Scientists believe that biochar, the partially burned remains of plants, has been used as fertilizer for at least 2,000 years in the Amazon Basin. Since initial studies published several years ago promoted biochar, farmers around the world have been using it as a soil additive to increase fertility and crop yields. But a new study casts doubt on biochar's efficacy, finding that using it only improves crop growth in the tropics, with no yield benefit at all in the temperate zone.

A team of researchers from the United Kingdom, Portugal, The Netherlands, Canada and the U.S. gathered data from more than 1,000 experiments conducted around the world that measured the effect of biochar on crop yield. By using meta-analysis, an advanced statistical technique that analyzes many studies at the same time, they performed rigorous testing to determine whether the beneficial effect of adding biochar depends on geography.

That's when the surprising result emerged. Although previous studies assumed that the beneficial effects of biochar are universal, applying to soils in all climate zones, researchers proved that geography matters by assembling and analyzing this very large dataset.

"We saw a huge boost for crops grown in the tropics, but zero results for crops in the temperate zone," said Dr. Bruce Hungate, Director of the Center for Ecosystem Science and Society at Northern Arizona University and co-author on the study. "Given all the talk about the benefits of biochar, we were really surprised."

The idea of biochar was inspired by a rare type of humanmade soil found in the Amazon Basin, terra preta -- Portuguese for 'black earth', so named because the soil is rich in black carbon generated by 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 new study supports that view. "It makes sense that biochar can benefit farms in low-nutrient, acidic soils such as in the tropics," explained Dr. Hungate, "But many temperate zone soils are higher in nutrients and more moderate in pH. And if, on top of that, farmers add nutrients through fertilizers, there isn't much room for biochar to increase yield."

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 them may still hold true 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 Dr. Hungate, "but our analysis, summarizing more than 1,000 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."

The report has been published, open access, in Environmental Research Letters.