ROCKS AND SOILS 



77 



SOIL EROSION 



The general topic of erosion was treated previously. 

 All that need be added is the usual cycle of soil 

 erosion and the specific erosion factors. 



The soil erosion cycle encompasses the removal of 

 soil from an area. Therefore, it is associated v^^ith 

 depressions, or erosional land forms; remnants, or 

 residual forms; and transported sediments, or deposi- 

 tional forms. In the usual cycle, water is the most 

 important erosion factor. If water flows over a 

 relatively large surface area, there is a fairly even 

 removal of surface soil by a process called sheet 

 erosion. Moreover, progressive restriction of the area 

 afTected by a given amount of water causes nil, gully, 

 and stream erosion. The terms "rill," "gully," and 

 "stream" refer also to the land forms associated with 

 each erosion type. The entire soil erosion phe- 

 nomenon shows two definite associations. First, there 

 is a correlation between the amount of water and the 

 size of area it erodes, because any particular amount 

 -size relationship determines whether the erosion 

 process is rill, gully, or stream. Second, erosion 

 processes often follow a three-step cycle of rill, gully, 

 and stream erosion in a given area. 



Although water is most important, the environ- 

 mental factors of slope, wind, ice, and plant cover 

 also modify soil erosion. Some of these contributing 

 factors, stated in terms of how erosion rate is 

 minimized, are as follows: greater soil permeability, 

 lesser angle of slope, slower melting of snow, an even 

 distribution of light rains, vegetation with a dense 

 root system and surface cover, lack of overgrazing, 

 neither clearing nor burning lands, and cultivating on 

 contours rather than down slopes. 



CLASSIFICATION 



Soils can be classified on the basis of site of origin, 

 texture, and so-called natural means. On the basis of 

 site of origin, sedentary and transported soils are 

 recognized. A sedentary soil is one that develops from 

 all factors of formation operating at the site of the 

 mature soil. A transported soil has its materials 

 brought to the site of soil formation. The other 

 criteria of classification are considered in greater 

 detail below. 



TEXTURAL CLASSIFICATION 



Soils are classified according to textural composi- 

 tion, the proportions of its various-sized particles. In 

 textural composition, the important soil particles are 



gravel, over 1/25 inch; sand, 1/500 to 1/25 inch; silt, 

 1/5000 to 1/500 inch; and clay, below 1/5000 inch. 



If a substrate is 90 per cent or more gravel, the sub- 

 strate is not soil. No matter what the textural com- 

 position of such rocky areas, all are called rubble. 

 The term "soil" is reserved for surfaces having less 

 than 90 per cent gravel. 



Textural Basis of Classification. In the textural 

 classification of soil, gravel is analyzed separately 

 from sand, silt, and clay and is used to provide a de- 

 scriptive adjective for the soil texture name which is 

 based upon the proportions of sand, silt, and clay. 

 Therefore, to classify and name soils according to 

 texture, one first determines the gravel content and, 

 then, the finer particle content. 



An adjective describing the gravel of a soil is used 

 only ij the soil contains more than 15 per cent gravel and is 

 derived from one of eight gravelly-nature classes. 

 These classes are diagnosed on the basis of shape, 

 size, parent rock, and amount of fragments (Table 

 5.4). 



TABLE 5.4 DIAGNOSES OF THE GRAVEUY-NATURE 

 CLASSES OF SOIL 



Textural Classes. These soil classes designate the 

 percentages of sand, silt, and clay content, but each 

 possesses certain physical characteristics that aids in 

 its identification (Table 5.5). 



NATURAL CLASSIFICATION 



Soil scientists still seek a better system of soil 

 classification. Among other things, a satisfactory 



