El Dorado, the rumored ancient kingdom in the middle of the Amazon, was considered a myth until the discovery of Terra Preta, a dark human-made layer of soil laid on top of the Amazon's pale and infertile ground. Terra Preta contains twice as much black carbon than surrounding soils and has remained fertile for more than a millennium now. This black carbon is called biochar. In this EDGE Insight, we explored the modern applications of this fascinating ancient technique.

Biochar is a charcoal-like residue from burning organic material in an oxygen-limited environment. Why limit oxygen? Oxygen is a catalyst for combustion and if allowed in abundance, will turn anything into ash. Cutting down the oxygen supply permits only partial combustion, leaving a biochar residue that is 80%–90% pure carbon. This process is called “pyrolysis.”

Plants absorb and store carbon dioxide from the atmosphere through photosynthesis, but this carbon is unstable and could return to the atmosphere when the plant dies and decomposes, burns from wildfires, or is cut down and used as firewood. Pyrolysis stabilizes this carbon in the form of biochar—so it won’t decompose or break down further. 

How is biochar different from charcoal? The main difference is the type of biomass used. Charcoal is destined for the barbecue grill, which requires big lumps of hardwood that burn hot and slow. In contrast, biochar can be made from a range of different feedstock including smaller plant prunings, wildfire residue, rice husks, sawdust, food waste, and even sewage sludge. Another key difference is the temperature at which they are made. Charcoal is made at roughly 400°C, whereas biochar is made between 600°C–1,000°C. Higher temperature also results in greater porosity. Let’s find out why porosity is important for biochar.

1. Agriculture: The most common use case for biochar is to simply stick it in the soil. Biochar’s porous structure can hold moisture for long periods while aerating and oxygenating the surrounding soil. Biochar is also negatively charged and has strong cation exchange properties. This means it can effectively bind positively charged nutrients like calcium, potassium, and ammonium needed for fertile soil. Due to its pores, a few ounces of biochar can have an internal surface area similar to the size of a football field This, coupled with Biochar’s longevity (biochar can last for thousands of years in the soil), provides a healthy breeding ground for microorganisms that stimulate plant growth and improve plant immunity. Read more about how microorganisms are used in agriculture in our EDGE Insight on biofertilizers.