Comparison of soil water potential sensors: a drying experiment

The soil water retention curve (WRC) plays a major role in soil’s hydrodynamic behaviour. Many measurement techniques are currently available for determining 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 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. This paper reports on a comparison of 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 range. 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. Recommendations are provided for future tests. 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.