Biochar is a type of charcoal produced from biomass. In some cases, the term is used to distinguish biomass charcoal produced via pyrolysis.

Biochar is employed most commonly as a soil amendment. It has several qualities that make it suitable to this task:

• It is largely inert. Microbial composting action leaves charcoal largely unaffected.
• Highly porous. Serves both to retain water in the soil or improve drainge , and provide a large surface area for microbes.
• It is of neutral or alkaline pH . Reducing the acidity of , or sweetening the soil .
• Very high cation exchange capacity . Increasing the uptake of minerals such as Ca , K , Mg and P .
• It is a good insulator . Reducing average soil temperatures in hot climates .

There is some empirical evidence that low-temperature biochar produces more robust growth in plants when compared to high-temperature biochar. It is speculated that it retains organic matter that is desirable to beneficial microbes (like mycorrhizal fungi), resulting in higher nutrient availability to the plants.

Biochar is the main (and likely key) ingredient in the formation of terra preta, or Amazonian dark earth. Efforts to recreate these soils are being undertaken by companies such as Eprida, Best Energies, and Dynamotive Energy Systems. Research efforts are underway at Cornell University, the University of Georgia, Iowa State University, and The University of Hawaii at Manoa. One focus of this research is the prospect that if biochar becomes widely used for soil improvement, it will involve globally significant amounts of carbon sequestration, remediating global warming.

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