As more gardeners and farmers add ground charcoal, or biochar, to soil, 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, published in PLOS ONE, offers the first detailed explanation for the hydrological mystery.
“Biochar is light and highly porous,” said lead author Rebecca Barnes. “When biochar is added to clay, it makes the soil less dense and it increases hydraulic conductivity, which makes intuitive sense. Adding biochar to sand also makes it less dense, so one would expect that soil to drain more quickly as well; but in fact, researchers have found that biochar-amended sand holds water longer. … By adding our results to the growing body of literature, we show that when biochar is added to sand or other coarse-grained soils, there is a simultaneous decrease in bulk density and hydraulic conductivity, as opposed to the expected result of decreased bulk density correlated with increased hydraulic conductivity that has been observed for other soil types.” The study was summarized in the January 2015 issue of Acres U.S.A.