EFFECTS OF SLUDGE AMENDMENTS ON SOIL CATION EXCHANGE CAPACITY, ORGANIC MATTER, AND SUBSEQUENT HEAVY METAL UPTAKE BY VEGETABLES

Abstract

Sewage sludge compost or leaf compost was applied to garden vegetable plots at the rate of 110 metric tons/hectare per year for four consecutive years to determine the influence of soil amendments on soil cation exchange capacity and on soil organic matter. The effect of soil amendments on zinc and cadmium uptake by various species of garden vegetables was also evaluated in light of changes of these soil properties.

Sludge compost treatments increased soil cation exchange capacity, whereas leaf compost did not. In contrast, soil organic matter increased in both leaf compost and sludge compost-treated soil compared to control. Cadmium and zinc levels in soil increased following sludge treatment, but applications of leaf compost caused no significant change.

Swiss chard (Beta vulgaris L., cv. Cicla, Fordhook Giant) absorbed less zinc and cadmium from sludge compost-amended soil compared to control. Similarly, beet (Beta vulgaris, L., cv. Red Ball) leaves grown in sludge compost soil showed lower zinc and cadmium content than those grown in control plots. Metal content of leaves or roots in radish (Raphanus sativus L.) was not affected by compost-amended soil.

The results indicate that applications of leaf compost increased soil organic matter, thereby improving soil structure without increasing cadmium and zinc concentrations. Although sludge compost increased soil heavy metal concentrations, the concomitant increase in cation exchange capacity appeared to restrict metal uptake by several vegetables. Additional work is necessary to determine if sludge applications will cause heavy metal toxicity to other crops, micro-organisms, and ground water.