Predict the fate of soil water

Moisture release curves answer critical questions such as: will water drain through the soil quickly or be held in the root zone? They are powerful tools used to predict plant water uptake, deep drainage, runoff, and more. In this live Q&A session, learn how to use a moisture release curve to analyze individual soil behaviors with respect to water.

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.

Next steps:

• Lab versus in situ soil water characteristic curves
• Soil Moisture Release Curves: Why you need them. How to use them.
• Request a ZENTRA Cloud live demo

Questions?

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Transcript

BRAD NEWBOLD 0:01
Hello, everyone and welcome to office hours with the METER Environment Team. Today’s session will focus on soil moisture release curves. And we’re shooting for about 45 minutes of Q&A with our experts Leo Rivera and Chris Chambers, who I’ll introduce in just a moment. But before we start one housekeeping item. If you’re 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. Someone from our science and support team will get back to you with an answer via email. All right, with all of that out of the way, let’s get started. Today our panelists are research scientists and application specialists Leo Rivera and Chris Chambers. Leo 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 got his master’s degree in soil science. There he helped develop an infiltration system for measuring hydraulic conductivity used by the NRCS and Texas. Currently, Leo is an application expert for METER’s hydrology instrumentation, including the HYPROP and WP4C. 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. Let’s get started taking some questions here. And this first question is a basic introduction to soil moisture release curves. They’re basically just asking, Can you explain what they are? And how you can use them?

LEO RIVERA 1:36
It’s a good question.

CHRIS CHAMBERS 1:36
This is I mean, this is kind of more relevant now than it used to be right? Because you think back to soil moisture released curves, and you needed specialized equipment, you needed specialized expertise to understand them. Yeah. And generally, you found labs that were designed for this purpose. And yeah, they weren’t that common. Almost every time we go, we go to one of these places, they’re updating their instruments, you know, we take a tour of how you used to do it, and it was time intensive, labor intensive, and just took a lot of expertise and specialized equipment to be able to make these curves.

LEO RIVERA 2:17
That’s true, and I think what we’ve tried to do is make it more accessible and easier to do. And along the way, we’ve made tools that are just so much more powerful to actually make these measurements.

CHRIS CHAMBERS 2:27
And so now you might wind up with in situ soil water retention curves from like a TEROS 21 and a TEROS 12 located right next to each other, and not quite realize the power of the data that you’ve got in your hands.

LEO RIVERA 2:41
Yeah, I think the best way to address this question too, is just show your basic soil moisture release curve. So I’m going to pull up a jam board here. And we’re gonna color Yeah, trying like our toys. Okay, so let’s just share the screen right here. So, so much released curve, in its basic form, is the relationship between water content and water potential. We see these curves for other things as well, we see them for water content, hydraulic conductivity, hydraulic conductivity, to water potential, these curves can all be really powerful. But they really act as you know, we often say they act as a tool of fingerprinting the soil. And the reason we say that is there are so many factors that impact the soil moisture release curve, and it’s really unique to that specific soil that you’re measuring it in. It’s not just soil texture, it’s not just the amount of organic matter. There’s structure, bulk density, the amount of bio pores that are in there, so many things that impacted and what that looks like. And but I think it’s really helpful to just break it down in this basic form where you see, let’s just say we have three basic curves here we have a lonely fine sand, find sandy loam, and silt loam. And we’re looking at their curves. And you see that they’re all different. And these

CHRIS CHAMBERS 4:02
Break it down for us, we got it looks like water potential on the x axis?

LEO RIVERA 4:06
Exactly. Thank you. Good point. So we have on the x axis, we have water potential. In this example, it’s in kilopascals. And we put negative in there because it’s a logarithmic scale,

CHRIS CHAMBERS 4:18
I hate log scale. Yeah, they’re great, I’m just lying.

