It seems to me we shouldn't use any kind of pebbles, etc in the bottom. there has been discussion on AFN recently about exactly this. So I have decided to question Auto Pots about their reasoning. Maybe they have a better answer, maybe they are just plain wrong and are assuming its a good practice!
Here is the email I just sent:
"I'm trying to fathom your reasoning for putting clay pebbles, etc in the bottom of the pot. doing so is contrary to the hydraulic movement of water.Water movement stops at the interface between differing mediums and resists further movement until overcome by gravity. It would seem to be even more of an issue when moving water against gravity.
I've used Modular Hydro's A.I.T ( Air Injection Technology ) unit in a 20 gallon pot or organic soil, with great results. Why not run straight medium ( soil, coco, etc ) in auto pots?"