[APD] Iron Revisted - oxidations states, chelators and availability etc.

["Chris White" <criffa at tiscali_co.uk>]

Hello everyone!
 
I’m a bit of a newbie when it comes to this sort of thing, and new to
the list, so my first plan was to trawl through the archive, and have
done so for many hours now – I’ve got to admit, I have a headache and am
no clearer, if not more confused, on the topic.
 
Oxidation states:  I’ve been reading in some threads that plants only
use iron in the roots as ferric (III) and subsequently get reduced to
ferrous (II) for the ion uptake process.  Additionally, there seems to
be some confusion whether plants can uptake iron directly through the
leaves as ferrous (II) or not at all.  I can understand the train of
thought in that they can’t as obviously terrestrial plants can’t take up
iron through the leaves!  But what about the case of non-rooting plants
such as Riccia or Hornwort?
 
Chelating agents: The old gluconate vs EDTA debate:  It again seems
suggested that the chelating agents are used to keep the iron in
suspension but at the same time, I’ve read that light breaks down the
EDTA (not too sure if the same applies for gluconate).  If the chelating
agent breaks down liberating the iron (either as (II) or (III)) would
this have the same effect of adding straight FeSO4 to the tank?
 
Availability:  After our chelating agents liberating and due to the
oxygen level in the water, iron (II) (chiefly from the gluconate I’d
assume) would become oxidised leaving the same result as iron derived
from the EDTA.  I may be on the wrong track here, but aren’t iron (III)
compounds generally insoluble?  If this is the case, how would using
commercial products differ from adding, say rust (!), to the
water/gravel?
 
Finally, I’ve probably mis-read something somewhere which has lead to
this confusion and may have mis-interpreted some of the information.
Can someone help me clear up this mess, please? And apologies if I have
got any of the nomenclature wrong :o)
 
Chris

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