ROTATION OF CROPS 233 



the best advantage, but the soil is relieved of excessive 

 demands on special elements. For example, wheat 

 and clover take different amounts of potash and lime 

 from the soil. Wheat has the power of feeding upon 

 silicates of potash which clover cannot assimilate, 

 hence wheat and clover in rotation relieve the soil of 

 excessive demands on the potash. 



318. Nitrogen-consuming and Nitrogen-producing 

 Crops. It is possible in a five-course rotation to main- 

 tain or even increase the nitrogen of the soil without 

 the use of nitrogenous manures. In Section 134 an 

 example is given of a rotation which has left the soil 

 with a better supply of nitrogen than at the begin- 

 ning. When a soil produces a good clover crop once 

 in five years, and stable manure is used once during 

 the rotation, the soil nitrogen is not decreased. By 

 means of rotating nitrogen-producing and nitrogen- 

 consuming crops grain can be sold from the farm 

 without purchasing nitrogenous manures. The con- 

 servation of the nitrogen and the humus of the soil 

 is one of the most important points to consider in 

 the rotation of crops. 



319. Influence of Rotation upon the Mechanical 

 Condition of Soils. With different kinds of crops, 

 the mechanical condition of soils is constantly under- 

 going change. Grain crops and hoed crops tend to 

 make the soil open in texture. Grass crops have the 

 opposite effect. All soils should undergo periodic 

 compacting and loosening. Some require more of one 

 treatment than of the other. In a rotation the 



