In this METER Environment Office Hours Q & A session, join research scientist Leo Rivera and application expert Chris Chambers as they tackle your toughest water potential measurement questions.

In this episode on water potential measurements and application, they answer questions about:

• The difference between water content and water potential
• When to measure water potential vs water content
• Choosing the right sensor
• Installation
• When to use moisture release curves and when to measure
• And more

Presenters

Chris Chambers operates as the Environment Support Manager and the Soil Moisture Sensor Product Manager at METER Group, the world leader in soil moisture measurement. He specializes in ecology and plant physiology and has over 10 years of experience helping researchers measure the soil-plant-atmosphere continuum.

Leo Rivera operates as a research scientist and Hydrology Product Manager at METER Group, the world leader in soil moisture measurement. He earned his undergraduate degree in Agriculture Systems Management at Texas A&M University, where he also got his Master’s degree in Soil Science. There he helped develop an infiltration system for measuring hydraulic conductivity used by the NRCS in Texas. Currently, Leo is the force behind application development in METER’s hydrology instrumentation including HYPROP and WP4C. He also works in R&D to explore new instrumentation for water and nutrient movement in soil.

Questions?

Our scientists have decades of experience helping researchers and growers measure the soil-plant-atmosphere continuum.  

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Transcript

BRAD NEWBOLD 0:03
Hello, everyone and welcome to office hours with the METER Environment Team. Today’s session will focus on soil water potential, and we’re shooting for about 45 minutes of Q&A with our experts, Leo Rivera and Chris Chambers, whom I will introduce in just a moment. But before we start, we’ve got one housekeeping item. If you are watching this video and you think of a question you’d like to ask our science experts, we encourage you to submit your question on our website at metergroup.com. And then someone from our science and support team will get back to you with an answer via email. All right. With that out of the way, let’s get started. Today our panelists are research scientists and application specialists Leo Rivera and Chris chambers. Leo Rivera operates as a research scientist and hydrology Product Manager at METER Group. He earned his undergraduate degree in agriculture systems management at Texas A&M University, where he also earned his master’s degree in soil science. There he developed an infiltration system for measuring hydraulic conductivity used by the NRCS and Texas. Currently, Leo is the force behind application development in METER’s hydrology instrumentation, including the HYPROP and WP4C. He also works in R&D to explore new instrumentation for water and nutrient movement in soil. Chris Chambers operates as the Support Manager at METER Group. He specializes in ecology and plant physiology and has over 13 years of experience helping researchers measure the soil plant atmosphere continuum. So thanks for joining us, you guys.

BRAD NEWBOLD 1:33
Alright, let’s get started taking our questions. And our first question of the day will be, what is the difference between soil moisture and soil water potential?

CHRIS CHAMBERS 1:46
So they’re both soil moisture, right? They both give you different pieces of information about your soil moisture. Yeah. So let’s kind of break it down a little bit more, and delineate them by water content and water potential.

LEO RIVERA 2:01
Yep. Yeah. So water content tells us purely the amount of water that is there. Okay, it doesn’t tell us anything about the availability of that water. Whereas water potential is your energy state.

CHRIS CHAMBERS 2:16
The differences in water potentials is going to determine which way the water in your soil is moving. So you really want to know both of these to have the complete water picture of your soil so that you have the mass or the volume, and also the energy state so that you know which way the water is going to move. And how much energy has to be applied to move it.

LEO RIVERA 2:35
Yeah, exactly. An often used analogy is to temperature and heat content. And it’s the energy state versus the amount or the intensive versus extensive property.

CHRIS CHAMBERS 2:52
Here you go again, I knew- how far are we into this? Yeah, 2 minutes in and he’s got to bring out the

LEO RIVERA 2:59
intensive versus extensive, and we have more resources about this on the website. But if you want to dive deeper into it, but really, it just boils down to how available is the water and how much water is there?

BRAD NEWBOLD 3:11
All right, next question, which is the best sensor for measuring water potential lower than -1 atmosphere for research purposes?

LEO RIVERA 3:20
Okay, so for those of you that aren’t familiar with atmosphere as a unit of pressure, it’s meant 100 Yeah, it’s essentially equal to one bar minus one bar or minus 100 kilopascals (kPa). And in that case, anything between zero and minus 100 kPa, the best sensor to make that measurement is a tentiometer.

