CHEMISTRY OF SOILS 131 



Oxidation and Reduction. The processes of oxidation 

 and reduction are chiefly confined to the organic matter 

 and minerals containing compounds of iron and man- 

 ganese. Oxidation may be considered complete when all 

 the iron is converted into the ferric state represented by 

 the oxide Fe 2 3 . 



The oxidation of iron pyrites, sometimes found in soils 

 derived from slates .and shales, may be shown thus 



2 FeS 2 + 15 + H 2 = Fe 2 (S0 4 ) 3 + H 2 S0 4 



It takes place much more rapidly in the presence of lime 

 which combines with the acid thus 



2 FeS 2 + 4 CuC0 8 +150 = Fe 2 3 + 4 CaS0 4 + 4 C0 2 



Pyrites. Ferric oxide. 



Magnetite and ferrous compounds, commonly present in 

 the minerals of the basaltic rocks, readily undergo oxida- 

 tion. , , 

 2Fe 3 4 + = 3Fe 2 3 



Magnetite. 



2 FeO + = Fe 2 8 

 Ferrous oxide. Ferric oxide. 



Ferric oxide is easily formed and also easily reduced 

 again to the ferrous state. It therefore acts as an oxidising 

 agent or carrier of oxygen and produces a stimulating effect 

 upon the soil. It accelerates the oxidation of organic 

 matter and causes the nitrogen and other substances con- 

 tained in it to become more rapidly available to plants. 

 The presence of ferric oxide and organic matter in soils, 

 therefore, generally indicates a high degree of fertility. 



There is, however, another side to the question. The 

 oxidation of organic matter by ferric oxide involves the 

 reduction of the latter, and, if the conditions do not admit 

 of rapid re-oxidation, the ferrous compounds produced 

 have a deleterious effect upon the grow,th of plants. la 



K 2 



