From: london@sunSITE.unc.edu (Larry London)
Subject: News from ATTRA: Cover Crops
Organization: University of North Carolina, Chapel Hill
Date: Thu, 19 Nov 1992 04:33:40 GMT

APPROPRIATE TECHNOLOGY TRANSFER FOR RURAL AREAS, P.O. Box 3657,
Fayetteville, AR  72702; 1-800-346-9140


          ATTRA Summary:  Cover Crops and Green Manures


     The practices of cover cropping and green manuring can serve
a number of important functions in cropping systems.  Any field or
forage crop grown to provide soil cover is a "cover crop."  "Green
manures" are crops tilled into the soil while green, or soon after
flowering, to improve the soil.  Since a crop grown as a cover
crop may later be soil-incorporated as a green manure, the two
practices are often referred to interchangeably.

     Cover crops are grown mainly to prevent erosion of soil by
wind and water.  But cover cropping and green manuring practices
perform other important functions in cropping systems which:

     - improve soil quality and moisture-retention capacity
     - add fertility and cycle plant nutrients
     - aid in pest management

     The types of crops that can be managed as cover crops and
green manures include annual, biennial, or perennial herbaceous
plants.  They can be legumes, such as clovers, vetches, and field
peas and beans, or non-legumes like rye, buckwheat, and
sorghum-sudangrass.  Legumes that fix nitrogen are often selected
as cover crops and especially as green manures to obtain the
benefit of added nitrogen for subsequent crops.

     Cover crops and green manures may be included in cropping
systems before or after a primary or "cash" crop.  They may also
be included during non-crop periods -- in some periods of
traditional fallow, for instance, or included as a regular step in
a cropping rotation.  In some applications, cover crops are grown
together with primary crops through forms of intercropping.  Cover
cropping and green manuring practices are used in a wide variety
of farming systems.  In many cases, they play a fundamental role
in farming systems using or making a transition toward sustainable
farming methods.


Principal Applications

     The principal applications described below show the varied
ways cover crops and green manures are used in cropping systems:

     1)  "Winter Cover Crops" are planted in late summer or in the
fall to protect soils over winter.  They must be cold tolerant. 
Legumes are often chosen in order to gain fixed nitrogen.  Some
applications combine a cool-season legume in a mixed stand with a
winter cereal grain.

     2)  "Summer Green Manures" are warm-season crops which
provide soil cover for part of the summer before being tilled in. 
They may be grown to fill a rotation interval, to improve poor
soils, or to prepare land for a perennial crop.  Summer green
manures smother weeds, provide organic matter, improve soil tilth,
and -- in the case of legumes -- add soil nitrogen.

     3)  "Living Mulches" are cover crops intercropped with annual
or perennial cash crops to suppress weeds and to reduce erosion,
enhance fertility, and improve water infiltration of soils.  In
annual cropping systems, a legume might be overseeded as a living
mulch into a grain crop.  Or, a legume may grown as a planting bed
for vegetables that are transplanted into tilled or chemically
killed strips in the legume stand.  In perennial systems, sods of
grasses, legumes, or grass-legume mixes are interplanted as living
mulches between rows of berries or orchard trees.

     4)  "Catch Crops" are cover crops established to retain
nutrients in cropping systems by reducing nutrient leaching from
the soil following the harvest of primary crops.  Catch crops may
also double as winter cover crops.  Short-term cover crops in a
crop rotation are also commonly called "catch crops."

     5)  "Forage Crops" can function as cover crops while grown
for grazing or haying, and then as green manures when later tilled
in.  A variety of forage-legume or legume-grass sods can fill this
dual function.  Forage crops used as green manures benefit soils
most if allowed to accumulate topgrowth before being turned under.


Benefits

     Cover crops and green manures perform important soil
conservation functions in cropping systems.  They also contribute
much toward sustainable management of soil structure and moisture,
crop pests, and fertility.  Green manures in particular help fill
crop fertility needs directly as well as improve soil structure.

     In large-scale farming systems, cover cropping and green
manuring practices may be among the most effective ways to reduce
soil erosion.  Used to establish and cycle fertility on-farm, they
may also be the most practical means available to large-scale
operations for developing systems for sustainable fertility
management.  In small-scale, highly diversified, or intensively
managed operations, cover crops and green manures may be one of
several viable strategies for developing and cycling fertility
on-farm.


