Soil organic matter, texture, and drying temperature effects on water content

Authors: Franzluebbers, Alan

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/31/2022
Publication Date: 6/30/2022
Citation: Franzluebbers, A.J. 2022. Soil organic matter, texture, and drying temperature effects on water content. Soil Science Society of America Journal. 86:1086-1095. https://doi.org/10.1002/saj2.20425.
DOI: https://doi.org/10.1002/saj2.20425

Interpretive Summary: A balance between soil water and air is needed to optimize soil microbial activity in laboratory incubations. Some fraction of water holding capacity (WHC) is often used to optimize water during controlled laboratory incubations, and yet this level may differ among soils. This study was undertaken to develop relationships among gravimetric and volumetric water content, WHC, and water-filled pore space (WFPS) among a variety of soils, representing eight soil orders, 46 subgroups, total soil N from 0.46 to 4.68 g kg–1, and clay concentration from .072 to .491 kg kg–1 (5–95% range). Gravimetric soil water content was determined for each sample under the following sequence of conditions: (a) from dry storage, (b) when dried at 55 °C, (c) near saturation, and (d) when drained to WHC. Gravimetric water content was calculated at 50% WFPS (.50 m3 m–3). Like that of WFPS, WHC varied strongly with soil clay and total N concentrations. To achieve 50% WFPS, water content was 69 ± 10% of WHC and 58 ± 3% of near saturation. Although drying soil to only 55 °C for routine soil microbial processing led to overestimation of soil bulk density by 0.012 ± 0.006 Mg m–3, it was a bias. Soil water content calculations are important for obtaining best estimates of soil-test biological activity and other soil C and N fractions to assess soil health, and this research demonstrated that using drying temperature of 55 °C was practical but created a small and consistent bias.

Technical Abstract: A balance between soil water and air is needed to optimize soil microbial activity. Some fraction of water holding capacity is often used to optimize water content during controlled laboratory incubations, and yet this level may differ among soils. This study was undertaken to develop relationships among gravimetric and volumetric water content, water holding capacity (WHC), and water-filled pore space (WFPS) among a variety of soils, representing eight soil orders, 46 subgroups, total soil N from 0.46 to 4.68 g/kg, and clay concentration from 0.072 to 0.491 kg/kg (5 to 95% range). Gravimetric soil water content was determined for each sample under the following sequence of conditions: (1) at condition of dry storage, (2) when dried at 55 °C, (3) at near saturation, and (4) when drained to WHC. Gravimetric water content was calculated at 50% WFPS (0.50 m3/m3). Like that of WFPS, WHC varied strongly with soil clay and total N concentrations. To achieve 50% WFPS, water content was 69 +/- 10% of WHC and 58 +/- 3% of near saturation. Although drying soil to only 55 °C for routine soil microbial processing led to overestimation of soil bulk density by 0.012 +/- 0.006 Mg/m3, it was a bias. Soil water content calculations are important for obtaining best estimates of soil-test biological activity and other soil C and N fractions to assess soil health, and this research demonstrated that using drying temperature of 55 °C was practical but created a small and consistent bias.