Effect of pH on the uptake of cadmium, copper, and zinc from soilless media containing sewage sludge

Swiss chard (Beta vulgaris cicla ’Fordhook Giant'), bean (Phaseolus vulgaris ’Hawksbury Wonder'), and Agrostis capillaris ’Parys’ were grown in soilless media composed of Pinus radiata bark and 0–41% by volume of dried sewage sludge containing [in mg/kg] cadmium (Cd) [22], copper (Cu) [1,116], and zinc (Zn) [1,121]. Sublots of all mixtures were adjusted to pH 4.5, 5.5, or 6.5. Shoot weights were not reduced by sludge addition but those of Swiss chard and bean shoots declined with pH. Shoot contents of Cu, Cd, and Zn increased with decreasing pH. Shoot Cu concentrations were always sub‐toxic, and in Swiss chard toxicity symptoms were not observed until shoot Zn content reached 1,300 mg/kg. Cadmium in Swiss chard shoots reached 22 mg/kg. Metal concentrations in the shoots were well correlated with total and extractable metals in the media only if pH was included in the regression equations.