FERTILIZERS 147 



valley should yield good crops without commercial fertilizers 

 for over a thousand years. l l From 7 to 135 pounds of nitrogen, 

 from 3 to 55 pounds of phosphoric acid, and from 3 to 36 

 pounds of potash are sold with every ton of produce leaving the 

 farm." Eighty billion pounds of nitrogenous material en- 

 tered into the creation of one harvest in the United States in 

 the early nineties. The cereals annually took from the earth 

 nearly 3 billion pounds of phosphoric acid, and the loss of potash 

 was not less than 4 billion pounds. 



1 Yearbook U. S. Dept. Agr., 1897, p. 301. 