     Reduced soil erosion:

     Soil erosion is a major environmental concern facing U.S.
agriculture.  Cover crops and green manures reduce the
vulnerability of soils to erosion in many ways.  The foliage
canopies of cover crops directly block wind and rain erosion of
soil.  When used to maintain year-round soil cover, cover crops
are effective in curtailing the erosion of soil in a wide range of
farming systems.

     Cover crops that quickly produce canopies covering a high
percentage of ground area yield the greatest soil protection
benefit.  Most grass and non-legume cover crops, like rye and
buckwheat, fulfill this need well.  The soil cover provided by
living cover crops or by the mulch from cover crop residues can
improve water infiltration of soil and limit surface soil erosion
by rainwater.

     Cover crop root systems also limit soil erosion by actively
holding soil in place.  By adding organic matter, green manuring
reduces soil vulnerability to surface erosion by improving soil
structure and the capacity of soil to retain moisture.


     Soil improvement:

     Improving the physical structure of soil benefits field
preparation and growing conditions by enhancing soil tilth,
aeration, and water infiltration qualities.  Cover cropping or
green manuring practices for adding organic matter to soils can be
highly effective in improving soil structure.  By loosening and
aerating soils, cover crops with extensive root systems also help
remedy problems of soil compaction.

     Sod-forming grasses or grass-legume plant mixtures used as
cover and forage crops are excellent suppliers of organic matter
to soils in cropping systems.  Frequent cultivation of soils
accelerates organic matter decomposition.  As part of a crop
rotation, sods can help replenish organic matter lost from soils
due to annual cultivation.  To gain an increase in soil organic
matter, however, several years of an established sod may be
required.  When established for the long term, sod systems such as
native grass stands or mixed pastures can greatly improve soil
structure.

     Consistent green manuring in more-frequently tilled soils is
also an important way to replenish and maintain soil levels of
organic matter.  Whether from sods or green manures, organic
matter supplies to soil are instrumental in building and renewing
soil structure.  As soil microorganisms break down organic matter
from a freshly incorporated green manure, for example, compounds
such as gums and lignins that resist decomposition are formed. 
These compounds, plus other substances the microorganisms produce,
bind together soil particles as granules, or "aggregates," which
provide the basis for good soil structure.


     Fertility:

     Cover crops and green manures help supply and cycle plant
nutrients in cropping systems in a number of ways.  Nitrogen
accumulated and contributed by legumes plays a key role in
sustainable farm fertility management.  Amounts of nitrogen (N)
fixed and accumulated by legumes can range from 40 to 175 pounds
per acre.  When legumes are used as green manures, about 40 to 60
percent of the nitrogen (N) they accumulate in plant tissue is
usually available to a following crop.  In rotations, forage
legumes are valuable because they produce income from grazing or
haying in addition to N from shoot regrowth and root residues.

     The nitrogen available from legumes depends on the total
biomass produced and on the percentage of N in the plant tissue. 
Cultural and environmental conditions that limit legume growth --
such as delayed planting, poor establishment, or drought -- will
reduce the amount of N produced.

     In addition to the nitrogen legumes fix, legume and
non-legume cover crops accumulate and cycle phosphorus, potassium,
calcium, magnesium, sulfur, and other important plant nutrients on
the farm.  When cover crops tilled in as green manures decompose,
the nutrients accumulated are released and made available to other
crops.  Some cover crops, like buckwheat and sweetclover,
accumulate certain minerals in their tissues even when soil
availability of the minerals is low.  Deep-rooting cover crops
extract and translocate subsoil nutrients, making them accessible
in surface rooting zones to subsequent crops.

     Green manuring stimulates greater plant nutrient availability
indirectly, as well.  Acids produced as green manures decompose
react with insoluble minerals in surrounding soil material and
release these in usable form as exchangeable plant nutrients. 
Another product which results from the partial decay of plant and
animal materials in the soil is "humus."  Humus also enhances the
availability of plant nutrients.


     Water management:

     A key aspect of the soil conservation and improvement
functions of cover crops and green manures in cropping systems is
better management of water.  Cover crop canopies reduce moisture
evaporation from the soil surface.  In low- and no-till systems,
the mulch left from dead cover crop residues also limits
evaporation.  Soil moisture conserved through reduced evaporation
helps keep surface soil from crusting.  Less soil crusting in turn
helps maintain the capacity for water to infiltrate the soil
surface rather than be lost or cause erosion as run-off.

