112 



THE PRINCIPLES OF AGRICULTURE. 



4. It secures the varied advantages of cultivating the 

 soil. 



When land is continually occupied with one crop, like 

 grass, it may become too hard and compact, and the for- 

 mation of plant food may be checked. By stirring such 

 soil, and exposing it to the atmosphere in cultivation, we 

 hasten the chemical changes, and unlock the supply of 

 food which nature has in store. 



There are many old meadows and pastures, regarded as 

 worn out and worthless, which, if subjected to thorough 

 cultivation, might yield profitable returns. 



5. It prevents the increase of weeds and injurious 

 insects. 



In cultivating certain crops, we may supply conditions 

 favorable to the growth of certain varieties of weeds. If 

 the crop is continued in the same soil, these weeds will 

 naturally increase. Proper management requires a 

 change to some other crop with which such weeds will 

 not thrive. 



It is well known that injurious insects, like the potato 

 beetle, will tend to increase on land which is repeatedly 

 occupied by the particular crop upon which they are 

 accustomed to prey. 



The following are examples of rotations which are 

 sometimes adopted : 



Indian corn 

 Potatoes 

 Wheat . . 

 Clover . . 



Clover . . 

 Tobacco . 

 Wheat . 



