Agriculture Reference
In-Depth Information
solution as in non-submerged soils. The first consideration is the concentration
of non-adsorbed anions in solution because this determines the total strength of
the solution. In submerged soils the principle anion is generally HCO
3
−
.The
next consideration is what proportions of the exchangeable cations balance the
anions in solution, and for this some form of empirical relation is necessary.
There have not been many attempts to apply ion exchange equations to sub-
merged soils (Pasricha and Ponnamperuma, 1976, 1977). But in principle the
same equations should apply as for non-submerged soils.
Considering the monovalent-divalent exchange reaction
soil—B
+
2A
+
=
soil—2A
+
B
2
+
(
3
.
55
)
and applying the law of mass action:
(
soil—2A
)(
B
2
+
)
(
soil—B
)(
A
+
)
2
=
K
E
(
3
.
56
)
where
K
E
is the equilibrium constant for the reaction and the terms in parentheses
are activities. Rearranging Equation (3.56) gives
(
soil—B
)
=
K
E
(
A
+
)
2
(
soil—2A
)
(
3
.
57
)
(
B
2
+
)
Because there is generally a large reserve of exchangeable cations on the solid,
a small change in A
+
results in little change in the ratio on the left-hand side.
Hence the 'reduced activity ratio'
(
A
+
)/
(
B
2
+
)
tends to remain constant. The
activity coefficients for the ions in solution can be evaluated with Equation (3.3).
Because of the complexity of soils, there are no general relations between the
proportions of two cations on the exchange complex and their reduced activity
ratio in solution. But two equations are commonly used:
the Gaines and Thomas
(
A
+
)
(
B
2
+
)
N
A
√
N
B
=
K
GT
(
3
.
58
)
and the Gapon
(
A
+
)
(
B
2
+
)
N
A
N
B
=
K
G
(
3
.
59
)
where
N
A
and
N
B
are equivalent fractions of the total exchange capacity and
K
GT
and
K
G
are exchange constants. When there are three or more competing ions, as
there generally will be, it is not practical to determine the exchange isotherms for
all possible combinations of the ions. However Bond and Verburg (1997) have
shown that ternary and higher order exchanges can be predicted from the binary
exchange isotherms of the component ions.
