phosphate diffusion

> From: krandall at world_std.com (Karen A Randall)
> Subject: is anaerobic good?
> Stephen Pushak wrote:
> > Crypts grow in a natural environment rich in dissolved iron and
> > there is a muddy bottom. Karen Randall, our Crypt expert, uses p
> > soil and micronized iron.
> Gee, I'm flattered, but I'm not sure I'd consider myself a Crypt
> "expert", just a Crypt afficionado.<g> 

I think you're being modest! ;-)

> Not that I'm strenuously arguing with much of what you are saying,
> but I don't have my Crypts growing in potting soil and micronized
> iron.  I pot my Echinodorus sp. and my R. macrandra this way
> because I have trouble growing them if I don't. 

I stand corrected. Anyway, here are two other plant types which
benefit from soil & high iron.

> Most Crypts do fine in my "normal" substrate, which is #1 gravel
> with laterite in the bottom 1/3-1/2.  I also have one shallow tank
> (my paludarium) set up with peat plates and laterite under the
> gravel.  This is the _only_ tank I have real success with C.
> affinis in, perhaps because the peat softens my moderately hard
> water.  There are other Crypts in this tank that do well too.

As a point of interest, have you tried them with soil? and are you
saying peat + laterite is about the same for Crypts? I've sure noticed
a difference with soil + vermiculite over plain gravel with only
Terrapur cones as the only substrate enhancement (old tank).

> > > Is it a good idea to add solid fertilizer (of the kind contain
> > > nitrates and phosphates) tablets or sticks in a substrate? Whe
> > > would be the ideal level for these? bottom, middle layer?
> >
> > I'm experimenting with this and other people use fertilizer or p
> > tablets with good success (notably Jim Kelly). I expect the
> > best place is in the middle layer, not in an anaerobic zone.
> >
> > > Is there
> > > a danger of those macro-nutrients getting into the water level
> > > will the plants consume all the nutrients if the only substrat
> > > circulation system is the roots of the plants?
> >
> > I would presume that all these macro-nutrients get used up by
> > the plants as they are drawn through the plants circulation
> > system. It should be safe to use macro-fertilizers if they are
> > confined to the substrate.
> I'm not arguing here, but I _am_ questioning.  I have been told by
> others that phosphate (and presumably other substances as well)
> will diffuse out of the substrate and into the water.  This would
> certainly jibe with my experiences the few times that I used DAI
> plant tabs in strongly lit tanks... I immediately got high
> phosphate readings and algae growth in the tanks, which only
> abated after a couple of months of stepped-up, larger than normal
> water changes.

Hmm... I recall a heated discussion about rates of nutrient
diffusion into substrates before. I'd been suggesting that the
use of vermiculite would make the substrate more permeable and
facilitate diffusion. The authoritative opinion was that nutrient
diffusion in substrates is very, very slow.  I suppose that there
are mechanisms for diffusion other than the random kinetic
jiggling of molecules and that diffusion rates are also
dependent upon molecular diameters, electro-valent polarities
and other technical jargon outside my domain of expertise. ;-)
Your observations suggest something was happening to make
the phosphate come out. I assume there was no artificial

My hypothesis is that the flow rates into the substrate and
into the plant roots will generally exceed phosphate ion diffusion
against the flow. If anyone knows differently, please contradict
me. ;-)

On the subject of the high relative ratio of Fe in the Dupla
products, I have a couple of observations that might explain
this. Lots of Fe in solution and in the substrate will help
precipitate phosphorus and prevent the formation of H2S as
noted by Dr dave in his recent comments. Perhaps those little
under gravel fertilizer tablets should get a wrapping of
super-fine steel wool or a jacket of some iron compound???
Any thoughts chemists?

And a question for our aquatic plant academics, how do we
determine the optimal nutrient ratios for plants? I suspect:

1) chemical analysis of plant composition
2) observations of natural habitat
3) by controlled experimentation

Is it also true that the relative concentrations of some
nutrients is far less important than others; that is, plants
can successfully tolerate a scarcity of certain elements
better than others? I understand that one method to determine
the importance of nutrients is to grow plants in an
environment with reduced amounts of a specific element until
differences can be observed.

Steve        Vancouver BC, CANADA