Stephen Pushak (Stephen.Pushak at hcsd_hac.com) wrote about lots of substrate
subjects on which I would like to comment (I tried to squeeze all into
subject line) :-) 
>Water movement through the
>substrate might be overemphasized. Roots also supply oxygen and
>if the substrate is permeable (i.e. VERMICULITE!), some O2 will penetrate by

I agree. I have many plant tanks, and none have circulation systems. But I
might some day :-). I am also trying vermiculite in 2 tanks (to experiment
w. CEC potential and help keep substrate permeable), but do not think this
is the only way to get O2 to penetrate (e.g. coarse gravel or shallow
substrate are others).

>Avoiding too much organic material in the substrate may solve the
>problem of anaerobic decomposition there!

With organic matter, anaerobic problems may also be avoidable with added
nitrogen fertilizer during initial setup, and with use of certain organic
materials (e.g. I am more successful with peat than potting soil, but am not
sure why; maybe because of low pH, different bacteria and slower decompostion?)

> Too much circulation will mean
>many useful reduction reactions can't take place!

Only a problem without chelated nutrients.

> The stuff found in stream
>beds and lakes has already undergone considerable decomposition in an 
>oxygen environment before it becomes part of the substrate.

True, same for mulm.

> It's value is
>it's CEC and as a source of minerals and as a host for countless bacterial
>and chemical actions. Lake mud may be an ideal substrate addition for this

Apparently so, since aquatic botanists (i.e. ones that publish in peer
reviewed journals) like to use it for their plant growing experiments.

>Don't forget
>that many people get acceptable results w/o any substrate circulation at least
>for a year or two.

Or three or four... :-)

> Of course, I believe with mud or clay you want to have
>some kind of substrate circulation though. 

I tend to agree. Some O2 is needed but, this can be provided through roots
of actively growing plants.

>Too much of any
>one mineral such as iron or aluminum can mean that other important micro
>nutrients get displaced from the CEC sites and therefore can't be absorbed
>as efficiently by the roots. This is an excellent argument for the use of
>the prepared nutrient mixes.

Or for using a good substrate additive, like peat.  I am not sure why, but
peat seems to provide a balanced set of nutrients, at least in combination
with my local water.

>Some clays contain carbonates and phosphates I think which are leached out
>of tropical laterite. I'll let someone else comment on that. 

Carbonates are not necessarily bad.  A good source of Carbon for many
plants; can be an alternative to CO2. Phosphates are another story; but, I
do not have any knowledge or comments about phosphates found in clay.
However, Jim Kelly (the 'father' of vermiculite) suggested that clay in
aquariums including laterite may actually attact phosphates, and with
substrate circulation, may remove it from the water column. Could this be
the real value of laterite?