Soaked some decomposing oak leaves for 36 hours in preboiled water with some yeast flakes. Stirred, put 1-2 ml into test tubes with 5 ml of malt beer ("Karamalz", I just picked a random brand), filled up with equal amounts of 5 % vinegar and whisky (didn’t have any vodka at hand). Put a thin layer of neutral vegetable oil on top.
Should keep out the vast majority of bad microbes and provide a mostly oxygen free environment for yeast.
IIRC, I’ve also read on suigeneris brewing that oak leaf litter has a 50 - 80 % chance of catching bretanomyce, which I’d be thrilled to find.
I did this a few years ago. Dissolved some spray malt in hot water with some hop pellets. Placed in a jar and covered with muslin fabric and a rubber band. Left overnight. Brought them in and put an airlock on. Few days a yeast cake started forming
The hops already help prevent some nasties and boiling the water first helps clear out anything that may have been in there.
I'll probably do that as well at some point. In my research though, I've come across the suspicion that hops would hinder the growth of Brettanomyces, so I deliberately left them out here. For whom it may concern, here is one of my sources: https://www.youtube.com/watch?v=G2nhUM5PIrg
I don't know this, but it's interesting. I guess it depends how wild you want to go with your yeast too :)