Comparison is made on the basis of equal adaptability of soil and climate to clover and the cowpea. Going southward, the cowpea has the advantage, and northward the clover gains. It is in the overlapping belt that both should be freely used. The cowpea has distinct advantage over the clover in its ability to supply nitrogen and organic matter within a few months, and in its adaptation to very poor soils where clover would not make much growth. As a catch crop it has great value.
Affecting Physical Condition.--The cowpea has marked influence upon the physical condition of heavy soils, even when the vines are not plowed down. This is due in some degree to the roots, and probably more to the mulching effect of the vines during their growth. Heavy soils are made much more mellow by the cowpea, and when the crop is removed for hay, the stubble-land is easily prepared for a seeding to grass or small grain. When the growth is plowed down, the soil may be made too loose for seeding to small grain, but is put into prime condition for a tilled crop.
Planting.--The land should be fitted as it is for corn. Light, sandy soils require little preparation, and too often the seeding is made in a woefully careless manner, the chief dependence being placed upon sufficiently deep covering to insure germination. The ground should be fitted as well as it is for a cash crop, being made fine and smooth. A grain drill makes the seeding in a satisfactory manner, and the seed may be drilled solid or in rows for cultivation. When the crop is grown as a fertilizer or for hay, solid drilling is good, and about five pecks of seed gives a good stand of plants if peas are sound. Much cowpea seed is low in germination power, and the buyer should exercise caution. When a seed crop is wanted, two to three pecks of seed per acre, placed in drills 28 to 32 inches apart, make an excellent seeding, as cultivation can be given. The amount of seed varies with the variety. In northern latitudes a warm soil is to be desired, and cultivation gives better results when a seeding to wheat will be made on the pea-stubble.
There is evidence that the cowpea can make a heavy growth in soils too deficient in lime for red clover, and it gained its first prominence in southern Ohio on land that was failing to grow clover. It is the plant of adversity as well as prosperity, adding rich organic matter to thin soils, but making its full returns under better conditions. Lime applications on acid soils give increase in yields. Its one absolute requirement is heat, and in a cold summer its northern limit is markedly depressed.