Here is a picture of a tree I started repotting, then realized what was going on with the mychorazzae:
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This tree had been half bare rooted the previous year. That's the white half. The soil was Boon Mix, and the mychorazzae exploded!
The dark half was the old soul. There's some mychorazzae in there, but nothing like the bottom half!
Anyway, I removed the old dark soil and replaced with Boon Mix and put it in a bonsai pot.
Boon doesn't add mychorazzae to his trees. The spores are in the air. If the conditions are right, it will grow.