Comparison of Soil Water Potential Sensors: A Drying Experiment

The soil water retention curve (WRC) plays a major role in a soil's hydrodynamic behavior. Many measurement techniques are currently available for determining the WRC in the laboratory. Direct in situ WRC can be obtained from simultaneous soil moisture and water potential readings covering a wide tension range, from saturation to the wilting point. There are many widely used soil moisture probes. Whereas near-saturation tension can be measured using water-filled tensiometers, wider ranges of water potential require new, more expensive, and less widely used probes. We compared three types of soil water potential sensors that could allow us to measure water potential in the field, with a range relevant to water uptake by plants. Polymer tensiometers (POTs), MPS-2 probes, and pF meters were compared in a controlled drying experiment. The study showed that the POTs and MPS-2 probes had good reliability in their respective ranges. Combined with a soil moisture probe, these two sensors can provide observed WRCs. The pF meters below -30 kPa were inaccurate, and their response was sensitive to measurement interval, with greater estimated suction at shorter measurement intervals. In situ WRC can provide supplementary information, particularly with regard to its spatial and temporal variability. It could also improve the results of other measurement techniques, such as geophysical observations.