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 Bjorn Straube Wrote:

  Just a quick question about CEC.  The discussion about
aluminum in substrates kicked loose a question about how CEC
works.  What is the primary quality about a given substance
that allows for a good CEC?  Is it particle size or chemical
composition?  I had considered using a 325 mesh silica as an
additive at one point in time, and as mentioned the recent
thread brought it back to my attention.

A bit of both actually.  The chemical charge is the key here and the
strength of the bond.  So having many small particles with a large surface
area is better than the same material with less surface area.  The second
part deals with chemical composition.  The quoted (do not know if
scientifically proven) CEC value for laterite is around 10 and vermiculite
is around 100 thus these two types of clay have a chemical composition that
allows the vermiculite ion bonding strength to be stronger vs. the
laterite.  This is _not_ to say that one is better than another
(experiments under way here).  I personally think that CEC has a lot to do
with substrate longevity and performance thus I am a hefty advocate for CEC
values.  There has been a rather length discussion in the past that finally
concluded with the response, "we need to do the actual experiment instead
of doing 'thought experiments.'"  Thus my goal is to perform a set of
experiments looking at this, and other, questions.

Matthew T. Mason                              "What is the path?
Doctoral Student                                    There is no path."
The Ohio State University                     -- Niels Bohr
Department of Molecular Genetics        (Physicist)
mason.163 at osu_edu