Hey Christopher...Mycorrhizae are fungi that form a symbiotic relationship with many, many plants. There are three main types: Ericoid mycorrhyzae, endo-, and ectomycorrhizae. Ecologically speaking, a mutualistic relationship means that both parties will benefit. In this case the fungi will get sugars from the plant and the plant will get more nutrients (usually phosphorus) from the fungi. These fungi associate themselves with the plants roots and will essentially increase the surface area that the roots are exposed to the soil. In addition, the fungi have a few tricks up their sleeves to make nutrients more available for absorption. You can inoculate your soil with these fungal spores, but they more or less are found naturally in nature. Many plants cannot thrive without them now. In fact acid rain has a detrimental effect on the growth and infection rate of these mycorrhizae and many scientists feel that trees suffer more from lack of mycorrhizal infection than from the acid rain itself.