Agricultural lime benefits the environment

by Dr. W A. Adams, The University of Wales, Aberystwyth

Agricultural soils in the lowlands of the UK are limed on a regular basis to maintain a pH of 6 or higher depending upon the cropping system. This general situation of neutral or near neutral soils does not apply to extensive areas of hill and upland. Here the benefits to herbage production enabled by liming may only be economic on small areas of the farm. Historically the practice of liming has moved up and down the hills depending upon the profitability of the livestock sector.

Soils in most of the uplands have low natural reserves of calcium. High rainfall and 'acid rain' have resulted in the loss of bases by leaching and a fall in soil pH to values of 4 or lower. The upland areas of Wales originating in unlimed rough grazing or forest pass are typical and in the very acidic soils, aluminium accounts for over 70% of the exchangeable cations and calcium a few per cent only. In limed soils, even when the pH is no higher than 5.5, there is no exchangeable aluminium and calcium is dominant. Exchangeable cations are the immediate source of these. There is no need on agricultural grounds to increase ions in the soil solution which are available to plants and liable to loss by leaching. Aluminium is negligible in the soil solution of limed soils, but is toxic to many plants in acid soils. It is necessary to lime soils as part of upland pasture improvement to prevent aluminium toxicity in ryegrass and clover.

The way pasture improvement has progressed in the uplands has resulted in a mosaic of limed and unlimed areas on many catchments. The patchwork of improved and unimproved pasture has important consequences for the area as a whole and the impact of liming should not be viewed as affecting agricultural production alone.

Limed soils support not only the sown herbage species, typically perennial ryegrass and white clover, but also bentgrasses, fine fescues and herbaceous species which are intolerant of extreme acidity. Thus botanical diversity as a whole is increased. Earthworms which are a food source for many birds and animals, including badgers, cannot survive in unlimed soils.

Beneficial effects of liming are not restricted to the land itself because the water draining from limed soils, especially those in coniferous woodland, are very acidic, high in aluminium and often low in bases. Biological diversity is restricted over a wide range of species ranging from algae to insects to fish and the birds and animals which depend upon them as a food source. A dramatic improvement in stream water quality is usually observed as streams originating in unlimed rough grazing or forest pass through improved pasture.

There is no need on agricultural grounds to increase the area of improved land in the uplands but the substantial contribution made to environmental quality by the current mosaic of limed and unlimed areas should be recognised. A strategy for liming in the uplands is needed to exploit its potential contribution to catchment management.