CHRIS CHAMBERS 3:41
Well, I mean, most of them cavitate close to around minus one right? But if you need accuracy below that rate, right, there’s nothing that’s going to beat the accuracy of a tensiometer. I mean, above that range. Sorry, yes. Thank you.

LEO RIVERA 3:56
So anything above -100 kPa. Now some tensiometers can go beyond, go lower than minus 100 kPa when they’re decast. properly. We have like the HYPROP and the TEROS 31. Those are all able to extend or delay that boiling phase.

CHRIS CHAMBERS 4:18
And there’s an asterisk there though, right? You’ve got to get a really good fill. Yeah, we have to.

LEO RIVERA 4:23
Yep. But ultimately, there’s the best sensor is going to be the best accuracy in that range. And there’s not really anything that can beat it. Nope, not for accuracy. Yep.

CHRIS CHAMBERS 4:35
But if you get much drier than that, then you’re going to need something like the TEROS 21, a matric potential sensor that doesn’t require, that doesn’t depend on the strength of the water column to take the measurement, exactly.

BRAD NEWBOLD 4:48
Okay, next one here. One of the most important equipment missing in irrigated agriculture is a reliable, reasonably priced, tensiometer with pressure transducer. Is METER Group working towards providing that to growers and researchers?

CHRIS CHAMBERS 5:04
Do you agree with that statement?

LEO RIVERA 5:06
I definitely agree that it’s an important tool. I mean, there are other ways to get around it, which most people do right now. But I do think there’s a lot of power in being able to have that measurement. I don’t agree with the statement that there isn’t an option available. The TEROS 21 Gen two is a phenomenal option, that is lower price than a tensiometer.

CHRIS CHAMBERS 5:31
It doesn’t quite hit the tensiometer accuracy. It doesn’t. But as far as a price point accuracy trade off, it has a pretty strong value proposition.

LEO RIVERA 5:41
Yeah. And I think for the purposes of irrigated ag, and just trying to control your irrigation to stay within that range, I think its accuracy is well suited for that. It’s not like we’re trying to measure water movement in the capillary range, right. It’s really well suited for that. And we’re working on a newer version too, that’s going to be easier to install and be installed for your good AG purposes where oftentimes, we can’t leave sensors in the room. So there is more work going into this. But I do think there are great sensors on the market for measuring water potential within the price range that is suitable for most irrigated ag. Right.

BRAD NEWBOLD 6:24
Right. Our next question, when should you use a water potential sensor versus a water content sensor?

CHRIS CHAMBERS 6:32
This sounds great. Let’s have some fun with this one. Okay. Um, water balance, creating a water balance. Which sensor do you choose?

LEO RIVERA 6:41
To get a water balance? Water content

CHRIS CHAMBERS 6:43
Water content. Yep.

CHRIS CHAMBERS 6:46
To know whether or not your plant is going to go into drought stress.

LEO RIVERA 6:50
Water potential.

CHRIS CHAMBERS 6:51
Water potential. Okay. One for water content, one for water potential. What about what if you’re studying freeze thaw dynamics soil? Stumper

LEO RIVERA 7:07
I mean, I guess it depends. If you’re looking at freeze thaw dynamics, I mean, you’re probably primarily just interested in water content, how much water is there. But there is some very interesting dynamics that happen due to freezing and thawing action that creates this huge gradient and matric in water potential. Because as well as soil freezes, it actually dries down the water potential really low. And so oftentimes, when you go through these freeze thaw cycles, sometimes you’ll notice that the ground the next day, after a freeze thaw cycle looks like it’s gotten really wet. There’s been no rain right.

CHRIS CHAMBERS 7:44
The water potential may actually get because of automation. They’re driving that up.

LEO RIVERA 7:48
Now, having said all that, it’s really hard to measure water potential when it’s frozen, right? But if you have temperature, you can actually damage some of your equipment. Yeah, it can. But if you have temperature, you can actually determine based on how much it dropped below zero, right? What the water potential was. So that’s kind of an interesting use case.

CHRIS CHAMBERS 8:06
Okay, next one, measuring soil health. So water content or water potential?

LEO RIVERA 8:12
I’m gonna want both, both. Exactly.

