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Soil Science Society of America Journal Abstract - Soil Physics

Assessing Errors and Accuracy in Dew-Point Potentiometer and Pressure Plate Extractor Meaurements


This article in SSSAJ

  1. Vol. 77 No. 1, p. 19-24
    Received: Jan 16, 2012
    Published: December 14, 2012

    * Corresponding author(s): colin@decagon.com
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  1. Paulo Ivonir Gubiania,
  2. José Miguel Reicherta,
  3. Colin Campbell *b,
  4. Dalvan José Reinertc and
  5. Neiva Somavilla Gelainc
  1. a Soils Dep. Federal Univ. of Santa Maria (UFSM) Av. Roraima 1000 97105-900 Santa Maria, RS, Brazil
    b Research and Development Decagon Devices, Inc. Pullman, WA 99163
    c Soils Dep. Federal Univ. of Santa Maria (UFSM) Av. Roraima 1000 97105-900 Santa Maria, RS, Brazil


Soil water potential can be determined quickly with a WP4 device, which uses a chilled mirror dew point technique. The instrument can be used to check the soil water potential of samples drained in a porous plate extractor (PPE). However, soil water potential measured in both devices on the same soil sample may not be in agreement, mainly in the wet-end of the water retention curve. This work investigates this problem, and further evaluates how equilibrating soil samples in a PPE with different permeable material at the sample bottom (filter paper [FP], polyester fabric [PE], and synthetic knitwear [SK]) affects the final water potential. Soil samples (soil fraction <2 mm) of an Ultisol and an Oxisol were drained at 0.5 and 1.5 MPa pressure in a PPE and the water potential of these samples was measured in a WP4. Another fraction of the same soil was used to obtain a WP4 water retention curve. The permeable materials caused large differences in water retention at 0.5 MPa pressure (0.006 g g−1 in the Oxisol and 0.021 g g−1 in the Ultisol), and the FP allowed more water drainage than PE and SK, indicating the latter two should be avoided when using a PPE to determine water extraction. In both soils, the water potential measured in the WP4 was lower than −0.5 MPa for samples under 0.5 MPa pressure and greater than −1.5 MPa for samples under 1.5 MPa pressure. The increase of residual (fitted minus measured) in the WP4 water retention curve indicated that above −0.7 MPa the precision of WP4 measurements decreased drastically.

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