Soil Quality in Organic Agriculture Systems - Transition to Organic Partnership Program

Building and maintaining soil quality is the basis for successful organic farming. Topics of crop rotations, soil amendments, soil health, carbon sequestration, organic agriculture philosophies, and relevant related field research are included.
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2003

Soil Quality
I N

O R G A N I C

A G R I C U L T U R A L

S Y S T E M S

Organic Farming Requires Quality Soil

Building and
maintaining
soil quality
is the basis
for successful

organic farming.

Building and maintaining soil quality is the basis for successful organic farming.
However, before developing a soil management plan focused on soil quality in
organic systems, farmers should become knowledgeable regarding the overall
philosophies, legalities, and marketing opportunities in organic agriculture. A brief
overview of organic agriculture follows, but for further details, see Iowa State
University Extension publication Organic Agriculture (PM 1880). (See page 8 for
ordering instructions.)

USDA NRCS

Periodic soil testing
will help determine
soil quality.

PM 1882 August 2003

Soil Quality
I N

O R G A N I C

A G R I C U L T U R A L

and barley), should be
rotated with row crops
For an organic crop
(corn, soybeans, amaranth,
to be certified,
and vegetables) to ensure
a healthy system. A typical
a crop rotation plan
six-year rotation in Iowa is
must be in place
corn (with a cover of winter
rye)-soybeans-oat (with an
to protect against
underseeding of alfalfa)pest problems and to
alfalfa-corn-soybeans.
Soybeans fix nitrogen and
maintain soil health.
can generally be grown
without fertilizer in the first
year. Subsequent crops
must include rotations of
grain crops and nitrogen-adding cover crops to
maintain adequate fertility. Horticultural crops
should be rotated with a leguminous cover crop at
least once every five years.

What Is Organic Agriculture?

In order to sell your crop as certified organic, you
must follow USDA National Organic Program rules,
and be certified by one of the accredited agencies
listed in Organic Agriculture (PM 1880). State of Iowa
organic certification rules will require the following:
• No synthetic fertilizers for 36 months prior to
the certified organic crop’s harvest.
• No synthetic pesticides (fungicides, insecticides,
herbicides) for 36 months prior to the certified
organic crop’s harvest.
• Crop rotations, including a soil-building legume
or small grain/legume mix following row crops,
to break weed, insect, and disease cycles and
maintain soil fertility.
• No synthetic hormones or antibiotics for
livestock may be used, and organic feeds and
pastures must be fed.
• Soil fertility in organic systems is maintained
primarily through crop rotations (usually cornsoybeans-oat-alfalfa or some variation of this
system) and through applications of composted
or raw manure. Seaweed, fish emulsion, or
plant/animal-based products, such as alfalfaand feather-meal, can be applied as soil and
foliar amendments in organic systems.

Soil Amendments

Naturally mined lime products are used to adjust
the soil pH to within a range of 6.0 to 7.0
(depending on crop requirements). In addition to
lime, manure and composted manure are the most
common forms of soil amendments in organic
operations. Raw manure may be obtained from
organic or conventional farms, provided the manure
is applied at least 3 months prior to the harvest of
an agronomic crop, or at least 4 months prior to the
harvest of a horticultural crop. These rules were
developed in order to provide adequate time for
decomposition of manure and avoid bacterial contamination of produce. To prevent contamination of
waterways, raw manure cannot be applied to frozen
or snow-covered ground. Organic certification
agencies recommend manure should be composted
prior to land application (see photo on next page).
Composting is the preferred method of stabilizing manure. Composting is a controlled process in

Crop Rotations

For an organic crop to be certified, a crop rotation
plan must be in place to protect against pest
problems and to maintain soil health. A good
general rule is that no more than four out of five
years should be in row crops, and it is required that
the same row crop cannot be grown in consecutive
years in the same field. Legumes (e.g., alfalfa, red
clover, berseem clover, and hairy vetch) alone, or in
combination with small grains (e.g., wheat, oats,

S Y S T E M S

The basis for all organic
farming systems is the
health of the soil.

K. DELATE

which nitrogen-containing
materials (e.g., manure,
yard waste, or kitchen
Naturally mined
waste) are mixed with a
carbon-containing source
lime products
(e.g., corn stalks or cobs,
are used to adjust
straw, and wood chips) to
produce a mixture preferthe soil pH to
ably with a carbon-towithin a range
nitrogen ratio (C:N) of 30
to 1. The compost mixture
of 6.0 to 7.0
must reach and maintain a
(depending upon
temperature of ≥ 140° F for
at
least three days during
crop requirements).
the composting process in
order to limit bacterial
contamination. Adequate
moisture and temperature
are required for proper composting. Most organic
farmers utilize front-end loaders or windrow turners
to construct outdoor composting systems. Other
composting systems include vermi-composting

Compost can be made in a 50-gallon barrel, with a front-end
loader or with a commercial windrow turner.

