Frequently Asked Questions
Soil Moisture Sensor
- Soil Moisture Sensor - TEROS 11 - Volumetric Water Content - TEROS 12 How can I calculate dielectric permittivity from the TEROS sensors? Dielectric permittivity is the primary variable used to measure water content in soils. Past METER sensors converted their raw electrical reading to dielectric permittivity internally through individual calibration. Dielectric was reported as sensor output and was then converted to water content with the Topp equation or other empirical relationship between dielectric and water content. This method was a step forward for 2009. However, it sometimes resulted in compounding error by fitting two layers of empirical calibration, one from sensor raw output to dielectric and then to one from dielectric to water content.With our TEROS 11/12 sensor line we achieve very low sensor to sensor variability by normalizing each sensor electronically. We also eliminate potentially compounding error by converting directly from sensor raw output to water content, skipping the intermediate dielectric permittivity step. You can still convert sensor raw output to dielectric using the dielectric calibration equation in your TEROS user manual. However, it is not recommended to convert sensor raw output to dielectric and then convert dielectric to water content for the reasons mentioned above.
- Water potential - TEROS 32 - Soil Moisture Sensor - T8 Tensiometer - T4 Tensiometer - TEROS Is a tensiometer the only option for container media?
Yes, currently the best option for measuring water potential in container media is a tensiometer. The soilless media holds most of its water at a very high water potential—typically between 0 and -30 kPa. Once they drop below this, they tend to get very dry fast.
- Water potential - Soil moisture release curves - TEROS 32 - Soil Moisture Sensor - T8 Tensiometer - T4 Tensiometer - TEROS Is there some relationship between the potential readings with tensiometers and the volumetric water content?
Yes, the soil moisture release curve would be used to describe this relationship between the water potential readings from a tensiometer and the volumetric water content of the soil. Read more about it here.
- Water potential - TEROS 32 - Soil Moisture Sensor - T8 Tensiometer - T4 Tensiometer - TEROS What is the physical meaning of a positive reading from a tensiometer?
A positive reading from a tensiometer indicates that the soil is saturated and has a positive pressure head from a water table. As the water table increases above the sensor, the reading will become more positive.
- Water potential - TEROS 32 - Soil Moisture Sensor - T8 Tensiometer - T4 Tensiometer - TEROS How sensitive is a tensiometer? If the pressure changes only one millimeter, can you measure it?
This sensitivity depends on the sensitivity of the pressure sensor used in the tensiometer. Some tensiometers are only able to resolve pressure changes to 1 hPa which is approximately 10 mm of water. Newer sensors like the TEROS 32 have higher-precision tensiometers which, in theory, are able to resolve down to the mm level. However, they are not calibrated to be accurate down to this level. So you can resolve these changes, but the accuracy might not be as good as a level sensor for a piezometer.
- Water potential - TEROS 32 - Soil Moisture Sensor - T8 Tensiometer - T4 Tensiometer - TEROS How do sodium and bicarbonates in water affect a tensiometer reading?
Salts in the water do not interact with the measurement from a tensiometer. Because salts and bicarbonates primarily affect the osmotic potential of the soil, the tensiometers will not see this effect because salts are able to pass freely through the permeable ceramic. One piece we don’t have a lot of experience with are precipitates. If the bicarbonates were to precipitate on the ceramic this could potentially be an issue.
- TEROS 21 - Water potential - TEROS 32 - Soil Moisture Sensor - T8 Tensiometer - T4 Tensiometer - TEROS Are water potential sensors available that measure at 2″ and 5″ at the same time?
Currently, there is not a profile-type water potential sensor. The only way would be to place individual sensors at the desired measurement depths. A profile probe could be a powerful tool for this measurement and is something we may approach in the future.
- TEROS 21 - Water potential - TEROS 10 - TEROS 32 - Soil Moisture Sensor - TEROS 11 - TEROS 12 - T8 Tensiometer - T4 Tensiometer - TEROS Is there a paper you can refer me to concerning the effects of digging a trench on the soil at a site?
