| Summary: | Abstract Optimizing soil microbial activity requires an equal balance between water‐ and air‐filled porosity, that is, 50% water‐filled pore space (WFPS). However, many soil biological investigations report water as some fraction of water‐holding capacity (WHC). This study was conducted to fill a quantitative gap between WFPS and WHC. Soil samples (n = 198) from 10 eastern U.S. states and one state in Brazil provided a wide distribution of clay (0.064–0.487 kg kg−1) and soil organic C (SOC, 5.2–52.0 g kg−1) concentrations (5–95% range). Gravimetric soil water content (SWC) was determined at WHC and at saturation. Both clay and SOC concentrations strongly influenced SWC; the effect of SOC was strongest and nonlinear. To achieve 50% WFPS, gravimetric SWC was 0.69 ± 0.10 times that of WHC and 0.59 ± 0.03 times that of saturation. For soil biological assays, 50% WFPS could be accurately and simply achieved with calculations using gravimetric SWC at saturation multiplied by 0.59.
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