The plant does
not use them in their simple elementary form but in various compounds.
These compounds must be soluble in water or in weak acids.

Of these seven elements of plant food the nitrogen, phosphorus, and
potassium and calcium are of particular importance to the farmer,
because they do not always exist in the soil in sufficient available
quantities to produce profitable crops. Professor Roberts, of Cornell
University, tells us that an average acre of soil eight inches deep
contains three thousand pounds of nitrogen. The nitrogen exists
largely in the humus of the soil and it is only as the humus decays
that the nitrogen is made available. Here is another reason for
keeping the soil well supplied with organic matter. The decay of this
organic matter is hastened by working the soil; therefore good tillage
helps to supply the plant with nitrogen.

If the nitrogen becomes available when there is no crop on the soil it
will be washed out by rains and so lost. Therefore the soil,
especially if it is sandy, should be covered with a crop the year
through. Many lands lose large amounts of plant food by being left
bare through the fall and winter, especially in those parts of the
country where the land does not freeze. The phosphorus, potassium and
calcium also exist in most soils in considerable quantities, but often
are not available; thorough tillage and the addition of organic matter
will help to make them available, and new supplies may be added in the
form of fertilizers. Calcium is found in nearly all soils in
sufficient quantities for most crops, but sometimes there is not
enough of it for such crops as clover, cowpea, alfalfa, etc. It is
also used to improve soil texture. The entire subject of commercial
fertilizers is based almost entirely on the fact of the lack of these
four elements in the soil in sufficient available quantities to grow
profitable crops. The plant gets its phosphorus from phosphoric acid,
its potassium from potash, and its calcium from lime.

There is a class of plants which have the power of taking free
nitrogen from the air. These are the leguminous plants; such as
clover, beans, cowpeas, alfalfa, soy bean, etc. They do it through the
acid of microscopic organisms called bacteria which live in nodules or
tubercles on the roots of these plants (Figs. 34-35). Collect roots of
these plants and find the nodules on them. The bacteria take nitrogen
from the air which penetrates the soil and