Sustainable Agriculture Research and Education - Publications - Building Soils for Better Crops- Chapter 10

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.

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.

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.

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

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.

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.

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.

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.

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 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.

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.

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).

Figure 10.2 Timing cover crop growth for winter grain, early vegetable, and vegetable-grain systems.

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.

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.