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Re: Fe2+ / Fe3+ [long but hopefully interesting]

>> On George's Aquatic Concepts web pages now found at
>> http://www.cco.caltech.edu/~aquaria/AquaticConcepts/ he discusses
>> several of the functions of heating coils. He says that [heating coils]
>> "Provide warmth in the substrate to speed up biochemical processes." 
>> [snip] 
>> I told Neil Frank I thought heating coils might be helping provide iron
>> in George's laterite substrates and he countered: "I don't buy this....
>> maybe he can test it out by not using his daily dupla drops which is
>> essentially only iron." This is a reasonable experiment and maybe George
>> could try it for a few weeks.
>Oh, sure, not adding Dupla drops for a few weeks is VERY reasonable.  Let
me run 
>right home and start killing my plants. Good idea Neil. Note the lack of a 

I actually was smiling to myself when I told Steve (in an off-line
conversation) that you might do such an experiment. BTW, this out of
context statement was responding to Steve's assertion that heat coils were
the ONLY source of iron to all plants.

>To my fevered brain, and as clearly stated on my website, the heating
coils are 
>MAINLY there to create circulation currents which pull the chelated iron
>the water column into the substrate where it is bound by the sufficiently
>CEC of the laterite until it can be adsorbed by plant roots.  If the warmer 
>substrate helps with other chemical processes or with delicate plants,
that's a 
>bonus not a design goal. 

IMHO, the role of the laterite together with heating coils is NOT to bring
iron to substrate (it is already there!), but instead to circulate water
and chemically bind phosphates. This is one of the important advantages of
ALL iron-bearing substrate materials. This helps to keep phosphates low in
the water column to starve algae. BTW, this concept is now mentioned in
DUPLA's publications.

>I have no doubt that the laterite provides lots of iron initially (the first 
>month or so after setup).  I typically measure iron levels in the water of
>mg/l or more right off the bat.  After the tank is established, I only see
>levels corresponding to the addition of drops. I once measured iron levels
>after a particularly brutal uprooting and pruning where the water was
>red for a few hours. No spike in iron concentration as I would have
suspected. I 
>think there is some available iron in laterite but most is "locked up for
>in strong bonds with other minerals (of course, that's purely conjecture).  

Yes, it appears that the iron from the laterite is kept out of the water
column... except during initial setup and other disruptions of the substrate.

I agree with George that laterite can be a source of iron to plants with
roots. However, laterite/latersoils CAN be a source of iron to ROOTED
plants, but only UNTIL the substrate becomes root bound or otherwise too
aerobic (from O2 thru the roots). Then the substrate iron cannot be reduced
and be generally available to plants. Presumably some (but perhaps not all)
 plants can overcome this with humic acids from roots (acting the same as a
chelator). In either case, I agree with George that the plants need roots
to make this happen.... not relevant for recently cut stem plants or
unestablished rooted plants. For these, iron in the watercolumn will help
to keep the plants reestablishing and growing.

>I think the long term purpose of laterite in the substrate is to act as a 
>bonding site for nutrients brought into the substrate by whatever
circulation is 

Yes... mostly P. I have not seen anything written to mention any other

Neil Frank, AGA