Evaluation of a Fully-Automated Analyzer for Rapid Measurement of Water Vapor Sorption Isotherms for Applications in Soil Science

The characterization and description of important soil processes such as water vapor transport, volatilization of pesticides, and hysteresis require accurate means for measuring the soil water characteristic (SWC) at low water potentials. Until recently, measurement of the SWC at low water potentials was constrained by hydraulic decoupling and long equilibration times when pressure plates or single-point, chilled-mirror instruments were used. A new, fully automated vapor sorption analyzer (VSA) helps to overcome these challenges and allows faster measurement of highly detailed water vapor sorption isotherms. In this technical note we present a comprehensive evaluation of the VSA instrument for a wide range of differently textured soils and discuss optimal measurement settings. The effects of operation mode, air-flow rate, sample pretreatment, test temperature, sample mass, and mass trigger point on resultant sorption isotherms were evaluated for a relative humidity (RH) range from 0.10 to 0.90. Both adsorption and desorption branches were measured for all soils within a reasonable time period (10-50 h). Sample masses larger than 3.5 g resulted in incomplete adsorption and desorption, while oven-dry (105°C) samples of coarse-textured soils exhibited water repellency characteristics. The required measurement times were strongly correlated with clay content and influenced by high organic carbon content.