Just as breeders study what animals are best for their climates, so farmers in the dry belt should study what crops are best suited to their lands. Some crops, like the sorghums and Kafir corn, are peculiarly at home in scantily watered lands. Others do not thrive. Experience is the only sure guide to the proper selection. To sum up, then, farmers can grow good crops in these lands only when four things are done: first, the land must be thoroughly tilled so that water can freely enter the soil; second, the land must be frequently cultivated so that the water will be kept in the soil; third, the crops must be properly rotated so as to use to best advantage the food and water supply; fourth, humus must be freely supplied so as to keep the soil in the best possible condition. SECTION LXIX. IRRIGATION Irrigation is the name given to the plan of supplying water in large quantities to growing crops. Since the dawn of history this practice has been more or less followed in Asia, in Africa, and in Europe. The Spanish settlers in the southwestern part of America were probably the first to introduce this custom into our country. In New Mexico there is an irrigating trench that has been in constant use for three hundred years. [Illustration: FIG. 286. PUMPING WATER FOR IRRIGATION] The most common source of water for irrigating purposes is a river or a smaller stream. Artesian wells are used in some parts of the country. Windmills are sometimes used when only a small supply of water is needed. Engines, hydraulic rams, and water-wheels are also employed. The water-wheel is one of the oldest and one of the most useful methods of raising water from streams. There are thousands of these in use in the dry regions of the West. Small buckets are fastened to a large wheel, which is turned by the current of a stream. As the wheel turns, the buckets are filled, raised, and then emptied into a trough called a flume. The water flows through the flume into the irrigating ditches, which distribute it as it is needed in the fields. In some parts of California and other comparatively dry sections, wells are sunk in or near the beds of underground streams, and then the water is pumped into ditches which convey it to the fields to be irrigated. Engines are often used for pumping water from streams and transferring it to ditches or canals. The canals distribute the water over the land or over the growing crops. [Illustration: FIG. 287. THE MAIN DITCH OF AN IRRIGATION PLANT]