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Re: QYNGA - ORP levels for Freshwater Planted Tanks

On Thu, 20 Nov 1997 10:32:41 -0500, "Louis Lin" <lhclin at aw_sgi.com>:

>David Aiken wrote:
>> Other comments made are that as the redox potential falls, ammonia
>> becomes favoured over nitrate between +450 and +300 mV ( a later comment
>> says at less than +350 mV), and Fe2+ over Fe3+ at between +300 and +200
>> mV.
>> Does +500 mV sound like a reasonable ball park figure to aim for? I hope
>> so because I'd hate for you to stop reading this list <g>.
>But plants cannot take the Fe3+, which means all liquid iron
>supplement will oxide quickly.  You better have a good substrate that
>can supplement the iron or reduce the Fe3+.

I accept the point about Fe3+, but if you keep the redox potential low 
enought to favour the Fe2+ you will also have it low enough to interfere 
with nitrification. One could ask which is worse.

Actually, I don't think we get that much of a choice in a real life 
situation. I suspect that if the redox potential was low enough to favour 
Fe2+, dissolved oxygen levels would also be very low - possibly/probably 
too low. George indicated in his post that his tanks were running at 
+500-560mV _without_ the use of ozone, i.e. these were naturally 
occurring levels in a well run tank. They are that high in part because 
plant photosynthesis drives the water to supersaturation with oxygen 
daily - all those little bubbles coming off the leaves in the last half 
of the day.

I suspect there's less of a problem with the availability of iron than 
there appears at first glance. The chelating agents used in liquid 
fertilisers are supposed to protect Fe2+ in the water column long enough 
for plants to break the chelation and obtain sufficient iron at 
recommended dosing levels.

The other factor with iron availability is that redox potentials will 
always be significantly lower in the substrate than in the water column 
(thereby favouring reduction), simply because substrates become 
anoxic/anaerobic at a depth of only a few mm. I would think that 
diffusion would ensure that some Fe3+ from the water column is 
continually being reduced to Fe2+ in the substate anyway, and some of 
that Fe2+ will end up back in the water column while some goes to the 
roots. Admittedly substrate heating (if used) and water diffusion through 
the substrate would affect that to a degree, but the water movement is 
slow and the redox potential will still drop significantly from the 
levels found in the water column even if substrate heating is used. The 
presence of laterite in the substrate would also help in maintaining Fe2+ 

Given the above, I think that it is possible to maintain sufficient Fe2+ 
levels in a well oxygenated tank with a redox potential above +300 mV. 
After all, people do seem to be doing it! While I haven't seen George's 
tanks (Colorado is a little too far away from Australia to just pop 
around the corner and take a peek <g>), I wouldn't want to argue with the 
kind of results the photos on his home page show. In summary, I don't 
think iron availability is a real problem at redox levels favouring Fe3+.

Now, if someone brought out a reliable cheap redox test, I'd suggest that 
we all go out and test the redox potential of our tanks and report 
(there's an interesting research project for someone). I'm prepared to 
bet that the potential in healthy tanks with good plant growth (including 
yours - you also have some nice pictures on your home page) is above the 
+300mV level, and probably above +400 mV. In fact, I wouldn't be 
surprised if George's readings are within +/- one standard deviation of 
whatever the mean is.

David Aiken