PREDICTION OF AVAILABLE HEAVY-METALS BY 6 CHEMICAL EXTRACTANTS IN A SEWAGE SLUDGE-AMENDED SOIL

In a field experiment conducted during three years in a sandy-loam, calcareous soil, one aerobically digested sewage sludge (ASL) and another anaerobically digested sewage sludge (ANSL) were applied at rates of 400, 800, and 1,200 kg N/ha/year, and compared with mineral nitrogen fertilizer at rates of 0, 200, 400, and 600 kg N/ha/year in a cropping sequence of potato-com, potato-lettuce, and potato, the first, second, and third year, respectively. Results showed that the highest values of soil extractable metals were obtained with aqua regia, whereas the lowest levels with DTPA. All metal (Zn, Cu, Cd, Ni, Pb, and Cr) gave significant correlations between metal extracted with the different extractants and metal loading applied with the sludges. The metal extractable ion increased over the control for Zn, Cu, Cd, Ni, Pb, and Cr extracted with DTPA, EDTA (pH 8.6) and 0.1 N HCl, for Zn, Cd, Ni, Pb, and Cr extracted with EDTA (pH 4.65) and AB-DTPA, and for Zn, Cd, Ni, and Cr extracted with aqua regia. The level of metal-DTPA extractable resulted highly correlated with that obtained by the other methods, except the Ni-aqua regia extractable. The soil extractable elements which showed significant correlations with metals in plant were: Zn, Cu, Cd, and Ni in potato leaves, Cd, Ni, and Pb in corn grain, and Zn and Cd for lettuce wrapper leaves. In general, all the chelate based extractants (DTPA, EDTA pH 4.6, EDTA pH 8.6, AB-DTPA) were equally useful as indicator of plant available metals in the soil amended with sludge.