When you say need to replace the soil after 3-4 years how much are you talking about? Bare root to the core? Just around the edges? One side at a time again?
The question relates to trees established in your Boon mix already not transitioning into it.
Nice tree, I like it and what you've done. For some reason I was curious how it would look with left branches trending up and right trending down. So I scratched at it a bit to see and got this far.
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As you know, bare rooting conifirs is a bad idea. So, we Half Bare Root them to ease the transition from whatever they were in to Boon Mix.
As an aside: Boon didn't name his mix "Boon Mix". Others did. He was merely trying to replicate the mix he used as an apprentice in Japan.
But he DID invent the HBR technique. It's not done in Japan. Because they don't have to. Their bonsai are already in something similar to Boon Mix.
When Boon returned to the US after his apprenticeship, he saw that trees didn't fare well if they were repotted into Boon Mix. The roots just wouldn't grow into it. But he knew that trees loved it in Japan. So, as an experiment, he did a half bare root repot, and the trees grew new roots in the new mix like crazy. He's never turned back. (A year or two later, you HBR the other half.)
So, you will never see this in any book or magazine unless it's about Boon, or one of his students.
When Warren Hill was curator of the National collection, 7 bonsai died. Another dozen were very sick. In desperation, they called Boon for help. He found they all had bad organic based soil. He repotted the dozen sick trees in Boon Mix and half bare rooted them. All but one recovered fully.
So... once the tree is in Boon Mix, you don't have to HBR. It's in good soil. But you can. If you are changing into a new pot, or changing the shape of the root ball, or changing the planting angle, or simply you want to refresh the soil, a HBR is a good idea.
I did a HBR on this juniper just because I noticed it was draining slowly. It's been in Boon Mix for decades. Didn't change the pot or anything:
Also, doing the HBR allows you to really get a good look at the nebari, and you can fix problems. I eliminated a couple problem roots during this repotting. (I also removed all the wire and thinned it. Will probably rewire next fall.)