Re: is anaerobic good?

Hmmm... nobody else is going to talk about this, so I might as well
talk to myself. ;-) Warning: long technical article. Skip to the
end for the summary if you're easily bored. ;-)

What I'm trying to get at here is an optimal soil substrate for
promoting plant growth especially by providing soluble iron and

Here is an extract from some material by Paul Krombholz which was
published in TAG, Vol. 7 No. 4: "Mineral nutrition of Aquatic Plants,
Part 2 Underwater Soils and Nutrient Availability"

"The availability of some nutrients, particularly iron, in aquatic systems
is complicated by the presence of mud (let's call it underwater soil) where
anaerobic conditions (oxygen is absent) exist.  In the process of cellular
respiration, most bacteria use, as we do, oxygen (O2) as an electron
acceptor.  They reduce oxygen by adding electrons and hydrogen atoms to it
to form water.  (Reduction is the addition of electrons, and oxidation is
the removal of electrons)  If oxygen is unavailable many species of
bacteria can switch to other molecules as electron acceptors.  They can
reduce sulfate (SO4-2) to sulfide (S-),ferric iron (Fe+3)to ferrous iron
(Fe+2) and nitrate (NO3-) to nitrite (NO2-) and then to nitrogen gas (N2),
which is unavailable for use as a nutrient.  This latter process is called
denitrification, and is responsible for considerable losses, on a global
scale, of nitrate from waterlogged soils.  Under aerobic conditions (oxygen
is present), iron compounds exist as ferric compounds, where iron is in the
Fe+3 state.  These compounds are all quite insoluble, and the iron is
mostly unavailable to plants.  Under anaerobic conditions, however, iron is
reduced by bacteria to the Fe2+ (ferrous) state.  Many ferrous compounds
are soluble in water.  The result is that iron is much more soluble in soil
under water than it is in the water, assuming that oxygen is present in the
water but not in the soil.  Many aquatic plants become iron deficient if
they do not have their roots in some anaerobic soil  Manganese, another of
the micronutrients, is similar to iron in that it also becomes more soluble
under anaerobic conditions."

On Wed, 20 Mar I wrote:
> 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.

Crypts grow in a natural environment rich in dissolved iron and where
there is a muddy bottom. Karen Randall, our Crypt expert, uses potting
soil and micronized iron.

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

I think it could. In an aerobic environment unoxidized Fe will not
be rapidly oxidized, however it will be relatively available chemically.
Ferrous oxide is relatively soluble (the reduced state). Ferric oxide
can be reduced by bacteria in an oxygen deficient environment to
the Ferrous state. Steel wool or other iron compounds should be in
the very bottom layer of the substrate where circulation is minimized
to prevent a situation where excessive amounts of iron could be in

> What conditions favour the production of hydrogen sulfide?

The same anaerobic conditions favourable to reducing iron, the
bacteria and the presence of sulfates. To avoid toxic H2S formation,
we need to minimize rich sulfer compounds in the anaerobic zones.
This would include manure, manure compost and especially, any
solid fertilizers containing sulfer compounds.

> 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 state?

There should be some organics in the anaerobic zone to support
bacterial activity. Without bacteria, Fe is not reduced.

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

In nature the soil depth is virtually unlimited. In an aquarium,
eventually the substrate becomes densely filled with roots which
provide an abundance of oxygen. At this point, Fe can no longer be
effectively solubilized. Obviously the deeper the substrate, the
longer before this occurs. A dense lower layer of clay will resist
root penetration. A 6" substrate or deeper would not be too much.

> 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 reduced level?

Yes. The soil may not be able to supply all Fe requirements but it
can greatly improve the availability of Fe between dosing. Plants
grown in my soil aquarium show less variation in red coloring when
the Fe supplementation is temporarily discontinued than in the gravel
only substrate.

> Is it a good idea to use vermiculite or other similar materials
> to promote permeability?

I think it is important to maintain an aerobic substrate zone
above the anaerobic one. This will provide a region where hydrogen
sulfide diffusing out can be oxidized to sulfates and where there
can be a higher concentration of organics and macro-nutrients
such as fertilizer for the benefit of the plants. Sulfur containing
fertilizers should be avoided. The aquarium should be densely
planted immediately after set-up since this is the time when
the substrate starts to loose its oxygen. The plant roots will
rapidly penetrate and begin to induce water circulation and
provide oxygen via their internal air channels.

> Nutrients in soil can be used up. How long does this take?

It would depend upon the amount of plants grown and the overall
growing conditions. One can remove a large amounts of bio-mass
from a good aquarium almost weekly. Some plants like Crypts and
Echinodorus grow much slower. A soil substrate might become
deficient in 6 months to a few years. I have little empirical data
to substantiate this guess.

> 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?

I'm experimenting with this and other people use fertilizer or pond
tablets with good success (notably Jim Kelly). I expect the
best place is in the middle layer, not in an anaerobic zone.

> 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?

I would presume that all these macro-nutrients get used up by
the plants as they are drawn through the plants circulation
system. It should be safe to use macro-fertilizers if they are
confined to the substrate.

Here is my proposed "ideal" substrate:

Bottom: 2" fine clay or clay-like dirt. Powdered laterite
would be a good choice here. Optionally a very small amount
of super-fine steel wool (<5 gms) Pure clay should be mixed
2:1 with ordinary dirt to provide minimal organics.

Lower middle: 1.5" soil with composted manure. 20% organics.

A few low sulfur fertilizer sticks broken into small pieces.

Upper middle: 1.5" soil/manure/vermiculite.

Top: 1" to .5" layer of dark silica or quartz coarse sand.

Vancouver Steve