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Re: iron and TMG etc

In one of the more fact filled posts on this rusty topic, Roger S.
Miller wrote, in part:

"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 thought that was the whole point of chelation. . .the chelator holds
the iron for a while, until the chelator breaks down, which happens
rather easily when exposed to, for example, UV in sunlight or bulb
light.  The breakdown of the chelator is a steadier slower release than
if available iron was put in all at once, faster than the plants could
use it.  So your point is that you need to add lots of chelated iron
for the plants to have a little iron?

Roger S. also wrote, in part:
"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
about it and I never measure."

Again this points to one end of the spectrum of choices.  If I am
reading Tom's and Roger S.'s posts right, the following is generally

(1) plants need very very little (available?) iron and if the substrate
has iron (flourite, laterite, iron tablets), then the plants (or only
the substrate-rooted plants) will get enough.
(2) you must add lots of (chelated) iron to the water column to make a
little of it available to plants.
(3) if you add lots of iron, then the plants will Look (grow?) even
better than if you didn't add the iron.
     (3a)-- This might be so despite (1), perhaps because what plants
need and what they can use is very different.  In other words plants
can benefit form more than they need OR
     (3b)-- This might be so because of (2), in other words, adding up
to 0.5, 0.7, or even 1.0 ppm (total iron?) levels gets the iron that's
most readily available to the plants up to the required level OR BOTH
     (3c)-- This is only so in high growth setups, added CO2, macros,
and other traces.
(4) Walstad's observations, seem to me to be consistent with (1) - (3).

And lastly, we have the Barr Conjecture, that 
(5) Adding lots (0.5, 0.7, 1.0 ppm) of (total?) iron doesn't (or
needn't) encourage long term increased presence of algae. 

I don't know if (5) is generally agreed upon, but does anyone have any
counterexamples to compare with Tom's examples?  My own example that
started this round of posts fails as a counter example, at a minimum,
because I didn't "power through" until the algae growth rate subsided.

Scott H.

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