Soil Development
PHYSICAL GEOGRAPHY 101

Soil Horizons

- distinctive horizontal layers that differ in physical, chemical and organic composition
- develop as a result of climate, living organisms, and configuration of land surface
- distinguished by color
- collection of horizons at a given location is called the soil profile

Soil Profile

The following is a typical sequence in a mid-latitude forest.
Not all soil profiles will contain all of the soil horizons listed below.

Organic Horizon - O

Accumulated organic material.
- Oi - identifiable organic material - undecomposed material.
- Oa - amorphous organic horizon - decomposed beyond the point of recognition - humus.

Solum Horizon

Mineral horizon which includes the A, E, and B Horizons
A Horizon - eluvial horizon - leaching of material from the soil.
E Horizon - horizon of maximum eluviation - typically has a light color, washed out look.
B Horizon - illuvial horizon - material from A and E is washed into this layer.

C Horizon

"calcareous" horizon - unconsolidated parent material - not affected much by organic processes.

R Horizon

regolith - loose bedrock material.

Bedrock

Solid rock - not considered to be part of the soil profile but underlies all soil profiles

Soil Forming Processes

Soil Enrichment - process of adding new material to the soil from outside sources.

Soil Removal or Depletion - removal of material from the soil - erosion and leaching

Translocation - movement of materials within the soil profile. Includes:
- Eluviation - washing out of material
- Illuviation - washing in of material
- Decalcification - removal of calcium
- Calcification - addition of calcium
- Salinization - evaporation of water leaves salts

Transformation - changing of the material within the soil - primary mineral weather to become secondary - organic matter decomposes to humus (humification)

Soil Temperature

Controls soil development by regulating the rate of chemical and biologic activity
Biologic activity
- below 0°C there is little or no activity
- below 10°C activity is slow
- decomposition and chemical activity is slowed by low temperature

Slope Configuration

Degree of the slope
- steep slopes = thin soil profile - more rapid removal due to erosion
- gentle slopes = thick soil profile - deposition can occur - erosion is slower

Orientation of the slope
- how is the slope oriented relative to insolation and wind.
- orographic precipitation, microclimates and insolation will influence soil development

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