The trick is to inoculate the
roots, this is where mycorrhizal activity is most beneficial. We usually inoculate when we are adding
trees to the orchard, putting in pollinator attractor plants and when we make a new vegetable
garden bed.
If we have to dig an area, as in prepping a new vegetable bed (our
land is chock full of rocks), then we inoculate once all the disturbance is over since we most likely destroyed the living habitat of the hyphae that were there.
Other than these instances, we don't do any disturbing of the soil strata when we plant, if we find hyphae around the planting hole then we don't add inoculant since the living hyphae will come in and attach to the new roots from the area surrounding the new plant.
Mycorrhizal fungi seem to do a great job of spreading as long as you aren't disturbing their habitat. Our testing shows that we can expect a spread rate of 2 feet per month of leaving the soil undisturbed.
Our hog pastures are only disturbed by the hogs rooting and I have noticed that the hyphae come back rather quickly into these rooted up areas once the disturbance is stopped. (we rotate our hogs so the pasture they were on can recover between grazing periods)