Sewage sludge as a soil amendment, with special reference to Cd, Cu, Mn, Ni, Pb and Zn— comparison of results from experiments conducted inside and outside a glasshouse

Abstract

Lettuces and onions were grown in pots in a glasshouse, in mini-plots, 1 m × 1 m, outside that glasshouse and in a market garden on the same soil amended with 0, 1, 3 and 9 t ha −1 dried sewage sludge from a treatment plant that serves the semi-industrial northern part of Adelaide, South Australia. Edible parts of lettuce tops and onion bulbs were analysed for Cd, Cu, Mn, Ni, Pb and Zn. The sludge applications brought about a sharp increase for most of the metals in the plant material from the glasshouse, whereas there was generally little increase in the concentrations in the field plants. Results of the mini-plots were in line with those of the field. The experiment has shown that the glasshouse experiments, even with pots containing 16 kg dry soil, can give completely erroneous indications of the probable uptake of heavy metals by vegetables on sludge-treated soils under field conditions. Some of the implications of the uses of sewage sludge on farmland and in commercial glasshouses are discussed.