SWEET-POTATO . 435 



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

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

 ciuahty, and hable to be of inferior appearance bj- reason 

 of adhering particles of soil. ^loreover, harvesting is 

 more laborious in clay than in sandy soils. 



405. Humus. — If sweet -p<jtatoes must be growm where 

 there is much clay, there should be also an abundant supplj* 

 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 -nith sweet-potatoes. A favorite method of apply- 

 ing it, especiall}' 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 supphing humus, 

 and with it nitrogen, consists in plo\\ing under a growth 

 of crimson clover a few weeks before setting sweet-potato 

 shps. 



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 fertilizer should be rich in potash. 

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

 potatoes, is. generally more deficient in potash than is 

 stiffer land. 



This crop makes heavj- demands for nitrogen also. The 

 cheapest means of supplying it consist in growing a pre- 

 ceding crop of crim.son clover, cowpeas. or other legumes. 



Acid phosphate has also been found by experience to 

 be needed in fertilizer formulas for sweet-potatoes. 