LEO RIVERA 4:21
But there are many other ways, units, that people use to express water potential and I like kilopascals, so we stick to that because that’s what we’re used to. And then on your Y axis, you have volumetric water content. And we see a couple lines here to designate some key factors. So right here, we have a lot did not mean to do that. Let’s get the pen out. So we have a line right here. That is our field capacity point. So it’s just gonna put an FC right here for field capacity in really poor handwriting. And we have this other line here representing permanent wilting point. And we’ll probably dive deeper into a little bit about these different points. But what’s beautiful about a tool like— That is not a P.

CHRIS CHAMBERS 5:13
That’s all right. But the main thing is here, you’ve got a couple of kind of waypoints for people to visualize the different parts of the curve, right?

LEO RIVERA 5:20
Exactly. And when you draw that line through all these different curves, or these different curves, you see that they mean very different things for these different soil types. And I think we’ll dive a little bit deeper into that as we get deeper into the questions. But really what this does, it helps us further our basic understanding of various properties of the soil, how available the water is, and how we can expect plants to perform in it. And so it just gives us a lot of that basic information to support the science that we’re doing.

CHRIS CHAMBERS 5:47
Okay. So I tell people all the time, they’re taking water content measurements, I tell them all the time, make sure you get a soil sample where you’re at. Does that mean you don’t have to do that anymore?

LEO RIVERA 5:55
No, you still should get your soil sample. Because

CHRIS CHAMBERS 5:58
That’s what I thought.

LEO RIVERA 5:59
you like I said that this changes with different soil types.

CHRIS CHAMBERS 6:04
And it doesn’t give you any information about hydraulic conductivity, other parameters that you might need.

LEO RIVERA 6:09
Now we can characterize that with these tools, but we need the soils to do it. So, anyways, that is your basic introduction into the soil moisture release curve. And I think part of the question is, you know, how can you use a soil moisture release curve? There are so many ways. There’s so much information that can be pulled out of a soil moisture release curve.

CHRIS CHAMBERS 6:31
Okay name one. Because I’m here to find out some of these answers.

LEO RIVERA 6:37
Well, the most basic one is this example right here is like, Okay, what’s our plant available water range? That’s the most basic one. And that’s that point between field capacity and permanent wilting point.

CHRIS CHAMBERS 6:46
But let’s look at it, you could get two points in the field, and basically know that. So you don’t need the effort to do the entire curve, right?

LEO RIVERA 6:53
You can.

CHRIS CHAMBERS 6:54
Okay.

LEO RIVERA 6:55
But and actually maybe it’s a good thing I put this other example in here, is this example right here. So this is actually a soil moisture release curve for soil moisture— for a soilless media sample. And we dive into this a little bit deeper in another video, but what they were finding is plants are stressing in a range that was way higher at a much higher water content than they would have expected it. And it’s because let’s just say you only had two points, let’s say we have our fuel capacity point right there, which isn’t exactly right. And our permanent wilting point was over here. Okay. And again, I’m just showing examples. If we were to only collects those two points, we will just assume, Okay, cool, this is a perfect curve, and it’s going to come down just like that all nicely.

CHRIS CHAMBERS 7:50
And that’s the size of your tank.

LEO RIVERA 7:51
Yeah. But this inflection right here tells us there’s something else going on. And this is a gap graded substrate. And so what they were finding is that when they were getting to this point right here, the plants, this is where the plants are starting to stress. And once we looked at the curve, we’re like, Oh, okay, now we understand why they’re starting to stress at this point. And it’s because of this gap graded substrate, you’re actually limiting how the water can move through the sample. And we’ll talk a little bit more about that. I think it’s some of the other questions, but we want to see that with those two points. And with tools like the HYPROP, we were able to figure this out and see what was going on, and understand, okay, this is why the plants are starting to stress, and then it gave us another tool to better characterize substrates. And companies have started running with this and actually using this to characterize their mixtures. Yeah, so you can do some really cool things with it. Let’s see what happens if we clear the frame. Hopefully, it takes the picture. Oh, I knew that was gonna happen.

CHRIS CHAMBERS 8:52
The old undo button.

LEO RIVERA 8:54
Yep. All right. Okay, Brad, we’ll let you move us to the next question. And we can stop sharing this for now.

