It is a signal honour to be invited to give the Leeuwenhoek Lecture, and I would like to thank the authorities of the Royal Society for their invitation to me to give this lecture on the chemical aspects of soil microbiology. Leeuwenhoek clearly perceived from his pioneer work that a vast world of micro-organisms lies in the soil. The importance of its study, both in the purely scientific domain and in the practical fields of agriculture and medicine, is only now being realized. I will try in this lecture to give a short description of some recent studies of the chemical aspects of soil microbiology with a few observations on their practical implications. Some conception of the enormous population of micro-organisms in soil is provided by the fact that there may be as many as 5000 million bacteria per gram of soil. This corresponds to a weight of over 4 tons of bacterial substance per acre of soil. The numbers of bacteria fluctuate very greatly depending on availability of food supply, moisture, aeration conditions, temperature, hydrogen-ion concentration of the soil, and other factors. Protozoa may reach figures of the order of 1 million per gram of soil and algae may exceed a hundred thousand per gram. Actinomycetes and fungi, the exact numbers of which are difficult to estimate, may have a combined weight in the soil equal to that of the bacteria. Kluyver (1953) makes a comment in his recent Leeuwenhoek Lecture that the weight of microbial protoplasm on earth surpasses that of animal protoplasm by almost twenty times. It is obvious, therefore, that in dealing with the subject of soil metabolism we are dealing with the metabolism of vast numbers of living cells, greatly exceeding the number of cells found in animal life. The organisms of soil do not develop in an unrestricted chaotic manner. They are, on the other hand, greatly dependent on each other, sharing many factors in common. Soil may be considered to be a complex biological system in which hosts of organisms compete with each other often for a limited supply of nutritional material. They exercise profound effects on each others’ development and chemical activities, and they establish between themselves a dynamic equilibrium which continually changes with the changes in the physical and chemical environment of the soil.