The purpose of this volume is to give the broad outlines of our present knowledge of the relationships of the population of living organisms in the soil to one another and to the surface vegetation. It is shown that there is a close relationship with vegetation, the soil population being dependent almost entirely on the growing plant for energy material, while the plant is equally dependent on the activities of the soil population for removing the residues of previous generations of plants and for the continued production in the soil of simple materials, such as nitrates, which are necessary to its growth. It is also shown, however, that the soil population takes toll of the plant nutrients and that some of its members may directly injure the growing plant. The soil population is so complex that it manifestly cannot be dealt with as a whole in any detail by any one person, and at the same time it plays so important a part in the soil economy that it must be seriously studied. Team work therefore becomes indispensable, and fortunately this has been rendered possible at Rothamsted. Each group of organisms is here dealt with by the person primarily responsible for that particular section of the work. The plan of the book has been carefully discussed by all the authors, and the subject matter has already been presented in a course of lectures given at University College, London, under the auspices of the Botanical Board of Studies of the London University. The interest shown in these lectures leads us to hope that the subject may appeal to a wider public, and above all to some of the younger investigators in biological science. They will find it bristling with big scientific problems, and those who pursue it have the satisfaction, which increases as the years pass by, of knowing that their work is not only of interest to themselves, but of great importance in ministering to the intellectual and material needs of the whole community. CONTENTS. CHAP. PAGE I. DEVELOPMENT OF THE IDEA OF A SOIL POPULATION 1 Sir E. JOHN RUSSELL, F.R.S., Director. II. OCCURRENCE OF BACTERIA IN SOIL--ACTIVITIES CONNECTED WITH THE ACQUIREMENT OF ENERGY 20 H. G. THORNTON, B.A., Head of the Department of Bacteriology. III. CONDITIONS AFFECTING BACTERIAL ACTIVITIES IN THE SOIL--ACTIVITIES CONNECTED WITH THE INTAKE OF PROTEIN BUILDING MATERIALS 39 H. G. THORNTON, B.A., Head of the Department of Bacteriology. IV. PROTOZOA OF THE SOIL, I. 66 D. W. CUTLER, M.A., Head of the Department of Protozoology. V. PROTOZOA OF THE SOIL, II. 77 D. W. CUTLER, M.A., Head of the Department of Protozoology.