Plant Availability of Heavy Metals in a Sludge‐Treated Soil: II. Metal Extractability Compared with Plant Metal Uptake

Abstract

AbstractA greenhouse study was conducted to investigate the uptake of Cd, Ni, Pb, Co, and Zn by fodder rape (Brassica napus L. ‘Kenton’) and extractability of these metals from sewage sludge‐treated soils at different levels of sludge application and soil pH. Metals extracted by several extractants were correlated to their uptake by rape. Total uptake of Cd, Ni, Pb, and Zn increased and that of Co decreased as the rate of sludge application increased. Uptake of all metals except Pb generally decreased as soil pH increased.The extractability of most metals was in the order: HNO3 > aqua regia > HCl > NH4OAc (pH 4.8) > DTPA > H2O. It was governed by rates of sludge application but was little affected by change in soil pH. The percentage of exchangeable form of all metals was < 1% and 90 to 99% of the total metals in sludge‐treated soils was bound in organic and sulfide forms.Cadmium extracted by HCl, NH4OAc (pH 4.8), and diethylenetriaminepentaacetic acid (DTPA) as well as that determined after digestion of soil with HNO3 and aqua regia showed significant correlation with its uptake by rape. The same was true for Pb except for HNO3, where correlation coefficient was not significant. For Co, Ni, and Zn, none of the extractants used gave an adequate prediction of their plant availability, as most of the correlation coefficient values obtained were low. Of the extractants used in this study, though none gave a reliable prediction of plant availability of all the metals, DTPA yielded significant r values for Cd and Pb. Thus, DTPA may be considered better than other extractants for predicting the availability of Cd, Pb, and possibly Zn in fodder rape under varying levels of sludge application and soil pH.