I’ve noticed that my seedlings usually die off from mildew when I use potting soil witout mixing it with regular garden soil. I’ve always wondered why. One would assume that sterile potting mix would be better than regular earth that isn’t sterile. But I also notice that when the weather warms up, the pots with regular soil will become hardened bricks if I don’t water them constant. This is how one very nice and very fragrant Golden Angel X Geranium Red died. I missed just one day of watering.
I think this year I will buy an irrigation system and continue mixing garden and potting soil. It’s much more cheaper like that, and seedlings don’t die off as quickly.
I use a mixture of vermiculite, perlite with “garden mix” (which has a lot of bark and manure in it too for my outdoor pots. Pure potting soil or pure garden earth is not a good idea due to various issues. The benfit of doing it this way is that it is cheaper because the bulk of it is “garden mix.” If I had a large pick-up Id buy it in landscape bulk that usually has some sand and mushroom compost mixed in, too.
I suppose that it is possible that beneficial microrhizae may be the difference. I actually use lots of peat in my soil for its purported antibiotic benefits. That said, my seedlings do get a good dose of composted organic trash pretty early on when I first pot them on, including composted chicken and cow manure, alfalfa meal, fish, bone and blood meal and other good crap, so I have little doubt that they have microbes all over the place. They seem to thrive on it.