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Re: Aquatic Plants Digest V3 #658

On Sat, 21 Nov 1998, Greg Morin wrote:

> I realize there is a strong anti-commercial posting clause in the 
> list charter so let me just say from the outset I intend to abide by that 
> in the strictest sense.

The anticommercial clause in the charter is aimed (I think) at avoiding
commerical promotion.  Personally, I welcome non-promotional information
and discussion from commercial sources as long as they are clearly
identified as such.  Of course the line between information and promotion
is very hard to draw and some people may simply take exception to the
whole idea.

> (2) With regard to this statement: "In an oxic environment "reducing 
> agents" will not prevent the oxidation of ferrous iron." I think the answer 
> in (1) above with the little experiment shows that reducing agents not only 
> prevent the oxidation of ferrous iron, but actually will reduce ferric back 
> to ferrous, even with a relatively mild reducing agent such as ascorbic 
> acid.

I think that the original statement refers to oxidation in the aquarium,
and much of your discussion seems more relevant to oxidation in the
bottle. Certainly reducing agents and complex stability can stabilize
ferrous compounds in sufficiently concentrated solution, but I doubt that
can be extended to the dilute, oxidized and biologically active
environment of a planted aquarium.

> (3) With respect to this statement: "The charge on chelated iron is 
> irrelevant to iron uptake." It may be irrelevant in that both _can_ be 
> adsorbed, however it is relevant in that Fe+2 is much more easily taken up. 

My understanding is that the Fe+2 ion is readily taken up.  Ferrous
gluconate is a salt containing Fe+2.  In order for the plant to access the
Fe+2 ion in the ferrous gluconate either the plant must take in the entire
salt molecule or the salt must be broken down before the Fe+2 is adsorbed.  
The first mechanism isn't specific to the oxidation state of the iron; it
works with ferric chelates as well.  The second, unless it happens in the
rhizosphere, will probably just lead to oxidation and precipitation of the
iron, not to plant uptake of the FE+2 ion.

> (4) The biological relevance of EDTA-Iron vs Gluconate-Iron. From the above 
> Baensch reference on page 151 is where the idea must be coming from that 
> EDTA complexed iron is available to plants (he says exactly that).

EDTA-chelated iron (as well as DTPA chelates and a few others) have been
in horticultural use for decades with a record of apparent effectiveness.
Either the chemical industry has foisted a huge hoax on the agricultural,
horticultural and hobby industries and we've all been naively duped or
EDTA chelated iron is biologically useful.  I tend to think that EDTA-iron
is useful.

I have no reason at all to believe that ferrous glauconate isn't similarly

> (5) With respect to the statement that Flourish Iron simply "oxidizes and 
> precipitates as rust": although I can't rule out the possibility that some 
> of it is doing just that, the only precipitates we have observed are 
> ferrous compounds (probably carbonate), this is based on their green 
> coloring (all ferrous compounds have a green coloring). Even though 
> Flourish Iron is targeted as a supplement for the stems and leaves, 
> whatever material that does fall out before being adsorbed by the stems and 
> leaves can still be adsorbed by the root system of the plants.

It sounds to me like the precipitate you're describing might be found in
the bottle.  Ferrous carbonate is the natural mineral siderite, which is
usually brown, not green; maybe a fresh precipitate would be green.  I
doubt that you would see iron carbonate in aquarium use, where the iron
would be too dilute and the pH too low to precipitate the carbonate.

I've read that gluconates have become the favored form of complexed
metals in the European industrial community specifically because it does
break down quickly.  EDTA in particular breaks down so slowly that its
presence in industrial effluents might effect the availability of metals
in the sediments of receiving waters; gluconates don't last long enough
to do that.

How is ferrous gluconate "targeted" to the leaves and stems?

> (7)
> >I think there is this opinion on Seachems part that EDTA is
> >two strong ( actually they commented that "The amount of EDTA-iron
> >that the plants are able to use is so small as to not really be useable"  )
> >This is a pretty strong statement,
> All I can say is this statement is in line with the known chemical and 
> biological facts concerning EDTA as I've outlined above. If anyone has any 
> evidence otherwise, I would be more than happy to take a look at it.

EDTA-iron has been in common use for decades.  That looks to me like 
pretty strong evidence.

> >And I believe there are pH dependencies here clouding any
> >blanket statements about either product.
> Actually, pH plays no role (in the aquarium) with either EDTA or gluconate, 
> The pKa's of the 4 carboxylic acids of EDTA are 0.26, 0.96, 2.00 and 2.67 
> and for gluconic acid it is 3.86 (source Handbook of Biochemistry and 
> Molecular Biology, 3ed, Vol 1, p. 310 Fasman, Gerald D. Ed, CRC Press 1976) 
> Since plant tank pH is going to be well above pH 4 all the acids are fully 
> ionized.

My reference puts the pKa of EDTA at 2.008, 2.683, 6.098 and 10.181. These
values would indicate considerable importance for pH on EDTA chemistry.
The values I sited are listed by James Plambeck on the University of
Alberta web site at:


I haven't tried to discuss the differences with Dr. Plambeck.

Virtually every source I've read on iron chelates indicates that the
stability of the complex is pH dependent.  For EDTA, in particular the
complex starts to break down at pH over about 6 and it's generally not
useful at pH well over 7.  Other chelates are more stable at higher pH

> (8)
> >While it may be true that only Fe+2 is taken up by plants, the charge on
> >the chelated Fe is irrelevant to its uptake.
> I'm sorry, I don't follow here. If as you're saying plants only uptake Fe+2 
> and the chelated iron is Fe+3, then how is the plant going to uptake the 
> Fe+3?

I found the original statement a little confusing, too.  I interpreted it
to mean that iron in an ionic form is taken up only in its Fe+2 state.
Uncomplexed Fe+3 iron is virtually non-existent under aquarium conditions.
Iron in organic complexes may be either Fe+2 and Fe+3 and the oxidation
state of the iron in the complex isn't important to plant uptake.

There's an excellent British web site that details iron uptake mechanisms
in plants.  I liked the document so much that I downloaded it locally, and
I don't have the URL anymore.  I posted the URL on this list a few weeks
back and more recently Steve Pushak announced that he provided a link to
that document on his web site.

Very briefly, plants have several mechanisms available for using Fe+3 as
long as the iron is in an organic complex.  Information on the site is
also very relevant to the occasional arguments on this list over
availability of iron in laterite.

> > The idea of the chelating agent is to keep the iron in solution, not to
> >maintain a II oxidation state.
> But if you can do both, then wouldn't that be better?

Probably yes, but certainly that isn't clear-cut.

> I'm sorry for the length of this message, but I wanted to be complete and 
> thorough and to try and answer everyone's questions regarding gluconate 
> iron as best I could.

I can't accept your arguments that EDTA-chelated iron is an ineffective
source of iron.  It does appear to me that ferrous glauconate might be a
better choice at pH > 7; other chelated iron products might also be good

My tanks generally run at pH > 7, so maybe next time I see ferrous
gluconate for sale I'll give it a try.

Roger Miller
in pleasantly cool, dry and sunny Albuquerque.