On 08/04/2012 17:47, Bob Hobden wrote:
"stuart noble" wrote ...

, AL_n wrote:
I'm planning to make some self-watering planters, using a bed of water-
absorbent granules (vermuculite would work). I will then cover the
bed of
granules with a synthetic filtering membrane and drill some overflow
holes
just below the top surface of the granule bed. I will then add a filler
tube going down into the bed of granules, before filling the
container with
compost, and planting.

The granules could be corse vermiculite. I wondered if anyone can
suggest
an alternative material that is less expensive? Broken up building
blocks
(the aerated lightweight type) might be one option. Anything
absorbent that
doesn't cantain harmful minerals or chemicals and is non-perishable
might
work.



Try Rockwool. Ridiculously cheap at B&Q. You may need to prevent the
insulation becoming compressed by the soil.


I understand Rockwool comes in two types, water absorbent and the
opposite. I know a chap out Hampshire way that was experimenting with it
many years ago for growing orchids and similar and was using both types
together to provide both air and moisture to the roots.
On thinking about it, that is where a specialist book of mine I've been
looking for went, he borrowed it and moved.

Rockwool holds moisture but doesn't absorb it (if that makes any sense).
A J cloth works in a similar way, as do microfibre cloths etc.