SOIL AND THE FARMER The Relation between Soil and Plant Food.—The poets some times speak of the earth as the mother of all things. They mean that practically all living things depend upon the soil for existence, the plants directly, and animals and man indirectly. To suppose, however, that the soil supplies the main food of plants is a mistake. From 93 to 95 per cent of the dry weight of plants consists of carbon, oxygen, and hydrogen derived from water or from the carbon dioxide of the air. The water comes to the plants through the soil, but its amount depends primarily upon climate and relief. The relative abundance of water has more effect upon the distribution and char acter of life than has any other factor except temperature. The carbon dioxide of the air, on the contrary, although as essential as water, has little effect upon the distribution of life or upon varia tions in character from place to place, because the supply of air is sufficient practically everywhere, and the percentage of carbon dioxide varies very little except in a few extreme cases. If water is abundant, plants can get carbon from the air anywhere, and some species can grow in almost any soil.
The nitrogen which forms an appreciable percentage of all plants also comes originally from the air. Only certain nitrogen-fixing bac teria, however, are positively known to take nitrogen directly from that source. They thrive on the roots of leguminous plants such as clover. All other plants obtain nitrogen from the soil, but even this comes chiefly from decayed organic matter. In addition to this the soil supplies from 1 to 4 per cent of the weight of the average plant in the form of mineral matter or ash. This small percentage is absolutely essential. In a certain way it bears to the main elements a relation analogous to that of salts, acids, and vitamines to the carbohydrates, fats, and proteids which form most of the food of man. It should be noted that the various chemical elements derived from soil, air, and water are not exactly plant foods when first absorbed by the plants. Before they can nourish the plant they must pass through the chlorophyl cells and be changed into starch, sugar, proteids, fats, and other substances like those which nourish animals.
The soil also serves as a base or foundation upon which the plants can grow and in which they can spread their roots. Thus the func
tions of the soil may be summed up as follows: (1) to act as a physical support of vegetation, (2) to serve as a medium for storing water and bringing it in contact with the roots, and (3) to supply a small but essential percentage of the materials which are converted into plant food by means of light. This last fact makes the soil a great problem and involves an expense of millions of dollars each year. Although water and carbon dioxide supply most of the bulk of plants there is no danger of exhausting the supply. The necessary chemicals of the soil, on the other hand, can be rapidly exhausted. Each year we put back only part of the fertilizing elements that we take away, so that year by year the soil becomes less fertile and the earth poorer. Some day we shall realize that among the things that man wastes or destroys none is more important than the common soil beneath our feet.
How Soil is Formed.—(a) Mechanical Agents.—All soil is derived from rocks. The first process is the breaking of the rock into frag ments by mechanical agencies. (1) When rocks are heated by the sun and cooled by the wind and rain, they alternately expand and contract. This causes them to crack like a piece of glass in hot water, although not so violently. The Sphinx in Egypt has been chipped in this way. (2) The rocks are also broken into fragments by move ments due to the earth's contraction. The largest of these move ments are earthquakes.- Still other cracks are formed because the rocks have contracted while cooling and hence have split a little and then settled downward. Even where the rocks appear solid their upper parts are broken by innumerable cracks. Into cracks formed in any of these ways rain water percolates sooner or later. If it freezes, it expands and thus pushes open the cracks a tiny bit and forms new ones. When it melts, the water settles into the enlarged openings. Then if it freezes again, they are enlarged still more. 3) Where small particles of rock lie on the surface running water, waves, and winds all move them and thus grind them still finer. At the same time the removal of the soil by these agencies exposes new rock. In places like Cape Cod, the wind sometimes carries the loose sand along with such violence that windows are etched so that people cannot see through them.