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Re: Iron precipitation (was APD V3 #891

Hello Christopher,

First of all, Fe++ is *oxidized* to Fe+++, not reduced.

The entire point of using iron chelates is to slow the formation of
"free" iron ions in solution, i.e, slow your #1 to #2 transition and
hence their oxidation. With a good chelate, the dissociation reaction is
very slow.

The precipitated Fe+++ (as Fe(OH)3.xH2O) is not totally lost to plants,
because it is the form most probably available to them in nature. In the
mud, where conditions are anaerobic, bacteria reduce Fe+++ to Fe++ and
solubilize it, making it available to plant roots.

"Free" Fe++ in solution is very short-lived. In dilute solution the
oxidation occurs in seconds. In more concentrated solutions it is
limited by the diffusion of oxygen in water and takes longer.

There is also a stage between your #2 and #3 -- the iron is already
oxidized to Fe+++, but exists in a colloidal form, "waiting" to
precipitate. Under certain conditions this stage can last hours or even

I don't have any experience with iron phosphate, which is about 1000
times more soluble in water than iron hydroxide.



> I have read that iron will combine with orthophosphate and
precipitate and also that ferrous iron in oxygenated environments
will form ferric and similarly precipitate .... This got me thinking
about the various pathways for iron in our tank and ultimately, the
factors determining how much ferrous is available for plants.

I see the pathways of iron in our systems being as follows:

   1) starts out as a chelate ( FeEDTA, FeDTPA,
        FeEDDHA, etc. )
   2)  becomes unbounded as Fe++
   3)  precipitates as a result of combining with
        orthophosphate or being reduced to Fe+++

It would  seem to me that progression to stage 2 only happens in
response to one of the events in stage 3 and as a result stage 2 is

Since plants prefer Fe++,  it seems to me that it would be available
to them during stage 1 ( as a result of uptaking the chelate and
it further ) or during stage 2.

It seems to me that if the iron is taken up during stage 2, that this
must be done quickly as Fe++ in an oxygenated environment becomes
reduced to Fe+++ very quickly.

Regardless of the method of uptake ( stage 1 or 2 ),  it seems to me
a competition for Fe++  exists between plants and stage 3 events which
vary from system to system depending on the particular levels of
and reductivenes.


Christopher Coleman
christopher.coleman at worldnet_att.net