Sand for potting mixes

Jane McGary
Sat, 15 Nov 2014 14:41:55 PST
I think "silica sand" is recommended in British books to steer people 
away from some other sources that might be bad for plants. I don't 
know what those are. However, ocean beach sand should be avoided. You 
also wouldn't want sand derived from ultramafic ("serpentine") rocks, 
which are toxic to some plants.

The sand I use is a mixture of various minerals, primarily quartz 
along with basalt and other volcanic material. The important thing is 
that the particles be of various sizes, as Bob Nold mentioned, and 
that they be angular rather than rounded. Some sand is loaded with 
organic fines and has to be washed before being used with plants that 
don't tolerate that. The sand I use comes from a quarry on a river 
close to the Cascade range and the fines appear to be more mineral 
than organic. You can improve the consistency of lowland river sand 
by mixing in some kind of angular grit, such as quarter-ten crushed 
rock or ground pumice. If you use crushed rock, it should not have 
any fines in it (i.e., don't use the kind intended for compacting 
into paths, etc.).

Different kinds of sand provide different levels of water retention 
and more or less air space in the mix. Unwashed sand direct from a 
quarry may also contain a significant amount of mineral nutrients 
useful to plants, especially plants that naturally grow in soils low 
in organic content.

Jane McGary
Portland, Oregon, USA
with apologies for mentioning our profligate use of pumice

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