Utilization of Dry Areas.—Among the distant regions those that are cold may be dismissed briefly. Their climate renders their soil almost useless. This leaves the rich soil of dry regions and the poor soil of tropical regions. The dry regions have the advantage not only of wonderful soil, but of a climate which is almost ideal for many plants provided they have water, and which is fairly good for man— better at least than that of the tropical regions. Their great need is irrigation. At present about 19 million acres are irrigated in the United but nearly 32 million more might be irrigated if capital were available. At best, however, only about one-fifteenth of the 750 million acres of arid lands in the United States can be irrigated so long as we must rely on present methods. The great need is cheap power so that water can be pumped long distances. If power were cheap enough not only could all the water of rivers like the Colorado be used, but sea water could be distilled and pumped hundreds of miles. At present the prospect of securing such cheap power appears very slight.
Utilization of Tropical Soils.—The most hopeful source of new land in the immediate future seems to be the vast unused areas within the tropics. Since the warm climate enables more than one crop to be raised each year such land might yield two or more times as much per acre as similar land in temperate regions. One of the greatest difficulties is the poor health of the white man and the inefficiency of the natives. This will be discussed in another chapter. A second difficulty is the rapid and continuous growth of weeds, bushes, and especially tough grasses which hinder the growth of everything else. If tropical people were energetic, this could be overcome by plowing, but the plow is a rare implement in many tropical countries. Moreover, tropical draft animals are generally small and weak, or else, like the water buf falo, can work only in special surroundings, and hence cannot plow the tough sod formed by the luxuriant tropical grasses and other growths. Imported animals like the horse and ox deteriorate rapidly. Never theless, if labor were efficient, machine plows could be used on a large scale.
The third great difficulty is the poverty of the soil. This can be partly overcome by plowing, which turns up the lower soil, but this is a temporary expedient. The only effective way to maintain or increase the fertility of tropical soils and thereby gain the advantage of the highly favorable climate, appears to be to supply abundant fertilizers. The poverty of the upper soil and probably still more the rapid and persistent growth of weeds are responsible for the fact that in Central America many fields are cultivated only one or two years and then lie fallow from three to six ostensibly to permit the soil to decay still further and thus acquire new fertility. This involves an enormous waste of energy, for bushes 10 or 20 feet high grow in the fallow fields and must be cut before each crop is planted.
The Improvement of Old Lands.—We have seen that it is possible to increase the world's supply of food and raw materials not only by using new lands but by improving old lands which have not been well cared for. Nature's care of the' soil is far better than man's. Accord
ing to nature's method all plants decay where they grow. Thus they not only return to the soil all that they have taken from it, but the legumes add new materials from the air in the form of humus in which the most valuable part is not the carbon which gives the dark color, but the nitrates made from atmospheric nitrogen. When man reaps the crops and carries them away without using fertilizers he removes materials which nature cannot replace for a long time. Thus many soils show signs of exhaustion, and the crops grow poorer and poorer. For example, tobacco exhausts the soil so rapidly that the crop raised in the United States in a single year takes from the soil more than 28 million pounds of nitrogen, 29 million pounds of potassium, and about 21 million pounds of phosphorus. Ordinary food products con tain so much plant food that each year the sewage of the United States carries off 600 to 1200 million pounds of nitrogen, 200 to 400 million pounds of potassium, and SO to 300 million pounds of phosphorus. If we could follow the wise Chinese example without detriment to health, we should save not only all the chemicals contained in sewage, but in street sweepings, weeds, and all sorts of waste products which we now throw away or burn.
The Problems of the Fertilizer nitrogen, potassium, and phosphorus are by no means the only plant foods, they are the most important. Oxygen, hydrogen, and carbon, to be sure, far exceed them in bulk in all plants, but can always be secured easily from air or water. Calcium (lime), iron, and sulphur are also important, but are present in most soils in sufficient amounts, or can easily be added. Nitrogen, potassium, and phosphorus, on the other hand, are so difficult to procure that the purchase and sale of nitrogen in the form of nitrates, potassium in the form of potash, and phos phorus in the form of phosphates is a highly important business. The nitrates involve commerce with Chile and with the Guano islands off the coast of South America where the supply of this nitrogenous phos phate is nearly exhausted. They demand factories where cotton hulls, garbage, the waste of slaughter houses and the slag from coke ovens are treated; they require farmer to plant and plow into the soil certain leguminous crops such as clover and alfalfa which form little nitrogenous nodules on their roots. In recent years the need of nitrates has led to the erection of plants like the one at Muscle Shoals on the Tennessee River in Alabama, for the extraction of nitrogen from the air by using water power to develop electricity and thereby combine the nitrogen with lime or other products. The need of potash, which formerly came almost wholly from the vast beds of Stassfurt in Germany, has led to a search of the dry parts of the United States and to the establishment of plants where potash-bearing brines are treated, especially in Nebraska and in the arid states of the Southwest. Large amounts of potash have also been discovered in deep-lying ancient desert beds of Peruvian age in Texas where oil wells were being drilled.