380 ROTATION OF CROPS. 



lated by the profits of the cultivation of this grain, we con- 

 tinue to grow it year after year, without intermission. The 

 result is that sooner or later, often within two or three 

 years we find the yield steadily diminishing. One reason 

 for this is that we have been constantly robbing the soil of 

 undue amounts of phosphoric acid, and (without rendering 

 it unfertile for some other crops, such as potatoes) we have 

 seriously impaired its capacity for the production of wheat. 

 If, instead of raising wheat the second year, we had raised 

 potatoes, or clover, or some plant of an entirely different 

 character from wheat, we should have drawn more evenly 

 on all of the resources of the land, and should have post- 

 poned the exhaustion of its stock of available phosphoric 

 acid. 



Here then comes in play, also, another element which it 

 is necessary for the farmer to consider, namely : there are 

 constantly going on in the soil (which may be considered a 

 natural chemical laboratory) certain chemical and mechani- 

 cal processes, whose effect is to continually set free from 

 other combinations and prepare for the use of plants the 

 various minerals which constitute their ashes. Therefore, 

 if we bring a grain crop into the rotation only once in four, 

 five, or six years, the simple action of these processes will, 

 in the intervening time, set free enough phosphoric acid for 

 a second crop. Soils differ, not only in their composition, 

 but in the rapidity with which their elements are set free ; 

 consequently we find some soils on which the same crop may 



