84 



CHEMISTRY OF FARM PRACTICE 



differ considerably, and the area occupied by the root 

 system limits the area from which the plants feed. The 

 cultivation of crops differs, and the influences of the culti- 

 vation of a previous crop must be considered when we 

 plan a rotation. 



The practice of the proper systems of rotation makes 

 it possible to maintain, at low cost, the supply of organic 



FIG. 30. Field of Cowpeas ready to plow under. (Farmers' Bul- 

 letin 278, U. S. Dept. Agr.) 



matter in the soil. Organic matter may be supplied in 

 the form of animal manures, but this source of supply 

 is very limited when we consider the total area in culti- 

 vation. Some organic matter is also accumulated in pas- 

 tures and in woodland, but these methods of incorporating 

 organic matter are quite slow. The incorporation of resi- 

 dues from field crops, especially the leguminous crops, is 

 the best method for increasing the amount of organic 

 matter in the soil. 



