Organic production and consumption has increased to a $39.5 billion industry in the United States with over 22,000 organic farmers. Over 5.4 million acres are in organic production in the U.S., including 164,403 acres of organic vegetables, valued at $1.3 billion. The majority of organic vegetable growers incorporate crop rotations, composting, and cover crops in their operations. The following information offers a guide for including these practices to meet certified organic rules and increase the long-term sustainability of an organic farm.
Midwest
PDF
4836, 4836, HORT3052, HORT3052.pdf, 792187, https://cms.organictransition.org/wp-content/uploads/2024/02/HORT3052.pdf, https://cms.organictransition.org/resource/crop-rotations-composting-and-cover-crops-for-organic-vegetable-production/hort3052/, , 7, Organic production and consumption has increased to a $39.5 billion industry in the United States with over 22,000 organic farmers. Over 5.4 million acres are in organic production in the U.S., including 164,403 acres of organic vegetables, valued at $1.3 billion. The majority of organic vegetable growers incorporate crop rotations, composting, and cover crops in their operations. The following information offers a guide for including these practices to meet certified organic rules and increase the long-term sustainability of an organic farm., , hort3052, inherit, 4835, 2024-02-12 15:43:27, 2024-02-12 15:43:35, 0, application/pdf, application, pdf, https://cms.organictransition.org/wp-includes/images/media/document.png
2016
Crop Rotations, Composting
and Cover Crops
FOR ORGANIC VEGETABLE PRODUCTION
Introduction
Organic production and consumption has increased to
a $39.5 billion industry in the United States with over
22,000 organic farmers. Over 5.4 million acres are in
organic production in the U.S., including 164,403 acres
of organic vegetables, valued at $1.3 billion. Consumers
purchase organic foods for several reasons including
a reduction in pesticide residues (organic production
prohibits the use of synthetic pesticides) and perceived
health benefits in terms of phytonutrients and certain
vitamins. Organic consumers support local farmers who
rely on environmentally-friendly methods of production
that help protect groundwater and waterways. The
majority of organic vegetable growers incorporate crop
rotations, composting, and cover crops in their operations.
The following information offers a guide for including
these practices to meet certified organic rules and increase
the long-term sustainability of an organic farm.
Crop Rotation
Crop rotations involve a systematic farm plan, where the
crop planted in one field on the farm changes every year
or every season. The history of crop rotations in farming
systems dates back to ancient Greece and Rome, where
Pliny described the benefits of incorporating legume crops
to enhance soil and crop quality in grain crop systems that
included wheat, barley, and emmer. In the U.S., George
Washington established a seven-year crop rotation of
grain and legume crops in the 1700s, along with carrots,
cabbage, peas, potatoes, pumpkins, and turnips, to
enhance soil quality on his Mount Vernon farm. While soil
quality derives from inherent parent material, climate, and
topography, human-mitigated operations, including tillage
and crop rotation, can also affect soil fertility. More diverse
crop rotations include more crops in the rotation and tend
to have better soil quality. Longer crop rotations have been
shown to improve the soil’s physical properties, decrease
erosion, reduce nitrogen (N) leaching potential, improve
soil organic matter, and provide competitive crop yields.
Certified organic farmers are required by law to practice
crop rotation under the USDA-National Organic Program
(NOP). This standard dictates that the producer must
implement a crop rotation, including, but not limited to,
sod, cover crops, green manure crops, and catch crops
that aid in soil
quality and pest
management.
Among the many
benefits of crop
rotations, the
USDA-NOP
recognizes that
crop rotations
HORT 3052 August 2016
MUSTARD COVER CROP
can improve soil organic matter, supply necessary
plant nutrients, and provide erosion control. In the
NOP standard on pest, weed, and disease management
(§205.206), crop rotation is specifically stated as the first
method used in managing pests and expressing the linkage
between healthy soils and healthy plants. Because organic
certification verifies that only organic practices were used
for a minimum of 36 months prior to certification, farmers
must complete an Organic System Plan (OSP), which
provides information on the history of crop rotations
planted in the last three years for every organic field. Thus,
determining crop rotations for the next few years will
greatly assist the certification process and lead to better
farm management.
wheat, hairy vetch, and red or white clover, due to
their quick establishment, ability to over-winter, weed
competitiveness, and ease of mechanical termination.
Because of their ability to fix N, leguminous cover crops
provide the greatest potential for improving yields. Cereal
crops generally result in higher levels of soil organic matter
helping suppress weeds, immobilize soil nitrogen, and
reduce nitrate leaching during winter months. Planting
small grains and N-fixing cover crops together may be
an effective management strategy to increase soil C and
improve the N cycling processes, thereby reducing N
leaching while maintaining robust yields. An example of
a crop rotation plan from an organic farm in the northeast
United State is shown in Figure 1.
