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

Summary

This article features a simple evaporation experiment using a hydraulic property instrument for a soil sample. Through efficient means, measurements can be taken over a series of days to generate high-quality data.

Abstract

The measurement of soil hydraulic properties is critical in understanding the physical components of soil health as well as integrated knowledge of soil systems under various management practices. Collecting reliable data is imperative for informing decisions that affect agriculture and the environment. The simple evaporation experiment described here uses instrumentation in a laboratory setting to analyze soil samples collected in the field. The soil water tension of the sample is measured by the instrument, and tension data is modeled by software to return soil hydraulic properties. This method can be utilized to measure soil water retention and hydraulic conductivity and give insight into differences in treatments or environmental dynamics over time. Initial establishment requires a user, but data acquisition is automated with the instrument. Soil hydraulic properties are not easily measured with traditional experiments, and this protocol offers a simple and optimal alternative. Interpretation of results and options for extending the data range are discussed.

Introduction

Soil water retention and hydraulic conductivity within natural and human-altered environments help us understand and observe changes in soil health and functionality. Quantifying hydraulic properties through the soil water retention curve (SWRC) and soil water conductivity curve offers insight into key drivers of soil physical behavior and water movement characterization¹. The relationship between volumetric water content (θ) and matric head (h) is represented within an SWRC, and the ranges within the curve describe the saturation point, field capacity, and permanent wilting point². Soil management practices, amendments, agroecosystem types, and environmental conditions can all have an impact on soil hydraulics^3,4. These factors can in turn influence solute transport⁵ and plant available water⁶, soil respiration and microbial activity⁷, as well as wetting and drying cycles⁸. As an important piece in quantifying healthy and functioning soil, proper analysis of the SWRC is imperative for obtaining an informed understanding of soil hydraulic properties.

A variety of measurement techniques currently exist for developing a reliable SWRC, with the hanging-water column and pressure plate methods being common traditional approaches for determining the pore size distribution of soil². Traditional methods can be time-consuming, usually taking weeks or months to analyze a small set of samples⁹. Moreover, once analysis is complete, these methods result in only a few data points that inform the SWRC⁹. Additionally, the accuracy of producing representative data using traditional methods such as pressure plates can become a concern at lower matric potentials, in particular, with fine-textured soils^10,11. More modern techniques, which involve the simple evaporation experiment approach using tensiometers and the chilled-mirror dew point method, tend to deliver more reproducible data across a wide range of soil textures². Initially developed by Wind in 1968, the simple evaporation experiment involved measuring water mass changes and tension changes through tensiometers in the soil sample over time¹². As evaporation occurs, soil sample mass measurements are taken at specific time intervals to create an SWRC. Later refined by Schindler (1980), the method involved only two tensiometers placed at different pressure heads within the soil sample. The modified method was then tested and validated as capable of being used in scientific analysis^13,14. A key benefit of the simple evaporation experiment is the potential to easily produce data across a large portion of the soil moisture curve (0 to -300 kPa), with more data points than with traditional methods.

These modern methods involve automated instruments that take numerous data points throughout the sample analysis period and produce data using a software interface. The hydraulic property instrument is a contemporary instrument that creates water retention curves and conductivity curves from sample data¹⁵. By employing a simple evaporation experiment using the hydraulic property instrument, the relationship between water content and water potential in the soil can be evaluated¹. In this experiment, water present within the tensiometer shaft exists in an equilibrium with water in the soil solution. As evaporation of soil water occurs and the soil sample dries, cavitation takes place in the tensiometer, and the experiment ends. There is a limitation of the hydraulic property instrument in the dry range of the SWRC, as the instrument is only capable of operating within matric potentials of 0 to -100 kPa. This can be remedied with the inclusion of data generated with a chilled-mirror dew point experiment using a soil water potential instrument¹⁶, which can extend the data range to -300,000 kPa or the permanent wilting point. All these data are brought together in the modeling software post processing to cohesively inform the SWRC from null tensions to higher tensions even beyond wilting point. The SWRC and hydraulic conductivity curves are then generated based on matric potential data points taken throughout the measurement period, allowing a complete curve projected from saturation to permanent wilting point to be generated.

