Soil Horizons

Soil horizons are layers within the soil profile, like those shown in the drawing.

You can find out how they are formed in the Soil Genesis module. Horizons are designated by a letter/number code.

Horizon nomenclature:

O horizon:
O horizons occur at the soil surface and are made up mainly of organic material. They are uncommon in Australian soils but are found in rainforests where decomposing leaf litter accumulates.

A horizon:
A horizons are surface mineral horizons that are darkened by an accumulation of organic matter.

A2 horizon:
A2 horizons are pale eluvial horizons from which iron oxides (which give red, yellow, and brown colors to soil) have been removed. To see how this happens, look at Fe mobilization in the Soil Genesis module.

B2 horizon:
Soil material like clay, iron or aluminum oxides, or humus may be washed down the soil profile by percolating rainwater, where it accumulates to form an illuvial horizon (look at eluviation/illuviation in the Soil Genesis module). A horizon where such accumulation appears to have occurred is called a B2 horizon.

C horizon:
Underlying many A and B horizons is a layer of weathered rock, which has been softened by weathering processes, but has not been subjected to other soil-forming processes like soil mixing or the formation of peds. Typically, the remnant structure of the rock is visible. This is a C horizon.

R horizon:
The underlying bedrock.