8: Soil Colloids (8/11) Flashcards
Cation exchange capacity (CEC)
Quantifies the capacity of colloids to adsorb cations
Expressed as the number of centimoles of (positive) charge that is adsorbed in the colloid per unit mass
What is cmolc?
The units of CEC which are expressed as cmolc/kg
1 cmol c = 6.02 x 10^21 charges
Soil cation capacity
Normal range
What does it depend on?
Normal range in soils is 0-180 cmolc/kg
Depends on the total amount of negative sites or negative charges:
- pH of soil matters, it changes CEC
- The relative abundance of different
colloids
- The CEC of those colloids
4 principles governing CEC reactions
Reversibility (cations are not permanently bound)
Concentration of charge (ratio law): chemical equilibrium with soil solution
Cation selectivity
Principle of equivalence