Crypts and clay coating of roots, substrate reactions

I had started some Crypts in a small experimental tank about a month or
two ago using a clay/soil/sand substrate enriched with clay balls with
fertilizer prills. On the weekend I was moving these Crypts back into
their original home, the newly redone 50 gal and I noticed something
I had not seen before with Crypts in gravel substrates. Most of the
roots where they had been growing in sand had a "typical" white
appearance. Wherever the root had grow through (or somehow gotten into)
one of the clay balls, there was an uniform coating of clay clinging
around the root of perhaps 1mm thickness. This clay jacket seemed so
consistent it puzzled me and I could only think of one or perhaps two
possible explanations:

My first thought was that perhaps the clay somehow stimulated the root
to send out many small root hairs as if to oxygenate the surrounding
clay or to extract mineral nutrients from it.

A second theory I came up with was that perhaps the root absorbed the
water out of the surrounding clay causing it to form a thicker, drier
layer of clay just around the root.

Has anyone observed this phenomenon with Crypts or have an explanation?

As I was rooting through the mucky clay, soil mixture, I did notice
several gas bubbles escaping. I couldn't detect the characteristic
H2S smell but I surmised that these gas bubbles could either be
gaseous nitrogen or NO2 which I understand is another product of
swamp muck decomposition.

On the subject of substrate anaerobic reactions here is some information
which a few of us interested in substrates have been sharing.

Sikora & Keeny, "Further aspects of soil chemistry under anaerobic 
conditions" 1983 in Mires: swamp, bog, fen and moor. Elsevier, 
Amsterdam, The Netherlands. Table 6.1 (referenced from another

Possible systems operating in flooded environments as related to
redox potential (Takai & Kamura 1966 etc...)

System                 Redox (mV)**2     Micro-organisms involved
O2 disappearance      +500 - +350        aerobes
Nitrate disappearance +350 - +100        }
Mn2+ formation        below +400         } facultative anaerobes
Fe2+ formation        below +400         }
Sulfide formation     0 - -150
Hydrogen, methane form. below -150       obligate anaerobes

Paul Krombholz added some further notes from an older article:
MORTIMER, C.H., 1941-42.  "The exchange of dissolved substances 
between mud and water in lakes."  J. Ecol. 29: 280-329.30: 147-201.

Mortimer made a graph of redox potential versus substrate depth in mud from
an eutrophic lake and also in mud from an oligotrophic lake.  In the first
2 cm. of the eutrophic mud the redox potential went from 600 mv to about 0
mv.  It reached a negative 100 or so mv. at about 5 cm. and then gradually
increased a little with increasing depth to about 0 again.  The redox
potential in the oligotrophic mud dropped to about 150 mv. at  5 cm. and
then stayed the same thereafter.  He gives ranges for verious reductions of
plant nutrients that differ a little from those you cite above:

NO3-----> NO2        0.45 to 0.40 volt
NO2-----> NH4        0.40 to 0.35 volt
Fe+++ ------> Fe++   0.3 to 0.2 volt
SO4 ------> S        0.1 to 0.06 volt.

Note that the sulfur reduction is to S, not S--.

Dave Huebert added some comments:

"These redox reactions are all biologically mediated. Bacteria use
the oxidized forms of these elements as the terminal electron acceptors in
the generation of energy."

Q. does pH (hydrogen ion concentration) play a role in H2S formation?

"There are such things as Eh/ Ph graphs which relate the redox (Eh)
to the pH. Basically, the more H+ that is present, the higher the Eh at
which a particular reaction takes place."

Could anyone clarify what materials are being used for electrodes in these
experiments? Paul Sears has suggested possibly a platinum electrode in the
substrate and a calomel electrode as a reference in the tank?

The objectives of the discussion about redox center around two goals:

o To stimulate plant growth through fertile substrates without introducing
  nutrients into the water column where they would be more available to
  algae (both macro and mineral micro nutrients)
o To prolong the useful period of fertility of substrates

As we can see from the above, the more fertile a substrate is, the
greater opportunity there is for some undesirable bacterial activities.
Some reactions are desirable, reduction of Fe+++ -> Fe++,
NO3 -> NO2 -> NH4 which is easier for plants to metabolize,
Mn2+ formation.

Since Fe solubalization probably occurs best at 100 - 200 mV it seems
to me that this should best occur in the lowest layer of the substrate
and in a dense layer which tends to inhibit the penetration of root
hairs. This layer should _not_ contain the organic materials which
provide a source of nitrogen since the redox would be too low and 
would cause de-nitrification. Ideally, I think, the organic materials
should lie in an upper layer (just as in nature) where the Eh does
not get too low. We should like to have the Eh close to or above
350 mV although this might be difficult to achieve in a thick layer.
Of course having lots of plants will help by two mechanisms:

o oxygen supplied by the plant roots

o (possibly) by induction of water into the substrate and
  through the vascular system of the plants

One theory to explain why peat seems to work well is that peat is
quite acidic and this acidity inhibits the decomposition process
so that one has a longer, more gradual release of nutrients. Peat is
composed of lignins and cellulose and these are the last materials
to be broken down during decomposition.

Steve P   in Vancouver BC