plant life. The nitrogen of the soil can be restored if we add to it a
fertilizer containing nitrogen compounds which are soluble in water.
Decayed vegetable matter contains large quantities of nitrogen
compounds, and hence if decayed vegetation is placed upon soil or is
plowed into soil, it acts as a fertilizer, returning to the soil what
was taken from it. Since man and all other animals subsist upon
plants, their bodies likewise contain nitrogenous substances, and
hence manure and waste animal matter is valuable as a fertilizer or
soil restorer.

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 satisfacto