[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: Pumice contents

On Fri, 12 Feb 1999, Robert H wrote:
> Not that I doubt what anybody said, but here is the information I got
> from Cornel universities Something to grow on WEB pages on Pumice: I
> also remember reading that Amano uses pumice or volcanic ash as his
> substrate.
> Somebody fill in the blanks here!

I see no reason to get overly concerned with details of pumice.  I think
its main value lies in its light weight and porous texture.  But if you
insist, I'm usually willing to oblige:

According to my old igneous petrology text (Carmichael, Turner and
Verhoogen, 1974, McGraw-Hill) pumice is "...the glassy froth formed from
rapidly extruded magma, frequently of rhyolitic composition."  The average
composition of rhyolite expressed as the oxides is (same source):

SiO2	73.66%
TiO2	 0.22%
Al2O3	13.45%
Fe2O3	 1.25%
FeO	 0.75%
MnO	 0.03%
MgO	 0.32%
CaO	 1.13%
Na2O	 2.99%
K2O	 5.35%
P2O5	 0.07%
H2O	 0.78%

(This composition is an average for the rock type; there are many

There are some nutrients in this composition, but many, many other types
of rocks contain higher levels of iron, magnesium, calcium and phosphorus.
Potassium is higher in a rhyolitic rock than it is in most other kinds of

The nutrient content probably isn't important, because the nutrients are
tied up in the glass and in a few (usually tiny) crystals within the foam.
The nutrients are released only when the glass and minerals decompose and
that happens slowly.  Most of the pumice you will ever see has already
been around for a million years or more.  It's already a little decomposed
and the concentration of a lot of the readily leached components
(potassium in particular) will probably already be on the decline.

As long as you're willing to work with its light weight and light color
there's probably no problem using pumice as a substrate, but don't count
on its chemical composition being of much value to your plants.

Roger Miller