In the Amazon basin of South America, where heavy rains leach soils of nutrients very quickly, the indigenous Indians found that adding charcoal soaked with beneficial liquids prevented this. This soil additive is now known as biochar. Ready-to-use biochar is available commercially, but can be expensive. In this article, Dirk S, a member the Tasmania’s Food Garden Group, explains how he makes biochar himself at no cost.

There are many articles and blog posts about the benefits of biochar. This one isn’t about that, rather about a simple way to make it. First, you make charcoal from woody garden waste, and then you activate the charcoal to make biochar. All you need is enough room to safely light a fire in your backyard and enough garden prunings.

We use any branches larger than about 2cm diameter in our wood heater in the house. The left-over smaller branches pile up over time and get burned off periodically out the back of our garden.

To comply with Tasmanian fire regulations (more stringent for fires more than one cubic metre in size) and to avoid harming any small animals using the pile as a home, I always light a fire next to the pile of branches and twigs. I then keep feeding the fire until the pile is gone.

I used to let the fire burn itself out, but one time I hosed it down earlier for fire safety. I ended up with a pile of fine charcoal, which was a nice surprise.

So how does this work? Charcoal is formed when you burn wood with insufficient oxygen for complete combustion, because the charcoal burns slower than the volatiles. The nice thing about the method I am describing here is that the low oxygen conditions are made simply by continually adding fuel. There are many other ways of making charcoal for biochar, but they involve more equipment or effort.

Here is a step-by-step guide.

• Collect a large pile of combustible garden waste. Branches, twigs, bark all work great. You can of course burn rubbish wood, but then you end up with rusty nails and residues of burnt paint and glue, which I wouldn’t recommend for the veggie garden.
• In a suitable spot with sufficient clearance, on a fire-safe day and with the garden hose at the ready just in case, light a fire. Once established, start feeding.
• You can minimise smoke by not loading the fire with too much fuel at once. Wait for the flames to burn through before putting more fuel on.
• At some point, my fire gets a bit messy, with long branches sticking out on all sides. I let them burn in half and then I pick them up with a garden fork and put them on top of the fire.
• Once my pile is gone, I keep moving any unburnt fuel from the edge of the fire to the middle.
• Then it takes another half hour or so before all the fuel is reduced to charcoal. At this point there are no more flames and little smoke. This is the moment we’ve waited for: the fire is now burning charcoal and it is time to hose it down,
• Turn on the hose, stand on the upwind side of the fire and from a safe distance start putting water on the upwind side of the fire. There will be a lot of steam and hissing! You should see that the white ash covering the fire is only a thin layer, under which there is the charcoal we are after.

• Putting the fire out completely takes a lot of water so don’t stop hosing too soon. When there is no more steam coming off, you are getting close.
• With a garden fork or rake, flatten out the pile. If it looks dry or hot, more hosing! If the fire isn’t properly extinguished, there is a fire hazard, and you will lose at least some of your charcoal.
• Come back a few days later and collect the charcoal. It will be definitely cool by now and probably a bit dryer and lighter.

A nice benefit from using small branches is that the charcoal is also small. No need for crushing!

Sometimes I find I overdo the low oxygen by either feeding too fast or hosing down too soon, resulting in partially burnt material mixed in with the charcoal. This isn’t really a problem as it will simply rot away over time in the garden. But I get better with practice.

So now you have nice, fine charcoal, ready to make into biochar.

Biochar is great for your garden soil. I soak the charcoal in a Seasol solution and then put it through the compost heap with manure. The internet has other suggestions on how to activate (charge) it before garden application.

There is a lot written about biochar on the internet. As always, some of it is best served with a (liberal) grain of salt. The claim that biochar has to be made in a modern pyrolysis system is at odds with the terra preta (black soil) found in the Amazon. One article suggests that hotter is better. While I haven’t measured, I am pretty sure that the technique described above burns much hotter than traditional charcoal making methods. And even if backyard biochar perhaps isn’t quite as effective as manufactured biochar on a per-handful basis, that doesn’t matter. This stuff is free and easy to make lots of.

There is a lot of attention on carbon in the atmosphere at the moment. So how does biochar fit in with that? Trees and shrubs take CO2 from the air to grow, and eventually that carbon is returned to the atmosphere when the tree dies or prunings are composted or go to landfill; this is the natural carbon cycle. When you make biochar you are removing a little carbon from the carbon cycle, or at least slowing its return to the atmosphere down a lot, which is a good thing.

Max Bahrfeldt started The Food Garden Group a decade ago after retiring from many years of full-time teaching, managing and designing courses for adult education. The aim of the group is to create an active community of Tasmanian food gardeners, who freely share their knowledge, surplus produce, seeds and plants. Meeting other food gardeners face to face is an important part of the group, and through this many food gardeners have found new friends with like-minded interests. Members include beginners, experienced food gardeners, and some horticultural professionals. This article first appeared on The Food Garden Group blog (foodgardengroup.blogspot.com).