| Summary: | Bibliography: pages 79-86. === Acid soils have long been associated with increased mobility of metals and the subsequent nutrient toxicity to plants, the cause of which is believed partly to be a reduction in cation exchange capacity (CEC). At the same time the reduction in CEC would result in a decline in the soil's ability to retain cations. The reduction in CEC is said to result from the development of positive charge at low pH in soils with variable charge which is enhanced by the amphoteric behaviour of Fe and AI oxides as well as the presence of organic matter. Aluminium tends to form stable complexes with organic matter and also competes with lower valency cations for exchange sites, in which way it interferes with the measurement of CEC in soils. Variation of charge with pH has important environmental implications, e.g. groundwater contamination with heavy metals as the soil becomes less capable of retaining them at low pH. Origin of variable charge in soil, factors affecting electric charge variation with pH and the methods employed in determining CEC appear to play an important role in understanding the concept of electric charge variation with pH as well as its environmental significance.
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