CATION EXCHANGE CAPACITY (CEC)

Defined as - the number of exchangeable cations in which soil solids can absorb.

Ie, you have a soil sold- clay, which is currently linked to the cation potassium, (K). One adds different cation, i.e. Salt (Na)

These cations will exchange and eventually change the concentration of the soil (i think?)

My question is how do we know when the exchange of cations will stop? (MAX CEC) Because cec is defined by the limit of exchanges, is that limit due to the ending of addition of cations? or is the limit due to the fact that clay particles are unable to exchange any more cations? and if the clay solids fill all the exchanges they can handle, wouldn't the reaction occur again in the opposite manner to balance the concentration of excess K ions now in the soil solution?

Any help would be appreciated !!!! thanks TL

Cation exchange capacity (CEC) is total number of cations that a soil can adsorb on its solid (charged) surfaces. Pls note that this is not the same as absorption, which implies that cations entered into something. Anyhow, the total number of cations adsorbed corresponds to the total number of negative charges present on soil solids.

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)