Along with bags of potting soil, mulch, and compost, you soon may see bags of biochar for sale at your local nursery. To explain what biochar is, we need to return to the Amazon basin circa 450 a.d. Indigenous people didn’t practice slash-and-burn farming as they do now. They practiced slash-and-char agriculture, roasting wood and leafy greens in “smothered” fires, in which lower temperatures and oxygen levels resulted in the production of charcoal instead of ash. The charcoal was buried in fields where crops were grown.
But then, with the arrival of Europeans and their diseases, the Amazon civilizations, some with cities of more than 100,000 people, collapsed. Slash-and-char agriculture was forgotten, as were the fields of buried charcoal. But they weren’t gone. In the 20th century, huge expanses of black soil were rediscovered, although at first no one knew what they were. Then, in the 1990s, scientists determined that these soils were manmade. They were dubbed terra preta (“dark earth” in Portuguese). And they were extensive. Some estimates put the total acreage covered by the charcoal-enriched soil at twice the size of Great Britain.
Most amazingly, the soils extended up to 6 feet deep in many places. Scientists have theorized that terra preta soils are self-propagating and have grown in depth since they were first made. The charcoal, acting a lot like humus, had been colonized by myriad microbes, fungi, earthworms, and other creatures; these soil organisms produced carbon-based molecules that stuck to the charcoal, gradually increasing the soil’s carbon content. Carbon in decomposing plants, which would otherwise escape into the air as greenhouse gases, was sequestered by the biologically active charcoal in the soil. Scientists theorize that the charcoal was originally laid down in thin layers and that earthworms chewed through the layers and mixed them deeply into the soil.
That is just the beginning of the benefits of this strange soil. It appears that the carbon will be sequestered for a thousand—possibly thousands—of years, unable to contribute to global warming in the form of greenhouse gases. Green charcoal, or biochar made from agricultural residues or renewable biomass, appears to hold the most promise as a carbon sink. Every ton of this biochar in the soil is capable of capturing and holding at least 3 tons of carbon.
Biochar also stimulates mycorrhizal fungi—those fungal symbionts that live on plant roots, scour surrounding soil for hard-to-find phosphorus, and deliver it back to their host plants. According to scientists studying the soils, microbial growth of all kinds is substantially improved. And so is the soil’s ability to hold nutrients until plants need them, then dole them out at the optimum rate for plant health. Crops have been shown to grow 45 percent greater biomass on unfertilized terra preta soil versus poor soil fertilized with chemical fertilizers.
You can also make your own biochar. One method of making biochar: pile up woody debris in a shallow pit in a garden bed; burn the brush until the smoke thins; damp down the fire with a one-inch soil covering; let the brush smolder until it is charred; put the fire out. The leftover charcoal will improve soil by improving nutrient availability and retention. You want the fire to brun fast and hot at first, but the reason the fire is then covered is to reduce the temperature of the fire, limit its oxygen and have it burn more slowly. You can also make biochar in a 55 gallon drum: