Rodger Whitlock totototo@pacificcoast.net
Fri, 15 Oct 2004 12:47:31 PDT
On 14 Oct 04 at 18:27, ConroeJoe@aol.com wrote:

> The pumice I can get is larger, the size used for landscape mulch. 
> It is just fine for pots of 1-gallon in size or larger--it works
> just as well as 1/4- to 1/2 inch-sized pumice I used to get in
> Pennsylvania.  
> I actually use the pumice mostly only for cacti and succulents--I do
> love the air that it holds  under the surface (cactus roots love
> that in wet weather). 

Everybody interested in pumice should be aware that the stuff sold
for landscaping is not necessarily pumice. I have several bags each
of black and red "volcanic cinders"; these are not pumice, but
others might mistake them for pumice because of their volcanic

Pumice forms when water- or gas-laden magma rises to the surface of 
the earth fairly quickly, whereupon the water and/or gases expand 
with the release of pressure (the water turning to steam), and you 
end up with frothy stone, with minute bubbles all through it. All the 
pumice I've seen has been quite pale: bone white to buff or almost an 

Volcanic cinders (for which there is undoubtedly a technical term) 
are little blobs of lava that were thrown into the air and hardened 
before landing. They are dark, have sharp edges, and are definitely 
not "frothy". 

From a horticultural point of view, these cinders lack the 
water-holding capacity of pumice. I've tried them in soil mixes and 
they worked very poorly. Might as well add rocks.

Just a caution to those of you not blessed with local volcanoes.

Rodger Whitlock
Victoria, British Columbia, Canada
Maritime Zone 8, a cool Mediterranean climate

on beautiful Vancouver Island

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