Vc. Farmer et Dg. Lumsdon, Interactions of fulvic acid with aluminium and a proto-ianogolite sol: thecontribution of E-horizon eluates to podzolization, EUR J SO SC, 52(2), 2001, pp. 177-188
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.