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Re: Iron, chelates, and pathways

For those interested in how Fe uptake occurs please checkout the
following URL:


Christopher Coleman wrote:
> > Iron is also reduced to ferrous iron (Fe++) in the anoxic
> > conditions in the substrate and can be recombined with various
> > organic complexes to become available to plant roots
> do you know what types of complexes may be typical?

No I don't. I would assume that a great many of these chemicals have a
phenolic base group (the benzene ring) and are loosely described as
phenols. Greg Morin might answer this question.

> > and move into the water to be available to plant shoots and
> > leaves. It's not necessary for the iron to be complexed only
> > by the stable (EDTA, EDDHA or DTPA) chelating chemicals
> > to be used by plants. There are plenty of natural humins which
> > can do the job, particularly if there is plenty of aged organic
> > material (peat or detritus).
> > Steve
> Interesting.  Upon returning to the water column,  are these complexes
> stable?

They are stable in the sense that they can exist for several minutes or
hours but they are not as stable the commercial chelators. Remember that
these chemical processes are in dynamic equilibrium and the
concentrations of humins in the water will play a large factor in how
long Fe stays in solution or rather how much there is at "equilibrium".
It also depends if there is a source of chemically available Fe such as
clay sized particles in the substrate.

> I think what I am hearing is that a soil substrate can have a tendency to
> turn the iron pathway I described into a cycle.   And I am reminded from
> George S. that in the substrate ( soil or gravel ) that bacteria in the
> substrate
> can reduce fe+++  to fe++.  Thus an updated pathway would be:
>    1) starts out as a chelate ( FeEDTA, FeDTPA,
>         FeEDDHA, etc. )
>    2)  becomes unbounded as Fe++
>    3)  forms a colloid / eventually precipitates as a result of
>         combining with orthophosphate or being oxidised to Fe+++
>    4)  In the substrate, becomes reduced to Fe++ by
>         bacteria and is available to roots
>    5)  In a soil substrate, combines with humins to
>         form organic complexes which may return
>         to the water column, forming a cycle
> I suspect that this too is still an over-simplification.  What
> started my thinking about this was some recognition that the
> iron we supplement for the benefit of our plants faces quite
> a bit of competion from other aspects of our systems before
> it actually gets to them.

If there were no other sources of Fe other than small doses being added
to the water, I think that most of the Fe would become captured in one
chemical form or another and would not tend to be "recycled". When there
is Fe in the substrate from laterite, clay or soil, there is a source of
Fe. Only a tiny portion of the overall Fe in the system gets involved
with the cycle.

Greg Morin wrote:
> >    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+++

> I would just point out that when the iron becomes unbound from these 
> particular chelates it is in Fe+3, not Fe+2.

The reference I gave above seems to indicate that it is Fe+3 which is
the chelated soluble form which is transported to the root hair just as
Greg points out. Free Fe+3 results from decomposition and Fe reductase
supplied by the plant root (or leaf in the case of foliar absorption)
reduces the Fe+3 to Fe+2 which is the ONLY form of Fe which can pass
through an Fe channel. There is an excellent diagram showing this and
other Fe mechanisms on the web reference. Reductase is important for
foliar absorption too!

Steve Pushak                              Vancouver, BC, CANADA 

Visit "Steve's Aquatic Page"      http://home.infinet.net/teban/
 for LOTS of pics, tips and links for aquatic gardening!!!