(utilization of earthworms in “beds” to decompose
manure and other wastes), in-vessel digesters, and
anaerobic systems. Additional information on
composting practices is listed in the references.
Many soil amendments are available for organic
farming. The key, however, is that the material is

Soil Quality
I N

O R G A N I C

A G R I C U L T U R A L

K. DELATE

naturally based, and that no prohibited substances,
such as hexane, are used in the processing or
collection of the materials. In addition to manurebased fertilizers, many organic farmers rely on fish
emulsion and seaweed preparations to supply
nitrogen and other elements. When phosphorus
and potassium limit crop production, rock phosphate and naturally mined potassium chloride are
allowed. It is imperative that farmers check with
their certification agencies before applying any
materials. A farmer’s certification may be revoked
for three years if it is discovered that he or she has
applied a material found to be contaminated with
prohibited materials.

Hairy vetch (Vicia villosa) and rye can supply up to 120 lb. of
nitrogen per acre to crop fields.

protection provided by crop residues, manure or
compost, and cover crops reduce soil erosion losses
and increase water-holding capacity and aeration.
Maintaining soil organic matter content at levels
that are consistent with the natural characteristics
of the soil (i.e., loamy soils will generally have
higher organic matter than sandy soils) helps soil
biological activity and the healthy microbial and
macrofaunal populations that are required for
efficient nutrient cycling. These populations include
bacteria, fungi, actinomycetes, nematodes, and
earthworms. Crop rotations (required for all organic
operations) are crucial for organic systems because
the legume crops (e.g., alfalfa and red clover)
provide nitrogen (N) and also help recycle nutrients,
such as phosphorus (P) and potassium (K).
Including crops with deep root systems in the
rotation helps extract nutrients from lower soil
depths and return them to the surface when the
vegetation dies. Crop residues also provide the
carbonaceous biomass upon which soil microfauna
(e.g., earthworms and beetles) and microorganisms
depend on for survival.
Cooperative research by scientists at Iowa
State University and USDA-Agricultural Research
Service (ARS) is being conducted to evaluate soil
quality within organic and conventional crop
production systems. After one growing season,
several soil quality indicators showed a positive
response to organic management (Table 1).

Soil Health in Organic Systems

Soil organic matter, created through decomposition
Soil organic matter,
and recycling of plant and
created through
animal residues, has many
important roles in organic
decomposition and
systems, including supplyrecycling of plant
ing the necessary elements
for plant growth. The first
and animal residues,
step in formation of soil
has many
organic matter is fixation
of carbon dioxide (CO2)
important roles
by plants. Capturing the
in organic systems,
energy of sunlight and
efficiently recycling it
including supplying
through various forms of
the necessary elements
different soil organic
matter is, therefore, a
for plant growth.
basic goal associated with
organic production systems. The soil organic
matter or carbon (C) inputs
improve soil physical properties, such as aggregate
stability, and provide food, habitat, and shelter for
billions of soil organisms. Increased aggregate
stability, improved soil structure, and surface

S Y S T E M S

Table 1

First-Year Effect of Organic and Conventional Management Practices on Selected Soil Quality Indicators
at the Neely-Kinyon Farm, Greenfield, Iowa
Carbon Pools 1
Farming System

Total

POM

-1

Conventional (n=8)
Organic (n=28)

Nitrogen Pools 2
Biomass

PMN

-1

Mg ha to 30 cm

NO3-N

-1

kg ha to 7.5 cm

kg ha to 30 cm

Other Indicators 3
WSA

PH
dS cm-1

82.1 a 4

10.7 a

36.6 b

136 a

29.6 a

20.6 a

240 a

6.4 a

85.0 a

12.1 a

83.2 a

144 a

14.8 b

22.6 a

223 a

6.4 a

POM—particulate organic matter; Biomass—microbial biomass

PMN—potentially mineralizable nitrogen; NO3-N—nitrate nitrogen

WSA—water stable aggregation; EC—electrical conductivity

Means followed by the same letter are not significantly different at p=0.10.

