Re: Oxygen diffusion vs. nutrient diffusion

> From: Charley Bay <charleyb at hpgrla_gr.hp.com>
> Seriously, though, I realize that vermiculite has almost no nutrients
> to offer initially (the stuff is sterile).  Laterite has high Fe+,
> and a host of other elements in macro- and micro- form.  This is
> the greatest appeal that laterite has for me:  It's hard to go wrong
> when you know you have such good concentrations in the substrate.

This point should be clarified; the Fe in laterite is insoluble and
bound by oxygen. IMHO it does not represent a sufficient source of Fe.
That is why we use Fe-EDTA (chelated iron) supplements and test Fe
concentration regularly. The big benefit from what I read is that
the iron oxides tend to bind up phosphates and tends to fix the
phosphorous atoms where they don't foster algae growth.
Do you concur George?

> > Could someone explain exactly why water will "passively circulate" in
> > the verimculite?  This seems to violate some Law of Nature.  What
> > provides the energy for this circulation?  A reply on sci.aquarium
> > stated that "diffusion" will be ineffective at distances greater than
> > a few millimeters. 

So far there seems to be quite a disagreement about the slowness of
diffusion. The soil people seem to be saying that we have diffusion
rates of a few centimeters per hour in undisturbed water and in
strong concentration gradients however this is also dependent upon
the ionic charges of the particles. Just inside the substrate, in 
the absence of induced flows, diffusion is the only mechanism of
nutrient migration. Now, if there are already some nutrients there,
(such as from pond fertilizer tablets or other solid tabs) we
don't require that much nutrient migration. After all, there are
lots of plants which absorb directly from the water. IMHO CEC
substrates enhance a substrate's performance provided that they
do not overly restrict flow and diffusion. They are not a panacea.

> I think there are at least two issues. (1) Will the substrate go
> anaerobic without circulation intervention (passive diffusion is 
> sufficient, no heat coils), and (2) will nutrients enter the 
> substrate passively (through diffusion) in sufficient quantities 
> to replenish those nutrients adsorbed?

Going anaerobic or having a low oxygen environment is not necessarily
all bad. It is very bad in the situation where there is organic
decomposition in the substrate which places high demands upon
oxygen AND produces toxic gaseous by products. It is these H2S
and H2SO4 gases which are most destructive.

Now in a rather dense lower substrate, I think a low oxygen environment
can be tolerated provided there isn't a lot of undecayed organics like
peat. I would suggest that peat works best when there is sufficient
flow as when the plant roots are drawing lots of oxygenated water
down into the substrate and the substrate itself is quite loose and
permeable. Roots also inject oxygen in gaseous form into the soil.

Duplarit-G, vermiculite(predominate) and gravel substrates all possess
something in common; lots of spaces between the particles. While 
vermiculite particles are smaller than the other two granules, they
are flat and have very high surface to volume ratios. It will not
compact to the degree that a substrate containing mud, fine clay,
dirt or high amounts of aquarium sludge will. This makes it easy
for diffusion to occur (to the slow degree that it does), allows 
mass flow (induced by roots or artificially) and allows the roots
to penetrate!! This penetration by the roots is the key factor here.
Please note that hence forth I will always make a distinction between
Duplarit-G which is granules and lateritic clays or fine powder clays
of all types.

We are trying to explain why these various substrates obviously
work in many applications. In non-flow enhanced substrates there
is argument that diffusion alone cannot provide enough nutrients
and oxygen. We also need to consider the flow induced by root uptake
and the oxygen injected by roots. For heavy root feeders where
nutrient infusion is not sufficient, it is also a wise practice to
put solid nutrient tablets in the substrate.

I don't think it would ever hurt to have heating cables too but
since George at one point in his career didn't use 'em I'm interested
to hear if he and Karla were able to quantify any differences upon
using them?

> > Well, I guess this discussion has deteriorated to a religious war. Bummer.
> > 
> > George "Patron Saint of Laterite"

Oh, I don't know that! After all, I've been learning a lot from all
this discussion. I kid George that he is a Laterist (a proponent of
the philosophy of Laterism) and a follower of the prophets
Horst and Koehler.

Steve (the Vermiculist ;-) follower of the prophet Jim Kelly.