Maintaining Fertility.--A heavy clover sod gives assurance that a good crop of corn or potatoes can be grown. If the amount of plant-food in the sod is not excessive, a heavy crop of wheat can be produced. The condition of the soil favors many crops. The clover has placed it upon a productive basis for the time being.
The object that should be kept in view, when a scheme of soil fertilization is worked out, is the maintenance of such a state of fertility that the land can be depended upon for whatever crop comes round in the rotation. When a 3-10-6 fertilizer, or a 3-8-10 fertilizer, is used, the effect upon a thin soil is to restore it temporarily to this good-cropping power, the size of the application varying with the crop. A richer soil may want the phosphoric acid and potash without the nitrogen. A manured soil may need only the phosphoric acid. The purpose of the fertilizer in any case is maintenance or increase of fertility, and when this object has been secured, the crop may be whatever the rotation calls for. It is this rational scheme that gives success to the Pennsylvania station's methods on some of its test plats. A given amount of plant-food is put upon the land, which is under a four-years' rotation. One half of it is applied every second year. The corn gets one half because it can use it to advantage. The oat crop that follows finds enough fertility because the soil is good. Next in the rotation is the wheat, and the wheat and timothy and clover plants can use fertilizer with profit. There is no change in its character because it is the soil that is getting the assistance, and not primarily just one crop in a rotation. The land in this experiment that is well fertilized is more productive than it was thirty years ago, although no manure has been applied, and it is the general productive condition that assures good yields, and not chiefly any one application of fertilizer.
Fertilizer for Grass.--A fertile soil will make a good sod. A thinner soil should have a liberal dressing of complete fertilizer at seeding time, and the formula that has been suggested is excellent for this purpose. If a succession of timothy hay crops is desired, the problem of maintaining fertility is wholly changed. The nitrogen supplied by the clover is soon exhausted, and the timothy sod must be kept thick and heavy until broken, or the soil will not have its supply of organic matter maintained. Nitrogen must be supplied freely, and phosphoric acid and potash must likewise be given the soil. The draft upon the soil is heavy, and at the same time the effort should be to have a sod to be broken for corn that will produce a big crop without the use of any fertilizer. The grass is the natural crop to receive the plant-food because its roots fill the ground, and the corn should get its food from the rotting sod, when broken. Station tests have shown that a sod can be caused to increase in productiveness for several years by means of annual applications of the right combinations of plant-food in the early spring. A mixture of 150 pounds of nitrate of soda, 150 pounds of acid phosphate, and 50 pounds of muriate of potash is excellent. This gives nearly the same quantity each of nitrogen, phosphoric acid, and potash, and is near a 7-7-7 fertilizer. The only material change in relative amounts of plant-food constituents, when compared with a 3-10-6 and 3-8-10 fertilizer, is in the increase of nitrogen, due to the heavy drafts made by continuous mowings of timothy. This fertilizer should be used as soon as any green appears in the grass field in the spring after the year of clover harvest. The large amount of nitrogen makes this an unprofitable fertilizer for clover, and its use is not advised until the spring of the year in which timothy will be harvested.