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; (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. [Illustration: Figure 2 THE THREE MAIN TYPES OF SOIL PROTOZOA. Magnified 300-400.]