I'm in Southern Arizona in the U.S. and I'm going to be trying equal parts bark, pumice and lava rock on my trees this year. I've been using a mix of lava, gravel, and potting soil, but the potting soil is settling at the bottom of the pot, and the gravel doesn't really add anything good to the mix.
I think that by going with the new mix, I'll have the bark for water retention and CEC, the pumice for water retention, and the lava rock for drainage and air. I'll be adding some moss on top to help keep everything from drying out too quickly as well.
Hoping to put one of my junipers in this mix in the next few months, to get it out of the muck it's in. My water is also really hard, and I'm hoping that the slightly acidic bark will help.