ROTATION OF CROPS. 157 



Certain kinds of weeds are found in certain 

 kinds of crops; then, if the field is weedy, this 

 particular crop should not be grown until these 

 weeds are killed out. 



II. Exterminates Insect Pests. 



Again, certain crops are more apt to be in- 

 fested with particular insect pests (see " Enemies 

 of Plants"), or fungous (parasitic) plants. If it 

 is known that such enemies have even a start 

 upon a certain field, that crop should not be 

 grown upon it the following year, nor until the 

 pest, whatever it may be, is eradicated. Co- 

 operation of neighbors can greatly facilitate this 

 work. 



C. PROFIT IN ROTATION. 



If there is one crop which can be grown upon 

 a field that is more profitable than another crop, 

 it is the first one to be considered in the system 

 of rotation. This crop, however, should not be 

 repeatedly grown, but such a rotation should be 

 chosen as will best fit the ground for the largest 

 yield of the best-paying crop. 



D SELECTING THE COURSE IN ROTATION. 



I. What Can Be Successfully Grown? 



This will depend upon the kind of soil, the 

 climate, and the seasons. The poorer the soil 

 the shorter the course, and the richer the soil 

 the longer the course of rotation may be. 



