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EFFECTS OF SOIL TYPE, LIMING, AND SLUDGE APPLICATION ON ZINC AND CADMIUM AVAILABILITY TO SWISS CHARD1

We investigated the effects of sludge application, liming, and soil type on Zn and Cd availability to Swiss chard (Beta vulgaris L.) in pot culture under field environmental conditions. Increasing sludge application reduced soil pH, increased the amounts of Zn and Cd extractable by MgCl2 and DTPA, and increased metal accumulation in the plants. Total metal levels in soils did not predict the accumulation in plants as well as did levels of extractable metals, even though the coefficient of determination (R2) between extractable and total levels was high. Soils containing high amorphous Fe oxide levels generally had lower metal extractability at a constant rate of sludge application than those with low amorphous Fe oxide levels. Although pH was important in influencing metal availability for any particular soil, its influence was often less than that of amorphous Fe oxide content when availability was compared among soils. Inclusions of both pH and amorphous Fe oxide content, along with total metal content, gave the best prediction of metal availability. Soil organic matter had little relation to metal availability. Results suggest that soils high in Fe oxide content, in conjunction with soil liming, may give the best control of metal availability for accumulator species, such as Swiss chard.