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CEC exchange sites
In APD 327, Roger Miller discusses circulation, aerobic and anaerobic (anoxic)
conditions in the substrate. THis brings to mind a couple related questions I
keep forgetting to ask.
1) We add clay (Amano's product line uses peat) to the substrate because it
has a high CEC, or cation excheange capacity, in molecular locations we like
to call "exchange sites". The nutrients attach themselves to these sites, and
then the plant detaches them by exchanging hydrogen into the site. Does this
exchange site become useless once the hydrogen is on it, or do natural forces
like electronegativity cause the hydrogen to them be replaced by the nutrients
we are attempting to provide? And if the latter, how does the plant overcome
these forces to get the hydrogen back on and the nutrient back off? Or is
that second question too complex to answer in a single email?
2) Roger discusses the need for some kind of circulation, but not too much
circulation to get all this nutrient-into-the-substrate stuff done. I
understand how UGFs tend to provide too much circulation, but what if it was
used a little differently? In a non-planted tank or a primarily-fish tank we
try to get 3-5 <tank volumes> per hour flow rate through our filters, whatever
the type, in order to allow the bacteria to do their job quickly enough to be
effective. This implies, to me at least, that molecular adsorption and
absorption can happen at these faster flow rates no problem. So, deductively,
the low-flow theory behind striated heater cables is to allow the proper
balance between aerobic and anaerobic sites. What if the UGF were only one of
2 filters, and its flow rate were significantly reduced? If I add clay,
whether laterite, kitty litter, SeaChem Flourite, or just plain red Georgia
clay, would this not restrict the water's ability to get through? and if I
used an airdriven Reverse-flow setup, which would allow me to extrapolate flow
rates using the dye-marker/time-through-the-tube estimation method, could I
not establish both aerobic and anaeobic sites? Especially in light of Roger's
eloquent discussion of "micro-environments" in the pores of the substrate
material? Yeah, yeah, why not just go with the tried and proven heater
cables? Well, gee, what would be the fun in that? And a filter is cheaper
and simpler and less risky than electricity in my tank. (Though strips of
heater cable under the tank might not be so bad- at least I would be less
likely to get zapped while digging around in the tank.) Also, PMDD, or any
other liquid fertilizer, could be introduced straight into the down-tube of
the RUGF. But what is the correct flow rate per square inch of <floor>?
Anyone know what flow rate the Dupla system is striving for with the cables?
Bob DIxon
In Boise, and obviously with too much spare time on my brain.