76 



PETROIOGY AND PEDOLOGY: 



These three factors are cHmate, topography, and 

 time, three physical features of the environment. 

 Their contribution is to decompose the source ma- 

 terials into soil. In the decomposition process, 

 climate works mainly through weathering, water, and 

 wind; topography, through slope. As for the final 

 factor, time, several hundred years often are needed 

 for an inch of soil to form. Of these environmental 

 factors, climate often is most important in determin- 

 ing the mature soil type. In fact, the two most im- 

 portant factors in the kind of soil formation normally 

 are climate and vegetation. 



SOIL PROFILE AND HORIZONS 



When a mature soil is formed, it shows vertical 

 layering of materials. The layering constitutes a soil 

 profile which is a cross-section of the substrate from 

 the surface to bedrock. Each individual layer, or 

 zone, is called a soil horizon. The four major soil 

 horizons are A, the topsoil; B, the subsoil; C', 

 weathered parent material; and D, unweathered 

 parent material. These major horizons display a pro- 

 gression of soil formation from /) to A. The major 

 soil horizons and their subdivisions are shown in 

 Figure 5.1. Note that the organic layers, although 

 designated Aqq and Ag, are not really part of the A 

 horizon. 



Organic Layers 



/Topsoi I 



Soil; 



Subsoi 



Weathered Rock 



Parent Rock 



undecomposed 

 decomposed 

 dark color 

 light color 

 transitional 

 transitional 

 deeper color 

 transitional 



Figure 5.1 The sot! profile. 



Some confusion may exist because the term "hori- 

 zon" is not limited to the four major subdivisions. 

 The term is not precise. One could speak of the soil 

 horizon and imply both A and B. Those layers con- 

 stitute "the soil"; the underlying materials are 

 weathered and unweathered rock. Also, "horizon" 

 can be applied to finer subdivisions of any of the 

 four major zones. Therefore, more detailed con- 

 sideration of the soil profile treats the subdivided 

 horizons. 



Above the topsoil there may be two organic hori- 

 zons, 01 and 02. Although these organic horizons are 

 not a part of the A horizon, 01 and 02 often are 

 designated respectively as A„q and A^, 01 consists of 

 loose leaves and other organic debris in which the 

 original form of most vegetative matter is discernible. 

 02 consists of partially decomposed or matted organic 

 debris in which the original nature of most plant and 

 animal matter cannot be recognized. 



The true topsoil, .-1, has three subdivisions. /I, is 

 a dark horizon containing mostly humus, but some 

 minerals. In the A^ horizon the most characteristic 

 feature is the intimate association of humus and fine 

 rock particles. This layer when present and mixed 

 with humus is granular, porous, and crumbly. When 

 a distinct humus layer (/If,) occurs, the A^ horizon is 

 absent and there is an abrupt transition from matted 

 and compacted humus to mineral soil. .-I, 'S a light 

 horizon, representing the region of maximum leach- 

 ing. The unique aspect of .-Ij is the loss of clay, iron, 

 and aluminum, with resultant concentration of 

 quartz or other resistant minerals of silt to sand 

 size. .-Ij is a transitional horizon to B, but is more 

 like .1. Although they constitute the possible hori- 

 zons within the .-1 major horizon, not all of these 

 subunits need be present in a given soil profile. 



Subsoil, B, also may have three layers, fi,, like 

 .1,, is a transitional horizon to B but is more like B. 

 /i, is usually a deeper-colored horizon, representing 

 the maximum accumulation of leached minerals and 

 clay; it has low humus content, /i, is a transitional 

 zone between B and C. 



Weathered parent material, C,', may be absent if 

 soil formation closely follows weathering. In some 

 grassland soils, calcium carbonate accumulates here. 



The unaltered parent material, /), is any layer be- 

 neath A, B, and C horizons. Therefore, in addition 

 to bedrock, D can be a zone of boulders, gravel, sand, 

 or clay above the true bedrock. However, the 1) layer 

 is always unaltered parent material. 



