Soil moisture sensors—How they work. Why some are not research grade
TDR, FDR, capacitance, resistance: A comparison of common soil moisture sensing methods, their pros and cons, and their unique applications.
Which soil sensor is perfect for you?
Among the thousands of peer-reviewed publications using METER soil sensors, no type emerges as the favorite. Sensor choice should be based on your needs and application. Use these considerations to help identify the perfect sensor for your research.
Which soil moisture sensor should you choose?
Deciding how to measure soil moisture can be complicated, but METER’s easy, plug and play soil sensors simplify the process. When choosing a METER soil moisture sensor, the first thing to know is that all METER soil sensors have the same research-grade accuracy with minimum sensitivity to salts (<10 dS/m bulk EC), temperature gradients, and soil texture. Among the thousands of peer-reviewed publications using METER soil sensors, no soil moisture sensor or water potential sensor emerges as the favorite. Thus sensor choice should be based on your needs and application. Use the following considerations to help identify the perfect soil moisture sensor or water potential sensor for your research. Explore the links for a more in-depth look at soil moisture definitions, terms, and topics.
HOW TO MEASURE SOIL MOISTURE—CHOOSE THE RIGHT MEASUREMENT
Volumetric water content
If you want to measure the rise and fall of the amount (or percentage) of water in the soil, you will need a water content sensor (soil moisture sensor). Soil is made up of water, air, minerals, organic matter, and sometimes ice (Figure 1). As a component, water makes up a percentage of the total. To directly measure soil water content, you can calculate the percentage on a mass basis (gravimetric water content) by comparing the amount of water, as a mass, to the total mass of everything else. However, since this method is labor-intensive, most researchers use soil moisture sensors to make an automated volume-based measurement called volumetric water content (VWC).
METER soil moisture sensors use high-frequency capacitance technology to measure the volumetric water content of the soil, meaning they measure the quantity of water on a volume basis compared to the total volume of the soil. Applications that typically need soil moisture sensors are watershed characterization, irrigation scheduling, greenhouse management, fertigation management, plant ecology, water balance studies, microbial ecology, plant disease forecasting, soil respiration, hydrology, and soil health monitoring. Learn more about volumetric water content here.
Water potential
If you need an understanding of plant-available water, plant water stress, or water movement (if water will move and where it will go), a water potential soil sensor is required in addition to a soil moisture sensor. Water potential is a measure of the energy state of the water in the soil, or in other words, how tightly water is bound to soil surfaces. This tension determines whether or not water is available for uptake by roots and provides a range that tells whether or not water will be available for plant growth. In addition, water always moves from a high water potential to a low water potential, thus researchers can use water potential to understand and predict the dynamics of water movement. Learn more about water potential here.
Download the “Researcher’s complete guide to water potential”
Understand your soil type and texture
In soil, the void spaces (pores) between soil particles can be simplistically thought of as a system of capillary tubes, with a diameter determined by the size of the associated particles and their spatial association. The smaller the size of those tubes, the more tightly water is held because of the surface association.
Clay holds water more tightly than a sand at the same water content because clay contains smaller pores and thus has more surface area for the water to bind to. But even sand can eventually dry to a point where there is only a thin film of water on its surfaces, and water will be bound tightly. In principle, the closer water is to a surface, the tighter it will be bound. Because water is loosely bound in a sandy soil, the amount of water will deplete and replenish quickly. Clay soils hold water so tightly that water movement is slow. However, there is still available water.
Note: Use the PARIO soil texture analyzer to automate soil type and texture identification.
Two measurements are better than one
In all soil types and textures, soil moisture sensors are effective at measuring the percentage of water. Dual measurements—using both types of soil sensors: a water potential soil sensor in addition to a soil moisture soil sensor—gives researchers the total soil moisture picture and are much more effective at determining when, and how much, to water. Water content data show subtle changes due to daily water uptake and also indicate how much water needs to be applied to maintain the root zone at an optimal level. Water potential data determine what that optimal level is for a particular soil type and texture. Read more about how this works here.
Get the big picture with moisture release curves
Dual measurements of both water content and water potential also enable the creation of in situ soil moisture release curves (or soil water characteristic curves) like the one below (Figure 5), which detail the relationship between water potential and water content. Scientists and engineers can evaluate these curves in the lab or the field and understand many things about the soil, such as hydraulic conductivity and total water availability. Learn more about soil moisture release curves here.
