The cowpea is perhaps the most important leguminous plant grown for soil improvement in the South.  It will grow anywhere south of the Ohio River and can be grown with fair success in many localities farther north.

It is a tender annual, that is, it is killed by frost and makes its entire growth from seed to seed in a single season.  It should therefore be planted only during the spring and summer.  This crop not only has power like the other legumes to take nitrogen from the air, but it is also a strong feeder, that is, it can feed upon mineral plant food in the soil that other plants are unable to make use of.  For this reason it will grow on some of the poorest soils, and is a good plant with which to begin the improvement of very poor land.  It is a deep-rooted plant.  On the farm of the Hampton Normal and Agricultural Institute cowpea roots have been traced to the depth of sixty-one inches.

Cowpeas will grow on almost any land that is not too wet.  From one and one-half to three bushels of seed are used per acre.  These are sown broadcast and harrowed in or are planted in drills or furrows and cultivated a few times.  Aside from its value as a green manure crop the cowpea is useful as food for man and the farm animals.  The green pods are used as string beans or snaps.  The ripened seeds are used as a food and the vines make good fodder for the farm animals.

“Experiments at the Louisiana Experiment Station show that one acre of cowpeas yielding 3,970.38 pounds of organic matter, turned under, gave to the soil 64.95 pounds of nitrogen, 20.39 pounds of phosphoric acid and 110.56 pounds of potash.”—­Farmer’s Bulletin, 16 U.S.  Dept. of Agriculture.

“It is now grown in all the States south of the Ohio River, and in 1899 there were planted nearly 800,000 acres to the crop.  Basing our estimate on the amount of nitrogen stored in the soil by this crop, it is fair to say that fully fifteen million pounds of this valuable substance were collected and retained as a result of the planting of the cowpea alone.  This at fifteen cents per pound (the market price of nitrogen) would be worth something more than $2,000,000 for nitrogen alone.”—­Year Book of the Department of Agriculture, 1902.

The Clovers.—­These are the most extensively grown plants for green manure purposes in the United States.  They are deep-rooted, and are able to use mineral food that is too tough for other plants.  They furnish large crops of hay or green forage and a good aftermath and sod to turn under as green manure, or the entire crop may be plowed under.

Red Clover is the most widely planted (Fig. 80).  It is a perennial plant and grows from the most northern States to the northern border of the Gulf States.  It grows best on the loams and heavier soils well supplied with water, but not wet.  It is sown broadcast at the rate of from ten to twenty pounds of seed per acre.  In the North it is generally sown in the spring on fields of winter grain.  In the South, September and October are recommended as the proper sowing times.  It is the custom to let it grow two years, cutting it for hay and seed, and then to turn the aftermath and sod under.