BRAD NEWBOLD 9:06
Okay, this next question is, How can you use release curves to predict water movement?

LEO RIVERA 9:15
That is a really good question.

CHRIS CHAMBERS 9:16
But at the base level, you don’t actually need a release curve for the full thing, right. Just the matric potential gradient will give you some information about where water is going to move.

LEO RIVERA 9:25
Exactly. So matric potential is a governing factor that’s going to tell us where water is going to move to in the soil. What a soil moisture release curve allows us to do is to actually get that model. So we can take a soil moisture release curve and generate a hydraulic conductivity function and a soil moisture release curve. So the Van Genuchten parameters, for example, and take that and put those parameters into a hydrology model. And say HYDROS is an example. And then we can use those parameters to actually predict at various water contents and the various conditions

CHRIS CHAMBERS 10:05
What your major potential will be. And after drawing down so much water, what water is still available.

LEO RIVERA 10:11
Yeah, exactly. And so say for example, especially when you have, so when you have horizons with very different textures, so if we jump back into that,

CHRIS CHAMBERS 10:19
Which never happens.

LEO RIVERA 10:20
Never happens. So let’s just say we jump back in here, and we’re gonna pull that graph up one more time. And just bear with me, there we go. Getting faster with this. Okay, we’re gonna jump back over to this guy right here. So again, we can use these moisture release curves, to identify, Okay, let’s say our water our soil is at 30%.

CHRIS CHAMBERS 10:49
And that’s our loamy fine sand. Right?

LEO RIVERA 10:51
Yeah. We’re gonna do that really quick. Okay, so let’s just say we’re at 30%. Okay, and let’s just say we have that’s a terrible line. But let’s just say we have our fine sandy loam and loamy fine sand. If we’re at 30%, we have very different water potentials for these two soil types. So obviously—

CHRIS CHAMBERS 11:17
If we look at the change in water potential, you’re gonna get a drastic change in water content. So vice versa, it’s actually vice versa, right? Yeah.

LEO RIVERA 11:25
So let’s just say we know water potential moves from areas of high to low water potential, right? So let’s just say high is going to be anything closer to zero. And lows just say we’re getting down to minus 100, minus 1000 kilopascals. Water’s always gonna move this direction. And so we can use that to kind of quantify okay, with these different textures, this substrate is going to pull water this way. And a good example of what can happen if it’s the opposite of that. Let’s just say, we have a coarse layer below a fine textured layer, this actually creates a capillary barrier and results in the, it actually increases the water holding capacity of the layer above it. And so it actually completely changes how that layer retains water.

CHRIS CHAMBERS 12:16
And so while the matric potential, everything is in equilibrium, right. So you know, your gradients are going to be moving, going to be evening out compete equilibrium, but your water contents are going to be drastically different.

LEO RIVERA 12:28
Yeah, they’re different stages. Yeah, exactly. So things always want to come to equilibrium. So these, let’s just say start at 30%, they’re not going to stay at 30%, they’re going to come to an equilibrium in terms of water potential, which is going to be something like this, let’s just say this is our equilibrium point. And so we might see the core section soil down at like, 6, 7%. And the fine sandy loam, much higher around 42, 46%.

Speaker 2 12:58
And so if you’re just trying to interpret water content data, say, between these two different points. And you’re going to be scratching your head, like what the heck is going on here?

LEO RIVERA 13:08
Exactly, yeah. And so you really need to understand these characteristics to really understand what’s going on there. Hopefully, that helps answer that question, although.

CHRIS CHAMBERS 13:16
I love it.

LEO RIVERA 13:19
All right.

BRAD NEWBOLD 13:20
Okay, next question. If we have a sensor for both volumetric water content and water potential, which measurement should prevail on the decision to add more water? And how can a moisture release curve help?

LEO RIVERA 13:43
Well, I mean,

CHRIS CHAMBERS 13:44
It really ties into that last question really well.