CROP ROTATIONS TO ENHANCE SOIL FERTILITY
Many organic farmers are striving for a closed, integrated
farm, relying on on-farm or locally produced inputs and
techniques, such as crop rotations, to meet crop nutritional
needs. Building or maintaining soil carbon (C) and
nitrogen (N) pools for subsequent crop use is an important
consideration in developing sustainable organic farming
systems. Incorporating crop residues from crop rotations
and manure has led to greater soil carbon sequestration,
improvements in soil function, nutrient cycling, and pest
control. This has led to greater water holding capacity,
higher microbial biomass of C and N, enhanced soil
respiration, and greater potentially mineralizable N relative
to nitrate-N; these results were attributed to the use of
diverse crop rotations that included cover crops and
applications of organic-based amendments.
In organic production systems, high N-demanding
crops, such as sweet corn, are usually planted in a field
following a soil-building crop such as oats, barley, rye,
2
Cover Rotations, Composting and Cover Crops for Organic Vegetable Production
Figure 1.
Typical organic
vegetable crop
rotation on
Northeast U.S.
farm (from C.
Mohler and S.E.
Johnson, 2009:
Crop Rotation on
Organic Farms: A
Planning Manual,
NRAES 177).
While legumes may provide a significant amount of N
(20-120 pounds/acre depending on the species mixture),
the contribution may not meet the complete needs of the
cash crop. Soil testing in the fall following crop harvest
can help determine the need for further amendments.
Before planting in the spring, producers can apply welldecomposed animal manure, or preferably, a manurebased compost in an amount that will provide the full
complement of N necessary for vigorous plant growth.
Many NOP-compliant fertilizers, such as fish emulsion,
humates, humic acids, surfactants, bioactivators, or
Biodynamic™ preparations can also be used. However,
these amendments may be viewed as cost-prohibitive
on a large scale and must be compatible with marketing
requirements in order not to limit marketing options.
Maintaining a soil pH of 6.0-7.0 is also critical for optimal
crop production. Various agricultural liming materials
can be used to neutralize the acidity of soils and to
provide calcium and magnesium, but concern over soil
magnesium buildup from dolomitic lime applications
has led to the popularity of naturally mined calcium
carbonate (limestone) in organic systems. Soil testing will
help determine the need for lime and other rock mineral
powders, such as rock phosphate. Hard-rock phosphate
varies considerably in soil reactivity while soft-rock or
colloidal phosphate has greater applicability. Gypsum
is used on many organic farms to supply calcium and
sulfur, especially on high-pH and sodic soils. Elemental
sulfur can be used to lower pH so growers should test for
this, especially when using compost every year. There are
several organic-compliant commercial fertilizers and soil
amendments that can be used for supplemental potassium,
including sulfate of potash-magnesia (e.g., Sul-Po-Mag®)
and naturally mined potassium sulfate, but all must be
approved by a certification agency before application.
Research conducted through the Iowa State University
Long-Term Agroecological Research (LTAR) experiment
(Figure 2) has demonstrated excellent organic corn yields
in the range of 120-209 bushels/acre when rotations with
soil-building legumes preceded corn crops. Soil quality has
remained high in these systems even with multiple tillage
operations. High yields have been achieved by preceding
organic corn crops with legumes, such as alfalfa, and
composted manure applications. Many organic farmers
seek optimal yields, based on the limits of their farm’s
internal resources, as opposed to maximum yields which
are achieved through external inputs. Vegetable crops
strictly relying on crop rotations, or cover crop residues,
usually require additional compost applications to equal
conventional yields. A systems experiment from 19982003 comparing organic and conventional bell pepper
production demonstrated similar growth and yield of
conventional and organic crops, when 100 pounds/acre N
was applied as compost. Using a rotation of hairy vetch/rye
preceding the pepper crop, without compost additions,
resulted in reduced pepper yields compared to
conventional yields 50 percent of the time. Soil analysis
revealed higher N in plots where cover crops were tilled
compared to strip-tilled plots, leading to recommendations
for side-dressing N in strip-tilled fields. Thus, most organic
growers use a combination of crop rotations, cover crops,
and compost to achieve the highest yields.
Figure 2. Overview of the 44 fields of the Long-Term
Agroecological Research (LTAR) experiment, which
examines biological and economic outcomes from five crops
in four crop rotations over time, at the ISU Neely-Kinyon
Farm, Greenfield, Iowa.
EFFECTS OF CROP ROTATIONS ON WATER QUALITY
The issue of water contamination from excess N
applications has become increasingly critical for the
future of farming. A significant proportion of the
NO3-N in the Mississippi River comes from agricultural
land in the Midwest, and high nitrate levels in waters
from agricultural lands flowing into a municipal water
plant is the subject of a 2015 lawsuit in Iowa. Relying
primarily on crop rotations (legumes) to supply N to
the vegetable crop can assist with alleviating potential
leaching problems associated with excess N applications,
even those from manure sources. Using composted
manure, which is in a more stable organic form, will
Cover Rotations, Composting and Cover Crops for Organic Vegetable Production
3
help reduce leaching loss compared with fresh manure
or synthetic nitrogen. A study in Ames, Iowa, has shown
that using longer rotations of corn-soybean-oats/alfalfaalfalfa can help reduce nitrate leaching by 50 percent
compared to a conventional corn-soybean rotation.
