Liming effects on the toxicity to perennial ryegrass of a sewage sludge contaminated with zinc, nickel, copper and chromium

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Up to 12% of the dry matter of a sewage sludge containing 0·65% Zn, 0·20% Ni, 0·50% Cu and 0·42% Cr was mixed with an acid soil limed to give a pH range from 4·3 to 6·8 and cropped with ryegrass in pots. Yields of two cuts of grass were reduced by the largest amount of sludge at soil pHs less than 6. Concentrations of more than 350 μg/g Zn, 160 μg/g Ni and 40 μg/g Cu in the grass dry matter were associated with lowered yields. The treatments had no effect on Cr and Pb concentrations in the grass. 0·5MM acetic acid-soluble metals in the cropped air-dried soils were increased by the maximum sludge additions to 350 mg/kg Zn, 70 mg/kg Ni and 100 mg/kg Cu, equivalent to 48% of the total applied Zn, 25% of the Ni and 18% of the Cu. At these high rates, the pH of the sludge (5·3) significantly affected soil pH. Sludges used on agricultural land should be adjusted to pH 7 before spreading so as to minimise any possible heavy metal toxicities to crops.