Frequently Asked Questions
- 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? How 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 u 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 really 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 firstname.lastname@example.org for details about the soil-specific calibraiton service.
- Hydraulic conductivity - HYPROP - GENERAL Is hydraulic conductivity at near saturation unreliable for higher suctions?
It’s not that unsaturated hydraulic conductivity near saturation is unreliable but that some measurement methods have difficulty making that measurement, for example: the flow cell vs. the HYPROP. The HYPROP has issues near saturation because there is not enough difference between its two tensiometers. The flow cell approach does a better job of measuring unsaturated hydraulic conductivity near saturation. So it just depends on the method you use.
- Hydraulic conductivity - GENERAL What is more significant to measure for irrigation: saturated or unsaturated hydraulic conductivity?
Saturated hydraulic conductivity is more significant in this application because it’s important to understand what will be the most limiting factor. As the soil is wetting up, there are matric forces that help pull water into the soil. But as we approach saturation near the surface, there may be ponding of water which could ultimately lead to runoff, and that will affect what your irrigation rates can be.
- Hydraulic conductivity - HYPROP - GENERAL If field and lab measurements are different, is it recommended to complete the hydraulic conductivity curve obtained with HYPROP in the lab with field measurements in the HYPROP-Fit?
This will depend on your research goal. If your goal is to understand individual types of soil (i.e., what are the intrinsic soil properties of a B horizon) then a better approach is to take lab measurements because you have better control. But if your goal is to understand interactions in the field, a better approach is to make field measurements.
- Hydraulic conductivity - HYPROP - GENERAL - KSAT - MINI DISK INFILTROMETER Can you refer me to a reliable measurement method to estimate and predict unsaturated hydraulic conductivity?
- Hydraulic conductivity - GENERAL What is the difference between infiltration and unsaturated hydraulic conductivity?
Infiltration and hydraulic conductivity are related. Infiltration is a measure of a soil’s ability to infiltrate water from the surface, and it will change as the soil’s moisture conditions change. Infiltration rate is not typically corrected for three-dimensional flow and is not referenced to specific moisture condition. Hydraulic conductivity is a one-dimensional value that is specified to a specific moisture condition and can be used when trying to model water movement in soil. The equation that shows the relationship between infiltration and hydraulic conductivity is shown on slides 10 and11 of this webinar.
- Hydraulic conductivity - GENERAL Is it appropriate to measure hydraulic conductivity at plot level season after season?
Season-to-season variability will depend a lot on management. From a land management/treatment perspective, if there are things being done that will ultimately lead to improved soil structure and hydraulic properties, you may need to look at seasonal differences. If the land is always being managed the same, you may not see much of a difference in soil hydraulic properties season to season.
- Hydraulic conductivity - GENERAL What is the importance of hydraulic conductivity at plot level, for example, 5×5 plots with the same land use (e.g., a corn field)?
Hydraulic conductivity can be important, even at the plot level. It depends on the variability of the soils within the area. Even if the site is under the same land use, we can see variability in the soils across a small area which can result in a difference in hydraulic properties.
- Hydraulic conductivity - GENERAL What parameters are the most important to consider a soil as “healthy”?
There are many parameters to consider, and each is important. Typically, researchers look at parameters that relate to a more stable soil: improved aggregate stability, improved nutrient levels, and more biological activity. There are several groups working in this area, I would recommend looking at the work being done by the Soil Health Institute as starting point.
- Hydraulic conductivity - SATURO - GENERAL - MINI DISK INFILTROMETER How does soil structure affect K values? How are they related?
Soil structure and aggregate stability will have a large impact on the hydraulic conductivity values. Depending on the type of soil structure that is formed and the strength of that structure you will see a larger development of macropores within the soil which will have the ability to transmit more water than if the soil had poor structure or no structure. Learn more about this here.
- Hydraulic conductivity - SATURO - GENERAL - MINI DISK INFILTROMETER How would you measure horizontal hydraulic conductivity in field conditions? How would you disaggregate the horizontal component of hydraulic conductivity from the vertical one?
This depends on whether you are measuring saturated or unsaturated hydraulic conductivity. It can be difficult to try and do this for saturated hydraulic conductivity in the field. In theory, if you took a sample from the field and measured this in the lab, horizontal vs. vertical should be the same in terms of hydraulic conductivity. Hydraulic conductivity is independent of whether the flows are vertical or horizontal because these components are corrected for. If you want to measure unsaturated hydraulic conductivity in the field and look at how it changes in the horizontal vs. vertical position, this could potentially be done with tensiometers and water content sensors positioned in a grid to look at the movement of water and change in water potential. Find more information on how to measure hydraulic conductivity here.
- Hydraulic conductivity - GENERAL - MINI DISK INFILTROMETER What influence do high soil temperatures associated with forest fires have on short term unsaturated conductivity?
