petrocalcic horizon. It is enriched with
calcium carbonate (k) and is strongly
cemented (m). Petrocalcic horizons occur
only in the southwestern United States in
environments that have had enough moisture
to leach carbonates part way down, but not
entirely out of, the soil. Subsequently these
carbonate deposits have crystallized to
completely cement the horizon.
Bkqm Horizon—This horizon is called a
duripan. It is enriched with calcium carbon-
ate (k) and silica (q), and it is strongly
cemented (m). Duripans are most common in
soils that contain some volcanic ash in
regions of limited rainfall that have distinct
rainy and dry seasons. Lime and silica
leached from the upper 10 to 20 inches are
deposited in the Bkqm, cementing the soil
grains so firmly that it's just like rock. The
duripan is usually only 6 to 10 inches thick,
but it is cemented so strongly that neither
roots nor water can go through it.
Bs Horizon—This horizon is called a spodic
horizon. It's common in sandy soils devel-
oped under coniferous vegetation in cool,
moist climates. The leachate from the litter at
the soil surface is very acid, causing iron,
aluminum, and organic matter to be removed
from upper horizons and deposited in the Bs
horizon. The color is usually bright yel-
lowish-brown or reddish-brown, and it fades
with depth. Sometimes there is a thin black
horizon at the top of the Bs, and often a white
E horizon is above it.
Bw Horizon —This is a weathered horizon, also
called a cambic horizon. It is altered enough
to have structure, more intense color, or to
have been leached, but it does not have
enough accumulation of secondary minerals
to be a Bt, Bs, or Bk horizon. Bw horizons
are common in cool region mountain soils
under high rainfall, in arid soils, and in
relatively young soils.
Bx Horizon—This refers to a special feature
called a fragipan. It is a massive, dense, but
not cemented soil horizon. It is often mottled
and has gray streaks of silt scattered through-
out. The density is so high that neither roots
nor water can penetrate effectively, except in
some of the silt streaks.
Cr Horizon—Weathered bedrock, or rock that
is soft enough to slice with a knife or a spade,
is called Cr. It's rock material, and you can
often see original rock structure, but it's not
hard enough to be designated R.
62
-------
Estimating AWHC and CEC Values
APPENDIX B
Calculation of AWHC from
Soil Properties and
Estimated Values
Steps in the procedure for calculating
estimated AWHC are as follows:
1. Identify the horizons present in the
soil profile.
2. Measure the thickness of each horizon.
3. Determine the depth to a root-limiting layer.
4. Determine the texture and the coarse
fragment content for each horizon.
5. Find the AWHC rate that corresponds to
the texture of each horizon (see Key 4).
6. Multiply AWHC x Depth x Percent fine
earth for each horizon.
7. Total the products from Step 6 for all
horizons within the depth of rooting
(see Key 5).
Key 4.—AWHC rates
Sand,
Loamy sand .06 in./in.
Sandy loam .12 in./in.
Clay, Silty clay,
Sandy clay,
Sandy clay loam .15 in/m.
Loam, Silt loam,
Clay loam, Silty clay loam .20 in./in.
Key 5.—Sample calculations of AWHC
Horizon
Depth
Texture
% coarse
fragments
AWHC
Thickness
Fraction
fine earth
AWHC
A
0-12
Silt loam
0
.2
X
12
X
1.0
=
2.4
BA
12-20
Silt loam
0
.2
X
8
X
1.0
s
1.6
Bt
20-36
Silty clay
loam
0
.2
X
16
X
1.0
—
3.2
BC
36-48
Silty clay
loam
0
.2
X
12
X
1.0
35
2.4
C
48-60
Silt loam
0
.2
X
12
X
1.0
=
2.4
Total soil AWHC
s '
12.0
A
0- 4
Loam
0
.2
X
4
X
1.0
SB
0.8
BA
4-10
Clay loam
0
.2
X
6
X
1.0
SB
1.2
Bw
10-18
Grav. clay
loam
30
.2
X
8
X
.7
S
1.1
Bkqm
18-28
(Duripan)
100
...
...
...
0
Ck
28-40
Loam
10
...
...
...
0
Total soil AWHC
SB
3.1
-------
Sample Calculation of
Estimated CEC from
Clay and Organic Matter Data
The four factors that influence CEC are amount
of clay, type of clay, amount of organic matter,
and pH. Data on the amounts of clay and organic
matter, and the pH, are usually available in soil
survey reports. The type of clay can be estimated
from soil color and stickiness. Red soils common in
the southeastern U.S. are dominated by kaolinitic
clays. Brown soils elsewhere in the U.S. are
dominated by clays in the hydrous mica group.
