60 ACTION OF ELEMENTS OF SOIL. 



79. The fact most important to the farmer in these changes 

 is, that these compounds are continually, in all soil, becoming 

 salts. Whenever iron pyrites, or sulphuret of iron, is 

 found, and it is very widely diffused, exposure to air and 

 moisture acidifies the sulphur, it forms oil of vitriol, or sul- 

 phuric acid. This immediately combines with iron, and 

 forms copperas, or sulphate of iron, or with alumina, form- 

 ing alum, or with lime, forming plaster of Paris, or with 

 magnesia, forming Epsom salts ; all these are salts, and 

 liable to be decomposed by any free alkali which may be 

 produced by the decomposition of silicates. 



80. Among the most abundant salts in soil, arising frorrx 

 the actions (79), are those which are very insoluble in water, 

 and not liable, therefore, to be drained oflt* when not required 

 by plants. These are sulphate of lime, and phosphates of 

 lime, and of alumina, and iron. The sulphate of lime is 

 partially soluble, and hence, is found in all river and spring 

 water ; but phosphates are more" insoluble, and are always 

 found in soil. 



81. That sulphate of lime might possibly exist in soil, has 

 been admitted by all who understood the actions (79), and 

 adding to this the fact of the gradual decomposition of the 

 silicates by carbonic acid, the function of sulphate of lime in 

 soil was easily admitted. The double silicates of lime and 

 potash are universally diffused, and in the order of affinities 

 sulphates of alkalies and of lime result. 



82. It is not so easily understood how phosphate of lime 

 should exist in soil. The true source, both of sulphate and 

 phosphate of lime, and of the solubility of silica is yet to be 

 detected by exact chemical analysis. It is to be looked for 

 in the sulphurets and phosphurets of silicon, which probably 

 exist in rocks. The action of sulphuret of iron, as explained, 

 ^ould demand its universal diffusion,' to account for the 



