118 AGRICULTURE 



the growing corn plants to improve the soil. Figure 72 

 shows that the nitrogen in the stubble of a previous crop 

 of vetch, a leguminous plant, increased the yield of corn 

 planted as soon as the vetch was cut. 



Another reason for rotating crops is to diversify the 

 farm products. The farmer who grows several crops has 

 his labor better distributed over the entire year than the 

 farmer who grows only one or two crops and is less injured 

 if storm, accidents, or low prices cut off his profit on one 

 crop. 



Rotation to avoid diseases and insect pests. Every 

 cultivated plant has its own special diseases and insect 

 enemies that do not attack most other farm crops. Many 

 kinds of disease germs and insects remain alive in the 

 soil for one or more years, ready to do injury when the 

 proper plant is grown on that field. If the plants that 

 would be attacked by these pests are kept away from 

 that field for a few years, all or most of the germs or 

 insects die of starvation. If, however, the plant subject 

 to attack is put on the same land again, the pest increases 

 and does more and more harm. 



Examples of rotation. Rotation must vary with the 

 kinds of crops to be grown and with the number of acres 

 given to each. Here is a rotation in which about one third 

 of the land is to be used for producing cotton : - 



Plant one third of the land in cotton ; one third in 

 corn, with cowpeas sown later between the corn rows 

 (Fig. 73) ; and one third in oats or wheat, planting cowpeas 

 in June after the grain is harvested. This is called a three- 

 year rotation, because at the end of three years each 



