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constant, nearly 0.5. From this Farmer (1979) of the Macaulay Institute in
Aberdeen (U.K.) and then Farmer et al . (1980) proposed a revolutionary
scheme:
Mineral-organic complexes will hardly play any role in the migration
of Al and Fe. In the first stage, silicates would be decomposed by
protons and will release soluble gels of relatively well defined structure
( Fe 2 O 3 -Al 2 O 3 -SiO 2 -H 2 O ) called proto-imogolite . These gels will migrate
and will precipitate in the B horizons. Then, in a medium that has been
depleted of flocculating ions, organic acids would migrate in their turn,
by themselves, and would be trapped in the B on these gels already
deposited there.
The Farmer theory was rejected by the scientific community (Buurman
and Van Reeuwijk 1984). The hypothesis of solubility of aluminosilicic
gels in a system crammed with silicate minerals and organic matter is
not credible (Macías-Vásquez et al . 1986). The synthesis of imogolite is
otherwise blocked by the high concentration of organic acids (Inoue and
Huang 1984) [also see Chap. 10]. Later on, Farmer modified the model
slightly, going back to mineral-organic migrations interacting with silicic
gels (Farmer and Lumsdon 2001).
F.C. Ugolini and R. Dahlgren of the University of Washington identified
the weathering processes occurring in the upper horizons and those in
the lower horizons:
In the A, there will be an acidocomplexolysis conforming to the
classic theory. It gives very mobile compounds:
Al-silicate + H-fulvic acid Æ Al-fulvic acid + H 4 SiO 4 + cation-fulvic acid
In, the B, on the contrary, there will be an ordinary hydrolysis linked
to a high partial pressure of CO 2 :
Al-silicate + H 2 CO 3 Æ Al-residue ( insoluble ) + H 4 SiO 4 + cation-HCO 3
The H 2 CO 3 cannot complex Al that, therefore, is precipitated in
situ .
In this model, part of the aluminium precipitated in the B is
autochthonous and does not come from the A or the E. Imogolite and
allophane are formed in the B. Considering what has been said above
(limitations 10 and 11 of the classic theory), Ugolini and Dahlgren (1986)
were right to construct and defend a model minimizing redistributions.
Furthermore, the model explains the Bh/Bs superposition, with the Bh
being rich in complexed Al and Fe, and the Bs in inorganic Al and Fe.
The Ugolini and Dahlgren model (1986)
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