I don’t have a specific paper to refer to on this topic. The concern with large trenches is the way it affects water movement through the soil near the sensor. Depending on how the trench is repacked you can wind up with preferential flow paths which will result in faster water migration through the soil profile. For more information on this topic, see our article: “5 ways Site Disturbance Impacts Your Data“.
- TEROS 10 - Soil Moisture Sensor - TEROS 11 - Volumetric Water Content - TEROS 12 - TEROS Where could I find the protocols and good experimental design to publish scientific research articles for watering requirements and watering optimization?
I would focus on a literature review that takes in papers on water requirements and optimization and carefully study their protocols and match your efforts with their designs, with improvements. Generally speaking, good sensor-to-soil contact and carefully derived models for water uptake along with weather data for water use using crop coefficients and ET are things to consider.
- TEROS 10 - Soil Moisture Sensor - TEROS 11 - Volumetric Water Content - TEROS 12 - TEROS How can you use the relationship between satellite images and sensors for intelligent irrigation?
This is a critical area or research at the moment. There are several institutions currently with projects trying to relate the two. Currently, I am involved in a project where we are using satellite data like Normalized Difference Water Index and ECOSTRESS to correlate with individual soil moisture sites in the field. We will use the trending information from the field data with the infrequent snapshots of the satellites to combine for a complete picture (we hope). Since they are massively different scales, this effort will be challenging.
- TEROS 10 - Soil Moisture Sensor - TEROS 11 - Volumetric Water Content - TEROS 12 - TEROS How is it possible to remove the sensors from the soil at the end of the season ?
Most water content sensor installations are permanent because removing the sensor is difficult. In the agricultural setting, the sensor and cable are often installed below the working layer. However, there are some rod type profile sensors that extend above the surface and can be removed each year. The accuracy of these sensors isn’t great, but sometimes it is good enough.
- TEROS 10 - Soil Moisture Sensor - TEROS 11 - Volumetric Water Content - TEROS 12 - TEROS What are good soil moisture sensors for a park?
- TEROS 10 - Soil Moisture Sensor - TEROS 11 - Volumetric Water Content - TEROS 12 - TEROS How is the sensor-to-sensor repeatability with TEROS sensors?
For the TEROS 11 and TEROS 12 is it extremely tight. We normalize each sensor to make sure it reads like every other TEROS 11/12. Our tests show that we maintain repeatability to well within 1% water content. The TEROS 10 doesn’t have a microprocessor to allow the normalization procedure, so we have to depend on really tight manufacturing processes. We can still hold that entire population to within about 2% water content.
- TEROS 10 - Soil Moisture Sensor - TEROS 11 - Volumetric Water Content - TEROS 12 - TEROS Which sensors are adequate for measuring water in individual potted plants and the whole nursery?
The dielectric sensors are commonly used in individual pots. You’ll just need to pick a sensor that will fit in the chosen pot size. The TEROS 12 is a really popular choice for potted plants because it measures water content and electrical conductivity, which is an indicator of fertilizer level. The trick with measuring in potted plants is to pick plants that are representative of the larger nursery or irrigation zone because it is generally far too expensive to instrument each pot.
- TEROS 10 - Soil Moisture Sensor - TEROS 11 - Volumetric Water Content - TEROS 12 - TEROS Is there a reason the TEROS 12 sensors are placed perpendicular to the soil instead of horizontal?
The only reason is to avoid having the body of the sensor impede water flow through the soil. That effect is pretty minor, but it can cause a time lag in the soil moisture signal as the water has to redistribute around the sensor.
- TEROS 10 - Soil Moisture Sensor - TEROS 11 - Volumetric Water Content - TEROS 12 - TEROS What is your water content measurement advice for tree plantation? Are there any limitations? Any patterns?
There are no problems with measuring water content in that scenario. One consideration is getting a measurement that is representative of the root zone. This is pretty easy if you’re relying on rain or using overhead irrigation. Sensor placement becomes more important if you’re using a drip irrigation system though. Many people place the sensors directly under the emitters if using drip.
- TEROS 10 - Soil Moisture Sensor - TEROS 11 - Volumetric Water Content - TEROS 12 - TEROS What type of sensors work best in substrate growing?
Generally, the dielectric soil moisture sensors are used. Often fertilizer levels are important, so growers will use a combination water content and electrical conductivity sensor to measure both water and fertilizer. The TEROS 12 is a really popular choice for substrate growing.
