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Re: nutrient diffusion into vermiculite

Hey, a small controversy! Great! Hope we can "stir up" some interest ;-)

> From: Charley Bay
> 
> Jim's reference to local diffusion gradients ("less than a 
> millimeter"?) makes me a little wary about jumping to the assumption
> that simple (passive) diffusion into the substrate will be
> sufficient.  Certainly, substrate depth must be a factor

Jim is refering to nutrient uptake into the roots. As Steven Petsch
pointed out, these effects are not instantaneous and occur over a span
of hours. Certainly diffusion is slower in a water/solid mixture
but occur in nature without benefit of substrate heating or substrate
flow devices. Steven quotes diffusion rates in centimeters/minute.
Even at two orders of magnitude difference in a substrate; 
centimeters/day is quite acceptable!! Don't forget, once a nutrient
ion is in there, its going to stay there for a long time until it
eventually (hopefully) gets absorbed by a root.

In nature things have reached a long term equilibrium; organic 
materials decay; roots penetrate. We have to avoid creating
unacceptable conditions such as putting too much undecayed organic
material in the substrate. That is why peat and peat moss mixtures
like potting soil are to be avoided entirely.

The question is "Does a high vermiculite; moderate depth substrate
require nutrient flow in addition to natural diffusion rates"?
My feeling is no since we already have a medium with higher permeabilty
than what is typically available in nature. So long as there are not
excess undecayed organics, there should be no danger of anaerobic
decay. Nutrients will diffuse into the substrate and be absorbed
by roots at some rate probably superior to those conditions found in
the natural habitat of the plant. For spectacular growth, we need to
improve on natural conditions after all.

> I'm a little dubious that simple passive diffusion 
> into the substrate will really be all that quick or uniform, 
> because the cation will bind to the first negative site it hits 
> (and fresh vermiculite is loaded with un-bonded sites).  I would 
> guess that it may be some time before supplemental nutrient doses 
> into the aquarium body are efficiently diffused into the substrate, 
> and that the deeper substrate will consistently have fewer 
> concentrations of these nutrients.  

It doesn't need to be particularly quick, after all plants themselves
can't consume it at high rates either. In addition, the rootlets
don't need to go foraging to great depths in quest of nutrients
when they are to be found easily in the first few centimeters. Crypts
don't have particularly deep roots; they spread out laterally.
The ones which go deeper such as Valisneria may have other reasons
for this such as providing anchorage in strong currents and to prevent
uprooting via underwater erosion. As to fresh vermiculite being
loaded with "un-bonded" sites; this should not be a problem after
the first few days after water has a chance to penetrate the new
substrate. The largest ion concentration is NH4+ and then probably
K+ and Ca+ and Na+. The percentage of each ion type at CE sites is 
in dynamic equilibrium with the concentrations of those ions in the 
aqueous solution. CE substrates simply provide a higher local 
concentration of those nutrients.

> As an aside--how come nobody ever mentions "pearl-ite"?

Don't have any information on it. I don't know if it has the layer
structure that vermiculite has so crushing it up to remove trapped
air (buoyancy) probably gets dust instead of tiny flakes. Anybody
know anything else about it?

> From: Neil Frank <nfrank at nando_net>
> 
> I agree that Dupla laterite is not a fine clay (it settles out quickly), but 
> other "laterites," like the old Aquarium Products stuff, are fine clay 
> and can remain in permanent suspension.

It would appear that Dupla decided a granular material was more
desirable, ostensibly to promote permeability (substrate diffusion
and bulk flow) at the expense of higher CEC which could be had from
powder.

> Anyway, in Kelly's article, Jim also mentions am interesting property of 
> laterite: binding of phosphates. 
> With water circulation (by slow flow UG heating or other mechanisms), this 
> may be the primary long-term function for this nutrient poor material - 
> ie. keep certain nutrient concentrations trapped in the substrate and low in 
> the water column. 

Yes and he also pointed out that you can achieve this by using ordinary
iron oxides! Jim, you're becoming something of a mythical legendary
character here! ;-)

> From: Steven T Petsch <stpetsch at minerva_cis.yale.edu>
> [snip] What most likely controls nutrient uptake by 
> plants is most likely a combination of reaction (how fast the plant can 
> actually 'pick up' a nutrient ion) and diffusion.
> [snip]  And on the scale of millimeters or 
> less, flow is not going to speed up diffusion much.  

