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# 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(meq) per 100 g or cmol/kg Clays are aluminosilicates in which some of the aluminium and silicon ions have been replaced by elements with different valence or charge For example aluminium (Al3+) may be replaced by iron (Fe2+) or magnesium (Mg2+) leading to a net negative charge This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 Turner RC and Clark JS 1966 Lime potential in acid clay and soil suspensions Trans Comm II & IV Int Soc Soil Science pp. 208-215 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 eg 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

• liming

## References

• ISRIC (International Soil and Reference Information Centre)
• Robert Lippert Clemson University Extension Service
• Microsoilcom Cation Exchange Capacity
• David B. Mengel Department of Agronomy Purdue University
• "One Hundred Harvests Research Branch Agriculture Canada 1886-1986" Historical series / Agriculture Canada - Série historique / Agriculture Canada Government of Canada
• "A study of the Lime Potential R. C. Turner Research Branch Canadian Department of Agriculture 1965