A Rapid Test for Plant-Available Water-Holding Capacity in Soil-Biochar Mixtures

Standard methods for measuring plant-available water-holding capacity can require days or weeks, making it difficult and expensive to monitor how soil-biochar mixtures change over time. This report describes and evaluates a faster, less expensive method that measures the water retained by a saturated sample after 90 minutes of centrifugation. Researchers calibrated the method against conventional pressure-plate and dew-point measurements before applying it to 72 mixtures representing nine Washington soils, four biochars, and two biochar application rates. The results indicate that biochar generally increases plant-available water-holding capacity, although the increase is not proportional to the amount applied. Soil texture and possibly mineralogy influence the response, with sandy soils receiving the greatest proportional benefit. Interactions between soil and biochar particles contributed more to increased water-holding capacity than the biochar’s internal porosity alone. Further monitoring is needed to determine how long these benefits persist as biochar ages.