Green Manure and Cover Crops

T. Hayden Rogers and Joel E. Giddens.

A green manure crop is grown and plowed under for the purpose of improving the soil. A cover crop is grown primarily to prevent or reduce erosion.

Both types of crops often are used for the same purpose, as well as for temporary grazing or for forage or grain.

In this discussion of the effects these crops have on the soil and on the following crop, we use the term "green manure crop" to include cover crops.

Green manure crops are usually annuals, either legumes or grasses. Some advantages generally attributed to them: They add nitrogen to the soil for use by the following crop. They increase the general level of fertility by mobilizing minerals and building up the organic matter and the nitrogen content. They reduce losses from erosion. They improve the physical condition of the soil and permit a more efficient use of plant nutrients. They conserve nutrients by cutting down the losses from leaching.

Some disadvantages: A risk is involved in obtaining a satisfactory stand and growth of the crops. The cost of growing them may be more than the cost of commercial nitrogen. An increase of diseases, insects, and nematodes is possible. They may exhaust the supply of moisture. They may adversely affect the stand of the next crop.

Green manuring has been practiced a long time as a way to supplement animal manures. The efficiency of the practice depends on the soil, climate, and the crop.

It is often said that green manuring is not effective in regions with an annual rainfall of less than 20 inches.

In an experiment near Fargo, N. Dak., from 1929 to 1954, sweetclover, plowed early, did not result in an increase of flax as compared to straight fallow. When sweetclover was allowed to stay on the land during a large part of the fallow season, the yield of flax was less than when no sweetclover was used. Moisture was probably the limiting factor.

Green manure crops are grown most extensively in the Southeastern States. Usually they are planted in the fall and turned the following spring before the summer or cash crop is sown. About 13 million acres of green manure crops were grown in the Southeast in 1940. The acreage has since declined.

The rate of decay of the organic residues is determined by conditions for microbial growth. Warmth, proper aeration, and ample moisture increase microbial growth. The decomposition releases carbon dioxide and weak acids, which act on insoluble soil minerals and may release nutrients for plant growth.

The decomposing crop residues also release inorganic plant nutrients in addition to nutrients from insoluble minerals. The chemical composition of the crops affects their value for green manuring.

An increase in potato yields in Maine following green manures was attributed to the nutrients added in the organic materials. Vetch used as a cover crop in North Carolina increased the available nitrogen, potassium, calcium, and magnesium of soil in both cotton-peanut and cotton-corn rotations. The cover crop did not increase the exchange capacity of the soil--the ability of the soil to hold or exchange plant nutrient elements. Alfalfa used as a green manure crop in Ohio increased the availability of residual soil phosphorus.

Some green manure crops with the approximate nitrogen percentages on a dry basis:

An experiment in progress at Athens, Ga., for 8 years showed no increase in Corn yields or in soil organic matter from green manure crops vetch, Austrian winter peas, and crimson clover if adequate amounts of nitrogen, phosphorus, and potassium were added.

Experiments in all parts of the country have indicated that if soil nitrogen is deficient, leguminous green manure crops supply the needed nitrogen; the amount of nitrogen they fix depends on the crop and amount of growth.

Green manure crops have little influence on soil organic matter if cultivation is continuous. In cooler sections, green manure increases organic matter and nitrogen somewhat. Such crops in warmer areas have a temporary effect, with little or no overall increase in soil organic matter because only a relatively small amount of plant material actually is grown and returned to the soil by most cover crops and because cultivation speeds up the rate of decomposition. Even soils in forests lose their supply of organic matter rapidly when they are first cultivated.

Perennial sod or cover crops increase soil organic matter and nitrogen.

Four years of cropping to kudzu in Mississippi brought an increase of 43 percent in nitrogen and 85 percent in carbon over the nitrogen and carbon present when corn was grown all the time. A long-term rotation, including alfalfa, in Ohio gave increased yields, but the top yield levels were reached quickly. In a 5-year rotation of corn, oats, alfalfa, alfalfa, alfalfa, no sustained benefit accrued once the first round of the rotation was completed.

Green, easily decomposable organic materials added to soils may speed up decomposition of the soil organic matter already present. F. E. Broadbent and A. G. Norman, at Iowa State College, found by using isotopic carbon and nitrogen that adding young Sudangrass increased the rate of loss of native carbon in soil.

Green manure crops can be expected to have more effect on soil organic matter when the soil clays are the montmorillonitic type (expanding clay with high exchange capacity) than when they are of the kaolinitic (nonswelling with low exchange) type. Proteinaceous organic materials have been shown to react with clays, especially montmorillonitic clays.

F. E. Allison and others at Beltsville, Md., found that more carbon was retained after adding soybean plants and corn stover to sand-bentonite clay mixtures than in sand-kaolin mixtures or in pure sand. Perhaps one reason that soil organic matter is low in the Southeast is that the soil clays are largely kaolinitic.

COVER CROPS prevent leaching of nitrogen and potassium and possibly other elements from soils. Studies throughout the United States have shown that losses from leaching are reduced greatly by soil cover a matter of great importance when soils are light and sandy.

The time of turning green manure crops under is important from this standpoint. Fall-turned crops may lose most of their nitrogen by leaching before the following crop can utilize it. Soybeans turned into a sandy soil in Alabama in the fall lost 70 percent of the added nitrogen by leaching, but the loss was 38 percent when soybeans were turned in the spring.

Cover crops improve the soil physical properties of aggregation, porosity, bulk density, and permeability. The effects are more pronounced on fine-textured soils than on coarse ones.

Any material that supplies a readily available source of food for soil microorganisms will cause an increase in soil aggregation. Rapidly decomposing organic residues are more beneficial than slowly decaying materials.

G. M. Browning and F. M. Milam in West Virginia showed that cane sugar was more effective in producing aggregation of a Gilpin silty clay loam than alfalfa, rye, and vetch, or wheat straw. Aggregation no doubt is associated with the gums, slimes, and other products of soil micro-organisms, which tend to cement soil particles together. The benefits of decomposing cover crops generally are of rather short duration, because the rapidly decaying materials remain briefly in the soil.

Good ground cover reduces soil erosion. The crops slow down the beating action of rain on soil particles and so lower the runoff and erosion.

Many fall-planted green manure crops, however, make too little growth to check erosion before winter rains set in. Residues from the preceding summer crop and from weed growth might do a better job of preventing soil loss by runoff than plowing and planting a new crop.

Data from the Georgia Coastal Plain Experiment Station indicate that soil loss is greater on land that has an annual winter cover crop than on land on which corn stubble is left on the ground. Seeding without disturbing the surface residues may be the answer.

Because most of the soil loss from cultivated fields by water occurs in June, July, and August, summer crops would be best for controlling erosion. When corn followed a winter cover of rye and vetch in South Carolina, the soil loss was less than when no cover crop was used.

Cover crops can help control wind erosion, but often the crops are not on the land during periods of high winds.

Cover crops increase the rate at which water filters into soils. Leaves and stems catch the rain, and the roots open channels for the water. Improved aggregation, which usually follows green manuring, also allows better penetration. The improved permeability of soils to water does not mean, however, that more nutrients would leach away. The growing crop would absorb a large part of the nutrients.

Deep-rooted crops, such as some of the legumes, help open up soils with restricted subsoils. Crops grown for a short time usually have little effect on heavy subsoils.