Cation-exchange capacity

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, available for exchange 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 of dry soil(meq+/100g), or the SI unit 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. One way to increase CEC is to favor the formation of humus.

In general, the higher the CEC, the higher the soil fertility.

Calculation of CEC

The CEC is the number of positive charges that a soil can contain. It is usually described as the amount of equivalents necessary to fill the soil capacity. In soil science, an equivalent is defined by the number of charges in terms of a given number of hydrogen ions. As hydrogen ions have only one positive charge (H⁺), this makes calculations relatively simple. An equivalent of Al⁺³ would have one third as many ions for the same amount of charges, and Ca⁺² would have half as many ions.

Translation from meq/100g to an every day unit, like lb/acre of available nutrients, can be made via calculation, that considers the atomic weight, the ion's valence, and by estimating the soil depth and its density. Mengel gives the following correspondence for 1 meq/100g:^[1]

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,^[2] 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 soil with high cation-exchange capacity takes longer time to acidify (as well as to recover from an acidified status) than a soil 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)^[3] and claimed to be successfully in use on over a million acres (4,000 km²) of farmland worldwide.

pH and CEC

Many soils' CEC is dependent upon the pH of the soil. This is due mostly to the lyotrophic series, which describes the relative strength of various cations' absorption, and is generally as follows:

Al⁺³ > H⁺ > Ca⁺² > Mg⁺² > K⁺ = NH₄⁺ > Na⁺

As soil acidity increases (pH decreases), more H⁺ ions are attached to the colloids and push other cations from the colloids and into the soil solution (CEC decreases). Inversely, when soils become more basic (pH increases), the available cations in solution decreases because there are fewer H⁺ ions to push cations into the soil solution from the colloids (CEC increases). ^[4]

Aluminum and CEC

Many heavily weathered or oxidized soils, especially in the wet tropics, have a high concentration of Al⁺³. Since aluminum is toxic in high quantities for most plants, there are certain advantages and disadvantages to this. For one, due to the relatively high absorption rate of aluminum to soil colloids, it is taken out of the soil and the plant cannot use it. On the other hand, because it has 3 positive charges, it takes up a large amount of charge in a CEC. For example, Al⁺³ fills the same space as three NH_4⁺ ions. This makes many aluminum heavy soils relatively infertile. There is no easy way to remove Al ions from the soil colloid and free the CEC for other ions.

Organic Matter

Organic materials in soil increase the CEC through an increase in available negative charges. As such, organic matter build-up in soil usually positively impacts soil fertility. However, organic matter CEC is heavily impacted by soil acidity as acidity causes many organic compounds to release ions to the soil solution.

Anion Exchange Capacity

Similar to the CEC, the Anion Exchange Capacity is a measurement of the positive charges in soils affecting the amount of negative charges which a soil can absorb. There are relatively few anions that are restrictive in agriculture, but they are important, such as sulfur or phosphorus. The anion lyotrophic series is:

H₂PO₄^- > SO₄^-2 > NO₃^- > Cl^-

In contrast to CEC, AEC generally will increase when pH drops and decrease when pH rises.^[4]

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)^[5] for the cation-exchange capacity of minerals in meq./100g at pH of 7.

References

^ Mengel, David D., Department of Agronomy, Purdue University. "Fundamentals of Soil Cation Exchange Capacity". Retrieved 2011-05-03.{{cite web}}: CS1 maint: multiple names: authors list (link)
^ 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
^ NCat Soil Management
^ Havlin, Tisdale, Beaton, Nelson (2011). Soil Fertility and Fertilizers. New Delhi: PHI.{{cite book}}: CS1 maint: multiple names: authors list (link)
^ Carroll, Dorothy (1959). "Ion exchange in clays and other minerals". Geological Society of America Bulletin. 70 (6): 749‐780. doi:10.1130/0016-7606(1959)70[749:IEICAO]2.0.CO;2.

[1] Mengel, David D., Department of Agronomy, Purdue University. "Fundamentals of Soil Cation Exchange Capacity". Retrieved 2011-05-03.{{cite web}}: CS1 maint: multiple names: authors list (link)

[2] 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

[ll-3] NCat Soil Management

[4] Havlin, Tisdale, Beaton, Nelson (2011). Soil Fertility and Fertilizers. New Delhi: PHI.{{cite book}}: CS1 maint: multiple names: authors list (link)

[5] Carroll, Dorothy (1959). "Ion exchange in clays and other minerals". Geological Society of America Bulletin. 70 (6): 749‐780. doi:10.1130/0016-7606(1959)70[749:IEICAO]2.0.CO;2.

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