An appropriate size bark makes a small measure of nutrients available as the bark breaks down, and makes the soil less expensive on a per volume basis. Both are minor attributes a grower can do w/o.

I think that if you eliminate the bark, you'll need to change the 1:1 ratio of granite:Turface to something like 3:2 or 2:1 in favor of the granite. The reason is, the granite and bark combined make up the largest fraction of the soil, and those components are both larger than Turface. Even screened Turface is small enough to support about an inch of perched water. The Turface will tend to filter in between the grit particles and make for fewer large pores than if there was a suitable size bark in the soil. It's nothing that can't be worked around with a minor adjustment.

Al

If you omit the bark you are basically using the Missouri Gravel Bed formula of pea gravel and calcined clay. They found that somewhere about 30-40% fraction of calcined clay maximizes water retention without impacting aeration. I use 2:1 ratio of pea gravel to turface. I keep all kinds of things in that mix. The succulents seem to like it.

Thanks for the replies. Al, good points, as always. Nil, I had never heard of the MGB but, yes, it seems similar to the granite/Turface, only scaled to nursery production. Interesting stuff!