Re: nutrient diffusion into substrate.

> > Date: Fri, 25 Aug 95 8:35:37 MDT
> > 
> From: Charley Bay <charleyb at hpgrla_gr.hp.com>
>> George may be
>> right in stating that laterite is "not a fine clay", but I think he
>> must admit that laterite compacts over time (thus restricting
>> circulation) to a point at least a magnitude greater than vermiculite
>> does.
George responded:
> Nope, I won't "admit" that at all.  The only place I notice any kind
> of "compaction" is on the very bottom of the substrate on top of the
> glass.  And that layer is just fine "dust" laying there.  Not
> compacted.  Remove from your mind the myth of laterite "compacting".

Oh.  I concede that point, then.  I remember seeing the "dust" on
the bottom, but am not quailified to comment on compaction since I
do not have a laterite-substrate tank of my own.

That said, though, I *know* that vermiculite exhibits a *strong*
macro-structure form with excellent aeration.  (I'm trying to
remember back a decade to my soils class...is that called a 'colloid'?)
Anyway, does laterite intrinsically exhibit this well-aerated macro
form?  I do not have the perception that it does; so even if laterite
does not compact, I would naively bet that it does restrict flow
(possibly) a magnitude greater than vermiculite.

Of course, this may not be bad.  It has already been discussed here
that slow circulation of oxygen-poor (but not anoxic) water through
the substrate may best allow nutrients the proper time to bind on 
particle sites without being oxydized or washed from the substrate.

I believe the point that Stephen.Pushak at hcsd_hac.com was trying
to make was that he *could* trust vermiculite to passive circulation
only, whereas he could *not* trust laterite to passive circulation
only (or heat coils would be unneccessary).  Magnitude or not,
these "clays" appear (to me) to be on opposing sides of some line.

charleyb at gr_hp.com	or	charley at agrostis_nrel.colostate.edu