- TEROS 10 - Soil Moisture Sensor - TEROS 11 - Volumetric Water Content - TEROS 12 - TEROS How reliable are measurements in saturated and highly variable soil conditions, such as peatlands?
The measurements are fine in such a scenario, with one limitation. Once the peat is saturated at the level of the sensor, more water can be added to increase the ponding height, but the sensor will still only measure the saturated water content at its level until the water recedes. In peat, you’ll likely want a substrate-specific calibration for best accuracy, since the organic material is a bit different from mineral soil. See our calibration instructions for some step-by-step instructions for the substrate-specific calibration if you’re interested.
- TEROS 10 - Soil Moisture Sensor - TEROS 11 - Volumetric Water Content - TEROS 12 - TEROS If roots grow between the sensor needles over time, how does it influence the measurement? How do we work around it?
The dielectric measurement will measure all the water in its measurement zone, including the water in nearby roots. So, it is possible to get enough rooting density in the measurement volume to affect the measurement. This might lead to a bias in the measurement, but there is really no work-around. In practice, the effect is pretty small, so water content sensors are used in agricultural/irrigation settings extremely often with no noticeable problems.
- TEROS 10 - Soil Moisture Sensor - TEROS 11 - Volumetric Water Content - TEROS 12 - TEROS How do you measure water content at shallow depths of 1-2 cm? Most sensors average over a sphere of 5-10 cm diameter.
This is difficult with most sensors, as you point out. It is possible to measure the volumetric heat capacity of soil using a dual needle heat pulse sensor on the 1-2 cm spatial scale. Volumetric heat capacity is linearly related to water content, so the conversion is pretty easy. Making that heat capacity measurement is pretty involved though, so only a few researchers use it for this purpose, but it has been done. Contact customer support if you want more details.
- TEROS 10 - Soil Moisture Sensor - TEROS 11 - TEROS 12 - TEROS How is dielectric measured? In which units?
That’s a trick question! But only because dielectric is a unitless quantity. It is ratio of charge storage in a medium to the charge storage in free space. It can be measured in many ways, including travel time of a pulse (TDR, TDT), charge time of a capacitor, or resonance frequency. Various soil moisture sensors make use of these different measurement techniques.
- TEROS 10 - Soil Moisture Sensor - TEROS 11 - TEROS 12 - TEROS Is it a challenge to keep the water content sensor in place while you backfill with soil?
That is a good question and one I also had when we were developing the concept of the installation tool. Fortunately, it has not proven to be a problem except in dry, coarse-textured soils. The pins on the TEROS sensors do a pretty good job of anchoring the sensors in place while the soil is repacked behind them. But, in dry sand, it is difficult to even keep the auger hole intact, not to mention keep the sensors in place.
- TEROS 10 - Soil Moisture Sensor - TEROS 11 - TEROS 12 - TEROS How does soilless organic media such as biochar or coco coir affect dielectric sensor accuracy? Does the physical shape and size of the pores holding the water being measured affect the measurement since it affects the electrical path between the anode and cathode?
Fortunately, the shape and size of the pores has little effect on the dielectric measurement. The electromagnetic field will polarize all the water molecules within the volume of measurement regardless of pore geometry. But, with organic materials, the dielectric permittivity of the low-density material is generally lower than that of mineral soil. So, accuracy can suffer, with the dielectric sensor measuring water content biased low. With these unique materials, I always recommend a substrate-specific calibration. We have some detailed instructions on how to create this calibration here. You’ll notice that there is a special procedure for coir since it is pretty difficult to work with.
- TEROS 10 - Soil Moisture Sensor - TEROS 11 - TEROS 12 - TEROS Are there any special considerations for very rocky soils or areas where the soil water content is typically very low as in the Mojave desert?
Low water content is not a problem and can be measured accurately by the dielectric sensors. Rocky soils are difficult for all soil sensors though. Best-practice installation techniques of inserting sensors into undisturbed soils may not be possible in rocky soils. You may have to remove some rocks and install the sensor into re-packed soil with no rocks. This will affect accuracy some, but precision should still be good.
