Specialty Gardening: Container Soils - Water Movement and Retention III , 1 by tapla

tapla wrote:
Turface is calcined (fired at a high temperature) clay. The special clay is heated until the tiny particles fuse, so they are like hard little ceramic sponges. After screening, their large size allows water to flow through the particles, leaving large air pores between the particles, with water trapped INSIDE the particles for plants to use. The result is lots of aeration, no perched water, and good water retention.

In the picture, you can see Turface on the left and crushed granite on the right. Pay no attention to the soil on the top. It's a handful of soil from my raised beds & too fine (water-retentive) to be suitable for use in containers.

Very good Timmijo! You definitely 'got it'. The only thing I would add to what you said is that drainage layers don't eliminate PWTs, they only serve to raise them in the pot. E.g., if you have a soil that supports 3" of perched water in a 4" deep pot, after a thorough watering, 75% of the soil will remain completely saturated. If you put that soil on top of a 1" deep 'drainage' layer, it will leave you with the entire soil mass saturated.

Al