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Re: iron

Tom Barr wrote:

> Roger wrote:
> > One simple conclusion seems unavoidable; the plants are *not* using the
> > chelated iron that we diligently add and carefully measure.  Rather,
> > they are using the much lower levels of ionic iron that are released as
> > the chelate breaks down.
> I think you got it. Or they use the Fe3+ when they need it and convert it
> themselves once they pull it in. Some let the bacteria do the work or the
> acidic sub's.
> It takes less energy to use the Fe2+ but it IS a trace element and does not
> require a huge energy expenditure relative to the whole plant.

I'm not sure that the difference between Fe2+ and Fe3+ really makes that
much difference to the plant.  I think it's fairly common for Fe3+ at
very low levels in soil water to be the primary source of iron.  Plants
use it and transport it in several different organically-bound states
where the oxidation state isn't very well defined and where it shifts
from one state to another without much ado.
> >  Plants probably won't use the chelated iron
> > directly unless they are already iron-stressed, which appears as a
> > visible iron deficiency.
> This could be plant species specific?

Absolutely.  Broadly, monocots and dicots differ in how they respond to
iron deficiency.  There has to be a big range in the response, but I
doubt that very many plants use chelated iron when there's enough
non-chelated iron around to keep them supplied. I think that breaking
down the chelate to get the iron out requires some specialized chemical

> > Further, some aquarists keep substrate
> > conditions (generally, acidic) that might make it unnecessary to *ever*
> > add iron.
> Well not in higher lighted tanks.

Not for all plants either.  Floating plants and surface-attached plants
like Java Fern aren't going to get any iron out of an acidic substrate. 
Also, stem plants that are frequently cut and replanted would be at a
disadvantage because new cuttings wouldn't have much of an iron source
until they regrow their roots.  So the advantage would go mostly to the
rosette plants.

> I've tried this now a few times. It does
> depend on plant species to some degree. Lower lighted tanks in the 2 watt
> range certainly I'd say yes to this. Maybe 3 watt a gallon. But the higher
> lighted tanks need both unfortunately.  I've tried to get around it but I
> just don't think it's possible. To have good growth at higher lighting
> levels, we need both sources(gravel and water column). But I get better
> growth even on the lower lighting tanks with good high CO2 and nutrients but
> less than the higher light tanks on the traces.

You would probably need very weakly buffered water to make the low-pH
substrate into a sufficient iron supply for a high-growth tank.  The
substrate pH would need to go down to 4 or so, and with well-buffered
water that may never happen.  In weakly buffered water you might get a
low enough pH from peat and from the plants' natural hydrogen pump to
provide plenty of iron.  But then again, only to rooted plants.

> > If you need to add iron at levels that are pushing 20000 times the
> > essential concentration then maybe it's time to look for be a better way
> > to fertilize your tank.
> Perhaps, but it works. Why? Why not do this also?

Well, because of the $cost of over-fertilizing, the hassle of having to
frequently dose with more iron several times a week and the need for
large water changes to guard against toxic trace element buildup.  Do we
need more reasons?

Maybe a good alternative would be to use an acidic substrate along with
more moderate levels of chelated iron in the water column.

Roger Miller