ROTATION OF CROPS. 381 



safely be tried every second or third year, and others on 

 which we must allow a much longer interval. 



The same rule that applies to the soil holds good also with 

 regard to manures. These almost always contain various 

 matters which go to feed plants, and we must study to so 

 arrange our crops as to make profitable use of all that they 

 can yield ; and, if they are of a sort to need time and the 

 action of the chemical and mechanical influence of the at- 

 mosphere and of the soil for the complete development of 

 all of their constituents, we must adjust our crops, so far as 

 possible, to take up these constituents as they are prepared 

 for use. 



The foregoing is the basis of the chemical theory of rota- 

 tions. 



In addition to this, we must consider the influence exerted 

 on the soil by the roots which are left in the ground when 

 the crop is removed. This element of the influence which 

 plants exert on plants which are to follow them in the same 

 soil is especially important in the case of clover, which is 

 so active in its fertilizing effect, that it may be assumed that 

 we have overcome our great difficulty in bringing up a poor 

 soil when we have enabled it to grow a good crop of clover. 

 One especial virtue of this plant is that it sends its roots far 

 into the subsoil, and thus appropriates, by means of its vigo- 

 rous feeding powers, useful materials which were out of the 

 reach of the roots of plants of other species. These materials 

 are deposited in the substance of the plant, and (on its decay 



