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 hydraulics3,4. 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 soils10,11. 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 analysis13,14. 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 agroecosystems3,17,18,19, 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.
- Sample collection
- Excavate the top few centimeters above the desired sampling depth to remove unwanted debris, particularly loose organic litter and soil surface crusting.
- 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.
- 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.
- Dig around the metal sampling core; then, dig underneath the core to remove it from the soil.
- 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.
- Add a sample label to the metal core.
- Sample storage and use
- Store samples in a refrigerator of approximately 4 oC prior to analysis.
- 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
- Tensiometer preparation
- 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.
- Seal the container holding the tensiometers to limit atmosphere diffusion into the water.
- Sensor unit degassing by vacuum pump method
NOTE: Complete the following steps for each sensor unit that will be used in the campaign.- 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.
- 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.
- 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.
- Tensiometer refilling
- Place a stage-mounted cup into both available positions on the degassing unit, then fill ¾ of the cup with degassed deionized water.
- 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.
- Begin vacuum degassing
- Ensure all connections are attached tightly to prevent leakage.
- 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.
- Tap the bottom of the sensor unit assembly on a thick towel to remove air bubbles from the tensiometer ports in the sensor unit.
- 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.
- 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
- Preparation of sensor units
- Plug the sensor unit assembly into the system connection cords.
- Place an absorbent material such as a towel under the connected sensor unit and remove the acrylic top by releasing the metal clips.
- 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.
- Click the data measurement software to open the program and click the Show Devices icon.
- Ensure that all the sensor units that are connected appear in the sidebar of the software.
- Tensiometer installation
- 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). - 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.
- Move the black O-ring on the tensiometer to the center of the threads.
- Invert the tensiometer into the standing water present on the sensor unit while keeping the convex meniscus intact.
- 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.
- 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). - 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.
- Repeat steps 3.2.1 to 3.2.7 for each tensiometer and sensor unit used in the campaign.
- 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.
- Sample placement on sensor units
- Remove a saturated sample and the corresponding saturation plate from the container of degassed deionized water and place them on a working surface.
- Place the auger guide on top of the sample ring.
- 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. - Remove the auger guide and ensure that the soil sample has not collapsed in the hole.
- 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. - Align the holes in the sample core with the corresponding tensiometer height on the sensor unit.
- Invert the sample core and place it on top of the sensor unit, fitting the sample onto the tensiometers.
- Remove the coffee filter and saturation plate. Secure the soil core with metal clasps located on the side of the sensor unit.
- Repeat steps 3.3.1 to 3.3.8 for each of the samples.
- System campaign initiation
- 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 Explorar to save the location of the file.
- Click Start.
- 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.
- 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
- Software termination
- Take a final weight measurement for each sensor unit once the sample has reached the air entry point.
- Click Stop and disconnect the connection cord from each sensor unit.
- Campaign disassembly
- 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. - Remove the silicone disk and clean the top of the sensor unit with a wet towel if necessary.
- Carefully remove each tensiometer from slots. Clean the tips of the tensiometers with a soft bristled toothbrush and water if dirty.
- Clean the sensor unit surface by inverting the unit and spraying water from a safety wash bottle.
- Clean the tensiometer shaft port by inverting the sensor unit and squirting water with a syringe into the port.
- Remove the sample core from the sensor unit. Place all soil material into a container, and oven-dry the soil sample.
5. Data analysis
- Obtain the dry soil weight of each sample and the corresponding metal core.
- 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.
- 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.
- 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.
- Click on the Fitting tab | apply the model that best suits the data.
- Click on the Export tab, choose a file path, and ensure the file is exported in .xlxs format.
- Repeat steps 5.1 to 5.7 for each soil sample.
Representative 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 results should have an output curve containing a clear cavitation and air entry point, so as best to inform a SWRC. If there is no clear cavitation and air entry point, the data can still be used but the SWRC becomes less accurate and more postprocessing within the software is required.
The analysis software contains multiple models that users can employ to analyze tension measurements21,22,23,24. In most cases, the van Genutchen/Mualem model is an effective choice for data; however, users have the option to select the model that best suits their data15. The software then applies this model to the tension curve, and modeled curves become available in the output tab. Water retention, θ(h), and unsaturated hydraulic conductivity K(h) or K(θ) are all generated by the modeling software, where θ is the volumetric water content, K the hydraulic conductivity, and h the matric potential14. A representative example of the SWRC and hydraulic conductivity data output can be seen in Figure 3. Output data that are fit with the van Genutchen/Maulem model can be seen in Figure 4.
The resulting output curve informs the modeled output curves which are the basis of many soil descriptive and hydraulic data. Mainly, the SWRC is generated from the initial data from this method, but other parameters such as porosity, bulk density, saturated water content, and field capacity of the sample soil can be extrapolated from these data. Results (Figure 4) can be compared to understand soil properties that are present and compare across treatments.