CHRIS CHAMBERS 8:14
Yeah, I vote for both, too. Yeah.

LEO RIVERA 8:15
Because you need to know the care, especially in situ, if you can characterize the moisture release curve, the soil, the way you’re impacting soil health is going to have a big effect on that. Right. On that characteristic curve. Right. Exactly.

BRAD NEWBOLD 8:30
All right. This one is asking for the tensiometer, if you’re looking to measure potential at a specific root depth, how do you know what depth to install the sensor with a long water column?

LEO RIVERA 8:43
So this is kind of referring to older style tensiometers that have long water columns. And yeah, most of our water most of our tensiometers, except for the TEROS 31 have small water columns. Very small water columns only held within this surrounding.

CHRIS CHAMBERS 8:56
Yeah, so for your field ones that we’ve kind of moved beyond that question. And if you have a long water column, like the TEROS 31, you can compensate for it. Yeah.

LEO RIVERA 9:06
So yeah, it’s a pretty simple correction. It’s just based on the height. Gravitational height. So if you install it an angle, you need to know what your gravitational height of the water column is. The total length of the water column, right. But the beautiful thing is that most newer tensiometers you don’t really have to worry about this for the field. We don’t typically use that tensiometer like the TEROS 31 on the field, because it’s just not as well suited for that and we have better tools.

CHRIS CHAMBERS 9:30
That’s right. TEROS 32 is going to be a better option. And this isn’t a problem. Yeah. Yep. Or TEROS 21. It’s like getting my numbers mixed up. Yeah, the TEROS 21 is an option as well where this isn’t a problem. Yep. Yeah.

LEO RIVERA 9:46
So most of these stuff you don’t really have to worry about it. If you do have an additional tensiometer. It’s just some basic math to make the correction. Or it may be time for an upgrade. Exactly.

BRAD NEWBOLD 9:57
This next person asks what sensor might be appropriate in more arid environments, when the soil water potential might be very low for most of the year?

CHRIS CHAMBERS 10:05
This is kind of a tricky one to get into. Tensiometers out of the question you can assume. So TEROS 21 is going to be a good option. We’re getting pretty good accuracies down to permanent wilting point and beyond. Yep. And what else heat dissipation sensor would probably be a good call, if you have the patience to calibrate them. Exactly. Yep.

LEO RIVERA 10:30
So no, I think that’s the right call there. Is this your non maintenance type, water potential sensors that give you the accuracy that you need in that range, and right now, the only ones that I know of or the ones that work well, is that something like the TEROS 21 or the heat dissipation sensor.

CHRIS CHAMBERS 10:48
Beyond that, unless you’re going to take samples and run them in a WP4C, Yeah, in which case, you lose the temporal variability there in a temporal resolution in your data. Yeah, yeah.

LEO RIVERA 11:00
Beyond that, I would probably also want to measure water content along with that, just because I think you might get a little more information.

CHRIS CHAMBERS 11:07
It’s a little more sensitive at the dry range. And you could generate a soil water retention curve and actually approach the problem that way. Yeah.

BRAD NEWBOLD 11:16
Our next question is, can the relationship between soil moisture and water potential in the range of accuracy of the water potential sensor be used to infer water potential from soil moisture readings in drier conditions?

LEO RIVERA 11:30
Yeah, absolutely. I mean, this is what people do we use this soil moisture release curve all the time to make this correlation. I personally, I don’t hate it. But you need to understand some of the potential drawbacks of this.

CHRIS CHAMBERS 11:46
Now, how much does hysteresis play in this range? Exactly. When you get to this area, it’s soil dependent, right?

LEO RIVERA 11:51
It is, it’s soil dependent. And hysteresis can play a big impact whether you’re on the wetting or drying cycle. And then in between that the scanning curve, where you’re going between these wetting and drying cycles within that hysteresis band, you’re going to get differences. So you just need to know that there’s going to be potential errors in your water potential estimate because of hysteresis. And because of what are you using a lab based dry down curve, a lab based wetting curve? There will be some potential error because of that now, and

CHRIS CHAMBERS 12:26
most of the time, we’re going to be interested in the drying curve, right? Because your wetting curve is generally really rapid.

