fermentation.

Compost heaps serve as homes for weed seeds, insects and plant
diseases.

Nevertheless, all waste organic matter on the farm should be saved and
made use of as manure. These materials when not too coarse may be
spread on the surface of the soil and plowed under; they should never
be burned unless too coarse and woody or foul with weed seeds, insects
and disease.

[Illustration: FIG. 81.--SOY BEANS IN YOUNG ORCHARD.]

[Illustration: FIG. 82.--A YOUNG ALFALFA PLANT JUST COMING INTO
FLOWER.]

CHAPTER XX

FARM MANURES--CONCLUDED

GREEN-CROP MANURES

Green-crop manures are crops grown and plowed under for the purpose of
improving the fertility of the soil.

The main object of turning these crops under is to furnish the soil
with humus. Any crop may be used for this purpose.

By growing any of the class of crops called Legumes we may add to the
soil not only humus but also nitrogen. Cowpeas, beans, clover, vetch
and plants having foliage, flowers, seed pods and seeds like them are
called Legumes.

Most of the farm plants take their nitrogen from the soil. This
nitrogen is taken in the form of nitric acid and nitrogen salts
dissolved in soil water. The legumes, however, are able to use the
free nitrogen which forms four-fifths of the atmosphere. This they do
not of their own power but through the aid of very minute plants
called bacteria or nitrogen-fixing germs. These germs are so small
that they cannot be seen without the use of a powerful microscope. It
would take ten thousand average sized bacteria placed side by side to
measure one inch.

These little germs make their homes in the roots of the legumes,
causing the root to enlarge at certain points and form tubercles or
nodules (Figs. 34 and 35).

Carefully dig up a root of clover, cowpea, soy bean or other legume
and wash the soil from it. You will find numbers of the little
tubercles or nodules. On the clover they will be about the size of a
pin head or a little larger. On the soy bean they will be nearly as
large as the beans. These nodules are filled with colonies or families
of bacteria which take the free nitrogen from the air which penetrates
the soil and give it over to the plant in return for house rent and
starch or other food they may have taken from the plant.

In an experiment at Cornell University Agricultural Experiment
Station, in 1896, clover seeds were sown August 1st, and the plants
were dug November 4th, thre