Some Heavy Metals in Soils Treated With Sewage Sludge, Their Effects on Yield, and Their Uptake by Plants

The possible use of sludge with high heavy metal concentrations and at high rates in calcareous soil was demonstrated in this study. Mixtures of two sludges were added to soils in various proportions up to 4% sludge content. One sludge was rich in Ni and Cd, while the other was relatively poor in heavy metals. Three soils varying in pH from 7.7 to 5.5 were tested. The concentrations of Cd, Ni, Cu and Zn in the DTPA and saturation extracts of the soil-sludge mixtures were determined and correlated with their uptake by plants and the yield of Swiss chard (Beta vulgaris L., cv. Ford Hook Giant) grown on these mixtures. The metal-poor sludge hardly affected the yield of the relatively salt-resistant Swiss chard. The metal-rich sludge reduced the yield drastically in noncalcareous soils after a critical amount of that sludge (1.5%) was added to the soils. Yet, even 4% of this metal-rich sludge increased the yield of Swiss chard, as compared with the sludge-free control in a calcareous soil. The best fit to yield was obtained by multiple regression with metal content in the soil saturation extract. The solubility in soil solution of Cd, Ni and Zn was strongly affected by the pH. The uptake of Ni and Zn by plants was significantly larger in the acid soil than in the calcareous soil. The difference in the uptake of Cd and Cu between the soils was smaller. Plant uptake of the metals was generally predicted better by the total metal addition or concentration in the DTPA extract than by metal concentration in the soil saturation extract. In noncalcareous soils the total metal addition correlated as well as metal content in the DTPA extracts with the metal concentration in the soil solution, with the uptake by plants and with the yield. It has been known for some time that heavy metals introduced as mineral salts do not exhibit in soil the same behavior as heavy metals released from the sludge itself. The heavy metal contents in the soils were varied, therefore, by mixing the two sludges rather than by adding mineral salts to one sludge. Results of the relatively recent studies in which spiked sludges were used should be interpreted with care.