CULTIVATIOX. Ill 



If potatoes should be raised upon the same land con- 

 tinuously, the supply of available potash would soon be 

 exhausted, while there might still remain in the soil an 

 excess of phosphoric acid. 



In order to continue to raise potatoes upon this land, 

 it would l)e necessary to undergo the expense of adding 

 a su})ply of potash. At the same time, the surplus of 

 phosphoric acid, for which the potatoes have no use, 

 might be washed out of the soil and wasted. If some 

 crop, like wheat, requiring more phosphoric acid should be 

 substituted for potatoes, a good croj) might be obtained, 

 and in the mean time the decomposition going on in the 

 soil would render a new sujjjjly of potash available. 



2. It keeps the land occupied. 



Many of the cultivated crops have a short season of 

 growth. After they are harvested, the land remains un- 

 occupied until the following season. 



During this time the formation of plant food in the 

 soil is continued, and is liable to be lost, as there are 

 no growing crops present to make use of it. 



By following the crop immediately with some other, 

 as winter grain, or grass, such hjss may be avoided. 



3. It prevents the loss of substances which have been 

 washed doum into the subsoil. 



The roots of some plants naturally grow near the sur- 

 face of the soil. In such cases, some of the elements of 

 food may ha washed down below the reach of these roots. 

 By following with a crop whose roots tend to penetrate 

 farther downward, these may be gathered up and saved. 



Clover, which has a deep tap-root, will generally grow 

 well after crops having branchiug roots, like wheat or 

 barley; and, on the other hand, these will thrive well 

 after clover. 



