The following information on green manures and cover crops comes from Five Acres and Independence by M. G. Kains. Five Acres and Independence is also available to purchase in print.
Green manures are crops grown solely for the improvement of the soil. When sown toward the close of the season, either alone or among other crops as these are approaching maturity, they are often called cover crops because they are intended to cover the ground during winter and thus prevent loss of plant food through washing over the surface (“sheet erosion”) or by seepage to lower levels and drainage. In the latter cases they are always dug or plowed under in early spring before they have made much growth. Otherwise they might become so woody they might decay slowly and thus, for a time, be a detriment to the soil.
Plants used for green manures are of two classes: Nitrogen gatherers, those that work over atmospheric nitrogen from the air in the soil; and nitrogen consumers, those that cannot perform this function but use what nitrogenous compounds are already in the soil. The former are generally the most important because they increase the supply of this essential element of plant growth—the most expensive to buy and the one most easily lost from the soil.
The principal nitrogen-gathering crops are clovers, vetches, peas, cowpeas, and soy beans; the consumers, buckwheat, rye, cowhorn and common turnips, and Dwarf Essex rape. When the former are dug or plowed under they add important quantities of nitrogenous material to the soil as well as other organic compounds; when the latter are similarly treated they add no plant food but what they took up from the soil—no nitrogen but what they were able to save from loss.
Often these crops are sown together with one or more of the first group so as both to save nitrogenous compounds already in the soil and to manufacture new supplies. One of the favorite combinations is rye and winter vetch; another buckwheat and crimson clover. Sometimes all four of these are sown together in July after an early vegetable crop has been harvested, or even while the crop is still occupying the ground. In the former case the ground is dug or plowed and made fine before sowing; in the latter only the surface is made loose by a cultivator and the seed sown either broadcast or in drills between the rows of standing crops. Though buckwheat plants are killed by the first frost and though winter may kill crimson clover the vegetable matter these crops develop will be just as good as if alive when turned under. Rye and vetch which will probably live through the winter must be dug or plowed under before they get 12″ high or the job will be difficult and the effects may not be as good as if the plants were more succulent.
When fresh or rotted manure is available it is highly advantageous to apply liberally just before a cover crop or a green manure crop is turned under because the bacteria these contain will help to break down the buried plants and thus make their plant food material more quickly available to the succeeding crops.
A green manure is measured for efficiency by its effect on the crops that follow. However, a failure should not always be laid to the crop since any one of many factors may be influential in the decomposition of the buried plants. For instance, most soil bacteria require a temperature of at least 65º F. to work properly. In spring the soil is usually too cold for their rapid work. Shortage of water in the soil also slows or even stops their action. Under most favorable conditions succulent green manures decompose in about a week but under lower temperatures and less moisture twice as long or longer.
Decomposition of green manure converts much of the plant matter into carbon dioxide which either passes into the air as such or combines with water in the soil to form carbonic acid. This acid converts more or less various soil minerals into forms which plants can utilize or which pass out of the soil in the drainage. Other parts of the green manures are converted into ammonia which remains in the soil and through bacterial action is converted into nitrates. Hence the larger the percentage of nitrogen in the plants buried the greater will be the quantity of nitrates. Hence also the high value of legumes—clovers, vetches, etc. Conversely, when the content of nitrogen is low (as in rye) the bacteria may use all of it for their growth. However, when these die and decay they give this nitrogen back to the soil.
When the quantity of nitrogen is very low, as in straw, harmful effects may result from an application because the bacteria may take their nitrogen from the soil and actually compete with the crop plants being grown.
During winter nitrate formation practically ceases, in early spring while the ground is cold and wet action is slow but during summer, provided the soil is moist, formation is greatest. From this it is evident that under ordinary conditions losses are greatest while the ground is wet and bare during late fall, winter and early spring; least during summer when it is dry and also when liberally covered with a growing crop.
This indicates that in order to prevent losses of nitrates by seepage the management of the soil must be such as to prevent the accumulation of nitrates during summer and early autumn. One way to do this is to plan the crop rotation so that one will be in active growth while, or soon after, nitrate formation is most active. Should it not be convenient or profitable to do this with a “money crop,” then a green manure or a cover crop may be added to the rotation so as to keep the surface covered with verdure. Such a crop will not only utilize the nitrates as formed or soon after and produce a lush growth, but will prevent their losses in drainage water.
Choice of crop will depend upon the time of year when the money crops will reach harvesting development. Some kinds such as cowpeas and buckwheat require warm or hot weather for their development and are tender to frost; others such as rye and winter vetch need cool weather and are resistant to frost damage. The time to sow need not necessarily be after the money crop has been harvested; it need not be postponed until the ground can be plowed and harrowed or dug and raked prior to sowing. On the contrary, it is not only often possible but advisable to sow the seed of the green manure or cover crop among and several weeks before the money crop will be ready for harvest. The amount of loss of plants due to tramping while harvesting the money crop will be more than offset by the avoidance of fitting the land for the green manure or cover crop and the extra saving of nitrates and other soluble plant foods in the soil.
Choice of the green manure or cover crop will also depend upon whether or not an increased supply of nitrates is desired in the soil. When such an increase is wanted the choice should be a nitrogen gatherer—some legume; when not, it should be a nitrogen consumer. For summer use cowpeas, soy beans, velvet beans and summer vetch are most popular among the legumes; for fall and winter, crimson clover, hairy or winter vetch and Canada field peas. The former are generally sown during late spring or early summer to be plowed under during late summer or early fall; crimson clover is sown during midsummer, winter vetch between July and September and Canada field peas during August or September.
Sweet clover or melilot has notable value as a green manure, especially on heavy soils, because of its deep rooting habit and the abundance of its foliage. However, if the soil is acid it may fail unless lime or superphosphate is applied shortly before seeding. Also it may fail if “unscarified” (machine scratched) seed is sown late—after the ground has become dry in spring. Such seed gives best results when sown in late fall or on the snow during winter. In these cases the plants get an earlier start than the weeds which they choke out. Scarified seed cannot be safely used in this way because it germinates too early. Like other legumes, the seed should be inoculated unless the soil has already grown this plant. When the plants are 8″ or 10″ high is the best time to plow them under for green manure.
Though the plants are grown for improving the soil they may need fertilizer to get a good start and to make a strong growth. When properly inoculated they get all the nitrogen they need from the air, so usually no nitrogenous fertilizer need be applied. Yet when the soil is suspected to be lacking in this element a little nitrate of soda or sulfate of ammonia may be applied, especially when seeding is delayed until May. This will help overcome the handicap. A dressing of 50 pounds to the acre may make a difference of 4,000 pounds or more of plant growth to the acre.
The fertilizers usually most needed by melilot are phosphates, even though sweet clover is famous for assimilating crude phosphates from the soil and converting them into other forms which become available when the plants decay. Yet if the soil is acid the plants seem to be unable to use the crude phosphates. Hence the advisability of applying lime or basic slag which are strongly alkaline and thus neutralize the acidity.
On sandy soils lacking in potash an application of muriate or sulfate of this element will often prove helpful.