Agriculture Reference
In-Depth Information
Following flooding, O
2
dissolved in the soil solution is consumed according to
Reaction (4.27). There is no pH change, the CO
2
pressure being constant, and
pe is poised by the O
2
-H
2
O couple:
O
2
(
aq
)
+
4H
+
+
4e
−
=
H
2
O
i.e.
1
pe
=
21
.
45
+
4
log[O
2
]
L
−
pH
(
4
.
33
)
Once all the O
2
has been used up Fe(OH)
3
is reduced according to Reaction
(4.30) and the pe is poised by the Fe(OH)
3
-Fe
2
+
couple:
3H
+
+
e
−
=
Fe
2
+
+
Fe(OH)
3
(
s
)
+
3H
2
O
i.e.
pe
=
16
.
54
−
log[Fe
2
+
]
L
−
3pH
(
4
.
34
)
Once pe falls sufficiently, precipitation of Fe
3
(
OH
)
8
commences and Fe
3
(
OH
)
8
is formed at the expense of Fe(OH)
3
according to the reaction
3Fe(OH)
3
+
CH
2
O
+
8H
+
→
Fe
3
(
OH
)
8
+
CO
2
+
11H
2
O
The pe is now poised by the Fe(OH)
3
-Fe
3
(
OH
)
8
half reaction:
H
+
+
e
−
=
3Fe(OH)
3
(
s
)
+
Fe
3
(
OH
)
8
+
H
2
O
i.e.
pe
=
1
.
46
−
pH
(
4
.
35
)
Hence for a given generation of Fe
2
+
in Fe(OH)
3
reduction, and for a specified
initial soil CEC and concentration of non-carbonate anions in the soil solution
(
[X
−
]
L
)
, we have five unknowns: the soil pH and the concentrations of Fe
2
+
and
M
2
+
in the soil solid and solution; and these can be found from the following
five equations:
(1) Equation (3.69) for the electrical neutrality of the solution, with [HCO
3
−
]
found from
p
CO2
and pH;
(2) Equation (3.70) for the electrical neutrality of the solid, with changes in
acidity in the solid related to changes in pH with the soil pH buffer capacity;
(3) Equation (3.72) for divalent - divalent cation exchange; and
(4) and (5) two equations like Equation (3.73) for conservation of M
2
+
and Fe
2
+
.
These equations can be solved simultaneously with Equations (4.33) - (4.35) to
obtain values of pe, pH, [O
2
]and[Fe
2
+
] over the course of reduction. Figure 4.7
shows results for realistic flooded soil conditions, expressed in terms of the
amounts of CH
2
O oxidized in the different reactions. Figure 4.7(a) gives results
in the absence of pH and cation buffering by the soil; Figure 4.7(b) - (d) gives
results for different values of
b
HS
,CECand[X
−
]
L
.
