Introduction to Soil Profiles

A soil profile is a column of soil extending from the soil surface to the bedrock or parent material. They are easily observed in road side cuttings and erosion gullies, but can also be seen in a specially dug pit or extracted using a coring machine.

A soil profile is usually composed of three major layers or horizons designated the A, B and C horizon. The A & B horizons have been formed over time through weathering. These are sometimes called the solum. The C horizon consists of decomposing parent material and the A,B and C horizons combine to form the regolith. The A horizon may also be covered with a layer of organic matter termed the O horizon or P horizon if the soil constantly wet such as in swamps.

The main horizons and their sub divisions are shown in the table below:

Horizon / Definition
P
O
O1
O2
A
A1
A2
A3
B
B1
B2
C
D / Surface layer dominated by decomposing organic matter in very wet soils eg swamps
Surface layer dominated by decomposing organic matter usually present in forests and undisturbed grasslands. May be divided into O1 and O2.
Surface layer of undecomposed plant material.
Layer of unrecognisable partly decomposed organic material.
The top layer of mineral soil. It can be broken into A1 and A2
Surface layer of mineral soil referred to as top soil. Dark colour with high biological activity. Contains the majority of plant nutrients . May be further subdivided into A11 and A12 as necessary.
Layer of paler colour (bleached) than A1 and B horizons. Has poorer structure, lower organic matter and often impeded drainage. Higher erosion risk.
Transitional layer between A and B horizons but dominated by the overlying A1 or A2 horizon.
Layer below the A horizon. Usually higher in clay content, denser and more strongly coloured than the A2.Can be divided into B1 and B2.
Transitional horizon dominated by properties similar to the underlying B2
Layer of concentration of clay minerals, aluminium iron and/or translocated organic material. Structure usually different to the A horizon. May be further subdivided into B21 and B22 etc based on colour, structure or texture.
Consists of weathered parent material.
Layers below the soil profile that are markedly different to the parent material and the profile. May be buried soils, or depositional layers that have not influenced the soil profile.

An example of a typical soil profile is shown overleaf.

Profile Types

Organic. A soil dominated by organic matter – mainly found in peat bogs or swampy areas. The surface 30cm will contain 20% or more organic matter when the clay content of the soil is less than or equal to 15% ie in sands, loamy sands and some sandy loams loams and silty loams. If the clay content is higher than 15% eg clay loams and clays, the top 30 cm must have more than 30% organic matter.

Uniform A soil where there is little if any change between the A and B horizons. The texture range through out the solum is not more than the equivalent of one texture group*.

Gradational A soil where there is a gradual change increase in clay content between the A horizon and the B horizon. Horizon boundaries are diffuse and the texture difference throughout the solum is one texture group*.

Duplex (or texture contrast)

Soils where there is a sharp change in texture between the A and B horizons eg a sandy loam overlying a sandy clay. There is a texture contrast of more than one texture group* between the A and the B horizon. The horizon boundaries are clear and the change from the A horizon to the B horizon occurs over a distance of 10cm or less. (See notes on soil texture)

For more information on texture groups see you soil texture notes.

For photographs of differing soil profiles view the Soil Profile Powerpoint available on the moodle.

Reference:

Charman and Murphy (2001), Soils: their properties and management. OUP, South Melbourne.

2 / Introduction to soil profiles v2 2014