After crops are harvested, large amounts of stems, husks, and other unusable plant material are left behind. That crop residue can be baked in special oxygen-free furnaces to convert it to a form of charcoal called biochar. The baking process, known as pyrolysis, locks much of the plant’s carbon into a solid form that doesn’t easily decompose, preventing the trapped carbon from decaying for decades or even centuries. When added back to croplands, some types of biochar can also improve soil health in various ways.
In this analysis, we considered only plant material left over from existing agricultural activity, and we did not consider agricultural residues already used for other activities (such as livestock feed). The calculations also account for the need for part of these residues to remain in the field to help maintain healthy levels of organic matter in cropland soils.
Converting part of the world’s unused agricultural residues to biochar could cost-effectively prevent the release of 331 million tonnes of carbon dioxide equivalent per year (MtCO2e/year) from decaying biomass. That’s comparable to the emissions from nearly 70 million passenger vehicles per year.
Reaching this scale will require operations converting half a billion tonnes of crop waste to biochar each year. That’s about 2-3 times the scale of current global charcoal production for fuel, although almost all of this currently comes from wood.
To create a market for biochar among farmers, we must first demonstrate its value as a soil amendment. To make that case, we need a better understanding of how this material will affect different types of agricultural soils.
There are also economic challenges to implementing biochar use on a wide scale. In many parts of the world, agricultural residues that can be easily collected are already gathered for feeding livestock or are burned to produce energy. In other places, the cost of harvesting agricultural residues can be too high to justify the production of biochar.
Initially, biochar will probably be most feasible in developed areas where there is better infrastructure to harvest crop residues, better access to new technology and fewer competing demands for agricultural residues. However, as this technology matures, biochar could have a role in restoring degraded soils worldwide. It also has promise for storing water in soils in dry regions.
Because of its high cost, biochar production isn’t yet ready to be put into practice on a large scale. With further research and market development, however, it could become a viable pathway relatively soon.
Spotlight: Carbon Gold in Belize
The Toledo Cacao Growers Association represents 1,200 small-scale cacao farmers in Belize who supply cacao beans to Kraft Foods. Since 2009, the members have been working with Carbon Gold, a UK-based biochar producer, to explore the benefits of using biochar from cacao tree trimmings to enrich the soil.
The project has shown that using a biochar/fertilizer blend improved germination of cacao seedlings and increased production of cacao fruit. What’s more, seedlings growing in soils that received biochar survived a recent water shortage, while large numbers of seedlings in untreated soils perished during the drought.
The biochar industry is growing rapidly, from 200 active companies in 2014 to 326 in 2015, according to the International Biochar Initiative. Existing biochar kilns can accept a wide range of agricultural residues, including wood chips, rice husks and coffee husks, and could be used in agricultural regions around the world.