- TEROS 10 - Soil Moisture Sensor - TEROS 11 - TEROS 12 - TEROS What is the best sensor to track the excessive moisture content in the soil? What are the moisture ranges for these sensors?
Good question. Our TEROS 10,11, 12 water content sensors will tell you how much water is present, so they can characterize the degree of saturation, which is an indicator of excess moisture. The TEROS 32 tensiometer will characterize the soil suction, and maybe even more importantly positive pore water pressure, both of which are important for slope stability and soil engineering projects. I’m not a civil engineer, but my understanding is that the combination of degree of saturation from the water content sensors and soil suction from the TEROS 32 is the optimal combination for understanding soil strength. Both sensor types work great in the excess moisture range, but the TEROS 32 will fail in dry soil.
- TEROS 10 - Soil Moisture Sensor - TEROS 11 - TEROS 12 - TEROS What are the applications for soil moisture sensors in asphalt pavements?
Here is a case study about soil moisture sensors in asphalt: https://www.metergroup.com/meter_knowledgebase/compression-testing-of-soil-moisture-sensors-embedded-in-asphalt/.
- TEROS 10 - TEMPOS - Soil Moisture Sensor - TEROS 11 - TEROS 12 - TEROS What was your experience developing soil moisture sensors for NASA’s JPL Phoenix Rover? Why did the sensor also record thermal conductivity? Were there any interesting findings?
Don’t get us started! The experience was great overall. The team we worked with at JPL were really good scientists and engineers. The thermal properties measurements were intended to be a ground-truth for remotely-sensed regolith thermal properties data, which are key to understanding the depth of penetration of solar heat. All of the measurement functions on the TECP worked well, and the project is considered highly successful. Maybe the most important finding was the vapor phase migration of water into the regolith as the regolith cooled with Martian winter approaching. The increase in dielectric permittivity that TECP measured was far larger than expected, probably due to water interacting with perchlorate salts in the unfrozen phase. We shot a video with the lead JPL researcher a while back. You can check it out here.
- TEROS 10 - Soil Moisture Sensor - TEROS 11 - TEROS 12 - TEROS How do you deal with extremes in salinity – high or low?
Low salinity is generally not a problem for most water content sensors. Extremely high salinity can be a problem. With TDR, high salinity can attenuate the signal to the point where no water content measurement is possible. With some of the capacitance type sensors, the accuracy can be really poor in high salinity soil. A soil-specific calibration can fix this for the capacitance sensors.
- TEROS 10 - Soil Moisture Sensor - TEROS 11 - TEROS 12 - TEROS Which is better for water content sensors: vertical installation or installation at an angle?
Either installation is fine. Find more installation tips here.
- TEROS 10 - Soil Moisture Sensor - TEROS 11 - TEROS 12 - TEROS If wanting to irrigate from a minimum percentage of soil moisture, which depth should we take into consideration?
The most meaningful depth is typically the depth with the highest root density. But, multiple depths do bring additional information. Often growers will place two sensors in the root zone and one below the root zone. The third sensor below the root zone helps control leaching fraction.
- TEROS 10 - Soil Moisture Sensor - TEROS 11 - TEROS 12 - TEROS Do you think dielectric is a more accurate option than a pressure chamber for almonds?
The dielectric measurement gives you a nice time-series of soil water content that you can monitor remotely. The pressure chamber will give you the water potential of the almond tree itself. The pressure chamber water potential measurement is a far better indicator of the water stress status of the almond tree. But, the downside is that collecting pressure chamber data is difficult and time consuming. Many growers use the pressure chamber measurement to “calibrate” their soil water content measurements, and figure out what water content starts to cause too much water stress. This makes the time-series water content data really powerful and convenient for quantifying water stress.
- TEROS 10 - Soil Moisture Sensor - TEROS 11 - TEROS 12 - TEROS How difficult is calibration of dielectric sensors?
The process is not difficult, but it does take some care. We have some detailed step-by-step instructions online here. If you don’t have the equipment, time, or desire to do this procedure yourself, we also offer a service to do the calibration for you if you send us a sample of your soil/substrate. You can contact email@example.com for details about the soil-specific calibration service.