LEO RIVERA 12:31
Yeah, typically, yes. That’s correct. And that’s what I would use.

CHRIS CHAMBERS 12:35
If you had to pick just one. Yep. instead of a sophisticated analysis. Yeah,

LEO RIVERA 12:39
I would use a drying curve. Just know that there are some drawbacks there that come with that. And so as long as you understand that, then I think it’s safe to use this approach. Great.

BRAD NEWBOLD 12:51
Okay, this one says, Are there effects from digging a trench on the soil at the site?

CHRIS CHAMBERS 12:58
This is kind of disappointingly not clear. To me, I feel like we should have a better handle on this.

LEO RIVERA 13:03
Yeah, I think it depends. I mean, well, there certainly can be. Because when you’re digging a trench, it’s a pretty large disturbance, yeah, to the site. And depending on if you’re on the uphill or down, especially if you’re on a slope, say you dig a trench uphill from where you’re installing sensors, you’re going to alter the hydraulic properties of that soil.

CHRIS CHAMBERS 13:28
Even on the downhill though, you’ll you’re probably going to have an effect.

LEO RIVERA 13:31
You are but I’d rather have my trench on the downhill side. I mean, if you have to pick between the two, having your trench on the downhill side minimizes the impact on water movement, in that

CHRIS CHAMBERS 13:44
where gravitational water is going to wind up by your sensors.

LEO RIVERA 13:49
The other thing is when you put the soil back into the pit, there’s two things that I always try to do as best as possible. And put it back in the same layer, same arrangement that you pulled it out great advice, pack in layers, to try and get as best as you can to the original bulk density the soil, you’re not going to be perfect, it’s really hard to get that back to the exact same bulk density. Never leave a depression on the site, if you dig a trench, and then you have a depression there. Or even if you’ve gotten it packed flat, there’s a good chance that over time it’s going to form a depression, right? So you want to make sure that you have a little bit of a mound of soil on there to allow for that settling that we know is going to happen. And try to avoid that, or if you can use different tools like our borehole installation tool, which minimizes the site impact, then that really can help as well.

CHRIS CHAMBERS 14:48
That’s right, you’re disturbing less of the vegetation, you’re disturbing less of the soil structure. Basically just keeping your site closer to the condition that you would like to study.

LEO RIVERA 15:00
Yeah, so if you can minimize site impact, it’s always, always a good idea. Yep.

BRAD NEWBOLD 15:05
Our next question asks, Are there case studies for plants and turf available?

LEO RIVERA 15:10
Yep. So we have a couple of case studies that we’ve done and published, working with some researchers at BYU, Brian Hopkins, and Neil Hansen, doing some work in turf, and also a case study on potatoes. So there are quite a few of those available. But there are plenty of other researchers that have done some of this work. And also, one of the best things to look at is some of the the papers from Sterling Taylor, where he has some of these water potential ranges published. So yeah, there’s quite a few out there. And you can look on our site and look at some of the case studies we have available, or dig deeper into literature.

CHRIS CHAMBERS 15:56
There’s lots of instances in the literature of water potentials at which plants, different species different soils are happy. Yep.

BRAD NEWBOLD 16:05
All right, this one is asking, are sensors available that measure at two inches and five inches at the same time, to better know how the soil water potential is changing in the soil profile?

CHRIS CHAMBERS 16:18
Soil profiles, pretty important to know what’s happening in there for a lot of questions. There are some profile probes available not so much for water potential or matric potential,

LEO RIVERA 16:31
Not for water potential. And that’s definitely on the list of things. It’d be really nice to make. And maybe one day we’ll make one down the road or somebody else will, we’ll see. It’s not an easy thing to make. So right now, your best approach is just to install your sensors at the depths that you need to monitor. But yeah, I do absolutely agree. Understanding profile dynamics is really important. But there’s no profile sensor for matric potential available. That’s right. Yeah, we have released the TEROS 54, which is our new profile soil moisture sensor for water content. Yep. And so that is our first stab at a profile sensor that we’re pretty excited about. But depending on what depths you want to measure at, sometimes you’re better off just doing an individual sensor, especially if you have very specific depths that you want to measure. That’s right.

BRAD NEWBOLD 17:27
This one says, How can you measure capillary water potential?

