The What For Lennon and McCartney. Saturdays and sleeping in. Buzz and Woody. Some things just belong together. Like this mix of biochar and frass. Frass is an all-natural fertiliser made from insect manure and bursting with nutrients and good microbes. Biochar is a charcoal-like substance that comes from heating raw materials without oxygen. Can be stirred through new potting mix when repotting, or sprinkled around the root ball to bring life to old mixes. Also great to add to composts to speed up decomposition. Here are a few reasons to add this double act to your Plant Care kit: * You don’t have to be rich but your soil should be. Frass holds about 240 million microbial colony forming units per gram.