My only experience with "decomposed granite" was in my brother's garden
near Monterey, California. There it's used for surfacing paths. As
available there, it contains a lot of fines and sets up like concrete
when dry, but will soften when wet. However, "granite grit," which is
available here in sacks and is used for various things such as
topdressing pots and (I think) as turkey grit, does not contain fines.
So David in California should be careful to use a "washed" product, not
one with fines. On the other hand, I prefer unwashed pumice for the grit
component of my potting mixes, because the fines provide some nutrients,
and it doesn't set hard even when used without other materials, such as
for rooting cuttings. I think only washed pumice is available in small
sacks, and only one supplier even here offers the unwashed product. I'm
told the pH of this material is near neutral. Another good sort of grit
is quarter-ten crushed rock, but the resulting pots are very heavy.
There is a very detailed discussion of soil mixes ("composts" in British
usage), by Louise Parsons, in the book "Rock Garden Design and
Construction," which I compiled for NARGS some years ago. It's probably
available used.
Jane McGary, Portland, Oregon, USA
On 10/21/2020 8:41 AM, Bob Hoel via pbs wrote:
> Another term for decomposed granite that is used here in the midwest is rotted granite. I will use it in my soil mixes to provide good drainage.
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