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Re: Iron: a factoid and two questions
ac554 at freenet_carleton.ca (David Whittaker) commented:
> One wishes to measure only the iron available to the plants. An acid
> digestion might catch iron which is normally not available.
True. This raises my other question: Does anyone know what _is_
available to the plants? With a proper lab, a person might try, for
instance growing plants in a well-aerated hydroponic setup with good
supplies of all trace elements, but iron available only from a chelate
of Fe+++. Or unavailable, as it might prove to be. Sounds like the sort
of thing somebody would have done, but we don't know the right graduate
student to make a project of finding out.
> people have claimed that the test they use requires an extended wait
> to obtain results with EDTA. Check the Archives.
I saw some of these. I gave my LaMotte kit much longer than the
recommended time to spot that 10 ppm of chelated iron, but it still
> Personally I think
> that the Emperor has no clothes, but then I'm not a fashion expert.
Heck, I'm not even sure which Emperor to look at.
> Craig Bingham suggested using one of the Hach kits with a potassium
> persulfate digestion to oxidize the chelating agent. It would be best
> to read his post on this. I'd love to test for iron, but like you, I
> am unsure of what each is measuring.
But we know that whatever Dupla is using is measurable and works. If
only I jnew where to go from there.
> The following are found in various micronutrient mixes.
> EDTA(ethylene-diamine tetra acetic acid)
> DTPA(diethylene triamine pentaacetic acid)
> EDDHA(ethylene-diamine dihydroxyphenyl acetic acid)
> Iron gluconate
> Iron sulfate
> lignosulfonic chelants of iron
And Shawn Keslar <keslar at wvunrcce_nrcce.wvu.edu> says,
> Subject: Iron
> I believe that I have seen iron chelated with NTA (nitrilotriacetic
> acid) and DTPA (diethylenetriaminepentaacetic acid - say THAT fast five
> times) in commercial mixes.
> ... We adjust our iron conc. by bringing an acidified sample
> here to work, where we have a plasma sectrometer that's usually running.
> One of the luxuries of being a chemist
Great solution! Wish I could do that. However, see above, do we know
what's available iron and what isn't?
> ....with respect to the ferrous/ferric question, I have seen some
> references (on the Krib, or on Steve Pushak's page, maybe) to some
> plants having an enzyme named ferric reductase. This enzyme reduces
> ferric iron to ferrous iron that the plant can use.
Now that makes sense. You'd expect plant life to be smart enough to do
that. Raises another matter, though: where would they get ferric ion to
work with, given its low solubility at reasonable pH? Maybe by breaking
down complexes? Like some humic-acid equivalent of EDTA?
[He asked his hairy uncle the baboon, and he spanked him with his hairy
paw. He asked -- but I've forgotten who the Elephant Child's other
uncles and aunts are.]
(But since you're an actual chemist -- Have I slipped a cog when I look
at the solubility product for Fe(OH)3, which is about 1e-36 (10 the minus
36) and conclude that at pH 7 you can have up to 1e-15 molar Fe+++ in
solution, and not more?)
dd at dandrake_com