LEO RIVERA 13:47
Yeah, they’re both important. We need to understand water potential to be able to understand how available the water is, how much water is there? Yeah. Then, but we then need the water content to know okay, if we’re trying to get to a certain threshold, we need to irrigate, say, to get an additional 5% to get to that threshold, but we need to know both, right? So I always argue that both are key. But we need to understand water potential to understand its availability. And we need to understand water content to understand the amount of water that’s actually available.

CHRIS CHAMBERS 13:47
That’s right.

LEO RIVERA 13:52
I think they’re both really key factors. And that’s why you’re starting to see more and more people measuring both

CHRIS CHAMBERS 14:26
And that curve will help you, the soil water retention curve will help you understand how the soil water dynamics will change as you went up or dry down.

LEO RIVERA 14:37
Exactly. And oftentimes, if you don’t have both measurements available to you, at least if you have the moisture release curve, then you can use one or the other.

Speaker 2 14:44
That’s right. Yep. How much of a difference do you think hysteresis plays in that? Whether you’re wetting or drying.

LEO RIVERA 14:52
Good question. So hysteresis can play a bigger factor in

CHRIS CHAMBERS 15:01
It’s not the same across all soils, right?

LEO RIVERA 15:03
It’s not the same across all soils, but it can play a factor in any of these. It’s maybe anywhere from 3% and up to 6 or 7% difference. And it really depends on where it starts on the curve too. So what we’ve seen with hysteresis curves is they’re very different depending on where you start your wetting and drying points within the curve. So it’s pretty dynamic.

CHRIS CHAMBERS 15:33
And the relevance for this is that your curve can be slightly different whether you’re on the wetting or drying. So it kind of depends on which direction you’re going, you might be taking a slightly different road to get to the same endpoint.

LEO RIVERA 15:46
Yep, exactly.

BRAD NEWBOLD 15:50
All right. Okay. We’ve got a couple of questions here regarding the HYPROP. And so this first one is asking, what is the impact of the evaporation rate on the HYPROP measurement?

LEO RIVERA 16:04
Yeah, so this is a really specific question, but it’s an important one. The HYPROP measurement is built on some assumptions, right? It’s assuming that the evaporation rate is in what we call phase one drying and to break that down, phase one is where your evaporation rate is consistent, and your change in water content is fairly consistent. And you can get to a point if your evaporation rate is too high, that you’re changing water content, because instead of being a nice, consistent linear curve, it will start to change over time. And that’s because your evaporation rate is so high that the water is no longer able to move, migrate back up from the bottom of the core up to the top of the sample. And, so it starts limiting that water movement. And we start seeing a decrease in the evaporation rate over time. So that’s our phase two, drying. So phase one, we expect consistent and phase two is where it’ll start to change over time. And the assumptions behind the evaporation method are that we’re in phase one drying. And so you don’t want to exceed a certain evaporation rate.

CHRIS CHAMBERS 17:18
And so but people ask this question all the time, you know, how quickly water evaporates drives how quickly my sample run is done. And so, you know, you don’t want to just apply a bunch of heat and bake it, this is going to change the dynamics of everything. Desiccant fans, what do you, what’s your, do you have recommendations there?

LEO RIVERA 17:41
Yeah. So the best way to, if you’re trying to expedite the measurement process is, in my opinion, is a desiccant chamber. So I’ve seen people that have used fans blowing air over the sample, there’s two issues that causes. One, the balance, it messes with the weight measurements. Two, it actually increases that boundary layer conductance and that evaporation rate too high to where we actually violate those assumptions.

CHRIS CHAMBERS 18:11
And the evaporation isn’t really what’s happening.

LEO RIVERA 18:13
Yes. Yeah. And so the desiccant chamber, I have found to be the best way to do it. And it does increase it pretty significantly, right. I think I can take a measurement that would normally take, let’s just say, seven days, and shorten it down to three days,

Speaker 2 18:25
and you’re still using that diffusive evaporation rather than force convection, or something like that.

