THE ROTATION OF FARM CROPS 63 



the land is shown by the fact that eighty bushels of corn 

 have been grown on alfalfa sod when fifty bushels could not 

 be grown on this land before it had been put in alfalfa. 



(We tell of our own experience in growing alfalfa in the 

 next chapter.) 



ROTATION KILLS INSECTS AND CHECKS PLANT DISEASES 



Rotation not only gives opportunity to improve the phys- 

 ical condition and increase the fertility of the soil, but it 

 may also be made to head off many kinds of insect enemies 

 and plant diseases. If one kind of crop is grown year after 

 year on the same field, its insect enemies are likely to multi- 

 ply rapidly since they are continually supplied with the par- 

 ticular kind of food upon which they thrive best. Because of 

 the fact that changing cuts off this food supply for a time, 

 intelligent crop rotation has been found ;more effective than 

 all other methods combined in the economical checking of 

 insect and fungous pests. (In the chapter entitled "Dis- 

 eases and Insects" we are telling in detail how crop rotation 

 is effectively checking the corn root worm.) 



Crop rotation is as effective in checking many of the 

 smuts, rust, and blights as it is in checking the insect pests. 

 Since the annual damages to the crops from insects alone 

 amounts to several millions in each state, too much stress can- 

 not be laid on any method that will check them. Even if 

 crop rotation were not essential to the maintenance of soil 

 fertility it would be necessary to rotate to keep in check 

 the insect pests. 



ROTATION DISTRIBUTES FARM WORK 



Another very important reason for practicing crop rota- 

 tion is that it distributes farm labor evenly over a long period 

 of time. When a rotation such as corn, oats, wheat and clover 



5 



