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Effect of Phosphorus and Zinc Fertilization on the Solubility of Zn²⁺ in Two Alkaline Soils¹

ABSTRACT

Concentrations of Zn²⁺ in equilibrium with two alkaline soils were calculated from the reactions of EDTA (ethylenediaminetetraacetic acid) chelates. The initial speciation of EDTA was varied to allow equilibrium with Zn²⁺ concentrations in the soil to be approached from above and below. Fertilizer Zn was added at 0 to 5 mg kg^–1 as ZnSO₄ and reacted thoroughly with the soil before additions of P or EDTA chelates. Phosphorus was added at 0 or 62 mg kg^–1 as a mixture of KH₂PO₄ and K₂HPO₄. Concentrations of soluble Zn²⁺ in unfertilized soil were extremely low, approximately 5 x 10^–10 M in the noncalcareous soil at pH 7.2 and 1 x 10^–10 M in the calcareous soil at pH 7.6. These Zn²⁺ concentrations were raised 7- to 14-fold by 5 mg kg^–1 of added Zn, and the higher levels persisted for at least 11 weeks. Buffering of Zn²⁺ by the soils was substantial, ranging from about 15 to 45 (nmol g^–1)/(nmol L^–1). Additions of P caused small decreases in the concentrations of Zn²⁺ and other divalent cations in solution. Only in the Zn-fertilized calcareous soil were the percent decreases in Zn²⁺ greater than those of the major divalent cations, Ca²⁺ and Mg²⁺. This effect of P in the calcareous soil is of special interest because of its apparent selectivity for Zn, but the effect was small as shown by reductions of only 12 to 19% in the [Zn²⁺]/[Ca²⁺ + Mg²⁺] concentration ratio.

¹ Contribution from the U.S. Plant, Soil, and Nutrition Lab., USDA-ARS, Ithaca, NY, and the Dep. of Agronomy (paper no. 1572), Cornell Univ., Ithaca, NY 14853.

² Soil Scientist, USDA-ARS; Visiting Fellow, Agronomy Dep., Cornell Univ. and Adjunct, Academy of Agriculture, Bydgoszcz, Poland; Chemist, USDA-ARS.

Received for publication May 5, 1986.