Another study evaluating nitrate-N leaching in an organic
vegetable system amended with composted poultry litter
demonstrated that nitrate-N concentrations in lysimeter
leachates were generally below 10 ppm during 52 months
of monitoring. Thus, an ongoing challenge for organic
growers is to synchronize nutrient release from various
crop residues and amendments with crop needs, which
in turn will help reduce both N2O emissions and NO3-N
leaching.
fields, can be avoided through the use of quality seed, crop
rotation (especially away from previously infected fields)
and planting when soils are warm (above 50° F) to ensure
quick germination.
CROP ROTATIONS FOR DISEASE
PEST MANAGEMENT
Vegetable growers around the world are cognizant of the
need for separating or rotating fields of vegetable crops
of the same family in order to avoid soil born diseases
prevalent in one particular family (e.g., a 3-year rotation
to avoid Verticillium wilt in Solanaceae crops). In organic
systems, this separation is even more critical, as synthetic
fungicides are not allowed and prevention is the main tool
utilized for disease management. Typically, organic farms
maintain greater spatial and temporal diversity of crops
than conventional counterparts, as green manure and
perennial legume crops, in addition to vegetable crops, are
often part of the OSP.
CROP ROTATIONS FOR INSECT
PEST MANAGEMENT
Habitat diversification, which includes rotations of crop
types across space and time, has been recommended as
a strategy to enhance biological control and subsequent
insect pest reduction, either through resource provisioning
for natural enemies or spatial interference from a mixture
of host-crop and non-host-crop species. As an example,
corn rootworms (Northern and Western types of
Diabrotica spp.) are not generally problematic on organic
farms where 3-4 year crop rotations are practiced. There
are also many natural enemies of prominent lepidopteran
pests in vegetable systems, including predators that feed
on eggs and larvae, such as lady beetles, lacewings, bigeyed
bugs (Geocorus spp.), damsel bugs (Nabis spp.), minute
pirate bugs, and others. The most significant parasitic
wasps against European corn borer are Macrocentris
grandii, a braconid larval parasite, and Trichogramma
ostriniae, an egg parasite. Pathogens of corn borer include
Nosema pyrausta and Beauveria bassiana. A diverse habitat
has been found to support natural enemies through
provisioning of nectar, pollen, and insect pest (host)
sources, as some host must be maintained for natural
enemy survival.
Other recommendations include a 3-4 year rotation to
avoid blackleg in brassicas, and up to 20 years for white
rot in allium crops. Variety selection should include
cultivars designated as V, F, and N, which signifies
resistance to Verticillium wilt, Fusarium wilt, and
pathogenic nematodes. In a properly rotated organic
field of the most resistant or tolerant vegetable cultivars
available, planting at the proper time to permit quick
germination and growth will generally keep disease
and insect pests below economic injury levels. Because
disease inocula can survive on infected crop residue,
crop rotation can break the disease triangle (pathogen–
host–environment) by changing to a non-host that does
not support the growth of that particular pathogen. As
with fertility regimes, a systems approach, including
crop rotation and tillage, can limit continued spread of
pathogenic organisms. Seed-corn maggots, the legless
fly larvae that attack corn seeds particularly in cool, wet
CROP ROTATIONS FOR WEED MANAGEMENT
In areas where soil fertility is adequate, weeds are
considered the greatest constraint in organic vegetable
production. Weeds generally occupy the same ecological
niche as the annual or perennial crop plants where they
grow and thus can be reduced through crop rotations
utilizing crops with different life cycles and management
requirements, such as deep-rooted, perennial legumes with
annual, shallow-rooted vegetable crops. Cultivar selection
can also impact crop competitiveness over weed species,
as quick-germinating, taller, and leafier plants tend to be
more competitive in their resource utilization. Longer
crop rotations (three years or more) have been found to
be instrumental in disrupting weed establishment and
growth. In a study in Greenfield, Iowa, the shorter twoyear organic rotation of soybean-wheat had, on average,
two to three times the weed population of the three and
four-year rotations of grain crops with oats/alfalfa.
4
Cover Rotations, Composting and Cover Crops for Organic Vegetable Production
RYE COVER CROP
With the focus in organic crop production on weed
prevention, establishment and growth of weed seeds
can be greatly managed through both crop rotations and
allelopathic cover crops. Rye is particularly important
in crop rotations in helping prevent weed proliferation
through its allelopathic properties. Weed reductions of
as high as 99 percent were observed for lambsquarter
when soybeans and sunflowers were planted into
killed rye compared to tilled plots with no mulch. In
Iowa, weed populations in organic tomato plots with a
rolled or crimped hairy vetch/rye mulch were lower or
statistically equivalent to tilled plots with no mulch. Other
fast-growing, high-biomass cover crops, like sorghumsudangrass and sunnhemp (warmer climates) can provide
excellent weed control when used in rotation with
vegetable crops, and are particularly useful when rotating
out of sod crops like bermudagrass or bahia grass.
