Interactions of fulvic acid with aluminium and a proto-ianogolite sol: thecontribution of E-horizon eluates to podzolization

The podzolization process is examined in the light of measurements of the s olubility characteristics of aluminium fulvate, the extent of dissolution o f a proto-imogolite sol by fulvic acid, the adsorption capacity of proto-im ogolite for fulvic acid and aluminium fulvate, and published evidence. Fulv ic acid at 500 mg l(-1) acting on a proto-imogolite (PI) preparation contai ning 0.95 mmoll(-1) Al as PI did not bring enough Al into solution at pH4.5 -5.0 over 4-15 months to cause significant precipitation of the fulvic acid . As allophanic Bs horizons of podzols typically have pH greater than or eq ual to4.8, fulvic acids entering them in drainage water cannot be quantitat ively precipitated by dissolution of Al from the allophane. They are, howev er, 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 co ntributes, along with colloidal humus and root decomposition, to the format ion of a Bh horizon. We conclude that fulvic acid plays no active role in podzolization, but onl y recycles Al and Fe, that have been transferred by biological processes to the O horizon, back to the Bh horizon. The podzolization process, which le ads 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 mo re readily weatherable minerals. Inorganic acids, particularly episodic flu xes of nitric acid, could play a major role in this, as well as attack by r eadily metabolized complexing acids such as oxalic and citric acids release d by roots and fungi. In addition to throwing light on the podzolization process, the experimenta l 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 e quilibrium 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 calcu lations overestimate by 70-85% the amount of Al-fulvate formed over 4 month s at pH4.5-5.0 in Ca-containing fulvate-imogolite systems.