PHYSIOGRAPHIC FACTORS 147 



numbers and activities increase as more complex organisms, par- 

 ticularly higher plants, gradually occupy the surface. 



Soil Profile.— Processes resulting in the formation of soil mate- 

 rial also contribute to soil development. As weathering proceeds, 

 fine materials in suspension and solution are carried downward by 

 percolating water to a lower level, where they gradually accumu- 

 late. As a soil develops, therefore, a rough stratification becomes 

 apparent in which the horizons characteristically have different 

 physical and chemical properties. These horizons, collectively 

 called the soil profile, are designated and recognized as follows: 



A Horizon. The upper layer of soil material from which 

 substances have been removed by percolating water. 



B Horizon. The layer below the A Horizon in which these 

 materials have been deposited. Layer of accumulation. 



C Horizon. The underlying parent material, relatively un- 

 weathered and not affected as above. 



Litter accumulated on the surface of the mineral soil may be 

 termed the Ao Horizon. It is often convenient to subdivide the 



ROOT DISTRIBUTION ON OUTSIDE WALL OF SOUTH TRENCH 

 TRENCHEO PLOT NO 2 JUNE 10,1932. 



A, 



A > • : — 1 5 s" 



9 5' 



i 5 



B. • •.••. - 215* 



360* 

 LEGEND 

 • = root.0'-or ..root 0.2*- 0.3" ■ SCALE , 

 .»roo10.f-0.2' •=root0.3'-0.4* "cot 



FlG. 76. An illustration of root distribution in soil horizons and of a 

 method for mapping roots in the wall of a soil well.— From Korsticm and 

 Coiled 



major horizons as Ai and A2, Bi and B2, etc. Ai is a particularly 

 useful subdivision, for it is applied to the portion of the A horizon, 

 distinguishable by its darker color, in which organic material has 

 become incorporated. 



Soil profiles may be observed in any fresh road cut. When 

 studied in connection with vegetation, a rectangular pit is usually 



