THE PRODUCTIVITY OF THE SOIL The Familiar Effects of the Soil on Production.—Soil, water, and air are the great sources of wealth. Aside from the metals all the great products listed in Chapter II are derived from them directly or in directly. Plants alone yield perhaps 50 billion dollars worth of products each year, or 5 times as much as all the mineral wealth. In ordinary food products and fibers about 1 per cent of the total weight is derived from the soil, and in articles like beans this rises to about 4. The remainder, consisting of carbon, nitrogen, oxygen, and hydrogen, is derived from water or air and hence depends upon climate. Never theless, the quality of the soil has a most important effect. The quality varies enormously from place to place. Even in the same climate and with the same water supply one garden may produce the finest lettuce, beets, potatoes, and celery; while in another where the soil is poor the vegetables are stunted and stringy. The poverty of the crops in the sandy quartz soil of the Pine Barrens of New Jersey keeps the population relatively sparse, backward, and unproductive; it fosters lumbering and the cultivation of blueberries, one of the typical bush products characteristic of poor soils as well as of rugged relief.
But even a poor soil can be made to yield good crops if well fertilized. Western Long Island has poor sandy soil, while the eastern part has good soil. Yet the great market of New York City offers such.advan tages, the cost of transportation is relatively so small, and 'fresh vege tables can be delivered with so much less deterioration than is suffered even in cold storage, that the western end of the island contains some of the world's finest truck farms, while the eastern part is only mod erately developed. It pays better to transport fertilizer and make the soil rich than to transport vegetables which lose their freshness on the way to market.
The Conditions that Make a Soil Valuable.—The value of a soil, including the subsoil, depends on its fineness, levelness, depth, and mellowness, and on the amount of available plant food. The first three qualities depend largely on relief. The soil of a plain is almost always fine-grained, level, and deep. The next quality, mellowness, or the ease with which the soil can be broken up and penetrated by roots, depends partly on the kind of rocks from which the soil is derived, for a granite soil is usually mellow while a limestone soil, though fertile, may be clayey and tough. It also depends on age, for an old soil is likely to have tough, clayey subsoil or hard pan which acts almost like rock, for the soluble parts are gradually leached away, leaving beds of almost insoluble clay. Finally the amount of plant food depends partly on climate, as explained below, and partly on the origin of the soil, for pure quartz, pure limestone, or any other single mineral rarely makes so good a soil as that which is derived from a mixture of diverse kinds of rocks. All over the world there are local
areas, often of great size, where the quality of the soil is largely deter mined by the underlying rock. For example, fine sandstone disin tegrates into grains of sand, which, being practically insoluble, form an infertile soil. Of this type are the sandy belts of the Atlantic Coastal Plain, and the central part of Wisconsin. Limestone is more apt to yield a fertile soil, as in the Blue Grass region of Kentucky, the Nashville Basin in Tennessee, and the Shenandoah Valley in Virginia. In certain tropical regions, such as Hawaii and Guatemala, an uncommonly fertile soil results from the recent weathering of lavas. In all parts of the world. each type of rock forms its own special kind of soil, so that a great variety of soils may be found within small areas. • The Great Divisions of the Soil. (1) Desert from innumerable local divisions due to the underlying rock and the relief, three other great types of soil deserve study: (1) the extremely rich , soils of deserts; (2) the fairly rich new soils of glaciated regions; and (3) the poor, leached, dehumified soils of many moist regions especially within the tropics. Desert basins commonly contain unusually good soil, as well as much that is gravelly, clayey, and saline. Several con ditions improve much of the desert soil: (1) such soil has been trans ported so that portions derived from many different areas are usually well mixed, giving a good proportion of the necessary minerals. (2) It is fine-grained, deep, and friable because it has been carried far enough to be thoroughly pulverized and thoroughly exposed to the air. (3) It still contains- almost all its original plant food, for the scanty rainfall does not leach out the valuable constituents. These qualities cause newly irrigated regions to produce wonderful crops, for in all dry regions the soil has something of the quality of desert soil. For the same reason, loess, like that of North China which is brought by the wind from a desert, is highly fertile. One of the arguments for large irrigation schemes is that the excellence of the soil reenforces the abundance of both water and sun in enabling huge crops to be raised from small areas with relatively little work. Of course the fertility of desert soil gradu ally diminishes under cultivation, but in the United States, even with out special fertilization, the average yield of 12 main crops on irrigated land is about 30 per cent more per acre than on other lands.