Agriculture Reference
In-Depth Information
Sodicity
When the salt concentration in the soil solution is high, Na
ions gradually re-
place Ca
2
and Mg
2
ions on the charged surfaces. This is an example of cation
exchange (section 4.6.1). The
exchangeable Na
percentage (
ESP
) on the charged
surfaces is defined as
7.2.3
Na
ESP
100
(7.2)
(Ca
2
Mg
2
K
Na
)
where the ion concentrations are expressed in cmols charge (
) per kg soil. As
the
ESP
rises, the soil may become
sodic
. The critical
ESP
value to define sodic-
ity ranges from
6 in Australia to
15 in the United States. The lower
ESP
Box 7.4
Cation Exchange and the Gapon Equation
As irrigation water is concentrated by evaporation, Na
ions gradually replace
Ca
2
and Mg
2
on the clays. The change in the proportions of cations on the
clays is gradual because, for a given volume of soil, there is a much larger quantity
of exchangeable cations than cations in solution. Take, for example, the exchange
reaction
(Na
)
1
/
2
(Ca
2
)
(Ca-clay)
(Na-clay)
(B7.4.1)
The equilibrium constant (
K
ex
) for this reaction can be written as
(Na-clay)(Ca
2
)
1/2
K
ex
(B7.4.2)
(Ca-clay)(Na
)
where the ion activities in solution are in molarities. When rearranged, this
expression is identical to an empirical equation—the
Gapon equation
—which was
derived from many measurements of cation exchange in soils:
[Na-clay]
[Na
]
K
Na,Ca
(B7.4.3)
[Ca-clay] [Ca
2
/2]
1/2
In this equation, the ion concentrations are expressed in cmols charge (
) per
kg clay and mmols charge (
)/L in solution. The Gapon coefficient
K
Na,Ca
is
approximately constant, provided the concentration of Na
on the clay is small
relative to that of Ca
2
. Because Ca
2
and Mg
2
have similar adsorption affinities
on most clays, which differ from the affinity for Na
, equation B7.4.3 can be
written as
[Na
]
K
Na
Ca,Mg
[Na-clay]
Ca
2
Mg
2
1/2
(B7.4.4)
[Ca,Mg-clay]
2
From this equation, we know that the ratio of exchangeable Na
to
exchangeable Ca
2
and Mg
2
ions will be governed by the ratio in solution of
[Na
]
Ca
2
Mg
2
1/2
2
which defines the
sodium adsorption ratio
(
SAR
).
