We know that mycorrhizal association helps the plant to absorb nutrients.
Why can't this association be exploited in agriculture ?
Can't we add spores of the specific fungi to specific plant and enjoy the benefit of this association as we would not have to add many fertilizers ?( We could add organic manure whose main drawback is the time it takes to release nutrients. The fungi could probably solve the problem.)
Mycorrhizal fungi are often used in small-scale agriculture for just that reason. If you search mycorrhizae on amazon they sell it by the tub. I think its especially used in organic gardening for just what you said, the slightly less-processed organic fertilizers are harder to take up by plants without their mycorrhizae partners.
I'm not sure about how widespread the practice of applying mycorrhizae is in large-scale non-organic farming. Judging by how big of a problem fertilizer runoff is, my guess is that a lot of companies have decided that their bottom line is best served by just swamping everything with artificial fertilizers. I'm not sure how mycorrhizae is produced either, it might really be prohibitively expensive to scale production of it up that much.