Lab for Soil Physics and Peat

In the PeatLab hydraulic properties are determined for the saturated, wet (pressure head range down to -800 hPa) and dry (pressure heads below -10000 hPa) range of the water retention and hydraulic conductivity curve.

Under saturated conditions, hydraulic conductivity is measured under constant head, imposing steady state conditions (Reynolds et al., 2002). In the wet range, hydraulic properties are measured with evaporation experiments (Gardner and Miklich, 1962; Schindler, 1980; Dettmann et al., 2019). Thereby, the water retention and hydraulic conductivity curve are obtained simultaneously from one intact soil sample. Additionally, bulk densities and porosities are determined at the end of the experiment by standard mass calculations. In the dry range, the water content near the permanent wilting point (pF 4.2 = log10(15848) hPa) is adjusted by applying a positive pressure to the air phase of the disturbed samples and afterwards determined by weighting and drying of the samples.

The saturated hydraulic conductivity is measured with the “Hood permeameter according to HARTGE” (Umwelt-Geräte-Technik GmbH, Müncheberg, Germany) with intact soil samples under constant head conditions. Thereby a constant pressure gradient is applied, imposing stationary flow. The water volume flowing through the soil sample during a specific time is determined at the end of the experiment. Saturated hydraulic conductivity is calculated following Darcy’s law (Darcy, 1856).

Hydraulic properties are determined with the “ku-pF-apparatus” (Umwelt-Geräte-Technik GmbH, Müncheberg, Germany) by evaporation experiments (Gardner and Miklich, 1962; Schindler, 1980; Dettmann et al., 2019). The measurement principle is based on the simultaneous measurement of pressure heads (suction, matrix potential) and sample mass. The water retention curve is determined by relating the column-averaged water contents to pressure heads (measured with tensiometer). The hydraulic conductivity curve is calculated by relating the vertical water flux in the middle of the soil column to the gradients of the hydraulic potential between the measured pressure heads. Hydraulic properties are determined for the measurement range of the tensiometer, which is between 0 to approximately -800 hPa.

With the “Pressure membrane apparatus” (Eijkelkamp, Giesbeek, The Netherlands) pressure heads can be adjusted between  1000 hPa to 16000 hPa by applying a positive pressure to the air phase of a soil sample (Dane and Hopmans, 2002). In the PeatLab, the device is used to determine the water retention near the permanent wilting point (pF 4.2 = log10(15848) hPa).

For the measurement, two porous, saturated cellophane membranes are placed in the “Pressure membrane apparatus”, by avoiding any air inclusions. Field moist samples are then packed in plastic cylinders (Ø: 3.50 cm, height: 1.00 cm) and placed on the membranes.  Afterwards, the “Pressure membrane apparatus” is closed and a positive pressure (e.g. 15848 hPa) is adjusted and applied to the samples. The experiment ends after samples reach hydraulic equilibrium. The water content is determined afterwards gravimetrically by drying the samples in an oven. More details can be found in Bechtold et al. (2018), Dettmann et al. (2019) and the user manual (Eijkelkamp, 2022).

The “pF pressure step module” (ecoTech Umwelt-Meßsysteme GmbH, Bonn, Germany) can dewater samples for pressure heads between approximately -3 to -750 hPa. Therefore, steel cylinders are placed on an airtight membrane where a negative pressure is applied to the water phase of the samples. After the sample reaches equilibrium, the water content is determined gravimetrically. Thereby, individual points of the water retention curve can be determined. By adjusting specific pressure heads, samples can also be prepared for further laboratory experiments, e.g. for the determination of sample volume with a 3D scanner or incubation experiments.