Zinc Absorption by Wheat Seedlings: II. Inhibition by Hydrogen Ions and by Micronutrient Cations

At very low concentrations of Ca 2+ in solution both H + and Cu 2+ retarded Zn 2+ absorption through two distinct effects—toxicity and inhibition. Concentrations of 350µ M Ca 2+ overcame toxic effects of H + at pH 3 but even 500µ M Ca 2+ did not prevent toxicity of Cu 2+ at 10µ M . At Ca 2+ concentrations commonly encountered in soil solutions no toxicities appeared. Copper inhibited Zn 2+ absorption strongly and competitively, Co 2+ competitively but weakly, and Mn 2+ and Fe 2+ had no effect. Also, Ca 2+ had no effect on Cu 2+ inhibition. Increasing H + concentration from pH 7 to 3 in the presence of 350µ M , 10m M , or 50m M Ca 2+ severely inhibited Zn 2+ absorption. Hydrogen ions inhibited Zn 2+ absorption by two mechanisms—one Ca 2+ ‐sensitive, the other Ca 2+ ‐insensitive. The Ca 2+ ‐sensitive mechanism only operated at very low Ca 2+ or under very acid conditions. At the usual H + and Ca 2+ concentrations of soil solutions, H + inhibited Zn 2+ absorption only by the Ca 2+ insensitive mechanism; in this mechanism H + was non‐competitive with Zn 2+ . In soils the effects of Ca 2+ , Cu 2+ , and of H + would be additive. However the effects of Cu 2+ and especially of H + on soil chemical reactions may operate in the opposite direction to their effects on Zn 2+ absorption. More information is needed on the behavior of chelating compounds before the effects of Cu 2+ on Zn 2+ absorption from soils can be assessed.