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

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 log(10){Al3+} + 3pH values near 9.4 at 8 degre es 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 re acting hydroxy-aluminium precipitates present, including proto-imogolite al lophane, but can release Al3+ much more rapidly than the inorganic precipit ates in laboratory equilibrations and soil leaching episodes that yield lys imeter waters. Equilibrium concentrations of Al reported in a range of Bs h orizons indicate that the allophanes present are less soluble than proto-im ogolite sols prepared in the laboratory and matured for up to 2 years.