Why soil moisture sensors can’t tell you everything you need to know
Accurate, inexpensive soil moisture sensors make soil VWC a justifiably popular measurement, but is it the right measurement for your application?
Lab vs. field instruments—why you should use both
Lab and field instruments used together can provide researchers a symphony of information and can be used as powerful tools in understanding data and predicting a soil’s behavior over time.
When researchers measure soil hydraulic properties in the lab or in the field, they’re likely only seeing part of the picture. Laboratory systems are highly accurate due to controlled conditions, but lab measurements don’t take into account site variability such as roots, cracks, or wormholes that might affect soil hydrology. In addition, when researchers take a sample from the field to the lab, they often compress soil macropores during the sampling process, altering the hydraulic properties of the soil.
Field experiments help researchers understand variability and real time conditions, but they have the opposite set of problems. The field is an uncontrolled system. Water moves through the soil profile by evaporation, plant uptake, capillary rise, or deep drainage, requiring many measurements at different depths and locations. Field researchers also have to deal with the unpredictability of the weather. Precipitation may cause a field drydown experiment to take an entire summer, whereas in the lab it takes only a week.
| Lab | Measurement | Field |
|---|---|---|
| PARIO | Soil texture | |
| HYPROP | Water potential (wet range) | Tensiometers (TEROS 32) |
| WP4C | Water potential (dry range) | TEROS 21 |
| HYPROP, and oven drying method | Water content | Volumetric water content sensors |
| KSAT | Saturated hydraulic conductivity | SATURO (field saturated) |
| HYPROP | Unsaturated hydraulic conductivity | Mini disk infiltrometer |
The big picture—supersized
Researchers who use both lab and field techniques while understanding each method’s strengths and limitations can exponentially increase their understanding of what’s happening in the soil profile. For example, in the laboratory, a researcher might use the PARIO soil texture analyzer to obtain accurate soil texture data, including a complete particle size distribution. They could then combine those data with a HYPROP-generated soil moisture release curve to understand the hydraulic properties of that soil type. If that researcher then adds high-quality field data in order to understand real world field conditions, then suddenly they’re seeing the larger picture.
Below is an exploration of lab versus field instrumentation and how researchers can combine these instruments for an increased understanding of their soil profile. Click the links for more in-depth information about each topic.
| Strengths | Limitations | |
|---|---|---|
| Lab Instrumentation |
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| Field Instrumentation |
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Particle size distribution and why it matters
Soil type and particle size analysis are the first window into the soil and its unique characteristics. Every researcher should identify the type of soil that they’re working with in order to benchmark their data. If researchers don’t understand their soil type, they can’t make assumptions about the state of soil water based on water content (i.e., if they work with plants, they won’t be able to predict whether there will be plant available water). In addition, differing soil types in the soil’s horizons may influence a researcher’s measurement selection, sensor choice, and sensor placement.
Particle size analysis defines the percentage of coarse to fine materials that make up a soil. With this knowledge, a researcher can estimate how strongly a particular soil will hold on to water. Particle size analysis goes beyond the simple definition of soil type. The particle size analysis acts more like a soil fingerprint, showing the unique distribution of particle size across the sand, silt, and clay fractions. This information can help a geotechnical engineer understand how a shrink-swell soil will react over time, or it might influence a grower’s irrigation decisions. Particle size distribution also may provide insights into how the soil formed, or eventually will form structure, and it influences saturated hydraulic conductivity: the more coarse the material is, the more easily water will move.
Historically, researchers identified soil texture using crude methods such as the ribbon test, the pipette method, or the time-consuming hydrometer technique. PARIO now automates the process of soil texture analysis, saving time and increasing accuracy. PARIO gives researchers a complete particle size distribution analysis, including a breakdown of the fraction of fine silt, middle silt, clay, and sand. After obtaining the analysis, the software automatically computes its location on the USDA soil texture triangle to accurately identify the soil type.
Researchers should use PARIO as a first step in understanding their soil, before they decide what other parameters to measure. This will help them determine which lab or field instrumentation will be most effective for their research goals.
A moisture release curve is a soil’s Rosetta Stone
Each soil type has a different moisture release curve (or soil water characteristic curve). Researchers use moisture release curves to comprehend how soil and plants would react if the moisture changed in a particular soil over time. It tells them how quickly the amount of water (water content) will change, compared to how much water is available (water potential).
![A graph showing a Soil moisture release curve (or soil water characteristic curve [SWCC])](https://cdn-images.metergroup.com/app/uploads/2023/05/soil-moisture-release-curve-1-1.jpg)
Moisture release curves help researchers predict if water will move and where it’s going to go. A release curve also illustrates how much water will be available to plants at different water contents over time. For example, in a sand near saturation, water content will change quickly over time while water potential will only decrease slightly. This is because the large pores and low surface area of the sand does not hold water tightly, making it more available. Conversely, in a clay near saturation, the amount of water changes more slowly while water potential changes relatively quickly because clay’s higher surface area and smaller pores hold water more tightly, releasing less for plants or water movement. A moisture release curve illustrates the relationship between water content and water potential and shows researchers how the soil will behave in any condition.
Learn more about why and how to create a moisture release curve
Download the “Researchers complete guide to water potential”
HYPROP: the expert on soil moisture release curves
The easiest way to create a moisture release curve is in the lab. The HYPROP is a unique lab instrument that uses the Wind/Schindler evaporation method to generate moisture release curves on soils with water potentials in the tensiometer range—the range of most water movement. Using two precision tensiometers, it automatically produces over 100 data points in the 0 to -100 kPa range. The curve takes three to five days to complete, but the instrument operates unattended. The HYPROP’s range is limited by the range of tensiometers, but can be combined with the WP4C to produce a moisture release curve over the entire moisture range.
