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Re: My plants can't get enough iron!

Tom Woods wrote:

> It's my understanding
> that a properly constituted trace element mix is made such that every trace
> element is present in the same ratios that they are used by the plants. So,
> if all the iron is used up by the plants (and not lost to other processes)
> then all the other trace elements will be used up as well, because they are
> used in lockstep with iron in the ratio they are present in the mix and in
> the plants. Isn't that the underlying assumption to using iron test kits in
> the first place?

Yes, that is the underlying assumption.  The problem is that iron *is*
lost to other processes, so the assumption may be wrong.

Iron(ferric)-chelates are broken down by light and the iron can then
form compounds that don't stay in solution and aren't biologically
available.  I think that iron is the only trace element that is effected
that way.  Iron can drop out of solution faster than other trace
elements, so it is a poor indicator; if you use iron as an indicator for
other trace elements then the other trace elements may build up in

How much of a problem this might be depends on details of the trace
element mix -- like the amount of excess ligand in the mix -- that
aren't usually posted on the label.  If it does happen, then it could be
difficult to detect.  A nutrient toxicity typically has the same
symptoms as a deficiency in another nutrient.  If you do run into a
problem and continue to increase your fertilizer dose to fix the
deficiency then the problem would only get gradually worse.

My description comes from data for ferric-EDTA; ferric-DTPA behaves
similarly.  I think that Seachem's Flourish line uses ferrous gluconate,
and that shouldn't act quite the same way.  But, since it was Dr. Morin
who brought up the subject I suppose the there might be some analogous
behavior with ferrous gluconate.

Roger Miller