200 ROTATION OF CROPS. 



CHAPTER XXI. 



O P ROTATION OP CROPS. 



710. By Rotation of Crops is meant raising a scries of 

 different crops in regular succession. A farmer turns 

 up a lot of his pasture land, and raises, this year, a crop 

 of potatoes ; next year, on the same land, a crop of corn ; 

 next, a crop of rye ; next, clover and grass. This is a 

 common four-fold rotation. 



The object of rotation in crops is to make a field 

 or a farm, yield, with a certain amount of labor and of 

 manure, the greatest possible amount of valuable crops, 

 with as little exhaustion of the soil as possible. 



The reason for a rotation of crops is that no two 

 plants, of different kinds, require the same substances, in 

 the same proportion, for their nourishment. The grains 

 and the grasses may soon exhaust the supply of silica. 

 They should, therefore, not immediately succeed eacli 

 other in a rotation. They should be each followed by a 

 crop which needs less of silica but more of potasli or some 

 other mineral salts. A field which would not yield a 

 second good crop of wheat, may, even without manure, 

 give a very good crop of clover, of turnips or of carrots. 



711. The Important Principles in the rotation of crops 

 are 1st, that though a soil may contain all the mineral 

 sulislances necessary for the nourishment of every variety 

 of cultivated plants, there is only a limited supply of the 

 mineral food necessary for a particular plant ; 2d, some 

 plants, like the grains, draw their nourishment from 

 near the surface ; others, like carrots and parsnips, draw 