High shrink-swell soils have montmorillonitic clays.
Steps in the calculation of estimated soil
CEC are as follows:
1. Determine the most likely kind of clay
and select the corresponding CEC rate
from Key 6.
2. Divide the CEC rate by 100 to give meq
per gram of clay.
3. Multiply the clay CEC in meq/gm times
the percent clay in the soil. This gives the
clay contribution to the CEC of a 100 gm
sample of soil.
4. Assume the CEC for organic matter is
200 meq/100 gm at pH 7.0. Assume the
pH-dependent effect reduces the CEC by
25 meq/100 gm for each 1/2-unit
decrease in pH. Then adjust the organic
matter CEC according to the soil pH.
5. Divide the adjusted CEC rate for organic
matter by 100.
6. Multiply the adjusted organic matter CEC
in meq/gm by the percent organic matter
in the soil.
7. Add the CEC contributed by the clay to the
CEC contributed by the organic matter.
Key 6.—Approximate CEC's for clays and
organic matter
meq/100gm
Example 1
A sandy loam soil contains 10% clay and 1%
organic matter. The soil color is yellowish brown
and the soil pH is 7.2.
Assume, from the color, that the clay CEC is 30
meq/100 gm clay.
Assume, from the pH, that the O.M. CEC is 200
meq/100 gm O.M.
Soil CEC from clay =
30 meq/100 gm clay x 10 gm clay = 3 meq
Soil CEC from O.M =
200 meq/100 gm O.M. x 1% O.M. = 2 meq
5 meq
CEC = 5 meq/100 gm soil
Example 2
A silty clay loam soil has 35% clay and 4%
organic matter. The soil is very sticky, and the
pH is 5.5.
Assume, from the stickiness, that the clay CEC is
60 meq/100 gm clay.
Assume, from the pH, that the O.M. CEC is
125 meq/100 gm O.M.
Soil CEC from clay =
60 meq/100 gm clay x 35% clay = 21 meq
Soil CEC from O.M =
125 meq/100 gm clay x 4% O.M. = 5 meq
26 meq
CEC = 26 meq/100 gm soil
Fe, Al oxides 4
Kaolinite 8
Hydrous mica 30
Montmorillonite 60
Organic matter
200
-------
Sample Clauses for Use
in Permits
APPENDIX C
The following clauses are examples of the
kind of information that can be included in
permits. Underlined words or phrases indicate
places where the permit writer needs to substi-
tute information specific to a given application.
This permit is issued in accordance with an
application submitted by the City of Central
Point for Gary Jones (site owner) at Tax Lots
403 and 405. Sections 12 and 13. Township 24
S.. Range 53 E.. Missouri Meridian, dated
March 17. 1990. and is subject to the following
conditions:
1. This permit applies only to those areas
(Fields 1 through 9) mapped by the USDA
Soil Conservation Service as belonging
to the Hillsboro soil series (approximately
620 acres) highlighted on the site map
submitted under that application.
2. Sludge volatile solids shall be reduced
by 38% or more by the anaerobic
digestion process prior to land spreading.
3. Prior to sludge land spreading, sludge
quality shall be assessed to determine
pH, percent total and volatile solids,
nitrate nitrogen, ammonia nitrogen, TKN,
phosphorus, potassium, and metals
(arsenic, cadmium, copper, chromium-
lead. mercury, nickel, and zinc).
4. Sludge land spreading shall be via
pressurized distribution plate application.
5. Sludge shall be transported by tank
trailers equipped with valves adequate
to prevent sludge leakage. Each tank
trailer shall have a current Department
sewage sludge permit number posted at
all times on the doors of the "motorized
vehicle" as defined by United States
Department of Transportation Regula-
tions. Title 49. U.S.C.
6. Immediately following land spreading,
sludge tankers shall be cleaned on-site
to prevent drag-out of sludge onto
public roadways.
7. Central Point's annual sludge land
spreading rates specifically delineated
for each crop are indicated below.
Application rates shall not exceed
those indicated in Keys 7 and 8.
