ROTATION OF CROPS. 379 



The principle on which rotations are based may be readi- 

 ly understood from the following illustration : 



What are known as the root crops contain, in their ashes, 

 a very large proportion of potash. The average amount of 

 this substance contained in the ash of potatoes, turnips, 

 beets, and carrots, is fully fifty per cent, of the whole ; that 

 is, they contain as much of this single ingredient as of all 

 the other mineral ingredients combined. Wheat, rye, oats, 

 and barley, on the other hand, contain an average of only 

 twenty -five per cent., or only one-half as much of this as of 

 all the other ingredients. 



If we examine their content of phosphoric acid, however, 

 we shall find the case quite different. For instance, the 

 four root crops above named contain an average of only 

 about thirteen per cent, of this element, while the four 

 grain crops contain an average of about thirty-seven per 

 cent. 



Again, lime forms but about three per cent, of the ash of 

 most root crops, while it exists in clover and most of the 

 fodder plants to the extent of about thirty-five per cent, of 

 their ash. 



If we were to follow through the whole range of the 

 mineral constituents of our crops, we should find similar 

 variations in the amounts appropriated by the different 

 plants which are commonly grown on our fields. 



Now, suppose that on a field of average quality we find 

 that wheat or some other grain grows to advantage. Stimu- 



