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

In the substrate, in the interstitial (between substrate particles)
water where there is very little free oxygen, Fe2+ is more soluble than
Fe3+. It can exist in unchelated form in free solution or loosely
attached to a cation exchange site.

I think we assume that most of the iron used by rooted, mature plants
comes from the water when we have a laterite or iron containing
substrate and are adding PMDD or Dupla daily drops or something similar.
Is this assumption justified??

Comments by Kaspar Horst in TAG 11:1 (Jan-Feb 98) indicate that the
natural substrates of Cryptocorynes is very high in dissolved iron. I
quote: "The soil samples were taken directly from plant Biotopees
[biotope - a region uniform in environmental conditions as a habitat]
and delivered to the Agricultural Research Station of the State of Hesse
in Kassel-Herleshausen for testing and subsequent evaluation. The
comparison between natural Biotopee and pure aquarium gravel speaks for
itself. Whereas the tropical substrates show an iron potential of
between 58 and 1366 mg/l in our soil tests the iron content of quartz
gravel is close to zero." 

He goes on to discuss other mineral nutrients in the natural substrates.
I'm not sure if this iron potential refers to iron dissolved in
interstitial water since lateritic soils could typically have total iron
concentration around 30%. It could mean that he is referring to
chemically available iron since much iron could be locked inside
relatively large granules (i.e. sand sized particles) as opposed to clay
sized particles (< 2 microns).

Later on in the article Kasper summarizes the function of laterite in a
section titled "The tasks of Duplarit in an aquarium". He says:

"Duplarit (laterite) performs a similar task in the aquarium as in
tropical regions. Under slightly anaerobic conditions in the aquarium
substrate as well as through the plant root' active dissolution of the
substrate's iron (through the escape of plant specific organic acids)
the nutrient absorption through the roots is provided with sufficient
nutritional iron."

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." 

[digression] There are many important biochemical processes in the
- Mineralization - breakdown of complex organic compounds like proteins
and carbohydrates into simpler molecules which can be used by plants.
- making phosphates soluble
- reducing nitrates to ammonia
- reducing manganese to the more soluble ionic form Mn2+
- reducing iron to the more soluble ionic form Fe2+

In TAG 5:5 (Sep-Oct 1992) "Some Assembly Required - Part 3, The
Environment" by George and Karla Booth:
"Another reason [substrate heating coils are an important element in the
substrate] is that for every 10 degrees higher temperature between 50
and 90 degrees F there is a 2-3 increase in the rate at which reduction
reactions take place."

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.

So I wonder what would happen if you did stop adding chelated iron?? I
have done this with two substrates in my two large CO2 injected tanks
currently. Both of these tanks contain sand mixed with a little soil or
subsoil (no organic matter) and enriched with micronized iron. I added
micronized iron because I couldn't be certain of the iron content or
availability of the soils I had. One tank has a little peat mixed in and
the other tank has more peat in the middle layer (about 1/4 by volume or
about 5% by weight). I stopped adding chelated trace nutrients and the
plant growth continues to be very good. Sometimes growth has slowed in
either tank but the addition of other nutrients (calcium in solution,
nitrogen and phosphates in clay balls) has always restored vigour. In
the aquarium with less peat, the growth of floating plants like Salvinia
is much slower and the leaves are smaller and a little yellowish. In the
tank with more peat Salvinia propagates quickly and shows good leaf
formation and colour.

Do aquatic plants in laterite tanks get most or at least a good portion
of their iron from the substrate? For those of you using PMDD, it may be
an option to reduce the amount of chelated nutrients to discourage the
growth of algae.

Steve Pushak