CROP ROTATIONS 187 



(3) Some crops increase the organic matter of the 

 soil. The roots of grass crops are much more extensive, 

 at least in the surface soil, than those of the grain crops. 

 The grass forms a "sod", the grain crops, a stubble. 

 Brome grass has some disadvantages, but it adds more 

 root' fiber to a soil than any of our other crops. Western 

 rye grass and timothy add root fiber also, but much less 

 than brome. 



(4) Some crops increase the nitrogen as well as the 

 organic matter of the soil. Legume crops such as alfalfa, 

 sweet clover, red clover and peas, if inoculated, may 

 leave the soil richer in nitrogen than before they were 

 grown. Crops with large fleshy roots like alfalfa and 

 sweet clover also add much organic matter to the soil. If 

 a quarter of the land is occupied by one of these crops, 

 and the stubble and roots plowed under, and if an oc- 

 casional catch crop of some similar legume is plowed 

 under, it is possible to maintain the nitrogen and 

 organic matter content of the soil. 



(5) Changing crops lessens plant diseases. Certain 

 diseases such as flax wilt, potato scab and clover sickness 

 are carried over in the soil. These diseases may cause a 

 partial failure in the crops upon which they prey, but 

 have no effect upon other classes of crops. By practising 

 a rotation all may be grown upon the same farm with 

 little danger from this cause. 



(6) Suitable rotations may lessen soil drifting (1) by 

 having the ground covered with a growing crop at the 

 season when drifting occurs or (2) by increasing the 

 cohesive power of the soil. Soil drifting is usually most 

 serious here in May. If the ground is covered with grass 

 or other hay crops, or even winter rye, it will be pro- 

 tected. If the soil is rich in organic matter, as after a 



