256 CROP PRODUCTION 



nitrogen from the air and to fix it so that as the root- 

 stems and leaves decay the nitrogen becomes available 

 for the growth of other crops. There is also thus added 

 to the soil a large amount of green vegetation which 

 helps to form humus, one of the most important elements 

 in the make-up of a soil. 



Another advantage of crop rotation is found in the 

 fact that the roots of different plants penetrate to differ- 

 ent depths in the soil. Consequently, it is possible to 

 plan a rotation so that each crop will derive most of its 

 food from a different region below the soil surface. Such 

 a grass as timothy is known as a shallow feeder, because 

 its roots generally remain within six inches of the surface. 

 Alfaffa, on the other hand, is a deep feeder, its roots 

 commonly penetrating to a depth of from four to twenty 

 feet. Obviously, there is an advantage in including 

 crops with such different root systems in a rotation. 



Another advantage of crop rotation is found in the 

 fact that different plants take different amounts of 

 chemical substances from the soil. While it is not now 

 believed that this fact is of as much importance as was 

 formerly supposed, it still serves as a good reason for 

 practicing an intelligent system of crop rotation. 



In most rotations the different crops require very 

 different methods of cultivation. Thus a hoed crop 

 like corn is often foil wed by a drilled or broadcast 

 crop like wheat. Because of this it is easier to keep 

 in check many sorts of weeds which become unduly 

 abundant when one crop is planted in the same field for 

 many successive seasons. In a somewhat similar way 

 various fungus diseases are checked by rotation, and the 

 injuries of many forms of insect life are prevented. 



