[dice]

Burning off everything on a piece of land ("slash and burn")
and leaving it in place or digging it in before planting
is a traditional subsistence farming practice in 3rd world
countries, especially Africa. But they don't do it for the
charcoal, particularly, it is probably the released potassium
that adds fertility to the soil. (The charcoal in the ash may
improve drainage a little.)

Soil like that may be great the first year or two, but it
gets "tired" fast, and the farmers that practice that kind
of agriculture need to move to a new field and do it all
over again every couple of years to maintain productivity,
allowing weeds to take over the old field until they get
back to it again years later.

I've dug into soil where someone had cleaned
out a fireplace a few times and dumped the ashes
there. It did not seem to be popular with the local
earthworms. Mix it into a compost pile instead,
and it does not seem to bother the worms at all.

If I were adding charcoal, I would add it just for
the drainage improvement in a heavy clay soil,
and I would mix it with compost or manure first.

Figure that it adds carbon to the soil, so it may induce
bacteria trying to digest it to consume more nitrogen
from the soil than they would have without it.