How Climate Influences the Soil.—Other conditions affect the soil quite as much as does glaciation. Before the soil can be used by plants its valuable portions for plant food must be dissolved. In tropical countries, where rain is abundant and the chemical processes active because of the heat, the more soluble elements are sometimes wholly dissolved and carried away, thus practically ruining the soil. In dry regions, on the contrary, the rainfall is so scanty that the plant foods are neither carried away, nor used by vegetation. In fact, materials from lower layers of the soil are often brought to the sur face so that the soil grows richer and richer. This is because much of the rainfall of dry regions sinks into the ground only to come out again by evaporation. As it evaporates it leaves the dissolved chem icals behind. If the farmer irrigates such soils he finds them unusually fertile. This partly accounts for some of the wonderful crops in New Mexico, Arizona, southern California, eastern Washington, and Oregon.
In ordinary temperate climates with a fair amount of rain the plant foods are not washed away as they are in the wet, warm tropical regions, nor do they accumulate as in dry regions. Hence ordinary vegetation usually has enough of each kind, but when a single crop is cultivated for a number of years certain chemicals are used up more rapidly than they are prepared by the processes of weathering. There may be enough of them, but they are not ready for use.
The Unwise Use of Soil.—Since all life depends on the soil it is evidently of supreme importance to preserve or renew its fertility. In a state of nature most plants die where they grow. The materials which they contain are thus returned to the soil through decay. Moreover, there are usually many varieties of plants on the same area, so that the same kind of food is not demanded by all. On farms, on the contrary, it is usually necessary to devote the whole of a given area to a single crop at any given time. When the crop is reaped, it is carried away and consumed somewhere else. Thus there is a great drain on the soil. For example, many early settlers of the great plains of our Central and Western States wanted to get rich as quickly as possible. Accordingly, they planted wheat or some other favorite and profitable crop year after year, and returned nothing to the soil. At first the crops were wonderfully abundant, but soon the soil began to show signs of exhaustion, the crops fell off, and the value of the farms declined. They forgot that one of their duties is
to see that the fields are passed on to their descendants in good con dition. In the Southern States, also many farmers have injured their lands by planting nothing but tobacco, which speedily exhausts the phosphorus of the soil, or cotton, which does the same thing more slowly. These crops bring good prices and are an easy way of getting ready money, but to sell the fertility of the soil along with the crop is like killing the goose that lays the golden eggs.
The Wise Use of the Soil.—(1) Rotation of Crops.—The wise farmer lessens this drain on the soil in two ways: (1) by rotation of crops, and (2) by using fertilizers. Rotation of crops means that the farmer plants different crops from year to year, so that .on a given area the same elements are not constantly required in large amounts. It is called rotation because after a few years the same series of crops is planted over again. In planning a rotation the object is not only to use crops which do not require the same food, but to include some, such as buckwheat and clove; which can be plowed under to serve as fertilizer. For instance, beets need a great deal of potash, while wheat in proportion to its bulk requires only half as much, but needs nearly twice as much nitrogen. Clover, and peas, on the other hand, do not require much nitrogen from the soil. Indeed they actually take nitrogen from the air and give to the soil. Hence beets, wheat, and peas would make a proper rotation.
The rotation of crops has still another value, as the people who raise cotton found out in the early part of the Great War. As Eng land prevented the shipment of cotton to Germany and Austria, the market for the crop was restricted and the price fell very low. As the farmers had no other important crop to sell many of them could not pay their debts, even though they had large supplies of cotton. If they had practiced rotation of crops, part of their land would have been in corn, part in beans, peanuts, or sweet potatoes. They could have sold these crops at good prices and thus have been able to get along for a year or more until cotton again rose to a profit able price. In 1915 they began to learn this lesson, and planted far more corn than ever before. The rotation of crops also helps in checking the ravages of insects and of various plant diseases due to bacteria and other causes. Wise farmers find that a variety of crops is as valuable in peace as in war.