Some people buy fungi to add to soil directly, but all dirt has some in it, and it spreads well in most conditions. With some moisture it can grow in most soils, and forms a relationship with plant roots, helping each other absorb and move nutrients. Sometimes I add a little soil from my previous plants to my new pots, to ensure some live bacteria and fungi for the new plants.
You are right, I will have to worry about depleted nutrients toward the end of the plants lives. Thank you for the warning!