Bluestem sod-qrass 



Permanenlly moist 



FIG. 11-4 Relation of lime hardpan to types of pr 

 (Shanti 1923). 



vegetation extending from west to east in central North America 



SOIL PROFILES 



As a result of the specific circumstances of 

 weathering, biotic reactions, and cHmatic influences 

 it has experienced, a mature soil has a definite struc- 

 ture characteristic of its different environment (Lutz 

 and Chandler 1946). The living plant draws nutrient 

 materials from the deeper layers of the soil, but those 

 materials are deposited on the surface soil, where 

 the dead plant decays. Rain falling on the ground 

 surface carries the nutrients and salts back down into 

 the soil, at least as far as the water percolates. This 

 sequence of events produces a definite layering of the 

 soil. Each layer is called a soil horizon, and the series 

 of horizons characteristic of a soil is called the soil 

 profile. 



Horizons 



The A horizon is one of organic decomposi- 

 tion and leaching; the B horizon, one of precipita- 

 tion of materials carried down from above; the C 

 horizon is the layer of parent soil material; the D 

 horizon is the underlying stratum of rock or sedi- 

 ment. 



Horizon A can be subdivided to reflect phases in 

 the decomposition continuum. Aoo, the L horizon, is 

 fresh litter or litter only slightly altered. Ao com- 

 prises an F horizon of fermentation where plant ma- 



terials are partly decomposed but still identifiable, 

 and an H horizon where decomposition has brought 

 the organic material into an amorphous mass. The 

 Ai horizon, directly below, is dark-colored and of 

 flocculent texture, a mixture of organic material and 

 the mineral soil. A2 is light-colored and of coarser 

 texture, since leaching is maximum. Occasionally an 

 A3 is recognized as a transition to B. The B horizon 

 is sometimes also subdivided ; suffice it to say that the 

 salts and humus leached out of A are deposited here. 

 The horizon is often brownish or yellowish in color 

 and columnar in structure. The A horizon is com- 

 monly called topsoil ; the B horizon, subsoil. 



A mature or fully developed soil profile is char- 

 acteristic of climax or late serai stages of a succes- 

 sion. Horizons are not fully expressed in early serai 

 stages, so these profiles are called immature or un- 

 developed. Mature profiles are found only under 

 virgin vegetation, for erosion and cultivation disturb 

 horizons. Profiles are best developed in humid cli- 

 mates where abundant precipitation carries humus 

 and salts well into the soil. 



Hardpan 



In arid regions, evaporation may be in excess of 

 rainfall. Moisture in the soil has no opportunity to 

 percolate downwards ; rather, it rises to the surface 

 of the ground and is lost. Where rainfall is inade- 



170 Ecological processes and dynamics 



