is anaerobic good?

Dave Gomberg wrote:

> When (not if) I redo this tank in July or so, I will lay down 3
> pounds or so of laterite, cover with small pebbles, drift on 5 pounds
> green sand, cover with 2 inches of #3 aquarium gravel, put in a
> custom split ugf with heater in one riser and airstone in the other,
> and then 1-2" more gravel.  This is intended to be anaerobic on the
> bottom (thanks dr dave) and aerobic (biological filtration) on the
> top.  Best of all worlds (I hope).  

The idea of an anaerobic, non-organic lower layer sounds good. Have
you considered mixing soil in the middle layer? The difference in
root growth and plant growth is quite dramatic especially with Crypts.

I wonder if iron oxide or steel wool could be used as an alternative
or supplemental Fe source for laterite?

Reading Baench Atlas vol 2 (recent acquisition) and they reiterate
the old adage that soil, compost, peat and organics have NO place in
a planted aquarium due to the danger of hydrogen sulphide poisoning.
I found this statement odd in what I would consider a recent and
rather complete reference. The value of at least some soil (or
equivalent) is well documented and the results are easy to duplicate.

dr dave has mentioned 20% organics (presumably already well composted)
is an ideal compromise for plants. Could some of the substrate experts
give us some more info on the merits and dangers of anaerobic "zones"
in a substrate??  What conditions favour the production of hydrogen
sulphide? I suspect the dangers of this are not exaggerated (or are

Is it better to have a dense clay zone at the bottom or should there be
some soil to support bacterial activity in the anaerobic zone if the
goal is to reduce Fe to the usable form?

Is it realistic to expect that a specially designed substrate could
support an optimal level of Fe? How deep would it need to be?

Is it still wise to add an appropriate micro-nutrient supplement
for other minerals and how often should that occur? Do we still
need to add chellated Fe, perhaps at a reduce level?

Different plant families have different optimal substrates; what
generalizations can we make about them? From my experience,
Ludwigia species and Crypts really prefer the "mud" substrate
(soil mixture). Aponogetons are said (in some books) to prefer
less organic substrates. Is that true?

The big benefit I can see of adding vermiculite is to improve
the permeability (ability for water + nutrients to penetrate and
be induced by the roots) of the soil. Soil provides excellent CEC
so I don't think vermiculite will contribute much additional CEC
benefit however soil could get rather dense. Is it a good idea to
use vermiculite or other similar materials to promote permeability?

Nutrients in soil can be used up. How long does this take? Is it
a good idea to add solid fertilizer (of the kind containing
nitrates and phosphates) tablets or sticks in a substrate? Where
would be the ideal level for these? bottom, middle layer? Is there
a danger of those macro-nutrients getting into the water level or
will the plants consume all the nutrients if the only substrate
circulation system is the roots of the plants?

Hope I can "stir up" a healthy discussion! ;-)

Steve                      in cloudy old Vancouver BC