Soils, Topography and Climate in the Island of Ireland

by

Brian S Coulter, Teagasc, Johnstown Castle Research Centre

SOILS

In Ireland, the principal soil forming factors are parent material, the climate and the rate of decay of organic material. Most Irish soil originates from glacial drift, predominantly calcareous in nature. Histosols and Gleysols predominate in the north with Cambisols on lower ground and Luvisols in central parts. Podzols are extensive in the mountainous regions, particularly in the south. Blanket and Basin peat accumulation is very important throughout western southern Ireland. In contrast with the blanket histosols which developed in the west due to high rainfall or where the sub-soil is sufficiently impermeable to give a high water table, the raised histosols and fens developed in the midlands in lake basins and open bodies of water and were then elevated by the growth of sphagnum moss.

The soil classification system used in Ireland is a modification of the system established by the United States Department of Agriculture. An indication of the categories used in this system with particular reference to the mapping units used on the General Soil Map is given in Table 1.

Table 1: Soil classification scheme used in Ireland and the occurrence of great soil groups.

Podzols mainly found on high ground in the north-west, west and south-west and on the Wicklow Mountains in the east. They are generally poor soils with high lime and fertiliser requirements. They are usually formed in hill and mountain areas where mechanical means of reclamation and cultivation are not feasible. For these reasons they are often devoted to forestry.

Brown Podzolics are predominantly found inland in the north, south and south-east of Ireland. They are somewhat similar to the podzols and have been formed under the influence of the podzolisation process. Because of their desirable physical characteristics, Brown Podzolics are often devoted extensively to cultivated cropping and pasture production. Their inherent low nutrient status is easily overcome by addition of lime and fertiliser.

Figure 1 Irish Soils

Grey Brown Podzolics are found in the north-east, south-east and south. They are usually formed from a calcareous parent material, which counteracts the effects of leaching. The lighter textured cambisols are good all-purpose soils, while the heavier textured members are highly suited to pasture production, responding well to manurial and management practices.

Brown Earths are very extensive and are found throughout the midlands. They are relatively mature, well-drained, mineral soils possessing a rather uniform profile, with little differentiation into horizons. Most Brown Earths occur on lime-deficient parent materials, and are, therefore, acid in nature. Brown Earths in general, possess medium textures (sandy loam, loam, sandy clay loam) and this, together with their friability, desirable structure and drainage characteristics, accounts for the fact that they are amongst the most extensively cultivated soils. Although often of relatively low nutrient status, they respond well to manurial amendments.

Gleys occur extensively throughout the country particularly in the north and south-west. They are soils in which the effects of drainage impedance dominate and which have developed under the influence of permanent or intermittent waterlogging. The impedance may be due to a high water-table, to a ‘perched’ watertable caused by the impervious nature of the soil itself, or to seepage or runoff from slopes. Most gleys soils have poor physical conditions which make them unsuitable for cultivation or for intensive grassland farming. Their productive capacity is also affected by restricted growth in spring and autumn.

Rendzinas (Shallow Brown Earths) are shallow soils, usually not more than 50 cm deep, usually derived from limestone parent material. They occur mainly in counties Galway, Mayo, Clare and Limerick. The use range of Rendzinas is often limited by their shallow depth. They are suited mostly to extensive grazing but where sufficiently deep they can also be excellent tillage soils.

Lithosols are found mainly in western counties and are skeletal, stony soils (often of an organic nature) overlying, in most cases, solid or shattered bedrock. Generally, such areas have frequent rock outcrops. They are most often associated with podzols and climatic peats at the higher levations. Their use-range is limited to rough grazing, or occasionally to forestry.

Regosols are alluvial soils which occur in small amounts in counties Carlow, Kildare and Wexford and to a lesser extent in other counties. They are mineral soils which are immature and show no distinct horizon development. The soils occur mostly in lowlying flat areas along river courses and at river estuaries, but they are also found on young deposits such as Aeolian (windblown) sands. Depending on the source of the deposits such soils may vary in nutrient status and also in physical and drainage characteristics. They have a wide use range exception that they are subject to flooding.