     By improving soil structure, organic matter supplied by green
manures and sod-forming cover crops further expands the capacity
of soil for water to infiltrate and be retained.  Finally, by
opening channels into subsoil or hardpans, the rooting action of
deep-rooted cover crops encourages water penetration to deeper
soil levels.  Together, all of these effects improve the
capacities of cropping systems to take advantage of moisture from
rain as well as irrigation water.


     Pest management:

     The primary pest management benefit of cover crops and green
manures is weed suppression.  Cover crops take up space and light,
thereby discouraging weeds that flourish on bare soil.  By
loosening soil, deep-rooting cover crops help reduce populations
of weeds that thrive in compacted soils.  In plantings of primary
crops, cover crops interplanted as living mulches function as
"smother crops" to suppress weeds by competing with them for
light, moisture, and nutrients.

     Along with living mulches, cover crops such as rye and other
small grains which exude natural toxins or "allelochemicals" are
gaining importance as alternative forms of weed suppression for
sustainable farming systems.  In no-till systems, the mulch that
results from mowing or chemically killing allelopathic cover crops
can significantly suppress weeds.  Rotating cover crops and green
manures with primary crops creates additional opportunities for
breaking pest life cycles.  Cover crops can also aid insect pest
management by serving as habitat for beneficial insects.


Selecting and Managing Cover Crops and Green Manures

     Some important factors to consider when selecting crop types
and varieties to use as cover crops and green manures are:

     1)  What cover cropping or green manuring use is planned?

     2)  What cover or green manure crops are best adapted to the
soil and climatic conditions (including rainfall) of the farm site
-- especially for the planned season of use?

     3)  Will irrigation be used?

     4)  How might the crops being considered interact with farm
pest cycles?

     Climatic conditions that affect available moisture are
important in determining when cover crops can be planted if no
irrigation will be used.  Soil and pest conditions, crop rotation
stage, labor availability, and other factors will also determine
the best time to plant.  Correct seeding rate and, in cultivated
systems, proper seedbed preparation are important to ensure good
establishment.  Because equipment and seeding methods used may be
different (e.g., broadcasting vs. drilling), seeding rates may
differ between large and small farm operations.  When legumes are
selected to supply nitrogen, the right rhizobial bacteria must be
sufficiently available in the soil or used to inoculate seed at
planting for nitrogen fixation to occur.

     Vegetation management is a basic requirement of using cover
crops and green manures.  In conservation tillage systems,
herbicides are the most common means of suppressing a cover crop
in preparation for the next stage in a crop cycle.  In general,
methods for managing cover crop vegetation range from conventional
herbicide use to non-chemical or alternative practices like
disking, mowing, rolling, and flaming.  Methods of vegetation
management may also vary between large- and small-scale farming
operations.


Limitations of Cover Crops and Green Manures

     Successfully including cover crops and green manures in a
cropping system involves planning for establishment and other
costs as well as for anticipated benefits.  Whether benefits
sought are immediate -- such as nitrogen from legumes, or
long-term -- such as sustained soil health, farm plans to include
cover crops and green manures should account for start-up and
periodic costs of seed, establishment, and cultural management. 
They should also account for any short- or long-term crop income
foregone by growing cover crops on acres that might otherwise be
used for cash crops.

     Cover crops and green manures add both biological diversity
and management complexity to a farm.  In general, increased
biological diversity can benefit farming systems in many ways. 
But some cover crops harbor pest communities that may pose
problems for certain crops or geographical locations.

     Managing a greater number and variety of crops also entails
increased farm cultural inputs, time, and expense.  Water consumed
by green manure crops can also be a concern.  However, even in
low-rainfall areas, native and adapted crop varieties may still be
viable replacements for cultivation or herbicides during summer
fallow.


For More Information

     This publication summarizes material from the ATTRA
Information Package:  "Cover Crops and Green Manures," prepared by
ATTRA Technical Specialists Steve Diver and Preston Sullivan.  To
request the full Package, or for help with a specific question,
call ATTRA toll-free at:  1-800-346-9140.


          ATTRA Summary:  Cover Crops and Green Manures

     Written by:  Holly Winger
     Editorial and Project Director:  Christine Rugen
     (PageMaker Version Design/Layout:  Jim Tracy)

     March 1992


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