114 The Sweet Potato 



has been grown without alternation. A thousand 

 pounds of sweet potatoes contain 2.9 pounds of nitrogen, 

 .9 pounds of phosphoric acid and 5.1 pounds of potash, 

 while a similar amount of Dent corn contains 16.2 

 pounds of nitrogen, 6.9 pounds of phosphoric acid and 4 

 pounds of potash.^ Thus it is seen that sweet potatoes 

 remove from the soil about two-thirds more potash than 

 nitrogen, while corn removes more than four times as 

 much nitrogen as potash. It would appear that a con- 

 tinuous cropping of corn would quickly exhaust the 

 nitrogen supply, while successive crops year after year 

 of sweet potatoes would tend to deplete the supply of 

 potash more quickly^ than the other food elements. 

 However, this consideration in favor of crop rotation 

 is not considered of as much importance as formerly for 

 the increased yields resulting will ordinarily take from 

 the soil more actual plant-food than would be used by 

 the smaller crops were rotation not practiced. The fact, 

 however, that plant-food in the soil is continually being 

 made available and that deficient crops use the elements 

 in varying proportions may be of importance. 



Boot excretions. 



The accumulation in the soil of certain root poisons 

 or toxins may be avoided by rotation. Frequently when 

 the soil has been well fertilized and diseases kept out, 

 small yields of sweet potatoes are secured on a piece of 

 ground after several years continuous cropping. 

 Although little definite information is available on the 

 subject, it is known that the roots of a plant throw off 

 waste matters and that these excretions seem to be self- 

 poisoning to the plant. Other plants, however, are not 



1 " Feeds and Teeding," Henry and Morrison. 



