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[APD] Re: Substrate iron

George wrote:
> Steve wrote:
> >To tell you the truth, I'm not sure how much iron
> >the laterite provides long term. (George is going
> >to tweak my toes for that comment).
> I don't know for sure but it probably provides less iron than garden soil,
>  which if I remember right, will provide about 10,000 years worth of iron.
> My theory is that the laterite will provide some (or a lot of) iron in
> solution
> when a tank is initially set up.

Diana was basing this estimate upon the total amount of iron, not the
chemically available iron. It depends upon the chemical composition &
texture of the iron in the laterite. If there's iron oxide in the laterite,
I think its chemically available at ordinary substrate redox. If its all
iron aluminum silicates, then I don't think it would be available.

I think the best way to ensure long-term iron availability from the
substrate is to have peat mixed with some type of soil (dirt, clay, or
laterite). The peat should ensure that the redox potential in the substrate
stays low. If there is oxygenated water flow into the substrate from heating
cables, wouldn't that oxidize all dissolved iron?

What if the heating cables weren't really circulating water much at all but
instead speeding up biological processes in the substrate, thus lowering the
redox? Plenty of chemical & biological processes work better at warmer

Peat also has the advantage of providing plenty of humic acids, which are
natural chelators. Not all plants are capable of excreting their own acids
from their roots to obtain iron. The humic acids will carry iron into the
water & thus make iron available to all the plants, even those floating or
which have not been permitted to grow roots into the substrate as would be
their natural condition.

If you are adding chelated trace nutrients, then you're not going to notice
a difference in iron availability from the substrate.

> Longer term, the laterite will "chelate",
> or
> more properly, bind nutrient ions such as Fe++ so plant roots can adsorb
> them.
> Assuming, of course, you have provided some way to add nutrients like iron
> to
> the water and have provided some way for the ions to reach the laterite
> layer
>  in the substrate.

Assuming that you have calcium & magnesium in the water, these cations would
displace any Fe++ at the CEC sites.

Steve P in sunny Vancouver

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