98 FIELD CROPS FOR THE COTTON-BELT 



110. Green-manures and the supply of organic mat- 

 ter. Ordinarily, cowpeas, soy beans, and crimson 

 clover will yield at least !}/ tons of dry matter to the acre 

 in tops and roots. This dry matter when plowed into the 

 soil is equivalent to an application of six tons of average 

 barnyard manure, containing 25 per cent dry matter and 

 75 per cent water. Very few farmers in the cotton -belt 

 produce a sufficient amount of barnyard manure to enable 

 them to apply six tons of manure to every acre of cul- 

 tivated land on their farms once every four years. Prac- 

 tically all of them can easily add the equivalent of this 

 much manure to their soils once every three or four years 

 by the use of green-manures. Whether or not the entire 

 crop should be plowed into the soil, or merely the roots 

 and stubble, will be determined largely by the needs of the 

 soil for organic matter and nitrogen. On soils that are 

 quite deficient in organic matter, it will in general be a 

 good practice to return the entire crop. Otherwise, the 

 crop should be harvested for hay and the manure returned 

 to the soil. 



The Alabama Agricultural Experiment Station reports 

 an increase in yield in one case of 696 pounds of seed cotton 

 to the acre, or 83 per cent, due to plowing under a crop of 

 cowpea vines on land which had been in cotton the pre- 

 vious season. 



111. Green-manure crops and the nitrogen supply. 

 Nitrogen is the most costly constituent of commercial 

 fertilizers, its commercial value usually ^being more than 

 three times that of either phosphoric acid or potash. For 

 this reason the farmer should attempt to secure from the 

 air (which is 4 /5 nitrogen) the greatest part of the nitrogen 

 needed in the production of his crops by the introduction 

 of legumes into his cropping system. 