Soil microbial biomass carbon was 228 percent
higher in plots fertilized with compost than in those
receiving inorganic N fertilizer. The conventional
treatment also had 50 percent higher residual
nitrate nitrogen (NO3-N) and excessive levels of
NO3-N in basal corn stalk samples collected at
physiologic maturity. (For more information on corn
stalk sampling, see Iowa State University Extension
publication PM 1584, Corn Stalk Testing to Evaluate
Nitrogen Management.)
Soil quality evaluations generally include
indicators such as microbial biomass carbon (MBC)
and particulate organic matter carbon (POM-C)
because these carbon pools are more responsive to
changes in soil and crop management practices
than total organic carbon. Changes in these
indicators occur more quickly because MBC and
POM-C are associated with nutrient cycling and
turnover throughout the year. Potential nitrogen
mineralization (PMN) also showed a slight response
(+8 percent) to organic management, which
coupled with the lower NO3-N concentration in the
soil, suggests the N applied through compost was
being incorporated into biologically active soil
organic matter. The 15 percent increase in water
stable aggregation (Table 1) also suggests the
slight increase in soil organic matter was beginning
to have positive effects on soil physical properties.

Relevant Field Research

Organic Soybeans and Cover Crops
for Organic Vegetable Production

Building improved soil
quality is laudable, but a
moot point, if organic
Organic soybean
production systems are
not profitable. Organic
production is
soybean production is one
one of the
of the most lucrative crops
for organic farmers in Iowa
most lucrative crops
today. A typical crop rotafor organic farmers
tion that includes organic
soybeans is corn followed
in Iowa today.
by a winter cover of rye,
soybeans, and oat with an
underseeding of alfalfa or
red clover in the third year.
In the spring, rye that is
less than 8″ in height can be killed with a field
cultivator. If plants are taller, rye should be mowed
or cut with a stalk chopper before cultivating. A
second cultivation may be necessary if there are
remaining rye plants. Sample soil in the fall to
determine if soil conditions are adequate for
soybean production. Adjustments to a proper soil

Soil Quality
I N

O R G A N I C

A G R I C U L T U R A L

K. DELATE

pH of 6.5 to 7.0 can be made through applications of
lime in the fall or spring. Iowa soils usually do not
require dolomitic lime. For additional details on
organic soybean production, see the Iowa State
University Extension publication Growing
Organic Soybeans on CRP Land (PM 1881).
Cover crops should be grown at least once
every five years in certified organic horticultural
operations in Iowa. Therefore, to help sustain or
improve soil quality and to remain in compliance
with the organic standards, many organic farmers
rotate to a soil-building legume cover crop after
every vegetable crop. Successful cover crops in
Iowa include hairy vetch, alfalfa, clover, and
medics. The annual legumes typically have 3.5 to 4
percent nitrogen in leaves before flowering and 3 to
3.5 percent at flowering. After flowering, much of
the nitrogen is directed to seed production.
Therefore, it is recommended that legumes be
incorporated at the start of flowering. To estimate
nitrogen additions from a cover crop, take cuttings
from several areas in the field and dry and weigh
them. Use a yardstick or metal frame of set dimensions (e.g., 1 square foot) and clip the plants at
ground level. After drying for several days, use the
following estimate to determine the per acre yield
of dry matter:

S Y S T E M S

Red or crimson clover, in addition to alfalfa, is used as a
legume plowdown on many organic farms. To estimate
nitrogen additions from a cover crop, follow the instructions
on this page.

If sufficient biomass is
produced by cover crops,
there may be adequate
nitrogen for the subsequent crop. If the cover
crop growth is limited by
poor weather, however,
additional nitrogen, from
compost or manure, may
be needed. Details on
cover crop additions can
be found in Managing
Cover Crops Profitably
(EDC 201), which is available through Iowa State
University Extension.)

Cover crops
should be grown
at least once
every five years
in certified
organic horticultural
operations in Iowa.
Successful
cover crops
in Iowa include

Yield
Total weight of dried sample (lb.)
43,560 sq. ft.
x
=
(lb./acre)
3 sq. ft. you sampled
1 acre

hairy vetch, alfalfa,
clover, and medics.

Divide this quantity from the above equation
by 2 if the cover crop will be conventionally tilled
or divide by 4 if it is to be left on the surface in
a no-till system. To estimate the total N in
green manure:
Total N
Biomass yield from above calculation
% N (in leaves)
x
=
(lb./acre)
(lb./acre)
100

Philosophies in Organic Agriculture

E
Inputs

USDA NRCS

Environmental, economic, and food safety concerns
are among the many reasons why some farmers
choose organic production. Likewise, organic
producers differ in the methods they use to achieve
the ideal system. Some organic farmers completely
shun external inputs, and these farmers enhance
the native biological insect control on their farms by
conserving beneficial insects’ food and nesting
sites instead of importing natural pesticides.
Compost is created on the farm for their fertilization
needs. Other organic farmers do not make a
distinction in inputs, and they rely on imported
inputs for soil fertility and pest management. This
philosophy of “input substitution” is discredited by
many long-time advocates of organic agriculture
who believe that a truly sustainable method of
organic farming would seek to eliminate, as much
as possible, reliance on external inputs. Organic
certification, however, is based on the use of
allowable substances, regardless of their origin.

