The sensitivity of rocks and soils to acidification is best described by their acid-base titration curves. These curves tend to be relatively featureless but can be usefully summarized in terms of their initial pH, the buffer capacity at this pH, and the total acidity to pH 8.3. Unfortunately, the shapes of soil titration curves depend on the type and concentration of background electrolyte and the rate of titration. Therefore, while a set of standard titration conditions can be defined, the resulting titration curves will not simulate those applicable to in situ field conditions (as required for modelling purposes). To progress, it is necessary to gain a better understanding of the sources of, and sinks for, acidity in rocks and soils. For soils, partitioning of the acidity into soil solution, neutral salt extractable, and solid phase (but not neutral salt extractable) components provides a promising approach. This is demonstrated using a series of soil samples derived from an extremely acid humus-iron podzol overlying Eocene sand near Wokingham, Berks.

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