Cation exchange capacity

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The cation exchange capacity (CEC) of a soil is a measure of the total number of sites available for ion exchange of positively charged ions, i.e. cations, or the total amount of exchangeable cations. Closely related to cation exchange capacity is the base saturation, which is simply 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 and acidified status) than a site with a low cation exchange capacity (assuming similar base saturations). The long term resistance to acidification, however, is determined by the weathering rate.

There are two standardised [ISRIC] methods for determining CEC:

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

There exists 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

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