Liming Effects on the Yield and Composition of Perennial Ryegrass Treated with Sewage Sludge Contaminated with Heavy Metals

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Up to 12% of the dry matter of a sewage containing 0.65% Zn, 0.20% of Ni, 0.5% Cu, and 0.41% Cr was mixed with an acid loamy sand limed to give a pH range from 4.6 to 7.0 and cropped with ryegrass in pots. The results obtained were similar to a previous experiment (2). Yields of three cuts of grass were reduced by the largest amo unts of sludge at soil pH's less than 6. However, grass yields were increased by medium amounts (8%) of sludge at soil pH more than 6. The increases in yield were caused by increased supplies of N, P, Mg, and S from the sludge. Concentrations of more than 300 ug/g Zn, and 200 ug/g Ni in the grass dry matter were associated with lowest yields. The treatments had only small effects on Cu concentrations. Acetic acid-extractable metals in the cropped air-dried soils were increased by the maximum sludge addition to 300 mg/Kg Zn, 100 mg/Kg Cu, 67 mg/Kg Ni, equivalent to 30% of the total applied Zn, 17% of Cu, and 28% of Ni. The pH of the sludge (5.0) significantly affected soil pH. Sludge used on agricultural land should be adjust to pH 7.0 before spreading so as to minimize any possible heavy metal toxicities to crops.