Make Biochar — this Ancient Technique Will Improve Your Soil

Image Gallery To make biochar, pile up woody debris in a shallow pit in a garden bed. Burn the brush until the smoke tins and then damp-down the fire by covering it with about an inch of soil. Let it smolder until the brush is charred, then put the fire out. 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.