 TOPOGRAPHY

Since topography governs the position of a soil on the landscape it is important in many respects, especially in its effect on water runoff and drainage. The amount of water that moves through a soil is less on steep than on gentle slopes, and low-lying and flat areas generally receive more water. This accounts to some extent for the preponderance of poorly drained soils in low-lying areas. Soils of poor drainage, however, may be found on good slopes where the lower soil horizons or parent material are of poor permeability, leading to retardation of water movement.

Elevation, with its attendant climatic and vegetational changes, has a strong influence in conditioning the soil development pattern. Other features such as erosion and those related to aspect are also associated with topography. Apart from its influence in soil formation, topography can be an important deciding factor in the use of soils.

In Ireland there is a close relationship between lithology and relief. The Carboniferous limestone of the Midlands forms a gently undulating “Central Plain” which is almost always below the 120 m contour. Pre-Carboniferous and Tertiary igneous rocks form a peripheral discontinuous upland rim. Quartzite and the Old Red Sandstone, because of their hardness, contribute significantly to Ireland’s geomorphology. Quartzites are widely scattered amongst the ancient schists of Galway, Mayo and Donegal and in parts of Wicklow. Quartzite outcrops are not extensive, but they form some 12% of the peaks over 600 metres in height e.g. Mount Errigal, Croagh Patrick and the Great Sugar Loaf mountain.

Old Red sandstone supports the topography of hills and mountains mainly in the south. Although it underlies only 10% of Ireland’s surface, it forms almost 50% of the 190 peaks that rise above 600 metres.

The granites in some places give rise to mountains and in others they underlie extensive lowlands. Granite forms the bold massif of the Mourne mountains, the Leinster mountain chain and the erryveagh mountains of Donegal, but in Galway, it underlies the subdued topography of the Connemara lowland.

CLIMATE

The Atlantic Ocean has a significant effect on the climate Ireland. Although the country is as far north as Labrador in Canada, it has a mild climate throughout the year due to the warming effect of the Gulf Stream. This phenomenon gives Ireland a cool-temperate maritime climate with mild, moist winters and cool cloudy summers. The prevailing winds are westerly to south-westerly; these winds blowing off the Atlantic Ocean bring large amounts of moisture to the island. Average relative humidity is high. Annual average precipitation, which exceeds evapotranspiration by over 500 mm, is highest on the west coast and in inland areas of high relief.

Temperature : Average temperatures range from 9.0°C in the north-east to 10.5°C in the south-west. The pattern of the isotherms reflects the ameliorating effect of the Gulf Stream so that it has a warming effect in winter and a cooling effect in summer. Winter temperatures are higher in Ireland than in most other parts of the world at the same latitude. Soil temperatures are favourable for grass growth for about 11 weeks longer per year in the coastal districts of the south and south-west compared to the midlands.

Rainfall: Rainfall is highest in the western hills where it can exceed 3,000mm and lowest along the east coast where it is generally less than 1,000mm (see Figures 2 and 3). The 1000 mm isohyet divides the country fairly evenly into two; the western half varies widely from the extremes of 3000 mm in the high ground to 1200 mm in costal regions. The eastern half is much drier with rainfall ranging from 800 mm in the east to 1000 mm inland. This intensity and variability in rainfall has had a marked effect on soil development.Although the driest month is April and the wettest is December, rainfall is rather evenly distributed throughout the year, the driest month having more than 40mm. The number of days on which rain falls is high, ranging from less than 200 in the south east to 270 in the west

Sunshine: The frequency of cloud cover and rainfall limit the amount of sunshine received. The average duration of bright sunshine per day varies from more than 4 hours along the south and east coasts, to less than 3.5 hours in much of the north-west. May and June are the sunniest months.

Frost: This occurs on an average of less than 25 days per year in coastal districts of the west and on more than 50 days in the interior of the country.

Fig. 2: Mean annual rainfall                              Fig. 3: Average number of raindays per year.