178 



ELEMENTS OF AGRICULTURE 



with shallow growing roots should be rotated with 

 crops that have deep roots. Deep-growing roots draw 

 much food from the subsoil. 



3. Some crops require specially large, amounts of 

 some particular plant food. Thus tobacco requires 

 about ten times as much potash as phosphoric acid, 

 and if such a crop be grown continuously upon a soil 

 the potash becomes exhausted before the phosphoric 

 acid. Tobacco should be rotated with some crop that 

 requires large amounts of phosphates — wheat, for in- 

 stance. 



4. From the growth of some crops the soil becomes 

 infested with weeds. Thus daises or wild carrots may 

 become very numerous in a pasture or hayfield. When 

 weeds become too numerous the field should be planted 

 in some cleanly cultivated crop, such as corn, tobacco, 

 or cotton. 



5. Many crops have special insect enemies that feed 

 upon them. Thus tobacco has the tobacco worm, 

 wheat the Hessian fly, potatoes the potato beetle, and 

 almost every crop grown has its special enemies among 

 the thousands of insects found on the farm. If the 

 same crop be grown upon a field year after year, these 

 insect enemies increase, forihey are supplied with just 

 the food they require. If, however, the kind of crop 

 be changed, the insects suffer. A tobacco worm hatched 

 in a wheat field finds nothing to feed on, and a potato 

 beetle starves in a tobacco patch. 



G. Most crops are subject to attacks of some special 

 disease which may not affect other crops. Wheat 

 suffers from a disease called rust, but tobacco is not 



