148 MANURES. 



an excellent absorbent of ammonia and other gases 

 arising from decomposition hence its usefulness in 

 destroying bad odors, and in preserving fertilizing 

 matters for the use of crops. 



It may be used like plaster, or in the decomposi- 

 tion of organic matters, where it not only hastens 

 decay, but absorbs and retains the escaping gases. 



Lime in combination with phosphoric acid forms 

 the valuable phosphate of lime, of which so large a 

 portion of the ash of grain, and the bones of animals, 

 is formed. This will be spoken of more at length 

 under the head of " phosphoric acid." 



MAGNESIA. 



Magnesia is a constituent of vegetable ashes, and 

 is almost always present in the soil in sufficient 

 quantities. 



ACIDS. 



SULPHURIC ACID. 



Sulphuric acid is a very" important constituent 

 of vegetable ashes. It is sometimes deficient in the 

 soil, particularly where potatoes have been long culti- 

 vated. One of the reasons why plaster (sulphate of 

 lime) is so beneficial to the potato crop is probably 

 that it supplies it with sulphuric acid. 



Sulphuric acid is commonly known by the name 

 of all vitriol, and may be purchased for agricultural 

 purposes at a low price. It may be added in a very 



