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Re: Oxidation of trace elements

	I have noticed many times in the APD concerns about oxidation
of trace elements for plants, and a few recent posts have brought the
subject up again.  For almost all of the things plants need, oxidation
cannot be a problem.  I'll go through the list:

K+, Ca++, Mg++, PO4---, NO3-

Not really "trace" elements, and fully oxidised in the forms above.
Cannot be oxidised further.

Cu++, Zn++

No higher oxidation states available, (Cu+++ does exist, but will not
be generated in an aquarium).

Mo, B

Present as molybdates and borates, i.e., already fully oxidised.

Fe++

Can easily be oxidised to Fe+++ in solution, because the Fe(III)oxo-
hydroxide has very low solubility.  The solution is to chelate the
stuff, but I suspect the chelate used is the Fe+++ one.  The problem
to be dealt with is insolubility, not the oxidation state.  Dave Huebert
mentioned that there was no evidence that Fe+++ was useless to plants,
and I'm not surprised.  That was a question that I asked a while ago,
and that was not answered - I'm glad it has come up now.

Mn++

Generally present as Mn++.  There are higher oxidation states (e.g.,
permanganate), but the only one that could be of interest to us is Mn(IV),
which _could_ form as MnO2, because that has very low solubility.  It
is a pretty strong oxidising agent in acidic solutions, and I don't
think we have to worry about it forming anywhere there is organic
material.


I have drawn a distinction between the ion M++ and the oxidation state
M(II), say, because in a lot of cases the metals aren't there as ions,
there are covalent bonds to other atoms (borates are a good example
definitely B(III), but not B+++).

Quit worrying about it!

-- 
Paul Sears        Ottawa, Canada