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Re: iron and TMG etc

Scott H. wrote:

> I thought that was the whole point of chelation. . .the chelator holds
> the iron for a while, until the chelator breaks down, which happens
> rather easily when exposed to, for example, UV in sunlight or bulb
> light.

Most of the lights we use don't produce any UV.  I don't know about
gluconate, but from my reading neither EDTA or DTPA is particularly
sensitive to light.  There is at least one commerical chelator (EDDHA)
that is sensitive to UV, but I don't think I've ever heard of EDDHA
being used for aquariums.

From my reading, the purpose of the chelator is to provide a slow steady
dose by making some free iron available in equilibrium with the
dissolved complex.  That seems to be the basis for "official"
recommendations on chelated iron doses.  To the extent that the chelator
does break down over time it will provide additional iron.  But that
breakdown rate is going to be even more unpredictable then the
equilibrium concentration because it depends on so many variable
factors.  The breakdown just magnifies the problem that levels found
sufficient in one tank may not be sufficient or may be wasteful in
another tank.
> So your point is that you need to add lots of chelated iron
> for the plants to have a little iron?

That's how it works.  Moreover, the amount that you measure with a test
kit is fairly meaningless to the plants.

> If I am
> reading Tom's and Roger S.'s posts right, the following is generally
> agreed:
> (1) plants need very very little (available?) iron and if the substrate
> has iron (flourite, laterite, iron tablets), then the plants (or only
> the substrate-rooted plants) will get enough.

I'm not sure that's entirely true.  I think most people using laterite
and Flourite still dose with iron.  I don't know if they need to or not,
but they seem to do it anyway.  I haven't read The Optimum Aquarium, but
my understanding is that Dupla doesn't regard laterite as a significant
source of iron; the laterite is there to fulfill other functions.  Could
a more Dupla-aware reader resolve that question?  James?  George?  Dan? 
Anyone else?

Iron tablets provide a slug of available iron, but I don't know how long
they last.  I only know that I don't have to use them very often. 
Otherwise, I think that certain chemical conditions are necessary to
make substrate iron available to plants, and those conditions aren't
automatically attained just by having iron in the substrate.

In order to make natural forms of substrate iron available the substrate
has to provide at least microenvironments where the redox potential
and/or the pH is low.  That usually means restricted circulation and/or
organic material in the substrate.

> (2) you must add lots of (chelated) iron to the water column to make a
> little of it available to plants.
> (3) if you add lots of iron, then the plants will Look (grow?) even
> better than if you didn't add the iron.
>      (3a)-- This might be so despite (1), perhaps because what plants
> need and what they can use is very different.  In other words plants
> can benefit form more than they need OR
>      (3b)-- This might be so because of (2), in other words, adding up
> to 0.5, 0.7, or even 1.0 ppm (total iron?) levels gets the iron that's
> most readily available to the plants up to the required level OR BOTH
>      (3c)-- This is only so in high growth setups, added CO2, macros,
> and other traces.

I checked my usual reference on deficiency and toxicity symptoms.  Oddly
the primary symptom of iron toxicity is dark green growth.  It can also
force deficiencies in manganese and zinc.  Perhaps those continuously
"improving" conditions are actually the onset of iron toxicity.

On the other hand, that same reference (which is a hydroponics article)
says that the nutrient solution needs to have 2-3 mg/l chelated iron to
avoid iron deficiency.  They may have been refering specifically to
EDTA-chelated iron, because not all iron chelate complexes work the
same.  Also, they're talking about a pretty saline solution with a lot
of possible interferences, not about fresh water.

Maybe people who depend on chelated iron in the water column and
maintain lower concentrations actually have iron-deficient plants.  They
just don't recognize it because the plants look normal to them.

For my part I have put 250 mg of gluconate-complexed ferrous iron into
the substrate at the base of a small plant -- presumably overloading any
possible iron demand -- and the plant didn't get particulary greener or
grow much faster than it was to start with.  I'm not a big believer in
the benefits of excess.

> (4) Walstad's observations, seem to me to be consistent with (1) - (3).
> And lastly, we have the Barr Conjecture, that
> (5) Adding lots (0.5, 0.7, 1.0 ppm) of (total?) iron doesn't (or
> needn't) encourage long term increased presence of algae.

I think the list of things that can be agreed to is probably a little

1) Plants need low levels of biologically available iron.
2) There are a number of methods to provide the necessary iron.
3) The commonly advised methods are safe for plants and fish in normal

So, your choice of approaches is a matter of personal preference.  

Roger Miller