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What is cation exchange?

In a previous article we’ve discussed cation exchange capacity, or CEC, and why this is critical to the availability of plant nutrients in soil. In this article we look at how it works.

Surface adsorption

The surface of a clay particle or organic colloid is negatively charged. Therefore, it attracts and adsorbs positively charged ions, or cations. These cations can be exchanged between the particle surface and the soil solution. In solution they become available to plant roots.

The most important exchangeable cations are aluminium (Al³⁺), calcium (Ca²⁺), magnesium (Mg²⁺), potassium (K⁺), ammonium (NH₄⁺), sodium (Na⁺) and hydrogen (H⁺).

To become available to a plant, a cation adsorbed on a soil particle must be replaced by a cation present in the soil solution (to balance the charge). Plant roots facilitate this process by excreting H⁺ into the soil solution. This displaces a cation and simultaneously acidifies the soil. The cation is then freed to be taken up by a root.

Clay dispersion

As well as their plant nutrient role, adsorbed cations also influence the behaviour of clays. The more strongly a cation is attracted to a surface, the greater is its chance of adsorption. The strength of attraction is a function of valence (i.e. charge). For this reason, trivalent (3+) cations such as Al³⁺ and divalent (2+) cations such as Ca²⁺ and Mg²⁺ are adsorbed nearly 3 times and 2 times as strongly as monovalent (1+) cations such as K⁺ or Na⁺. Thus, an Al³⁺, a Ca²⁺ or an Mg²⁺ ion stays close in to the clay particle, and does not wedge apart aggregated particles. This is why gypsum (CaSO₄), which supplies Ca²⁺, reduces soil dispersion.

Among cations with equal charge, the cation with the smallest hydrated radius (that is, the radius of the ion in solution) is more strongly adsorbed. For example, K⁺ has a smaller hydrated radius than Na⁺ and hence is more strongly adsorbed. In comparison, Na⁺ is so loosely held and so easily hydrated that high-sodium soil will disperse, because the Na⁺ ions effectively wedge the clay particles apart. Such soils are called sodic soils.

Size matters

The net result is that highly charged and smaller cations are held more tightly and are less likely to be desorbed from the surface. The order of attraction is Al³⁺ > Ca²⁺ > Mg²⁺ > K⁺ > NH₄⁺ > Na⁺ > H⁺. Thus, Na⁺ and H⁺, located furthest from the surface, are most likely to be leached down the profile. Conversely, Al³⁺, Ca²⁺ and Mg²⁺ will move the slowest.

In addition, if the concentration of a cation in the soil solution is high, more of that cation will be adsorbed. This is why gypsum is added to sodic soil. The concentration of Ca²⁺ in the soil solution increases, increasing the quantity of Ca²⁺ on the particle surface, and thus blocking access by Na⁺.

Further information

Wikipedia: http://en.wikipedia.org/wiki/Cation_Exchange_Capacity

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