The acidity of a soil depends on the parent material – the rock from which it was formed. Soils over limestone, or those on boulder clay derived from limestone rock, in the Midland basin and North Munster, are limy.
Deciduous azaleas need acid soil
Limestone soil in Ireland is usually high in clay content with the result that these soils of often tend to be heavy and fertile, unlike the thin, chalk soils of other countries. Most plants thrive on these fertile limy soils, except the lime-haters such as pieris, rhododendron, most heathers, camellia, enkianthus and witch hazel.
These limy soils can have a layer of neutral or even acidified soil at the surface if old grassland has not been disturbed, or limed, for decades. They can be acidified by the addition of large quantities – 10 kilograms per square metre – of organic material, or by applying sulphate of iron at 100 grams per square metre and repeat this as required.
Over red sandstone, in South Munster, shale, in South-east Leinster, and granite, in Wicklow, Carlow, Kerry, Connemara and Ulster, soils tend to be acid, as are peat soils. Acid soils can be fertile if they can be classed as loams with plenty of organic material present, but they can also be low in nutrients, especially at higher altitude and in areas of high rainfall along the west coast.
Acid soils can be neutralised by applying lime at rates of about 500 grams per square metre. Simple kits are available to test soil acidity. Hydrangeas are good indicator plants; on acid soil, the flowers are blue; on limy soil, the flowers are red or pink.