Re: clay, laterite and vermiculite

> From: Stephen.Pushak at hcsd_hac.com
> Date: Thu, 24 Aug 95 16:34:47 PDT
> Subject: nutrient diffusion into substrates
> George and I have been conducting an interesting discussion on the
> merits of clay, laterite and vermiculite in substrates. I was saying 
> how vermiculite is so wonderful because it increases the permeability 
> of the substrate so well whereas fine clays would restrict it. 

FWIW, I agree. I started 2 tanks with vermiculite substrate last winter. 
I saw vermiculte as a 
potential solution to a problem that I had been having with compacted 
sand/soil substrates. I still do not use UG heating, but recognize the
need for intra-substrate movement of fluids. Certain slower growing 
plants may not be able to provide enough O2 to their localized area.

(Sorry that I have not had time to participate in these interesting 
discussions more regularly)

> countered saying Dupla laterite is not a fine clay and challenged
> my scientific foundation for this "ionic diffusion" claim.

I agree that Dupla laterite is not a fine clay (it settles out quickly), but 
other "laterites," like the old Aquarium Products stuff, are fine clay 
and can remain in permanent suspension. A consistent definition of this 
mystical product would be useful, but I have not been able to find one.  I 
once checked with soil science professor from NC State University who had 
travelled extensively thru the tropics and he said there is not such 
thing as laterite.  I seem to remember that he used the term latersoil. 

Anyway, in Kelly's article, Jim also mentions am interesting property of 
laterite: binding of phosphates. 
With water circulation (by slow flow UG heating or other mechanisms), this 
may be the primary long-term function for this nutrient poor material - 
ie. keep certain nutrient concentrations trapped in the substrate and low in 
the water column.