Rocks and stuff

Pat Colville
Sun, 28 Jul 2002 03:25:03 PDT
Decomposed Granite-----If you live in an area where the rocks in the
mountains are granitic, the chances are that the sand you buy is just
finer grades of the local decomposed granite. This is certainly the case
here in Southern California. It is unlikely we will find a pure silica

I use a rather coarse ungraded sand in my potting mixes that I get in Home
Depot when I can find it. Its composition is around 70% feldspar minerals,
25% quartz and 5% misc, which is what we get when granite decomposes. The
feldspars will further decompose but that takes a very long time. Acids in
the soil or mix help this process. Potassium and some trace elements are
released in this process and clay may form. The type of clay will depend on
the environment. Around here we get some kaolin but out in the desert I
notice that clays like bentonite are more prevalent. 

It is always best to find nearby sources of heavy material like rock. I have
recently been looking for high grade silica sand sources in Northern
California for another purpose and have found very few. The construction
sands there are mostly granitic. For instance, I have an analysis from RMC
Pacific Sales, Pleasanton,( one big supplier) that is around 40% feldspar.
The California Geological survey recently reported a new mine of crushed
granitic construction materials was permitted near Lincoln, CA so a new
source may soon be available. Unfortunately these guys do not market to
people like us. They are interested in tons per day sales 

Pumice---I have been using pumice, especially when filling large pots. The
pumice we have available here comes from volcanic deposits in the Eastern
California desert and in Nevada. I have tested it and found it is mainly
just foamed glass. There is a very small amount of crystalline material
present. I haven't been paying attention to it breaking down but will check
some pots in another year or so for clay. Of course I could use some clay
in my soil (decomposed sandstone, siltstone) so maybe I should be adding
some pumice there too.

Glassy, foamed volcanic rock tends to break down fast in most environments
because, as Paul says, it has all that surface area exposed to the weather.
I recall seeing lava flows in Hawaii only a few years old with plants
growing on them already. That is what a tough environment does to a weak

Just thinking about this stuff is causing a guilt attack over being so far
behind in repotting.

Pat Colville

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