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Re: nutrient diffusion into vermiculite

> From: Stephen.Pushak at hcsd_hac.com
> Certainly diffusion is slower in a water/solid mixture
> but occur in nature without benefit of substrate heating or substrate
> flow devices.

I've been wondering about this.  Because the bottom of a lake/pond
essentially has infinite insulation (depth to the center of the
earth), doesn't this function like substrate heating?  On cold days,
the ground is almost always warmer than the atmosphere (and
eventually the water).  It seems that this could be very good 
substrate heating given geologic time.

I don't really know if the quest for "optimal" plant growth (which I 
bet does NOT occur in nature all that often) requires warm roots or 
not, aside from the obvious need for some type of circulation.

> 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.

Ok, I need to go back to my text book.  If this is true, then you've
answered my primary worry:  "The *real* question here is whether or
not passive substrate diffusion is sufficiently speedy enough that
algae or opportunistic plants in the main aquarium body cannot take
up (the majority of) the nutrients in our daily drops BEFORE THEY
HAVE A CHANCE to diffuse into the substrate and benefit the roots
of our Echinodorus magalenensis."

> 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.

This makes sense to me.  

Ok, so back to the issue:  If i've read it right, the argument is
that good plant growth can be achieved with a porous substrate that
inherently (passively) allows for adequate circulation and diffusion
of nutrients to plant roots.  Questions:

* Do you see a need for directly taking nutrients directly to the
  substrate to avoid high nutrient concentrations in the aquarium 
  water?  (High nutrients in water could force an algae bloom; the 
  substrate is low volume compared to the rest of the tank, and the
  principle of diffusion suggests nutrient concentrations in the
  substrate will merely achieve the same proportion as those
  concentrations in the water.  In short, can we get the nutrients
  in *sufficient quantities* where we need it, or *only* where we 
  need it?)

* Do you see a need for any type of substrate circulation?  (To
  ensure even nutrient distribution, or minimize the eventual
  restriction of substrate circulation due to mulm or debris
  accumulation?

Of course, we're looking for a tank that will last 5+ years.  :^)
BTW--my 180 needs a substrate.  :^>

--charley