ABSTRACT
Different crops remove from and contribute to the soil elements of different kinds or in different proportions. Some crops also contribute to the soil elements needed by others, as, for example, leguminous plants fix nitrogen which becomes available for wheat. Rotation of crops, especially when grass is introduced, will soon return the soil to its proper physical condition and prevent blowing. As a rule the pioneer farmer in a new country never practices much rotation of crops. Before the twentieth century, American agriculture consisted mainly in raising cheap crops, and little attention was given to resulting effects upon the soil. After the soil was robbed of its fertility, various devices were resorted to in order to get a paying crop. Irrigation is more or less extensively practiced by all of the great nations of the globe, even in subhumid and humid regions. There are two sources of water for irrigation: Surface waters, such as streams and lakes, and subterranean waters.