ECONOMIC CONSIDERATIONS
OF CROP ROTATIONS
Both farm and field considerations are involved in
determining the economic viability of specific crop
rotations. The balance between financial and biological
considerations should be considered before long-range
crop rotation plans are established. This includes both
short-term (annual) and long-term (multi-year) farm
management decisions. Several factors can often override
rotation plans, including weather, market opportunities,
and crop failure. Organic certification agencies have the
authority to grant variances in crop rotation plans when
unforeseeable events, such as an extended cold and rainy
season cause germination to fail in spring vegetable crops,
leading to a summer crop planting instead. Economic
decisions are often based on growing the most profitable
CHECK FOR PRESENCE OF N-FIXING
NODULES IN LEGUME COVER CROPS
crop for the area, such as heirloom tomatoes, which can
return $547.21 over all costs for a 4 x 100-foot bed. Not
rotating tomato crops, however, can be detrimental to
the long-term viability of the farm if diseases, such as
late blight, severely affect yields and profits. Growing
3-6 signature vegetables which provide the main income
for the farm is recommended while also rotating other
less-profitable vegetable crops and non-vegetable crops
which are useful for the ecosystem services they provide
like N fertilization and other nutrients, beneficial insect
habitat, weed management, and potential mitigation of
greenhouse gases. Economic analysis shows higher returns
in longer crop rotations that include grain crops and
legumes compared to a two-crop rotation, and a general
equivalency between vegetable crops grown with cover
crops in rotation and those without cover crops. When
crops like hairy vetch/rye are grown strictly for soilbuilding purposes and nothing from this crop is marketed
off the farm, the cost of cover crop seed must be off-set by
the additional gain in yield or, ideally, “green payments”
for their carbon sequestration benefits. The soil-building
properties of these cover crops and other benefits they
provide to the whole farm, however, can be considered a
type of bank to support long-term farm viability.
Organic Fertility Sources
Manure and Composted Manure
In addition to crop rotations, organic farmers rely on
animal manure or compost to provide additional soil
fertility. Proper use of manure and compost in cropping
systems is essential from both a crop production and
environmental standpoint. According to USDA–NOP
rules, raw manure cannot be put on vegetable crop
fields unless it is properly composted or applied at least
Cover Rotations, Composting and Cover Crops for Organic Vegetable Production
5
four months before vegetable harvest. Raw manure and
compost should be applied after the soil has warmed in the
spring so microbial activity can immediately commence,
as opposed to applying to frozen or cold ground where
manure can run off more easily, potentially causing water
pollution. Incorporation is always the best strategy for
the most effective use of these amendments, but if time
or resources permit only a broadcast application, it is
even more essential to wait at least four weeks before
planting. Manure that is applied too soon before planting
(at least two weeks with incorporation) can burn plant
tissue and increase problems with insect pests, like seedcorn maggots. Compost that is not properly cured can
detrimentally affect plant growth by tying up nitrogen
and producing harmful compounds that can stunt plant
growth or even kill sensitive plant species. Applying very
low rates can lead to nutrient deficiency and reduced
yields. On the other hand, excessive applications can lead
to nitrate leaching, phosphorus runoff, and excessive
vegetative growth of some crops. Thus, understanding the
process of composting, compost nutrient composition and
availability, and proper handling techniques is important
when utilizing compost or manure as a major source of
nutrients.
COMPOST USE IN ORGANIC
VEGETABLE PRODUCTION
Organic systems heavily rely on organic matter based
amendments such as manure and compost to meet crop
nutrient demand. Composts and manures are applied
to agricultural lands as a source of essential microbial
populations, as plant nutrients, and as a source of organic
matter. Composts have also been successfully used in
organic vegetable transplant production. Organic fertility
amendments such as compost and manure have been
known to improve the physical, chemical, and biological
properties of soil and produce yields equivalent to
conventional cropping systems. A 2012 study proved that
compost amendments in organic vegetable production
systems can result in increased soil microbial biomass
and enhanced microbial diversity. Soil fertility and crop
nutrient management standards are set forth by the USDANOP and regulate practices required for management
and application of plant and animal materials in organic
production. Compost made in accordance with NOP rules
may be applied in organic production systems without
restriction on the time interval between application and
crop harvest. Because many producers do not have the
equipment, time, or resources to follow the rigorous NOP
6
COMPOST PILES
rules, they opt to follow the raw manure rule and apply their
compost at least four months ahead of vegetable harvest.
Numerous studies have shown benefits of using compost
in organic vegetable production systems. Studies
conducted on organic pepper production have shown
better growth and yields in compost-based organic
fertilizer treatments than their conventional counterparts.
Similarly, a study conducted in organic cucumber
production showed enhanced crop growth and higher
marketable yields in compost amended soils. In another
study, yields of peppers grown with dairy leaf compost
produced similar yields as conventionally grown peppers.
After three years of compost additions, yields of three
Spanish onion cultivars from the compost plots were
significantly greater than un-amended plots. Compost
application rates in organic vegetable production systems
vary depending upon the N content of the compost, N
demands for the crop, and soil test results. Although
some research has shown certain composts deliver only
10-25 percent of compost N as plant-available during the
first year of application, because estimates for P and K
availability in the first year are 40 percent and 60 percent,
respectively, most growers apply based on total N rates
in the compost to avoid problems of excess phosphorus
pollution.
