24 NATURE AND PROPERTIES OF SOILS 



and drainage, as well as an accumulation and distribution of 

 organic materials. Lichens, mosses, and algae play their parts 

 in a similar manner. It must be noted, however, that while 

 plants tend to preserve and improve the soil tilth, their action 

 in this respect is not wholly physical. Decay due largely to 

 bacterial action is necessary before the accumulated organic 

 matter can improve to any marked degree the physical con- 

 dition of the soil. This is only one of the many examples 

 illustrating the cooperation of physical and chemical changes 

 incident to soil formation. 



Animals influence the soil physically by their burrowing 

 propensities. Gophers, squirrels, ants, and the like mix and 

 open up the soil, thus providing for the circulation both of 

 air and water. Other soil forces, both physical and chemical, 

 are markedly encouraged thereby. Earth worms produce 

 similar effects. They not only pass great quantities of soil 

 through their bodies, but they carry much to the surface. 

 This has been estimated as amounting to one or two surface 

 inches in a decade. Man also is producing important physical 

 changes on the soil and soil material. The plowing under of 

 green-manures, crop residues and farm manure, the addition 

 of lime and fertilizers and the tillage incident to cropping 

 have much to do with the physical changes, which are con- 

 tinually occurring in the soil. 



16. Oxidation and deoxidation. — Scarcely has the disin- 

 tegration of rock begun than its decomposition is also appar- 

 ent. This is especially noticeable in humid regions where the 

 chemical and physical processes of soil formation are par- 

 ticularly active and markedly accelerate each other. Of the 

 chemical forces, oxidation is usually, especially near the sur- 

 face, the first to be noticed. It is particularly manifest in 

 rocks carrying iron in the sulfide, carbonate or silicate forms. 

 The sulfide, although widespread, is less important in pro- 

 moting rock decay than the other combinations. The oxida- 

 tion of iron in any form is indicated by a discoloration of the 



