We know in a general way that earthworms effect the distribution of the plant residues in the soil, and serve to disintegrate them; there is no evidence, however, that they play any indispensable part in the decomposition. Many root and other fragments do not go through this process; observation shows that fungi can force a way in, and they may be followed by nematodes which continue the disintegration. Possibly some of the flagellates help, and certainly the bacteria do. After that nothing is certain. We cannot, with certainty, assign any particular reaction in the decomposition to any specific organism, with the exception of the oxidation of the phenolic substances, the conversion of ammonia to nitrite and nitrate, and the fixation of nitrogen. With these exceptions many organisms seem capable of bringing about the reactions, and indeed some of the reactions may be purely chemical and independent of biological agencies. The relationships between the soil population and soil fertility are readily stated in general outline, but they are by no means clear cut when one comes to details; fertility is a complex property, and some of its factors are independent of soil micro-organisms. The general relationship between plants and soil organisms is one of complete mutual interdependence. The growing plant fixes the sun’s energy and converts it into a form utilisable by the soil organisms; without the plant they could not exist. The plant is equally dependent on the soil organisms in at least two directions: their scavenging action removes the dead vegetation which would, if accumulated on the surface of the soil, effectively prevent most plants from growing. Further, the plant is dependent on the soil population for supplies of nitrates. Nothing is known about the relative efficiencies of the various soil organisms as scavengers. Numerous fungi and bacteria are effective producers of ammonia, the precursor of nitrates; it is not known, however, whether flagellates and such higher forms as nematodes act in this way.