247.  Bacteria as Nitrogen Gatherers.  Soil from which crops are removed year after year usually becomes less fertile, but the soil from which crops of clover, peas, beans, or alfalfa have been removed is richer in nitrogen rather than poorer.  This is because the roots of these plants often have on them tiny swellings, or tubercles, in which millions of certain bacteria live and multiply.  These bacteria have the remarkable power of taking free nitrogen from the air in the soil and of combining it with other substances to form compounds which plants can use.  The bacteria-made compounds dissolve in the soil water and are absorbed into the plant by the roots.  So much nitrogen-containing material is made by the root bacteria of plants of the pea family that the soil in which they grow becomes somewhat richer in nitrogen, and if plants which cannot make nitrogen are subsequently planted in such a soil, they find there a store of nitrogen.  A crop of peas, beans, or clover is equivalent to nitrogenous fertilizer and helps to make ready the soil for other crops.

[Illustration:  FIG. 162.—­Roots of soy bean having tubercle-bearing bacteria.]

248.  Artificial Fertilizers.  Plants need other foods besides nitrogen, and they exhaust the soil not only of nitrogen, but also of phosphorus and potash, since large quantities of these are necessary for plant life.  There are many other substances absorbed from the soil by the plant, namely, iron, sodium, calcium, magnesium, but these are used in smaller quantities and the supply in the soil does not readily become exhausted.

Commercial fertilizers generally contain nitrogen, phosphorus, and potash in amounts varying with the requirements of the soil.  Wheat requires a large amount of phosphorus and quickly exhausts the ground of that food stuff; a field which has supported a crop of wheat is particularly poor in phosphorus, and a satisfactory fertilizer for that land would necessarily contain a large percentage of phosphorus.  The fertilizer to be used in a soil depends upon the character of the soil and upon the crops previously grown on it.

[Illustration:  FIG. 163.—­Water cultures of buckwheat:  1, with all the food elements; 2, without potash; 3, without nitrates.]

The quantity of fertilizer needed by the farmers of the world is enormous, and the problem of securing the necessary substances in quantities sufficient to satisfy the demand bids fair to be serious.  But modern chemistry is at work on the problem, and already it is possible to make some nitrogen compounds on a commercial scale.  When nitrogen gas is in contact with heated calcium carbide, a reaction takes place which results in the formation of calcium nitride, a compound suitable for enriching the soil.  There are other commercial methods for obtaining nitrogen compounds which are suitable for absorption by plant roots.

Phosphorus is obtained from bone ash and from phosphate rock which is widely distributed over the surface of the earth.  Bone ash and thousands of tons of phosphate rock are treated with sulphuric acid to form a phosphorus compound which is soluble in soil water and which, when added to soil, will be usable by the plants growing there.