Cation-exchange capacity

In soil science, cation exchange capacity (CEC) is the capacity of a soil for ion exchange of cations between the soil and the soil solution. CEC is used as a measure of fertility, nutrient retention capacity, and the capacity to protect groundwater from cation contamination. Cations can also be easier to understand by just adding the group number.

The quantity of positively charged ions (cations) that a clay mineral or similar material can accommodate on its negatively charged surface is expressed as milli-ion equivalent per 100 g, or more commonly as milliequivalent (meq) per 100 g or cmol/kg. Clays are aluminosilicates in which some of the aluminium and silicon ions have been replaced by elements with different valence, or charge. For example, aluminium (Al³⁺) may be replaced by iron (Fe²⁺) or magnesium (Mg²⁺), leading to a net negative charge. This charge attracts cations when the clay is immersed in an electrolyte such as salty water and causes an electrical double layer. The cation-exchange capacity is often expressed in terms of its contribution per unit pore volume, $Q_{v}$ .

Approximate measures

For agricultural soils, CEC is ideally between 10 and 30 meq/100g

Base saturation

Closely related to cation exchange capacity is the base saturation ^[1], which is the fraction of exchangeable cations that are base cations (Ca, Mg, K and Na). The higher the amount of exchangeable base cations, the more acidity can be neutralised in the short time perspective. Thus, a site with high cation exchange capacity takes longer time to acidify (as well as to recover from an acidified status) than a site with a low cation exchange capacity (assuming similar base saturations).

Laboratory determination

There are two standardised International Soil Reference and Information Centre methods for determining CEC:

extraction with ammonium acetate; and
the silver-thiourea method (one-step centrifugal extraction).

There exist slightly conflicting ideas on which mechanisms to include in the term, "cation exchange", in soil chemistry. From a theoretical point of view, one should distinguish cation exchange from ligand exchange, and exchange of diffuse layer adsorbed cations. On the other hand, from a practical point of view, e.g. in forest and agricultural management, what is important is the soils' ability to replace one cation with another rather than the exact mechanism by which this replacement occurs. What is included in the term, "cation exchange", in soil science thus varies with the scientific context.

References

^ Turner, R.C. and Clark J.S., 1966, Lime potential in acid clay and soil suspensions. Trans. Comm. II & IV Int. Soc. Soil Science, pp. 208-215

[1] Turner, R.C. and Clark J.S., 1966, Lime potential in acid clay and soil suspensions. Trans. Comm. II & IV Int. Soc. Soil Science, pp. 208-215

[1]

Approximate measures

Base saturation

Laboratory determination

See also

References