pH and solubility of aluminium in acidic forest soils: a consequence of reactions between organic acidity and aluminium alkalinity

Summary Data from two Podzol O and E horizons, sampled in 1‐cm layers at 13 points within 2 m × 2 m plots, were used to test the hypothesis that the composition of hydrogen ions (H) and aluminium (Al) adsorbed to the solid‐phase soil organic matter (SOM) determines pH and Al solubility in organic‐rich acidic forest soils. Organically adsorbed Al was extracted sequentially with 0.5 m CuCl 2 and organically adsorbed H was determined as the difference between total acidity titrated to pH 8.2 and Al extracted in 0.5 m CuCl 2 . The quotient between fractions of SOM sites binding Al and H (N Al /N H ) is shown to determine the variation in pH and Al solubility. It is furthermore shown that models in which pH and Al solubility are linked via a pH‐dependent solubility of an Al hydroxide and in which cation exchange between Al 3+ and Ca 2+ , rather than cation exchange between Al 3+ and H + , is the main pH‐buffering process cannot be used to simulate pH or Al solubility in O and E horizons. The fraction of SOM sites adsorbing Al increased by depth in the lower O and throughout the E horizon at the same magnitude as sites adsorbing H decreased. The fraction of sites binding the cations Ca 2+ + Mg 2+ + K + + Na + remained constant. It is suggested that a net reaction between Al silicates (proton acceptors) and protonated functional groups in SOM (proton donors) is the long‐term chemical process determining the composition of organically adsorbed H and Al in the lower part of the O and in the E horizon of Podzols. Thus, in the long term, pH and Al solubility are determined by the interaction between organic acidity and Al alkalinity.