Cation exchange capacity

Calculation of CEC

The CEC is the number of positive charges (cations) that a representative sample of soil can hold. It is usually described as the number of hydrogen ions (H⁺) necessary to fill the soil cation holding sites per 100 grams of dry soil. Alternatively, an equivalent amount of another cation (Al³⁺ or Ca²⁺) can be used in the measure. 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 Al³⁺ that could be held would amount to one third as many of those ions, and Ca²⁺ would have half as many ions.

Translation from meq/100g to an applicable unit, for instance lbs/acre of available nutrients, can calculated, but must consider the atomic weight, the ion's valence, and using a reasonable estimate of soil depth and 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 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). A rain, with its load of acidic hydrogen ions, upon a soil that has a high CEC will be quickly returned (buffered) to its original pH in a very short time. A rain on a low CEC soil, such as in the Amazon Basin with its acid soils, will not be restored and the pH will drop sharply and remain there for a relatively long time.

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.

pH and CEC

For many soils, the CEC is dependent upon the pH of the soil. This is due mostly to the Hofmeister series (lyotrophic series), which describes the relative strength of various cations' adsorption to colloids, and is usually given as:

Al³⁺ > H⁺ > Ca²⁺ > Mg²⁺ > K⁺ = NH₄⁺ > Na⁺

As soil acidity increases (ie, as pH decreases), more H⁺ ions become attached to the colloids. They will have pushed the other cations from the colloids and into the soil water solution. Inversely, when soils become more basic (pH increases), the available cations in solution decrease because there are fewer H⁺ ions to push cations into the soil solution from the colloids (CEC increases).

Various colloids and soils' CEC

The CEC of various soils and soil constituents varies greatly.

Standard values

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

Aluminium ions and CEC

Many heavily leached or oxidized soils, especially in the wet tropics, have a high concentration of Al³⁺ occupying the soil colloids cation exchange sites. Since aluminium is toxic in high quantities for most plants, there are certain advantages to this. Due to the relatively high adsorption rate of aluminium to soil colloids, it will be taken out of the soil, hence the plant cannot be adversely affected by it. On the other hand, because it has three positive charges, it takes up a large amount of charge on a colloid. For example, Al³⁺ fills the same space as three NH₄⁺ ions. As a result, the ammonium is left in the soil water solution where it can be washed away by a heavy rain. This makes many aluminium heavy soils relatively infertile. There is no easy way to remove aluminium 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⁻

Converse to CEC, AEC generally will increase when pH drops and decrease when pH rises.

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.