CHRIS CHAMBERS 17:31
It’s a really good question, take a step back and define that for us.

LEO RIVERA 17:34
So capillary water potential has to do with the actual pores in the soil, pulling the water up, like acting like capillaries

CHRIS CHAMBERS 17:42
And different size capillaries have a larger effect. Right? Exactly.

LEO RIVERA 17:46
So it depends on the pore size distribution of your soil. What that looks like. However, the capillary range that we typically define in salt water potential is zero to -100 kPa. Okay, so you’re ultimately measuring multiple things, you’re not just measuring the capillary water potential. But if we know that that’s the capillary range, then we can look at the water potential changes within zero to minus 100 kPa. And know that those changes are primarily due to changes in the capillary range.

CHRIS CHAMBERS 18:22
Do you generally want to know something about your soil structure in this case, because it’s largely a function of structure? Is it not?

LEO RIVERA 18:29
Absolutely it’s a function of structure, its functional structure and pore size distribution, of course pore size distribution So

CHRIS CHAMBERS 18:35
that’s the most important variables. Yeah, if you really want to nail this down,

LEO RIVERA 18:39
yeah. But if you’re measuring within that range, then you’ll know that that is all capillary water changing. When you get beyond that, then we get into the adsorbed film range. And then beyond that, we get into the really inner layer, changes of water potential and those are really low water potentials. Right, right. And the only people that are interested in that are going to be geotechnical engineers, or, or soil mineralogists, things like that. But yeah, measuring within the zero to minus 100 kPa range gives you the information you didn’t know about changes in capillary water.

BRAD NEWBOLD 19:15
All right, this question is asking how do sodium and bicarbonates in water affect tensiometer readings?

CHRIS CHAMBERS 19:22
So mostly tensiometers are going to ignore solids and soil? Yeah, right. The ceramic is gonna let them diffuse in and out. Sothere’s no potential difference. It’s going to equilibrate and it shouldn’t really matter. Depending on the method you’re using, so for tensiometers, it’s, you probably won’t see anything. The TEROS, something like the TEROS 21 that actually measures water content. If you get a high enough salt level, then that’s going to mess with the water content reading. And since we infer the matric potential from a water content, then you can run into problems. So this kind of sensor dependent tensiometers generally aren’t going to worry about solutes in the soil. But pay attention to the base measurement of some of your other sensors out there.

LEO RIVERA 20:15
Yeah. And Chris, your chemistry is probably a little bit stronger than mine is like bicarbonates, do we worry about those precipitating out?

CHRIS CHAMBERS 20:25
That really depends. This is pH dependent, I believe. So it kind of depends on what else is happening in your soil. Yeah. If everything is in equilibrium, probably don’t have to worry about it. Things in soil change. So it’s always good to know what’s happening in your soil.

BRAD NEWBOLD 20:46
Our next question is, how sensitive is a tensiometer? If we install it in one place, and the pressure changes, say to only one millimeter, can we measure it?

CHRIS CHAMBERS 20:57
If you want to get the best possible resolution, the tensiometer is the way to go? Yep. What is the resolution of a TEROS 31?

LEO RIVERA 21:04
I believe it is. It’s four or five honestly 6, is I believe you can resolve I think to .1 hectopascal cool way, which is about one millimeter, approximately, of changes in water potential. So I say for them. Yeah, I mean, that one millimeter change is pretty dang precise. I don’t know if we’re going to be as accurate because our accuracy is about plus or minus half a kilopascal. Okay. But the resolution is .1. So we can definitely see those changes, it’s not going to be as accurate. Now, we are working on a project with some researchers at Utah State where they’re trying to deploy these on a space station project nice, and they are looking for much higher precision, and it is absolutely doable, the pressure transducers on those tensiometers have really high resolution when used on third party systems like a Campbell Scientific data log, right. And so we’re working with them to actually utilize that system to give them the resolution that they need.

CHRIS CHAMBERS 22:15
So the reader has a part to play.

LEO RIVERA 22:20
Yeah. So yeah, we can definitely see that. Most people don’t need that type of resolution, except in some of these niche cases,

CHRIS CHAMBERS 22:29
studying plants and water movement in space just sounds fun. Yeah. Yeah.

LEO RIVERA 22:34
And, yeah, I’m excited to see how that one turns out.

