On Fri, 20 Dec 2002 20:41:47 +0000, Neil Trotter
wrote:

Anyone reading the subject who *didn't* see the above programme would be
forgiven for thinking this a little off-topic, but please read on.


A programme summary can be found at

http://www.bbc.co.uk/science/horizon...eldorado.shtml


and further information (though the site seems a little scrappy) here

http://www.geo.uni-bayreuth.de/bodenkunde/terra_preta/


The programme is based around the remarkable nutritional qualities of
"terra preta" in the Amazon.

A quick excerpt from the above BBC link:

"Bruno Glaser, from the University of Bayreuth, has found that terra
preta is rich in charcoal, incompletely burnt wood. He believes it acts
to hold the nutrients in the soil and sustain its fertility from year to
year. This is the great secret of the early Amazonians: how to nurture
the soil towards lasting productivity. In experimental plots, adding a
combination of charcoal and fertiliser into the rainforest soil boosted
yields by 880% compared with fertiliser alone."

I'm impressed.

Has anyone here experimented with charcoal as a soil additive? Better
still, has anyone managed to get their hands (here or abroad) on some of
this magic stuff?

I too found it very interesting. I (attempt to) grow South African
heathers and proteas etc. outside in the very mild climate in west
Cornwall. When growing them on from seed in pots, I like to add about
10% of crushed charcoal to the very open and free draining compost,
mainly because I saw it recommended to help reduce fungal infections
to which these plants seem very prone. They grow well enough in my
mix, but when I plant then into the soil, heavily enriched with gritty
sand (more grit/sand than soil in reality), they don't do so well and
tend to be chlorotic, despite the soil being acid, and need regular
feeding etc.

The fynbos in South Africa is frequently swept by fire. Indeed, smoke
is a key factor in getting seeds of fynbos plants to germinate. From
what I've seen on our local heather and gorse moors when they are
burnt, although much white ash is produced, there's also a lot of
charcoal from the sticks and twiggier stuff. I assume the same thing
happens on the fynbos. From now on, I'm going to try digging in a
generous amount of charcoal when I plant them.

I've also wondered about the mechanism by which charcoal is supposed
to keep soil in pots 'sweet'. The explanation that most easily comes
to mind is that the charcoal, having a very high surface area, absorbs
the mould and fungi spores. But I'm not entirely convinced, and wonder
if some of the soluble pyrolysis products in the charcoal don't have a
disinfectant action.

What I didn't understand was the type of Terra Preta that seemed to be
able to renew itself after most of it was dug away. This couldn't have
been due to charcoal, as no fresh material was added, apparently. My
impression was that it was simply due to the decomposition of organic
matter that accumulated on the surface over the years, i.e. leaf-mould
or compost in our terms. In which case, was it that unusual?