I don`t know if we speak about the same thing but I used selfmade Terra preta for some container Citrus. So far it is a complet failure. No growth at all. Charcoal percentage is probably around 30 %. Charcoal were soaked with nutrients for 1/2 year. Probably it is a matter of dosage. At least pH value, nutritions in my specific mix are not suitable for Citrus. No idea if it works at all. It needs further tests.