From Edwin S. Holmes, U. S. Department of Agriculture

A Steam Plow in the Great Valley of California

products of rock disintegration are carried by moving ice and deposited therefrom, they are called glacial drift. When they are carried by wind, as is often the case, they are called æolian soils. The above are some of the varieties of soils as determined by their method of formation. Soils are also classified in regard to their chemical characters; as, for instance, when formed from the decay of carbonate of lime, they are called limestone soils. When arising from the disintegration of granite, they are called granitic soils. When formed chiefly from particles of silex, they are called sandy soils. When consisting mostly of silicate of alumina, they are called clay soils, and so on.

But for agricultural purposes the soil consists of more than decayed mineral matter. By the decay of organic matter there is introduced into the soil the element, humus, which is one of its principal characteristics from an agricultural point of view. The soil is filled with millions of organisms of a lower form, without whose activity the growing of crops would be impossible. The soil, therefore, not only contains. the mineral matters which are necessary to sustain the life of plants, but also those organic elements without which these mineral matters would not be available for plant growth. The three principal mineral foods of plants are potash, phosphoric acid, and nitrogen. Lime, magnesia, iron, and many other mineral substances are also found in plants, but these are not absolutely essential to plant growth. If, however, either nitrogen, potash, or phosphoric acid be entirely removed from the environment, it is impossible to produce a matured plant. The great bulk of the material of which plants are composed is not drawn, however, from the soil, but is taken from the air and water. Great as have been the chemical achievements of man, no chemist has yet arisen whose skill can be compared to that of

the plant itself. Any chemist who to-day, with all the appliances which science has placed at his disposal, could make by synthesis the various organic compounds of which plants are principally composed would rival the fame of Berzelius, Liebig, Hoffman, Berthelot, Gibbs, or Remsen. Thus the soil must be regarded as that part of plant life. which furnishes the chemical support for the growing plant, supplies it with the mineral foods essential to its growth and maturity, and favors best those conditions which enable the plant cell to elaborate the organic matters of which the matured plant is chiefly composed.

THE UNITED STATES AN AGRICUL-
TURAL COMMUNITY

Having thus briefly described how the soil originated, we pass to the consideration of the second part of the subject, namely, the crops which grow therein.

The United States is essentially an agricultural community. The basis of its wealth lies not so much in the products of its mines and manufactures as it does in those of its fields, gardens, orchards, and forests. The territory of the United States, including its new possessions, represents every variety of soil and every character of climate. It has agricultural lands in the tropics, in the subtropics, in the temperate zone, and in the sub-boreal regions of Alaska. latitude its agricultural lands extend half way around the world. Agricultural crops are grown in the United States subject to all the vicissitudes of climate, to excessive rainfalls, to prolonged drouth, to intense heat, and to alternating frosts and sunshine.

In

Within the borders of the United States are grown every agricultural crop known to the world. It produces immense quantities of the cereals; of fiber plants, including especially cotton and flax; of sugar-producing plants, including sugar cane, sugar beets, sorghum,