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SOIL PHYSICS

The Adequacy of Pressure Plate Apparatus for Determining Soil Water Retention

CSIRO Land and Water, GPO Box 1666, Canberra, ACT 2601, Australia

^* Corresponding author (Hamish.Cresswell{at}csiro.au).

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.

Abbreviations: EC, electrical conductivity • ESP, exchangeable sodium percentage • LS_mod, modified linear shrinkage