Response of corn to high levels of CuSO₄ and ZnSO₄ applications

High levels of Cu and Zn application to agricultural soils are considered to pose a potential hazard to plants and animals. The levels of Cu and Zn which can be safely added to cropland have yet to be established. This study was conducted on a Davidson silty clay (Rhodic Paleudult) to determine the response of corn (Zea mays L.) to cumulative application of up to 469 kg Cu and 1032 kg Zn ha⁻¹ as sulfates over the 22-year period from 1967 through 1988. Neither corn grain nor silage yield was affected by the metal additions even though the cumulative amount of Cu and Zn added exceeded the maximum allowable Cu and Zn loading rates based on the U.S. Environmental Protection Agency guidelines (i.e., 250 kg Cu and 560 kg Zn ha⁻¹) for this soil. The twenty-first annual application of Cu as CuSO₄, increased Cu concentrations in ear leaves. However, Cu concentrations in ear leaves were unaffected by the twenty-second year of Cu application. Concentrations of Zn in ear leaves were increased by the high level of Zn application during the two years of the study. Twenty-first year Cu and Zn concentrations in com grain were not increased by the high levels of Cu and Zn sulfates. All grain and ear leaf Cu and Zn concentrations were within the normal ranges from the high amount of metal application. The DTPA extractable Cu and Zn in the soil increased with an increase in level of applied Cu and Zn. More Cu and Zn were extracted from the soil by the Mehlich-3 method than by the DTPA method. This higher rate of extraction was attributed to the ethylenediaminetetraacetic acid (EDTA) in the Mehlich-3 solution.