I use Pro Mix with MycoRise, which is a mycorrhizal fungi so my seedlings are grown with it. But it takes about 2 weeks for them to be activated so it doesn't start to interact with the roots until then. The fungi feed on organic matter and a lot of potting soils don't have a lot, Pro Mix has almost none, so I like to give them a mild feeding at around 2 weeks. I also add some Diamond Nectar humic acid which also helps activate and feed them.