The method described here presents a succinct operating procedure for soil analysis with a hydraulic property instrument. This method has been conducted in a number of scientific settings, including quantification of soil health in a broad range of agroecosystems^3,17,18,19, and efforts have been made to understand best practices beyond the instrument user manual²⁰. Here, a standardized protocol is outlined for all steps of the procedure, including field sampling, sample preparation, software function, and data processing. Following this method will ensure a successful campaign that results in reliable data. Critical steps for ensuring quality data, common challenges, and best practices are presented to ensure proper implementation.

Protocol

1. Soil sampling and sample preparation

NOTE: A schematic diagram of the workflow of this method can be found in Figure 1.

1. Sample collection
   1. Excavate the top few centimeters above the desired sampling depth to remove unwanted debris, particularly loose organic litter and soil surface crusting.
   2. Place the metal sampling core level on the surface of the exposed soil, with the sharp edge side facing the soil surface; then, place the hammering holder on top of the ring.
   3. Hit the top of the hammering holder repeatedly using a rubber mallet until the top of the metal sampling core aligns with the soil surface.
   4. Dig around the metal sampling core; then, dig underneath the core to remove it from the soil.
   5. Level both sides of the metal sampling core with a trowel or knife once it is removed from the soil; then, place plastic covers on either side of the core.
   6. Add a sample label to the metal core.
2. Sample storage and use
   1. Store samples in a refrigerator of approximately 4 ^oC prior to analysis.
   2. Saturate samples at least 24 h before analysis by placing the sample cores in a large plastic container with degassed deionized water. First, remove the plastic cover that is on the flat edge side of the metal sampling core and place a paper coffee filter on top, followed by a saturation plate. Then, invert the core and saturation plate into the container and fill it with degassed deionized water within 1 cm of the top of the soil sample. Refill the container with degassed deionized water as needed until the soil has reached saturation.
      NOTE: Saturation occurs when water is visible on the exposed surface of the soil.

2. Sensor unit and tensiometer establishment

1. Tensiometer preparation
   1. Soak the tensiometers for 24 h in degassed deionized water: one tall (50 mm length) and one short (25 mm) tensiometer for each sensor unit that will be used in the analysis campaign.
   2. Seal the container holding the tensiometers to limit atmosphere diffusion into the water.
2. Sensor unit degassing by vacuum pump method
   NOTE: Complete the following steps for each sensor unit that will be used in the campaign.
   1. Fill both tensiometer shaft ports with degassed deionized water to the top of the port using a 20 mL syringe and fine-tip needle. Ensure the pressure transducer is clean by shining a light into the port and assessing if there are debris present.
   2. 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 to just below the top of the acrylic head.
   3. Attach the acrylic top to the degassing unit by inserting the 'T' tube on the degassing unit to the top of the acrylic head.
      NOTE: Two sensor units with acrylic tops can be attached to each degassing unit.
3. Tensiometer refilling
   1. Place a stage-mounted cup into both available positions on the degassing unit, then fill ¾ of the cup with degassed deionized water.
   2. Screw the tensiometers into the threaded acrylic holders; then, move the black O-ring to meet the top of the acrylic holder. Place into the degassed deionized water held within the stage-mounted cups.
4. Begin vacuum degassing
   1. Ensure all connections are attached tightly to prevent leakage.
   2. Turn on the vacuum pump until -0.4 Bar is reached; then, turn off the vacuum pump and let the system equalize. Turn on the vacuum pump again until -0.8 bar is reached.
   3. Tap the bottom of the sensor unit assembly on a thick towel to remove air bubbles from the tensiometer ports in the sensor unit.
   4. Keep the system under vacuum for at least 24 h. Check for leaks by turning off the vacuum and ensuring it remains at pressure. Turn on the vacuum pump at regular intervals to bring the vacuum to pressure, since it will slowly lose pressure as the system equalizes.
   5. After 24 h, remove the tubing from the acrylic top, and remove all tensiometers from the acrylic holders. Place the short and tall tensiometers into separate beakers of degassed deionized water.