BRAD NEWBOLD 22:38
This next person is asking what is the physical meaning of a positive result from a tensiometer?

LEO RIVERA 22:45
That is a great question. So typically, if we see a positive reading on a tensiometer, that means we’re saturated. Right? And then some Yeah, and then some and so the more positive that reading goes, that indicates that we have a higher water column above the tensiometer. So oftentimes, this is really the beauty in tensiometers is especially if you’re doing let’s say, for example, you’re studying slope stability, where when you see these positive pore pressures, which is what they typically refer to it, that’s a bad thing, right, or that indicates a higher risk of a slope failure. And so they use tensiometers. In these cases, to look when the pore pressure is negative as they it’s typically referred to as geotech. Or when we have normal water potentials. And then when it goes positive, we look at those readings and are treating the tensiometers, like a pesometer, which is a tool that we use to measure water levels, right. In the saturated zone. So So yeah, this is a really cool thing about tensiometers, because they can give us information in saturated conditions as well. Yep. So yeah, it’s not a error or anything with the tensiometer. That’s just a normal reading, and you’ve just got a positive water column above the sensor.

CHRIS CHAMBERS 24:01
That’s right. And it is unique. Tensiometers if your TEROS 21 gives you a positive reading, give me a call.

BRAD NEWBOLD 24:09
All right, this one is asking are sensors available that measure at two inches and five inches at the same time to better know how the soil water potential is changing in the soil profile.

CHRIS CHAMBERS 24:21
So talking about soil water retention curves here. Yep. And so what is your what do you think is most interesting about soil water retention curves? Like it’s neat, it’s cool, it’s like here, look, we can do this with the TEROS 21 and the TEROS 12. Why is that so important?

LEO RIVERA 24:44
Yeah. So yeah, there absolutely is this relationship and it’s important because it tells us a lot about the soil properties. There’s I mean, we can go really deep dive into all the information that you can get out of soil moisture, release curve. There’s actually quite a bit from soil wall capacity of soils to specific surface area now, that’s diving really deep and and maybe not what we need to do for this. But ultimately it just tells us it’s we often refer to this as the fingerprint of the soil, right? Because depending on changes in bulk density changes in organic matter, structural changes in the soil, these all impact that moisture release curve, right, and we need to understand that to understand much water can be stored in the soil. How much water is available for plants? What else? I mean, there’s just so much information that you can’t get out of there. But

CHRIS CHAMBERS 25:42
if you have the soil water retention curve, do you even really need to know like the soil texture? Or do you still want to have that information?

LEO RIVERA 25:49
Oh you’re still gonna want to have that information. Because, again, there’s so many factors that impact, it’s not just texture on its own impacts, the more

CHRIS CHAMBERS 25:56
that you can account for it, in your analysis, the better Yeah, exactly.

LEO RIVERA 26:00
So. So it’s great. And measuring in situ is really is an ideal way to do it. If you can, it’s still hard to do. But it’s more doable. Now with the sensors that we have available. It’s sometimes hard to compare lab based retention curves, with the field based soil moisture release curves, I think Colin is going to present on one in his webinar. And so we’ll see how that comes out. And we’ve seen some people doing that.

CHRIS CHAMBERS 26:31
Is that mostly because of the control of the process or the completion or how complete the curve is, because like in the field, you’re only going to get a partial curve most of the time.

LEO RIVERA 26:40
Exactly is how some of it has to do with the dynamics in the field, because you’re going through the scanning and scanning curve. So you’re going through this frequent wetting and drying curves. Also, if your lab curve isn’t based on an intact core sample, you’re gonna have differences due to the fact that the sample really isn’t the exact same as a field, right? So things like that can impact it. And also, it’s very rare that you truly get fully saturated in the field,

CHRIS CHAMBERS 27:10
Right, where it’s got some air trapped somewhere. Yeah,

LEO RIVERA 27:13
exactly. And whereas with a lab based retention curve, we completely saturated that soil sample. So so you’ll see differences in the actual Max water content that you hit with a lab base curve. So just things like that.

BRAD NEWBOLD 27:27
All right, this question. The only option for container media is a tensiometer, isn’t it?

