Tutorial:Soil Profiles

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Soil profiles and horizons
Typically soil profiles consist of an organic rich top soil (A horizon), a minerogenic sub-soil horizon (B horizon), and an unaltered layer of partly weathered parent material (rock or sediment) that is called the C horizon. Not all soils exhibit the full A, B, C sequence of horizons, some have no, or poorly developed A or B horizons, and in other cases the A and/or B horizon may be further subdivided so that more than three soil horizons are present.

The properties of the A, B, and C horizons depends on the nature of the parent material and the nature of the soil forming processes that are operating (or have operated in the past). However, there are some common characteristics that are associated with the different horizons and knowing these can help the archaeologist to:
 * 1) Identify the presence of a soil profile.
 * 2) Identify where in a soil a particular context or horizon lies.
 * 3) Predict and identify features associated with soil formation as opposed to anthropogenic activity.

More information on specific horizon and profile characteristics can be found in the section on soil classification, accessible here

A horizons (top soils)
Most top soil horizons are dominated by the input of organic matter as litter and from roots and soil organisms, the action of soil fauna such as earthworms, and the actions of humans (agriculture etc.). The result is a dark organic or organo-mineral surface horizon characterised by plant roots and biological activity, although the actual characteristics vary greatly according to the balance of different processes.

Exposure to the oxygen rich atmosphere and the actions of water percolating through the profile are also important leading to oxidation and weathering of minerals and the leaching out of nutrients. In some cases the evaporation and capillary rise of groundwater can result in the deposition of salts and the development of salty, "salinised" topsoils.

B horizons (sub soils)
The B horizon is a zone of transition and is often the most diagnostic in terms of soil processes and soil type. There are many types of B horizon each with very different properties. Some are zones of accumulation where organic matter, silt, clay, and/or iron and aluminium, washed down from the topsoil accumulate resulting in dark organic, reddened or clay rich sub-soils. In others salts, gypsum or calcite can accumulate as groundwaters bring nutrients up through the profile. In some soils waterlogging (gleying) results in characteristically grey sub-soils with reddish mottles of iron.

In general B-horizons contain less organic matter and root densities are lower than in the top soil. The structure also tends to be coarser blocky or prismatic.

C horizons (parent material)
The nature of the C horizon is entirely dependent on the parent material. In some areas the parent material is solid rock, but large areas of the land surface are coated by drift deposits in which soils form. The parent material is very important in determining soil type as it helps control drainage, mineraology and nutrient status. Although the C horizon consists only of partly weathered parent material is may be altered by gleying (waterlogging) resulting in the characteristic grey, mottled colour. Most C horizons are mineral in nature, although this will depend on the parent material.