LAB TEST TO GET ACCURATE RATES
Obtaining an accurate soil, compost, or manure nutrient
test from a reputable lab is essential in determining
application rates for crops (Table 1). An example analysis
with rates used for organic vegetable crops is a compost
composed of poultry manure and organic straw that
had an analysis of 3 percent nitrogen (3-2-3 N-P-K). In
order to achieve a rate of 100 pounds of nitrogen per acre
(100 pounds N/acre), a recommended amount if soil is
relatively fertile as most Iowa soils are, would require
applying 3,333 pounds/acre of the compost. While this
number may seem large, when such an amount is applied,
Cover Rotations, Composting and Cover Crops for Organic Vegetable Production
the compost itself on the fields may be imperceptible, often
ranging from only a quarter-inch to half-inch in depth.
Table 1. Labs for testing Iowa soils
A & L Labs
111 Linn St, Atlantic, IA 50022
712-243-6933
www.alheartland.com/Html/Contact_us.html
Iowa State University Plant and Soils Lab
Department of Agronomy – Kerry Culp
2104 Agronomy Hall, Ames, IA 50011
515-294-1360
http://soiltesting.agron.iastate.edu
Midwest Laboratories
335 4th Ave SW, Plainview, MN 55964
507-273-3339
www.midwestlabs.com
Woods End Research Laboratory
PO Box 297, Mt Vernon, ME 04352
207-293-2457
https://woodsend.org
THE COMPOSTING PROCESS
Composting can be defined as the decomposition of
organic matter into a humus-like substance and minerals
by the action of microorganisms under aerobic conditions,
combined with chemical and physical reactions.
Composting is predominantly an aerobic or oxygenrequiring process in which microorganisms consume
oxygen while feeding on organic matter. In doing so they
produce carbon dioxide, water, heat, and miscellaneous
gaseous byproducts to create compost (Figure 3).
Composting stabilizes the nutrient content of manure
and other organic materials and releases nutrients slowly,
minimizing nutrient loss and potential environmental
contamination. Depending on the raw material used,
the time required to produce a mature batch of compost
in Iowa could range from 6-8 months. To be successful,
the composting process must be carefully managed;
from the mixing of the initial ingredients through the
high temperature phase to the maturation phase when
the compost is deemed ready for use. Preparation of
high quality compost requires appropriate raw materials,
proper temperature and moisture management, and an
understanding of the science behind the composting process.
The composting process begins when the appropriate raw
materials and water are mixed and brought together in
a pile. In the presence of oxygen, microorganisms begin
to decompose the organic matter in the pile. The major
group of microorganisms that participate in composting
are bacteria, fungi, and actinomycetes. Bacteria tend to
flourish during the early stages of composting, with higher
populations of mesophilic bacteria active in the 95-115°F
range, whose activity and energy raises the temperature
of the compost pile. At 115°F the activity of thermophilic
bacteria (heat loving bacteria) increases the temperature
of the pile to 149-158°F. As oxygen levels are depleted
microbial activity and temperature decreases. Compost
piles must be turned and mixed to bring in oxygen and
restart the decomposition cycle. After successive agitations
and depletion of easily degradable compounds, bacterial
activity and population decreases. The compost enters
a final maturation phase and is inhabited by mesophilic
bacteria, fungi and actinomycetes feeding on resistant
organic materials that remain in the pile. The final phase
of composting is the curing phase where the compost no
longer reheats after agitation. During the curing phase,
microorganisms, protozoa, worms, insects, and other
large organisms that feed on microorganisms and organic
matter colonize the pile. The concentration of nitrate N
also increases as the compost pile cools down and enters
this phase. The curing time for compost varies based
on the length of the active composting phase which in
turn depends on raw material, composting conditions,
and management of the pile. The recommended time for
curing is around 30 days; a longer period is necessary if
active composting was not completed.
COMPOSTING METHODS
Depending on how the compost pile is handled,
composting can be broadly classified into three methods:
windrow, passive, and aerated static pile composting.
There are other methods used commercially, but these
three methods are common on organic vegetable
production farms. Windrow composting is the most
common of the three and in this method, the mixture of
raw material is placed in long narrow piles or windrows
that are turned on a regular basis. Typically, windrow
height ranges from 12-15 feet for fluffy material such
as leaves and from 3-6 feet for dense material such as
manure. It is important to maintain the correct height for
the windrow as large windrows can develop anaerobic
zones near the center and can lose heat quickly and are
not able to maintain the high temperatures needed to
evaporate moisture and kill harmful pathogens if too
small. Windrow width varies from 12-18 feet and should
be turned or mixed on a periodic basis to provide oxygen
Cover Rotations, Composting and Cover Crops for Organic Vegetable Production
7
CO2
HEAT
WATER
NITROGEN (NH3)
ORGANIC MATTER
[carbon and nitrogen]
WATER
OXYGEN
MICROORGANISM
ACTIVITY
COMPOST
COMPOST PILE
Figure 3. Schematic of composting process
throughout the pile to help rebuild the pore space in the
pile that is lost due to decomposition and settling of the
organic material, while releasing trapped heat, water vapor,
and gases. Turning also distributes water, nutrients, and
microorganisms throughout the windrow.
