Having trialled the addition of Carbon Gold's GroChar - containing mycorrhizal fungi, seaweed and wormcasts - to the root zone of newly planted horse chestnuts, the resulting trees "had deeper-green leaves and less leaf miner damage, so it has plant-protection qualities", he said.
Made by combusting green waste at high temperature in restricted oxygen, biochar increases cation exchange capacity in the soil, while its honeycomb structure provides a large surface area for mycorrhizae, "which is not necessarily advantageous", Percival explained. Ordinary charcoal, by contrast, is produced at lower temperatures and is less pure, still contains volatile chemicals and can be phytotoxic, he explained.
Biochar is already extensively used in glasshouse horticulture, he added. "It's not cheap, but it stays there in the soil. Five per cent by volume seems to be the sweet spot, though we don't know why. There's no magic bullet. It's about enhancing the trees' own defences."