Agriculture Reference
In-Depth Information
Table 7.7
(
continued
)
pe
0
∗
pe
0
pH 5
pH 7
1
2
HAsO
4
2
−
+
2H
+
+
e
−
1
2
H
3
AsO
3
1
2
H
2
O
=
+
14.9
4.9
0.9
(note also H
3
AsO
4
=
H
2
AsO
4
−
+
H
+
,
p
K
=
2
.
24; H
2
AsO
4
−
=
HAsO
4
2
−
+
H
+
,
p
K
=
6
.
94; HAsO
4
2
−
=
AsO
4
3
−
+
H
+
,
p
K
=
11
.
5; H
3
AsO
3
=
H
2
AsO
3
−
+
H
+
,
p
K
=
9
.
29)
Sb
1
2
SbO
3
−
+
3
2
H
+
+
e
−
1
2
Sb(OH)
3
=
11.3
3.8
0.8
(note also Sb(OH)
3
+
H
+
=
Sb(OH)
2
+
+
H
2
O
,
p
K
=−
1
.
42; Sb(OH)
3
+
H
2
O
=
Sb(OH)
4
−
+
H
+
,
p
K
=
11
.
82
)
Se
1
2
SeO
4
2
−
+
H
+
+
e
−
1
2
SeO
3
2
−
+
1
2
H
2
O
=
14.9
9.9
7.9
1
4
SeO
3
2
−
+
3
2
H
+
+
e
−
1
4
Se(s)
+
3
4
H
2
O
=
14.8
6.0
3.0
1
6
SeO
3
2
−
+
H
+
+
e
−
1
6
Se
2
−
+
1
2
H
2
O
=
5.3
0.3
-1.7
HSeO
3
−
+
2
.
4; HSeO
3
−
=
SeO
3
2
−
+
=
H
+
,
p
K
=
H
+
,
p
K
=
(note also H
2
SeO
3
7
.
9;
SeO
4
2
−
+
H
+
=
HSeO
4
−
,
p
K
=−
1
.
7)
I
IO
3
−
+
6H
+
+
6e
−
I
−
+
=
3H
2
O
18.3
13.3
11.3
1
2
I
2
(
aq
)
e
−
I
−
+
=
10.5
10.5
10.5
Sources
:pe
0
values calculated with Equation (4.8) using
G
f
values from Garrels and Christ (1965). pe
0
∗
values calculated with Equation (4.22) for conditions in submerged soil solutions: for trace element ions,
(
ion
)
=
M
,(
Mn
2
+
)
0.2mM
,(
Fe
2
+
)
10
=
=
1mM. Constants for hydrolysis equilibria from Baes and Maesmer (1976).
µ
Zn
2
+
becomes strongly immobilized as solid solutions (van Breemen
et al
., 1980;
Scharpenseel
et al
., 1983). The formation of [Ca,Mg,Zn]CO
3
solid solutions in
submerged rice soils would explain the association between Zn deficiency and
soils with high Mg:Ca ratios.
Cadmium
Like Zn, Cd is a Group IIB element and occurs in soils exclusively in the
+
2
oxidation state as the Cd
2
+
cation. Cadmium and zinc are often co-precipitated
with each other in sulfide minerals in rocks
(
p
K
CdS
=
27
.
0
)
. Hence Cd tends
to be highly immobile under anaerobic sulfate-reducing conditions, but under
acid, oxidizing conditions it is released in soluble and mobile forms. Hence soils
