CHAPTEE IV. 



THE EEACTIONS OCCUBEING IN SOILS. 



THE chemical changes occurring in a soil are numerous and complex. 

 To acquire a general knowledge of their nature and direction is diffi- 

 cult, especially as they are greatly influenced by circumstances, e.g., 

 temperature, strength of the solution with which the soil is moistened, 

 free or difficult access of air, and a number of other constantly 

 varying conditions. It is therefore only possible to indicate some of 

 the actions which take place and to describe, to some extent, the effect 

 thereon of variations in conditions. 



CHANGES IN THE INORGANIC MATTER. The inorganic 

 portion of the soil is subjected to the same actions of the air and water 

 as gave rise to its formation from the original rock and which have 

 been already considered. It is to be noted, however, that the changes 

 probably proceed at an accelerated rate because of the larger quantity 

 of carbon dioxide provided by the decomposing organic matter of the 

 soil. Fragments of felspar, mica and other minerals are thus exposed 

 to the action of air and of water rich in carbonic acid. Their disinte- 

 gration and the solution of the potash, lime and magnesia which they 

 contain thus proceed rapidly. 



The reactions which occur are probably of a type which may be 

 thus represented : 



Al 2 O 3 .K 2 0.6Si0 2 + C0 2 + 10H 2 O 



Orthoclase. 



= Al 2 3 .2Si0 2 .2H 2 O + K 2 CO 3 + 4H 4 Si 4 



Kaolin. Potassium carbonate. Silicic acid. 



Also 



Ca(HC0 3 ) 2 + Al 2 3 .K 2 0.6Si0 2 + 9H 2 



= Al 2 3 .2SiO,.2H 2 O+ CaC0 3 4- K 2 C0 3 + 4H 4 Si0 4 , 



the CaC0 3 acting as a carrier of CO 2 ; or if lime or magnesia be 



present, as in anorthite, proportionately more carbon dioxide is 



required : 



Al 2 3 .(CaO:MgO)2Si0 2 + 2C0 2 +3H 2 O 



Anorthite. 



= (Ca:Mg)(HC0 3 ) 2 + Al 2 3 .2SiO 2 .2H 2 

 Bicarbonate of lime or magnesia. Kaolin. 



The calcium carbonate present in the soil also becomes soluble 



CaC0 3 + H 2 + CO, = Ca(HC0 3 ) 2 



and is either absorbed by the plant or carried away in the drainage 

 water. 



(51) 



