Holding water with capacity to target porosity

Authors: Franzluebbers, Alan

Submitted to: Agricultural and Environmental Letters
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/20/2020
Publication Date: 10/1/2020
Citation: Franzluebbers, A.J. 2020. Holding water with capacity to target porosity. Agricultural and Environmental Letters. 5, Article 20029.

Interpretive Summary: Soil water content for laboratory incubations of soil microbial activity and net nitrogen mineralization has been reported with many different approaches. This study was conducted to see if a simple, standardized approach could be developed. A scientist from USDA-Agricultural Research Service in Raleigh NC determined soil water content at saturation, at water holding capacity, and at 50% water filled pore space, the latter of which should be most ideal for soil incubations. Clay and soil organic carbon concentrations greatly influenced water content at these three targeted levels of water retention. A fraction of soil water content at saturation provided the most stable estimate of achieving desired water content of 50% water filled pore space. Gravimetric water content at saturation times 0.59 is recommended for further studies. It provided a simple and stable estimate of optimum water for microbial activity. This information will be valuable for soil biological scientists to evaluate large numbers of samples for assessment of soil health.

Technical Abstract: 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 (0.06-0.43 kg/kg) and soil organic C (5.2-52.0 g/kg) concentrations (5-95% range). Gravimetric soil water content (SWC) was determined for each at WHC and at saturation. Soil clay and organic C concentrations strongly influenced SWC; the effect of soil organic C 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 following gravimetric SWC at saturation.