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Reactive Aluminum Estimated from the Formation of Magnesium-Aluminum Double Hydroxide¹

ABSTRACT

Reactive Al in volcanic ash soils of Chile was estimated from the buffering in the pH interval of 8.2 to 8.5 for potentiometric titration curves obtained from serial titrations of 5-g samples of field-moist soil suspended in 50 ml of 0.5N MgCl₂ using NaOH as the base. The period of time allowed for reaction was 24 hours. Reactive Al thus measured was highly correlated with the Al extracted by 1N NH₄OAc at pH 4.8, with the ratio of Al₂O₃ to SiO₂ dissolved by NaOH, with phosphate fixation, and with the CEC measured by saturating with KH₂PO₄, but not with organic matter content. The reactive Al in A horizons was highly correlated with that in B horizons. An alkaline soil containing no volcanic ash showed no buffer capacity in the pH 8.2 to 8.5 range when titrated in MgCl₂ solution. Volcanic ash soils titrated without MgCl₂ showed no buffering in this pH range. This method of measuring reactive Al appears to be promising but needs to be further tested.

Contribution from the Facultad de Quimica y Farmacia of the Univ. of Chile, Santiago, and from the Dep. of Soil Sci. and Agr. Eng., Univ. of California, Riverside 92502. The support of the Univ. of Chile-Univ. of California Program financed by the Ford Foundation is gratefully acknowledged.

² Professor of Chemistry, Univ. of Chile; Professor of Soil Science, Univ. of California; and former student, Univ. of Chile, respectively.

Received for publication March 1, 1972. Accepted for publication June 13, 1972.