I just use regular native soil which has quite a lot of gravel worked into it. On a few trees I have mixed this up with used bonsai soil but by and large it’s native soil. I am just making it up as I go along and experimenting, I have no real idea what is best!
The only real advantages I can think of in using inorganic soil in the field is it may help develop a better root system prior to collection and it should be definitely easier to transition conifers from field to container. (I am likely to bare root deciduous on collection)
Interestingly in one of the latest soil scence podcasts Ryan said the bonsai with the highest number of micro-organisms in them are the ones most recently collected which still have some field soil in the container. He didn’t say why this was and I think it would be difficult to do explain this with evidence. It seems logical to me that micro-organisms would prefer organic soils and native soil versus inorganic and this may have benefits on tree growth. I suspect you would have to fertilise the trees in inorganic soils as well, I haven’t felt the need to fertilise any of my trees yet and year 2 growth is far more than year 2.
This winter I am going to plant some bare root Hornbeam and Trident maples I will try and do an experiment to see which grows best between soil types.
If anyone of you guys have room and two of the same species why not experiment as well?