When rain floods pots in open frames, I depend on capillary action to draw the excess water into the sand below, and on into the ground. When the weather is dry and plants are in growth, I depend on moisture rising, from the sand, (capillary action) to maintain the moisture levels in the pots. This requires an open textured compost to allow fast drainage of excess water so that air is not excluded when it is wet, but fine enough that capillary action is maintained, and can assist the process in both directions. I do not know a lot about small particle physics, but I was not under the impression that clay content was part of this discussion on composts, I have been referring to granular particles. Peter (UK) On 23 January 2015 at 19:46, Tim Eck <teck11@embarqmail.com> wrote: > Typically capillary action is the enemy of drainage. The most extreme > example is bentonite clay formed from extremely fine airborne volcanic ash. > The capillary action is so strong that it is used in geo-fabrics to line > waste sites to prevent water exfiltration. > Tim > > > Normally for my purposes, and in my climate, I want maximum capillary > action > through my pots and into the surrounding sand. > Peter (UK) >