What is cation exchange capacity?

Why does the % of max. cation exchange capacity (CEC) that can effectively serve as CEC lower with lower pH?

• IN SOIL SCIENCE The only thing I can think of is the principles of zero point charge. something like.... as charge increases with changing pH cations are less able to adhere to -or in- a soil colloid. H+ would repulse cations. This in turn would mean a lowed CEC, which would lead to a less fertile, more acidic soil. please correct me, or confirm this. THNX!

• Answer:

  Since, %CEC = Ca + Mg + Na + K / Total CEC * 100 as you lower pH you have more H+ and Al+^3, which means you have less or have replaced (at least percentage wise) Ca+^2 and the other basic cations in the soil. this is why at pH > 7 we generally assume 100% CEC as far as CEC itself changing with pH;it is basically because of the acidic cations replacing negative charge (since you are not measuring the acidic ones the CEC would measure lower) which is the reason why when we measure CEC we do it at a buffered pH i.e. CEC-7. we do have methods (i use titration) to measure the hydrogen and acidity when trying to figure out how much lime (or other alkaline) is needed to neutralize pH. hope this helped