Soil Specific Surface Area and Clay Content Estimated From Vapor Sorption Isotherms.

Knowledge of the soil specific surface area (SA) and clay content (CL) is important to understand various essential soil processes, e.g. contaminant adsorption, colloid filtration, and soil aggregation. However, measurements of these two properties are tedious and expensive. Recent studies therefore suggest estimating both SA and CL from water vapor sorption isotherms at low saturations, a measurement presently challenged by hydraulic decoupling and long equilibration times. This study evaluates the methodology of a new fully-automated Vapor Sorption Analyser (VSA) for rapid measurement of vapor sorption isotherms at low saturations (-10 MPa to -480 MPa) and demonstrates estimation of SA and CL from obtained data. To test applicability of the obtained data, 150 variably-textured soils from different geographical areas were used. The VSA successfully measured both adsorption and desorption isotherms within reasonable time (1–3 days per sample). Parameterization of the Tuller-Or model using measured vapor sorption data yielded SA estimates comparable to that obtained from the ethylene glycol monoethyl ether (EGME) method. The CL estimates using the water content at a relative humidity of 50% showed remarkable similarity to measured data using the hydrometer method, except for soils with high organic matter contents. In addition, regression functions for clay content estimation using water contents at other RH values are presented. The results confirm and expand the possibility of rapid estimation of SA and CL from water vapor sorption data.