Soil Pedon

Pedons are a measureable area of soil six feet wide and as deep as the bedrock. This section, or profile, is representative of an area and is used to rate the quality of the land in a soil survey.

The pedon contains six soil horizons or layers that are rarely as clearly defined as the profile to the right. The layers blend in transition and numbers are added to the layer name to help define the degree of transition. Layers are defined by color, soil structure, texture, and the size and shape of aggregates. For now we will keep this simplified and focus on the clearly defined layers.

Layers
Organic layer (O)
It is not easy to define. It is the darkest layer and is part of the topsoil. Contains the highest amount of organic matter and is highly weathered.

Accumulated layer (A)
Organic matter is slightly lighter than the organic layer and is part of the topsoil. This layer has an increased rate of weather caused by roots from crops, some grasses, other plants, water, and many types of animals. Aggregates here have a granular structure that is porous and spherically shaped.

Mineral horizon or layer (E)
May have the widest range of color variation but often it is lighter or even a different shade of color than the layers above or below. Plant roots, water and some animals cause weather to this layer. Aggregates common to this layer are platelike caused by compaction and can even shield the layer below from water.

Obliteration or original rock layer (B)
Is the first layer of subsoil and is stable enough to be where the foundation of buildings is laid on. Tree roots and water are the main contributor of weathering in this layer. Aggregates common to humid are angular blocky (have clear edges) and subangular blocky (rounded edges) both may also occur in the A layer. In aird and semiarid regions, aggregates are columnar (rounded edges) and prismatic (flat angular tops).

Parent material (C)
Is a rock layer where the geological (parent) material can be reached by water and is beginning to weather.

Bedrock (R)
Is the lowest layer of the pedon