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Re: Shelf life of Flourish/Substrate iron

> From: "Christopher Coleman" <christopher.coleman at worldnet_att.net>
> Steve, I have recently spoke with Sechem regards the Flourite soil substrate
> product which is a calcined clay product.  It has iron ... ferric iron. 

As far as I can tell, Flourite is a low-temperature fired iron containing
clay, basically kiln-fired kitty litter.  It would be about as soluble as
crushed terra cotta flower pot.  Not to say that this is negligible -
try soaking a terra cotta flowerpot in a bucket of water with muriatic
acid and you may be surprised at how red the water gets.

> They also
> mentioned their Flourish and Flourish Iron fertilizers contain non-chelated
> iron ... ferrous gluconate.  Subsequent to the conversation I posted the
> message
> below which I had hoped could be answered by some of the chemists on the
> list as I felt the interest in Iron and Iron test kits is always popular
> here as indicated also by the current thread.

I'm not much of a chemist any more, but since nobody else has commented,
I'll have a go at it.

Btw, it is mildly amusing to find out that Flourish is ferrous gluconate.
I sometimes take an iron supplement to prevent anemia, and I buy a local
'house brand' of ferrous gluconate or fumarate about $5Cdn (~$3+US) for
100 tablets containing 300mg Fe gluconate each.  300mg of Fe gluconate
contains 34.7mg of elemental iron, according to the bottle.  This may be of
interest to anyone who wants to make their own Flourish-equivalent and has
access to a drugstore.  Personally, I bought 250g of Fe-DTPA for $3 some
years ago, and I expect it to last me far into the new millenium.

> I have just got off the phone with Seachem technical support and been
> informed that the Iron supplied in their Flourish line of products is non-
> chelated, that it is ferrous  gluconate, and that this makes it better than
> many other iron supplements which are chelated.

I wouldn't be surprised to learn that gluconate has a mild chelating or
reducing action that helps preserve the iron in the ferrous state, but 
in that case I'd expect Seachem to brag about it.

> I was then reminded of the many posts on the APD which indicate that
> neither the Lamotte or Hatch test kits test for chelated iron. Yet most of
> us get measurable numbers from these test kits.
> Could one of more chemically inclined APDers remove some of the
> confusion surrounding the statements the test kit manufacturers are
> making versus the results which we obtain in practice?  I have read that
> it is possible the test kits can still be accurate, that the measurements
> we get are as a result of the iron comming unchelated.
> But if the iron did become unchelated, it raises the question, how fast and
> how completly was it unchelated by the time we test it? (did it measure all
> the iron)

Ok, here's my take on this.  Btw, I have never used a test kit for iron.
If my plants don't look green enough for my taste, I sometimes add a tiny
pinch of Fe-DTPA to the tank.  This happens once or twice a year, but my
tap water has a bit of iron in it.

Chelation is an equilibrium process with a strong tendency to the chelated
form.  So a solution of chelated iron will have a very small amount of the
iron in the free form, since it is constantly dissociating and recombining.
Some people find that when they use iron test kits, they get a low reading
right away, but if they leave it a few hours, the solution gives a much
higher reading.  My guess is that the test reagent is grabbing the iron ions
whenever they are released by the chelator, so the immediate reading reflects
the level of dissociated iron at equilibrium, but as time passes and the 
reagents grab more and more of the iron before it can recombine with the
chelator, the reading reflects total iron.  Presumably the plants take up
ferrous ion from the constantly available minute amounts that are not 
chelated, although I think I've read that land plants, at least, may secrete
chelators from their roots and reabsorb the chelated minerals as such.

> Additional questions regards iron I have:
>   1)  in measurements I have made using Dupla drops in a
>        bucket of tap water, the iron levels remain stable
>        over the period of a period of a week.  This seems to
>        contradict what I have read that iron is unstable. Any
>        insight?

The chelating agent protects the ferrous ion from being oxidized to ferric
ion.  It's this oxidation that is referred to when ferrous ion is described
as being unstable in water.

> 2)   Does anyone know if the Dupla products are
>        chelated and by which chelate?

I would guess yes, and probably EDTA, maybe DTPA.

> 3)   Is there really any benefit to non-chelated iron
>        versus chelated ferrous iron?

Plants use ferrous iron.

I hope this helps.  Real chemists, please feel free to correct the above