Aeration of the windrow occurs at the time of turning and
by diffusion, wind, and convection between turnings. The
number of times a windrow pile is turned is determined
by many factors, including pile temperature, moisture
content, and porosity of the pile. For certified organic
production, the NOP stipulates that temperatures must be
maintained between 130 and 170°F for 15 days, during
which the materials must be turned a minimum of five
times. A common strategy is to turn the windrow when
the interior temperature falls below 122°F, resulting in
turning every 2-3 days for the first 2-3 weeks, followed by
weekly turnings for another 6-8 weeks. Growers often use
thermometers with a long stem to measure temperatures
at 50-75 foot intervals along the windrow. Turning the
windrow on small to moderate scale farms usually is done
with a front-end loader or a bucket loader. Specialized
equipment such as tractor-assisted windrow turners can
also be used as they are highly efficient and save time.
Many organic farms use the windrow composting method,
as it easily accommodates a wide range of feedstock,
equipment, farm size, and management strategies.
In passive composting, organic materials are placed in a
pile and left for extended periods of time to decompose.
Aeration, which is a critical factor for the composting
process, occurs passively by diffusion, natural air
movement, and thermal convection. Without active
8
aeration, a passive compost pile will take much longer
than a windrow pile to mature. As the temperature of the
pile rises, gases in the pile also heat and rise, creating a
vacuum in the pile that results in the movement of air
from the surrounding area. The moisture content of such
a pile may exceed levels required to maintain a porous
structure in the pile, which may lead to low temperature,
slow decomposition, and release of malodorous gases,
including hydrogen sulfide, from the pile. Passive
composting is not approved for certified organic
production. To meet NOP requirements, piles that are
passively composted must be aerated to sustain microbial
activity and adequate temperatures.
Growers often install perforated pipes at the base of the
pile and sometimes will install fans or blowers to force air
through the pile. This is the aerated static pile method.
The base of the aerated static compost pile is composed
of wood chips or chopped straw to provide porosity to
the pile. Underneath the base is a perforated aeration
pipe that provides oxygen and removes water vapor,
carbon dioxide, and other products of decomposition. No
turning or agitation of the pile occurs after the pile has
been set up. Growers also cover the static pile with a layer
of finished compost, straw, wood chips, or a breathable
compost cover to insulate the pile and retain heat. The
feedstock material for the compost pile needs to be well
mixed before being placed on the pile as there is no further
turning or agitation of the pile. Aerated static piles range
in height from 6-12 feet. The length of the pile depends
on the efficiency and uniformity of air distribution of the
pipe and ducts. Typically, the length of the pile ranges
from 90-200 feet. One of the advantages of this method
Cover Rotations, Composting and Cover Crops for Organic Vegetable Production
over the windrow method is that it requires less area and
the efficiency of aeration is higher, although uniform
distribution of forced air largely depends on the porosity of
the pile and how well feedstocks are mixed. This method,
if done properly, can produce high quality compost in 6-8
weeks.
RAW MATERIALS
Raw materials or feedstocks used in composting are
generally organic byproducts or waste materials. Some
commonly used raw materials for composting include
animal manure, crop residues, straw bedding, and food
wastes. The elemental composition of the final compost
largely depends on the chemical composition of the
feedstock. Of all the elements, the two most important
elements are carbon (C) and nitrogen (N). During
the composting process, the N concentration directly
influences microbial population growth, while C serves
as the energy source. The most important aspect of the
feedstock is the carbon to nitrogen ratio. Higher C:N ratios
in raw materials (more than 40:1) can immobilize nitrogen
and slow the composting process. Lower ratios lead to the
loss of N as ammonia, although higher and lower ratios are
debatable. The most accepted and agreed upon C:N ratio
of the feedstock is between 25 and 35. Table 2 outlines
C:N ratios of commonly used feedstocks for composting.
Table 2. C:N ratios of common composting materials
Raw material
Crop residue and
fruit/vegetable
Coffee grounds
Corn stalks
Cull potatoes
Fruit wastes
Hay
Straw
Vegetable wastes
Wood and yard waste
Bark-hardwood
Bark-softwood
Grass clippings
Leaves
Wood chips
C:N Ratio Raw material
C:N Ratio
Manures
20:1
60-73:1
18:1
18-49:1
15-32:1
48-150:1
11-13:1
Broiler litter
Dairy manure
Horse manure
Poultry
Sheep
Swine
14:1
19:1
30:1
6:1
16:1
14:1
223:1
496:1
17:1
54:1
600:1
Source: Rynk et al. 1992. On-Farm Composting Handbook, NRAES 54
In addition to considering the C:N ratio of the feedstock,
the biodegradability and bioavailability of organic
materials in the compost will depend on the form in which
carbon exists in the pile. Feedstocks also impact the odor
potential of the pile through presence of odorous raw
materials, ammonia generated during composting, and
anaerobic conditions within the compost pile. Feedstocks
that have higher odor potential include fish wastes, swine
manure, and other forms of liquid manure. Materials such
as crop residue, leaves, and sawdust present little or no
odor issues. A good mix of feedstock, appropriate moisture
content, and frequent turning or agitation reduce odor
problems. In aerated static piles, lining the surface with
peat moss or finished compost helps to trap odor-forming
gases.
