Bamboo Biochar

We made biochar on the weekend. About 1,000 litres of it. Thirty years ago we planted four rhizomes of a giant, clumping bamboo. Back then deforestation was the hot issue. We were thinking of it as a windbreak and as a sustainable building material in place of logging. And it is that. (The downside is that it rots very quickly in the ground but with some skill you can work with it in ways that prevent it being eaten by things that like wood.)

But over time the clumps have become large and unruly and it’s time to clean them up. And luckily, bamboo makes great biochar.

I’m always skeptical about the newest magical thing in the garden. Over the years I’ve seen so many of them come and go. But there’s some impressive science behind the idea that biochar, and especially bamboo biochar, might be a cheap, fast, effective way to remove huge amounts of carbon from the atmosphere and add it to the soil. And the reason it’s so effective is that it’s a no cost, big benefit method, and carrots are so much more pursuasive than sticks.

Biochar does such good things for soil quality. It improves water holding capacity, reduces leaching, adds carbon, stops compaction, but most importantly, biochar has a unique kind of texture with a truly enormous porous surface area. The combination is a perfect home and breeding ground for micro-organisms. Soil life. The basis of all successful farming and gardening.

And bamboo biochar is even lighter and more porous than hard-wood biochar.

Biochar is made by burning organic material in the absence of oxygen. So not really burning, more like kiln baking. The process is called pyrolysis and the result is a kind of charcoal that has a crystalline structure, almost like biscuit fired pottery. The trick to making it is to get the bamboo up to a temperature over 500 ^oC without turning it to ash. There are lots of Youtube videos of various methods. Ours is simple, low cost, low tech.

We used 205 litre steel drums with lids and clamp rings, bought cheap second hand. We laid them on their side and lit a kindling fire, then, when that was well alight, stood them up and started adding bamboo, cut into lengths that would fit horiontally in the barrel. The bamboo we used was dead standing culms or culms that were felled last year. As each layer caught alight, we added another, aiming to fit them closely. Every so often we would a use piece of green bamboo to smash down the partially charred bamboo pieces, and then use multigrips to grab the side of the barrel and shake vigorously to settle the layers. As the barrels filled, the fire was burning not wood but gases released from the underneath layers, with minimal smoke. This is a kind of el cheapo variety of flame curtain kiln. (The best explanation I found of the principles of flame curtain kilns is in this academic journal article).

When the barrels were full and the top layer alight, we put the lids on and clamped them down. They smoked heavily for a few minutes then the fire went out. Then it was just curbing my impatience to see how the burn had gone long enough to let it completely cool down overnight, before opening the lids and tipping the bins onto a tarp.

The result was about five wheelie bins full all up, which is probably not optimum yield but given that the original incentive was to clean up some large and unruly clumps of bamboo, I’m happy with that. Two of us could manage that – roughly 1000 litres – comfortably in a winter day’s work, and with the price of biochar, it makes it a good payday. I’ve made some potting mix with it today and I’ll plant some pots with it, and some with my standard mix to see if it makes a real difference. I’ll let you know how it goes. I think we might do quite a lot of biochar making this winter.