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Aluminium toxicity is a worldwide problem affecting growth of crop plants and yields on acid soils (see also Chap. 3.5.1 on forest damage): Al is the most frequent metal element in the earth's crust. In acid soils, i.e. at pH <5.5, the phyto-toxic Al3+ ion becomes soluble to an extent which inhibits root growth and, as secondary effects, decreases uptake of nutrients and water and thus growth of the plant. In only slightly acidic or neutral soils aluminium forms insoluble oxides or silicates. More than 30% of the (potential) agricultural land has a pH of <5.5 with aluminium toxicity being a serious problem. The widespread acidic red soils, called oxi-sols, of the tropics and subtropics are particularly affected. Aluminium toxicity also occurs in temperate climates because of acid rain or mist.
However, there is growing awareness that on acid soils several factors in addition to Al toxicity may limit plant growth, such as high concentrations of iron and manganese ions or deficien cy in several essential mineral elements, in particular of phosphorus.
According to present knowledge, aluminium is not a trace element required for the nutrition of plants. Though not belonging to the heavy metals, it is considered a toxin because of its negative effects on plant growth. Nevertheless -as with most xenobiotics - inheritable resistance to A1 ions in the root zone occurs, and possibly even tolerance of aluminium ions in the cell. Research into A1 toxicity and resistance mechanisms has been made possible by breeding of almost isogenic lines, e.g. of wheat, which differ only in their sensitivity to Al. Because of the multiplicity of secondary effects of Al toxicity, the interpretation of the physiological effects of Al ions is still very controversial. However, methods of analysis have become more sophisticated and hence interpretation is clearing up.
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