ask-tom-pictureTom –

I keep hearing about biochar and its many benefits. How can I make biochar on my farm?

— Bill in Burnsville

Bill –

I am hearing the same buzz about biochar. Farmer Pat Battle mentioned terra preta probably five years ago in a hallway conversation at the OGS Spring School. Farmer Walter Harrill mentioned it at a CRAFT tour this summer, and local permaculture activist Zev Friedman mentioned his experiments last weekend at the North Asheville Market.

Terra preta are high carbon soils in the tropics dating back to pre-Columbian times. Some archeologists believe that earlier civilizations used charcoal to improve their soil quality in tropical areas where organic matter often leaves soil very quickly, but this charcoal seems to have lasted for centuries. Some scientists believe that charred biomass (biochar) can be used to both build soil quality and to sequester carbon in an innovative way to counter climate change.

At an OGS CRAFT session earlier this summer Walter Harrill described his method of burning last years berry canes in a 55 gallon drum with holes in the bottom. Once the fire was burning actively he smothered it by pulling out supports under the barrel and placing a lid on the barrel. He indicated that he was not completely satisfied with his results and referred us to the work of John Rogers.

John developed a “top lit down draft” (TLUD) method similar to Walter’s with the biocharaddition of a half barrel on top and a stovepipe stack. The sketch at right may help, but in short, he removes the top of several 55 gallon drums, punches holes in the bottom, and fills them with loosely packed fuel. He builds a fire on the top of the fuel and once it is burning, he spreads the embers over the top of the packed fuel and puts on a half drum with stove pipe atop that he calls an “afterburner.” The stovepipe helps draw air in the bottom of the fuel barrel which feeds the fire that gradually burns down through the fuel. Most of the oxygen is consumed by the fire layer so the smoke puts out the fire higher in the barrel, leaving charcoal behind. When the burning layer reaches the bottom, he caps the barrel to put out the fire and then quenches it with water from a hose. While one barrel is burning he starts others so that he can produce batches of biochar on a continuous basis.

In searching around the web I found another family of biochar “reactors” involving one barrel inside another. In this approach fuel between the two barrels triggers pyrolysis in the inner container, leaving charred wood behind. One of the best explanations of that approach is by Peter Hirst. That process is more complicated than John’s but Peter believes that it generates a better product.

A more basic approach is to build a fire and then smother it with soil or dowse it with water. Charcoal makers have operated for centuries around the world, but only recently have we explored charcoal as a way to sequester carbon and to counter the effects of climate change.

So from a broad view biochar looks great. It:

  • Disposes of fruit trimmings or other farm biological “waste”
  • Destroys pests and diseases
  • Builds soil carbon
  • Produces few emissions with an efficient burner and
  • Reduces the ill-effects of global warming by sequestering carbon in the soil for centuries.

I am adding a John Rogers’ burner to my list of winter projects so that I will be ready for apple pruning in February. I also plan to attend the OGS Spring Conference where there will be a hands-on workshop on biochar. Be sure to check it out on the schedule, which posts online December 15th.

Let me know if you try it out.


— Tom


Author: Tom Elmore

Tom Elmore is co-owner and operator of Thatchmore Farm in Leicester NC. He has grown certified organic fruits and vegetables for 25 years and serves on the Boards of the NC Greenhouse Vegetable Growers Association and the Organic Growers School.