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Determination of Aluminum Ion Activity in Soil Extracts¹

ABSTRACT

A method is presented for calculating the Al³⁺ ion activity from measurements of the concentration of Al, pH, and the ionic strength of the soil extract. The colorimetric method for determining the concentration of Al in soil extracts makes use of the aluminon reagent with provisions for removing organic matter and silica and for preventing the interference of Fe by the addition of thioglycollic acid.

The validity of the method was established by verifying the existence of a theoretical equilibrium relationship between the Al³⁺ and the H⁺ activities in soil extracts. The value of (pH — pAl) for CaCl₂ extracts of three acid soils was found to remain remarkably constant as the concentration of CaCl₂ in the extracting solution was increased from 0.001 to 0.10M. This constancy of (pH — pAl) for soil extracts can be predicted from the Donnan and Gouy theories and does not necessarily indicate the presence in the soil of some crystalline form of hydrous aluminum oxide having a definite solubility product.

¹ Agronomy Paper No. 454, New York State College of Agriculture, Department of Agronomy, Cornell University, Ithaca, N. Y. This work was supported in part by funds from Tennessee Valley Authority, Wilson Dam, Ala.

² Formerly Graduate Assistant, now Soil Chemist, TVA, Willson Dam, Ala.; Professor of Soil Science, Cornell University; and formerly Assistant Professor of Soil Science, Cornell University, now Assistant Professor of Soils, University of British Columbia, Vancouver, Can., respectively.

Received for publication October 27, 1958. Accepted for publication January 8, 1959.