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 related with those of the others. During the activities of these organisms various organic and mineral substances are decomposed or transformed into materials, such as humus and the inorganic compounds of nitrogen, phosphorus, potassium, etc., necessary or helpful for the growth of plants. The protozoa (see Chapter I.) are the lowest and simplest forms of animal life, being mere specks of living matter. Three different groups of soil protozoa occur (Fig. 2). Some, like the amœba, progress by streaming movements, extruding temporary extensions of their substance in the form of finger or thread-like processes; the bodies of such protozoa may be naked, or enclosed in a shell-like covering secreted by the organism itself, or protected by an accumulation of particles of foreign matter. Some have a body of more definite shape and progress by means of the whip-like action of one or two thread-like processes, or flagella, arising from one end of the body. Such forms are the most numerous in the soil. Others, also of definite shape, control their movements by means of short, hair-like processes, or cilia, either distributed over the body or restricted to definite regions. The protozoa are widely distributed, being most abundant in the richer types of soil, especially during the spring and autumn. A great amount of research has been undertaken at Rothamsted, England, and elsewhere, on the part played by protozoa in soil fertility; the evidence thus secured points to the probability that some of these organisms may be detrimental in that they devour certain kinds of bacteria responsible for the production of nitrates and other substances of nutritive value to plants. The extent of this may be realised from the fact that in a definite weight of soil (about 1-28th of an ounce) the micro-population was calculated to include not only about 1,550,000 protozoa, of which 430,000 were amœbæ (Fig. 2), but also some 6,000,000,000 bacteria. Observations showed that a single bacteria-destroying amœba required about 400 organisms for its nourishment, so that the amœbæ, to say nothing of the other protozoa, present in the weight of soil above-mentioned, would be capable of destroying about 172,000,000 of the bacterial population. Since the partial sterilisation of soil by steam results in an increase of fertility, it is thought, on account of the sterilisation destroying the protozoa, being more susceptible, and not the bacteria, that protozoa inhibit the activities of the bacteria to such an extent as to reduce the fertility of the soil; but this is a subject as yet open to argument. Apart from the bacteria-destroying protozoa, there are other forms that are thought to have something to