In situ strength, bulk density, and water content relationships of a durinodic xeric haplocalcid soil

Compaction significantly reduces yield, quality, and profitability of irrigated crops in the US Pacific Northwest (PNW). Compaction assessment is usually done via bulk density measurement, even though crops respond negatively to excessive compaction largely because of root penetration (soil strength) limitations, not because of bulk density per se. For most soils, strength is thought to depend primarily on the interaction of water content and bulk density. We hypothesized that the soil strength (expressed as cone index) of an important PNW soil, Portneuf silt loam (Durinodic Xeric Haplocalcid), could be predicted for a given bulk density or water content and that it would increase with increasing bulk density and decreasing water content. To test this, the in situ cone index, the bulk density and water content profile of a 1.5-ha field was intensively sampled three times over a 2-year period, producing 688 data triplets. These data were used to produce soil water strength-bulk density response surface relationships using robust curve fitting. Cone index relationships were poor when derived from full-profile data sets but improved when data were segregated by depths. When grouped by depth intervals, cone indices of individual layers were always correlated strongly with soil water content, but not always with bulk density. The high calcium carbonate content of this soil was thought to have produced cementation effects on the cone index that varied with prolonged wetting versus prolonged drying. Variability among in situ strength penetrations and bulk density cores was also thought to reduce model accuracy. The difficulties inherent in developing the comprehensive relationships of soil strength to bulk density, and the overriding dependency of strength on the dynamic variable of water content, suggest great uncertainty when using bulk density sampling for realistic assessment of overall soil status affecting root restriction or crop performance unless sampling is extensive and the relationships between strength, bulk density, and water content have been intensively documented for an individual soil.