Soil Horizons

Soil horizon are layers of soil material that are approximately parallel to the land surface with properties different from horizons/layers below and/or above it. Horizons are designated by a letter and number code. You can find out how they are formed in the Soil Genesis module.

Horizon nomenclature:

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

A1 horizon:
A horizons are surface mineral horizons that are usually darkened by an accumulation of organic matter then the underlaying horizons. Is the zone of maximum biological activity.

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 Layer:
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 layer.

R Layer:
The underlying bedrock.