On 04/07/2012 09:16 PM, 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.

Yes, you can use cat litter if you find the right kind. What used to be
sold as "Tesco's Premium", I think it's now "Tesco Dust Free", is
produced as a horticultural additive, but sold for many times the price.

I use a cheap cat litter made from sepiolite, which works really well.
I have run an experiment over several years and it doesn't seem to break
down at all.

Generally recommended is diatomaceous earth, I think you want to avoid
clumping litters like Fuller's earth.

Here's a link that discusses it in the bonsai context. I don't do
bonsai, and it works just as well.

http://www.bonsai4me.com/Basics/Basicscatlitter.htm

HTH