Key 7.—Sludge from lagoon area 1
Gallons
Dry tons
per acre
per acre
Limited by
CRP ground (pasture)
36,500
5.7
Nitrogen, 50 lb/acre
Reclamation
41,625
6.5
Cadmium
Key 8.—Sludge from lagoon area 2
Gallons
Dry tons
per acre
per acre
Limited by
Crop: Alfalfa hay
35,840
5.6
Cadmium
Dry wheat
21,760
3.4
Nitrogen, 40 lb/acre
Dry barley
16,000
2.5
Nitrog&n, 30 lb/acre
Dry pasture
26,880
4.2
Nitrogen, 50 lb/acre
Field corn
35,840
5.6
Cadmium
Irrigated wheat
35,840
5.6
Cadmium
Irrigated barley
35,840
5.6
Cadmium
Irrigated pasture
35,840
5.6
Cadmium
Reclamation
19,000
3.7
Cadmium
65
-------
8. Sludge shall be applied at an annual rate
which assures that no more than 0.446 lbs
per acre cadmium are added.
9. Based on Central Point's sludge analysis
data and the average cation exchange
capacity of area soils, the Jones Site has
an ultimate loading of 56 dry tons per
acre. Cadmium is the pollutant which
limits loading. Should future analyses
show substantial changes in the charac-
teristics of Central Point's sludge metal
content, the ultimate loading rate and
allowable accumulation period may
have to be adjusted.
10. No sludge land spreading shall occur
within the 100 year flood plain of
Eagle Creek.
11. A 100 foot (minimum) setback shall be
maintained between Buckhorn Creek
and the nearest point of sludge land
application.
12. A 50 foot (minimum) setback shall be
maintained from all seasonal streams
and points of sludge land application.
13. A 300 foot (minimum) setback shall be
maintained between the Crestline
Recreation Trail and the nearest point of
sludge land application.
14. A 600 foot (minimum) setback shall be
maintained between areas of sludge
land application and the Oak Grove
Elementary School.
15. No sludge land spreading shall occur
within 100 feet of the shoreline of the
Beaver Flat Marsh.
16. No sludge land spreading shall occur in
areas where slopes exceed 20%.
17. Sludge land spreading shall cease when
precipitation exceeds 1/4 inch per hour.
18. Depth to groundwater shall be measured
from Piezometers 1. 2. 3. and 4 in
Fields 4. 8 and 9 prior to sludge land
spreading. No sludge shall be applied
when permanent groundwater is within
4 feet of ground surface.
19. Application of sludge is not permitted
from November 15 to April 15 due to
frozen soil conditions.
20. No sludge land spreading shall occur on
the site annually between November 1
and April 15 without separate case-by-
case written authorization from the
Department.
21. Areas where sludge has been applied shall
be clearly marked by flag pins or stakes
noting the date of application immedi-
ately following sludge application.
22. No food crops (for direct human
consumption) whose harvested parts are
grown below ground shall be planted on
any sludge amended field for at least 60
months following sludge land spreading.
23. No food chain crops for direct human
consumption whose harvested parts are
grown above ground (e.g., tomatoes)
shall be grown on fields authorized to
receive sludge for at least 18 months
following sludge land spreading.
24. The western perimeter of the sludge
land application site shall be posted bv
signs at 150 foot (maximum) intervals
and enclosed by a fence. Access to the
sludge land spreading area shall be via
locked gate.
25. Public access to the site shall be
restricted for at least 12 months after
sludge land spreading has ceased.
26. There shall be no storage or stockpiling
of sewage sludge at the Jones Site
without separate written authorization
from the Department.
27. In the event an odor problem is reported
to the Department after sludge has been
land spread at the Jones Site, immediate
steps, such as, but not limited to, the
addition of liming materials, must be
taken to counteract that condition.
28. Central Point WWTP shall keep site
records adequate to quantify the date,
location and amount of sludge applied,
segments of each field that received
sludge, pounds of arsenic, cadmium.
copper, chromium, lead, mercury.
nickel, and zinc applied to each segment
receiving sludge, and the type of crop
grown. These data shall be submitted to
the Department on a monthly basis
through the life of the permit.
66
/
-------
29. The Department shall have the right to,
at reasonable times, and upon presenta-
tion of credentials: enter Central Point
WWTP's place of recordkeeping to
review sludge management operations
and records; have access to and obtain
copies of any records required to be
kept under the terms of this permit;
inspect any monitoring equipment
required under this permit; inspect any
collection, transport, or land spreading
vehicles acknowledged under Central
Point's sludge management plan;
sample any ground or surface water,
soils, or vegetation from the Jones Site
and obtain any photographic documen-
tation or evidence deemed appropriate.
30. The Department shall be notified within
one hour of any spills or other threats to
the environment that may occur as a
result of sludge handling. Failure to
provide notification within one hour
may be considered cause for taking
enforcement action against Central
Point. Spills that occur after normal
working hours shall be reported to the
Emergency Management Division
(EMDi within one hour. The telephone
number for EMDis 1-800-452-1103.
