Cation-exchange capacity

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In soil science, cation-exchange capacity (CEC) is the maximum quantity of total cations, of any class, that a soil is capable of holding, at a given pH value, for exchanging with the soil solution. CEC is used as a measure of fertility, nutrient retention capacity, and the capacity to protect groundwater from cation contamination. It is expressed as milliequivalent of hydrogen per 100 g (meq+/100g), or centi mol per kg (cmol+/kg). The numeric expression is coincident in both units.

Clay and humus have electrostatic surface charges that attract the solution ions, and hold them. This holding capacity varies for the different clay types and clay-blends present in soil, and is very dependent of the proportion of clay+humus that is present in a particular soil. A way to increase CEC is to favor the formation of humus.

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

Translation into practical units

Translation from meq/100g to an every day unit, like lb/acre, can be made via calculation, considering atomic weight and valence, and estimating the soil depth and its density. Mengel gives the following correspondence for 1 meq/100g:

Calcium, 400 lb/acre

Magnesium, 240 lb/acre

Potassium, 780 lb/acre

Ammonium, 360 lb/acre

Base saturation

Closely related to cation-exchange capacity is the base saturation, which is the fraction of exchangeable cations that are base cations (Ca, Mg, K and Na). It can be expressed as a percentage, and called percent base saturation. 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).

The base-cation saturation ratio (BCSR) is a method of interpreting soil test results that is widely used in sustainable agriculture, supported by the National Sustainable Agriculture Information Service (ATTRA) and claimed to be successfully in use on over a million acres (4,000 km²) of farmland worldwide.

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.

Standard values

Kaolinite	3-15
Halloysite 2H2O	5-10
Halloysite 4H2O	40-50
Montmorillonite-group	70-100
Illite	10-40
Vermiculite	100-150
Chlorite	10-40
Glauconite	11-20+
Palygorskite-group	20-30
Allophane	~70

These are the values reported by Carroll (1959) for the cation-exchange capacity of minerals in meq./100g at pH of 7.

See also

• liming (soil)

References

• ISRIC (International Soil and Reference Information Centre)
• Robert Lippert, Clemson University Extension Service
• 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