I've been trying to learn as much as I can about mycorrhizae lately and I'm not sure if this matters in the discussion but one thing in the OP is
Mycorrhizal inoculation response disappeared beyond soil P levels of 25 ppm in Acacia nilotica and 30 ppm in Albizzia lebbeck. Levels of soil P greater than 25 ppm suppressed AM fungus colonization in both species. Soil P levels of 30 and 40 ppm caused negative mycorrhizal dependencies (MD) in Acacia nilotica and Albizzia lebbeck respectively."
I swear I ran across an abstract of a study that had something like this in it but I couldn't access the full document. Anyway, another thing I ran across stated that when the available (soluble) phosphorus levels in the soil were high that mycorrhizae won't colonize as aggressively, but this is at the direction of the plant. I guess the plant says "hey I can get the P by myself right now. you can chill."
In this
paper (PDF Link) they state:
With high levels of phosphorus in the soil (above 100 parts per million) in a form that plants can use, the roots don’t need help from the mycorrhizal fungi to take up the phosphorus. When levels are less than about 50 parts per million, the fungi play a key role in improving the plant’s ability to take up phosphorus. Thus, heavy fertilization with phosphorus makes the fungi unnecessary to the plant, so the plant has no advantage in helping to help keep the fungi alive.
If I understand that it's not saying that the myco will just die off, but that the plant will instead not do any of that symbiosis stuff where they work together. But the myco can still keep itself alive anyway.
This article is also interesting but I don't see any sourcing:
Oddly, many people have erroneously concluded that phosphate kills mycorrhizal fungi. In fact, there is no killing effect going on here. Instead, in soils having high available phosphate, the host plant apparently opts to restrict fungal colonization. The fungi and their spores are still alive. They are simply experiencing a higher level of restrictions from the host plant. In fact, there is ample evidence to show that the host plant has significant control over when and where mycorrhizal fungi may enter root tissue. Theoretically, under conditions of high available phosphate, the restrictions are increased. So the levels of soluble phosphate in the soil can affect whether the host plant opens the door to the root widely or narrowly. But the mycorrhizal fungi are not killed by phosphates. This misunderstanding has been promoted by various companies who attempt to blend and sell mycorrhizal products with no real grasp of their biology.
I gave the first one I linked a pass on sourcing because it was published by the University of Alaska so I'll assume their studies are legit.
Anyway just from these two things it sounds like if you feed with salts or any other soluble nutes you're probably not getting much of a benefit from mycorrhizae. Am I understanding this correctly?
One thing I am understanding more lately is that all these myco products tell you to use every so many days or weeks but I believe once the colony has established itself you won't need any further inoculations. That's why it's generally suggested to apply these early like in the seedling/clone stage or right before transplant. There are things you can do to hurt and maybe kill that colony but for annuals like cannabis there will be little you can do to completely kill the colony once it's established outside of sterilizing the soil. However, don't hesitate to feed those microbes with something like a friendly molasses.
This culture that we're making in the dirt beneath the plants is not unlike a sourdough starter. It's a colony of microbes. Those microbes have a finite lifespan but they're multiplying at an incredible pace (with the proper environment) so as many die out, many many more are being produced. Just remember to feed it. But of course if you're a company and you're selling a mycorrhizae product are you going to want people to use more or less of it? It's a perfect product for making money actually. You generally need a certain amount of it in order for it to be effective in a reasonable timeframe so there's a minimum baseline people would use, but using too much isn't going to hurt anything until you reach absurd amounts of 'too much'. So yeah on my label I might say to use it with every watering and maybe even say something fancy like "don't be afraid to sprinkle some as a top dress every x days."
Furthermore, unless you're using a sterile dirt or maybe some soilless medium you are probably going to have a certain amount of mycorrhizae in your dirt when you buy it. I think as an example
pro-mix amends some in to at least one of their products, probably more. So in this case once you transplant something into this particular mix the myco in there will begin colonizing (because they need a plant to begin this process. roots, to be more specific). You shouldn't need to ever amend with myco during the life of this plant.
I know I rambled a lot but trying to figure out how beneficial amending soils with mycorrhizae throughout a grow is required more reading that I readied myself for but enjoyed learning as much so far.
So as a response to the OP's statement that mycorrhizae isn't really beneficial...in some cases it probably isn't very. Like if you're running full salts there will probably be little benefit to a myco amendment, but probably not nothing. For instance, hydro.
I'd love to discuss this more and be told I didn't understand anything correctly. This was more interesting that I thought or I wouldn't have gone down this rabbit hole. I know I don't have a full grasp of this but I will strive to understand more.
