Nearly all soil, except heavily cultivated/disturbed soil, or one treated with fungicides, will contain mycorrhizal fungi. Without mycorrhizae, it's said that plants would have never been able to migrate outside of the ocean.

These mycorrhizae have a symbiotic relationship with plants, where the fungus uses plants sugar and carbon reserves as an energy source while at the same time supplying water and nutrients, such as phosphorus, to the plant. 

This relationship acts almost like a secondary root structure for the plant, as it is able to draw more moisture and nutrients than just by the roots alone!

There are two types of mychorrizae, "Endo" and "Ecto". Ectomychorrizae is primarily suited for larger plants, such as trees and shrubs. Endomycorrhizae, on the other hand, can be found on more than 80% of all plant species, and is the most useful form of the fungus for your annual garden.

You can buy it online or at your nursery, either as pellets you plant with or as a powder you can dilute and water with.