48 CHEMISTRY, GEOLOGY 



Crops of grain rapidly exhaust the soil of 

 phosphoric acid, magnesia, potash, and soda, as 

 will be seen by looking at the table showing the 

 constituents of the inorganic part, or ash, of 

 wheat, oats, barley, and rye. 



Unless phosphoric acid, magnesia, potash, and 

 soda, be constantly supplied in the form of ma- 

 nure, it will be seen that successive crops of 

 grain must soon take up all those substances the 

 soil contained, and thereby render it incapable 

 of producing further crops. 



The phosphoric acid may be supplied to the 

 soil by the addition of bone-dust, guano, or 

 other substances containing an abundance of 

 phosphorus. Common salt will supply soda 

 and a little magnesia, and wood-ashes will give 

 some potash. Bones also furnish lime to the 

 soil, a substance which oats require in pretty 

 large quantity. 



If the crops raised are always carried away 

 from the soil, and their inorganic elements are 



Questions. Of what substances do grain crops most 

 rapidly rob the soil ? How may the impoverishment of the 

 soil be prevented ? What substances will yield pho&phorus 

 to the soil ? How may soda, magnesia, and potash, be sup- 

 plied ? Do bones supply anything besides phosphorus ? 



