I will be repotting a collected Subalpine Fir fairly soon and am unsure of the appropriate soil mixture for this species (ratio and size range). It is currently potted in what appears to be almost entirely akadama although there are traces of pumic and lava.
I’ve always used the 2:1:1 akadama lava pumice mix. Larger collected tree. 15 years. Also-- use the sphagnum/ moss covering. Do not remove ALL of the original soil at one time. Thirds over three years.