Re: nutrient diffusion into substrates

Stephen.Pushak at hcsd_hac.com wrote:
> George and I have been conducting an interesting discussion on the
> merits of clay, laterite and vermiculite in substrates. I was saying 
> how vermiculite is so wonderful because it increases the permeability 
> of the substrate so well whereas fine clays would restrict it. I made
> the claim that vermiculite substrates might not even need any substrate
> flow enhancers such as slow RUGF or heating coils since diffusion of
> the nutrient ions via concentration gradients may be sufficient. George
> countered saying Dupla laterite is not a fine clay and challenged
> my scientific foundation for this "ionic diffusion" claim.

[snip, Jim Kelly's three mechanisms for nutrient uptake]

> To put that in context, Jim is talking about really local diffusion
> gradients probably less than a millimeter. Does anybody with a
> chemistry background know if ions rely upon actual circulation of
> water through a substrate or if diffusion through the substrate
> is predominant?? I believe or suspect that ion concentrations in a
> solution reach equilibrium very quickly, not dependent upon mixing.

As an aside, I would personally *love* to hear yours and George's
discussions.  David Webb and I have been carrying on similar
discussions for the merits of laterite/vermiculite.

I have seen George's tanks first-hand, and it's hard to argue against
what obviously works so well (TOA approach, heat coils/laterite).
I am trying to do a "cold-water" setup, however, and am trying to
find something other than heat coils for substrate circulation.

I agree with your statement "how vermiculite is so wonderful because 
it increases the permeability of the substrate..."  George may be
right in stating that laterite is "not a fine clay", but I think
he must admit that laterite compacts over time (thus restricting
circulation) to a point at least a magnitude greater than vermiculite
does.  Vermiculite that we purchase is composed of numerous static
structures that permit good circulation (diffusion potential), 
whereas laterite does not strongly exhibit any such "macro-structure".

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:  a 12" 
water head over a ten foot deep vermiculite substrate would suggest 
to me that passive diffusion of Fe ion supplements would not 
evenly penetrate to the bottom of the substrate.  (Unless we have 
infinite time and no loss at any level).

Re-charging the substrate with macro- and micro-nutrients seems 
to be a significant area of conjecture, but an intuitively 
logical one (a point that under-gravel coils is surmised to 
promote).  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.  

Further, there has been some discussion here of some plants' 
preferences at ion uptake from the water or through exchange on
some ion bonding site in the substrate.  It stands to reason that 
ion diffusion in the aquarium body will be far more uniform and 
speedy (because it is an open body, and there will ALWAYS be 
eddies or currents, even if we don't have fish) than diffusion 
into the substrate (far more resistance due to substrate 
particles, and currents on a *much* smaller scale).  The
*real* question here is whether or not passive substrate diffusion
is sufficiently speedy enough that algae or other 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 magdalenensis.  Even if we add tablets directly to
the substrate, my guess would be that diffusion would be uniform
into the substrate in all directions, but more rapid into the
aquarium body than into the substrate.  

I would suppose a 2" partial vermiculite substrate in my 10 gallon 
with only passive circulation might be adequate, but I would be 
afraid to try a 3-4" partial vermiculite substrate in my 180
gallon to passive circulation only.  The costs for being wrong
sound too painful...  (Obviously, we need someone with first-hand
experience here.)  However, I *really* like the direction you
are going that we lean more on passive systems and lower electric
bills.  :^>

I was fascinated by the discussion a couple months ago of directly 
(and uniformly) charging (re-charging) the substrate with macro- 
and micro-nutrients.  Recall that this is especially important 
in older tanks (where vegetative growth drops off), and in newer 
tanks with substrates short on nutrients (vermiculite, but not 

David Webb and I have been discussing a substrate circulation
system exactly for charging (re-charging) the substrate (the plan
is to use vermiculite and its passive circulation potential, and
no laterite).  Essentially, we are talking about a RUGF with 
reverse-diffusion from the bottom of the tank up into the substrate 
with high (we think optimal) nutrient use efficiency.

As an aside--how come nobody ever mentions "pearl-ite"?  A
synthesized compound used for terrestrial plant substrates.  Very
light, high CEC, same aeration as vermiculite.  Larger particle
size than vermiculite (5 mm or so), so won't stir up in water
as much.  It is bright white, though (until mulm gets it!  :-)
Is that why nobody uses it here?

Anyway--carry on!  I'd love to hear more about yours and George's

cbay at gr_hp.com		or	charley at agrostis_nrel.colostate.edu