I saw a video of a guy using this symbiotic fungus powder on legume root balls that he placed in a green manuring bed. Is there a point to using this stuff in general? Right now I'm a few days away from planting black/raspberry bushes from pot to 50/50 potting/native soil, in the ground. Should I put some of this stuff on the roots?
Edit: Okay, found some plant lists:
I have never heard of inoculating mycorrhizal fungi as that is pretty much present everywhere. Legumes however, have symbiotic relationships with nitrogen fixing bacteria. Once present in the soil, new crops will take them up and form root nodules to house them. Getting the correct inoculant for your legume ( each legume species has a specific bacteria species it pairs with ) is a good idea if you haven't done so before and your soil doesn't naturally contain the correct bacteria already.
I have had good results with green soybeans ( edemame ). My first crop had no root nodules, and so I inoculated the second crop and have had good nodulation and better yield since. Pull up a plant and look at the roots and if there are no nodules ( largish knot or ball like structures ) in the roots, then inoculating may help.
Of course, if there is already plenty of nitrogen in the soil ( because you are adding fertilizer ), inoculating probably won't make any difference. I have read studies that say the plants only cultivate the bacteria when they need some more nitrogen than they are getting from the soil.
Note that black/raspberry bushes are not legumes.
Mycorrhizae fungus do provide benefits to plants. They colonize on the root hairs of plants and effectively increase the root mass aiding in nutrient absorption.
As @psusi mentioned Mycorrhizae are generally present in most soils. In addition I read that plants secrete organic acids (Dr. Linda Chalker-Scott) to attract the mycorrhizae when they need it. They don't need to when the soil is nutrient-rich. Phosphorous seems to be a big determinant of whether or not mycorrhizae will colonize on the plant's roots.
I think the addition of mycorrhizae is overhyped and aggressively marketed but there are instances where it can be beneficial. In soil that has been previously managed with synthetic chemicals there is little biological activity and innoculating the soil with different beneficial bacteria and fungi seemed to work well for me.
A few years ago I did some digging into mycorrhizae when I was reviewing an organic starter fertilizer on my website. I used it a couple of times before overseeding. Good fertilizer and I like the other microorganisms they add but I don't think the mycorrhizae is a big deal considering the fertilizer is high in phosphorous.
If you have good healthy soil with good microbiological activity and don't use fungicides that will kill the beneficial fungi in your soil you probably don't need to add mycorrhizae to your plantings. If you're still cultivating your soil ecology, inoculating with various microorganisms might speed things up but adding organic matter (compost) along with the microorganisms in it is more important for sustainable results.
These presentation slides from Dr. Curtis Swift of the Colorodo State University Cooperative Extension titled Micorrhyza and its uses: Fact or Fiction is a good read with lots of references to studies.
