Growing medium
Jane McGary (Tue, 10 Jul 2012 11:39:11 PDT)

Sujit asked,

It keeps the surface of the soil from drying out too fast and keeps
it from forming an impervious layer, which can be a problem if the
mix contains peat and it gets dry. It also keeps the seeds from
washing around when watered overhead. The best sort of grit is
granite, but many people use aquarium gravel, which is more widely
available, and I sometimes use plain pumice if I don't have any
grantie grit on hand.

My preferred seed mix has always been 2 parts coarse, sharp sand, 1
part pumice, and 1 part sphagnum peat, but that type of sand is not
available everywhere, so some other kind of grit could be substituted
rather than use the kind of fine, eroded sand found in the lower
parts of rivers and near beaches. People who can't get pumice usually
use Perlite instead, but it is inferior to pumice in every respect
except weight (a large flat of 4-inch pots with my mixes is pretty heavy).

Jane McGary
Portland, Oregon. USA