Measurements of Soil Water Potential and Conductivity based on a Simple Evaporation Experiment using a Hydraulic Property Analyzer

At the Sustainable Land Ecosystems Lab, our research focuses on finding sustainable solutions to problems affecting land ecosystems, including croplands and grasslands. Solving this complex problem involves researching soil health, a key part of all agricultural land ecosystems. Obtaining a soil water retention curve that extends into the dry range is a challenge.

Currently, it is necessary to combine the data from this experiment with additional data on another instrument. This can be less than ideal for some research objectives. The main advantage here is the amount of data the instrument can produce with relatively low user time and input.

Compared to other techniques that attempt to create a soil water retention curve with few data points, this instrument provides more precise data. To begin, remove the plastic cover from the flat edge side of the metal sampling core, and place a paper coffee filter on top, followed by a saturation plate. Saturate the soil samples for at least 24 hours inside the sample cores in a large plastic container with degassed deionized water.

Invert the core and saturation plate into the container and fill it with degassed deionized water, leaving a one centimeter space from the top of the soil sample. Using a 20 milliliter syringe and fine tip needle, fill both tensiometer shaft ports with degassed deionized water. Shine a light into the port and check for debris to confirm if the pressure transducer is clean.

Place the acrylic top on the sensor unit and fasten the metal clips. Insert the syringe containing degassed deionized water into the opening of the acrylic top and fill just below the top of the acrylic head. To attach the acrylic top to the degassing unit, connect the T tube on the degassing unit to the top of the acrylic head.

Place stage-mounted cups into both positions on the degassing unit, and fill three quarters of each cup with degassed deionized water. Screw the tensiometers into the threaded acrylic holders and adjust the black O ring to the top of the acrylic holder. Place the tensiometers into the degassed deionized water in the stage-mounted cups.

Turn on the vacuum pump until minus 0.4 bar is reached. Then turn off the vacuum pump and allow the system to equalize. Turn on the vacuum pump again until minus 0.8 bar is reached.

After 24 hours, remove the tubing from the acrylic top and take out all tensiometers from the acrylic holders. Place the short and tall tensiometers into separate beakers of degassed deionized water. To begin, saturate the soil samples with degassed deionized water and prepare the tensiometers for installation.

For sensor unit setup, plug the sensor unit assembly into the system connection cords. Release the metal clips and remove the acrylic top. Click the data measurement software to open the program and click the show devices icon.

Click the refilling wizard icon to open the user interface and navigate to the appropriate sensor unit from the dropdown menu. Select a tensiometer from the beaker. Ensure there are no visible air bubbles in the shaft and a convex meniscus of water forms at the top.

Add more degassed deionized water with a syringe if there is no convex meniscus. Then move the black O ring on the tensiometer to the center of the threads. Invert the tensiometer into the standing water on the sensor unit while keeping the convex meniscus intact.

Install the short tensiometer in the port indicated with a short line and the tall tensiometer in the port with a long line. Carefully screw the tensiometer into the port while observing the pressure readings on the current readings tab. Next, place a silicon bulb filled with degassed deionized water on the top of the tensiometer to prevent the tip from desiccating during installation.

To begin, saturate the soil samples with degassed deionized water. After setting up the tensiometers and the sensor, place the saturated sample and the corresponding saturation plate on a working surface. Place the agar guide on top of the sample ring.

Insert the tensiometer shaft agar into the hole of the agar guide, and turn it completely to remove soil. Then remove the agar guide and ensure that the soil sample has not collapsed in the hole. Remove the silicon bulbs from each tensiometer and place the silicon disc on top of the sensor unit.

Place an absorbent material such as a towel under the connected sensor unit. Invert the sample core and place it on top of the sensor unit fitting the sample onto the tensiometers. Next, remove the coffee filter and saturation plate.

Secure the soil core with metal clasps on the side of the sensor unit. To begin, place the saturated soil samples in the tensiometer sensor unit. Once each sensor unit is set up, input the sample identification present on the metal core.

Then click Browse to save the file location. After clicking Start, take the initial weight reading after two tensiometer readings have been completed. First, unplug the connection cord from the sensor unit.

Place the unit on the weight scale when a dialog box appears on the software. Remove the sensor unit once the software indicates the weight reading has been taken, and plug it back into the connection cord. Once the sample has reached the air entry point, take a final weight measurement for each sensor unit and click Stop.

The measurement campaign revealed distinct tension curves for two soil samples, showcasing optimal and suboptimal results. Soil water retention curves and hydraulic conductivity data were generated from the experiment with optimal data, demonstrating clear air entry and cavitation points. The application of the van Genuchten-Mualem model to the output data provided fitted curves, which were used to derive soil properties.