interacting environmental factors. As normally defined, a soil must support 

 plant life. The concept is expanded to include substrates that could support 

 plant life. For various reasons, plants may be absent from areas that have 

 well-defined soils. 



40. A soil profile (Figure 2) consists of various soil layers described 

 from the surface downward. Most soils have two or more identifiable horizons. 

 A soil horizon is a layer oriented approximately parallel to the soil surface, 

 and usually is differentiated from contiguous horizons by characteristics that 

 can be seen or measured in the field (e.g., color, structure, texture, etc.). 

 Most mineral soils have A-, B-, and C-horizons, and many have surficial 

 organic layers (0-horizon) . The A-horizon, the surface soil or topsoil, is a 



DESCRIPTION 



ORGANIC MATTER CONSISTING OF VISIBLE VEGETATIVE MATTER. 



ORGANIC MATTER IN A FORM WHERE INDIVIDUAL COMPONENTS 

 ARE UNRECOGNIZABLE TO THE NAKED EYE. 



DECOMPOSED ORGANIC MATTER MIXED WITH MINERAL MATTER 

 AND COATING MINERAL PARTICLES, RESULTING IN DARKER COLOR 

 OF THE SOIL MASS. USUALLY THIN IN FOREST SOILS AND THICK 

 IN GRASSLAND SOILS. 



ZONE WHERE CLAY, IRON, OR ALUMINUM IS LOST GENERALLY 

 LIGHTER IN COLOR AND LOWER IN ORGANIC MATTER CONTENT 

 THAN THE A1 HORIZON. 



THESE HORIZONS ARE TRANSITIONAL BETWEEN THE A AND B 



HORIZONS. 



THE A3 HORIZON HAS PROPERTIES MORE LIKE A THAN B THE 



B1 HORIZON HAS PROPERTIES MORE LIKE B THAN A 



ZONE WHERE THE SOIL LACKS PROPERTIES OF THE OVERLYING A AND 

 UNDERLYING C HORIZONS GENERALLY THE ZONE OF MAXIMUM CLAY 

 CONTENT AND SOIL STRUCTURE DEVELOPMENT 



ZONE OF TRANSITION BETWEEN THE B AND C OR R HORIZONS, 

 BUT WITH PREDOMINANT CHARACTERISTICS OF THE B HORIZON. 



A MINERAL LAYER, EXCLUSIVE OF BEDROCK, THAT HAS BEEN 

 RELATIVELY LITTLE AFFECTED BY SOIL-FORMING PROCESSES 

 AND LACKS PROPERTIES OF EITHER THE A OR B HORIZONS, 

 BUT WHICH CONSISTS OF MATERIALS WEATHERED BELOW 

 THE ZONE OF BIOLOGICAL ACTIVITY. 



CONSOLIDATED BEDROCK. WHICH IS NOT NECESSARILY THE 

 SOURCE OF MINERAL MATTER FROM WHICH THE SOIL FORMED. 



Figure 2. Generalized soil profile 



28 



