SOIL FERTILITY 327 



6. Crop Rotation and Fertility 



The rotation of crops can not properly be said to in- 

 crease the fertility of the soil. For every crop removes 

 from the soil some quantity of each of the elements required 

 for plant growth. Certain very definite advantages come 

 from rotation, however, which at least save the rapid soil 

 exhaustion resulting from growing one crop continuously. 

 Rotation also brings increased yields. In this sense a proper 

 rotation may have the same effect as the application of a 

 fertilizer, though it can never serve as a substitute. 



What is meant by rotation of crops. By rotation of 

 crops is meant a regular order followed for a period of years, 

 and alternating on different fields. If this order is hit-and- 

 miss, or the result of whim or chance, it can not be called a 

 rotation. Rotations may be planned on a two-year, three- 

 year, four-year, or any other cycle. 



What is accomplished by rotation. Rotation of 

 crops improves the physical condition of the soil. Grasses 

 and legumes have a larger supply of roots than most cereals, 

 and so increase the organic matter. Different crops send 

 their roots to different depths, and so use new portions of 

 the soil. The cultivation of inter-tilled crops clears the soil 

 of weeds, and opens it up to air and moisture. 



Rotation also aids in destroying insects and other ene- 

 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- 



