The Mineral Contents Of The Soil

The basis of most farm soils is rock that has been ground into very fine particles by frost, air, water. etc., and mixed with the remains of plants and animals. The value of decayed vegetation, or humus, in a soil is so great, and the farm practices resting upon this fact are so important, that this matter is treated in a separate chapter (XII) . The mineral contents of a soil depend upon the kind of rock from which it has been made. These rocks are of many kinds; the nature of a soil may often be determined by seeing specimens of the rocks it contains, provided the soil is south of the region that was covered by the great soil mixers, the glaciers. There is no mineral in any soil that cannot be found in the rock from which it came; there is no mineral in any plant that is not in the soil from which it sprang. Soil is being made from many kinds of rocks, principally quartz, feldspar, mica, apatite, zeolites, hornblende; and various combinations of these, as granite, which is made of quartz, feldspar, and mica. Quartz and feldspar form the largest proportion of most soils.

The chief constituent of all soils that have been made from rocks is silica (pure sand), which is the principal ingredient of quartz. This is because silica is the hardest kind of rock material and hence it is not dissolved and lost as rapidly. The rocks of the earth, and the soils made from them, contain from 65 to 70 so-called "elements," the simple ingredients; as iron, carbon, oxygen. These elements, however, unite with one another to make innumerable "compounds," or combinations of several elements. This might be illustrated by saying that eggs, salt and milk are the elements or ingredients of a compound — omelet. It is the great number and the intricacy of these compounds that make geology and chemistry so complex. No one kind of rock contains all the elements, but all of the rocks from which fertile soil is made contain at least seven of them — nitrogen, potassium, phosphorus, calcium, iron, magnesium, sulphur.

No plant can grow unless these seven are present in the soil; they are the "plant foods," and constitute from 80 to 90 per cent of most fertile soils. The first four of these seven are much needed by plants and so the soil is most likely to be exhausted of them by continuous cropping; while the latter three are usually so abundant that the farmer is never concerned about how he may add them to his soil. The nature and sources of these four essential plant foods, nitrogen, potash, phosphorus, and calcium, which are the necessary constituents of fertilisers, are discussed in Chapters XI to XIV. Besides these seven elements in the soil which are absolutely necessary for the growth of plants, a number of others are frequently absorbed by the roots of plants and used by them. Of these the most common are chlorine, silicon, aluminium, and manganese. Numerous experiments have shown that plants thrive as well without these as with them, so they must be considered as accidental or unnecessary elements.

In considering the mineral contents of the soil as a supply of food for the growth of plants, we must not forget that the soil furnishes but a small part of the material out of which plants are made. We are so actively engaged in trying to keep up the fertility of our soils by checking their wastes, and by adding to them fresh supplies of the minerals that our crops have taken from them, that we are apt to think that the plant comes from the soil alone. Yet over 90 per cent, of the crops that we remove from a soil comes from the air. The air, not the soil, is the greatest storehouse of fertility.

From the air plants get, through their leaves, three other foods — oxygen, hydrogen and carbon. These are all gases, the latter being combined with oxygen in the form of carbonic acid gas. The supply of these plant foods is, so far as we know, inexhaustible. A friend once remarked, "That is mighty lucky. I have a hard enough time now, trying to supply my worn-out soil with enough potash, phosphoric acid and nitrogen to grow profitable crops; yet you say these are only side dishes of a plant's dinner. If I had to supply it with the main dishes, or fillers, as you might call these foods that it now gets from the air, I don't believe I could have raised my family of six on these forty rocky acres of New England soil."