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Cover Crops
Where no kind of manure is to be had, I think
the cultivation of lupines will be found the readiest and best
substitute. If they are sown about the middle of September in a
poor soil,
and then plowed in, they will answer as well as the best manure.
Columella, first century, Rome
Understanding the effect of cover crops on the soil
and the productivity of subsequent crops comes down to us from antiquity.
Chinese manuscripts indicate that the use of green manures is probably
3,000 years old. Green manures were also commonly used in ancient
Greece and Rome. There are three different terms used to describe
crops grown specifically to help maintain soil fertility and productivity
instead of for harvesting: green manures, cover crops, and catch
crops. The terms are sometimes used interchangeably and are best
thought of from the grower's perspective. A green manure crop
is usually grown to help maintain soil organic matter and increase
nitrogen availability. A cover crop is grown mainly to prevent
soil erosion by covering the ground with living vegetation and with
living roots that hold onto the soil. This, of course, is related
to managing soil organic matter, because the topsoil lost during
erosion contains the most organic matter of any soil layer. A catch
crop is grown to retrieve available nutrients still in the soil
following an economic crop and prevents nutrients leaching over
the winter.
Sometimes it's confusing to decide which term to use
green manure, cover crop, or catch crop. We usually have more than
one goal when we plant these crops during or after our main crop,
and plants grown for one of these purposes may also accomplish the
other two goals. The question of which term to use is not really
important, so in our discussion below the term cover crop will be
used.
Cover crops are usually incorporated into the soil
or killed on the surface before they are mature. (This is the origin
of the term green manure.) Since cover crop residues are usually
low in lignin content and high in nitrogen, they decompose rapidly
in the soil.
Effects of Cover Crops
The benefits from cover crops depend on the productivity
of the one that you're growing and how long it's left to grow before
the soil is prepared for the next crop. The more residue you return
to the soil, the better the effect on soil organic matter. The amount
of residue produced by the growth of a cover crop may be very small,
as little as half a ton of dry matter per acre. This adds some active
organic matter, but because most decomposes rapidly after it's killed,
there is no measurable effect on the total amount of organic matter
present. On the other hand, good production of hairy vetch or crimson
clover cover crops may yield 1½ to 2½ tons to over 4 tons
per acre. If a crop like cereal rye is grown to maturity, it can
produce 3 to 5 tons of residue.
A five-year experiment with clover in California showed
that cover crops increased organic matter in the top 2 inches from
1.3 to 2.6 percent and in the 2- to 6-inch layer from 1 to 1.2 percent.
Some researchers have found that cover crops do not seem to increase
soil organic matter. Low-growing cover crops that don't produce
much organic matter may not be able to counter the depleting effects
of some management practices, such as intensive tillage. Even if
they don't significantly increase organic matter levels, cover crops
help prevent erosion and add at least some residues that are readily
used by soil organisms.
Cover crops also supply nutrients to the following
crop, suppress weeds, and break pest cycles. Cover crops help maintain
high populations of mycorrhizal fungi spores, which helps improve
inoculation of the next crop. Their pollen and nectar are important
food sources for predatory mites and parasitic wasps, both important
for biological control of insect pests. A cover crop also provides
a good habitat for spiders, and these general insect feeders help
decrease pest populations. Use of cover crops in the Southeast has
reduced the incidence of thrips, bollworm, budworm, aphids, fall
armyworm, beet armyworm, and white flies. Living cover crop plants
and their residues also increase water infiltration into soil, thus
compensating for the water that cover crops use.
Selection of Cover Crops
Before growing cover crops, you need to ask yourself
some questions.
- Which type should you plant?
- When and how should you plant the crop?
- When should the crop be killed or incorporated into the soil?
When you select a cover crop, you should consider
what you want to accomplish, the soil conditions, and the climate.
- Is the main purpose to add available nitrogen to the soil
or to provide large amounts of organic residue?
- Is erosion control in the late fall and early spring your
primary objective?
- Is the soil very acidic and infertile, with low availability
of nutrients?
- Does the soil have a compaction problem? (Some species are
especially good for alleviating compaction.)
- Is weed suppression your main goal?
- Which species are best for your climate? (Some species are
more winter-hardy than others.)
- Will the climate and water-holding properties of your soil
cause a cover crop to use so much water that it harms the following
crop?
There are many types of plants that can be used as
cover crops, with legumes and grasses (including cereals) the most
extensively used. Leguminous crops are often very good cover crops.
