Three questions for future reference. First, are trunk development and root development part of the same step? Second, does air pruning facilitate both? Third, what type of soli is the tree put in when air pruning? Organic/potting soil? Bonsai soil? Or something like perlite?
Trunk and root development can be done at the same time with care. Maximum trunk development occurs with maximum growth (generally in the ground), but this often results in a small number of roots taking off to the detriment of the others. If a good radial root system is developed before planting in the ground you can get both. Grow bags can help contain the long running roots.
Air pruning of the roots occurs in growing containers and will generally lead to a more uniform root development. However, it will slow trunk development since the roots do not run as far.
To maximize growth you want a fairly coarse mix with a fairly large organic component. Nurseries use a fully organic mix (shredded/chipped bark). I like a mix of pumice and bark in the 3 - 9 mm range to maximize growth while giving a roots that are easier to sort out for repotting into a bonsai pot.
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Thank you! That’s very helpful.
Trunk and root development can as Marty says be done together. In the ground is normally best for trunk but without the roots needed for a bonsai pot. My experience with air pruning is trunk development can sometimes outstrip that gained in the ground which has happened again for me this year. I have a number of Japanese larch growing in 3ltr airpots and they have literally raced away gaining almost 3 feet of growth this year. The pots are literally full of roots. The trunks are normally thicker when grown in airpots than they would be if grown in the ground. They are good for both trunk and root development for deciduous but the nodes on any branches are usually quite far apart. I find they are excellent when lifting a tree from the ground in promoting rapid root growth, usually one year in the airpot is enough to grow sufficient roots to repot into a bonsai pot.
Not all trees are suitable or are happy in airpots.