Soil profiles

Many different soil types have been identified in Ireland including podzols, gleys, brown earths and peat, all of which vary in quality as discussed above. A soil profile is a vertical section of a soil comprised of a number of layers, which differ in colour and texture. There are usually four layers or ‘horizons’ in each profile, represented by the letters O, A, B and C.

O Horizon: This layer consists of organic matter which may or may not be degraded.

A Horizon: Also referred to as topsoil, this layer is rich in humus and living matter eg earthworms and bacteria. As it is close to the surface, it is easily reached by rainfall and thus is affected by leaching. The A horizon may be further sub-divided into A1, A2 and A3 on the basis of the colour of the topsoil.

B Horizon: Also referred to as the subsoil, this layer has less living organisms and less organic matter than the A horizon. It may be absent in some soils and as deep as 5m in others. Again depending on colour, the B horizon may be further subdivided into B1, B2 and B3.

C Horizon: This layer comprises the parent material from which the soil is derived and is made up of broken pieces of rock or glacial drift, giving it a yellowish-brown colour.

Note: Sometimes tiny small particles (clay) or minerals are carried down by water from the A horizon and accumulate in the B horizon in a process known as podzolisation. This accumulation of clay, iron and aluminium oxides cement together to form what is known as an iron pan. Iron pans prevent penetration by roots and water and in mountainous areas, if rainfall is high can result in the formation of peat. Podzols are generally poor quality soils unsuitable for tillage or grazing.