ents of plant food,
all productive soils contain decaying vegetable matter, generally
classified under the term "humus." Humus serves as a source of
acid-generating material which further breaks down soil particles and,
most important of all, serves as a food for millions of microscopic plants
which develop and die quite beyond the scope of human vision. These
constitute a type of bacteria which are distinctly beneficial and
essential to human life since they make possible the growth of larger
plants that serve as human food.

Green plants, straw or leaves, when plowed under or spaded in the soil,
are attacked by bacterial agencies which gradually turn these products
into humus. The same process occurs when a "compost" is set up. This is
made of leaves, manure, soil, straw and other materials thrown into a
heap and allowed to decay. Such compost is excellent for placing around
plants when setting them out, since it holds moisture, supplies fertility
and creates optimum conditions for young root growth. Under practical
field conditions, humus may be added to soils by spreading animal manures,
followed by plowing them down, or by the growing of heavy green crops such
as wheat, rye, cowpeas or vetch and turning the entire mass under with the
plow when they are at their height.

_Lime and Its Application._--Reference has been made to the fact that
calcium is an essential plant food and is frequently deficient in soils.
As a matter of fact, the great majority of soils are deficient in calcium
and their productiveness is inhibited thereby. Lime supplies calcium and
also magnesium as food for plants. Its application accomplishes many other
desirable things such as correcting soil acidity. The growth of beneficial
bacteria is greatly stimulated in a soil that has had its acidity
neutralized by the application of lime. This product, therefore, creates a
more congenial condition for the growth of bacteria, which, in turn, make
for better crop production. Lime is also beneficial through furnishing the
element calcium with which other plant foods combine chemically and
thereby become soluble in the soil water. Unless plant foods are in a
state of solution, they cannot be absorbed by plant roots. Lime is a
potent force in creating chemical reactions in the soil, resulting in the
stimulation of growth through increased absorption of essential elements
in solution.

Lime also benefits soils of a clayey nature through its ability to