31. The Department may impose any
additional restrictions or conditions
deemed necessary to assure adequate
sludge management. Any variations
from the approved sludge application
plan for the Jones Site must be
approved in writing in advance by
the Department.
32. This permit is subject to revocation
should health hazards, environmental
degradation, or nuisance conditions
develop as a result of inadequate sludge
treatment or site management. If
operations are not conducted in
accordance with terms specified under
this permit, the Department shall initiate
necessary remedial action.
-------
I 68
-------
GLOSSARY
69
I
-------
I
70
-------
Absorption
Filling up of soil pores with water, much as
a sponge soaks up water.
Adsorption
Retention of water in soil by attraction
between water molecules and the surfaces of
soil particles. Also, retention of cations in
soil by attraction between their positive
charges and the negatively charged surfaces
of clay and organic matter particles.
Aeration
The movement of air back and forth
between the atmosphere and the pores of a
soil. See Well-aerated soil.
Aerobic
Having oxygen gas as part of the
environment, or occurring only in the
presence of oxygen gas.
Agronomic loading rate
The amount of sludge that would need to be
applied to a site in order to supply the
recommended amount of nitrogen or
phosphorus for a growing crop.
Allowable accumulation period
The number of years that sludge can be
applied on a particular site. The allowable
accumulation period depends on the amount
of heavy metals contained in the sludge and
the amount of sludge applied each year.
AWHC
Available Water Holding Capacity. The
maximum amount of water a soil can store for
plant use. A good soil can provide 9 to 12
inches of available water. A poor soil might
provide only 2 or 3 inches of available water.
Beneficial use
Taking advantage of the nutrient content and
soil conditioning properties of an organic
waste product to supply some or all of the
fertilizer needs of an agronomic crop or
stabilizing vegetative cover.
Cation
A positively charged ion in the soil solution.
CEC
Cation Exchange Capacity. The amount of
positively charged cations such as calcium,
potassium, copper, and nickel that can be
retained in the soil by attraction to the
negatively charged surfaces of soil clays and
organic matter. CEC is expressed in terms of
milliequivalents per 100 grams of soil.
Channers
Small, flat, oblong coarse fragments in soils,
ranging in length of the longest dimension
from 2 mm to 6 inches.
Claypan
A restrictive layer that consists of a horizon
of clayey soil that is dense and massive, but
not cemented. Claypans usually lie directly
beneath a horizon of medium-textured soil,
and the change from that horizon to the
claypan is abrupt.
Coarse-textured soil
A soil whose texture is sand or loamy sand.
Coarse fragments
Rock fragments larger than 2 mm in diameter.
Gravel, cobbles, and channers are the most
common kinds of coarse fragments in soils.
Coarse-grained materials
Soils that consist of loamy sand or sand
textures and often contain large amounts of
gravel or cobbles. These soils have high
rates of saturated hydraulic conductivity and
rapid or very rapid permeability.
Cobbles
Large, rounded coarse fragments in soils,
ranging in diameter from 3 to 10 inches.
Contour cropping
Planting crops in rows that run across slopes
and around hills, rather than up and down
slopes. Contour crop rows slow down runoff
and help conserve soil.
Crop rotation
The sequence of crops grown on a field over
a number of years. Crop rotation cycles may
run from as few as 3 years to as many as 9
or 10 years.
Cross-slope farming
See Contour cropping.
Deep soil
Soil that is moie than 40 inches deep to hard
bedrock (R horizon), soft or weathered
bedrock (Cr horizon), or a cemented horizon
such as a duripan (Bkqm horizon) or
petrocalcic (Bkm) horizon.
Denitrification
Loss of nitrogen from soil by conversion of
nitrate (N03 ) to nitrogen gas (N2).
Denitrification occurs when parts of the soil
become reduced by exclusion of oxygen,
either under saturated or near-saturated
conditions, or as oxygen demand is created
during decomposition.
Diversion terrace
A raised berm of earth constructed
horizontally across a slope in order to
intercept runoff and divert it laterally to a
grassed waterway that can conduct the water
safely downslope without eroding the soil.
-------
Drainage class
The degree of wetness of a soil, as
determined by the depth to a water table and
the length of time the soil remains saturated.
Common drainage classes include
excessively drained, well drained,
moderately well drained, somewhat poorly
drained, and poorly drained.
Droughty
Incapable of storing much water in the soil
for plant use. Sandy soils, shallow soils, and
sloping soils on southerly aspects are likely
to be droughty unless they are resupplied
with rainfall or irrigation water at short
intervals.
