The pH‐dependent charge properties of the A and B horizons of two representative oxisols from the Cerrado Region, Brazil, were studied by a comparison of the potentiometric titration method of net charge measurement and direct measurement of anion and cation exchange capacity from K + and Cl ‐ adsorption. Most soil samples bore net negative charge at their native pH, but variability in the magnitude of this charge was attributed to the effect of Al‐blocked exchange sites or contributions from strongly acidic organic functional groups, since these soils had similar mineralogies. The soils also bore significant pH‐dependent positive charge at their natural acidic pH values, which was more evident in the B horizons than the surface horizons. Estimates of net surface charge by potentiometric titration and ion adsorption did not agree, especially as the soil pH was adjusted away from the point of zero charge. This lack of agreement is attributed to dissolution reactions of minerals and organic matter at high and low pH, which consume acid or base and overestimate surface charge.
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Marcano‐Martinez et al. (1989) studied this question.