Summer annual legumes, usually grown only during the summer, include
soybeans, peas, and beans. Winter annual legumes that are normally
planted in the fall and counted on to overwinter include berseem
clover, crimson clover, hairy vetch, and subterranean clover. Some,
like crimson clover, can only overwinter in regions with mild frost.
Hairy vetch, though, is able to withstand fairly severe winter weather.
Biennials and perennials include red clover, white clover, sweet
clover, and alfalfa. It should be noted that crops usually used
as winter annuals are sometimes grown as summer annuals in cold,
short-season regions. Also, summer annuals that are easily damaged
by frost, such as cowpeas, can be grown as a winter annual in the
Deep South.
One of the main reasons for selecting legumes as cover
crops is their ability to fix nitrogen from the atmosphere and add
it to the soil. Legumes such as hairy vetch or crimson clover that
produce a substantial amount of growth may supply over 100 pounds
of nitrogen per acre to the next crop. However, other legumes, such
as field peas, bigflower vetch, and red clover, may supply only
30 to 80 pounds of available nitrogen.
Nonleguminous crops used as cover crops include the
cereal grasses rye, wheat, oats, and barley, as well as other grass
family species, such as ryegrass. Other cover crops, like buckwheat,
rape, and turnips, are neither legumes nor grasses.
Some of the most important cover crops are discussed
below.
Legumes
If you grow a legume as a cover crop, don't forget
to inoculate seeds with the bacteria that live in the roots and
fix nitrogen. There are various types of rhizobial bacteria that
fix nitrogen. Some are specific to certain crops. There are different
strains for alfalfa, clovers, soybeans, beans, peas, vetch and cowpeas.
Unless you've recently grown a legume from the same general group
you are currently planting, consider mixing the seeds with the appropriate
commercial rhizobial inoculant before planting. The addition of
sugar water to the seed-inoculant mix helps the bacteria stick to
the seeds. Plant right away, so the bacteria don't dry out. Inoculums
are readily available only if they are commonly used in your region.
It's best to check with your seed supplier a few months before you
need the inoculant, so it can be special ordered, if necessary.
Inoculum Groups
red and white clovers
crimson and berseem clovers
alfalfa, sweet clover
pea, vetch, lentils
annual medics
cowpea, lespedeza |
Winter Annual Legumes
Berseem clover is an annual crop that is grown in
the South during the winter. Some newer varieties have done very
well in California, with "Multicut" outyielding "Bigbee."
It establishes easily and rapidly and develops a dense cover, making
it a good choice for weed suppression. It's also drought tolerant
and re-grows rapidly when mowed or grazed. Berseem is also grown
as a summer annual in the Northeast and Midwest.
Crimson clover is considered one of the best cover
crops for the southeastern United States. Where adapted, it grows
in the fall and winter, and matures more rapidly than most other
legumes. It also contributes a relatively large amount of nitrogen
to the following crop. Because it is not very winter-hardy, crimson
clover is not usually a good choice for the northern portions of
the South and further north. In northern regions, crimson clover
can be grown as a summer annual, but that prevents an economic crop
from growing during that field season. Varieties like "Chief,"
"Dixie," and "Kentucky Select" are somewhat
winter-hardy if established early enough before winter. Crimson
clover does not grow well on high pH (calcareous) or poorly drained
soils.
Hairy vetch is grown in the Southeast, but is winter-hardy
enough to grow well in the mid-Atlantic states and even in most
of the Northeast and Midwest. Where adapted, hairy vetch produces
a large amount of vegetation and fixes a significant amount of nitrogen,
contributing as much as 100 pounds of nitrogen per acre or more
to the next crop. Hairy vetch residues decompose rapidly and release
nitrogen more quickly than most other cover crops. This can be an
advantage when a rapidly growing, high-nitrogen-demand crop follows
hairy vetch. Hairy vetch will do better on sandy soils than many
other green manures, but needs good soil potassium levels to be
most productive.
Subterranean clover is a warm climate winter
annual that, in many situations, can complete its life cycle before
a summer crop is planted. When used this way, it doesn't need to
be suppressed or killed and does not compete with the summer crop.
If left undisturbed, it will naturally re-seed itself from the pods
that mature below-ground. Because it grows low to the ground and
does not tolerate much shading, it is not a good choice to interplant
with summer annual row crops.
Summer Annual Legumes
Cowpeas are native to central Africa and do well in
hot climates. The cowpea is, however, severely damaged by even a
mild frost. It is deep rooted and is able to do well under droughty
conditions. It usually does better on low-fertility soils than crimson
clover.
Soybeans, usually grown as an economic crop for their
oil and protein-rich seeds, also can serve as a summer cover crop.