Figure 1: Workflow overview. Preparation of both the soil sample and hydraulic property instrument is necessary for executing the protocol. Arrows indicate the workflow sequence, beginning with sample and instrument preparation and ending with data analysis and sample post-processing. Please click here to view a larger version of this figure.
Figure 2: Examples of optimal and suboptimal tension curves. (A) Optimal curve: regular measurement occurs, cavitation is reached, and tension in the tensiometer shaft abruptly drops to ambient pressure. Air entry occurs at ~ 700 hPa for both curves, and the measurements are stopped shortly after. (B) Suboptimal curve: regular measurement occurs and cavitation is reached, but tension in the tensiometer shaft does not abruptly drop and it is difficult to discern where the air entry phase has begun. This curve could still inform a soil water retention curve by using manual adjustment during data post processing, but it is less accurate than (A). Please click here to view a larger version of this figure.
Figure 3: Output curves from software. Soil water retention and hydraulic conductivity curves as displayed in the software before any model has been applied. (A) Soil water retention curve, and (B,C) hydraulic conductivity curves as they appear in the analysis software once sample analysis is complete. Please click here to view a larger version of this figure.
Figure 4: Output curves with applied model. Figure 4 output data can be fit to a model within the instrument software. Here the modeled data are shown; (A) Soil water retention curve, and (B,C) hydraulic conductivity curves with van Genuchten-Mualem model applied to data. Root Mean Square Error (A) 0.4%, (B) 7.09%. Please click here to view a larger version of this figure.
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 analysis processes to generate high quality data. When taking a sample, the soil within the metal core should remain undisturbed. The metal core should remain level throughout hammering to avoid vibrations or tilting that can disturb the soil, and the sample handled carefully throughout transportation.
Additionally, it is necessary to meticulously limit the amount of air that enters each part of the instrument system. Output tension curves will likely be reduced in their accuracy if air has been unintentionally introduced. There are many steps throughout assembling and executing this method that go beyond manufacturer’s recommendations with the aim of eliminating opportunity for air to enter the system. Such nuances include the utilization of degassed water, modified tensiometer refilling procedure, and continual re-application of water in soil sample saturation. It is vital to ensure that any water used in the experiment has been properly degassed, tensiometers have been refilled correctly, and that tensiometer ports in the sensor unit have no bubbles or air pockets within them. These are critical points to diligently observe in the protocol for enabling campaign success.
The measurement campaign is complete after cavitation when the air entry point is reached. This will appear as a sharp peak followed by tension readings rapidly declining to zero for both tensiometers on the measurement curve, as displayed in Figure 2. Following the measurement period and during data analysis, user modifications can be made to increase output data accuracy. For instance, within the data processing software, start, stop, and air entry points can be manually adjusted according to visual inspection and user discretion if there are obvious issues in the tension curve. There may be major fluctuations at the start of a measurement campaign as tensiometers begin taking readings, and water evaporation has not yet begun. This data noise can be easily removed at the post processing stage by moving the start line further right in the software interface. Further, if the tensiometer readings for cavitation have a plateau rather than a sharp peak, it may be difficult for the software to automatically discern where cavitation begins and ends. This challenge can be remedied by moving both the stop top and stop bottom line to scale locations points that capture a wide enough change in slope to indicate cavitation. Likewise, air entry points can be adjusted in a similar fashion, as they can be moved to the end of the plateau portion of the curve, ensuring that the proper portion of the data domain is being captured by the software.
Fewer data points towards the dry range of the SWRC generated from the hydraulic property instrument can create inaccuracies when applying models to the data. This situation can be augmented by incorporating other sources of data covering the dry range of water retention. Additional data points can be supplemented by analyzing the same soil samples with the soil water potential instrument and then manually inputting these points to the analysis software. The additional data increase the validity of estimates in the dry range and are relatively easy to use. Combining data from both instruments can become valuable if the research goal is concerned with the whole range of soil moisture retention. Further information about the soil water potential instrument operation, applicability of these measurements, and integration with the hydraulic property instrument can be found in existing literature2,16,17,25.
Soil water retention and hydraulic conductivity curves provide an important characterization of the physical and unsaturated hydraulic properties of a soil sample. Across all disciplines within environmental, agricultural, and of soil sciences, land management practices that change physical and hydraulic properties of soil can have a lasting effect on soil health3,17. Quantifying soil metrics and indicators within soil research and monitoring can help create a better awareness of the true effects of contrasting management practices on soil. Understanding properties such as field capacity, plant available water, pore size distribution, and water conductivity further an informed conclusion about preferable soil management practices for soil health26. Increasing the use of robust methods as described here can contribute to deepen and harmonize a unified body of knowledge concerning the paramount impacts of contrasting soil management options across a broad range of land systems.
Declarações
The authors have nothing to disclose.
Acknowledgements
The authors gratefully acknowledge the financial support provided by the Canadian Foundation for Innovation (John Evans Leadership Fund) in the acquisition of the hydraulic property analyser instrument.
Materials
| 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 |
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