Duripan
A restrictive layer, also denoted as a Bkqm
horizon, that is so thoroughly cemented with
silica, with or without calcium carbonate,
that it resembles a layer of rock in the soil.
Equivalent
The number of grams of any particular
chemical (calcium, copper, potassium) that is
equal in reacting power to 1 gram of hydrogen.
Field capacity
The moisture content of a soil when free
drainage immediately after a rain or an
irrigation has virtually ceased. It represents
the maximum amount of water a soil can
retain against the force of gravity.
Fine earth
All soil material that is smaller than 2 mm in
diameter. Fine earth includes sand (.05-2.0
mm), silt (.002-.05 mm), and clay (<.002 mm).
Fine-textured soil
A soil whose texture is silty clay, sandy
clay, or clay.
Floodplain
The nearly level surface next to a river that
is covered with water when the river floods.
Fragipan
A restrictive layer, also denoted as a Bx horizon,
that is extremely dense and compact but is not
cemented and is not very high in clay content.
Glacial till
An unstratified, heterogeneous mixture of
sand, silt, clay, coarse fragments, rocks and
boulders that was deposited at the margins
and beneath the ice of continental or alpine
glaciers.
Gleyed
Soil that is very wet for long periods of time
and is characterized by gray colors, with or
without mottles.
72
Gravel
Small, rounded coarse fragments in soils,
ranging in diameter from 2 mm to 3 inches.
Gravitational water
Water that fills large pores when soil is
saturated and drains away freely under the
influence of gravity. Gravitational water is
not available for plant use.
Hardpan
A generic term for a very dense or cemented
layer in soils. Very dense "hardpans"
include fragipans, claypans, tillage pans, and
compacted layers. Cemented "hardpans"
include duripans and petrocalcic horizons.
Heavy texture
A general term that refers to soils that have a
high clay content. The higher the clay
content, the "heavier" the soil. Clays, silty
clays, and some silty clay loams would all
be considered heavy soils.
Humus
The relatively resistant fraction of soil
organic matter that forms during biological
decomposition of organic residues. Humus
usually constitutes the major fraction of soil
organic matter.
Hydraulic conductivity
A quantitative measure of the rate of water
movement through soil. The most common
laboratory measurement determines the rate
of saturated flow in a vertical direction.
Classes of vertical, saturated hydraulic
conductivity are empirically related to
classes of soil permeability.
Immobilization
Conversion of an element from the inorganic
form to the organic form by microbes.
Immobilized elements are not readily
available for uptake by plants. Nitrogen and
phosphorus are common elements
immobilized in this way. Heavy metals are
also immobilized, but retention on the cation
exchange complex is also included in the
concept of heavy metal immobilization.
Inclusion
An area of soil that is too small to show
separately on a soil map at the scale at
which the map is being made. Inclusions are
present only because of limitations of the
scale of mapping, and they are normal parts
of the definition of map units. Inclusions can
be mapped separately only by making very
detailed maps at very large scales.
-------
Infiltration
The rate that water enters the soil. Infiltration
depends on the size of pores and the stability
of soil aggregates right at the soil surface. If
water cannot infiltrate, then it either ponds
on the surface or runs off over the surface.
Inorganic nitrogen
Nitrogen that is in the ammonium (NH/)
or nitrate (N03 ) form, either in sludge or in
the soil.
Internal drainage
The ability of free water to escape from a
soil. Internal drainage is characterized by the
depth to and duration of water tables in soils
and classified in terms of drainage classes.
Internal drainage class
See Drainage class.
Lacustrine
A term that refers to sediments that were
originally deposited at the bottom of a lake
and are now exposed due to uplift of the
land or lowering of the water level.
Leaching
Removal of soluble minerals, nutrients,
organic chemicals and pesticides from the
soil by water passing through the soil.
Legume
A crop that forms a specific association with
soil bacteria that are capable of fixing
nitrogen, that is, transforming nitrogen gas
to organically combined nitrogen. Common
legumes include alfalfa, clovers, peas, and
soybeans. Nitrogen fixation can provide
most of the nitrogen nutrition of the legume
crop, and it can provide large amounts of
residual nitrogen for succeeding crops.
Light texture
A general term that refers to soils that have a
very low clay content. The lower the clay
content, the "lighter" the soil. Loamy sands,
sandy loams, and some loams all would be
considered light soils.
Loam
A specific class of soil texture that contains
a balanced mixture of sand, silt, and clay.
Generally the sand content is between 30 and
50%, the silt content is between 30 and 50%,
and the clay content is between 10 and 27%.
