iv PREFACE 



Again, it is a purpose of this book to make available for 

 students of agriculture, a systematic outline of the work so far 

 accomplished in this particular field. It is to the students of 

 to-day from whom are to come the investigations of the near 

 future that the book is particularly addressed. Some of the 

 details presented in the following pages are matters on which 

 opposed opinions are now held strongly by different authorities, 

 and to the unbiased minds of the coming investigators must be 

 left the decision as to how closely the truth has been approxi- 

 mated in what is written to-day. The field of effort covered 

 by this book is one in which there is an increasing activity, and 

 new facts and deductions will inevitably bring modifications to 

 present opinions. To encourage this further acquisition of 

 knowledge is the main purpose of the book. 



The material brought together in this book has been presented 

 to the faculties and students of several of our Agricultural 

 Colleges, in the form of a short course of lectures. In large 

 part, moreover, it has been published in Volume XIV of the 

 Journal of Physical Chemistry. To make it accessible to and 

 more easily read by one familiar with the progress of technical 

 soil investigations, it has been recast in its present form. 



It has been assumed that the reader will have a fair working 

 knowledge of the concepts of modern chemistry. Nevertheless, 

 an effort has been made to avoid technical terms so far as this 

 can be done without undue sacrifice of lucidity of expression. 

 Free references have been made to the bulletins of the Bureau 

 of Soils, U. S. Department of Argiculture, because they are 

 generally accessible to the American student, and because in 

 them will be found detailed discussions and bibliographical 

 material pertinent to the subjects outlined here. To his coworkers, 

 the author is indebted for many criticisms and suggestions; 

 and more especially in the making of the book is he indebted to 

 Mr. S. C. Stuntz. 



Washington, D. C. 

 1911. 



