Agriculture Reference
In-Depth Information
Calculated Changes in Exchangeable Cations Following Soil Reduction
. We
can use these equations to calculate how the exchangeable cations and the com-
position of the soil solution will change following soil reduction. As we have
seen, precipitation of insoluble reduced compounds is often inhibited until a large
supersaturation is reached. Therefore for simplicity I assume no precipitation; the
effects of precipitation are considered in Chapter 4.
The major changes in the soil solid affecting exchangeable cations are: reduc-
tion and dissolution of Fe oxyhydroxide coatings (cf. Equation 3.44):
2[soil—2Fe
(
OH
)
3
−
m
]
+
CH
2
O
+
(
8
−
4
m)
H
+
−−−→
2[soil—]
2
m
−
+
4Fe
2
+
+
CO
2
+
(
11
−
4
m)
H
2
O
(
3
.
60
)
in which, for each mol of Fe reduced, the change in surface negative charge is
0
.
5
m
mol
c
and 2
−
m
mol of H
+
are consumed; reduction of structural Fe in
clay lattices (cf. Equation 3.47):
4[clay—Fe
(
III
)
OH]
+
CH
2
O
−−−→
4[clay—Fe
(
II
)
OH
1
−
n
]
(
1
−
n)
−
+
4
(
1
−
n)
H
+
+
CO
2
+
(
4
n
−
1
)
H
2
O
(
3
.
61
)
in which, for each mol of Fe reduced, the surface negative charge increases by
1
−
n
mol
c
and 1
−
n
mol of H
+
are released; and changes in the charge on
organic matter and variable-charge clays due to the changes in pH. If
ψ
is the
ratio of structural Fe reduced to total Fe reduced, the total changes in surface
negative charge and acidity are
[Z]
=
[cations]
S
+
[HS]
S
={
(
1
−
ψ)
0
.
5
m
+
ψ(
1
−
n)
}
[Fe(III)]
(
3
.
62
)
and
=−{
(
1
−
ψ)(
2
−
m)
−
ψ(
1
−
n)
}
[Fe(III)]
[HS]
(
3
.
63
)
where
[cations]
S
and
[HS]
S
are the changes in exchangeable cations and
acidity in the soil solid. The latter is related to
[HS] by
−
θ/ρ(
[H
+
]
L
−
[HCO
3
−
]
L
)
[HS]
S
=
[HS]
(
3
.
64
)
It is often found empirically that the change in soil pH for a given addition of
acid or base is constant over a wide range of pH and this relation is not greatly
altered by soil reduction. Hence the pH buffer power,
b
HS
, is constant and
pH
=−
[HS]
/b
HS
(
3
.
65
)
Hence
[H
+
]
L
and
[HCO
3
−
]
L
in Equation (3.64) can be found from
[HS].
We therefore have the basis for the calculation.
Consider the effect of Reactions (3.60) and (3.61) on the composition of a soil
solution containing exchangeable cations A
+
and B
2
+
balanced by the anion X
−
.
As Fe
2
+
and CO
2
are formed and H
+
consumed:
