Impact of Texture, Bulk Density, and Organic Carbon on the Soil Water Characteristic from Oven Dryness to Saturation.

The soil water characteristic (SWC) is important for understanding soil-plant-atmospheric relationships and crucial for modeling gas and water flow processes in soils. Measuring the SWC is time consuming, and hitherto the dry region soil water retention has commonly been excluded due to slow and inaccurate measurements. The present study tests and evaluates a recently developed model for prediction of the SWC from oven dryness to saturation based on the bulk density, organic matter content, and particle size distribution. The model was not tested on undisturbed highly structured soils and soils containing high percentages of organic matter. Sixty-six undisturbed Danish soil samples from different agricultural fields with clay and organic matter contents ranging from 0.05 to 0.47 kg kg^-1 and 0.03 to 0.14 kg kg^-1, respectively, were used for model evaluation.  Measuring the SWC from saturation to oven dryness was accomplished with sandboxes, the WP4-T Dewpoint potentiometer, and a fully automated vapor sorption analyzer. The results showed that the new model had considerably high prediction accuracy over the entire matric potential range. The model, however, showed somewhat higher deviations from measured data in the wet SWC region at lower bulk densities. A bulk density correction index was therefore introduced to increase the SWC prediction accuracy. Results also indicated that no global matric potential value should be used at zero water content, but the potential should be rather estimated as a function of the fine soil fractions (clay, organic matter and silt content).