go SCIENTIFIC AGRICULTURE. 



crops, result from the following considerations : In 

 the first place, different crops require elements in 

 different proportions ; one requires more potash than 

 another, or lime, or phosphoric acid, or nitrogen, or 

 some other constituent. By reference to the table 

 in paragraph 54, it will be seen that potatoes require 

 more potash than wheat or corn, while these require 

 more phosphoric acid ; clover and tobacco need a 

 great deal of lime, and so with other crops. A num- 

 ber of crops of potatoes in succession, without the 

 addition of potash in some form, will use up the 

 available supply, unless the quantity be very large. 

 Before this comes to pass, another crop that re- 

 quires less of potash but more of some other ele- 

 ment might grow well, while the potatoes would not 

 flourish. 



179. In the second place, rotation of crops gives 

 time for the disintegrating action of the atmosphere, 

 rain, and frost to prepare new material from the 

 rock-particles in the soil, and get it in a form to be 

 used by the plant. One crop may use up the avail- 

 able food of a particular kind faster than it can be 

 prepared by these natural agencies. 



1 80. In the third place, rotation, or change of 

 crop, when properly managed, enables one plant to 

 prepare food for another. Thus clover sends a long 

 tap-root deep down into the soil, and brings up food 

 to the surface. When the roots decay, the wheat- 

 plant that has surface-roots mainly can use the food 

 prepared by the clover. 



