Fe recycling in soil substrates

I wanted to share with you some interesting information that Paul
Krombholz has been given me concerning Fe in soil substrates.

Paul explained how in low oxygen environments many species of bacteria
switch from oxygen to other molecules as electron acceptors. Specifically
they can reduce ferric iron (Fe+3) to ferrous iron (Fe+2).  "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." The chemical processes
that occur in soil in an aquarium substrate are very complex; Paul gave
a short summary of some of these. Suffice to say that there seems to be
a complex chain of reactions which effectively "recycles" Fe to soluble
states which can be absorbed and utilized by plant roots. If there is
an undergravel filter plate or gravel at the bottom of the substrate
where inorganic precipitates would accumulate, this process would be
disrupted. Experimentation and observations corroborate this.

Another effect that occurs is the eventual penetration of the substrate
by roots and rootlets. Plant roots contain air channels which rapidly
induce oxygen into the soil (rate of oxygen diffusion in air is
100,000 times faster than in water). Observing the soil at the bottom
of the aquarium near the roots, there is a region of rusty looking Fe
precipitate on the glass. Fine root hairs, (~1/4" long) extend into
the anaerobic zones where soluble Fe is present and are able to utilize

In tanks with a large number of plants and well developed root systems,
some plants can develop symptoms of Fe deficiency. Supplementation with
chellated Fe resolves the symptoms. Cryptocorynes appear to be adept
at competing for Fe in root bound, oxygen rich substrates.

Don't be afraid to use organic materials in the substrate; this will
greatly stimulate plant growth by encouraging oxygen consuming bacteria
in the substrate which provide additional CO2 for the plants. Paul
(or was it Neil Frank) says that it is possible to provide most of 
the CO2 requirements for plants by this method under certain 
circumstances (presumably where there wasn't excessive demand such 
as in strong lighting). The bacteria also provide the anaerobic 
environment under the soil that promotes the reduction of Fe.

Another observation that I would make: it would be good to use an even
deeper layer of soil in the aquarium where the dimensions permitted;
6-8 inches might not be too much! Paul, have you ever tried a deep
substrate with a 50:50 manure/soil mix? Would this create a potential
for problems do you think? This would be a good option to try with
my awkward 28" deep tank where it's so difficult to reach the bottom
of the tank without wetting the face!