Physical Geography 101
Soil Development

Soil Development

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
Soil Profile - collection of horizons at a given location.

Soil Profile

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

Soil horizons diagram

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 A1, A2, and B Horizons
A1 Horizon - eluvial horizon - leaching of material from soil.
A2 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.


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
- Calcification/Decalcification - addition/removal of calcium
- Salinization/Desalinization - addition/removal of salts

Changing of the material within the soil
Primary mineral weather to become secondary (weathering)
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
Slope orientation relative to insolation and wind.
Orographic precipitation, microclimates and insolation will influence soil development