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 give it over to the plants.
Here is another reason for good soil ventilation.

This last fact brings us to another very important property of soils.
Soils have existing in them many very small plants called bacteria.
They are so very small that it would take several hundred of them to
reach across the edge of this sheet of paper. We cannot see them with
the naked eye but only with the most powerful microscopes. Some of
these minute plants are great friends to the farmer, for it is largely
through their work that food is made available for the higher plants.
Some of them break down the organic matter and help prepare the
nitrogen for the larger plants. Others help the leguminous plants to
feed on the nitrogen of the air. To do their work they need warmth,
moisture, air, and some mineral food; these conditions we bring about