Between the foredunes and the main dunes are dune slacks which are low lying depressions. In winter these are often close to or even below the water table. This produces particular plant associations. Plants in the dune slacks include wild strawberries, buttercup and violets with some flag iris and willow in the slacks at the rear of the main dune system.
In the slacks the vegetation cover is almost complete and a soil is starting to develop. A number of studies suggest that the rate of organic build-up in soils on dunes is very slow, being below 1% for the first 100 years, and than speeding up. The acceleration of humus formation was thought to be a consequence of the calcium carbonate being leached from the uppermost part of the soil profile, producing an acid reaction which encourages humus formation.
More recent research, at Magilligan Point, suggests that the organic matter can accumulate much faster than previously believed. Wilson (in "Soil formation on coastal beach and dune sands at Magilligan Point Nature Reserve, Co. Londonderry" (1987) Irish Geography) found 1% organic matter after only 12 years. He surmised that the relatively high annual precipitation at Magilligan Point might have allowed the growth of mosses (pleurocarpus - see photograph) at the expense of the marram grass. These mosses are better able to retain water and to add organic matter to the soil.
Follow the links in the diagram below to explore the dunes.