Heres a basic explination of mycorrhizae. There are two basic types of mycorrhizae, ectotrophic and endotrophic... Ectotrophic only penetrate intercellular spaces while endotrophic penetrates cortical cells of the roots.

It releases certain chemicals that increase the availability of both inorganic and organic phosphorus, because plants with the fungi need less phosphate, fert applications can be reduced. It also widens the areas of new roots hairs or older "not much used roots". they improve plant health, which lower pest problems and reduce applications of various pesticides, increases uptake of water and nutrients and also protect roots from pathogens... increase water retention, and releases nutrients of organic matter by ensymic means, trapping nutrients, and in death provides organic matter.

These fungi are common in 97% of all plants in their native environment thus going out and digging up some of your forest soils will be enough to do the job. No need to go out and buy this stuff, unless your using ALL organic mix, which excludes perlite and vermaculite which are both minerals soils...

I am leaning twords the use of sand silt and clay soil with about 1/3 to 1/2 organics. Any suggestions?