Biochar is a new word for a very old, familiar substance: charcoal. But biochar is special charcoal made by special processes to use in soil, not for fuel or filtration. Charred carbon is made by heating biomass with no or little oxygen. "Pyrolysis" is a technique to cook biomass (woody, weedy, manure) at low temperature (500-900 C) in a retort. This also produces heat, syngas, bio-oil, wood vinegar. Energy released to make biochar can heat a building, power a truck, or generate electric power.
Biochar is lightweight because it's empty inside. Seen in a microscope, biochar is very porous, filled with tiny microscopic pores (microphoto at right), because plants are 95% water, and their physical structures are mostly hollow tubes and pipes to move water around inside. Like a sponge, biochar micropores absorb and retain water and nutrients, and provide space for microbes to live. So, biochar keeps water and nutrients in the root zone, available to plants, reducing their leaching and loss.
Biochar micropores are ideal to add to sandy, or drought-prone soils. More than most carbon, biochar retains up to six times its weight in water. Biochar micropores also aerate & loosen heavy clay. Biochar's remarkable, huge internal surface areas boost soil Cation Exchange Capacity (CEC) to retain positive ions like Calcium, Sodium, Magnesium, Potassium, etc. But biochar has Anion Exchange Capacity (AEC) to capture negative ions, beginning with Nitrogen and Phosphorus ( N-P of N-P-K ). Biochar improves water quality by increasing retention of nutrients for plant and crop use -- more nutrients stay in soil rather than leach into groundwater or outgas.
Biochar micropores are also refuge, habitat and "home" for beneficial microbes, and encourages their growth to make nutrients more readily, easily available. Biochar boosts microbial activity and diversity. Microbes don't eat biochar; they live in it. Biochar anchors a strong, stable Soil Food Web that manages nutrients, maintains fertility and delivers energy, minerals and metabolites to plant roots.
Biochar isn't a fertilizer, although it retains some ash that's plant nutrients. Biochar is used with fertilizer and amendments, not as substitute, but as high-efficiency delivery system. Biochar adsorps nutrients in fertilizers to make them easily available to plant roots. This improves fertilizer efficiency and allows growers to reduce use of fertilizer.
Biochar can remain in soil over a thousand years, so it doesn't have to reapplied year after year. Eventually, after a few annual applications reach an ideal concentration of at least 5% in soil, only occasional small additions are required .
Biochar can sequester carbon (capture & long-term store) to mitigate global warming, slow greenhouse gas accumulation, reduce burning of fossil fuels. A mere 2% increase in the planet’s soil carbon content can offset 100% of all greenhouse gas emissions going into the atmosphere. Soil is one of the planet's largest carbon sinks.