THE BIOAVAILABILITY OF CADMIUM TO LETTUCE AND CABBAGE IN SOILS PREVIOUSLY TREATED WITH SEWAGE SLUDGES

The application of sewage sludges to soils may lead to increased soil-Cd levels. The bioavailability of Cd is determined by the interaction of a number of soil physico-chemical and plant variables, of which pH is the most important. Duplicate samples of sludge-treated soils were transferred to tubs in the field, one of each pair being limed to pH 7 +/- 0.5. Lettuce and cabbage were grown to maturity and analysed for Cd. Liming always reduced Cd uptake by the plants. Three soil extractants, 1 M NH4NO3, 0.05 M EDTA-(Na)2 and 0.05 M CaCl2 were used as indices of Cd bioavailability. CaCl2 proved to be the most effective for both lettuce and cabbage. Multiple linear regression equations were derived to describe the uptake and accumulation of Cd by both crops. The relative influence of soil variables differed between the two species. Unlike those of a number of pot experiments conducted in glasshouses, the data from this experiment are comparable with those of crop samples taken from the field.