Biochar is charcoal made from plant or animal residues that are burned at high temperatures and in the presence of little to no oxygen, in a process called pyrolosis. The carbon-rich cell walls of the original material remain behind as black carbon, which is very stable and can stay in the soil for hundreds of years. The specific properties of the biochar, including the pH and nutrient content, depend on which material was used to make it and the temperature at which is was burned.
Check out the scanning electro micrographs (pictures taken at very high magnification) below to see some visual differences between biochar made from different feedstocks (the product used to make the charcoal is referred to as feedstock).
Biochar itself generally does not have a lot of nutrients in it. But it has many other important benefits to farmers as it is highly adsorbent and reactive. (It’s no coincidence that activated charcoal is an antidote for people who have swallowed poison. The charcoal adsorbs the poison molecules.)
- It improves the soil’s ability to hold onto applied nutrients.
- It increases the water-holding capacities of the soil.
- It provides habitat for millions of soil microorganisms. (See all the little habitats in the micrographs above).
- It is useful for controlling soil acidity by raising soil pH since biochar usually has a pH between 8-10. In acid soils, biochar can make a difference in crop growth if you concentrate it in the furrow/near plant roots
- It improves crop germination and growth by reducing soil pathogens.
- It can help build up soil carbon and compensate for carbon lost through the removal of crop residues due to its high carbon content and persistence in the soil.
Application rates
Row cropping of maize or sorghum: In the planting furrow, apply one and a half 2-kg containers (or three 1-kg containers) per 5 meters of row. This rate assumes maize or sorghum plants are spaced at 25-30 cm within a row. For other planting arrangements, divide this amount of biochar among the planting stations of 20 plants.
Using charcoal dust of unknown origin
The following references explain more about dosing and also how to test for toxicity of charcoal dusts of unknown origin (buying the dust from a charcoal seller). To learn more, refer to the two references provided below from the International Biochar Initiative.