Geoscience Reference
In-Depth Information
acids can no longer capture it but, on the contrary, get precipitated on
contact with it.
Table 11.2
Approximate average properties of typical humic and fulvic acids
(Schnitzer 1984).
Humic acid
Fulvic acid
C
56.2
45.7
H
4.7
5.4
Elements, %
O
35.5
44.8
N
3.2
2.1
S
0.8
1.9
Total acidity
6.7
10.3
Acid functional groups
COOH
3.6
8.2
Phenolic OH
3.9
3.0
Presumed molecular weight, daltons
50,000-100,000
500-2000
Solubility in water
If pH > 6.5
At all pH
It is now possible to precisely define the organization of the B horizons
of Podzols (Table 11.3, from Anderson
et al
. 1982).
Organization of the accumulative horizons
Table 11.3
Typical composition of
horizons of accumulation in Podzols.
Upper horizon of accumulation:
Fe-humic-acid complexes (or Al-humic-
(Bh or Bhs)
acid complexes in water-saturated
medium); high aromaticity, few
chelating functions
Lower horizon of accumulation:
Soluble Al-fulvic-acid complexes, proto-
Bs (or BFe in old reports)
imogolite, imogolite, allophanes, free
oxides of iron and aluminium, aliphatic
functions, oxygen-containing functions
The Bh horizon is morphologically characterized by an accumulation
of humus. It is often named Bhs, referring to its high content of
sesquioxides of iron and aluminium.
The Bs horizon is the lower horizon; its brighter colour is related
to iron and systematically differentiates it in the field. It also contains
colourless oxides of aluminium. But often there is more iron in the Bh
(where it is masked) than in the Bs (where it is clearly visible)! This Bs
disappears in water-saturated systems where the iron content can fall to
practically zero. The B horizons are then composed of aluminium and
organic compounds.
