SOIL-FORMING PROCESSES 17 



superficial in comparison to the earth's bulk. Nevertheless, 

 the weathered mantle, in spite of its comparative insignifi- 

 cance, presents an effective medium for plant growth. The 

 agencies of formation, therefore, demand more than the brief 

 mention just given. These forces are geologic when the soil 

 is being evolved, but once the soil materials are in place, the 

 actions become localized and the influences may be considered 

 as soil processes rather than more broadly geological. 



The soil-forming processes 1 , while diverse both in action 

 and product, may be classified under two heads, mechanical 

 and chemical. The former is often designated as disintegra- 

 tion, the latter as decomposition. 



SOIL-FORMING PROCESSES 



I. Mechanical (disintegration) 



A. Erosion and deposition. 



Water, ice and wind. 2 



B. Temperature change. 



Differential expansion of minerals, exfoliation 

 and frost. 



C. Biological influences. 



Plants and animals. 

 II. Chemical (decomposition) 



A. Oxidation and deoxidation. 



B. Carbonation and decarbonation. 



C. Hydration and dehydration. 



D. Solution. 



11. The mechanical action of water. — From the time that 

 that water as rain beats down upon the solid earth until it 

 is finally discharged into the ocean, there to pound as waves 

 upon the bordering lands, it is moving, sorting, and rework- 

 ing the products of weathering. Water to erode must be 



1 For a complete and detailed discussion of soil formation, see Merrill, 

 G. P., Boclcs, Bock Weathering and Soils; New York. 1906. Also, 

 Em erson, H. L., Agricultural Geolog y; New York. 1920. 



2 Gravity is generally included in this group. While indirectly of 

 great significance in soil formation, its direct action is not of great 

 importance and is adequately disposed of in paragraph 27. 



