Measuring Soil Water Potential with Gypsum Blocks: Calibration and Sensitivity

Abstract

Gypsum block soil sensors have been a useful tool for measuring soil water for over sixty years. We improve their usefulness by 1) demonstrating a new gypsum block calibration procedure, 2) determining equilibration times for two types of commercial blocks (Delmhorst GS-1, and Bouyoucos) across a range of water potentials (-4.3, -1.2, 0.56, -0.2, -0 0, -0.8, and -0.02 MPa), 3) providing calibration curves for Delmhorst and Bouyoucos instruments, and 4) quantifying the sensitivity of gypsum blocks by finding which soil water potentials are statistically distinguishable. Our procedure yielded calibration curves which are appropriate for Delmhorst and Bouyoucos instruments in well aggregated soils. Dry blocks included in soils reached equilibration (variability of sensor readings stabilized) afler ~ 150 hours for Delmhorst, and ~ 300 hrs for larger Bouyoucos blocks. In the water potential series described above, sensor readings of blocks between -4.3 and -0.08 MPa were statistically distinguishable (a.= 0.05) for both Delmhorst and Bouyoucos blocks. While sensor readings from blocks at the two highest water potentials (-0.02 and - 0.05 MPa) were not significantly different for either Delmhorst or Bouyoucos blocks, readings for -0.02 MPa soils were significantly different from soils ≤ -0.08 MPa for both blocks types.