Lodging
A process whereby cereal grains, upon
taking up excess amounts of nitrogen, put on
excess vegetative growth, lose strength in
the stems, and tip over. Lodging reduces
grain yields because grain lying flat is
difficult to combine.
Map unit
A collection of all soil-landscape areas
shown on a soil map that have the same
name and the same kind or kinds of soils.
Map units contain one or two dominant
soils, plus small areas of other kinds of soils
of minor extent. Map unit names identify the
dominant soil or soils. The minor
components collectively are called
inclusions, and they are identified in the
map unit description.
Map unit description
The part of the text of a soil survey report
that describes the characteristics of the soil
and landscape of the dominant soils in a map
unit. The map unit description also tells the
kind and amount of the inclusions in the map
unit, and it gives general information on use
and management of the map unit for
agriculture, forestry, and urban development.
Matrix color
The dominant color in a variegated or
mottled soil. The color that occupies a
greater percentage of the exposed surface
area than any other color.
Medium texture soil
A soil whose texture is loam or silt loam.
Milliequivalent
One one-thousandth of an equivalent. In soil
science mi21iequiva!ents are used to quantify
the capacity of a soil to adsorb positively
charged ions independently of the particular
kind of substance.
Mineralization
Biochemical conversion of nitrogen in the
organic matter of soils and sludges to
inorganic nitrogen. Mineralization produces
nitrogen in the ammonium (NH4+) form,
which is then converted to the nitrate (N03 )
form by the nitrification process.
Minimum tillage
Preparation of a seedbed for planting a crop
without conventional moldboard plowing.
Tillage operations range from disking or
chisel plowing the soil before planting to
planting directly into soil that has not been
tilled since the harvest of the previous crop.
Minimum tillage usually leaves enough
plant residues on the soil surface to provide
a measure of erosion protection.
Moderately coarse texture soil
A soil whose texture is sandy loam.
Moderately deep soli
Soil that is 20 to 40 inches deep to hard
bedrock (R horizon), soft or weathered
bedrock (Cr horizon), or a cemented horizon
such as a duripan (Bkqm horizon) or
petrocalcic (Bkm) horizon.
-------
Moderately fine texture soil
A soil whose texture is silty clay loam, clay
loam, or sandy clay loam.
Moderately well drained
A soil that has a temporary water table for
short periods of time in the lower part of the
subsoil. The soil is usually mottled
somewhere between 24 and 40 inches.
Montmorillonite
A type of soil clay that has a very high
shrink-swell potential.
nh4-n
The amount of nitrogen in the ammonium
form. Each 100 pounds of ammonium-nitrogen
contains 78 pounds of actual nitrogen.
Nitrification
The biological conversion of ammonium
(NH4+) to nitrate (NO,") in soil. As the
nitrogen cycle operates in most soils, the
nitrification step follows the mineralization
step, in which organic nitrogen is converted
to ammonium (NH4+).
N03-N
The amount of nitrogen in the nitrate form.
Each 100 pounds of nitrate nitrogen contains
22 1/2 pounds of actual nitrogen.
Nutrient-supplying power
The ability of a soil to provide nutrients in
amounts needed for plant growth. Medium-
textured soils that are high in organic matter
generally have a high nutrient-supplying
power. Sandy soils that are low in organic
matter generally have a low nutrient-
supplying power.
Organic nitrogen
Nitrogen that is combined in the molecular
structure of organic compounds. Most of the
organic nitrogen in soils occurs as proteins
and amino acids or amine groups.
Ped
An aggregate of individual grains of sand,
silt, and clay into a single unit of soil
structure. Ped shapes include granular, platy,
blocky, and prismatic. Ped sizes may vary
from 1-mm granules to 40-cm prisms.
Permeability
The rate that water moves through the soil.
Permeability depends on the amount, size,
and interconnectedness of soil pores. These
in turn are related to soil texture, soil
structure, and soil density.
74
Petrocalcic horizon
A restrictive layer, also denoted as a Bkm
horizon, that is enriched with calcium
carbonate, and in which the calcium
carbonate has cemented the horizon into a
rock-like layer.
Poorly aerated
Soils in which air is not readily exchanged
between the soil and the atmosphere. Wet
soils are poorly aerated because air moves
more slowly through water than through air-
filled pores.
Poorly drained
A soil that is saturated at or near the surface
for long periods of time. Poorly drained soils
are usually gleyed, and they often have
mottles in the A or Ap horizon.
