Exchangeable cations and anions

Most secondary clay minerals contain a net charge associated with isomorphic substitution within the mineral or pH dependent charge occurring on hydroxylated surfaces. Organic matter contains pH dependent negative charge associated with dissociation of carboxylic functional groups and phenol groups at high pH (pH > 8). The amount of cation or anion exchange capacity is dependent primarily on the clay mineral type and concentration, organic matter amount and degree of decomposition, and the soil pH. Cation exchange capacity and anion exchange capacity show the following distribution as a function of secondary mineral composition:

CEC: 2:1 > 1:1 > oxides/hydroxides

AEC: oxides/hydroxides > 1:1 > 2:1

Cation exchange capacities for various secondary clay minerals and soil organic matter are shown in this table. On a weight basis, soil organic matter has the greatest cation exchange capacity. For variable charge surfaces, the amount of negative charge increases and the amount of positive charge decreases as soil pH is increased.

The negative and positive charge associated with clay minerals and organic matter are balanced by electrostatic attraction of cations and anions, respectively. The balancing ions are termed exchangeable cations or anions. Exchangeable cations and anions form outer-sphere complexes with the charged surfaces in which waters of hydration exist between the charged ion and the oppositely charged mineral surface. These bonds are relatively weak resulting in rapid replacement of one ion with that of another. The composition of the exchangeable ions is a function of their concentration in the soil solution and the affinity of an ion for the exchange site. The ion affinity is a function of the charge and hydrated size of the ion with small, highly charged ions being preferred over large, low charged ions.

Affinity series:

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

Anions: PO₄^3- > SO₄^2- > Cl^- > NO₃^-

Exchangeable ions are easily displaced into the soil solution making these ions readily available for plant and microorganism utilization. The exchangeable ion pool is the dominant storage pool for Ca²⁺, Mg²⁺, and K⁺. Ammonium is generally converted rapidly to NO₃^- leading to low concentrations of NH₄⁺ in solution and thus on the exchange capacity. Phosphate, and to a lesser degree sulphate, is generally retained by stronger sorption reactions (inter-sphere complexes) and as such are not truly exchangeable anions. The low amount of anion exchange capacity in soils of the temperate region coupled with the very weak affinity of chloride and nitrate for exchange sites leads to very small quantities of exchangeable chloride or nitrate.