Combine the HYPROP with the WP4C for a full moisture release curve
The WP4C is a lab instrument that measures water potential in the dry range by determining the relative humidity of the air above a sample in a closed chamber. Once the sample comes into equilibrium with the vapor in the WP4C’s sealed chamber, the instrument finds relative humidity using the chilled-mirror method. This method entails chilling a tiny mirror in the chamber until dew just starts to form on it. At the dew point, the WP4C measures both mirror and sample temperature with 0.001 °C accuracy. This allows the WP4C to deliver water potential readings with unparalleled accuracy in the -0.05 MPa to -300 MPa range.
The WP4C can be used in concert with the HYPROP to create a complete soil moisture release curve over the full range of moisture in soil. Watch the video to see how this works. This, combined with information extracted from the PARIO can be a powerful tool for understanding soil hydraulic properties.
Learn how to combine HYPROP and WP4C for a full moisture release curve
Moisture release curves in the field? Yes, it’s possible
The HYPROP and WP4C provide the ability to make fast, accurate soil moisture release curves (soil water characteristic curves-SWCCs), but lab measurements have some limitations: sample throughput limits the number of curves that can be produced, and curves generated in a laboratory do not represent their in situ behavior. However, lab-produced soil water retention curves can be paired with information from in situ moisture release curves for deeper insight into real world variability. Co-locating matric potential sensors and water content sensors in situ add many more moisture release curves to a researcher’s knowledge base. And, since it is primarily the in-place performance of unsaturated soils that is the chief concern to geotechnical engineers and irrigation scientists, adding in situ measurements to lab-produced curves would be ideal. A recent paper by Campbell et al. (2018), “Comparing in situ soil water characteristic curves to those generated in the lab”, given at the Pan American Conference of Unsaturated Soils shows how well in situ generated SWCCs using the TEROS 21 calibrated matric potential sensor and METER’s soil moisture sensors compare to those created in the lab. Field and lab SWCCs compared quite well, but a few factors reduced their agreement. In coarse-textured soils, removing living roots caused a divergence as the soil suction increased, and intact core samples compared more favorably than disturbed. In finer textured soils, comparisons were favorable but were also affected when laboratory samples were disturbed. Data suggest collocated in situ sensors could provide an important augmentation to laboratory data for developing a wide range of SWCCs in order to create a more robust understanding of unsaturated soil behavior.
Mix, match, and compare hydraulic conductivity in the lab or the field
Much like lab and in-situ soil moisture release curves, lab and field measurements for saturated and unsaturated hydraulic conductivity can be used in tandem for a more in-depth understanding of the hydraulic properties of any soil type. Comparing these measurements at different depths and locations can provide insight into various soil horizons and help researchers understand long-term infiltration data from each of those horizons (i.e., if one horizon becomes saturated, how will that change a model for runoff?).
Scientists can use field instruments to determine how water will infiltrate in the field, and they can add lab measurements to identify the most limiting horizon. For example, a surface horizon may be a sandy loam, but the PARIO could reveal that a deeper layer has a higher clay content with lower hydraulic conductivity. Using lab and field measurements together help determine which horizon is causing lower permeability during wetter periods.
Field saturated hydraulic conductivity data from the SATURO can be augmented with KSAT lab measurements. Unsaturated hydraulic conductivity field data produced by the Mini Disk Infiltrometer can be understood at a deeper level when a researcher pairs them with HYPROP unsaturated hydraulic conductivity data from the lab. HYPROP uses the same internal tensiometers that generate moisture release curves to automatically measure unsaturated hydraulic conductivity and then model saturated hydraulic conductivity. Typically, lab and field measurements won’t match up because of real world variability, but analyzing the information together provides greater insight.
Learn about combining hydraulic conductivity with other measurements
What a hydraulic conductivity curve will tell you
Researchers can also combine hydraulic conductivity data from two laboratory instruments, the KSAT and the HYPROP, to produce a full hydraulic conductivity curve (Figure 2). A hydraulic conductivity curve tells you, at a given water potential, the ability of the soil to conduct water (i.e., as the soil dries, what is the ability of water to go from the the top of a sample [or soil layer in the field] to the bottom). These curves are used in modeling to illustrate or predict what will happen to water moving in a soil system during fluctuating moisture conditions.
Learn more about hydraulic conductivity curves
In the video below, soil moisture expert, Leo Rivera teaches the basics of hydraulic conductivity and hydraulic conductivity curves.
More instruments create a symphony of data
Measuring only a single parameter such as water content may give researchers a starting point for understanding their soil, but they won’t understand what that percentage of water is telling them without knowing other information such as soil type, water potential, or hydraulic conductivity. For the deepest insight into soil, researchers can use particle size distribution, hydraulic conductivity curves, and moisture release curves in concert for the most accurate and comprehensive information. Using two different types of curves can even help researchers isolate obscure issues such as a dual porosity moisture release curve in a soilless substrate. Lab and field instruments used together can provide researchers a symphony of information and can be used as powerful tools in understanding data and predicting a soil’s behavior over time.
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Six short videos teach you everything you need to know about soil water content and soil water potential—and why you should measure them together. Plus, master the basics of soil hydraulic conductivity.
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