Pores
Spaces, or voids, between mineral grains
and aggregates in the soil. The amount, size,
shape, and continuity of soil pores control
the rates of air and water movement into and
throughout the soil.
Puddling
Formation of a dense, massive surface soil
when medium- to fine-textured soils are
tilled when they are too wet.
pH
A number that indicates the relative acidity
or alkalinity of a soil. A pH of 7.0 indicates
a neutral soil. Lower numbers indicate acid
soils; higher numbers indicate alkaline soils.
Readily available nitrogen
Nitrogen that is in the soil in the ammonium
(NH4+) or nitrate (N03 ) form. Ammonium
and nitrate are dissolved in the soil solution
and can be taken up and utilized by plants
immediately.
Reduced tillage
See Minimum tillage.
Residual nitrogen
Nitrogen that remains in the soil after the
harvest of a crop. Residual nitrogen is either
immediately available or will become
available to the succeeding crop. Sources of
residual nitrogen include inorganic nitrate
that is not leached from the soil, organic
nitrogen in crop residues, and organic
nitrogen in previous sludge applications.
Restrictive layer
A general term for any soil horizon that is
slowly or very slowly permeable and
underlies more permeable soil horizons.
Restrictive layers slow down or stop the
downward movement of water in soils, and
they impede plant root penetration.
-------
Runoff
Water that flows over the surface of soil
toward a stream or lake without sinking into
the soil.
Saturated flow
Movement of water in soil when all the pores
are completely full of water, that is, when the
soil is saturated. Rates of saturated flow through
large pores are relatively rapid, and the
potential for contamination of groundwater
with water-borne pollutants is high.
Shallow soil
Soil that is less than 20 inches deep to hard
bedrock (R horizon), soft or weathered
bedrock (Cr horizon), or a cemented horizon
such as a duripan (Bkqm horizon) or
petrocalcic (Bkm) horizon.
Shrink-swell potential
The tendency of a soil to change volume due
to gain or loss of moisture. Soils with high
shrink-swell potentials expand appreciably
when they wet up and contract appreciably
when they dry out.
Slaking
The breakdown of structural aggregates
under the impact of falling droplets of water.
Grains of silt and clay washed off peds clog
soil pores, creating a thin surface crust that
seals the soil, reduces infiltration, and increases
runoff and erosion. Silty soils that are low in
organic matter and have weak structure are
particularly susceptible to slaking.
Slickensides
Polished, shiny surfaces on clayey soil
aggregates, caused by the movement of two
masses of soil past each other as soil
expands upon wetting. Slickensides are
evidence of high shrink-swell potential in
soils that contain large amounts of
montmorillonite clays.
Soil
A natural body that develops in profile form
in response to forces of climate and
organisms acting on a parent material in a
specific landscape position over a long
period of time. Soil covers the earth in a thin
layer and supplies plants with air, water,
nutrients, and mechanical support.
Soil amendment
Anything that is added to the soil to improve
its physical or chemical condition for plant
growth. Lime, gypsum, inorganic fertilizers,
and organic materials, including sewage
sludge, are all soil amendments.
Soil conditioner
Any material applied to improve
aggregation and stability of structural soil
aggregates. Sewage sludge provides these
benefits, and is therefore a soil conditioner.
Soil drainage class
The degree of wetness of a soil, as
determined by the depth to a water table in
the soil and the length of time the soil
remains saturated. Common drainage classes
include excessively drained, well drained,
moderately well drained, somewhat poorly
drained, and poorly drained.
Soil horizon
A layer of soil that is approximately parallel
to the earth's surface. Each horizon results
from specific soil-forming processes, and
each is distinguished from horizons above
and below by a unique set of physical,
chemical, and biological properties.
Soil profile
A vertical exposure that allows you to see all
of the soil horizons that are present.
Soil profile description
A technical record of the soil properties that
can be observed and measured in the field for
each and every horizon in the entire soil profile.
Soil series
The set of all soils whose profiles are
essentially alike, within narrowly defined
ranges of variability. The soil series is the
lowest unit of soil classification. Names of
soil series are usually taken from geographic
entities in areas where the soil was first
described, for example, the Appling Series,
the Tama Series, the Willamette Series.
Soil slope
The inclination of the land surface,
expressed as percent. A 10% slope means
that the elevation changes by 10 feet for
every 100 feet of horizontal distance.
Soil structure
The arrangement of individual grains of
sand, silt, and clay into larger units called
aggregates, or peds. Plant roots, humus, and
soil clays all help to hold soil peds together.
Soil structure is characterized by the size,
shape, and strength of the peds.
Soil survey
The process by which a soil map is made.
