Here are the transparent 37 litre containers with an inch of unmilled biochar in the base. These will be watered in so they can slowly release water to maintain some humidity in the closed containers while the seeds are sprouting and seedlings grow.

How to mill small batches of biochar eg. for potting mix? I tried rolling a barrel over the biochar but failed to get a fine enough powder. I then tried using the top of a heavy duty sledgehammer and pounded the biochar with a vertical pounding movement. It worked great!