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Re: Aquatic Plants Digest V3 #213

Yves Durocher wrote:

> Date: Sun, 19 Apr 1998 14:45:52 +0000
> From: Yves Durocher <yves.durocher at nrc_ca>
> Subject: Fe2+ or Fe3+ ? that is the question.
> What is the best bioavailable form of iron for aquatic plants:
> Fe2+ (e.g. FeSO4) or Fe3+ (e.g. FeCl3) ?
> Also, is there any reported toxicity related to chelating agent (EDTA
> and others) accumulation in the aquarium ?

Hi, Yves,

Fe+2 is the bioavalable form of iron. Fe+3 is insoluble above pH, say,
6.0. It drops out of solution & forms a brownish gel.

Unfortunately, FeSO4 oxidises rapidly to Fe+3 [Fe2(SO4)2(OH)2] which
then goes by several steps to [Fe(OH)3]x - which is very insoluble!

Thus one HAS to use chelated iron+2, to prevent its oxidation in
solution. In nature, the available iron is chelated by organic acids of
plant origin & often is the limiting nutrient. This is why you can see
pictures of Crypt stands in red mud, full of iron +3 - ("rust"), but
iron limited.

I have never heard of EDTA being toxic to plants, but who knows? (I
suspect that commercial preparations do use EDTA to chelate the iron,
because it is a very good chelator and one of the least expensive.

FeCl3 - ferric chloride - in solution is very acidic, almost as acidic
as hydrochloric (muriatic) acid.

Hope this helps,