APPLY COMPOST AFTER STABILIZATION
Compost is considered stabilized when the temperature
of the pile does not rise, even after turning, and subsides
to near ambient levels. Fully mature compost is well
decomposed, stable, and has an earthy smell (Table 3).
During the curing phase, the C:N ratio decreases, the
pH of the pile shifts toward neutral, and conversion of
ammonium-N (NH4-N) to nitrate-N (NO3-N) occurs.
Plants absorb both forms of nitrogen, but higher
concentrations of ammonium can cause temporary
stunting and burning of foliage in susceptible young
plants. Vegetable crops absorb most of their N in the
nitrate form although in their younger stages they will
absorb the ammonia form as well. Fully mature compost
has gone through the curing phase and contains more
of the nitrate than ammonia form. Although not a
rigorous assessment, growers often test maturity of the
compost based on its color and odor. A pile that is not
fully composted usually smells foul and is considered
immature. Measuring the electrical conductivity (EC),
soluble salt concentration, and pH is another tool
growers often use to assess compost maturity. The pH
range for most finished compost is between 6.0 and 8.0.
Similar to pH, EC largely depends on the feedstock used.
Compost with high salt concentrations can affect seed
germination and stunt root growth. Usually, compost
with EC higher than 3 dS/m is considered phytotoxic for
seed germination. However, compost EC can be in the
8-10 dS/m range if the intended use of compost is soil
incorporation in the field. Additionally, certain vegetable
crops such as onions and beans are more susceptible to
higher salt concentration than others. Tests that measure
oxygen consumed and carbon dioxide released from a
finished compost pile can help determine the maturity
of the pile. The rate of compost respiration determined
over three days by carbon dioxide respiration at 98°F is a
standard method of measuring compost stability by the US
Composting Council. A rapid semi-quantitative test called
Cover Rotations, Composting and Cover Crops for Organic Vegetable Production
9
the Solvita® test involves the use of colorimetric pads
sensitive to carbon dioxide and ammonia placed into a jar
that contains a fixed volume of compost sample. The pads
are left in the jar for four hours where they absorb carbon
dioxide and ammonia and change color. The color change
on the pad surface is visually compared to a pre-calibrated
coded color chart.
Table 3. Suggested optimum qualities of compost for
on-farm use
Compost attribute
Optimum
Organic matter
Should range between 40-60%
C: N ratio
10-15:1
pH
6-8
Electrical conductivity
Below 10 dS/m
Phytotoxicity
Seed germination > 85%
Weeds
No or few seeds
Source: Cooperland 2002
Cover Crops
One practice that has been routinely used on organic
farms, and is now gaining traction on conventional farms,
is cover crops. Cover crops can have a profound impact
on soil health, as they add soil organic matter, enhance
soil structure and fertility, improve water-holding capacity,
suppress weeds, and reduce soil erosion. Cover crops help
support diverse and active soil biotic communities that
serve as a foundation for agricultural sustainability.
NITROGEN FIXATION
Legume cover crops, in addition to adding organic matter,
add nitrogen to the soil by fixing atmospheric nitrogen
through a symbiotic relationship with soil bacteria
(Rhizobium sp.) The bacteria, living in legume plant roots,
absorbs nitrogen from the air and transforms it into plantavailable forms. The amount of nitrogen contributed by
legumes varies by species. There are specific species of
bacteria that form symbiotic relationship with individual
legume cover crop species. Growers should inoculate
legume seeds with the proper nitrogen-fixing bacteria
strain to improve nitrogen fixation. The cost for the
inoculum packet is $5-10 and can usually treat 50 pounds
of seeds. Research has shown significant increases in cover
crop biomass and nitrogen-fixing potential in inoculated
legume cover crop systems. Legume-based cover crops,
such as hairy vetch and field pea, are best used before high
nitrogen-demanding crops, like corn and vegetables, with
rye being the best before N-fixers like soybeans. In warmer
10
climates like Florida, sun hemp is used extensively as a
cover crop.
WEED SUPPRESSION
Cover crops can be used to manage weeds in vegetable
production systems by reducing weed germination and
establishment by competing or producing allelochemicals
which suppress weed seed germination. Cover crops can
influence weeds either in the form of living plants or as
plant residue remaining after the cover crop is killed.
Cover crops such as cereal grins and grasses establish
quickly in the fall, cover the soil, and grow throughout the
winter, thereby suppressing fall and winter weeds. Smallseeded legumes that are seeded alone in the fall are not
a good choice for weed suppression as they grow slowly
during cold weather and weeds may out-compete them.
