Decomposed Granite

Jane McGary
Sun, 28 Jul 2002 03:25:03 PDT
No, it isn't. The decomposed granite I've seen, admittedly in the wild,
has more fines and is rougher than the chicken grit, which is crushed
granite. I use fine granite grit as a topdressing on seed pots, and a
larger size as topdressing on pots of alpines. It is heavy and, I think,
pretty nonreactive, not breaking down over the years. I get it in several
sizes from a rock company.

The soil where my brother lives between Monterey and Salinas is known as
decomposed granite, but it has a lot of fines and sets like concrete in
the summer. There are a number of native bulbs there, so obviously they are
happy down under the hard crust.

The traction sand Lisa mentions must be similar to what I get as pit run
sand from our local quarry. It is high up on a mountain river and is very
sharp and coarse -- quite different from the builder's sand sold just 30
miles downriver in Portland. Apparently it doesn't make very good concrete,
but it is beloved by plants.

Some alpine growers wash their sand to remove the fines, but I think this
is a bad idea for most plants, including bulbs, because the available
nutrients are likely to be in the fines. The vigorous growth of plants in
fresh sand here testifies to the presence of nutrients, probably not just
minerals but also organic material from snowmelt runoff. I have a sand and
gravel berm where the species Alstroemerias look every bit as happy as they
do on railroad embankments in Chile.

That was an interesting note by Paul Chapman on the crystalline structure
of granite and pumice. I'll have to look at some pumice under a microscope!

Jane McGary
NW Oregon
Land of volcanic rock

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