3. Initiating a campaign

1. Preparation of sensor units
   1. Plug the sensor unit assembly into the system connection cords.
   2. Place an absorbent material such as a towel under the connected sensor unit and remove the acrylic top by releasing the metal clips.
   3. Repeat Steps 3.1.1 and 3.1.2 for each sensor unit assembly for a multi-sensor setup, or continue to 3.A.iv for a single sensor setup.
   4. Click the data measurement software to open the program and click the Show Devices icon.
   5. Ensure that all the sensor units that are connected appear in the sidebar of the software.
2. Tensiometer installation
   1. Click the Refilling Wizard icon at the top of the software to open the user interface. From the dropdown menu, navigate to the appropriate sensor unit.
      NOTE: Transducers are in good working order if their initial readings are 0 hPa (± 5 hPa).
   2. Select a tensiometer from the beaker. Ensure that there are no visible air bubbles in the shaft, and that there is water forming over the top of the tensiometer shaft in a convex meniscus. Add more degassed deionized water with a syringe to the top of the tensiometer if there is no convex meniscus.
   3. Move the black O-ring on the tensiometer to the center of the threads.
   4. Invert the tensiometer into the standing water present on the sensor unit while keeping the convex meniscus intact.
   5. Install the short tensiometer in the tensiometer port that is indicated with a short line. Install the tall tensiometer in the tensiometer port that is indicated with a long line.
   6. Carefully screw the tensiometer into the tensiometer port while watching the pressure readings on the Current Readings tab. Obtain a tight seal by doing a half-to-full turn of the tensiometer.
      ​NOTE: After being installed on the tensiometer port, ensure that tensiometer readings are 0 hPa (± 5 hPa).
   7. Place a silicon bulb filled with degassed deionized water on the top of the tensiometer to prevent desiccation of the tip while other tensiometers are being installed.
   8. Repeat steps 3.2.1 to 3.2.7 for each tensiometer and sensor unit used in the campaign.
3. Sample placement on sensor units
   1. Remove a saturated sample and the corresponding saturation plate from the container of degassed deionized water and place them on a working surface.
   2. Place the auger guide on top of the sample ring.
   3. Insert the tensiometer shaft auger into the hole of the auger guide and turn the tensiometer shaft auger in a complete rotation to remove soil. Repeat for the second hole.
      NOTE: Keep track of the depth each hole makes in the soil sample as it corresponds to the height of the tensiometer.
   4. Remove the auger guide and ensure that the soil sample has not collapsed in the hole.
   5. Remove the silicon bulbs on each tensiometer and place the silicon disk on top of the sensor unit.
      NOTE: Ensure that no air is trapped under the silicon disk, and that the temperature sensor is not covered.
   6. Align the holes in the sample core with the corresponding tensiometer height on the sensor unit.
   7. Invert the sample core and place it on top of the sensor unit, fitting the sample onto the tensiometers.
   8. Remove the coffee filter and saturation plate. Secure the soil core with metal clasps located on the side of the sensor unit.
   9. Repeat steps 3.3.1 to 3.3.8 for each of the samples.
4. System campaign initiation
   1. Once each sensor unit has been set up, input the sample identification present on the metal core as it corresponds to each of the sensor unit serial numbers. Input a unique name for the field campaign, the click Browse to save the location of the file.
   2. Click Start.
   3. Take the initial weight reading after two tensiometer readings have been completed. First, unplug the connection cord from the sensor unit, wait for a dialog box to appear on the software, and place it on the weight scale. Remove the sensor unit once the software indicates the weight reading has been taken and plug it back into the connection cord. Repeat for all sensor units.
   4. Weigh samples 3x a day for the first 2 days of measurement, then 2x a day at regular intervals for the remainder of the campaign

