Jim Lewis wrote:

3. The so-called "function" of Mycorrhiza is to make hard-to-use
nutrients more accessible to the plant. For potted trees, we
supply all the nutrients they need, and in an easily useable
form, so there's no real need for Mycorrhiza (though Mycorrhiza
probably are there; Mycorrhiza tend to develop even on
bare-rooted plants).

As I mentioned the last time this came up, your typical balanced
fertilizer isn't made to deal with the nutrient needs of a non-mycorrhizal
plant of a normally mycorrhizal species. With species that rely heavily
on mycorrhizae, nutrient needs vary drastically between mycorrhizal and
non-mycorrhizal plants--a balanced fertilizer will generally work with the
former, but the latter may need up to four times as much phosphorus as
nitrogen.
OTOH, I don't know if mycorrhizal inoculation is beneficial for
bonsai... I haven't heard of anyone testing this, and suspect it would
vary greatly with circumstances. But the presence of mycorrhizae is
clearly beneficial--the question is whether or not an inoculum is a
significant factor in their establishment under normal cultural practices.

Patrick Alexander