Leaving Certificate Agricultural Science Soil Profiles H. Jones, St. Columba’s College

Horizons If one were to cut a pit in the ground your would notice that the soil is made of distinct layers or HORIZONS. In very simple terms there are three main horizons, A, B & C. The A Horizon is commonly called the Top Soil. It contains a large proportion of Humus (Organic Matter), roots, bacteria and living organisms (Nematodes and Earthworms).

A Horizon The humus gives the top soil its characteristic dark colour. In humid temperate climates like our own, the A-horizon or topsoil is subject to leaching (Losing minerals by washing out). Clay and humus can also be leached out of the A Horizon. Below the A-horizon is the B Horizon.

B Horizon It has intermediate characteristics and it often referred to as the sub soil. The B horizon owes it origin to the activity of earthworms. The earthworms swallow the humus from the topsoil (with it minerals etc) and transfer them all around the B Horizon. The B Horizon is also strongly influenced by the leached materials from the A Horizon. The B Horizon is often a strong colour (Orange in iron rich soils) when leaching is a factor. The B-horizon may also have a high percentage of clay than the A and C horizons.

C Horizon The C Horizon is the Parent Material or the original rocks from which the soil was formed. The main ingredient of the C Horizon is broken pieces of under lying rock. Certain soils, called Derived Soils contain material other than the original rock but material that were transported to that location by Winds, Rivers or Glaciers.

Further Division of Horizons To further complicate things the A, B and C Horizons can be subdivided further. When the A Horizon has been subjected to leaching, the lower part of the horizon is pale and light coloured and almost looks like it has been bleached. The upper half of the A horizon is still dark as humus is continuously being formed from the decaying organic matter. The upper layer is then called the A1 horizon and the leached area the A2. An A3 horizon may be present if you notice another layer between the leached area and the B Horizon.

Further Division of Horizons 2 The B Horizon is commonly recognised by the accumulation of leached materials from the A horizon. The area where most of the accumulation occurs is the B2 Horizon. The B2 Horizon is then referred to by what is accumulated. i.e. B2h stands for Humus B2ir stands for Iron B2t stands for textural. Where the B2ir has accumulated to such a serious extent that a layer of Iron Oxide has formed, which results in water logging, we refer the B2 Layer as the IRON PAN.

Further Division of Horizons - 3 There is also an O Horizon. This is the layer of organic matter on the surface of the soil. This can be further divided into the O1 and O2 Horizons. (Totally decayed and not decayed)