Ion exchange
• The substitution of one ion for another
on the surface or in the interstitial
spaces of a crystal
– ?? exchange (e.g., Ca2+ for K+)
– ?? exchange (e.g., H2PO4- for NO3-)
Ion exchange
• The substitution of one ion for another
on the surface or in the interstitial
spaces of a crystal
– Cation exchange (e.g., Ca2+ for K+)
– Anion exchange (e.g., H2PO4- for
NO3-)
What’s so great about ion exchange?
Name 2 reasons!!
“Next to photosynthesis and respiration, probably no
process in nature is as vital to plant and animal life
as the exchange of ions between soil particles and growing
plant roots.”
Nyle C. Brady
What’s so great about ion exchange?
• Retards the release of pollutants to groundwater
• Affects permeability, with implications for landfills,
ponds, etc.
• Affects nutrient availability to plants (constant
supply, protection vs. leaching)
“Next to photosynthesis and respiration, probably no
process in nature is as vital to plant and animal life
as the exchange of ions between soil particles and growing
plant roots.”
Nyle C. Brady
Definitions
• cation: An ion that carries a positive charge
• Name this process: A process - cations in
solution exchanged with cations on exchange
sites of minerals and OM
• Name this term: The total amount of
exchangeable cations that a particular
material or soil can adsorb at a given pH
Definitions
• cation: An ion that carries a positive charge
• cation exchange: A process - cations in
solution exchanged with cations on exchange
sites of minerals and OM
• cation exchange capacity (CEC): The total
amount of exchangeable cations that a
particular material or soil can adsorb at a
given pH
Exchange affinity
Held more strongly
Held more weakly
H+  Al3+ > Ca2+ > Mg2+ > NH4+ = K+ > Na+
This is referred to as the “Lyotropic series”
Strength of adsorption proportional to
valence ÷ hydrated radius
CEC depends upon
• Amount of clay and organic matter
• Type of clay minerals present
Examples of cation exchange
Ca2+
K+
+ Ca2+ 
+
2K+
K+
K+
+ 3K+
Al3+

K+
+ Al3+
K+
The interchange between a cation in solution and
one on a colloid must be CHARGE balanced.
The reactions are reversible, unless…
Adsorbed cations by soil order
Soil order “Acid cations”
“Base cations”
(H+, Al3+)
(everything else, e.g.,
Ca2+, NH4+, K+, etc.)
Ultisol
65
35
Alfisol
Mollisol
45
55
30
70
Adsorbed cations: area
Humid region soil
H+
H+
K+
H+
Al3+
Arid region soil
NH4+
Mg2+
K+
Ca2+
H+
Mg2+
Low pH (acidic)
High pH (basic)
A real-life application:
How lime raises pH -CaCO3 + 2H+  H2O + CO2 + Ca2+
OM has highest CEC
2:1 clays
1:1 clays
Non-clayey soils
CEC and weathering intensity
Alfisols,
Vertisols,
Argiudolls*
Ultisols
*remember nomenclature structure = “argi-ud-oll”
Oxisols
Soil Order CEC (cmolc/kg)
Soil order
CEC
Key factor
Oxisols
Low
High Al/Fe
oxides
Ultisols
3.5
1:1 clays
Alfisols
9.0
Low pH
Mollisols
18.7
Vertisols
35.6
Histosols
128.0
low
2:1 clays
O.M.
high