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Re: Sediments
> I wonder if steel wool or iron oxide could be used as an alternative
> or supplemental Fe source for laterite?
This would only work if the substrate is anaerobic. The solubility of
ferric iron (Fe+3) is vanishingly small. Ferrous iron (Fe+2) is much more
soluble.
> 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.
The operative word here is 'ordnung' or orderly control.
When you add a soil to your tank, you lose that orderly control. Laterite,
on the other hand, is safe, well tested and benign. It is clear, however,
that a soil substrate is the only substrate which supports optimal growth
of rooted aquatic plants. The research that has been done on this topic is
long-standing, clear,consistent and very dramatic. Fast growing aquatic
plants can double their biomass every 2 to 4 days ... this kind of growth
rate can not be achieved with anything less than a fertile substrate (for
rooted aquatic plants). This kind of growth rate may not be desirable and
from what I read on the APD, laterite produces excellent results ... so
its up to you how adventurous you want to be.
> dr dave has mentioned 20% organics (presumably already well composte)
> 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
> they?)
Increasing the organic content has been found to decrease growth. There
are several possible reasons; redox becomes to low, the organic compounds
produced by anaerobic processes are toxic, soil density decreases so that
even though the organic matter is fertile, there is too little in any
given root zone. Having said that, others in the APD have had great
success with much higher organic matter content in their soil. Hydrogen
sulfide needs three conditions to form: 1) redox must be low enough ...
just because the sediment is anaerobic doesnt mean that sulfur compounds
will be immediately reduced, 2) sulfur must be present in the soil, 3)
the sulfur content must exceed the iron content in the soil. Sulfides are
precipitated by metals, particularly iron.
> 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?
I think the most important factor in having a soil substrate is to have a
layer of clean sand or gravel over top. This allows for an aerobic zone
where the bacteria can oxidise all the potentially toxic substances which
are produced in the anaerobic sediment. Aquatic plants which normally
grow on anaerobic substrates are well adapted to thriving under those
condition ... organisms in the water column are not so there has to be a
separation.
> 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?
The limited amount of research that has been done suggests that the only
salts needed in the water column are Mg, Ca and K. All other mineral
nutrients (except CO2) can be extracted from the sediment by the roots.
Of course if you have nonrooted plants, then you need to fertilize your
water.
> 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?
This is perhaps one of the major problems with using a soil substrate. In
nature sediments are renewed by inputs from the watershed that are
incorporated into algae and plants and then sink down into the substrate.
This doesnt happen in aquaria so the soil has to be renewed in some
way ... I havent seen any sign of soil depletion in my tank yet. Perhaps
the best solution is to pot fast growing plants that require a rich
substrate. They can then be repotted periodically without tearing down
the tank.
> Hope I can "stir up" a healthy discussion! ;-)
>
> Steve in cloudy old Vancouver BC
Dr. dave (from the soapbox).