Transcriber’s Notes Text printed in italics has been transcribed _between underscores_; underlined text ~between tildes~. ^x and _{x} represent a superscript and subscript x respectively. Uppercase letters between square brackets (such as [A]) refer to footnotes (to be found directly underneath the paragraph or table), numbers between square brackets (such as [1] or [1_a_] refer to references at the end of the chapter(s) by the same author. More Transcriber’s Notes may be found at the end of this text. _~THE ROTHAMSTED MONOGRAPHS ON AGRICULTURAL SCIENCE~_ EDITED BY SIR E. J. RUSSELL, D.Sc. (LOND.), F.R.S. THE MICRO-ORGANISMS OF THE SOIL THE ROTHAMSTED MONOGRAPHS ON AGRICULTURAL SCIENCE. EDITED BY SIR E. JOHN RUSSELL, D.Sc., F.R.S. During the past ten years there have been marked developments in knowledge of the relations between the soil and the growing plant. The subject involves physical, biological, and chemical considerations, and its ramifications are now so wide that they cannot be satisfactorily dealt with in detail in any one book. These monographs collectively cover the whole ground. In “Soil Conditions and Plant Growth” the general outlines are presented: in the monographs the various divisions are fully and critically dealt with by the Heads of the Departments concerned at Rothamsted. A homogeneous treatment is thus secured that will, it is hoped, much facilitate the use of the series. SOIL CONDITIONS AND PLANT GROWTH, Fourth Edition. By SIR E. JOHN RUSSELL, F.R.S. 16_s._ net. The following volumes are in preparation:-- MANURING OF GRASS-LANDS FOR HAY By WINIFRED E. BRENCHLEY, D.Sc., F.Z.S. THE MICRO-ORGANISMS OF THE SOIL By Sir E. JOHN RUSSELL, F.R.S., and Members of the Biological Staff of the Rothamsted Experimental Station. SOIL PHYSICS By B. A. KEEN, B.Sc. SOIL PROTOZOA By D. W. CUTLER, M.A., and L. M. CRUMP, M.Sc. SOIL BACTERIA By H. G. THORNTON, M.A. SOIL FUNGI AND ALGÆ By W. B. BRIERLEY, S. T. JEWSON, B.Sc., and B. M. ROACH (Bristol), D.Sc. CHEMICAL CHANGES IN THE SOIL By H. J. PAGE, B.Sc. LONGMANS, GREEN AND CO., LONDON, NEW YORK, TORONTO, BOMBAY, CALCUTTA, AND MADRAS. THE MICRO-ORGANISMS OF THE SOIL BY SIR E. JOHN RUSSELL, F.R.S. AND MEMBERS OF THE BIOLOGICAL STAFF OF THE ROTHAMSTED EXPERIMENTAL STATION _WITH DIAGRAMS_ LONGMANS, GREEN AND CO. 39 PATERNOSTER ROW, LONDON, E.C. 4 NEW YORK, TORONTO BOMBAY, CALCUTTA and MADRAS 1923 _Made in Great Britain_ INTRODUCTION.