- TEROS 21 - Water potential - Soil Moisture Sensor - GENERAL - T8 Tensiometer - T4 Tensiometer - T5 Tensiometer How can you measure kPa or MPa? And what tools can you use for container production?
kPa and MPa are really just a preference. You convert between the two by moving the decimal point. In containers, you can use tensiometers which are highly accurate in the wet range but not in the dry range. Matric potential sensors such as the TEROS 21 also work well. They aren’t as accurate as a tensiometer in the wet end, but they give you a better range and require less maintenance.
- Soil Moisture Sensor - TEROS 11 - Volumetric Water Content - TEROS 12 - GENERAL - TEROS What tools can I use to measure soil moisture along a soil profile of 40cm depth?
- Soil Moisture Sensor - ECH2O 5TE - Volumetric Water Content - TEROS 12 - GENERAL - GS3 What will measuring electrical conductivity give you a metric of versus VWC?
EC can give you the solute concentration of the soil, but is trickier to interpret than VWC since the VWC can be a confounding variable.
- TEROS 21 - Water potential - Soil Moisture Sensor - TEROS 11 - Volumetric Water Content - TEROS 12 - GENERAL - GS3 - TEROS What are important considerations when thinking about measuring water content and water potential in peatlands (with organic soils)?
Variability of your substrate is a big one. There is a lot of variability in soils as well, but we have better mechanisms to capture and account for variability in mineral soils. Good substrate-to-sensor contact is critical and trickier to accomplish (good installation), but it is achievable. You will most likely require a custom calibration for water content.
- Soil Moisture Sensor - ECH2O 5TE - TEROS 11 - ECH2O 5TM - Volumetric Water Content - TEROS 12 - GENERAL - ECH2O EC-5 - TEROS What is the difference between the FDR method and the capacitance method of VWC measurement? Are they the same?
They are technically not the same. For the strictest technical accuracy, FDR makes a frequency scan and capacitance measures the charge time of a capacitor. From a practical standpoint there is little difference in the performance.
- TEROS 21 - Water potential - Soil Moisture Sensor - TEROS 11 - Volumetric Water Content - TEROS 12 - GENERAL - TEROS Would you agree that with the impact of soil moisture on the atmosphere, measuring water content alone is not enough?
It depends on your specific goals. If you are studying the impact of soil water on atmospheric impact then you would need water potential. There are plenty of cases where water content alone is sufficient if you also have information about your soil.
- TEROS 21 - Soil Moisture Sensor - GENERAL - TEROS If I use the TEROS 21 to measure soil water potential when planning irrigation, do I need to know the soil type?
No. With the TEROS 21 you just need to know the matric potential limits of your plants, and you do not need to worry about soil type.
- TEROS 21 - Water potential - Soil Moisture Sensor - WP4C - HYPROP - GENERAL What is matric potential?
Matric potential is the force that would need to be exerted to move a water molecule from the surface of a soil particle. For example, a matric potential of -100 kPa would require a force of -101 kPa to pull that water molecule off of the soil particle. It is one component of the total water potential. Learn more about the different components of water potential here.
- Soil Moisture Sensor - ECH2O 5TE - GS3 Can your 5TE or GS3 tell me the soil salinity or the pore water EC?
Our sensors measure bulk EC, not soil salinity or pore water EC. Pore water EC can be derived from bulk EC using various models, including the Hillhorst model. This is described more in our application note about soil EC.
- Soil Moisture Sensor - GENERAL Why doesn’t my soil moisture sensor read 100% VWC in pure water?
Testing your sensor in water can be a great quick check for basic sensor function, but it is a poor test for accuracy. The calibration of all METER soil moisture sensors is optimized for 0 – 50% volumetric water content (VWC), the usual range of soil water content.
Water will tell you if a sensor responds to a change in water content. But the reading itself will vary. Please contact firstname.lastname@example.org for recommendations to test the absolute accuracy of your sensors.
- Soil Moisture Sensor - GENERAL What does my soil moisture value in a frozen soil mean?
Volumetric water content (VWC) from a frozen soil will look much like dry soil. Your sensor will carry on merrily, but you will not see any change in water content until you start to get liquid water near the sensor again.