They require a fertile soil for best growth. As with cowpeas, soybeans
are easily damaged by frost. Soybeans, if grown to maturity and
harvested for seed, do not add much in the way of lasting residues.
Biennial and Perennial Legumes
Alfalfa is a good choice for well-drained soils, near
neutral in pH, and high in fertility. The good soil conditions required
for the best growth of alfalfa make it a poor choice for problem
situations. Where adapted, it is usually grown in a rotation for
a number of years (see chapter 11). Alfalfa
is commonly interseeded with small grains, such as oats, wheat,
and barley, and it grows after the grain is harvested. The alfalfa
variety "Nitro" can be used as an annual cover crop because
it is not very winter-hardy and usually winter kills under northern
conditions. Nitro continues to fix nitrogen later into the fall
than winter-hardy varieties. However, it does not reliably winter
kill every year, and the small amounts of extra fall growth and
nitrogen fixation may not be worth the extra cost of the seed compared
with perennial varieties.
Crown vetch is only adapted to well-drained soils,
but can be grown under lower fertility conditions than alfalfa.
It has been used successfully for roadbank stabilization and is
able to provide permanent groundcover. Crown vetch has been tried
as an interseeded "living mulch," with only limited success
at providing nitrogen to corn. However, it is relatively easy to
suppress crownvetch with herbicides to reduce its competition with
corn.
Red clover is vigorous, shade tolerant, winter-hardy,
and can be established relatively easily. Red clover is commonly
interseeded with small grains. Because red clover starts growing
slowly, the competition between it and the small grain is not usually
great. Red clover also successfully interseeds with corn in the
Northeast.
Sweet clover (yellow blossom) is a reasonably winter-hardy,
vigorous-growing crop with an ability to get its roots into compacted
subsoils. It is able to withstand high temperatures and droughty
conditions better than many other cover crops. Sweet clover requires
a soil pH near neutrality and a high calcium level. As long as the
pH is high, sweet clover is able to grow well on low-fertility soils.
It is sometimes grown for a full year or more, since it flowers
and completes its life cycle in the second year. When used as a
green manure crop, it is incorporated into the soil before full
bloom.
White clover does not produce as much growth as many
of the other legumes and is also less tolerant of droughty situations.
(New Zealand types of white clover are more drought tolerant than
the more commonly used Dutch white clover.) However, because it
does not grow very tall and is able to tolerate shading better than
many other legumes, it may be useful in orchard-floor covers or
as a living mulch. It is also a common component of intensively
managed pastures.
Grasses
A problem common to all the grasses is that if you
grow the crop to maturity for the maximum amount of residue, you
reduce the amount of available nitrogen for the next crop. This
is caused by the high C:N ratio, or low percentage of nitrogen,
in grasses near maturity. The problem can be avoided by killing
the grass early or by adding extra nitrogen in the form of fertilizer
or manure. Another way to help with this problem is to supply extra
nitrogen by seeding a legume-grass mix.
Winter rye, also called cereal or grain rye, is very
winter-hardy and easy to establish. Its ability to germinate quickly,
together with its winter-hardiness, means that it can be planted
later in the fall than most other species. Winter rye has been shown
to have an allelopathic effect, which means that it can chemically
suppress weeds. It grows quickly in the fall and also grows readily
in the spring.
Oats are not winter-hardy. Summer or fall seedings
will winter-kill under most northern conditions. This provides a
naturally killed mulch the following spring and may help with weed
suppression. As a mixture with one of the clovers, oats provide
some quick cover in the fall. Oat stems help trap snow and conserve
moisture, even after it has been killed by frost.
Annual ryegrass (not related to winter rye) grows
well in the fall, if established early enough. It develops a very
extensive root system and therefore provides very effective erosion
control, while adding significant quantities of organic matter.
It may winterkill in northern climates. Some caution is needed with
annual ryegrass, because it may become a problem weed in some situations.
Sudangrass and sorghum-sudan hybrids are fast-growing
summer annuals that produce a lot of growth in a short time. Because
of their vigorous nature, they are good at suppressing weeds. If
they are interseeded with a low-growing crop, such as strawberries
or many vegetables, you may need to delay seeding so the main crop
will not be severely shaded. Sun-dangrass is especially helpful
for loosening compacted soil.
Other Crops
Buckwheat is a summer annual that is easily killed
by frost. It will grow better than many other cover crops on low-fertility
soils. It also grows rapidly and completes its life cycle quickly.
