Plant Availability of Heavy Metals in Soils Previously Amended with Heavy Applications of Sewage Sludge

Abstract

Plant uptake is one of the major pathways by which sludge-borne potentially toxic metals enter the food chain. This study examined the accumulation of Cd, Cu, Ni, Pb and Zn in wheat, carrots and spinach grown on soils from 13 sites previously amended with sewage sludge. Winter wheat, carrots and spinach were grown consecutively under field like conditions. The results showed that plant availability of heavy metals differed widely among the crop species. The accumulation of Cd, Ni and Zn in the plants showed the greatest increases compared to their background levels. The Cu and Pb accumulation in the plants grown on sludge-amended soils showed only small increases compared to those grown on uncontaminated soils. Multiple regression analysis of various soil properties showed that the surest way to control the accumulation of metals in food plants is by controlling their concentrations in the soils. Furthermore, soils with a non-acidic pH and a clayey texture tended to achieve better control of metal accumulation in food plants compared to those with an acidic reaction and a coarse texture. Metal concentrations in the plants generally correlated well with those extracted from soils in 0·005 M DTPA, 0·05 M EDTA-(Na)2, 1 M NH4NO3 and 0·05 M CaCl2. The EDTA, however, proved to be a more reliable and consistent test in predicting the accumulation of metals in the plants. The results also showed that liming soils to pH 7 effectively reduced the metal contents in carrots and spinach, but liming to pH 6·5 had little effect on metal concentrations in wheat grain. © 1997 SCI.