PREFACE. 



I HAVE endeavoured in the following pages to give a concise 

 account of our present knowledge of the soil as a medium for 

 plant life. At first sight the subject appears very simple ; in 

 reality it is highly complex and trenches on several different 

 subjects with which no one individual can claim to have any 

 adequate knowledge, and, what is perhaps a greater disadvan- 

 tage, it has grown up very unsystematically. Chemists, 

 botanists, bacteriologists, geologists, and agriculturists have 

 all contributed something, but usually in connection with their 

 own special problems and not with the idea of developing a 

 new subject. It has usually been reckoned part of the some- 

 what vague mixture known as agricultural chemistry, and has 

 often been considered more suitable for farmers' lectures than 

 for pursuit for its own sake. 



As a result of its history the subject is now in a rather con- 

 fused state. Suggestions thrown out by men eminent in some 

 other branch of science have been accepted without much 

 serious examination ; illustrations used in farmers' lectures to 

 drive home some important point to an audience before whom 

 lucidity is above all things necessary, have acquired the force 

 of established facts ; whilst statements, and sometimes even 

 substances, have come to be believed in for no better reason 

 than that people have talked a great deal about them. 



In. recent years, however, its recognition as a basis of 

 national wealth has given the soil a high degree of technical 

 importance, whilst the remarkable constitution it appears to 

 possess, the variety of its microscopic inhabitants and their 

 close connection with plant life, all impart to its study un- 

 usual scientific interest. The time, therefore, seems ripe for a 

 critical examination of the foundations for our beliefs, and this 

 task is rendered easier by the advances made of late years on 

 the Continent, in America and in this country. As the foreign 

 literature is not generally available for English readers I have 

 given the evidence in some detail, so that my fellow agricultural 

 chemists may see it for themselves and the student of pure 

 science may be able to tell us how far we are justified in using 

 the data as we do. 



E. J. R. 



HARPENDEN, January ', 1912. 



