Screened compost is the bulk of it. I added finely divided clay for water retention, gypsum to add sulpher and calcium which prevents crusting, epsom salts for the magnesium, rock phosphate, ground bone, charcoal and small fragment broken pottery. I'm dosing it now with my fish emulsion and seaweed solution to encourage microbial growth. I also threw in some nitrogen fixing bacteria and fungi mycillium.
There aren't any viable seeds in it because of the heat of composting, so I don't need to sterilize it. I should have enough good bacteria and fungi in it to prevent damping off from becoming a problem.
/johnny