Soil water content determined at saturation and water holding capacity and calculated at 50% water filled pore space

Optimizing soil microbial activity requires an equal balance between
water- and air-filled porosity, i.e. 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 non-linear. 
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 reasonably accurately and simply achieved with
calculations using gravimetric SWC at saturation multiplied by
0.59.