ACKNOWLEDGMENTS 



The author desires to acknowledge his indebtedness 

 to the bulletins issued by the agricultural experiment 

 stations in the various states of the Union which 

 bear upon the subject discussed in the pages of this book. 

 He also desires to acknowledge his indebtedness to the 

 experiment stations of Michigan, Minnesota, Iowa, Illinois, 

 North Dakota and Washington for the sketches furnished 

 by these respective stations. 



