What Is Biochar? Biochar is a stable form of carbon,chemically equivalent to charcoal, that can be used as a soil amendment to increasemoisture, nutrient retention, and habitat for beneficial microorganisms, as well as sequestering carbon in soils.
Biochar can be producedusing several methods.One method suitable forsmall-scale agriculture is thetwo-barrel nested retort.
Building a two-barrel nested retort is quite simple. It takestwo barrels, one larger and one smaller. We used 55- and 30- gallon steel drums.
Be sure to purchase a steel lid for the 55 gal drum, as this becomes an important part of the retort.
The retort also requires a chimney.Chimney pipe in 6-or 8-inch diameter works well; its available in most hardware stores. Avoid roof flashing, as it willmelt in the intense heat.
Adequate airflow from the bottom of the large barrel isessential to a good burning process. To ensure this, air holes are required around the bottom edge.
An exhaust hole must be cut into the steel lid to accommodate the chimney.
We found it helpful to remove the bottom from a metal coffee can and fix it over the exhaust hole using steel brackets. This helps to stabilize the bottom of the chimney, which remains removable.
Our primary feedstock to date has been dead and downed hardwood branches, which are abundant on the ranch.Hardwoods generally contain fewer tars and other chemicals that might not be desirable in soil amendments.
Secondarily, we are using some fibrous wastes from our horticulture projects, such as the shells of under-sized birdhouse gourds.
Feedstocks for charring must also be dry. Green or wet materials will not work in this retort.
Choose a levelarea in the open,with no trees orelectrical linesdirectly above.Keep a gooddistance betweenthe retort andflammablebuildings, haybales, andanything else thatcould easily catchfire. We took theadded precautionof constructing abrick-enclosedsand pad.
Fill the smaller bucket with the feedstock. Leave space between the materials, and fill to the top.Limbs larger than 4” in diameter do not char completely, sotwigs and small branches are best. Pieces between 6’-18” inlength pack more easily into the retort, and make it easier to transfer the finished char afterwards.
Invert the larger barrel over the smaller barrel.
Reaching under the two barrels to press the smaller barrel upwards…
…and then invert it. The goal is to invert the barrels, leaving the rim of the smaller barrel pressed cleanly against the bottom of the large barrel with little or no gap.
Center the inner barrel within the outer barrel.
Fill the gap between the barrels with the same size of fuel wood that was used to fill the smaller barrel. Pack in asmuch fuel as possible, stuffing very small twigs into gaps.
Pack the top ofthe gap and thecharge barrelwith dry grass,straw or similarmaterial. Avoidusing starterfluids, diesel, orother petroleum-based fuels. Startthe fire usingmatches or alighter. A smallhand-heldpropane torch isan easy way toget the fire goingquickly.
Once the fire is successfully burning, place the lid on the outer barrel. Be certain to wear fire-proof gloves and eye protection.Then place the chimney on the lid, and secure it so it doesn’tblow or fall over. We use a few rocks or concrete blocks.
If all is working properly, air will be drawn through the bottom air vents and a clean, very hot fire will begin to develop.
This is a good time to wet or re-wet any grass or weeds that might be growing near the retort, to reduce fire hazard.
As the fire nears the bottom of theretort, it appears to burn even fasterand hotter. This is due to flammable gases that are cooked out of thefeedstock and leak out of the openbottom of the innerbarrel. These gases provide the final fuel necessary toensure that a good biochar product results.
When you judge it safe, removethe chimney and the lid. Be cautious of leftover coals and extinguishthem with water or let the retort sit a bit longer.
When safe, invert the retort and remove the outer barrel. There should be very little ash remaining in the outer barrel. The charge barrel,however, will be full of finished biochar.
Biochar contains measurable but small amounts of plant nutrients. It should not be viewed as a fertilizer, but as a soil amendment - one with the potential to make added fertilizers more stable and efficient.
Added directly to the soil, fresh biochar can act like a sponge and temporarily rob plants of nutrients. To prevent this from happening, we incorporate biochar into our compost first, whereit can absorb both water and nutrients. It is then spread on the soil on the same schedule as our compost applications.