Introduction to Soil Chemistry

84 soil basics iv
Soil
H
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H H
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H H
H H
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H H H H
H H H H
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H H
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H H
H H
H H
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H H
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H H
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H H H H
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H H H H
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H H
H
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H H A
H
H
H H H H
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H H
H
H
H H H
H
H H
H H
–
A –
A –
A –
A –
A –
M +
M +
M +
M +
M +
M +
M +
H H
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H
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H O
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Figure 4.8. A soil particle with a diffuse layer of hydrated ions around it. The dashed line represents
the boundary of the layer of tightly held cations. This diagram is not meant to be an exact
representation of the diffuse double layer around a soil particle.
also give invaluable information about the time and amount of extractant
needed for an extraction procedure.
Two types of distribution coefficients can be measured and used in describing
the distribution between solid and liquid phases. The first and simplest is
the distribution between total solid and liquid phases. This can be represented
by K d, as given by the following equations (where kg is kilogram and L is liter
of soil solution):
K
K
d
om
mg component kg soil
=
mg component L solution
mg component kg organic matter
=
mg component L solution
(4.2)
For K om 2 applied to soil, the numerator would be kg organic matter in soil.
Organic matter in soil has a much higher sorptive capacity than does the inorganic
component and so it is sometimes more useful to describe the distribution
between organic matter and a component of interest, particularly
organic components. This can be done using the distribution coefficient Kom,
which denotes the distribution between organic matter and water. The equation
for this distribution is also given in equations (4.2).
2 A Koc where oc = organic carbon could be calculated if organic carbon is substituted in the
equation.