Soil properties - Fundamentals of the Physical Environment - page 583

valence of the original one. For example, if a clay containing sodium as the exchangeable
cation is washed with a solution of calcium chloride, each calcium ion will replace two
sodium ions, and the sodium will be washed out in solution. This process is called cation
exchange or base exchange . It can be written as the chemical equation:

The total quantity of exchangeable cations held is the cation exchange capacity . The
predominant exchangeable cations in soils are calcium and magnesium, with lesser
amounts of potassium and sodium. Aluminium and hydrogen are common in acid soils.
The proportions of these cations found on the colloids of any particular soil are governed
by the parent rock and by the nature and intensity of weathering and leaching. Calcareous
soils over limestone will contain mostly calcium. Clays deposited in sea water will have
mostly magnesium and sodium. Leaching removes the cations which form bases
(calcium, sodium, etc.), leaving a clay with the acidic cations, aluminium and hydrogen.
The influence of hydrogen ions on the exchange sites was originally thought to give soils
acidic properties, but it was later found that acid clays had aluminium rather than
hydrogen as the exchangeable ion. In very acid soils the clay minerals themselves start to
dissociate, releasing aluminium which can then move on to the soil complex. The process
of cations fixing themselves on to exchange sites on colloids is termed adsorption . The
cations are not all held in a layer right at the clay surface but are present as a diffuse
double layer , as shown in Figure 18.9. The inner layer is the highest concentration of
cations at the colloid surface, attracted by coulomb electrical forces; the outer layer is a
diffuse 'cloud' of cations whose thermal energy makes them diffuse away from the
colloid surface.
Table 18.7 illustrates the cation exchange characteristics of five contrasting soils. The
values for the four

Table 18.6 Electrical charges on clay minerals

Charge (me 100g −1 )

Clay mineral Source of charge

Kaolinite 5-15 Broken bands

Ionization of OH

Illite, chlorite 20-40 Ion substitution

Montmorillonite 80-100 Ion substitution

Vermiculite 100-150 Ion substitution