DOI: 10.5061/dryad.tdz08kpxj
title: Soil water content determined at saturation and water holding capacity and calculated at 50% water filled pore space

description: 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. 
description: Total C and N were determined from soil subsamples on a Leco TruMac CN analyzer and assumed as organic since pH was acidic for all samples (5.6-6.6).   Sand and clay concentrations were predicted from near infrared spectroscopy (Model 5000 with WinISI version 1.5 software, Foss North America, Inc., Eden Prairie, MN) of pulverized soil (ball milled for 2 min).   Calibration was from 278 samples that were selected using ‘H’ statistic of 0.6.  Standard error of calibration was 0.156 and 0.060 kg kg-1 for sand and clay, respectively.   The middle 90% of calibration observations was 0.160 to 0.744 kg kg-1 for sand and 0.078 to 0.480 kg kg-1 for clay.   Statistical distribution of samples in this study at 5, 50, and 95 percentile limits was 0.064, 0.259, and 0.487 kg kg-1, respectively, for clay and 5.2, 19.7, and 52.0 g kg-1, respectively, for SOC. Soil passing a screen with 4.75-mm openings was heaped into a cylindrical metal container with total volume of 80 mL (54 mm diameter, 35 mm height).   Containers had five 1-mm diameter holes punched into the bottom.   The container was tapped vigorously 10 times to allow soil to settle, then a metal blade was used to shear off excess soil to exactly 80 mL volume.   Containers of soil were dried in a forced-air oven at 55 °C for 9 h before immersing into a pan of deionized water ~10 mm deep.   Water was allowed to wick up into soil for 14 h, at which time containers were wiped of outside water and weighed immediately to calculate SWC at saturation.   Containers were placed onto a paper towel, lid placed loosely on top to avoid evaporation, and allowed to drain freely for 9 h, at which time containers were weighed to calculate SWC at WHC.   Containers of soil were dried in an oven at 55 °C for 3 d and then further dried at 105 °C for 16 h and mass recorded. Density of sieved soil in the metal container was calculated as mass (variable among samples) per volume (80 mL fixed).   Total porosity was derived from density, assuming particle density of 2.65 Mg m-3 (Total porosity = 1 – Density / 2.65).   All other calculations were derivations from soil mass at 105 °C, gravimetric SWC at saturation and at WHC, and density of soil.