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Re: substrate iron (was[none])

Bob Dixon wrote:

> So what percentage of iron content is necessary in a substrate additive like
> laterite or other clay if one wants to maintain a tank without teardown over
> an extended period of time, say foive or six years?

It is not the amount of iron contained within a substrate that
determines how well or how long it provides iron in available form to
the plants. Diana Walstad pointed out that if all of the iron in a soil
substrate could be made available to the plants, it would be enough to
last thousands of years.

My understanding of the processes suggests that there are two dominant
factors:

1) chemical availability which really means that the soil particles are
very fine in texture such as iron oxide or hydroxide clay. Refer to the
links on my web page on soil science and the recently mentioned link
"Plant Nutrition Lecture List" for discussion of iron availability in
latersol and oxysol soils.

2) the presence of small particles of stable organic matter such as
peat. These peat particles maintain reducing environments
(micro-environments as described by Roger Miller in a previous email
several months back) which solubalize iron and provide a source of
organic Fe complexes which help solubalized iron be transported by
diffusion to root hairs. Only a very small amount of peat or other
stable organic material is necessary.

>  I realise this is
> conditional on the size of the tank, the amount of substrate additive, the
> types and quantities of plants, etc, etc, ad infinitum, but can someone
> generalize on an average tank of a nominal size, say 55 gallons, or maybe 100
> gallons with a given substrate additive level as recommended by say, Dupla
> (wouldn't that be like 10 g. per gallon, mixed into a one inch layer at the
> bottom of the substrate)?

I've found that initially these soil/clay substrates provide a large
amount of iron and with significant amounts of peat, the humic acids
released into the aquarium water actually provide more iron in solution
than I want. During this initial transition phase, I think it's
important to make frequent water changes or to provide regular carbon
filtration (with weekly replacements of the carbon media) in order to
-reduce- the amount of soluble iron in the water so as not to encourage
algae growth. I don't think it actually makes very much difference how
much substrate there is; Paul K has used soil substrates of 1/2" depth
for long periods. I recall that he observed that when he mixed peat with
the soil in these soil trays, that he never observed the plants to
develop iron deficiency. Presumably the peat prevents a condition where
the soil becomes so oxygenated by the plant roots that it no longer
provides sufficient reducing environments to solubalize iron.

> And has anyone actually tested the iron content in their substrate additive
> before setup and again after a couple years, in order to demonstrate that iron
> is indeed coming out of the clay?  Or are we all assuming that the iron thing
> Dupla has sold us on is actually happening?

In order for a soil to become depleted of available iron, I think it
would have to be leached for hundreds of thousands of years such as the
tropical weathered soils which would be predominantly aluminum
oxyhydroxides. I think the iron oxyhydroxides have the lowest chemical
activity. Perhaps Roger will correct or comment on this. Another
interesting observation is that many tropical soils of volcanic origin
do not qualify as laterite and are in fact very rich in iron and other
nutrients!

> Some folks, I think Karen is among them are not using substrate coils.  Do
> these tanks deteriorate after a given percentage of substrate iron is
> depleted?  

In my soil/clay substrate aquariums, I have never observed a deficiency
of iron. It is very difficult to make really long term observations. It
takes, well, years! ;-)

> Or do the keepers of these tanks simply recognize iron deficiency
> symptoms and suplement into the water column successfully?

I think you could recognize chlorosis and verify that it was an iron
deficiency by adding an iron supplement such as Fe-EDTA, Fe-DTPA or
better yet, Fe-HEDTA. I test for a shortage of iron and other micro
nutrients by making such additions at intervals and observing growth
rates subjectively. I've only seen chlorotic symptoms once in H stricta
and this turned out to be N deficiency.

Steve Pushak                              Vancouver, BC, CANADA 

Visit "Steve's Aquatic Page"      http://home.infinet.net/teban/
 for LOTS of pics, tips and links for aquatic gardening!!!