COMMERCIAL FERTILIZERS CONCLUDED 213 



These factors are: 



The crop. 



The soil. 



The system of farming. 



THE CROP 



Crop roots differ in their powers of feeding, or 

 their powers of securing plant foods. Some roots 

 can use very tough plant foods, while others require 

 it in the most available form. Some roots secure 

 nitrogen from the air. The cowpea roots, for ex- 

 ample, can take nitrogen from the air and they can 

 use such tough phosphoric acid and potash that it 

 seldom pays to feed them directly with fertilizers. 



A bale per acre crop of cotton requires for the 

 building of roots, stems, leaves, bolls, lint and seed : 



103 pounds of Nitrogen. 

 41 " " Phosphoric Acid. 

 65 " " Potash. 



and yet experiment and experience have proved that 

 the best fertilizer for such a crop contains the fol- 

 lowing amounts of plant food : 



Nitrogen 20 pounds 



Phosphoric Acid 7 



Potash 20 



This means that cotton roots are fairly strong 

 feeders of nitrogen and potash, but are weak on the 

 phosphoric acid side. 



The small grains, wheat, oats, barley and rye, 



