The solubility characteristics of organic Al precipitates in allophanic Bs horizons could mimic those of Al hydroxides, and indicate the solubility of associated allophanes

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Abstract
Summary: It is shown that Al‐humate and fulvate precipitates in Bs horizons of pH > 4.6 can be the source of the soluble aluminium which is rapidly released in equilibrium studies to give log10{Al3+} + 3pH values near 9.4 at 8°C, so that it is not necessary to postulate an anomalously reactive but sparingly soluble Al(OH)3 phase. These Al‐organic precipitates will have reached equilibrium in the natural soil environment with the more slowly reacting hydroxy‐aluminium precipitates present, including proto‐imogolite allophane, but can release Al3+ much more rapidly than the inorganic precipitates in laboratory equilibrations and soil leaching episodes that yield lysimeter waters. Equilibrium concentrations of Al reported in a range of Bs horizons indicate that the allophanes present are less soluble than proto‐imogolite sols prepared in the laboratory and matured for up to 2 years.