200 WALL'S MANUAL 



ing plants. The action of the carbonic acid upon 

 the silicates separates the potash or the alkaline part 

 of the silicate, and makes it soluble, to become food 

 for plants. 



The fact most important for the farmer in these 

 changes is that the compounds are present in all soils, 

 becoming salts and food for plants. Whenever iron 

 pyrites or sulphuret of iron is found, and it is present 

 in nearly all soils, exposure to air and moisture acid- 

 ifies the sulphur, and forms oil of vitriol or sulphuric 

 acid. This acid immediately combines with iron and 

 forms copperas ; or with alumina, forming alum ; or 

 with lime, forming sulphate of lime (plaster) ; or with 

 magnesia, forming epsom salts. All these salts are 

 liable to be decomposed by any soluble alkali, as 

 carbonate of potash, soda, etc., which may be pro- 

 duced by the decomposition of the silicates. 



Among the most important salts in the soil arising 

 from those actions, are sulphate of lime (or plaster), 

 phosphate of lime, and of alumina and iron. The 

 sulphate of lime is partially soluble, but phosphates 

 are more insoluble, and are always found in soil. 



It is not easily understood how phosphate of lime 

 exists in the soil, but that it does exist none can 

 doubt. The proof may be stated in a few words, 

 Jjoncs of all grazing animals contain about half their 

 weight of the phosphate of lime. It can be derived only 

 from their food, and that from the soil. The actual 

 result of chemical analysis confirms this statement. 

 Beets, carrots, beans, peas, potatoes, cabbages, etc., 

 afford phosphate of lime, magnesia and potash. Indian 

 corn, rice, wheat, barley, oats, rye, and cotton seed 

 and plant, contain sulphate and phosphate of lime. 



