How do I clean the WP4C?
See the WP4C cleaning video here.
We have a HYPROP and WP4C. What challenges can we expect with analyzing high-clay vertisol soils?
This is a really good question. One of the main issues you will see with the high clay vertisols will be the shrinking of the samples during the measurements. You shouldn’t have too much of an issue with contact during the measurement. Due to the volume change during the measurement, the VWC measured by the HYPROP won’t correlate well with the actual change in volume. The VWC and bulk density will be based on the saturated volume and density of the soil samples. One way to approach this is to convert to gravimetric water contents.
Does the 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.
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.
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.
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.
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.
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.
Now VWC trends are used to determine field capacity and stress onset. Is this 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.
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.
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.
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.
What are the main differences between WP4, WP4-T, and WP4C?
The WP4, first model, doesn’t have a couple of features of the newer dewpoint water potential models. The second model, WP4-T, has temperature control of the sample. The third model, WP4C, in addition to temperature control of the block, has improved accuracy in the wet range by being able to resolve temperature differences of 0.001 degrees between the sample and the mirror. The WP4-T can only resolve temperature differences of 0.01 degrees between the sample and the mirror. This results in an improvement in accuracy of 0.5 MPa in the WP4C. The range of the WP4C has also been extended to -300 MPa.
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.
What measurement mode should I use to read my samples?
It depends on the expected water potential range of your sample. Very dry samples (< -40 MPa) can be run in fast mode with no loss of accuracy. Precise mode should be used for optimum accuracy of samples up to ~ -0.50 MPa. Continuous mode is recommended for wetter samples that require extreme temperature equilibrium for maximum precision.

Please note that the time to completion is not determined in continuous mode; the user must determine when the reading levels off and the sample has reached equilibrium.
What causes long read times in my WP4C?
Contamination of the sample chamber is the primary cause of long read times. The WP4C relies on equilibration of water vapor in the chamber with the sample. A dirty sample chamber can have samples that adsorb or desorb water vapor. This can lead to longer read times but is usually rectified by a good cleaning.

Unstable temperatures can also be a problem. Take care to provide a stable temperature environment for your WP4C and to keep your samples close to the temperature at which you intend to read them.


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