Increase the fertility of your garden for FREE.
Make Biochar ‹ this Ancient Technique Will Improve Your Soil
By Barbara Pleasant
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 ³new² soil-building
technique that¹s at least 3,000 years old.
What¹s biochar? Basically, it¹s 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 ³dark earths² 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
Earth¹s 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
Biochar¹s 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 you¹re 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 neighbor¹s deck for a
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
you¹ll get the added benefits of this ³double-digging² 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 what¹s 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.)
I¹m part of the Smokey-the-Bear generation, raised on phrases like
³learn not to burn,² 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 it¹s 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 Earth¹s
soil would increase five-fold.
But there¹s 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,
Mother¹s 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 ³new² 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. (A the above web site)
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