Questions? Talk to an expert
CHOOSE THE RIGHT SOIL MOISTURE SENSOR
Life expectancy matters
It’s important for researchers to know how long an experiment is going to run so they can choose a soil moisture sensor that will meet their expectations. METER’s 10HS soil moisture sensor and EC-5 soil moisture sensor have an overmolding technology that lasts approximately 3 to 5 years in the field with typical use (less in warm/wet conditions). The ruggedized TEROS 11, TEROS 12, TEROS 10 and TEROS 54 soil sensors are made to last twice as long as our standard sensors due to an upgraded polyurethane (epoxy) fill. Lab tests indicate these sensors last 10+ years before water intrudes to the circuit board. If the research environment is tropical—warm and typically wet—always choose a long-life sensor.
Which sensor for what purpose?
METER’s soil moisture sensors have minimal sensitivity to temperature, but if the installation depth is shallow and the location is exposed, temperature effects need to be considered. The TEROS 11 and TEROS 12 soil moisture sensors and the TEROS 54 soil moisture profile probe have an onboard thermistor that measures temperature along with soil moisture. This eliminates the need for extra temperature sensors at every measurement site. The TEROS 54 profile probe measures soil water content and soil temperature at multiple depthsplug-and-play enabling you to make more measurements with each data logger port.
Researchers who want to measure bulk electrical conductivity (EC) in addition to water content should choose the TEROS 12 soil moisture sensor. This sensor enables users to measure the bulk EC response to salts and fertilizers in the soil. EC measurements will require good contact between the stainless steel electrodes on the sensor and the soil.
For soil moisture measurements only, many scientists prefer the TEROS 10. It’s easy to install, inexpensive, and reliable—perfect for big projects where sensors are needed in quantity. In difficult (hard or rocky) soils, potting soil, and soilless media, we recommend using TEROS series sensors to maintain good soil contact and compensate for air gaps in the soil or substrate.
All METER soil moisture sensors are plug-and-play with METER data loggers. They also integrate with third-party loggers using SDI-12protocol. Use the ZL6 for easy data collection from remote sites, or you can manually download data.
How METER water content sensors compare with other methods
In this 20-minute webinar, Dr. Colin Campbell demystifies the differences between soil water content measurement methods. He explores the scientific measurement theory and the pros and cons of each method. He also explains which technology might apply to different types of field research, and why modern sensing is about more than just the sensor.
Learn:
- Measurement theory behind the gravimetric method, capacitance, time-domain reflectometry (TDR), time-domain transmission (TDT), frequency-domain reflectometry (FDR), resistance sensors, and more
- Which technology applies to different field situations
- What factors matter when choosing a sensor type
- Why some methods are not research-grade
- How modern sensing is about more than just the sensor
- How to determine a good price-to-performance ratio for your unique application
Understanding variability can be difficult
Within the area of a study site, soil moisture variability arises from differences in soil texture, amount and type of vegetation cover, topography, precipitation and other weather factors, management practices, and soil hydraulic properties (how fast water moves through the soil). Researchers should consider the variability in landscape features to get a sense of how many sample locations are necessary to capture the diversity in soil moisture. Scientists often measure soil moisture at different depths to understand the effects of soil variability and to observe how water is moving through the soil profile. Large research areas or sites with high variability often require a large number of soil moisture sensors. The EC-5 soil moisture sensor or the TEROS 10 soil moisture sensor are economical choices for scientists who need a large soil sensor network. The TEROS 11 soil moisture sensor and TEROS 12 soil sensor, our newest sensors, have a larger volume of influence (1 liter), which can help smooth variability.
Installation impacts data
METER’s high-quality, research-grade sensors produce excellent data, but users must understand the site situation as they prepare to install. All dielectric probes are most sensitive between the prongs.
Any loss of contact between the probe and the soil or compaction of soil within the sensor measurement volume can result in measurement errors. Water ponding on the surface and running in preferential paths down probe installation holes can also cause measurement errors. These are issues to consider when choosing the best installation method for a particular site or soil type, and it means the needle shape, size, and durability of a soil moisture sensor will matter in difficult soils. The new TEROS Borehole Installation Tool used with the TEROS series sensors eliminates air gaps, soil disturbance, and preferential flow. Watch the video to see how it works.
If a soil is too rocky or hard for good soil to sensor contact, think about using the TEROS 21 water potential sensor. Water content can be calculated from water potential data using a soil moisture release curve, and the TEROS 21 can be backfilled or packed in. TEROS 10, 11, or 12 soil moisture sensors used with the installation tool is our unanimous recommendation for difficult soils.
Need higher accuracy in your soil moisture sensor?
