SWEET-POTATO 435 



some clay soils, especially if rich in lime, large yields are 

 made; but here the crop is later, of somewhat poorer 

 quality, and liable to be of inferior appearance by reason 

 of adhering particles of soil. Moreover, harvesting is 

 more laborious in clay than in sandy soils. 



405. Humus. — If sweet-potatoes must be grown where 

 there is much clay, there should be also an abundant supply 

 of humus, so as to make the soil mellow and free from a 

 tendency to bake. In fact, whatever may be the nature 

 of the soil, humus is an important constituent for the best 

 results with sweet-potatoes'. A favorite method of apply- 

 ing it, especially in regions where sweet-potatoes are grown 

 for market, consists in using pine or other leaves from the 

 woods, which are first employed for a number of months 

 as bedding in the stables or barn lots. 



A still more economical method of supplying humus, 

 and with it nitrogen, consists in plowing under a growth 

 of crimson clover a few weeks before setting sweet^potato 

 sUps. 



406. Fertilizers. — As shown in a previous paragraph, 

 both the roots and the vines of sweet-potatoes contain 

 much more potash than either nitrogen or phosphoric 

 acid. Therefore, the fertihzer should be rich in potash. 

 Moreover, sandy soil, the type usually selected for sweet- 

 potatoes, is generally more deficient in potash than is 

 stiff er land. 



This crop makes heavy demands for nitrogen also. The 

 cheapest means of supplying it consist in growing a pre- 

 ceding crop of crimson clover, cowpeas, or other legumes. ' 



Acid phosphate has also been found by experience to 

 be needed in fertilizer formulas for sweet-potatoes. 



