Apparently plant roots are not very good at taking up water and nutrients on their own.  They actually form a symbiotic (mutually beneficial) relationship with these fungi where the fungi help the plant to absorb water and mineral nutrients and in exchange receive sugars from the plant. I haven’t tried the powed (known as an innoculant) but I am going to now after researching this answer!    You can buy at any garden center or just google “buy mycorrhizal inoculant” for loads of places on the web to buy.