Leaving plant residue from the previous year’s crop
can increase the level of soil carbon sequestration in
organic fields.

farming are all management practices that can
enhance carbon storage
in soils. However, tillage
practices, which can
increase CO 2 emissions,
must be considered when
evaluating the tradeoffs
associated with organic
systems and carbon
sequestration. Additional
information on carbon
sequestration can be
obtained in the Iowa State
University Extension publication Impact of Tillage
and Crop Rotation Systems
on Soil Carbon Sequestration (PM 1871).

Organic Agriculture and
Carbon Sequestration

Recently, some world governments have promoted
soil carbon sequestration (storage) as a way to help
mitigate elevated levels of atmospheric CO2 caused
by burning fossil fuels and other sources of
industrial pollution. The use of crop rotations,
effective manure management, and green manure
crops as required for effective and efficient organic

The use of crop
rotations, effective
manure management,
and green manure
crops . . . are all
management practices
that can enhance
carbon storage in soils.

Soil Quality
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A G R I C U L T U R A L

S Y S T E M S

References
Bowman, G. (ed.). 1997. Steel in the Field—A farmer’s

IDALS (Iowa Department of Agriculture and Land

guide to weed management tools. Sustainable

Stewardship). 2000. Iowa Organic Certification and

Agriculture Network, USDA, National Agriculture

Organic Standards. Des Moines, IA.

Library, Beltsville, MD.

Magdoff, F. and H. van Es. 2000. Building soils for better
crops. Sustainable Agriculture Network, National

Delate, K., and C. Cambardella, 2000. Integrating organic

Agriculture Library, Beltsville, MD.

soybeans following CRP land. USDA-SARE Annual
Report, N.C. SARE, University of Nebraska, Lincoln, NE.

Rynk, R. 1992. On-Farm Composting Handbook. NRAES-

Delate, K., C. Cambardella, K. Taylor, and B. Burcham.

54. Northeast Regional Agricultural Engineering
Service. Ithaca, NY.

1999. Comparison of organic and conventional rotations at the Neely-Kinyon Long-Term Agroecological

Sustainable Agriculture Network (SAN). 1998. Managing

Research (LTAR) site: First year results. Leopold

cover crops profitably. 2nd Edition. Sustainable

Center for Sustainable Agriculture Annual Report,

Agriculture Network, USDA National Agriculture

Iowa State University, Ames, IA.

Library, Beltsville, MD.

LEOPOLD CENTER

This research project is partially funded by the
Leopold Center for Sustainable Agriculture at
Iowa State University and the USDA-ARS
Sustainable Agriculture Initiative.

Publications mentioned in this booklet can be ordered by
contacting any ISU Extension county office or the ISU
Extension Distribution Center at (515) 294-5247. There is a
charge, plus shipping and handling, for some publications.

Prepared by Kathleen Delate, Iowa State University;
Cynthia Cambardella and Douglas Karlen, USDA National
Soil Tilth Laboratory.
Edited by Jean McGuire, ISU Extension
Continuing Education & Communication Services.
Illustrated by Jane Lenahan.

File: Agriculture 2

[A]

. . . and justice for all
The U.S. Department of Agriculture (USDA) prohibits discrimination in all its programs and activities on the basis of race, color, national origin, gender, religion,
age, disability, political beliefs, sexual orientation, and marital or family status.
(Not all prohibited bases apply to all programs.) Many materials can be made
available in alternative formats for ADA clients. To file a complaint of discrimination, write USDA, Office of Civil Rights, Room 326-W, Whitten Building, 14th and
Independence Avenue SW, Washington, DC 20250-9410 or call 202-720-5964.

Soil Web graphic on page 2 reprinted with permission from
Soil and Water Conservation Society (SWCS). 2000. Soil
Biology Primer. Rev. ed. Ankeny, Iowa: Soil and Water
Conservation Society.
Some photos on the cover were provided courtesy of Iowa
State University College of Agriculture and the USDA-NRCS.

Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30,
1914, in cooperation with the U.S. Department of Agriculture. Stanley R. Johnson,
director, Cooperative Extension Service, Iowa State University of Science and
Technology, Ames, Iowa.

For the latest on organic agriculture from Iowa State University
go to http://extension.agron.iastate.edu/organicag/.