46 AGRICULTURE. [CH. 



plant through its several successive stages of growth 

 by the slow and gradual formation of the am- 

 monia. We have referred to the fact (56) that 

 many soils have little power for retaining ammonia 

 when it is added in a soluble form, and from these 

 soils much is lost by using ammonia in such a form, 

 for instance, as the sulphate of ammonia ; whereas, 

 by selecting fertilizing substances which will supply 

 ammonia slowly by their gradual decomposition, the 

 growing crop has a better opportunity for using it 

 without waste. 



88. Potash exercises an important influence in 

 promoting the fertility of the land. The nitrate of 

 potash, which is a most powerful manure, always com- 

 mands a price considerably higher than nitrate of 

 soda, because the former is used for the manufacture 

 of gunpowder, whereas the latter cannot be so em- 

 ployed. Some other source of potash had therefore 

 to be found (107). The ashes of wood fires yielded 

 small supplies, and their influence upon the land was 

 generally very marked, but the quantity obtained in 

 this way was very small for the purposes of manuring 

 land. Some few years since large deposits of potash 

 salts were discovered in Germany, and they were 

 introduced into this country under the name of 

 kainite. It generally contains about 25 per cent, of 

 sulphate of potash. In some cases the use of potash 

 in this form has been productive of most satisfactory 

 results, but as a rule its use has not realized the san- 

 guine expectations which were so generally enter- 

 tained. This in no way depreciates the value previ- 

 ously set upon a proper supply of potash ; it rather 

 indicates that we have in some degree failed to employ 

 the kainite so as to secure the best results which the 



otash it contains is capable of producing. 



89. Common salt is also largely employed as a 

 manure. It is a compound of chlorine and sodium, 

 and by its use we are enabled to supply to the soil a 



