The texture of the soil in growing vegetables, should not be so open as to leave air pockets, because the roots have minute hairs through which they absorb moisture and dissolved plant foods. These foods are needed in addition to carbon, oxygen and hydrogen. Chief of these is nitrogen, supplemented by phosphorus, potassium, calcium, sulphur and a long list of other minerals usually known as the minor elements. Only the merest trace of some of them may be necessary, but if, for example, beets are grown in a soil completely devoid of boron, the plants are apt to be stunted and blotched and the roots discolored and watery. The absence of an iron fraction in the soil may mean yellowed leaves and inefficient growth.
When some corn plants were analyzed, it was found that 70 per cent of their weight was water. They were then dried and the remainder was found to be as follows:
Oxygen 44.57% Carbon 43.70 Hydrogen 6.26 _____ 94.53%
Nitrogen 1.46 Silicon 1.17 Potassium .92 Calcium .23 Phosphorus .20 Magnesium .18 Sulphur .17 Chlorine .14 Aluminum .11 Iron .08 Manganese .03 Sodium and other elements .78 _____ 5.47%
*The magnesium in this analysis would amount to less than 3 ounces in 100 pounds. Other plants would show different ratios of the constituents, but the range would be equally wide.
Is the soil then a mass of minerals? By no means. Nor can the rootlets absorb raw rpineral elements. Of the fertile top soil, on which plants chiefly depend, disintegrated rock forms 65 to 95 per cent of the mass, and organic matter 2 to 5 per cent. The rest is soil air and soil water, which holds salts of the minerals in solution. Nor is the soil an inert mass; there are more plants and animals in it than there are above it. Except for the industrious earthworms and insects, they are microscopic, mostly bacteria.
As organic matter breaks down, with the aid of all of these, ammonia is released. It is turned into acid which unites with mineral bases to form nitrates which in solution, can be taken up by the roots as plant food. The beneficial bacteria in the soil require oxygen, which is another reason for tillage. Conversely, there are other bacteria which attack the nitrates and waste the nitrogen (from the gardener’s point of view) by releasing it into the air. These bacteria operate only in soils deficient of oxygen, usually wet soils from which the proper supply of plant food is missing. Therefore, a wet soil is to be avoided or drained when a garden is planned.