4. System campaign termination

1. Software termination
   1. Take a final weight measurement for each sensor unit once the sample has reached the air entry point.
   2. Click Stop and disconnect the connection cord from each sensor unit.
2. Campaign disassembly
   1. Remove the sample core from the sensor unit. Place all soil material into a container, and oven-dry the soil sample.
      NOTE: If working with fine-textured soils, wet each soil sample within an hour before removing it from the sensor unit.
   2. Remove the silicone disk and clean the top of the sensor unit with a wet towel if necessary.
   3. Carefully remove each tensiometer from slots. Clean the tips of the tensiometers with a soft bristled toothbrush and water if dirty.
   4. Clean the sensor unit surface by inverting the unit and spraying water from a safety wash bottle.
   5. Clean the tensiometer shaft port by inverting the sensor unit and squirting water with a syringe into the port.

5. Data analysis

1. Obtain the dry soil weight of each sample and the corresponding metal core.
2. Click the data analysis software to open the program and click on a sample file to open the data in the software. Input the weight of the metal core in the Parameter section of the Information tab.
3. Click the Measurements tab | Search Air Entry Point. To fine-tune the point of cavitation, move the dotted lines of the start and stop points in the tensiometer data range. Do the same for the air entry points if it needs to be specified for the software.
4. Click on the Evaluation tab, and under Calculation of water contents, ensure that From dry soil weight (g) is selected. Input the weight of the dried soil.
5. Click on the Fitting tab | apply the model that best suits the data.
6. Click on the Export tab, choose a file path, and ensure the file is exported in .xlxs format.
7. Repeat steps 5.1 to 5.7 for each soil sample.

Results

Upon completing a proper measurement campaign following the protocol above, it will be possible to view the data output of the experiment in the analysis software. Output curves originate from tensiometer readings that measure water tension (hPa) over time (t), and the initial curve of this data is generated immediately after termination of the campaign. Selected examples of tension curves of two soil samples can be examined to illustrate optimal and suboptimal results (Figure 2). Optimal re...

Discussion

The simple evaporation experiment approach using the method that is described here is an efficient means to develop the SWRC and hydraulic conductivity curves. Simplicity and accuracy of data measurement make it a viable alternative to more traditional methods¹⁴. The method described here goes beyond the user manual and current literature to synthesize and expand on finer points of this intricate instrument. Particular attention needs to be paid to the soil sample collection, transport, and analys...

Materials

Name	Company	Catalog Number	Comments
4 L Buchner Flasks (two)	Various	n/a	Containers for water degassing
20 mL Syringe, fine tip	BD	BD-302830
Coffee filter	Various	n/a	Prevents soil travel out of core while soaking
HYPROP Complete Set	Hoskin	110813/E240-M020210	tensiometer shaft auger, tube for vacuum syringe and refilling adapter, auger guide, HYPROP USB adapter, HYPROP sensor unit, tensiometer shafts (50 mm and 25 mm), saturation plate, refilling adapter, silicone gasket, set of o-rings, LABROS balance, software, cables
HYPROP Refill Unit	Hoskin	108899/ E240-M020258	vacuum pump, vacuum mount, beaker mount, refilling adapters
Large Plastic Tubs	Various	n/a	Holds water and soil cores during saturation
METER hammering holder	Hoskin	100255/E240-100201
Rubber Mallet	Home Depot	18CT1031	Sample collection tool used with hammering holder
Shovel	Home Depot	83200
Soil Sampling Ring incl. 2 caps	Hoskin	100254/E240-100101
Stir plate/ Stirring Bar	Various	n/a
Trowel	Home Depot	91365