CO-Horts Blog

Friday, November 22, 2013

Small scale biochar by Irene Shonle

Up here in the mountains, our soil is usually loose “sandy” decomposed granite.  It’s not particularly good for holding water or nutrients, and I’m forever recommending that people amend their soils (especially for vegetable gardening)

We also tend to accumulate a lot of slash (pine tree limbs with needles on them) as a result of cutting trees for fire mitigation.  Typically, the slash is considered to be a waste product, unfit for burning in wood stoves or for larger-scale woody biomass utilization.

These two things have led many people to consider biochar as an interesting strategy to not only make use of the “waste” product, but to capture carbon and increase soil fertility and water-holding capacity all at once.  There is even a fact sheet on it from CSU:  There is ongoing research on large-scale forestry/biochar applications or recovery from wildfire ( , but I’m interested in a very local garden scale.
Biochar --
What is biochar? According to the International Biochar Initiative (, biochar production is a” 2,000-year-old practice that converts agricultural waste into a soil enhancer that can hold carbon, boost food security, and discourage deforestation. The process creates a fine-grained, highly porous charcoal that helps soils retain nutrients and water.”  It was originally discovered by examining the unusually fertile soils comprising the ‘dark earth’ in the Amazon (terra preta). The soils are anthropogenic in nature, and are dark because of their very high charcoal content.  They were created by adding a mixture of charcoal, bone, and manure to the otherwise relatively infertile Amazonian soil. It is very stable and remains in the soil for thousands of years.  Modern day enthusiasts claim biochar will do everything from save the world from the looming climate crisis to producing the best vegetables your garden has ever seen!  What’s not to like?

I’ve been poking around , trying to learn a little more about how to make biochar.  For the most part, the theory is pretty straightforward – burn wood in an oxygen-starved manner (much like making charcoal), and then grind the resulting blackened chunks, mix with compost or manure – and add to your garden.

But, of course, the devil is in the details. I have found instructions on everything from digging a trench to much more elaborate commercial units.

The trench method involves  placing wood in a pit, setting it on fire, and then covering it with soil and letting it smolder a while before stopping combustion by drenching.  This method is spelled out here:  This method is also recommended by some  for farmers in developing countries:

While I like the simplicity of this technique, some say that this won’t produce enough of a controlled burn, and that the product will be less high quality.  This camp says you need some sort of kiln. There are many homebrewed versions of creating a biochar kiln (, and  many others) as well as variously priced commercial units ( , or other such units). 

I’m intrigued and may even try it this winter, given that the snow makes it possible to obtain an open burn permit up here, and I definitely have some slash I need to get rid of.  If I try it, I’ll be sure to report back.
Has anyone tried small-scale biochar?  If so, please let me know what you did in the comments!

1 comment:

  1. I've been making biochar below ground using a 2inch steel pipe as an air feed. Imagine a Dakota fire pit where the second hole is replaced using the pipe in the shape of a "J". I use a hairdryer fan, 12 volt H-bridge and an Arduino along with a length of thermocouple wire to control the temperature.

    I set the temperature to 750 - 850F for an automatic controlled burn and this seems to minimize ash while removing the oils.

    I keep a propane torch tip lit and position above the chimney to burn off the smoke that's produced. We're allowed to burn here and while it may seem a pity to loose all that energy at least I'm not smoking up the neighborhood (any more :)

    The difficult part is cutting everything burnable into 3 inch pieces and then there's the task of scooping the carbon out by hand. But it works real good - process about 2 five gallon buckets of tree trimmings per batch.

    I've been mixing the BC with composting worms and bone meal as our soil is decomposed granite and already neutral to slightly alkaline. I'll have to wait for summer 2015 to see how the charging goes.

    Hope to find a good 'charging' recipe soon!