nice oak tree si !
what's your soil mix ?
Hi Art, for this tree I used mostly a commercial brand cactus mix, which listed a content of aged fir barks, redwood compost, Canadian sphagnum peat moss, sand, lava rocks, and pumice. And then I mix in a little bit more lava rocks, akadama, and turface. I don't have a specific recipe, I just throw in whatever is at hand. I could have used 100% cactus mix too. A component of decomposed organic matter is good for oaks. My oaks are very healthy. A 100% inorganic mix would dry out too fast in my area.
Not all commercial brands of cactus mix are good, so I can't recommend it in general. I am lucky to have a local nursery that carry a good brand of cactus mix (E.B. Stone mix for cactus, palm and succulents). It's cheap! It has really tiny composted barks in it, and small sized lava rocks and minimal pumice and sand. I use it straight out of the bag for most of my trees and bonsai. The secret is to keep the cactus mix moist
before you use it. Don't use it if it is too dry, because you can never get the bark moist again once it is in your bonsai pot.