For higher accuracy, consider a soil-specific calibration. METER’s soil moisture sensors measure the volumetric water content of the soil by measuring the dielectric constant of the soil, which is a strong function of water content. However, not all soils have identical electrical properties. Due to variations in soil bulk density, mineralogy, texture, and salinity, the generic mineral calibration for current METER sensors results in approximately ± 3 to 4% accuracy for most mineral soils and approximately ± 5% for soilless growth substrates (potting soil, stone wool, coco coir, etc.). However, accuracy increases to ± 1 to 2% for soils and soilless substrates with soil-specific calibration. METER recommends that soil moisture sensor users conduct a soil-specific calibration or use our Soil-Specific Calibration Service for best possible accuracy in volumetric water content measurements.
| TEROS 12 | TEROS 11 | TEROS 10 | EC-5 | 10HS | |
|---|---|---|---|---|---|
| Measures | Volumetric water content, temperature, electrical conductivity | Volumetric water content, temperature | Volumetric water content | Volumetric water content | Volumetric water content |
| Volume of Influence | 1010 mL | 1010 mL | 430 mL | 240 mL | 1320 mL |
| Measurement Output | Digital SDI-12 | Digital SDI-12 | Analog | Analog | Analog |
| Field Lifespan | 10+ years | 10+ years | 10+ years | 3-5 years* | 3-5 years* |
| Durability | Highest | Highest | Highest | Moderate | Moderate |
| Installation | Installation tool for high accuracy | Installation tool for high accuracy | Installation tool for high accuracy | Install by hand | Install by hand |
CHOOSE THE RIGHT FIELD WATER POTENTIAL SENSOR
Making good water potential measurements is largely a function of choosing the right instrument and using it skillfully. In an ideal world, there would be one instrument that simply and accurately measured water potential over its entire range from wet to dry. In the real world, there is an assortment of instruments, each with a well-defined range. Figure 7 illustrates that METER’s TEROS 21 matric potential sensor is useful for measuring water potential in the plant available water range (field capacity to air dry). Lab and field tests indicate that it can make water potential measurements with acceptable accuracy at least as dry as permanent wilting point. The tensiometer has a much higher accuracy for measuring water potential in the wet range, which is where most water movement occurs. Only tensiometers have the ability to measure high water potential ranges directly.
Dr. Colin Campbell’s webinar (below) “Water Potential 201: Choosing the Right Instrument” covers water potential instrument theory, including the challenges of measuring water potential and how to choose and use various water potential instruments.
Matric potential sensors
METER’s TEROS 21 matric potential sensor is composed of a moisture content sensor and a porous substrate with a known moisture release curve. After the porous material has equilibrated with the surrounding soil, the moisture sensor measures the water content of the porous material, and the sensor uses the moisture release curve to translate moisture content into water potential.
- Range vs. accuracy: A matric potential sensor’s range depends on the variation in the pore sizes in the porous substrate; the wider the range of pore sizes, the bigger the measurement range. Commercially available ceramics are designed to have a uniform pore size, which limits their range. The TEROS 21 uses a ceramic specifically designed with a wide pore size distribution for wider measurement range. However, a sensor’s accuracy depends on how well the moisture release curve characterizes the porous substrate in that particular sensor. The more consistent the substrate is from sensor to sensor, the more accurate each sensor will be. Widely varied pore sizes lead to inconsistency from sensor to sensor, putting these two critical sensor goals in conflict.
- Calibration solution: This conflict can be resolved with a factory calibration of each individual sensor. However, this has always been a time-consuming and expensive process. The TEROS 21’s accuracy comes from breakthrough factory calibration methods that allow sensors to be individually calibrated using an automated calibration apparatus. These new techniques make the TEROS 21 the first low-cost matric potential sensor with research-grade accuracy.
Tensiometers
Water potential, by definition, is a measure of the difference in potential energy between the water in a sample and the water in a reference pool of pure, free water. The tensiometer is an actualization of this definition.
The tensiometer tube contains a pool of (theoretically) pure, free water. This reservoir is connected (through a permeable membrane) to a soil sample. Thanks to the second law of thermodynamics, water moves from the reservoir to the soil until its energy is equal on both sides of the membrane. That creates a vacuum in the tube. The tensiometer uses a negative pressure gauge (pressure transducer) to measures the strength of that vacuum and describes water potential in terms of pressure.
Tensiometers are probably the oldest type of water potential instrument (the initial concept dates at least to Livingston in 1908), but they can be quite useful. In fact, in the wet range, a high-quality tensiometer used skillfully can have excellent accuracy. And, as previously mentioned, a tensiometer is unaffected by soil heterogeneity.
