Organic matter in the soil is made up of living and dead organisms. Microorganisms living in the soil, such as bacteria and fungi, decompose animal and plant material and recycle nutrients. The stable product of decomposed organic matter is called humus and forms the top layer of many soils, providing the nutrients that plants need. The particles in the soil are held together by moist clay, organic matter and by fungal hyphae, which together form soil aggregates. Water and air is held in the pores between these soil aggregates.

Soil Great Groups. (Image Source: AgMet, UCD. )

In Ireland, the three main soil forming processes that determine soil characteristics and type are leaching, gleisation and calcification.

Leaching process: In this process the soluble elements in the soil are washed downwards as a result of waterflow in the soil. These elements may be lost to the top layers of soil.

Gleisation: This process results from restricted water movement in the soil, also known as water logging. It occurs in soils with a high water table or where the chemical property of the soil is not permeable. In waterlogged soils, lack of oxygen causes the reduction of iron and a display of a grey to blue colour.

Calcification: During this process calcium carbonate is redistributed in the soil, without its complete removal. It is mainly the result of low rainfall and calcium carbonate is accumulated at some point in the soil profile. The climate in Ireland prevents the calcification of Irish soils on a large scale but this process takes place to a smaller extent creating base rich soils.

• Podzols: these poor, acidic soils are formed in hill and mountain areas; they are grey in colour and low in humus and can be found in Counties Wicklow, Donegal and Kerry.
• Brown Podzolics: these soils are similar to the Podzols but less depleted; they have a good mix of mineral and organic matter in the surface layer and can be found in Counties Cork, Waterford and Tipperary.
• Brown Earth: these mature, well drained soils with a uniform, brown soil horizon show high fertility and can be found in Counties Clare, Wexford and Wicklow.
• Grey Brown Podzolics: these soils are formed on calcareous parent material, which counteracts leaching processes; they are all-purpose soils and can be found in Counties Meath, Westmeath and Kildare.
• Gleys: these soils developed under waterlogged conditions; they mostly show poor physical conditions and are grey/blue in colour and can be found in Counties Monaghan, Cavan and Leitrim.
• Blanket Peats: these soils have accumulated in areas of high rainfall and have poor drainage capacities; they have a characteristic high organic matter content, which can be over 30 percent and are at least 30 cm deep; they can be found in Counties Galway, Mayo and Sligo.
• Basin Peats: these soils have formed in hollows, river valleys or lake basins under conditions where the subsoil is not permeable; there are two types: raised bogs and fen peats; they can be found in County Longford.
• Redzinas: these shallow soils are not more than 50 cm deep and can be found together with Lithosols in Counties Dublin and Clare.
• Lithosols: these soils are very stony and are situated over solid or crushed bedrock and can be found together with Redzinas in Counties Dublin and Clare.
• Regosols: these soils show no horizon development and lack the B-horizon and can be found in County Waterford.

Click here to view a soil classification for Northern Ireland.