Comparing methods for measuring water retention of peat near permanent wilting point

Peat soils shrink and become very hydrophobic when dried. Both properties may cause inaccuracies when applying laboratory methods for soil hydraulic properties that have been developed and tested for mineral soils. This study aimed to compare different methods for the determination of the water retention of peat soils near permanent wilting point (pF 3.5 to 4.2). Three common methods were tested: two pressure apparatus (ceramic plate [Soilmoisture] vs. membrane [Eijkelkamp]) and a dew-point potentiameter (WP4C, Decagon Devices, Inc.), which is based on the equilibrium of soil water potential with air humidity. We used both field-moist peat samples and samples that had been rewetted after oven-drying. We found that there was no systematic difference between the two pressure apparatus. Low moisture variability among replicates and dew-point potentiameter measurements that indicated a drainage to pF 4.2 support the use of pressure apparatus for the determination of water retention near permanent wilting point. Despite a rewetting time of 2 wk including periodic mixing, rewetted oven-dried samples showed lower soil moistures at pF 3.5 and 4.2 than field-moist ones. This severe and long-lasting hysteresis effect was strongest for less decomposed peat samples. Thus, field-moist samples should be used. This makes the classical dew-point potentiameter measurement protocol, which is based on defined water additions to oven-dried samples, unsuitable for peat samples.