PREFACE TO THE THIRD EDITION 



AFTER an interval of eleven years, even though re- 

 search in Europe has been suspended for five of them, 

 considerable revision becomes necessary in a book 

 dealing with a subject growing so rapidly as the 

 science of the soil. During the period in question 

 the chief addition to our knowledge has been the 

 extended light upon the organisms of the soil and 

 their function that we have gained through the investi- 

 gations of Russell and Hutchinson at Rothamsted. 

 The current views upon the structure of clay and 

 many of the reactions of the soil in which the clay 

 takes part have been considerably modified through 

 the development of the conception of colloids which 

 has taken place of late years. But in many other 

 instances I have found it necessary either to record 

 some additional knowledge or to modify the point of 

 view. 



A. D. HALL. 



London, October 1919. 



