A field experiment was initiated in fall 1971 to compare 90‐, 180‐, and 360‐kg/ha rates of Zn supplied in ZnSO4 and in garbage compost or sewage sludge. The same treatments were reapplied to half of each plot in fall 1972. Forage yields of sweet corn (Zea mays L.) in 1972 and 1973 were higher with applied compost and sludge but were unaffected or reduced by ZnSO4 applications. Concentrations of Zn were much higher in forage with ZnSO4 than with the organic wastes; concentrations in grain were similar among all treatments. Reapplication of the treatments did not result in proportional increases in heavy metal concentrations in either forage or grain. However, concentrations of Cd in forage and grain were about threefold higher with compost and sludge.Bush beans (Phaseolus vulgaris), double‐cropped after sweet corn each year, were more sensitive to excessive Zn. All rates of ZnSO4 and sewage sludge depressed yields of mature pods, and dry matter yields of bean vines were lower with the high rate of ZnSO4 but relatively unaffected by sludge. Concentrations of Cd and Ni were slightly higher in vines and pods from sludge‐treated plots.
Two coarsely ground air‐dried municipal sewage sludges and a garbage compost were mixed with Hartsells fsl (limed to pH 5.5) at rates of 15, 75, or 300 g/3 kg of soil in two greenhouse pot experiments. Fine ZnSO4 and Na2Cr2O7 were each mixed with soil limed to pH 5.5 to 7.0 at rates up to 1,400 ppm of Zn and 320 ppm of Cr, equivalent to rates applied in the organic wastes. Three successive crops of corn (Zea mays L.) were planted 0, 7, and 18 months after waste application.Corn forage yields were increased only in Crops 1 and 3 by compost applications but were not affected by sludge applications. Concentrations of Zn in the corn forage increased with application rate and Zn content of the three waste products. In contrast, Cr concentrations in the corn were not affected by the 0.05% to 1.36% Cr in the wastes. Concentrations of both Zn and Cr in the corn forage were much higher from ZnSO4 and Na2Cr2O7 from each waste at similar Zn and Cr rates and soil pH. Levels of 0.5N HCl‐extractable Zn and Cr in the soil after each harvest increased with application rate of Zn and Cr in each organic waste and inorganic salt, but were not proportional to plant concentrations.Trivalent Cr as Cr2(SO4)3 was less toxic to corn than hexavalent Cr as Na2Cr2O7 at a rate of 80 ppm of Cr, while both sources were very toxic at 320 ppm. Toxicity resulted from ZnSO4 in pH 5.5 soil at 240 ppm Zn and at 960 ppm in soil above pH 6.5. Results of these experiments suggest that Zn and Cr contained in municipal wastes applied at high rates are not toxic to corn even after relatively long periods in the soil. Reduced plant uptake of these heavy metals in organic waste was not due entirely to their liming effect on the soil.
Fly ash residues from several sites in which solid municipal refuse is being incinerated for energy production were found to contain excessive amounts of cadmium and/or lead according to an empirical extraction test (0.5 N acetic acid) developed by the Environmental Protection Agency. Samples of associated bottom ashes contained acceptable levels of Cd and Pb, but several were marginal with respect to Pb content. Leaching tests clearly demonstrated that Cd and, to a lesser extent, Pb were mobile in soil when applied in a fly ash matrix. The high chloride content (10-12%), which is characteristic of these ashes, resulted in metal complexation and enhanced Cd and Pb mobility in soil. Normally these elements are relatively immobile in soils when applied as inorganic salts or contained in municipal sewage sludge. Plant uptake of Cd and Pb in a greenhouse test was marked when fly or finely ground bottom ashes were soil applied. Phytotoxicity of fly ash appeared to be associated with the high salt content rather than with a specific metal.
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