Often land which was once thought excellent is left to grow up in weeds. The owner says that the land is worn out, and that it will not pay to plant it. What does "worn out" mean? Simply that constant cropping has used up the plant food in the land. Therefore, plants on worn-out land are too nearly starved to yield bountifully. Such wearing out is so easily prevented that no owner ought ever to allow his land to become poverty-stricken. But in case this misfortune has happened, how can the land be again made fertile?
On page 24 you learned that phosphoric acid, potash, and nitrogen are the foods most needed by plants. "Worn out," then, to put it in another way, usually means that a soil has been robbed of one of these plant necessities, or of two or of all three. To make the land once more fruitful it is necessary to restore the missing food or foods. How can this be done? Two of these plant foods, namely, phosphoric acid and potash, are minerals. If either of these is lacking, it can be supplied only by putting on the land some fertilizer containing the missing food. Fortunately, however, nitrogen, the most costly of the plant foods, can be readily and cheaply returned to poor land.
Fig. 230. Alfalfa ready for the Third Cutting
As explained on page [32] the leguminous crops have the power of drawing nitrogen from the air and, by means of their root-tubercles, of storing it in the soil. Hence by growing these crops on poor land the expensive nitrogen is quickly restored to the soil, and only the two cheaper plant foods need be bought. How important it is then to grow these leguminous plants! Every farmer should so rotate his planting that at least once every two or three years a crop of legumes may add to the fruitfulness of his fields.
Moreover these crops help land in another way. They send a multitude of roots deep into the ground. These roots loosen and pulverize the soil, and their decay, at the end of the growing season, leaves much humus in the soil. Land will rarely become worn out if legumes are regularly and wisely grown.
From the fact that they do well in so many different sections and in so many different climates, the following are the most useful legumes: alfalfa, clovers, cowpeas, vetches, and soy beans.
Alfalfa. Alfalfa is primarily a hay crop. It thrives in the Far West, in the Middle West, in the North, and in the South. In fact, it will do well wherever the soil is rich, moist, deep, and underlaid by an open subsoil. The vast areas given to this valuable crop are yearly increasing in every section of the United States. Alfalfa, however, unlike the cowpea, does not take to poor land. For its cultivation, therefore, good fertile land that is moist but not water-soaked should be selected.
Fig. 231. Sheep fattening on Alfalfa Stubble