FERTILIZERS, ROTATIONS FOR COTTON 99 



A crop of cowpeas yielding 1J/2 tons of hay to the acre 

 will, if returned to the soil, increase the nitrogen supply 

 supply approximately 65 pounds to the acre. This is 

 assuming that the cowpea secures from 40 to 45 pounds 

 of nitrogen from the air for each ton of hay it produces, 

 the nitrogen contained in the roots and stubble being no 

 more than that furnished by the average soil. To add 

 this much nitrogen would require 930 pounds of cotton- 

 seed meal or 433 pounds of sodium nitrate. In addition, 

 the organic matter supplied by the cowpeas is usually of 

 greater value than the nitrogen. Similar yields of soy 

 beans and crimson clover would supply to an acre, 75 and 

 70 pounds of nitrogen, respectively. 



Non-leguminous green-manure crops, such as the small- 

 grains, millet, and the like, while not increasing the 

 amount of nitrogen in the soil are, nevertheless, nitrogen 

 savers, owing to the fact that they prevent loss from 

 leaching and erosion. 



112. Will crop rotation maintain fertility? It must 

 not be assumed that growing cotton in a rotation which 

 supplies the soil with an abundance of organic matter 

 will necessarily eliminate the need of commercial fertil- 

 izers. Such a system will render the use of nitrogenous 

 fertilizers unnecessary, and mineral fertilizers will not 

 be needed on soils that contain an abundant natural 

 supply of phosphoric acid and potash. However, much 

 of the soils in the cotton-belt are quite deficient in 

 phosphoric acid and, to a less extent, in potash. Maxi- 

 mum yields on these soils cannot be obtained without 

 the application of materials containing phosphoric acid 

 and potash. 



113. Rotations for cotton. A good rotation applica- 

 ble to the greater part of the cotton-belt is: first year, 



