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.

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.

These phenomena of plant establishment and succession, correlated with soil formation, were clearly demonstrated by the re-establishment of vegetation after the soil and plant life had been destroyed by the historic eruption in 1883 of Krakatoa, a volcanic island in the Straits of Sunda, between Java and Sumatra. The first plants to be established on the volcanic deposits were species of terrestrial algæ, which gradually spread and built up soil suitable for the development of soil organisms and for the growth of seeds brought to the island by birds and ocean currents. So rapid were the changes brought about by these influences, that within a period of twenty years after the eruption the barren ground was reclothed by a dense and varied plant covering.

Organisms that form part of the organic complex of the soil range from the more conspicuous species, such as slugs and snails, insects, spiders, wood lice, millepedes, earthworms and eelworms, to such microscopic forms as protozoa, fungi, algæ and bacteria, the last three being members of the plant kingdom. These organisms may be grouped as follows:—

(1) Temporary inhabitants that enter the soil for shelter, or to feed as scavengers on decaying organic matter, or both—​e.g., slugs, snails, wood lice, certain insects and some eelworms.

(2) Permanent inhabitants that are dependent on the soil for their development and supplies of food, either throughout or during most of their lives—​e.g., certain insects and spiders, millepedes, earthworms, eelworms, protozoa, fungi, algæ and bacteria.

The organisms in the first group play a comparatively minor part in soil development, and influence its fertility to an almost negligible extent, the temporary scavengers, perhaps, being of more importance since they aid in the reduction of vegetable residues. The forms in the second group, however, are invaluable as soil-making agents and in the production of plant food materials, the least important among them being the insects, spiders and millepedes. Many are merely scavengers, but some insects, such as grass-grubs and the caterpillars of certain moths, and millepedes, feed upon living plants and so add organic matter to the soil in their excreta, which also contains quantities of soil swallowed with the food, this latter mechanical action aiding in the pulverising and opening up of the soil; certain eelworms, too, that attack living plants play a somewhat similar part, in that they are primary causative agents in the decay of healthy tissues. Other forms of insects, together with spiders and some eelworms, are predaceous upon their fellows, the remains of the latter being added to the soil residual complex. Apart from the activities of all these organisms, however, it is the earthworms, protozoa, fungi, algæ and bacteria that have the most fundamental influence upon soil fertility.

Earthworms may be correctly called the great soil builders; they burrow through it, allowing the free passage of air and water; they swallow large quantities, which they eject on the surface in the form of “worm-casts,” the soil materials being well mixed in the process; they pull underground leaves and other parts of plants from the surface and so increase the supply of organic matter for the action of the micro-organisms that bring about decomposition. Further, by depositing their “casts” on the surface, earthworms soon cover the accumulations of dead vegetable matter, as has been illustrated by Darwin in his classic work on these animals. Without the aid of earthworms—​e.g., in sour soils in which they do not abound—​the plant residues accumulate on the surface, to form a partially decomposed, peaty mass, which only a limited number of plants can tolerate.

The protozoa, fungi, algæ and bacteria are all microscopic organisms, and are the agents responsible for the decomposition of the organic residues in the soil; they do not act as independent units, the processes of one group being dependent upon and intimately