Tillage, Fertilizing and Rotation 115 



noticeably injured by tbis poison and after a few years 

 cropping in other varieties the toxid effect is destroyed 

 by natural soil forces. 



Mechanical effect on soil. 



The prevailing opinion that " sweets " are hard on 

 land is not without foundation. Not only are large 

 amounts of nitrogen and potash removed but frequently 

 the crop is not dug until late and the ground is often 

 left bare all winter when the leaching rains rob the soil 

 of its available plant-food. 



Hogs are often left in the potato fields in wet weather 

 when their rooting causes the soil to become hard and 

 baked when it is dried out. The soil should never be 

 molested when muddy, either by animals or otherwise. 



On heavy soils, the vines, especially when heavily 

 manured, become very rank and when turned under 

 cause too much acid in the soil. Under such conditions, 

 an application of raw phosphate rock would be advis- 

 able. Sweet potato land should be amply supplied with 

 organic matter but it should be well decayed and the 

 soil should be loose and mellow. 



Proper rotation of crops provides for sowing a winter 

 cover-crop to prevent leaching; for the growth of 

 legumes to be turned under and allowed to decay 

 thoroughly ; and for the alternation of deep and shallow- 

 rooted crops which will preserve and improve, the 

 mechanical condition of the soil, making it easier and 

 more economical to cultivate and allowing the produc- 

 tion of a crop of the best quality. 



Insect, fungous and weed enemies. 



One of the most important advantages of crop rota- 



