Interactions of fulvic acid with aluminium and a proto‐imogolite sol: the contribution of E‐horizon eluates to podzolization

Summary The podzolization process is examined in the light of measurements of the solubility characteristics of aluminium fulvate, the extent of dissolution of a proto‐imogolite sol by fulvic acid, the adsorption capacity of proto‐imogolite for fulvic acid and aluminium fulvate, and published evidence. Fulvic acid at 500 mg l −1 acting on a proto‐imogolite (PI) preparation containing 0.95 mmol l −1 Al as PI did not bring enough Al into solution at pH 4.5–5.0 over 4–15 months to cause significant precipitation of the fulvic acid. As allophanic Bs horizons of podzols typically have pH ≥ 4.8, fulvic acids entering them in drainage water cannot be quantitatively precipitated by dissolution of Al from the allophane. They are, however, strongly absorbed on the allophane, and this must be the mechanism that removes most of the fulvic acid at the top of the Bs horizon, and which contributes, along with colloidal humus and root decomposition, to the formation of a Bh horizon. We conclude that fulvic acid plays no active role in podzolization, but only recycles Al and Fe, that have been transferred by biological processes to the O horizon, back to the Bh horizon. The podzolization process, which leads to the formation of an allophanic Bs horizon underlying a progressively deepening E horizon, requires the dissolution of Al‐humate and allophanic precipitates at the Bh–Bs interface as well as progressive attack on the more readily weatherable minerals. Inorganic acids, particularly episodic fluxes of nitric acid, could play a major role in this, as well as attack by readily metabolized complexing acids such as oxalic and citric acids released by roots and fungi. In addition to throwing light on the podzolization process, the experimental results provide an explanation of the lower limit to C:Al ratios reported in natural waters, and a check on the applicability of the WHAM chemical equilibrium model to Al–fulvate–proto‐imogolite equilibria. In Ca‐containing fulvate solutions, Al‐fulvate begins to precipitate when C:Al falls below 50, which is also the limiting ratio observed in natural waters. WHAM calculations overestimate by 70–85% the amount of Al‐fulvate formed over 4 months at pH 4.5–5.0 in Ca‐containing fulvate–imogolite systems.