permaculture

[Toby Hemenway <toby@patternliteracy.com>]

On Nov 18, 2011, at 11:31 AM, John D'hondt wrote:
> 
> I am well aware of the stability of charcoal but that it serves as living 
> space for microbes is pure nonsense. You can't have it both ways. If it 
> were 
> good for microbes it would not be stable. It is one or the other and you 
> really can't keep your cabbage and eat it at the same time.

Well, you don't eat your house, either, even though it's made of carbon just 
as your cabbage is. They are different substances, with different functions, 
just as biochar and humus are. 
> 
> And I also can't see it as a sink for soil minerals. As a slow sink for 
> mercury yes and organic pesticide molecules probably.

Yes, it is a sink for soil minerals. See, for example, "Black Carbon 
Increases Cation Exchange Capacity in Soils" from the Soil Science of America 
Journal. Abstract is at
http://www.biochar-international.org/node/157 though the article needs a 
subscription to view.

First, here's why biochar can house microbes and not break down. Microbes 
colonize surfaces. They do not always eat the surfaces they colonize. When 
they colonize gravel in reed beds, they don't eat the gravel. (Cred here: I  
ran a microbiology lab at a large biotech company. We used to colonize inert 
surfaces with microbes all the time, like porcelain and carbon fiber. They 
didn't eat it.) 

I'm seeing a fundamental confusion between the roles of humus and biochar 
here that I hope to clear up. Humus feeds plants and microbes in two 
different ways, while biochar feeds them in only one way.  They both house 
microbes. The first way humus feeds is by its ability to hold cations: 
positively charged nutrient ions such as calcium, iron, etc. These cations 
are not part of the humus; they are hydrogen-bonded to the oxygen atoms on 
the humus. The cations are donated to the plant or microbe, and are recharged 
(returned) to the humus by the breakdown of other organic matter. When 
organisms eat cations, the humus remains unchanged.

The other way microbes (not plants) are fed by humus is when the microbes 
break down the humus itself, extracting energy and carbon from the humus. 
This results in degradation and loss of humus. New humus must be formed by a 
constant supply of new organic matter. Humus is always breaking down in this 
way, sometimes slowly if there is ample OM or few microbes, but fast if there 
is a lack of OM. That's part of why OM must be constantly added to soil. 
Biochar lasts far longer than humus in soil where there is microbial feeding. 
Humus is not as good a carbon sink as biochar.

Next,  biochar holds minerals in two ways. They can be directly adsorbed onto 
the surface, and since it has so much surface, it can hold a lot of 
minerals.. Also, although the carbon in it is positively charged, and thus 
won't bond to other +-charged cations, it raises CEC by adsorbing negatively 
charged substances in the soil, which then bond to cation nutrients. This is 
documented. It then releases the bound nutrients to microbes and plants the 
way humus does--and thus doesn't break down, just as humus does not in these 
cases. It also houses microbes but does not itself feed them carbon; the 
carbon has been largely stripped of the type of bond needed to fuel microbes. 
Charcoal alone will not feed microbes or plants. There must be other OM 
present. This is why I am puzzled by this argument. Charcoal and OM work 
together, it is not a case of one or the other. Biochar provides a 
long-lasting carbon storage, unlike humus. And OM provides the food.

I hope not to hear the claim that biochar can't house microbes or donate 
nutrients any more. These are very well proven facts and have a very solid 
mechanism behind them.

I don't see biochar as a silver bullet. Like everything else, if made on an 
industrial scale, it be just as poor a product, and just as damaging, as 
industrial organic food or industrial medicine. But that is the fault of 
industrialization, not biochar.

Toby
http://patternliteracy.com
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