16.5  Soil acidity and toxicities

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Elements essential for plant growth generally occur at soil concentrations that range between deficient or adequate for plant growth. Instances do occur, however, where there are excess concentrations of either essential or non-essential elements. These toxicities may occur naturally, as saline soils (Section 17.1), in soils developed from parent materials high in heavy metals (Section 16.5.3), or through soil acidification (Section 16.5.1).

Human action can exacerbate natural processes. This has occurred with salinisation of the landscape as a result of clearing native vegetation or through irrigation. Heavy metals have accumulated in the landscape through industrial pollution, through the use of fungicides that contain Cu or the use of superphosphate which may contain Cd.

Accelerated acidification has occurred through certain farming practices and is a particularly serious problem on sandy soils with low buffer capacity. Application of N fertilisers and inclusion of pasture legumes in rotations increase acidification. Also, drainage of some estuarine soils for farming or urban uses, in which S is present in the reduced state (e.g. as iron pyrite) under waterlogged conditions (i.e. acid sulphate soils), results in oxidation of S and reduction in soil pH. Certain mine spoils are also acid, particularly those high in iron pyrite, which on oxidation result in acidification of spoil in situ or of runoff waters. Acid rain is a further problem, and in industrialised nations, especially in Europe and northeastern USA, there is considerable atmospheric pollution which produces acid rain containing dissolved oxides of N and S. This acid rain may fall long distances away from the source of pollution, and has led to international disputes.