Evaluating the Contribution of Soil Organic Carbon to Improved Water-Holding Capacity Through Increased Compaction Resistance: Final Report

This final BIOAg report evaluated whether increases in soil organic matter from long-term biosolids applications improve water-holding capacity by increasing resistance to compaction and preserving soil structure. Using a long-term dryland wheat system in Douglas County, Washington, the study compared three biosolids rates (0, 2, and 4.5 dry tons per acre) with field-compacted and uncompacted strips. Intact soil cores were analyzed for saturated hydraulic conductivity, bulk density, and full soil moisture release curves, and repacked cores were used to test whether macroscopic structure affected soil water retention.

Biosolids treatments generally increased saturated hydraulic conductivity and water content at saturation and lowered bulk density. Field compaction increased bulk density and reduced saturated hydraulic conductivity and water content at saturation, while increasing water content at field capacity and permanent wilting point. Despite these shifts, neither biosolids nor compaction changed plant available water. Water retention also did not differ between intact and repacked cores. Findings suggest biosolids can improve infiltration and some physical properties in coarse-textured soils, but may not reliably increase plant available water.