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.