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Buffer Intensities and Equilibrium pH of Minerals and Soils: II. Theoretical and Actual pH of Minerals and Soils¹

ABSTRACT

Equilibrium pH values of different clay minerals, oxides and carbonates at pCO₂ = 10^-2 atm. are between 4.5 and 8, i.e., in the range typical for most soils. The agreement between the calculated pH and the abrasion pH for most minerals is reasonable. The pH of a number of soils can be explained by partial equilibrium between the soil solution and kaolinite, allophane, gibbsite, montmorillonite or calcite. For illite the agreement is less satisfactory. The effects of reducing conditions and the possible role of pH buffering by organic matter are discussed briefly. Application of the phase rule indicates that the specific mineral assemblage of acid sulfate soils provides a nearly ideal pH-stat, i.e., addition of moderate amounts of sulfuric acid or lime hardly affects the pH.

¹ Contribution from the Department of Soil Science & Geology, Agricultural University, Wageningen, The Netherlands. The work was partly supported by the Netherlands Foundation for the Advancement of Tropical Research (WOTRO, grant 89-2).

² Soil Scientists, Dep. of Soil Science & Geology, Agricultural Univ. P. O. Box 37, Wageningen, The Netherlands.

Received for publication July 25, 1973. Accepted for publication October 8, 1973.