viii PREFACE 



To encourage the teacher, let me say that much of the science 

 of agriculture can be successfully taught without a field or a gar- 

 den, and even without complete knowledge of the art. Thus, you 

 may teach why clover should be grown and when it contains the 

 most nitrogen, but leave the farmer to determine for himself when 

 to plow it under, if he is the better judge of seasonal conditions 

 and of their probable influence upon his own soil and crop. 



CYRIL G. HOPKINS 



NOTE. Opportunity has been taken to insert in this edition the later data 

 from some of the most important long-continued field investigations. In addi- 

 tion, a list of questions relating to the most important facts in every chapter 

 has been prepared for the convenience of the teacher in helping the student 

 to acquire a thorough knowledge of the most essential facts and principles of 

 soil fertility, with economy of time and with conservation of mental energy. 

 These questions are supplied in pamphlet form by the publishers. C. G. H. 



