“If you could continually turn a lot of organic material into biochar, you could, over time, reverse the history of the last two hundred years…We can, literally, start sucking some of the carbon that our predecessors have poured into the atmosphere down through our weeds and stalks and stick it back in the ground. We can run the movie backward. We can unmine some of the coal, undrill some of the oil. We can take at least pieces of the Earth and – this is something we haven’t done for quite a while – leave them Better Than We Found Them.”

Bill McKibben, author, climate activist and founder of 350.org

What is Biochar?

Ancient Amazonian indigenous people created areas of incredible soil thousands of years ago. This terra preta, or 'dark earth', is significantly more fertile, productive and structural than the soil from surrounding areas which are often infertile and prone to erosion like most tropical soils. The terra preta was created, whether by accident or design, through charring organic matter and adding it to the soil over centuries, and it's benefits and carbon holding is still there today.  How it was created is still a bit of an unknown, maybe ritual charring, or from pottery making - it was not from people high on the local hooch cutting all the trees down and setting fire to it and having a flash in the pan party - a low oxygen, charring technique is needed to get the good stuff. What we do know is it allowed them to settle in the area rather than having to keep moving as the soil is exhausted elsewhere. It would appear that they did have to stop using the technique when everyone had to run around and stay on the move once some conquistadors appeared on the scene though. hmm.

Biochar carbon sequestration is the modern fancy pants term for this ancient practice. It is basically charcoal - a very stable form of carbon, that’s made by burning biomass in a specific low oxygen pyrolysis process. As the organic material burns it releases very few fumes and is converted into a stable locked away form of carbon. Bury that carbon and it is locked away back in the earth for centuries, and even perhaps for thousands of years.

Benefits of biochar 

A lump of biochar is around 60-90% carbon depending on the feedstock, highly porous with large surface area. This also allows it to hold some other bodacious properties. It is creates a perfect habitat for beneficial soil microbes. It aerates soil whilst simultaneously being a water and nutrient buffer.
It can also sequester the equivalent of 360% it's own weight in carbon dioxide emissions. (It's a bit more complicated than this, depending on the exact carbon %, and how long it is assumed to last in the soil for - the ICC rates this as at least a hundred years to be on the safe side, but a number of research papers indicate it could possibly last for thousands of years)
For an excellent in depth paper on the subject please see
Lehmann, J.; John Gaunt; Marco Rondon (2006) “Bio-char Sequestration in Terrestrial Ecosystems” Mitigation and Adaptation Strategies for global Change, 11: 403-427.

How much biochar should you add to your soil?

You only need a small amount of biochar for the beneficial properties to the soil, but a larger amount locks up more carbon. Too much though and it hoovers up all available nitrogen for quite some time. We found that 8% biochar was a real good balance between locking that carbon down and keeping the benefits to the soil structure and nutrient buffering.

Using it in our compost mix at Organic Plant Nursery means that there is extra aeration (important as our plants aren't mollycuddled inside a climate controlled polytunnel), more nutrient buffering (less leaching of fertilisers = don't need synthetic fertiliser topups) and more water retention (so less wasted water)

How much carbon does biochar sequester?

That 8% of biochar added to the compost means every 2 litre sized plant  contains 160 ml - around 80g of carbon. So if you plant that rootball in your garden you'll be locking the equivalent of 288g of carbon dioxide in the soil for yours, your children's and your children's childrens' lifetimes. Hot stuff! (pun intended) and then if you're planting perennial grasses, you're also sequestering more carbon over 10 years than planting the same space with trees!