Buckwheat can grow more than 2 feet tall in the month following
planting. It competes well with weeds, because it grows so fast
and, therefore, is used to suppress weeds following an early spring
vegetable crop. It is possible to grow more than one crop of buckwheat
per year in many regions. Its seeds do not disperse widely, but
it can reseed itself and become a weed. Mow or till it before seeds
develop to prevent re-seeding.
Rape is a winter-hardy member of the crucifer (cabbage)
family. It grows well under the moist and cool conditions of late
fall, when other kinds of plants just sit there and get ready for
winter. Rape is killed by harsh winter conditions in the North,
but is grown as a winter crop in the middle and southern sections
of the country. Members of the crucifer family do not develop mycorrhizal
fungi associations, so rape will not promote mycorrhizae in the
following crop.
Mixtures of Cover Crops
Mixtures of cover crops offer combined benefits. The
most common mixture is a grass and legume, such as winter rye and
hairy vetch or oats and red clover. Mixed stands usually do a better
job of suppressing weeds than a single species. Growing legumes
with grasses helps compensate for the decreases in nitrogen availability
for the following crop when grasses are allowed to mature. In the
mid-Atlantic region, the winter rye-hairy vetch mixture has been
shown to provide another advantage for managing nitrogen: When a
lot of nitrate is left in the soil at the end of the season, the
rye is stimulated (reducing leaching losses). When little nitrogen
is available, the vetch competes better with the rye, fixing more
nitrogen for the next crop.
A crop that grows erect, such as winter rye, may provide
support for hairy vetch and enable it to grow better. Mowing close
to the ground kills vetch supported by rye easier than vetch alone.
This may allow mowing instead of herbicide use, in no-till production
systems.
Timing Cover Crop Growth
If you want to accumulate a lot of organic matter,
it's best to grow a cover crop for the whole growing season (see
figure 10.1a). This means there will be no
income-generating crop grown that year. This may be useful with
very infertile, possibly eroded, soils. It also may help vegetable
production systems when there is no manure available and where a
market for hay crops justifies a longer rotation.
Most farmers sow cover crops after the economic crop
has been harvested (figure 10.lb.). In this case, as with the system
shown in figure 10.1a, there is no competition between the cover
crop and the main crop. The seeds can be drilled instead of broadcast,
resulting in better cover crop stands. In the Deep South and in
the country's mid-section, you can usually plant cover crops after
harvesting the main crop. In northern areas, there may not be enough
time to establish a cover crop following harvest. Even if you are
able to get it established, there will be little growth in the fall
to provide soil protection or nutrient uptake. The choice of a cover
crop to fit between main summer crops (figure 10.1b) is severely
limited in northern climates by the short growing season and severe
cold. Winter rye is probably the most reliable cover crop for these
conditions. In most situations, there are a range of establishment
options.
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Figure 10.1 Three ways to time cover
crop growth for use with a summer crop. |
The third management strategy is to interseed cover crops during
the growth of the main crop (figure 10.1c). This system is especially
helpful for the establishment of cover crops in short-growing-season
areas. Delay seeding the cover crop until the main crop is off to
a good start and will be able to grow well despite the competition.
Good establishment of cover crops requires moisture and, for small-seeded
crops, some covering of the seed by soil or crop residues. On the
other hand, cereal rye is able to establish well without seed covering,
as long as sufficient moisture is present. Farmers using this system
usually broadcast seed during or just after the last cultivation.
Aerial seeding, "highboy" tractors, or detasseling machines
are used to broadcast green manure seed after a main crop is already
fairly tall. When growing on smaller scale, seed is broadcast with
the use of a hand-crank spin seeder.
When used in winter grain cropping systems, cover
crops are established following grain harvest in late spring, interseeded
with the grain during fall planting, or frost-seeded in early spring
(figure 10.2a). With some early-maturing vegetable crops, especially
in warmer regions, it is also possible to establish cover crops
in late spring or early summer (figure 10.2b). Cover crops also
fit into an early vegetable-winter grain rotation sequence (figure
10.2c).
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Figure 10.2 Timing cover crop growth
for winter grain, early vegetable, and vegetable-grain systems.
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No matter when you establish cover crops, they are usually killed
before or during soil preparation for the next economic crop. This
is done by mowing (most annuals are killed by mowing once they've
flowered), plowing into the soil, with herbicides, or naturally
by winter injury. Good suppression of vetch in a no-till system
has been obtained with the use of a modified rolling stalk chopper.
