VOlcanic rock

Jane McGary janemcgary@earthlink.net
Wed, 24 Jul 2002 11:40:43 PDT
Marguerite and Cathy mentioned crushed volcanic rock and scoria.

We use a great deal of these products here in the Pacific Northwest of the
USA (because they're local and cheap, and also because they're great for
plants). The crushed horticultural pumiceis a pale gray or white pumice
and is the kind most people use. A geologist told me that it will break
down quickly (i.e., in 5 years or so) to a claylike substance if combined
with an acid ingredient such as peat. It is nearly neutral in pH and
contributes potassium to the soil, as well as being a superb physical
amendment -- hospitable to fine roots, retains just enough water, and is
angular for good drainage. European and British gardeners can get a similar
product that is mined in Iceland.

I use several cubic yards/meters of this pumice every year, purchasing it
in bags that weigh 60-70 pounds and cost $7 apiece. It is cheaper bought in
bulk but you need a dumptruck and a way to keep the weed seeds out of it
once it is dumped. My bulb potting mix is: 1 part crushed pumice, 1 part
humus, and 2 parts very coarse, sharp river sand. This is suitable for all
the smaller bulbs I grow but would be too lean for, e.g., lilies.

Scoria is a harder type of lava that is usually dark in color -- what we
get here is dark red. It is typically used as a mulch. I can't find any
crushed small enough to use in potting soil, but Loren Russell reports that
he goes to the rock yard and scrapes up the dust and grit that falls to the
bottom of the redrock bins.

Jane McGary
NW Oregon

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