Lesson plan: Investigating the effects of biochar on soil fertility

Download the lesson plan.

Biochar is a fine-grained, highly-porous charcoal produced from carbon-rich biomass feedstocks, including forestry waste, animal manures and agricultural waste products such as husks, shells and stover.(Stover is the dry stalks of crops such as corn, sorghum or soybean that remain in a field after harvest.) Biochar is produced by pyrolysis or gasification (heating the biomass with little or no air) in a process similar to the production of charcoal. However, the primary use of biochar is as a soil improver rather than a fuel. Biochar has been shown to add value to soils in terms of fertility, particularly to acidic soils. The exact details of the mechanism of action are not fully established, but it may act by filtering out and retaining nutrients from percolating water in the soil or by changing crumb structure. Biochar shows a mean residence time in the soil in the range of hundreds to thousands of years, making it a long-term carbon store (compared with uncharred biomass) and hence a long-term contributor to climate change mitigation. Also, some of the wastes proposed as feedstock for making biochar are currently burned or composted which returns all or part of their carbon to the atmosphere quickly and, in the case of anaerobic composting or landfilling, produces methane which is a more potent “greenhouse gas” than carbon dioxide.

There is a range of possible investigative work that can be done with biochar (or other substances that could affect soils and soil fertility). Some (as described in the IBI Biochar Trial Guide on the IBI website) is beyond the scope of schools. However, a medium-term student project at Advanced level could explore some biochar issues, and even at Introductory level, students could investigate the effect of adding biochar or charcoal to the growing medium used to grow seeds in pots.

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