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Form and Availability of Copper and Zinc following Long-Term Copper Sulfate and Zinc Sulfate Applications

Dep. of Soil Sci., Univ. of Florida, Agric. Res. Ctr., Ona, FL 33865;

D. C. Martens, C. Winarko and N. F. Perera

Dep. of Agronomy, Virginia Polytech. Inst. and State Univ., Blacksburg, VA 24061.

^* Corresponding author.

ABSTRACT

High levels of Cu and Zn application to agricultural land poses potential environmental hazards due to reduced plant growth and accumulations of undesirable levels of Cu and Zn in the food chain. The levels of Cu and Zn that 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 the cumulative application of up to 415 kg Cu and 897 kg Zn ha^–1 as CuSO₄ and ZnSO₄, respectively, over the 20-yr period from 1967 through 1986. Neither corn grain nor silage yields were affected by the metal additions, even though the cumulative added amounts of Cu and Zn greatly exceeded the maximum allowed Cu and Zn loading rates (i.e., 250 kg Cu and 560 kg Zn ha^–1) for this soil. Over the period from 1984 through 1986, Cu concentrations in leaf and grain tissue were generally unaffected by metal applications, whereas Zn concentration in ear leaves increased linearly with the cumulative level of applied Zn. Zinc concentration in corn grain was increased only by the highest rate of Zn application. Copper and Zn concentrations in the corn leaves and grain were within normal ranges for all treatments. Levels of DTPA-extractable Cu and Zn in the Ap horizon sampled annually from 1984 through 1985 increased linearly with the amount of metals applied (r² = 0.97 and 0.98, = 0.001, for Cu and Zn, respectively). Extractable Cu was also influenced by Zn applications with a larger amount of extractable Cu being present when both Cu and Zn were added compared to when only Cu was applied. Analysis of subsoil samples indicated limited downward movement of Cu and Zn. Sequential extraction data revealed that the applied Cu and Zn was distributed among both potentially plant-available (exchangeable, specifically adsorbed, and organically bound) and plant-unavailable (oxide associated) fractions. The results of this study indicate that application of 135 kg Cu and 337 kg Zn ha^–1 in excess of the limits set forth by USEPA guidelines was not detrimental to corn production on the Davidson soil with a pH > 6.5. The reasons for the lack of phytotoxicity of the metals include the relatively high soil pH maintained at the site, which limits the availability and the conversion of the applied Cu and Zn to plant-unavailable forms over time.

Contribution of the Dep. of Agronomy, Virginia Polytech. and State Univ., Blacksburg, VA 24061.

This research was partially funded by the Int. Copper Res. Assoc., New York 10017.

Received for publication September 21, 1987.