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Re: Using pumice

On Thu, 11 Feb 1999, Robert Paul H wrote:

> Since a lot of substrate methods are being discussed here, I thought I
> would throw in another one. I would like opinions on how Pumice could be
> used in a substrate. It is high in potassium and magnesium, but low in
> Fe. I also understand this is the prime ingridient of the Amano
> substrate. Doesnt this stuff float in water?  How could it be used?
> Mixed with clay?

I've never intentionally added pumice to my substrate but it is a part of
the river sand that I use.  Pumice is glass foam - naturally formed when
gas-charged, high silica lava reaches the surface and froths (usually with
a lot of big explosions, molten ash clouds, earthquakes and other
unpleasantries), then quickly cools and hardens.

Being a natural material, its composition is variable; I expect most
pumice will contain a few percent potassium, but I'm surprised to hear
that it might contain much magnesium.  It should never contain much iron.
The material is mostly glass, but it probably will contain a few crystals
and it probably will be weathered so that the glass is leached (which
lowers its potassium and sodium content) and partly broken down to more
stable minerals, including quartz and clay minerals.  The potassium and
other minerals are combined in fairly stable material so they aren't
readily available to plants. They may become available in low
concentrations over a period of time.

Being froth, pumice fragments have a rough surface with a lot of pores.  I
think its value in a substrate might come from the rough and porous
texture, which allows a large surface for bacterial colonization.  The
rough, porous surface may also be of some value for plant nutrient uptake.

When dry, pumice is very light and most will float on water.  Given time
it takes on water and sinks.  But even when waterlogged and under water it
is very light. This might be a good thing for your substrate (keeping it
fluffy and all), but I find it pretty annoying because it moves around.

All the pumice I've ever seen is also light colored - usually a light ashy
grey - which you may or may not enjoy.

Roger Miller