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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.

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