We only use the 10 x 20 trays for our breeding plants, but if we had the bench space, I think we would grow most of our Phrags that way given the success we've had with the besseae's. In cases where we grow Phrags in pots, we use pots just large enough to contain the root mass of the individual plant.
As far as our Phrag. medium, we've used the same formula for many years. It consists of:
8 parts seedling grade bark (1/8-1/4 inch particle size)
1 part #3 size charcoal approx (1/4-3/8 inch particle size)
1 part small perlite (approx 1/8 inch particle size)
1 part milled Canadian peat moss
For the long-petalled Phrag. species and hybrids, we replace the small perlite with Spong-Rok which has a larger particle size (approx 1/4-3/8 inch). For the Micropetallum species, we add a pinch of very fine dolomite lime to each pot. We don't use Orchiata because it just hasn't worked as well for us as our standard formula. Given the above formulation, I'd expect the mix to be acidic rather than basic, although we haven't done a pour-through pH.
Hope this helps. It works for us under our conditions, but that's not to say there aren't better formulas for particular circumstances. I've seen beautifully grown lady's slippers growing under what I would consider less than ideal conditions, so the plants are somewhat flexible. In the final analysis, what works for you, works for you.
