Definitions

cation: An ion that carries a positive electrical charge (e.g. the metallic element of salt compounds). A cation can combine with certain anions (which have negative charges).

cation exchange: A process in which cations in solution are exchanged with cations held on the exchange sites of mineral and organic matter, particularly on the surfaces of colloids of clay and humus.

cation exchange capacity: The total amount of exchangeable cations that a particular material or soil can adsorb at a given pH. Exchangeable cations are held mainly on the surface of colloids of clay and humus and are measured in centimoles of charge per kilogram (cmol_c/kg) of soil (or of other adsorbing material such as clay).

Concepts

Soil colloids have a net negative charge, therefore these particles are surrounded by a swarm of cations. The concentration of the cations is the highest in the Stern layer at surface of the clay particle, decreases in the Gouy layer and is the lowest in the bulk solution (Fig. 6.12).

Fig. 6.12. Impact of soil colloid (clay mineral) on the distribution of cation and anions in soil.

It is important to note that atoms of an element can exist in different form in soil. For example, aluminum is in a bound form in the crystal lattice (integral component of the octahedral layer and may be a substitute for silicon in the tetrahedral layer) and can be present as exchangeable cation and in soil bulk solution at certain soil pHs.

Table 6.4. Exchangeable cations and their ionic radius

The valance and ionic radius of exchangeable cations is presented in Table 6.4. The strength of the attractive force between a cation and a micelle depends on the charge of the cation, the charge density of micelle, and how close the cation can get to the micelle. The sequence of relative attractive forces, reflecting both the number of charges and the hydrated radius is Al³⁺ > Ca²⁺ > Mg²⁺ > NH₄⁺ ~ K⁺ > Na⁺ . This list, sometimes called the lyotrophic series, may be considered as a preferential adsorption sequence. Cation exchange sites adsorb more of an ion early in the sequence than of one later in the sequence of the two are present in equal amounts. Preferential adsorption, however, can be overcome by an ion that is much more concentrated than others in soil solution (Thompson and Troeh, 1973).

Cation Exchange

The interchange between a cation in solution and one on a colloid must be chemically balanced (Fig. 6.13):

Fig 6.13. Dynamics of cation exchange

The exchange of cation is a dynamic and a rapid process. In order to assess the cation exchange capacity (CEC), it is necessary to determine the sum of exchangeable cations that a material (soil) can adsorb at a specific pH. The CEC is expressed in cmol (+)/kg soil. It yields a measure of the capacity of a soil to hold nutrients against leaching and the amount of negative charge of a soil sample.

Cations such as Ca²⁺, Mg²⁺, K⁺ and Na⁺ are basic while Al³⁺ and H⁺ are acidic. Therefore, the base saturation in soil is a measure of the proportion of basic cations occupying the exchange sites of a soil.

Base saturation (%) =

x 100