26 NATURE AND PROPERTIES OF SOILS 



17. Carbonation. — The process of oxidation is almost al- 

 ways aecompanied by the action of carbon dioxide. This gas 

 is a constituent of the air and is a product of the organic 

 decay which vigorously progresses in most soils. It occurs 

 in large amounts in rain water, especially in warm climates. 

 It increases the solvent action of water by actively engaging 

 in chemical reactions, producing carbonates and bicarbonates 

 with the various rock and soil bases. The decomposition of 

 orthoelase and muscovite mica into kaolinite and carbonates 

 is as follows: 



2KAlSi,0s + 2H2O + CO2 = H^AlgSioO^ + K^COg + 4Si02 

 Orthoelase Water Carbon Kaolinite Potassium Silica 



Dioxide Carbonate 



2H.KAl3Si30,o + CO. + 4H2O = SH.Al^SiaOg + K.COg 



Muscovite Carbon Water Kaolinite Potassium 



Dioxide Carbonate 



Under certain conditions decarbonation may occur. When- 

 ever the processes of weathering produce either mineral or 

 organic acids carbonates are rapidly decomposed. The 

 presence of unsaturated aluminum silicates may also rapidly 

 promote decarbonation by absorbing the base and liberating 

 the acid radical. This latter reaction is of especial importance 

 in soil. 



18. Hydration, — ^AU the chemical transformations above 

 discussed depend on the presence of a certain amount of 

 water, especially if rapid changes are to occur. The illus- 

 trative reactions already cited indicate this. Oxidation pro- 

 ceeds but slowly in a dry atmosphere, w^ater being necessary 

 as a catalytic agent. In the carbonation of the potash of 

 orthoelase and mica, water enters into the reactions, produc- 

 ing not only kaolinite but also potassium hydroxide, which is 

 later changed to the carbonate. 



Water functions in the chemical changes of rock and soil 



