Research shines light on soil additive’s seemingly contradictory benefits
Jade Boyd – September 24, 2014 Rice University News & Media
As more gardeners and farmers add ground charcoal, or biochar, to soil to both boost crop yields and counter global climate change, a new study by researchers at Rice University and Colorado College could help settle the debate about one of biochar’s biggest benefits — the seemingly contradictory ability to make clay soils drain faster and sandy soils drain slower.
The study, available online this week in the journal PLOS ONE, offers the first detailed explanation for the hydrological mystery.
“Understanding the controls on water movement through biochar-amended soils is critical to explaining other frequently reported benefits of biochar, such as nutrient retention, carbon sequestration and reduced greenhouse gas emissions,” said lead author Rebecca Barnes, an assistant professor of environmental science at Colorado College, who began the research while serving as a postdoctoral research associate at Rice.
Biochar can be produced from waste wood, manure or leaves, and its popularity among do-it-yourselfers and gardening buffs took off after archaeological studies found that biochar added to soils in the Amazon more than 1,000 years ago was still improving the water- and nutrient-holding abilities of those poor soils today.
Studies over the past decade have found that biochar soil amendments can either increase or decrease the amount of water that soil holds, but it has been tough for experts to explain why this occurs, due partly to conflicting results from many different field tests.
In the new study, biogeochemists at Rice conducted side-by-side tests of the water-holding ability of three soil types — sand, clay and topsoil — both with and without added biochar. The biochar used in the experiments, which was derived from Texas mesquite wood, was prepared to exacting standards in the lab of Rice geochemist Caroline Masiello, a study co-author, to ensure comparable results across soil types.
“Not all biochar is created equal, and one of the important lessons of recent studies is that the hydrological properties of biochar can vary widely, depending on the temperature and time in the reactor,” Masiello said. “It’s important to use the right recipe for the biochar that you want to make, and the differences can be subtle. For scientific studies, it is critical to make sure you’re comparing apples to apples.”