The Adequacy of Pressure Plate Apparatus for Determining Soil Water Retention

Abstract

This study evaluated the accuracy of pressure plate apparatus for measuring soil water retention at −0.5 and −1.5 MPa matric potential. Samples from 35 contrasting Australian soils were wetted with distilled water and drained on pressure plate apparatus at −0.5 and −1.5 MPa. The soil matric potential of each sample was then determined using a thermocouple psychrometer, and water content was measured. Water content at exactly −0.5 and −1.5 MPa matric potential was determined independently by interpolating between replicates of matric potential–water content data measured using a thermocouple psychrometer. Water content of the soil samples at apparent equilibrium on pressure plates was compared with these “target” water contents. The 35 samples on pressure plates at −1.5 MPa equilibrated, on average, to 0.3% (w/w) wetter than the target water content, with mean matric potential of −1.10 MPa. Fifteen samples were significantly wetter than the target values. Soil samples on pressure plates at −0.5 MPa equilibrated, on average, to 0.2% (w/w) wetter than the target water content, attaining a mean matric potential of −0.48 MPa. Mean error in water content at −1.5 MPa on pressure plates was reduced from >0.5 to <0.1% (w/w) in a subset of 10 samples prone to dispersion by wetting with 0.01 mol L−1 CaCl2 Water contents of samples equilibrated on pressure plates at −1.5 MPa were good estimates of “true” −1.5 MPa water content for the nonswelling soils tested, provided CaCl2 was used to minimize dispersion. Vapor equilibrium measurement methods are recommended for swelling soils.