8 THE COW PEA. 



crop draws from the soil, and what even the least worn 

 fields must have returned to them in the shape of plant food 

 to prevent the soil from gradually losing its fertility. Let it 

 be carefully noted, however, that there is, of necessity, some 

 waste in the application of fertilizers; that not all the 

 plant food given a soil can be gathered up and realized 

 upon in crops. There is a loss in operation in the soil just 

 as there is in a machine or in applying any other chemical 

 or mechanical force, natural or otherwise. With ordinary 

 fertilizers, this loss in potash has been roughly estimated 

 at about one-third; in phosphoric acid, not less than one- 

 half. Therefore, the actual amount of potash and phos- 

 phoric acid to be provided, to enable cow peas to take up 

 and assimilate 65 pounds of nitrogen, is about 167 pounds 

 of actual potash and 40 pounds of phosphoric acid (equiv- 

 alent to 334 pounds of muriate of potash and 300 pounds of 

 acid phosphate). This is not given here as a fertilizer for- 

 mu)a for cow peas, but merely to show what an acre of the 

 crop must actually have. 



As already stated, and now repeated and emphasized, 

 plant growth can take place only when all three of the 

 necessary constituents are present in sufficient quantities 

 and in an available condition. The moment the supply of 

 any one of these essential constituents is exhausted, nor- 

 mal growth stops and there cannot be a full crop. After 

 the cow pea has developed a few leaves and so long as the 

 plant can continue to take up all of the potash and phos- 

 phoric acid required, it will in turn get all the nitrogen it 