CHRIS CHAMBERS 27:34
I might, in my experience, I agree with that statement. TEROS 21 I’ve had some really bad situations from my plants, trying using TEROS 21 and container media medium personally. The installation is tricky. It’s hard to tell exactly the full extent of the problems with it with something like the TEROS 21. But the tensiometer is just precise enough and responsive enough that you can get good information from it. In most media, something you need to watch out for and container media is the shape of the soil water retention curve. Like everything looks great for matric potential for a long time. But there’s a cliff that it’s waiting to fall off of if it gets dry enough. Yeah. So if you’re not paying attention to it, if you’re not getting your notifications, and you go just a little bit too far, you’re gonna have a small change in water content that drives off the cliff and there’s no water left for your plants, basically.

LEO RIVERA 28:40
Yeah, no, I have to agree with that. But when you actually want to measure water potentials, and most of these soilless mediums that are using containers, the tensiometers are the best option. I will say there’s a caveat to that you know, we did a study with a researcher at University of Georgia, this was like almost 10 years ago now, where we were seeing plants that were stressing at minus 10 kPa. Oh, man, so water potential only tells you part of the story. And what we found with those is that it was actually a limitation due to hydraulic conductivity. And so technically, the water potential where we were measuring was minus 10 kPa, or what you would assume was to be a minus 10 kPa, but right around the surfaces of the root because there’s less redistribution of what our soil redistribution of water, the plant was starting to stress and due to that, due to the limited unsaturated hydraulic conductivity, so it is something you have to be careful with and know what the optimal water potential ranges to stay within for that media are

CHRIS CHAMBERS 29:39
and in that case, something like your water content sensor, which samples a larger section of soil you don’t have to worry about something like that.

LEO RIVERA 29:46
Exactly. So yeah, soilless media can be a little tricky sometimes. Yep.

BRAD NEWBOLD 29:51
This next person is asking we have a HYPROP and WP4C, what challenges can we expect with analyzing high clay vertisols soils?

CHRIS CHAMBERS 30:02
It’s mostly a shrink swell thing, right?

LEO RIVERA 30:04
That’s the biggest factor is the shrink swell of the soil. None of these methods really handle that all that well, especially as you get it really dry you large cracks that form. And it shrinks, the shrinks away from the edge of the rings on the HYPROP

CHRIS CHAMBERS 30:20
But not just that you can lose contact with your tensiometers too.

LEO RIVERA 30:23
Yeah, yep. So yeah, these vertisols are tricky. I think for the most part within the range of the HYPROP, I wouldn’t push it beyond the -100 kPa range. If you try to go lower and use like our entry point, you’re really going to get

CHRIS CHAMBERS 30:38
The WP4C a little robust to that, right.

LEO RIVERA 30:41
Yeah, so that’s where I would just use the very wet end for the HYPROP, and then just jump over to the WP4C once you get past that, yep, once you get past that, wet range that the WP4C doesn’t do very well, you should be in good shape. Yeah, and the WP4C does great with the soils. And we use that same technology on the VSA as well to characterize the soils. And these are the types of soils where there’s actually a lot of information in that dry end of the soil moisture release curve, especially when we look at the hysteresis that occurs down there too. So it’s really, really, really interesting to look at, you just need to understand some limitations and how you are going to approach those measurements. Right.

BRAD NEWBOLD 31:25
This next question asks, How do you develop a soil moisture release curve in highly variable soils?

CHRIS CHAMBERS 31:33
Samples sample Yeah,

LEO RIVERA 31:34
I mean, it’s if they’re highly variable soils, you need to collect samples to represent that variability.

CHRIS CHAMBERS 31:39
And if you get a handle on the spatial variability of your soil types, then you’re in good shape.

LEO RIVERA 31:44
Yeah, yep. Now, some people might try to use petal transfer functions, which are tools that we use, where they pull in a large database of soils. And then you input some of these different, varied changes in variability, maybe its texture, maybe your density, and you can try to use those to kind of characterize some of those changes, but they’re not perfect. The best thing to do is just sample that variability.

CHRIS CHAMBERS 32:07
I’m sure AI will open that up at some point once the database is fully ingested.

