You can make biochar in a burn barrel. Just watch the smoke. When it thins, pop the lid onto the barrel to slow combustion.
ELAYNE SEARS
Last year, I committed one of the great sins of gardening: I let weeds
go to seed. Cleaning up in fall, I faced down a ton of seed-bearing
foxtail, burdock and crabgrass. Sure, I could compost it hot to steam
the weed seeds to death, but instead I decided to try something
different. I dug a ditch, added the weeds and lots of woody prunings,
and burned it into biochar, thus practicing a “new” soil-building
technique that’s at least 3,000 years old.
What’s biochar? Basically, it’s organic matter that is burned slowly,
with a restricted flow of oxygen, and then the fire is stopped when the
material reaches the charcoal stage. Unlike tiny tidbits of ash, coarse
lumps of charcoal are full of crevices and holes, which help them serve
as life rafts to soil microorganisms. The carbon compounds in charcoal
form loose chemical bonds with soluble plant nutrients so they are not
as readily washed away by rain and irrigation. Biochar alone added to
poor soil has little benefit to plants, but when used in combination
with compost and organic fertilizers, it can dramatically improve plant
growth while helping retain nutrients in the soil.
Amazonian Dark Earths
The
idea of biochar comes from the Amazonian rain forests of Brazil, where a
civilization thrived for 2,000 years, from about 500 B.C. until Spanish
and Portuguese explorers introduced devastating European diseases in
the mid-1500s. Using only their hands, sticks and stone axes, Amazonian
tribes grew cassava, corn and numerous tree fruits in soil made rich
with compost, mulch and smoldered plant matter.
Amazingly, these
“dark earths” persist today as a testament to an ancient soil-building
method you can use in your garden. Scientists disagree on whether the
soils were created on purpose, in order to grow more food, or if they
were an accidental byproduct of the biochar and compost generated in
day-to-day village life along the banks of the Earth’s biggest river.
However they came to be, there is no doubt that Amazonian dark earths
(often called
terra preta) hold plant nutrients, including
nitrogen, phosphorous, calcium and magnesium, much more efficiently than
unimproved soil. Even after 500 years of tropical temperatures and
rainfall that averages 80 inches a year, the dark earths remain
remarkably fertile.
Scientists around the world are working in
labs and field trial plots to better understand how biochar works, and
to unravel the many mysteries of
terra preta. At Cornell University in Ithaca, N.Y., microbiologists have discovered bacteria in
terra preta soils that are similar to strains that are active in hot compost piles. Overall populations of fungi and bacteria are high in
terra preta
soils, too, but the presence of abundant carbon makes the
microorganisms live and reproduce at a slowed pace. The result is a
reduction in the turnover rate of organic matter in the soil, so
composts and other soil-enriching forms of organic matter last longer.
In field trials with corn, rice and many other crops, biochar has
increased productivity by making nutrients already present in the soil
better available to plants. Results are especially dramatic when biochar
is added to good soil that contains ample minerals and plant nutrients.
Research continues (track it at
The International Biochar Initiative),
but at this point it appears that biochar gives both organic matter and
microorganisms in organically enriched soil enhanced staying power.
Digging in nuggets of biochar — or adding them to compost as it is set
aside to cure — can slow the leaching away of nutrients and help
organically enriched soil retain nutrients for decades rather than for a
couple of seasons.
Finding Free Biochar
Biochar’s soil building talents may
change the way you clean your woodstove. In addition to gathering ashes
(and keeping them in a dry metal can until you’re ready to use them as a
phosphorus-rich soil amendment, applied in light dustings), make a
habit of gathering the charred remains of logs. Take them to your
garden, give them a good smack with the back of a shovel and you have
biochar.
If you live close to a campground, you may have access to
an unlimited supply of garden-worthy biochar from the remains of
partially burned campfires. The small fires burned in chimineas often
produce biochar, too, so you may need to look no further than your
neighbor’s deck for a steady supply.
