Auk, in my experience, the recommended substrate varies wildly, depending on the seller and on the type of tree, I guess.
Some people say "Akadama"! Others show a wide range of mixes, almost all agree on inorganic, all vow to good drainage.
You see, I have been buying "bonsai soil" from various sources to test, and they are never the same.
Thanks for your answers, I won't fret about PH so much anymore!