CHAPTER II.
Soil Organisms and Soil Fertility.
In the first chapter the plants were referred to as the primary producers of life, and the animals as the consumers; the former not only furnish nourishment for their own growth, but also for the support of the animal world as a whole. Living plants (in reference to green plants) utilise the sun’s energy in the manufacture of their complex food materials from comparatively simple chemical compounds. These latter compounds are carbon dioxide, derived from the air through the agency of leaves, and a weak solution of various chemical compounds in water, derived by means of the roots from the soil, and carried up through the plant to the leaves, where the elaboration into the complex compounds to be utilised by the plants as food takes place.
These comparatively simple compounds from which the plants elaborate their nourishment are the raw food materials, and that they must always be available for plant growth, is evident when one considers the vast areas of vegetation that cover, with the exception of desert regions, the surface of the earth. Under moist climatic conditions it has been calculated that some 500 tons of carbon dioxide and 1,000,000 tons of water, having the raw food materials in solution, are used annually by one square mile of dense forest. For their development, therefore, plants require:—
(1) Sunlight as the source of energy for the carrying on of their life functions;
(2) Air for the supply of carbon dioxide, oxygen, and, indirectly, nitrogen;