Charcoal briquettes used in
grilling are probably not a good choice. Those designed to light fast
often include paraffin or other hydrocarbon solvents that have no place
in an organic garden. Plain charred weeds, wood or cow pies are better
materials for using this promising soil-building technique based on
ancient gardening wisdom.
How to Make Biochar
To make
biochar right in your gardens, start by digging a trench in a bed. (Use a
fork to loosen the soil in the bottom of the trench and you’ll get the
added benefits of this “double-digging” technique.) Then pile brush into
the trench and light it. You want to have a fire that starts out hot,
but is quickly slowed down by reducing the oxygen supply. The best way
to tell what’s going on in a biochar fire is to watch the smoke. The
white smoke, produced early on, is mostly water vapor. As the smoke
turns yellow, resins and sugars in the material are being burned. When
the smoke thins and turns grayish blue, dampen down the fire by covering
it with about an inch of soil to reduce the air supply, and leave it to
smolder. Then, after the organic matter has smoldered into charcoal
chunks, use water to put out the fire. Another option would be to make
charcoal from wood scraps in metal barrels. (For details, go to
Twin Oaks Forge.)
I’m
part of the Smokey-the-Bear generation, raised on phrases like “learn
not to burn,” so it took me a while to warm up to the idea of using
semi-open burning as a soil-building technique. Unrestrained open
burning releases 95 percent or more of the carbon in the wood, weeds or
whatever else that goes up in smoke. However, low-temperature controlled
burning to create biochar, called pyrolysis, retains much more carbon
(about 50 percent) in the initial burning phase. Carbon release is cut
even more when the biochar becomes part of the soil, where it may reduce
the production of greenhouse gases including methane and nitrous oxide.
This charcoal releases its carbon 10 to 100 times slower than rotting
organic matter. As long as it is done correctly, controlled charring of
weeds, pruned limbs and other hard-to-compost forms of organic matter,
and then using the biochar as a soil or compost amendment, can result in
a zero emission carbon cycling system.
Burning responsibly requires simple common sense. Check with your
local fire department to make sure you have any necessary permits, wait
as long as you must to get damp, windless weather, and monitor the fire
until it’s dead.
The Bigger Picture
If
global warming is to be slowed, we must find ways to reduce the loss of
carbon into the atmosphere. In the dark earths of the Amazon, and in
million-year-old charcoal deposits beneath the Pacific Ocean, charcoal
has proven its ability to bring carbon release almost to a standstill.
If each of one million farmers around the globe incorporated biochar
into 160 acres of land, the amount of carbon locked away in the Earth’s
soil would increase five-fold.
But there’s more. What if you
generate energy by burning a renewable biomass crop (like wood, corn,
peanut hulls, bamboo, willow or whatever), while also producing biochar
that is then stashed away by using it as a soil amendment? (For an
example, see the Archive article,
Mother’s Woodburning Truck,
about wood-gas generators.) The carbon recovery numbers in such a
system make it the only biomass model found thus far that can produce
energy without a net release of carbon. Research teams around the world
are scrambling to work out the details of these elegantly Earth-based
systems.
Much remains to be known about how biochar systems should
tick, but some may be as simple as on-farm set ups that transform
manure and other wastes into nuggets of black carbon that help
fertilizer go farther while holding carbon in the soil.
As
gardeners, it is up to us to find ways to adapt this new knowledge to
the needs of our land. To make the most of my bonfire of weeds, I staged
the burn in a trench dug in my garden, and then used the excavated soil
to smother the fire. A layer of biochar now rests buried in the soil.
Hundreds of years from now, it will still be holding carbon while
energizing the soil food web. This simple melding of soil and fire,
first discovered by ancient people in the Amazon, may be a “new” key to
feeding ourselves while restoring the health of our planet.
To learn more about this fascinating topic, read
Amazonian Dark Earths by Johannes Lehmann. And click
here for more articles on biochar research.
Contributing editor Barbara Pleasant
gardens in southwest Virginia, where she grows vegetables, herbs,
fruits, flowers and a few lucky chickens. Contact Barbara by visiting her website or finding her on Google+.
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