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 assequestering 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 thisbecomes an important part of the retort.

The retort alsorequires achimney.

Chimney pipe in 6-or 8-inch diameter

works well; it'savailable in mosthardware stores.

Avoid roofflashing, 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 toaccommodate the chimney.

We found it helpful to remove the bottom from a metalcoffee can and fix it over the exhaust hole using steel

brackets. This helps to stabilize the bottom of thechimney, which remains removable.

Our primary feedstock to date has been dead and downedhardwood branches, which are abundant on the ranch.

Hardwoods generally contain fewer tars and other chemicalsthat might not be desirable in soil amendments.

Secondarily, we are using some fibrous wastes from ourhorticulture projects, such as the shells of under-sized

birdhouse gourds.

Feedstocks for charring must also be dry. Green or wetmaterials 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.

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.

Fill the smaller bucket with the feedstock. Leave spacebetween the materials, and fill to the top.

Invert the larger barrel over the smaller barrel.

Reaching under the two barrels to press the smaller barrelupwards…

…tip the retort on its side…

…and then invert it. The goal is to invert the barrels, leavingthe 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 fuelwood that was used to fill the smaller barrel. Pack in as

much 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 theouter 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 thebottom 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 weedsthat might be growing near the retort, to reduce fire

hazard.

As the fire nearsthe bottom of the

retort, it appears toburn even faster

and hotter. This isdue to flammable

gases that arecooked out of the

feedstock and leakout of the open

bottom of the innerbarrel. These gases

provide the finalfuel necessary to

ensure that a goodbiochar product

results.

When you judgeit safe, remove

the chimney andthe lid. Becautious of

leftover coalsand extinguish

them with wateror let the retortsit a bit longer.

When safe, invert the retortand remove the outerbarrel. There should bevery little ash remaining inthe 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 andtemporarily 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.