First, you aren’t going to sterilize anything with isopropyl alcohol. All you are doing is sanitizing. Second, the whole step isn’t even necessary if your spawn is clean.
what's the yield on these and is it better to do 2 layers of colonized grain rather than just the one rather than use another tub or do you recommend the 2 layer technique?
I like the way I did it here works great for the small tubs. I do a 1 part spawn to 2 parts substrate. So 1 650g coir brick and 8 cups vermiculite I can do 5 tubs. When it comes to yield that's where genetics come to play.
