For years, biochar has been promoted as a soil additive to increase fertility and crop yields. But a new paper, co-authored by Diego Abalos and Jan Willem van Groenigen of the Department of Soil Quality, casts doubt on that view, finding that biochar only improves crop growth in the tropics, with no yield benefit at all in the temperate zone. “A quite remarkable result,” says Jan Willem van Groenigen.
An international team of researchers gathered data from more than 1000 experiments conducted around the world, each measuring the effect of biochar on crop yield. Then, they used meta-analysis, an advanced statistical technique that analyses many studies at the same time, to test whether the beneficial effect of biochar addition depends on geography. That’s when the surprising result emerged: “Location, location, location: it really matters for biochar,” said Simon Jeffery of Harper Adams University and lead author. “Biochar had a huge benefit in the tropics, a 25% increase in yield. But in the temperate zone, there was just no effect at all. We were really surprised.”
The idea of biochar was inspired by a rare type of soil that occurs in the tropics, ‘terra preta’ – Portuguese for ‘black earth’, so named because the soil is rich in black carbon, the partially burned remains of old plants, much like charcoal. ‘Terra preta’ is fertile, with favourable pH, unlike typical tropical soils which are low fertility and acidic. Initial experiments showed that adding biochar to typical tropical soils increased crop yields, making it possible for farmers to cultivate a plot of land in these soils for more than a few years.
The surprise (“a quite remarkable result”) came because past work has assumed that the beneficial effects of biochar are universal, applying to soils no matter where they occur. “Our study was the first to test whether geography matters, and we were able to do this because of the very large dataset we assembled,” says Jan Willem van Groenigen. “Our findings confirm that biochar can benefit farms in low-nutrient, acidic soils such as in the tropics, but in more fertile soils, such as those in the temperate zone, obtaining yield increases through biochar application is much less certain.”
Many other benefits have been claimed for biochar, including managing waste, storing carbon in the soil, and being more environmentally friendly than adding synthetic chemicals to the soil. The new study did not evaluate these other potential benefits, and the authors note that some of these may still hold in both temperate and tropical regions.
“There may be a potential upside to using biochar in the temperate zone, like promoting soil carbon storage to slow climate change,” said Diego Abalos, postdoctoral researcher at the Department of Soil Quality. “But our analysis, summarizing over 1000 observations, shows that the yield benefits just aren’t there. So if the goal for biochar application is boosting crop yield, stick to the tropics.”