(2) Use of Fertilizers.—The Southern farmer who raises cattle and pigs can use the second method of preserving the fertility of the soil. The seed from his cotton, after the oil has been extracted, makes good food for cattle, while corn is the best kind of food for pigs and hens. Thus much of the nutriment taken from the soil by the crops is returned in the form of manure. The Chinese not only return to the soil all the waste products of animals, but also human waste and sewage which we permit to pollute our rivers and harbors. Inevitably, however, if some products are carried away from the farm, there is a loss of fertility, even though there is a wise rota tion of crops and many animals are raised. If weathering is very rapid this loss may be supplied by the freeing of new materials in the lower parts of the soil, but generally the soil becomes steadily poorer unless artificial fertilizers are employed.
What Chemicals Plants Need from the Soil.—In order to pro vide the right kind of artificial fertilizers it is necessary to know (1) what chemical compounds plants need, and (2) how much of these the soil contains. A good soil must contain at least seven chemical elements in such form that they can readily be dissolved and absorbed by the plants. Three of these are magnesium, iron, and sulphur. The farmer need not worry about these, for they are practically always present in sufficient abundance for any kind of crop. The supply of the other four—calcium, phosphorus, potassium, and nitrogen—is often inadequate. Certain other elements such as silicon, chlorine, and sodium are also taken from the soil by plants, but do not seem to be essential, for growth goes on apparently unchecked without them. Therefore, among the many chemicals of the soil, only calcium, phosphorus, potassium, and nitrogen cause the farmer trouble because they are not available in sufficient quantities.
How the Farmer Knows what Kind of Fertilizer to Use.—The following table shows the amount of the chief elements used by an acre of beets each year, the amount of each in the upper part of an ordinary loamy soil, and the number of years that the element would last if the beets could get every bit of it. Evidently the farmer who is raising beets needs to provide fertilizers rich in potash and nitrogen rather than in phosphorus and calcium. It must be remembered, however, that the process by which nature prepares the soil ingre dients is slow. Hence if beets are raised each year they exhaust the available supply while large reserves are still waiting to be prepared.
Even after five or ten years, unless fertilizers are applied, the avail able nitrogen would be so scanty that the crop would not be worth raising. In other kinds of soil, some of the other chief elements may be exhausted. For example, in a sandy soil, the plants are stunted for lack of lime, Artificial Fertilizers.—Lime.--In searching for fertilizers other than manure, it is necessaryiiifind materials which are not unduly expensive and which will furnish lime, phosphoric acid, potash, and nitrogen in forms that the plants can readily assimilate. Lime pre sents no special difficulty. Almost all parts of the world contain limestone beds, and it is merely a question of finding the cheapest means of pulverizing the rock and making it easily accessible to the plants.
Phosphates.—Phosphates are not so easy to find. They are ob tained frorn four chief sources: (1) They occur abundantly in a few minerals such as apatite, but these are generally so intermingled with quartz, feldspar, and other materials that it is difficult to prepare the phosphates as a fertilizer. (2) The easiest source is the slaughter house, from which the bones and refuse meat of domestic animals are taken to ill-smelling fertilizer plants. In former years bone-hunters drove their wagons over our Western plains gathering the skeletons of buffalo and cattle that had perished in blizzards, by the wolf pack, or at the hand of the hunter.
(3) As the present supply of bones and other animal refuse is not sufficient, man draws on the past. One of the important sources of phosphates is great beds of guano or bird droppings on several dry islands of the South Pacific and West Indies. From about 1830 to 1880 guano worth about $600,000,000 was taken from the Chincha Islands off the coast of Peru and carried around Cape Horn to be sold in Europe and America at $30 to $60 per ton. The Peruvian government was largely supported by the taxes on the guano until the deposits were exhausted. Only a limited supply is now available. (4) To-day the world is drawing on still older deposits of animal bones and refuse in the form of phosphate rocks. The chief supply comes from Florida, South Carolina, and Tennessee, together with Tunis, Algeria, and some of the small islands of the sea, but much is available in Montana, Idaho, Utah, and Wyoming.