iv PREFACE 



those who have helped give authority to much that appears in 

 the following pages. 



Particular reference should be made to Mrs. Mildred Douthitt 

 Hiers, Mr. Charles Stephenson, and Mr. Clyde Hissong for advice 

 and suggestions as to arrangement of subject matter from the teach- 

 ing standpoint, and to Mr. Hissong, especially, for trying out the 

 material thus arranged in a Smith-Hughes high school; to Miss 

 Miss Grace Kiernan for compiling index and reading proof; to Pro- 

 fessor John V. Ankeney, of the University of Missouri, for reading 

 the manuscript and for several illustrations; to Professor Bruce 

 Fink, of Miami University, for critical reading of Chapters I, XIV 

 and XV; to Professor T. L. Harris, of the University of West 

 Virginia, for suggestions in preparation of Chapters XXVIII, 

 XIX and XXX; to Dean Alfred Vivian, of the Agricultural Col- 

 lege of Ohio State University, for comments on correct theories and 

 practices relating to soil fertility and use of fertilizers; to Professor 

 G. I. Christie, Director of the Agricultural Experiment Station, 

 Purdue University, and to members of the staff, for many help- 

 ful suggestions and for reading the proof of the entire book; to 

 Mrs. Emma Johnson Davis, wife of the author, for her great 

 help in preparing the manuscript for publication and in reading 

 the proof sheets. 



Acknowledgement is given in the description of the figures 

 as they appear in the text for those not made by the author or 

 under his direction. 



