Frequently Asked Questions
Volumetric Water Content
- 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.
- 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.
- 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.