Agriculture Reference
In-Depth Information
Also, the changes in pH will cause changes in the charges of variable-charge
clays and organic matter, thus the cation exchange capacity of acid soils will
tend to increase and that of alkaline soils decrease.
Changes in Fe
Large concentrations of Fe
2
+
develop in the soil solution in the weeks follow-
ing flooding, often several mM or tens of mM (Figure 4.5). Calculations with
chemical equilibrium models show that the ion activity products of pure ferrous
hydroxides, carbonates and other minerals are often exceeded 100-fold (Neue
and Bloom, 1989). Evidently precipitation of these minerals is inhibited, proba-
bly as a result of adsorption of foreign solutes, such as dissolved organic matter
and phosphate ions, onto nucleation sites (Section 3.7). However, once a suffi-
cient supersaturation has been reached there is a rapid precipitation of amorphous
solid phases, which may later re-order to more crystalline forms. Only a small
part of the Fe(II) formed in reduction remains in solution; the bulk is sorbed in
exchangeable forms or, eventually, precipitated.
The identities of the solid phases that form remain a mystery. Direct identi-
fication is difficult because Fe(II) and Mn(II) solid phases are readily oxidized
by O
2
and it is therefore necessary to maintain scrupulously anoxic conditions
to ensure that the material examined actually represents that in anoxic soil. An
alternative is to make indirect assessments through measurements of pe, pH and
[Fe
2
+
] in solution, but these too are difficult (see section on measurement of
redox potential in soil).
Some of the well-known solid phases that might form are shown in Table 4.3.
None of these appears to be quantitatively important, at least in the first few
Table 4.3
Some possible mineral phases in reduced soils and their equilibrium constants
at 25
◦
C
Compound
Equilibrium
log
K
2H
+
=
Mn
2
+
+
Mn(II) hydroxide
Mn(OH)
2
(
s
)
+
2H
2
O
15.13
a
MnCO
3
(
s
)
=
Mn
2
+
+
CO
3
2
−
−
10.39
b
Rhodocrosite
MnS
2
(
s
)
=
Mn
2
+
+
S
2
2
−
−
14.79
c
Hauerite
Fe(OH)
2
(
s
)
+
2H
+
=
Fe
2
+
+
2H
2
O
11.67
a
Fe(II) hydroxide
2H
+
=
Fe
2
+
+
Fe(II)Fe(III) hydroxide
Fe
3
(
OH
)
8
(
s
)
+
2Fe(OH)
3
+
2H
2
O
−
10.60
d
FeCO
3
(
s
)
=
Fe
2
+
+
CO
3
2
−
−
10.45
b
Siderite
Fe
3
(
PO
4
)
2
·
8H
2
O(s)
=
3Fe
2
+
+
2H
2
PO
4
−
+
8H
2
O3.11
c
Vivianite
FeS
2
(
s
)
=
Fe
2
+
+
S
2
2
−
−
26.93
c
Pyrite
Source
:
a
Calculated from
G
f
values.
b
Stumm and Morgan (1996).
c
Lindsay (1979).
d
Arden (1950).
