72 AMERICAN MANURES. 



with the nitrate of lime, magnesia, or soda, or 

 with their chlorides and sulphates. 



Potash, as before stated, exists in plants ; in 

 general, combined principally with organic 

 acids. This potash may be extracted from the 

 ashes of wood and plants, by lixiviation with 

 lime and water. If the water of the caustic 

 lye thus produced be evaporated, and the resi- 

 due be calcined, in order to eliminate any organic 

 matter remaining, crude caustic of potash is pro- 

 duced. This crude substance contains about 60 

 per cent, of potash, mixed with various impuri- 

 ties, together with a considerable amount of 

 chloride of potassium (muriate of potash), and 

 sulphate and silicate of potassa. This crude 

 potash is largely used in the manufacture of 

 soap and glass ; and, until the recent discovery 

 of extensive deposits of crude potash salts at 

 Stassfurth, in Prussia, was the source of most 

 of the salts of potassa used in the arts and in 

 medicine. 



The value of wood ashes as a manure was 

 known at an early period. The old Roman 

 farmers used it, and practised paring and burn- 

 ing the soil, and also burnt the stubble of their 

 wheat fields, in order to enrich the succeeding 

 crop ; a practice which was also prevalent 

 among the ancient Jews. Cato recommends the 

 burning of leaves and branches of trees, and 