Soil scientists walk over the land, observe
soil and landscape properties, classify the
soils, and locate soil boundaries in the field.
They use air photo base maps to record the
location of soil boundaries and label each
delineation with a map unit symbol.
-------
Soil survey report
A book in which the results of the soil
survey of an area, often a county, are
published. The soil survey report contains
three related components: the soil maps, text
that describes the properties and behavior of
the soils and map units, and tables that give
quantitative data and interpretations for soil
use and management.
Soil texture
The amounts of sand, silt, and clay that
make up a soil. Specific combinations of
sand, silt, and clay form textural classes,
each of which is named with a term such as
silt loam, clay loam, or sandy loam.
Somewhat poorly drained
A soil that is saturated in the upper part of
the subsoil for significant periods of time
during rainy seasons. The soil is usually
gray and mottled somewhere between 10
and 24 inches.
Starter fertilizer
Fertilizer applied to the soil at the time a
crop is planted to provide a source of nutrients
that will be readily available at the time the
plants are beginning to grow vigorously. Starter
fertilizers bridge the gap between planting
and subsequent availability of nutrients from
mineralization of organic matter.
Terrace
A landform consisting of a long, narrow,
nearly level surface at or near the margin
and above the level of a body of water.
Stream terraces are above the level of the
stream's floodplain, and they are usually
marked by an escarpment that descends
from the terrace to the floodplain.
Terracing
Construction of one or more di version
terraces across a slope.
Tillage pan
A compact, dense layer of soil at the base of
the surface layer of a cultivated soil.
Compaction occurs when the soil is plowed
or disked when it is too wet.
Traffic pan
A compacted layer beneath the surface layer
of a cultivated soil that occurs as the result
of the cumulative effects over time of
driving over the soil with heavy equipment.
Unsaturated flow
Movement of water through soil when some
soil pores, particularly the larger ones, are
filled with air. Unsaturated flow occurs as
water moves through films of water around
soil particles in response to an energy
gradient from moist soil to dry soil. Rates of
unsaturated flow are very slow.
Volatilization
Conversion of ammonium (NH4+) in the soil
to ammonia gas (NH;t) and escape of
ammonia into the atmosphere.
Water table
The top of a zone of saturated soil. Water
tables in soils are classed as perched,
apparent, or artesian. A perched water table
refers to a zone of saturation that is
underlain by unsaturated soil. Perched water
tables are associated with restrictive layers.
An apparent water table refers to a thick
zone of saturated soil in which there is no
evidence of restrictive layers. An artesian
water table refers to water under pressure
that is trapped beneath an impermeable
layer. The water table rises when the
impermeable layer is breached.
Weathering
The physical disintegration and chemical
decomposition of rocks in place upon
exposure to the atmosphere. Weathering
produces earthy material that, upon further
modification by chemical and biological
processes, is transformed into soil.
Well-aerated
Soil that allows easy exchange of air
between the soil and the atmosphere. Well-
aerated soils have plenty of pores that are
big enough and sufficiently interconnected
to provide pathways for air movement. They
usually have good structure and are well or
moderately well drained.
Well drained
A soil that is rarely saturated above a depth
of 40 inches. Well drained soils are well-
aerated and have brown, yellowish brown,
or feddish brown colors. They are not
mottled above 40 inches or so.
Wilting point
The moisture content of a soil at which a
plant can no longer extract water. Without
addition of water, plants will wilt and
ultimately die. Some soils, particularly
clayey soils, contain relatively large
amounts of water at the wilting point, but it
is held so tightly in the very small pores of
the clays that plants can't use it.
-------
The Oregon State University Extension Service educates Oregonians by delivering
research-based, objective information to help them solve problems, develop leadership,
and manage resources wisely.
Extension's agriculture program provides education, training, and technical assistance to
people with agriculturally related needs and interests. Major program emphases include
food and fiber production, farm business management, marketing and processing of
agricultural products, and resource use and conservation.
Extension Service, Oregon State University, Corvallis, O.E. Smith, director. This
publication was produced and distributed in furtherance of die Acts of Congress of
May 8 and June 30,1914. Extension work is a cooperative program of Oregon State
University, the U.S. Department of Agriculture, and Oregon counties.
Oregon State University Extension Service offers educational programs, activities, and
materials—without regard to race, color, national origin, sex, age, or disability—*s
required by Title VI of the Civil Rights Act of 1964, Title IX of the Education
Amendments of 1972, and Section 504 of the Rehabilitation Act of 1973. Oregon State
University Extension Service is an Equal Opportunity Employer.
------- |