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Response of Corn to Zinc and Chromium in Municipal Wastes Applied to Soil¹

ABSTRACT

Two coarsely ground air-dried municipal sewage sludges and a garbage compost were mixed with Hartsells fsl (limed to pH 5.5) 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.

Key Words: heavy metals • plant toxicity • sewage sludge • garbage compost

¹ Contribution from the Soils and Fertilizer Research Branch, National Fertilizer Development Center, Tennessee Valley Authority, Muscle Shoals, AL 35660. This research was supported in part by funds provided by the Solid and Hazardous Waste Research Laboratory, National Environmental Research Center-Cincinnati, Environmental Protection Agency. Presented in part before the Southern Branch, ASA, 6 Feb. 1974, in Memphis, TN.

² Research Soil Chemists.

Received for publication May 6, 1974.

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