LEO RIVERA 32:13
We’ll have a soil GPT at some point. Now, it’s really a I mean, this is something that we deal with in all types of measurements, not just soil moisture, release curves, hydraulic conductivity, even just changes in soil moisture spatially. Really, the best way to approach this is through sampling, but also characterizing where that variability in the field is coming from. And there are tools that we can use to help with that,

CHRIS CHAMBERS 32:40
If in doubt, buy more HYPROPs, and more samples.

LEO RIVERA 32:44
Yep, but yeah, just characterize where that variability is gonna come from, and take your samples from within those zones to try and characterize it as best as you can.

BRAD NEWBOLD 32:53
All right, we’re getting near the end. So we’ll do a couple more questions here. This one is asking, how is the temperature sensitivity of a water potential sensor? Are there any compensation equations for those sensors?

CHRIS CHAMBERS 33:06
So the potential the water potential is temperature, there is a temperature effect, right?

LEO RIVERA 33:11
Exactly. So water potential is temperature dependent. There’s also the potential for sensor sensitivity right to this. I know that’s something we spend a lot of time on the TEROS 21 gen two to try and limit that temperature sensitivity, right? Or the sensor, temperature sensitivity. But there is still water potential. That’s the water potential, just it changes with temperature. So some of those are just actual changes in water potential.

CHRIS CHAMBERS 33:38
So it’s gonna be a pretty small in a tensiometer. Yep. Where you are doing a much more direct measurement. Exactly. And in the, in the ones that measure indirect measurements that make indirect measurements like the TEROS 21 Then you’re gonna have to be more careful of well, how does temperature affect what the TEROS 21 is measuring? Yep. And also like the TEROS 21 relies on a soil water retention curve. And so if you get to a part of the curve where a small change in water content, save from a temperature change results in a large change in matric potential, then you’re gonna get noisier data. Yep.

LEO RIVERA 34:15
Yeah. And I, we’ve seen some papers published on doing some of these corrections with the TEROS 21 camera, which were this group was out of that published the Swiss or, or out of Italy. But we have some of those papers linked in the, on our literature sources on the website. Right. Really, really, I think most of the sensitivity comes in on the dry range. Right, exactly. That is important. Even for the TEROS 21 it’s not the on the lead and that’s right comes

CHRIS CHAMBERS 34:45
that comes into the what the soil water retention curve looks like. There. There’s there’s just a little buffer.

LEO RIVERA 34:50
Yeah, exact changes on the dry end there’s just so little water in this. And yeah, yeah. And so those little super changes in temperature can make a big difference. So yeah, so there’s definitely resources out there to look at this, and we can share them with you just send us an email, and we’ll get you some of those resources.

BRAD NEWBOLD 35:06
And then finally, it looks like we are close to the end of our time here. So this will be our last question. And this final question asks, we monitor soil moisture using water content. How can we integrate this into a soil moisture release curve?

LEO RIVERA 35:22
So yeah, so this is great. If you’re measuring soil moisture, you’re not necessarily going to integrate it into the soil moisture, release curve, but you can use soil moisture, release curve to then quantify the changes in water potential for the sites where you’re measuring soil moisture.

CHRIS CHAMBERS 35:38
Take your soil sample from when you installed your water content sensors. You did take a soil sample when you installed water content sensors. Take that sample, do the soil water retention curve on it and you can then you have a transfer function to get over to your water potential.

LEO RIVERA 35:57
Yeah, as Chambers said, it’s always a great idea to take a soil sample when you install sensors, just that way you can characterize it and understand what’s going on down there. Exactly. But yeah, if you have the sample to characterize the soil moisture, release curve, then you can use some of the different available functions out there. There’s the Van Genuchten equation. There’s Brooks and Corey. There’s the Fred Lensing equation. There’s several options out there that you can then take those values that you get from the soil measure release curve and apply the water content readings to that equation. And I’ll put a lot of potential. Yep. So yeah.

BRAD NEWBOLD 36:38
All right. That’s gonna wrap it up for us. Thank you again, everybody, for joining us today. We hope that you enjoyed this discussion. Thanks again for all your great questions. And again, if you have any questions we didn’t answer, please contact us via our website at metergroup.com. Finally, you can subscribe to the METER Group YouTube channel and accept notifications to see previous episodes of Office Hours and to get notified when future videos are available. So thanks again. Stay safe, and have a great day.