The object of composting these materials is to hasten their decay and render available the plant food in them.

There are certain disadvantages in composting, namely: 

Expense of handling and carting on account of bulk.

Low composition.

Loss of organic matter by 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, three months and four days after the seeds were sown.  The clovers were then weighed and tested and the following results were obtained: