Cation & Anion Exchange Flashcards
Cation exchange capacity (CEC): a property of soil for
Determining it’s fertility.
Cation exchange: clay and organic matter attract
Cations to restore electro neutrality.
CEC is the ability of a soil to
Absorb and exchange cations with the surrounding soil solution.
CEC isn’t the amount of negative charge, but the
Capacity of a clay minerals to exchange positive ions.
The greater the charge on the cation, the more
Strongly attracted it is to a negatively charged surface.
Soils with a high OM content and 2:1 clays have
A high CEC.
Cation selectivity of a soil depends on the nature of the cation exchanger, including it’s: (3)
Surface charge density.
Surface structure.
Type of clay / OM present.
Diffuse layer: heterogeneous, loosely held ions from a
Layer of electric charge between an exchange surface and the soil solution.
CEC is calculated by
Multiplying each exchangeable cation value by its charge and adding the values together.
(If already given the values, they only need to be added together)
Exchangeable cations: (2)
Held closer to surface in more highly concentrated soil solutions.
Clays with weak layers have a higher proportion of exchangeable cations.
Non-exchangeable cations:
Trapped in tightly held layers of illite so may not exchange.
Anion exchange capacity (AEC):
The capacity of a soil to retain anions.
As a soils mineralogy becomes dominated by oxide minerals, the
CEC decreases and the AEC increases.
Highly weather soils with a high proportion of
Hydrous oxide clays have a high AEC.
