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 Kinits INsects AND CHECKS PuANT 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 Distrisutes 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 
