ACTION OF ELEMENTS OP SOIL. 47 



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 al- 

 kali, which may be produced, by the decomposition 

 of silicates. 



80. Among the most abundant salts in soil, aris- 

 ing from the actions (79) are those, which are very 

 insoluble in water, and not liable, therefore to be 

 drained off, when not required by plants. These 

 are sulphate of lime, and phosphates of lime, and of 

 alumina, and iron. The sulphate of lime is easily 

 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 grad- 

 ual 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 solu- 