Are you saying that the natural rate of nutrient diffusion is
much higher in a substrate than flows which might be induced by
heating coils? or that the rate of nutrient infusion by roots won't
vary much regardless of flow?

One point I made to George was that given a specific density change of
.005% for 40 degrees F difference in water temperature, the induced
pressure difference between two adjacent "columns" of water 1" in
height (visualize heating coils 1" separation) in an ideal situation
would only be .005" of hydrostatic pressure!! I'm still bugging George
to try to measure induced flow in gravel, gravel/vermiculite with
heating coils, UHF plate and ink dye drops. That isn't going to provide
a whole lot of bulk circulation, BUT it would stimulate diffusion by
thermal agitation of the H2O molecules and promote metabolic activity
in the substrate!!!!

> Fine grained clays probably would serve to restrict 
> fluid flow and diffusion, simply because there is some much less pore 
> space for the water and ions to flow through, that diffusion of oxygen 
> into the substrate could not keep up with consumption of oxygen during 
> bacterial decomposition of waste, dissolved organics, and even other, 
> dead bacteria.  Even clays the size of vermiculite would most likely not 
> be 'open' enough to keep oxygen concentrations up in the substrate.  
> However, the mixing and mashing ala the Jim Kelly article on 'the 
> Krib' breaks up the alignment of the clay grains, creating more 
> porosity and space for fluid flow - so roots can access an oxygenated 
> environment richer in nutrients.  

I think you might be implying here is that the diffusion of limited amounts
of oxygen into the substrate are more important to the well being of the
plants than the rate at which they can absorb nutrients. We certainly 
don't want large amounts of oxygen as that tends to promote oxidation
reactions which consume Fe+ & Mg+. The Kelly recipe calls for the bottom
half of the substrate to be primarily just mashed vermiculite with just
enough loam (containing humus, sand & clay) to give it a gray color.
The mashed vermiculite looks like very thin flakes about 1-2 mm in dia.
As you say, this produces a much "fluffier" substrate!

Some of the plant roots that are visible at the sides of my 75 gal tank 
(Lilaeopsis) go down about an inch until they hit the vermiculite/humus
layer and then kind of branch out there. I've used earthworm poop in
a higher ratio than Jim's so if anybody starts to have anaerobic decay,
it should be me. I used about 70% gravel with 15% earthworm castings
and 15% vermiculite. I did set up a slow-flow RUGF system with a
tube filled with sand, steel wool, peat & humus. With a 1" pressure
head in the tube, I had only about 4mm/hr through the tube which is
about .005 mm/hr through the substrate. Pretty negligible.

What are the symptoms of anaerobic decay? Gas bubble formation? Do I
have to pull up the plants to look at their roots? So far, everything
appears ok and healthy. No serious algae problems on plants. Brush
algae grows well on the RUGF down tube and the trickle filter uplift
tube toward the top where there is high light intensity from the 
250w MH lamp. Ludwigia arcuata, red & green Cabomba, H poly, H diff,
Bacopa, Crypts, Aponogetons, Echinodorus a. & cordifolius, Water sprite,
Salvinia, Valisnaria are all growing very well. The A. madagascariensis
is producing leaves but at much reduced growth rates from initial
planting but steady growth is still encouraging. Of rooted plants
Valisneria is propagating and growing fastest. Crypts show steady
growth with new leaves (which may be considered successful for me
considering Crypts don't grow too fast). H poly, Bacopa & Ludwigia a
show very high growth rates, but these are not what I think of as
heavy root feeders although these plants do have good roots, white,
no signs of anaerobic decay. Cabomba is not a root plant and I think
the Milfoil is not either. They love the strong light!

So far I have good results with vermiculite, humus & gravel for about
two months.

Anybody who has not had good results using vermiculite? Sometimes failure
is far more interesting to learn from than success so if you can always
look on the bright side if sharing your results helps someone else! :-)
Unfortunately, success is hard to quantify when almost anything seems
to work to one degree or another.

Steve