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cation exchange capacity of vermiculite (technical)
Roger commented that positively charged ions can be easily exchanged at
CEC sites. This is true but a simplification. I think what Matthew is
pointing out is that different materials have different attachment
bonding strength and affinity for various cations. Calcium is bound much
more strongly for instance than sodium. Here is a partial list of
cations ordered by adsorption strength: Al > H > Ca > Mg > K > Na.
(Refer to the substrate article on my web page).
Cations can be exchanged at the CEC sites by random kinetic events (i.e.
the wiggling actions of molecules due to temperature). As the
temperature goes up, the rate of cation exchanges can increase. Which
cation ends up on the site is then a function of probability related to
the concentration of ions present and the binding affinity.
Some materials reduce their binding strength (bonding energy) with pH
while others have pH independent CEC. Vermiculite's CEC is pH
independent so it is difficult for plants to exchange nutrient cations
by secreting organic acids. Humus and other organic materials have a pH
dependent CEC so are more suitable for binding nutrients for plants.
Summary: vermiculite may not be the wonder substrate material as first
thought. The bigger problems are its tendency to make the soil too
fluffy and to create turbidity problems when uprooting. Also the little
vermiculite flakes tend to settle on plant leaves and look gross.
As George and I have pointed out, high CEC may not be as important as we
think and I think that's what Matthew is saying about vermiculite.
Steve P
Aquatic Gardeners Association