THE PLANT AVAILABILITY AND DTPA EXTRACTABILITY OF TRACE-METALS IN SLUDGE-AMENDED SOILS

The DTPA extractability of trace metals and their bioaccumulation by ryegrass (Lolium perenne L.), grown on two soils treated with sewage sludge, were investigated in a greenhouse pot experiment over a period of 1 year following sludge application. The experiments were carried out at ambient temperatures of 15-degrees-C and 25-degrees-C. The sludge application resulted in elevated concentrations of Cd, Cu, Ni, Pb, and Zn in ryegrass. The plants grown at 25-degrees-C accumulated significantly higher levels of all the metals than those grown at 15-degrees-C. A trend towards increasing plant metal accumulation over successive harvests was observed for all the metals except Cu. This appeared to be caused by the progressive decline in pH and organic matter status. The DTPA extraction data generally supported the results of plant metal concentrations for a given harvest or temperature; however, it was not consistent in predicting the changing metal uptake behaviour by the ryegrass over the residual period.