Water retained by various soil ingredients

Hannon othonna@gmail.com
Wed, 24 Oct 2012 13:50:07 PDT
The pumice available here, mined in eastern CA and in NV, provides good
surfaces for roots and water to adhere to but water and roots do not pass
through the particles-- the "holes" in the pumice are locked inside the
individual pieces. This material has been a staple for cactus industry for
years, but it should be noted that most wholesale production of cacti &
succulent utilizes only a mix of organics like SuperSoil and fine perlite.
There is often a sharp difference between successful approaches to nursery
production versus methods of maintaining plants longer term by hobbyists.

Since it is a natural product, what is sold as the same item (pumice) can
vary from very clean and sharp-edged particles to soft-edged rounded
particles with more dust-like fines included. I agree with Jane that the
fines are helpful to the roots, in part by coating them and protecting them
with a 'powder coating', especially when plants are dormant. Fines in
general, especially organic ones, help with cation exchange (places where
nutrients can be stored in the soil). It is unfortunate that pumice is so
difficult or expensive for growers to obtain in practical units regardless
of where they live.

Scoria sounds like what is also known as cinder. This is volcanic material
that is crushed in huge quantities to make cinder blocks and this seems to
be its nearly exclusive use. A fine grade of cinder (black or red) with
pieces 1-2mm is very clean and sharp and one of the best materials I have
found to start seedlings of cacti, bulbs, etc. Again, with horticultural
demand so low it is no wonder it is difficult to find a source for it.

Dylan Hannon

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