CATION EXCHANGE CAPACITY (CEC)

Thomas, I think you may be confusing two concepts: 1) exchange reactions and 2) CEC

As Maja points out CEC is the total number of cations that a soil can adsorb on its charged surfaces (i.e. the total number of negative charges present on soil solids).

Exchange reactions (between soil solids and soil solution) occur due to the mass ion effect.

For example K-clay + Na --> Na-clay + K

Note that ion exchange reactions are reversible, rapid and stoichiometric (charge for charge)