OP AGRICULTURE. 41 



stone, or other rock, in a peat or muck bog, where 

 these vegetable acids abound. The rocks, before 

 dark and solid, come out of the peat perfectly white ; 

 the granite has lost its felspar, and its mica is decom- 

 posed, and potash and soda extracted, while only the 

 pulverized mica remains. A rock containing lime is 

 soon deprived of the same, and cavities are left where 

 it formerly existed. In a similar manner all vegetable 

 mold acts upon the minerals in the soil, disengagign 

 from them their potash, soda, lime and magnesia. 



A still further change takes place more readily in 

 cultivated soils, the slow decay of vegetable mold 



J o 



producing carbonic acid gas, which is also an active 

 decomposer of the silicates, freeing potash, soda and 

 lime, and is a great solvent of the carbonate of lime. 

 This decomposing agent is constantly at work during 

 the plowing of the soil; but it is not the only agent 

 operating to renovate the soil, for at the same time 

 the clay and mold arc engaged in absorbing the 

 minute quantities of ammonia which descends with 

 the falling rain or snow, or is in the air. Thus, a 

 complicated but beautiful exchange takes place among 

 the salts in the soil. 



When we mix acids and alkalies together in the 

 soil, the result is neutral salts ; and since vegetable 

 acids have a great affinity for ammonia, with which 

 they readily combine, they prevent the escape of this 

 valuable gaseous manure, and preserve it in its most 

 available form as food for plants. It has been ascer- 

 tained, by experiment, that ammonia, in combination 

 with humic and other vegetable acids, is actually 

 absorbed and digested by plants. 



Chemistry has verified and justified the experience 



