Abstract
The AquaLab Vapor Sorption Analyzer (VSA) is an automatic isotherm generator to measure water po-tential for a sample with a chilled-mirror dew-point sensor while gravimetrically tracking the sample weight during wetting and drying processes. In this study, we applied the VSA to measure water retention curves for relatively dry soils ranging from —7.2 x 10 4 cm to —3.2 x 10 6 cm, and investigated the accuracy of measurement and properties of the VSA setting parameters. We confirmed that soil water potential was equilibrated with air humidity for the DVS method with 0.01 % h -1 of the weight trigger. The equi-librated water content by weight was almost identical re-gardless of the soil bulk density since the surface retention was dominant in the dry range. The newly developed DDI (Dynamic Dewpoint Isotherm) method was validated with the DVS method. The DDI method could accurately mea-sure successive retention curves for 50 h measuring time with the flow rate of 100 mL min -1. Hysteretic retention curves as well as scanning curves in a dry range could be also observed in a repeatable manner for Fujinomori silty loam soil, rokachi Andisol, and Tottori dune sand using the DDI method.
