Humus vs. laterite

Thank you for the Jim Kelly article Shaji; it's extremely interesting.

Shaji said:
> Jim's article pointed out that the Cation Exchange Capacity (CEC) of
> Vermiculite was quite high.  This means that Vermiculite is very good
> at making metal ions and NH4+ available for uptake by plants.  The
> problem of course, is that it is hard to make Vermiculite sink.
True. There's no way you could add it to an aquarium which had already been
set up. The descriptions of it's use indicate they put a layer of sand 
(gravel) over top in order to contain it. If you used pots, it also
shouldn't be a problem.
> The article also listed humus as having a high CEC, but humus is high
> in organics, which is often a disadvantage in aquaria. Jim also
> pointed out that laterite clays have a relatively low CEC.

That's what I found interesting too. He did point out there are several
other factors in addition to CEC however I have not had a chance to
fully absorb this article (high nutrient content, low brain solubility :-)

Regarding humus Jim did say:

> * HUMUS is the end product of the decomposition of organic matter.
> It is a complex substance consisting of material either modified
> from dead plant tissue or synthesized by soil organisms.  It is
> fairly resistant to furthur decay (in contrast to peat moss, which
> is relatively undecayed organic matter but which also has a high
> CEC) and thus forms the long-lived organic component of the 
> substrate (but not as stable a clay). <see the original for
> other technical details on humus - sp>

So the lesson here is that the more thoroughly composted is the organic
material, (I suspect) the less chance of subsequent problems with
anaerobic decomposition or production of large amounts of phosphates,
nitrates and other overly abundant algae fuels. Moderation would be
good since just a little will probably serve our purposes. It makes
me think again about the use of lake and pond mud as an additive.

> 8)  Nutrient Uptake by Plants
>      The uptake of a nutrient depends not only on solubility
> and being in an exchangeable position, but also on it being
> in close proximity of the root surface. <technical details snipped>
>    Root uptake of
> nutrients requires intimate root-soil association.  [Which means
> coarse gravel is not optimal.  The finer the substrate the
> better as long as there is aeration.]  Nutrient solubility is
> strongly affected by root exudates and microbial activity near
> roots [by the latter Brady is presumably referring to N 
> fixation by bacteria in terrestrial plants].  Finally, the
> entrance of soluble nutrients into root cells is stimulated by
> plant root metabolism -- respiration provides the fuel!  [Repeat
> the mantra:  "I *will* provide aeration for the soil."]

This gives us insight into the importance of low volume circulation
in the substrate (UG heating coils etc.) I also suspect that
microbial activity may play a more important role than we know in
aquatic plant nutrient absorption so again oxygenated water is
important. This is an excellent article and I recommend those interested
to have a look at it.