In a pot experiment with additions of Zn, Cu and Ni to a loam soil (pH, 6.3) at rates up to 480 ppm, sewage sludge eliminated the toxic effect of Cu and reduced the toxic effect of Ni on corn, reduced the concentrations of Zn and Ni in the plants, and reduced the amounts of DTPA-extractable metals and of 0.01 M CaCl2-soluble Zn and Ni in the soil. Lettuce grown with the metals and sludges was more susceptible than corn to the toxic effects of the three metals in the loam soil and to Zn in a clay soil (pH 5.9). In the latter soil, Ni was less toxic to lettuce than to corn. The concentrations of metals were considerably higher in lettuce than in corn. Liming of the clay soil to a pH of 6.5 eliminated the toxic effect of Ni on corn and of Zn and Ni on lettuce and reduced the concentrations of the metals in the plants and the amounts of DTPA-extractable and CaCl2-soluble metals in the soil. The order of magnitude of the DTPA-extractable metals was Cu > Zn > Ni whereas the order for CaCl2-soluble metals was Ni > Zn> Cu. Variability in the effect of the metals between crops and soils precluded the use, except as a preliminary guide, of a Zn equivalent concept based on the assumption that Cu is twice as toxic and Ni is eight times as toxic as Zn.
