Hi, Mike said: Metric: 118 ml of sand retained 28 grams of water, or .24g of water per ml of sand. To convert his results to a dimensionless ratio, multiply them by 0.24. When I did similar tests I got (as ratios). sand 0.28 vermiculite 0.48 perlite 0.28 peat 0.58 I tried to measure the amount of water retained and the amount of air after the pot drained. I measured how much water I could get into a fixed volume of the medium, then how much drained over 24 hours. Perlite showed the greatest air content. This is a good page about kitty litter and other substances which can be repurposed: http://bonsai4me.com/Basics/Basicscatlitter.htm/ -- David Pilling email: david@pilling.demon.co.uk web: http://www.davidpilling.net/