GregMartin:

There is also NAPA Floor Dry, Part #8822 that is calcined clay. It is much like turface, but cheaper.

I've used it in a soilless mix along with pine bark in a 4-5 part pine bark, to 1 part floor dry and 1 part perlite.

Thanks for the tip...a quick online search shows that it even survives multiple freeze/thaw cycles (at least the batch that experimenter tried), so it could likely be used outdoors instead of Turface as well. I'll have to buy a bag and see how it does.

Just curious, what is the role of the bark? Can the other ingredients not hold enough water?

Sorry, I had the ratio wrong. It is 5 pine bark : 1 peat : 1 : floor dry

I used the floor dry instead of perlite.

The following thread on GW may answer your questions.

http://forums2.gardenweb.com/forums/load/contain/msg0212444023053.html

Tonight I bought some NAPA #8822 and it is printed on the bag that it's diatomite (diatomateous earth) that has been calcined. The bag is 24 liters and weighs just under 20 lbs...very light. I weighed a liter dry and it came in at 386 grams. I then wet it out and let it drain out for 20 minutes (should probably have waited longer, but I was impatient and it's pretty course) and it weighed 831 grams...445 grams of water held in a liter...that's 44.5% of the volume in water...wow! I'll have to let it drain overnight and see how it does. I'll also have to get a look at it under the microscope. It does make sense that calcined DE should be able to absorb a lot of water...and absobing a lot of liquid is exactly what they made it for. The particles are also pretty tough. Looking forward to throwing this into the testing. Thanks.
By the way, it cost me $8.42 for that 6 gallon bag...not too bad.
I'm guessing it has near zero CEC, very unlike the Turface, we'll see if that makes any difference.

After the 8822 sat draining overnight (as a 1 liter, 9" collumn sitting on a stainless sieve) it lost another 10 grams of water....so it ended up holding 435 grams of water per liter. Visually you can tell that the 8822 has lots of air between it's gritty structure.