Geoscience Reference
In-Depth Information
The type of soil. If it is clayey, pore space is closed up during
saturation and makes the transmission of oxygen still more
difficult (Vizier 1984);
Temperature; we have indicated at the beginning of the chapter
that the solubility of oxygen depends on temperature.
Whereas anaerobiosis is established more or less gradually,
reoxidation of the medium is abrupt in soil cracks. In the mass of
aggregates, it is different. During the drying phase, the water containing
ferrous iron is displaced towards the drier zones. The iron is precipitated
there by oxidation resulting in formation of concretions in soils that
undergo alternate wetting and drying.
In the acid soils of old mangrove swamps, the pH-H
2
O, close to 4, can
rise to 6.5 when submergence is practised (Quantin
et al
. 2004). In the
laboratory, during percolation of water through a column of acid soil kept
in an atmosphere of nitrogen (Vizier 1978), a simultaneous diminution
of redox potential and rise in pH are observed. All this points to the
fact that most reduction reactions consume protons (cf. Table 12.2 of
reduction half-reactions).
Conversely, if one starts from a soil of high pH in which H
2
CO
3
is strongly dissociated, reduction will release Fe
2+
and Mn
2+
ions that
favour the following reactions:
Changes in pH
Fe
2+
+
H
2
CO
3
Æ
FeCO
3
+ 2
H
+
and
Mn
2+
+
H
2
CO
3
Æ
MnCO
3
+ 2
H
+
Thus the pH drops, because of release of protons. When salts are
absent in soils saturated with water, the pH goes to between 6 and 7,
then stabilizes near neutrality by the following reaction, related to the
simultaneous presence of water and CO
2
in large amounts:
H
2
CO
3
´
HCO
3
-
+
H
+
On the other hand, if the soil is drained, the pH may fall or rise.
In soils, ferric iron is solidly bound to the clay; it is strongly adsorbed
because it is a small and trivalent cation (Table 12.6).
When reduced, the iron ion becomes larger, less charged and soluble.
It moves away from the clay, which can then disperse and migrate. This
mechanism could be one of the causes of argilluviation.
Argilluviation (transport of clay)
