

SOIL-FORMING PROCESSES 25 



affected rock, which from the first is streaked with iron oxide. 

 The mica, amphibole, pyroxene and garnet groups are par- 

 ticularly affected, until, as the process continues, these min- 

 erals waste away into unrecognizable forms so weakening the 

 rock as to cause it to crumble easily. The way is now open 

 for vigorous chemical and physical changes of all kinds. Oxi- 

 dation may be illustrated chemically, using olivine as the 

 mineral decomposed. It is to be noted that the first step is 

 the assumption of water and the production of serpentine and 

 ferrous oxide. The latter quickly changes to the susquioxide. 



3MgFeSi0 4 +2H 2 0=H 4 MggSi 2 9 +Si0 2 +3FeO 

 Olivine Water Serpentine Silica Ferrous 



Oxide 

 4FeO + 2 = 2Fe 2 3 (red) 

 Ferrous Oxygen Ferric Oxide 

 Oxide 



Deoxidation is the reverse of oxidation, being a reduction 

 of the amount of oxygen present in the compound. With 

 hematite it might occur as follows : 



2Fe 2 3 — 2 = 4FeO 

 Ferric Oxide Oxygen Ferrous Oxide 



In a similar way, other oxides and salts may be reduced by 

 the withdrawal of oxygen. This action occurs in poorly 

 drained soils or in soil very rich in organic matter. It is 

 generally apparent in forest soils just below the organic sur- 

 face layer. Here the leaching downward of small quantities 

 of organic acids has been sufficient to develop a definite grey- 

 ish zone, varying both in color and depth. The bleaching of 

 sands, shales, sandstones, and clays may often be due to 

 deoxidation rather than the actual removal of ferric iron. 

 No great importance need be attached to deoxidation either 

 in soil formation or in the chemical processes which continue 

 to affect the soil after it is definitely developed. 



