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Re: iron and TMG etc

Scott H. wrote:

> A submeasurable level is all that's needed?  Others have said the 0.1
> ppm is plenty.  Tom Barr has suggested a rate of as much as 0.5 - 0.7
> ppm.  The range to choose from is awfully large.

If you read Diana Walstad's book you see that she believes that iron is
the last thing you ever want in solution.  Not only should it not be
added, but it should be actively discouraged.

I think that the short story here is that I don't know of anyone who has
ever reported iron toxicity problems in their tank, so it seems pretty
safe to dose as much as you want.  Unless you're following Ms. Walstad's
methods.  Unless you get tired of paying for and dosing all that
chelated iron.  The long part of the explanation follows.

Keep in mind that iron is a trace element.  Admittedly it is one of the
major trace elements but it's still a trace element.  Compositionally,
iron is about a hundredth of one percent of plant dry weight; by weight
the iron content of a plant is about 1/150th of its nitrogen content. 
Incidentally, about 96% of a plant's dry weight is carbon, hydrogen and
oxygen which come from -- y'all know this -- CO2 and water.

Plants use mostly dissolved, unchelated iron.  If the concentration of
dissolved iron get's too low then plants get iron deficient and resort
to other means to get their iron.  "Too low" according to the literature
I've seen is about 0.000000001 moles/liter, or 0.000056 mg/liter.

When plants are stressed by lack of iron they have a few different
tricks they can use to get iron.  There are at least two major
strategies, but neither of those involves the plant adsorbing
synthetically chelated iron.  Iron EDTA in particular might be about as
biologically available as rust.  Maybe less so.  I haven't (recently,
anyway) seen the stability constants for iron-DTPA or iron gluconate. 
It's rumored that iron DTPA may be even less available than iron EDTA
and that iron gluconate *is* biologically available.  I seem to recall
that Greg Morin has had something to say about that topic.

I think that when people cite high numbers for iron concentrations
they're referring to high *total* iron.  The iron is mostly chelated and
not directly useful to plants.  The high iron readings seem to be
intended to get one of two conditions; either the free iron in
equilibrium with the chelated complex is high enough to supply the
plants' needs, or the free iron that is released from the complex as the
organic ligand degrades (due to light, for instance) is enough to supply
the plants' needs.

Either way, it isn't the iron that they're measuring in the water that
feeds the plants.

There's really nothing wrong with using high levels of chelated iron,
but there are big problems translating sufficient levels from one tank
to another.  Take the case of iron EDTA, for instance.  The
concentration of free iron in solution at equilibrium with iron EDTA is
given by

[Fe] = (10^-27.5)*[FeEDTA]/[EDTA]

where concentrations are in molar units, [Fe] is dissolved free iron,
[FeEDTA] and [EDTA] are the concentrations of iron EDTA and uncomplexed
EDTA respectively.

[FeEDTA] is pretty much what you might hope to measure with a test kit. 
The concentration of uncomplexed EDTA is a complete unknown and it
should be an extremely tiny number.  It depends on the amount of
fertilizer solution you add and the composition of the fertilizer
solution, on the pH of the water and the concentration of other metals
-- especially calcium -- in the water.

Someone with very hard and alkaline water might actually need to use 2
or 3 mg/l of iron and dose daily to get enough dissolved iron to feed
the plants.  It would be a complete waste for someone with soft,
unbuffered water to use the same amount of iron, because the low pH and
low calcium content of their water effectively increases the amount of
iron that the plants can get out of the same concentration of chelated

Further, different chelating agents give vastly different results.  I
don't have the stability constants for iron DTPA or iron gluconate --
the other major agents in common use in aquariums -- but my
understanding is that it may be necessary to use more DTPA than EDTA and
that it may be necessary to use less (maybe orders of magnitude less)
iron gluconate than EDTA.  The gluconate would probably need to be dosed
more often. 
> This keeps getting more interesting, at least to me and hopefully to
> others that add, or consider adding, iron.  Have those that now say the
> submeasurable to 0.1 ppm iron is all that's necessary tried Tom's
> suggestion for going to much higher levels?

From my own experience I'd have to say that iron is not a big deal. I
don't think I've ever seen a dramatic response to dosing with iron. 
Obviously the iron has to be there, but maybe the small amount that
comes in with tap water and feeding is sufficient to prevent severe
problems.  I prefer to keep a complex substrate and to dose iron in the
substrate when I suspect a shortage. In between times I don't think much
about it and I never measure.

Roger Miller