Preface 



This is a book about the Hfe in the soil, the soil under our feet, the 

 soil in which our culti\'ated and unculti\'ated plants grow and from 

 which they derive most of their sustenance, and which in their turn 

 support animal life, including that of man himself. This book deals 

 with the numerous biological, physical, and chemical processes which 

 continuously go on in the soil and in which microbes are involved. 

 Without soil microbes, life on this planet would soon come to a stand- 

 still. 



The soil is not a mass of dead debris, merely resulting from the 

 physical and chemical weathering of rocks; it is a more or less ho- 

 mogeneous system which has resulted from the decomposition of 

 plant and animal remains. It is teeming with life. The numerous 

 living forms which spend all or part of their life in the soil range 

 from the submicroscopic viruses and phages, through the micro- 

 scopic bacteria, actinomycetes, fungi, algae, and protozoa, to the 

 lower animal forms, the worms, insects, and rotifers, many of which 

 can be seen and recognized with the naked eye. These organisms, 

 comprising both the living forms and their dead bodies, as well as 

 the products of theii- decomposition, such as carbon dioxide and or- 

 ganic acids, interact with the rock constituents to give rise to soil. 

 The soil thus gains the characteristic properties that make it a suit- 

 able medium for plant and animal life. 



The aims of this book are to survey the nature and abundance of 

 microorganisms in the soil, to review the important role that they 

 play in soil processes, and, so far as possible, to show the relation be- 

 tween them and soil fertility ( including plant nutrition and crop pro- 

 duction ) . I have attempted to point out some of the more promising 

 lines of advance in the field of soil microbiology and to suggest some 

 likely paths for future study. 



This book presents broadly a discussion of the soil microbiological 

 population, the general flora and fauna of the soil, and the mutual 

 interrelationships among microorganisms; the decomposition of plant 

 and animal residues and the formation of humus; the transformation 

 of various elements essential for plant growth; and the general appli- 