SOIL EROSION AND WATER QUALITY
Most vegetable growers use cover crops as a strategy to
reduce soil erosion in the fall and early spring. A cover
crop provides vegetative cover during periods when a
vegetable crop is not present and reduces the impact of
falling raindrops, which otherwise would detach soil
particles and increase erosion. They also slow the rate
of runoff, thus improving moisture infiltration into the
soil. No-tillage (commonly called no-till) and other
conservation tillage practices combined with cover crops
have shown to significantly reduce runoff and soil erosion
losses. Cover crops have also shown to improve water
quality by suppressing nitrate leaching. Research has
found that nitrate leaching was reduced by 50 percent in
plots having a rolled cover crop of cereal rye and hairy
vetch in organic broccoli and pepper production compared
to tilled cover crop plots of the same plants.
ORGANIC NO-TILL
The concept of no-till or reduced tillage has been proven to
provide multiple environmental benefits on conventional
farms, particularly in the area of soil conservation, while
also reducing machinery, labor, and fuel costs. On organic
farms, no-till systems had been constrained by the
prohibition of herbicides to terminate cover crops. The
Rodale Institute (RI) began investigating using a roller/
crimper in 2004 to crush cover crops in lieu of herbicide
termination to comply with organic rules. The cover crop
(typically rye alone, or a rye and hairy vetch combination)
is planted in the fall and then terminated with a roller/
crimper when the grass crop reaches anthesis (pollen shed).
Cover Rotations, Composting and Cover Crops for Organic Vegetable Production
ROLLING HAIRY VETCH AND RYE
anthesis, which has become more difficult in recent years
with global climate change creating cooler, wetter springs
that slows cover crop growth.
TRANSPLANTING IN NO-TILL
Weather conditions are extremely important in achieving
success with organic no-till. When spring and summer
rains fall evenly and adequately organic no-till soybean
yields ranged from 37-45 bushels per acre in Iowa,
which is an excellent organic tofu-variety soybean
yield, especially considering no post-planting tillage
was performed to manage weeds. Soil scientists at the
USDA-ARS in Ames, Iowa, have documented increases in
soil carbon and microbial biomass carbon and nitrogen
in the organic no-till system compared to the normal,
tilled organic system. The challenge remains to balance
improving soil quality while maintaining optimal yields.
Some farmers are experimenting with drilling organic
no-till soybeans on 7-inch rows, as opposed to normal
30-inch rows. The concern with moving to 7-inch rows
was the limited ability to perform any “rescue” cultivating
if needed.
The reliance on irrigation in vegetable systems can help
ameliorate unfavorable weather conditions during the
crucial period when cover crops are decomposing and
the cash crop needs additional moisture. Yields in the
organic no-till vegetable systems studied in Iowa (broccoli,
tomatoes, peppers, and lettuce) have been competitive
with conventional yields when sufficient biomass is
produced by the cover crop (planting at 1.5-2 bushels/
acre is best) and moisture levels are kept adequate through
irrigation. Sweet corn, however, performs best in tilled
systems, which was also found for field corn. Ideal results
occur when the cover crop can be crushed early in the
spring (before May 15) when the rye reaches, or is past
Cambardella has also found interesting results with the
no-till organic systems sequestering more soil carbon than
tilled plots. Nitrate leaching is also reduced in the cover
crop-based systems compared to completely tilled plots.
In Florida’s sandy soils, no-till summer squash yields were
equal or greater than tilled yields, with no significant
difference between no-till and plastic mulch. In warmer
climates, organic no-till holds the most promise, because
of the potential for early cover crop planting, continuous
cover crop growth over the winter months, and earlier
termination dates in the spring. Sandier soils also seem
to be more amenable to organic no-till, as has been
demonstrated in both Florida and Pennsylvania where
even no-till corn yields were high. A mechanical issue
(e.g., the ability of the roller/crimper to sink deeper into
the soil and crush the cover crop) may be the mechanism
here.
Conclusion
Crop rotations, composting, and cover crops can provide
numerous horticultural and ecological benefits in
vegetable production systems. A system’s approach to
production is necessary to identify and understand the
significance of the linkages between grower practices and
their implications for crop growth, productivity, and the
environment. Examples provided here are a starting point
and can be modified to fit grower needs, resources, and
Organic System Plans.
Prepared by Kathleen Delate, professor and extension
organic specialist in horticulture and agronomy with Iowa
State University; and Ajay Nair, assistant professor of
horticulture and extension vegetable production specialist
with Iowa State University.
Photo credits: Kathleen Delate and Ajay Nair.
Cover Rotations, Composting and Cover Crops for Organic Vegetable Production
11
Acknowledgements
This work was supported by the USDA-Organic Transitions
Program and Iowa State University College of Agriculture and
Life Sciences.
References
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Delate, K. H. Friedrich and V. Lawson. 2003. Organic pepper
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Copyright
This Iowa State University Extension and Outreach
publication is adapted from a chapter in the textbook
‘Organic Farming for Sustainable Agriculture’, Springer
International Publishing AG, Cham, Switzerland, copyright
2016.
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Cover Rotations, Composting and Cover Crops for Organic Vegetable Production