Because of the hydrophobicity effect from the high temperatures associated with forest fires, I expect you would see a significant impact on the unsaturated hydraulic conductivity of the soil. There are experts who have done work in this area. Researchers at the US Forest Service would be a great sounding board for this question. Pete Robichaud, in particular, might have some thoughts on this topic. We’ve highlighted his forest fire research here.
- Hydraulic conductivity - SATURO - GENERAL - MINI DISK INFILTROMETER Lab Ks measurements move from the bottom to the top. Is Ks similar if water goes from the top to the bottom of a sample?
As long as the calculations are done correctly there shouldn’t be a difference if water is percolated from the bottom or the top of the sample.
- Hydraulic conductivity - GENERAL What is the difference between liquids like water (polar liquids) and oils (non-polar or organic liquids) in terms of diffusion and the fate of liquids in soil. Does oil have different conductivity variables to measure?
There will be a difference in how non-polar liquids move through soil. The interaction with the soil surfaces will be different along with the differences in viscosity. There are ways to model these liquids, but you will need to consult an expert in this area.
- Hydraulic conductivity - SATURO - GENERAL Soil with rock fragments is a common soil type but difficult to measure with traditional methods. What do you suggest?
Rock fragments can make things difficult when trying to measure hydraulic conductivity. If it is possible to avoid the rocks, that is the best approach. This will depend on the size of the rock fragments. If you are measuring in a gravelly site, you can still use traditional methods, but you will likely need to repair the ring edges more regularly as they will get bent and dinged frequently. If you have very large fragments, avoid the large rocks by measuring in a smaller area or by trying to encompass the large rocks within the infiltrometer. In some cases, a borehole approach might work better for these situations.
- Hydraulic conductivity - SATURO - GENERAL In preparing for an infiltrometer test, I’m thinking of statistical significance so would like multiple sites. How far apart should my test holes be?
Some of this depends on the soil variability at the site. Typically, we take measurements in triplicate at a single site. We then try to cover the expected variability at the site. This could mean somewhere around 3 to 12 measurement sites within a location. One way to approach this is to start with three sites and see what the variability looks like.
- Hydraulic conductivity - SATURO - HYPROP - GENERAL How could one determine the change in hydraulic conductivity at a soil-spoil interface?
There are two approaches you could take. You could measure the two materials independently and see which one is going to be most limiting. Typically, hydraulic conductivity is governed by the most limiting layer. You could also just make the measurement in the field and see the interaction between the two layers. Again this rate will be governed by the most limiting layer.
- Hydraulic conductivity - GENERAL What is the relationship between soil/regolith profile and hydraulic conductivity in explaining the overall hydrology of the terrain?
Hydraulic conductivity through a profile is a small component used to explain the overall hydrology of the terrain. You need to understand hydraulic conductivity in order to understand how the water is going to move through the soil. But, if you want to understand this across a large landscape, you will need to know how the soil varies and how the hydraulic properties change across the site.
- Water potential - T8 Tensiometer - T4 Tensiometer - T5 Tensiometer How does a tensiometer deal with partially saturated soils?
A tensiometer is able to measure positive pressures along with the water potential measurements. If you use the right tensiometer, you can actually have a very good measurement near saturation.
- Water potential - Soil moisture release curves - WP4C - HYPROP - Volumetric Water Content - GENERAL - LABROS Does field capacity vary depending on whether the soil has previously been in a dry or wet state? If so, what error margin might that cause if I plan irrigation scheduling according to FC?
This is true. What you are looking at is the effect of hysteresis, which is generally not a big concern. Depending on the soil type and how big the hysteresis effect is, it can actually shift the field capacity point slightly. If you are concerned about this, you may want to use water potential to schedule irrigation, for example with the TEROS 21 or a tensiometer. If you’d like more information about this, contact customer support.
- TEROS 21 - Water potential - Soil moisture release curves - WP4C - HYPROP - Volumetric Water Content - GENERAL - T8 Tensiometer - T4 Tensiometer - T5 Tensiometer - LABROS How can you measure capillary water potential?
Capillary water potential is tied to matric potential. So if you are measuring matric potential with a tensiometer or a TEROS 21, you are essentially measuring the effect of the capillaries or those different pore sizes. You can also use the HYPROP. The WP4C will also work assuming the soil has a negligible osmotic potential.
- TEROS 21 - Water potential - Soil moisture release curves - WP4C - HYPROP - Volumetric Water Content - GENERAL - T8 Tensiometer - T4 Tensiometer - TEROS - T5 Tensiometer Do matric potential sensor readings include osmotic potential?
This depends on what type of instrument you are using to measure the potential. For example, tensiometers, granular matric sensors, and the TEROS 21 ONLY measure matric potential. So these sensors are blind to osmotic potential. Laboratory instruments like the WP4C measure both osmotic and matric potential. But in terms of field sensors, there aren’t any that give both components.
