Trees have shown the same pattern — we had a large, older tree go down, one which had quite a bit of fungus growing around it, & the trees planted near the old site did well while new trees on the other side of the yard did not, even with significant, regular watering.

https://www.merlinsheldrake.com/entangled-life

Note that fertilizer applications are not synergistic with mycorrhizal fungi, fertilizer applications typically prevent the benefits of mycorrhizal fungi, plants no longer need to rely on mycorrhizal fungi for nutrients. I have noticed an "explosion" of fertilizer and supposedly growth enhancing products touting their microbial benefits with labels that show long lists of various beneficial microorganisms, but it's mostly just marketing hype.

It takes hardly any artificial fertiliser (10g per sq meter) to eliminate beneficial fungus from the soil, at which point you are basically running an open air hydro system and have to artificially add nutrients and adjust pH. It takes upwards of 3-5 years to start getting proper results from organic farming methods when switching back and basically involves adding years of organic material to kickstart the organic cycle.

It is totally possible to get comparable results to fertiliser based farming when farming organically, you just need to focus on 'growing' the soil, not just providing nutrient to the plants. In my opinion the former is farming, the latter is hydroponic production. Both have their benefits, it's just that one leaves behind barren soil and the other enriches the soil and is part of a natural cycle that leaves the soil exceptionally fertile.

The RHS (Royal Horticultural Society) sells little packets of mycorrhizal powder that you can put into your garden if you feel it is lacking beneficial fungi. Another more natural route is to bury a kilo of cooked white rice near a very healthy tree, where the soil is soft and 'healthy' then retrieve it after a week. It will be mouldy, but with the right type of mould. Mix that into compost, grow tomatoes in that compost, then when they are finished, chop up their roots, mix it into the compost again, add fresh compost from your compost bin to make seed compost. Mix that seed compost into whatever you want to 'infect'. Some people grow just the fungus using sprouted barley and add the mouldy sprouted barley to their compost.

A bit more difficult is growing edible mushrooms along with the plants, for example Boletus edulis, which are symbiotic with oaks and other trees.

The difficulty lies in the waiting period, it can easily take years before any fruiting bodies appear.

I live in a rain forest. Every winter washes the soil clean of the majority of nitrogen that was built up the previous summer. To keep a raised bed alive I have to amend with bulk compost in the fall and spring. Without the fall amendment the soil starves and the spring amendment is less effective. That was until I added biochar(innoculated charcoal) to my beds. The charcoal prevents nutrients from being washed out of the soil when there are no plants replenishing it. In beds amended with biochar I get higher production and lower inputs. Biochar can help reduce inputs on chemical farms too. But, the combination of healthy living soil and biochar, for me, makes gardening easy. I am more concerned with whether a plant has enough sun than anything else these days.