The image shows carbonised oranges, before they will eventually be charged with comfrey liquid. Once charged they will be dug into the soil to slowly release the nutrients to the plant roots. Sounds simple enough but if not charged successfully the carbon can have the opposite effect and pull nutrients out of the ground. It is fair to say that experiments are still ongoing.
The phenomena has been observed in the Amazon - Terra
preta - literally "black soil" inPortuguese, is a type of
very dark, fertile anthropogenic soil found in the Amazon Basin.
Terra preta owes its name to its very high charcoal content, and
was made by adding a mixture of charcoal, bone, and manure to the
otherwise relatively infertile Amazonian soil. It is very stable
and remains in the soil for thousands of years.
Biochar is a name for charcoal when it is used
for particular purposes, especially as a soil amendment. Like all
charcoal, biochar is created by pyrolysis of biomass. Biochar is
under investigation as an approach to carbon sequestration to
produce negative carbon dioxide emissions. Biochar thus has the
potential to help mitigate climate change, via carbon
sequestration. Independently, biochar can increase soil fertility,
increase agricultural productivity and provide protection against
some foliar and soil-borne diseases. Furthermore, biochar reduce
pressure onforests, though the degree to which results offer long
term carbon sequestration in practice has been challenged. Biochar
is a stable solid, rich in carbon and can endure in soil for
thousands of years.
(source - http://en.wikipedia.org/wiki/Biochar)