It is a good idea to leave a week or two between the time a cover
crop is tilled in or killed and a main crop is planted. This allows
some decomposition to occur and may lessen problems of nitrogen
immobilization and allelopathic effects. It also may allow for the
establishment of a better seedbed for small-seeded crops, such as
some of the vegetables. Establishing a good seedbed for crops with
small seeds may be difficult, because of the lumpiness caused by
the fresh residues.
In drier areas and on droughty soils, such as sands,
late killing of a winter cover crop may result in moisture deficiency
for the main summer crop. In these situations, the cover crop should
be killed before too much water is removed from the soil. However,
in warm climates where no-till methods are practiced, allowing the
cover crop to grow longer means more residue and better water conservation
for the main crop. Cover crop mulch may more than compensate for
the extra water removed from the soil during the later period of
green manure growth.
In very humid regions or on wet soils, the ability
of an actively growing cover crop to "pump" water out
of the soil by transpiration may be an advantage (see figure 14.2).
Letting the cover crop grow as long as possible results in more
rapid soil drying and allows for earlier planting of the main crop.
Cover crops are sometimes allowed to flower to provide
bees or other beneficial insects with pollen. However, if the plants
actually set seed, the cover crop may re-seed unintentionally. Cover
crops that may become a weed problem include buckwheat, ryegrass,
crown vetch, and hairy vetch.
Intercrops
Growing a cover crop between the rows of a main crop
has been practiced for a long time. It has been called a living
mulch, an intercrop, polyculture (if more than one crop will be
harvested), and an orchard-floor cover. Intercropping has many benefits.
Compared with bare soil, a groundcover provides erosion control,
better conditions for using equipment during harvesting, higher
water-infiltration capacity, and an increase in soil organic matter.
In addition, if the cover crop is a legume, a significant buildup
of nitrogen may be available to crops in future years. Another benefit
is the attraction of beneficial insects, such as predatory mites
to flowering plants. Less insect damage has been noted under polyculture
than under monoculture.
Growing other plants near the main crop also poses
potential dangers. The intercrop may harbor insect pests, such as
the tarnished plant bug. Most of the management decisions for using
intercrops are connected with minimizing competition with the main
crop. Intercrops, if they grow too tall, can compete with the main
crop for light, or may physically interfere with the main crop's
growth or harvest. Intercrops may compete for water and nutrients.
Using intercrops is a highly questionable practice if rainfall is
barely adequate for the main crop and supplemental irrigation isn't
available. One way to decrease competition is to delay seeding the
intercrop until the main crop is well established. This is sometimes
done in commercial fruit orchards. Soil improving intercrops established
by delayed planting into annual main crops are usually referred
to as cover crops. Herbicides, mowing, and partial rototilling are
used to suppress the cover crop and give an advantage to the main
crop. Another way to lessen competition from the cover is to plant
the main crop in a relatively wide cover-free strip. This provides
more distance between the main crop and the intercrop rows.
Sources
Allison, F.E. 1973. Soil Organic Matter and its Role in Crop
Production. Elsevier Scientific Publishing Co. Amsterdam, Netherlands.
In his discussion of organic matter replenishment and green manures
(pp. 450451), Allison cites a number of researchers who indicate
that there is little or no effect of green manures on total organic
matter, even though the supply of active (rapidly-decomposing) organic
matter increases.
Hargrove, W.L. (ed.). 1991. Cover Cops for Clean
Water. Soil and Water Conservation Society. Ankeny, IA.
MacRae, R.J., and G.R. Mehuys. 1985. The effect of
green manuring on the physical properties of temperate-area soils.
Advances in Soil Science 3:7194.
Managing Cover Crops Profitably, 2nd Edition.
1998. Sustainable Agriculture Network, Handbook Series, #3. USDA
Sustainable Agriculture Research and Education Program. Beltsville,
MD. An excellent source for practical information about cover crops.
Miller, P.R., W.L. Graves, W.A. Williams, and B.A.
Madson. 1989. Cover Crops for California Agriculture. Leaflet
21471. Division of Agriculture and Natural Resources, University
of California. Davis, CA. This is the reference for the experiment
with clover in California.
Pieters, A.J. 1927. Green Manuring Principles and
Practices. John Wiley & Sons. New York, NY.
Power, J.F. (ed.). 1987. The Role of Legumes in
Conservation Tillage Systems. Soil Conservation Society of America.
Ankeny, IA.
Sarrantonio, M. 1997. Northeast Cover Crop Handbook.
Soil Health Series, Rodale Institute, Kutztown, PA.
Smith, M.S., W.W. Frye, and J.J. Varco. 1987. Legume
winter cover crops. Advances in Soil Science 7:95139.
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