The tensiometer’s range is limited by the ability of water inside the tube to withstand a vacuum. Although water is essentially incompressible, discontinuities in the water surface such as edges or grit provide nucleation points where water’s strong bonds are disrupted and cavitation (low-pressure boiling) occurs. Most tensiometers cavitate around -80 kPa, right in the middle of the plant-available range. However, METER builds tensiometers that are modern classics thanks to precision German engineering, meticulous construction, and fanatical attention to detail. These tensiometers have terrific accuracy and a range of up to -85 kPa.
Which tensiometer for what purpose?
Tensiometer choice should be based on application. The TEROS 31 is a small laboratory tensiometer used for spot measurements or column experiments. It’s possible to use the TEROS 31 in the field for spot checks but inconvenient because if it cavitates, it must be refilled in the lab. The TEROS 32 is a field tensiometer. It’s equipped with external refilling tubes which eliminate the need to remove the tensiometer from the ground for refilling.
Sensor choice will also depend on data collection requirements. The TEROS 32 provides continuous, near-real-time data remotely with ZENTRA Cloud. It’s designed to plug and play with the ZL6 data logger and the ProCheck handheld reader, making it the best choice for a large sensor network. The TEROS 31 also works with the ZL6 and ZENTRA Cloud and the ProCheck. Both TEROS 31 and TEROS 32 can interface with a Campbell Scientific (or similar) data logger.
Understand shaft, cable, and refill tube lengths
The TEROS 32 tensiometer is typically installed at an angle in the field. We recommend using a METER field auger that is specifically sized to install this tensiometer. We also recommend using an irrigation valve cover box to protect the tensiometer shaft.
The shaft length needed will be based on installation depth. If, for example, you want to measure at a one-meter depth and are installing at an angle, you’ll need to know what that angle is in order to calculate how long the shaft should be. Typically, it will be 10 to 20 cm longer than the desired installation depth. In addition, the refill tubes need to be accessible. The deeper the tensiometer is buried, the longer the tubes should be in order to reach the surface. Lastly, the tensiometer cable length will depend on proximity to the data logger.
| TEROS 21 Matric Potential Sensor | TEROS 31 Tensiometer | TEROS 32 Tensiometer | |
|---|---|---|---|
| Cost | $$ | $$$ | $$ |
| Accuracy | ±(10% of reading + 2 kPa) from −100 to −5 kPa | ±0.15 kPa | ±0.15 kPa |
| Range | −5 to −100,000 kPa (1.70 to 6.00 pF) | –85 to +50 kPa (up to –150 kPa during boiling retardation) | –85 to +50 kPa |
| Power Requirements | 3.6-15 V, 10 mA | 3.6- to 28.0-VDC | 3.6- to 28.0-VDC |
| Measurement Output | Digital SDI-12 | DDI serial, SDI-12 communication protocol | DDI serial, SDI-12 communication protocol |
| Method Used for Determining Water Potential | Calibrated method: Capacitance of a ceramic matrix, six-point calibration | Direct method: Piezoelectric pressure sensor, Wheatstone full bridge | Direct method: Piezoelectric pressure sensor, Wheatstone full bridge |
| Data Logger Compatibility | ZL6 logger(and ZENTRA Cloud), EM60 loggers, Campbell Scientific | ZL6 logger(and ZENTRA Cloud), EM60 loggers, Campbell Scientific | ZL6 logger(and ZENTRA Cloud), EM60 loggers, Campbell Scientific |
| Best For… | – Long term research studies – Natural environment monitoring |
– Column & spot measurements in the laboratory – Small point measurements |
– Long term field studies – Vadose zone hydrology |
Soil moisture definition resources
Kirkham, Mary Beth. Principles of soil and plant water relations. Academic Press, 2014. (Book link)
Taylor, Sterling A., and Gaylen L. Ashcroft. Physical edaphology. The physics of irrigated and nonirrigated soils. 1972. (Book link)
Hillel, Daniel. Fundamentals of soil physics. Academic press, 2013. (Book link)
Dane, Jacob H., G. C. Topp, and Gaylon S. Campbell. Methods of soil analysis physical methods. No. 631.41 S63/4. 2002. (Book link)
Questions about how to measure soil moisture?
Watch the webinar below to learn:
- Why soil moisture is more than just an amount
- Water content: what it is, how it’s measured, and why you need it
- Water potential: what it is, how it’s different from water content, and why you need it
- Whether you should measure water content, water potential, or both
- Which sensors measure each type of parameter
How to interpret soil moisture data
Take a deep dive into soil moisture measurement. In the webinar below, Dr. Colin Campbell discusses how to interpret surprising and problematic soil moisture data. He also teaches what to expect in different soil, site, and environmental situations.
Take our soil moisture master class
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.
Questions?
Our scientists have decades of experience helping researchers and growers measure the soil-plant-atmosphere continuum.
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