X PREFACE 



at Cornell University to the Graduate School of 

 Agriculture of the United States Department of Agri- 

 culture in July 1908. In this way, much of the sub- 

 stance of Chapters II., III., IV., V., and VI. has already 

 been printed in the Journal of the Society of Arts, the 

 greater part of Chapter VI I. has appeared in the 

 Journal of the Board of Agriculture, and Chapter X., 

 the last of my American lectures, was printed in 

 Science. 



I have drawn so freely upon my friends for informa- 

 tion, that it seems invidious to single out for thanks 

 some more than others : but I owe much to Dr E. J. 

 Russell of this Laboratory, who has read and criticised 

 parts of the text ; to Dr J. A. Voelcker, who has so 

 often placed the results of his wide experience at my 

 disposal ; to Mr H, Voss of the Anglo-Continental 

 Guano Co., and to Mr T. Elborough of the Lawes 

 Chemical Manure Co., who have furnished me with 

 many facts and figures respecting the trade in ferti- 

 lisers ; and once again, to Mr G. T. Dunkley of the 

 Rothamsted Laboratory, who has been indefatigable in 

 verifying references and in securing the accuracy of the 

 many figures and tables the book contains. 



A. D. HALL. 



The Rothamsted Experimental Station, 

 Harpenden, December 1908. 



