Effects of sludge application on the Cd, Pb and Zn levels of selected vegetable plants

Tissue of Swiss chard (Beta vulgaris L.), radish (Raphonus sativus L.) and tomato (Clycopersicon esculentum Mil.) were examined to determine the effects of repeated sludge applications on the accumulation of Cd and Zn. As many as six crops of Swiss chard and radish and three crops of tomato were cultured in greenhouses during the 15-month experiment. Results showed that the plants did not extract appreciable Cd or Zn from the soil when the soil metals were at indigenous levels. Following the initiation of sludge applications, elevated levels of Cd and Zn were detected in all harvested plant tissues. For Swiss chard and radish, the leaf Cd and Zn concentrations increased with the amount of applied sludge. In tomato, elevated Cd and Zn levels were observed following the initial sludge application; however, subsequent applications did not increase the levels significantly. When crops were planted in soils no longer receiving sludge application, plant tissue continued to exhibit Cd and Zn levels greater than non-sludged controls in all six croppings. If the soil received only a single large dosage of sludges at the beginning and then was continuously cropped for the remainder of the experiment, the highest Cd and Zn content of the plant tissue occurred at the first crop harvest. After the first cropping, the metal concentration in the plant tissue of successive crops was reduced to a stable, but still elevated level (a level greater than the non-sludged controls). There was no indication that the elevated plant-tissue Cd and Zn contents caused by the sludge applications would be reduced to the background level after the termination of sludge disposal.