Toxicity factors in acidic forest soils:: attempts to evaluate separately the toxic effects of excessive Al3+ and H+ and insufficient Ca2+ and Mg2+ upon root elongation

The acidic soils of temperate forests and some pastures on former forest land characteristically contain large amounts of aluminium and hydrogen ions and small amounts of calcium and magnesium ions. The relative importance of these potential toxicity factors are assessed from published data from soils collected in the United States and Europe. Activities of ions in the soil solutions and at the surfaces of root-cell plasma membranes were computed with electrostatic models. Activities of Al3+ in soil solutions ({Al3+}) peaked at pH 4.1, and Al3+ activities at the surface of the plasma membrane ({Al3+}(0) ) achieved a broad maximum between pH 4.1 and 4.8; thus, Al3+ intoxication is likely to be more severe in soils at pH 4.1 than in more acidic ones. Intoxication (assessed by root elongation) correlated somewhat ambiguously with ion activities, but Al3+- and H+-induced depletion of Ca2+ and Mg2+, or both, from the cell surface appears to play a role in toxicity. By contrast, experiments in solution culture, where intercorrelation among {Al3+}, {H+}, and {Ca2+} could be avoided, clearly demonstrated the following extrinsic and intrinsic effects. 1 The ions Al3+ and H+ are intrinsic toxicants. 2 They are also extrinsic toxicants because of the electrostatic displacement of Ca2+ from the surface of the plasma membrane. 3 They are extrinsic ameliorants because each electrostatically displaces the other from the surface of the plasma membrane. 4 The ion Ca2+ is an extrinsic ameliorant because of the electrostatic displacement of Al3+ and H+ from the surface of the plasma membrane. 5 It is an intrinsic ameliorant of intrinsic H+ toxicity, but not intrinsic Al3+ toxicity. 6 It meets an intrinsic requirement. 7 The ion Mg2+ resembles Ca2+ in item 4 but not items 5 and 6 in short-term cultures. In acidic soils, Al3+ may prevent H+ from becoming an intrinsic toxicant (item 3) and may induce an insufficiency of Ca2+ and Mg2+ (item 2). These findings have implications for the mechanisms by which woodland plants tolerate very acidic soils.