I've been doing some reading up on Mycorrhiza. mainly because ive seen it as
a product offered at local nursery.

I was skeptical about mycorrhiza in potted plants, so I contacted a forestry
person who researched references. it seems quite a bit of research has been
done. Mycorriza added when repotting is useful to all the conifers and possibly
some of the other forest type trees. It wouldn't hurt to add it routinely to
all your trees, but it is rather expensive for that. For the commercial nursery
trade, it was found that mycorrhiza is actually cost effective because it
reduces the amount of fertilizer needed. And of course its value in
reforestation, and hence growing in the ground, is well known. It is more
useful for juvenile trees, because adult trees gradually develop their own as
they find it in the soil. Many bonsai growers, when they repot an old pine,
save some mycorrhiza (you can easily spot it from the mycelium) like sourdough
for the next tree of the same or similar species to be repotted.

Iris,
Central NY, Zone 5a, Sunset Zone 40
"The trouble with people is not that they don't know but that they know so much
that ain't so."
Josh Billings (Henry Wheeler Shaw), 1818-1885