Land use, liming and the mobilization of potentially toxic metals

The liming of agricultural land to offset acidification from ammonium-based fertilizers, acid deposition and the removal of nutrient cations in harvested material has been practised for thousands of years. Current levels of pollution mean that more lime is now required to offset acidification, but extensification and afforestation an likely to result in a cessation of liming for economic reasons, and afforestation will result in increased acid deposition and acidification. The Classical Experiments at Rothamsted show how acidification leads to soil degradation and the release of aluminium (Al) and potentially toxic metals into the soil waters and plants, how regular liming can prevent these problems, and how liming already-acid soil can reverse Al and metal mobilization but not all of any acid-induced weathering. Better models for calculating lime loss and lime requirement are needed, together with dynamic, mechanistic models that include land use as an input and Al and heavy metal mobilization as an output. These will help to predict better the effects of changing land use and so determine better policy decisions. However, farmers and land managers must be convinced of the need to apply lime. (C) 1998 Elsevier Science B.V.