- Water potential - Soil moisture release curves - WP4C - HYPROP - Volumetric Water Content - GENERAL - LABROS We monitor soil moisture using water content. How can we integrate this into a soil moisture release curve?
One of the best ways to do this is to take some samples and measure the soil moisture release curve for that soil, generating a functional relationship. Then you can take that curve and use your water content values to set your irrigation points through your release curve function. Another option is modeling it. If you know some information about the soil type and pedology, there are pedotransfer functions you can use by inputting those variables, and it will predict a soil moisture release curve. This method is not as accurate, but it’s a possible option.
- Water potential - Soil moisture release curves - WP4C - HYPROP - Volumetric Water Content - GENERAL Which depths should I consider for active roots in maize for irrigation management?
You can refer to the literature for rooting depths of maize. As for the sensors, we recommend a combination of TEROS 12 soil moisture sensors and TEROS 21 matric potential sensors to get the whole picture.
- Water potential - Soil moisture release curves - WP4C - HYPROP - Volumetric Water Content - GENERAL - LABROS What modeling programs can you use to model soil moisture release curves?
There are a few different models out there to model soil moisture release curves. ROSETTA is a program from the US Salinity lab that has been around for a long time. Hydrus is another tool that can be used to model soil moisture release curves. One thing to remember is that these models don’t take into account all of the factors that can change a soil moisture release curve. So if you decide to model your soil moisture release curve, remember they aren’t perfect.
- Water potential - Soil moisture release curves - WP4C - HYPROP - Volumetric Water Content - GENERAL - LABROS Now VWC trends are used to determine field capacity and stress onset. Is this a more accurate method than water potential?
This is one approach to take. The issue with using water content measurements is you have to wait until you observe stress occurring to make this type of set point. We recommend a physical water potential measurement as a better way to determine a stress set point. As for field capacity, you can still use the physical measurements to set your field capacity point. The most important thing to understand is that the traditional -33 kPa point for field capacity is not a good rule of thumb to follow. Read why here.
- Water potential - Soil moisture release curves - WP4C - HYPROP - Volumetric Water Content - GENERAL - LABROS How do you develop a soil moisture release curve in highly variable soils?
If you have a site with highly variable soils you will need to generate a curve for each individual soil type. One approach would be to map out the site and select the soil types of most importance and then create soil moisture release curves for those soils.
- Water potential - Soil moisture release curves - WP4C - HYPROP - Volumetric Water Content - GENERAL - LABROS Do private chemical labs carry out soil water retention curve analysis? Or just university labs?
There aren’t many private labs that offer retention curve services, however, METER does offer soil moisture release curve services. Find it here.
- GENERAL Are METER products safe?
What is Prop 65?
In 1986, California voters approved an initiative to help them make informed decisions about the products they buy. That initiative became known as Proposition 65. Proposition 65 requires the state of California to publish a list of chemicals known to cause cancer/ reproductive harm. You can learn more about Prop 65 here.
Why do METER products have a Prop 65 warning?
METER is required by California law to post a warning on any product that might be sold in California if it contains or might contain one of the more than 900 chemicals known to the state of California to cause cancer and/or reproductive harm. Because consumers learn about our products from our website, we are required to post these warnings on the product web pages as well. California residents are used to seeing these notifications on many products, including household items, electrical wires, electronics, jewelry, dishes, flashlights, cosmetics, skincare products, and much more. People who live outside the state of California may be surprised and alarmed by these notifications. It is important to know that nearly all electronic products require the notification label even though the chemicals involved may only be potentially harmful when eaten in large quantities.
Are METER products safe?
A Prop 65 warning does not necessarily mean a product is in violation of any product safety standards or requirements. Because METER sources manufacturing materials from other suppliers, we provide the warning in order to acknowledge that METER products may contain one or more of the chemicals listed. However, it’s likely that those chemicals are in such small amounts that they fall into the no-significant-risk category. To be safe and compliant, we have elected to label all product web pages and packaging with this warning.
What types of chemicals are on the list?
You can find a list of Prop 65 chemicals here.
- 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.
- WP4C - GENERAL - LABROS How do you convert MPa to pF?
You can convert MPa to cm of suction by dividing MPa by -9.787×10-4. pF is then the log base 10 of cm of suction.
- 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.
- ECH2O 5TE - ECH2O 5TM - Volumetric Water Content - GENERAL - ECH2O EC-5 - ECH2O Why don’t your soil moisture sensors measure with 1% accuracy like some others spec’d?
There are two elements to the accuracy of time-domain reflectometry (TDR), frequency domain reflectometry (FDR), and capacitance sensors:
- How well the sensors measure the dielectric permittivity of the soil; and
- How well the calibration converts that measurement to water content.
Due to inherent variability in soil physical properties, it is impossible for a dielectric sensor to measure within 1% water content accuracy across a broad range of soil types, even with a perfect measurement of dielectric permittivity. Beware sensors that claim an unbelievable accuracy in all substrates.
- 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 email@example.com 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.