Re: Iron - Chelated - Ferrous - Ferric - lost and found

[Roger Miller <roger at spinn_net>]

On Wednesday 02 April 2003 05:05, Scott Hieber wrote:

> So the iron can be gotten by the plants (eventually),
> sooner or later, regardless of whether it's about the tank
> as chelated, ferrous, or ferric?  If you dose it, they can
> use it even if your (initial or edta) chelators are being
> photoreduced and you're photoreducing the "free" iron,
> etc.?

Yes, but with complications.  Some of the mechanisms that plants can use to 
get different forms of iron are not triggered until *after* the plant is 
already stressed for iron -- not till after the plant is already deficient in 
iron.  Some of the mechanisms plants can use are also very expensive to the 
plant in terms of the amount of chemical energy and fixed carbon they 
require.  It seems like that expense must result in a reduction in growth.

Further, different plants vary considerably in their ability to get iron when 
it is in low supply, so some plants can be failing with iron deficiency while 
other plants in the same aquarium show no deficiency problems.

Plants need something like 1x10-9 molar of biologically available iron.  
That's about 0.06 parts per *billion*.  I don't think even good test kits 
will measure concentrations that low.  Concentrations of chelated iron are 
generally not significant.  That's because the chelated iron isn't 
biologically available until after the chelate complex breaks down.  Test kit 
results that include chelated iron are only useful as a general indicator of 
the size of the dissolved iron "pool" that might be available to plants.  The 
amount of iron you need to register on the test kit varies a lot depending on 
the exact nature of the kit, the kind of chelating ligand used and other 
variables that I don't know or don't understand. 

For iron, I think the best approach is to throw away the test kits and just 
make sure that the plants have access to multiple iron sources -- in soil 
humus, substrate solids and low levels dissolved in the water.

Roger Miller
>
> Scott H.