SOIL FERTILITY AND PLANT GROWTH 249 



mies of plants, as we have already seen. The pests that 

 attack one kind of crop die out when another crop is raised 

 on the field. The growing of the nitrogen-gathering legumes 

 on every part of the farm in succession is allowed by rota- 

 tion, thus saving the necessity of buying commercial nitro- 

 gen fertilizers. 



The crops to use in a rotation. It is evident that no 

 universal standard rotation can be prescribed. Both the par- 

 ticular crops and the order must be decided by local con- 

 ditions and requirements. The rotation is usually based on 

 some one principal crop, the other crops being arranged to 

 favor this. For example, corn is the basis of rotation in 

 the states constituting the "corn belt." A five-year rota- 

 tion with corn occupying about forty per cent, of the farm 

 might be something as follows : 



It will be noted that this arrangement provides for two 

 fields of corn each year, and one field each of oats, clover, 

 and pasture. Of course other crops might be inserted in the 

 rotation. With alfalfa in the rotation, a longer cycle is 

 needed, since it does not pay to plow alfalfa up so soon after 

 planting. 



TOPICS FOR INVESTIGATION 



1. Has the yield of the principal crops increased or 

 decreased in your region within the last ten or twenty 

 years ? To make sure of this, ask your father, or some one 

 who has farmed in the vicinity for some time. 



2. Are there run-down farms in the neighborhood? 

 If so, is the land naturally poor, or has it been depleted by 



