(3) An ample supply of water required for the living tissues and as a vehicle for the transport from the soil of
(4) The raw food materials, in the form of various chemical compounds.
With the exception of the carbon dioxide derived from the air, all the raw food materials—water, nitrates, phosphates, sulphates, potassium, calcium, magnesium, iron, etc.—are present in the soil, though only a part of them is in a form suitable for imbibition by plants. In the formation of these food materials, which render the soil fertile, physical forces and the activities of living organisms play a leading part. Our immediate concern is with the influence of these organisms upon soil fertility, but it is advisable to give some consideration to the soil itself, since it is the environment in which the organisms live, and with which their existence is intimately associated; in this respect attention will be confined to the type of soil usually cultivated by the horticulturist, and to the uppermost layers—that is, approximately, within one foot of the surface.
Soil is the product of disintegrated and weathered rocks with which are mixed the residues of organic matter. Apart from the particles of disintegrated rocks, which form the matrix, soil contains chemical compounds of two kinds: those of a purely mineral nature derived from the inorganic components of the original rocks, and those of an organic origin derived either from the ancient remains of organisms, which, in the case of sedementary deposits, became incorporated in the rocks at the time of their origin, or from the remains of present-day plants and animals decomposed by soil organisms. In addition, there is the humus, which has a fundamental physical influence, and for the production of which soil organisms are responsible.
Figure 2
THE THREE MAIN TYPES OF SOIL PROTOZOA.
Magnified 300–400.
In the initial stages of soil formation during the disintegration and decomposition of rocks, the first type of soil to be formed is suitable for the growth of only certain plants; it is of a purely mineral nature, containing raw food materials derived mainly from the rocks and not from organic matter, unless from such organic residues as were incorporated in the rocks during their formation in ancient times. Such soil cannot sustain the higher types of green plants, nor is it populated by soil organisms; it furnishes suitable pabulum, however, for the nourishment and growth of the more lowly types of vegetation, which are able to convert to their benefit the limited supply of food materials available. The complex organic compounds that such primitive plants elaborate from these food materials of purely mineral origin, and incorporate in their tissues, are, after death, returned to the soil, which becomes correspondingly enriched, and a favourable environment for the establishment of organisms; the latter reduce these plant residues to humus, and during this process of decomposition produce food materials of an organic origin suitable for the nutrition of the sequential plant covering. So the process proceeds until a soil is formed of sufficient extent and quality for the support of a more extensive and increasingly complex vegetation; thus, in the cycle of life and decay, stores of organic compounds are elaborated by plants and returned to the soil, which they enrich, and where they are decomposed by organisms, and so maintain the supplies of food materials suitable for the maintenance of vegetation.