G2 AMERICAN MANURES. 



iron be present, as iron pyrites, a compound of 

 iron and sulphur ; the lime, by absorbing carbonic 

 acid and giving free access to the oxygen of the 

 atmosphere, causes a rapid decomposition of this 

 compound into peroxide, or rust of iron, and sul- 

 phuric acid. This acid at once unites with the 

 lime and forms land plaster or gypsum in the 

 soil. 



The great source of potash and soda in soils 

 is from the disintegration of rocks, such as fel- 

 spar and granite. No matter how fiue these 

 rocks may be reduced, their valuable elements 

 are scarcely at all soluble in water without 

 previous chemical action. In these rocks, known 

 to contain much potash and soda, there must be 

 a chemical decomposition before the potash and 

 soda can be available to plants. These silicates 

 are slow r ly decomposed by the action of carbonic 

 acid, but the action is much more rapid in the 

 presence of lime ; hence its great value for this 

 purpose. 



Another valuable property of lime is, that, 

 in the presence of decaying organic matter, it 

 will decompose common salt, forming carbonate 

 of soda, and chloride of calcium ; the latter being 

 the most soluble form in which lime can be pre- 

 sented to plants. 



The prominent chemical effects of lime in the 

 being stated, w r e proceed to notice its physi- 