LEO RIVERA 18:32
Yeah. And so it doesn’t violate any of the assumptions, and it speeds it up. And so I’m a big proponent of that approach, if that’s the way someone else wants to do it, and it’s pretty easy to set up. It’s really any plastic container and a tub full of desiccant is enough to dry it down. And you gotta be a little creative with cable routing, but I think you can make it work. And we’ve seen the most success with that.

BRAD NEWBOLD 19:02
All right. Okay, moving on down.

CHRIS CHAMBERS 19:06
Oh, I think we just hit that one.

LEO RIVERA 19:07
Yeah.

BRAD NEWBOLD 19:08
Yeah, we just took care of expediting the process for HYPROP experiments, because they do take a long time. So this next one, how can I properly interpret soil moisture curves for geotechnical purposes?

LEO RIVERA 19:21
Yeah. So this is actually a really fun area that I think we’re learning a lot more about every day. The geotech measurements, a lot of them are based on some of the fundamentals that we’ve found in soil science. And what’s really cool is we’re seeing even more, learning even more about what we can learn from a soil moisture release curve. It’s not just about plant available water, right? I don’t remember exactly when this was published. This is a long time ago, probably 30, 40 years ago, but McKean found that if we use the slope of the dry end of the soil moisture release curve, we can actually predict how expansive the soils are. So it’s a really good, interesting, geotechnical application.

CHRIS CHAMBERS 20:06
So the HYPROP might not be the best instrument for the job here.

LEO RIVERA 20:10
No, this is something where a tool like the WP4C or the VSA can be much more powerful for these applications. In most cases, the HYPROP really isn’t a tool for geotech applications because they’re worried about soil mechanics there. And a lot of that information that we’re finding is in the drier end of the soil moisture release curve.

CHRIS CHAMBERS 20:29
So it’s more the HYPROP is for the biological end, where you’re looking at growing things.

LEO RIVERA 20:35
Exactly. Anything where we’re worried about plant life or life in general, typically, the HYPROP is the better way to go. More the soil mechanical processes are typically found in the dry end range. And then what’s really cool is, you know, we’ve gotten beyond just that expansiveness, or how expansive soils are, which was a great application. But we’re also finding that there is information in the dry end of this soil moisture release curve about the soil specific surface area, kinda an exchange capacity. And they’re also looking at ways of replacing methods like the proctor tests that take a long, long time to do with, and we can get all of this information out of tools, like the VSA or the WP4C, and what’s really interesting is, we didn’t know this until we had tools powerful enough to actually see this. But there’s hysteresis on the dry end of the soil moisture release curve to which I don’t think anybody would have thought that. And so what’s interesting is, there’s a lot of information inside of that how big that hysteresis curve is for those, on the dry end. So researchers like Bill Likos and Ning Lu are putting a lot of work into building out the publications and the methods for some of these applications. So yeah, it’s really interesting.

BRAD NEWBOLD 21:56
Okay, this next one, is it possible to get a release curve using the TEROS 12 and TEROS 21?

CHRIS CHAMBERS 22:05
Absolutely.

LEO RIVERA 22:06
Yep. Yeah. I mean, I think Chris and I both have experience with this. But anytime you can co locate those measurements in the field, it’s great.

CHRIS CHAMBERS 22:15
Especially with the TEROS 21 Gen 2 where you’ve pushed the accuracy in the dry end down quite a bit further. That was kind of limiting in the MPS6, but now, we haven’t released an official number yet on they have we?

LEO RIVERA 22:27
On the dry end

CHRIS CHAMBERS 22:28
On way on our accuracy at permanent welting point. Yeah, getting really good looking data down there.

LEO RIVERA 22:34
We’ve seen major improvements there, I mean, and even on the wet end too with the Gen 2 being able to push it up to that minus 5 kPa. Whereas the Gen 1 MPS6 could only go up to minus 10 kPa. So that really helps a lot because you do see that minus 5 to minus 10 kPa a lot, for a long period of time in soils. And so just expanding that range a little bit. But they just go so well together. And I know Colin’s done a lot of work on showing how these field generated moisture release curves compare with some of the lab generated and there’s gonna be differences because you’re seeing more, a lab curve is typically just a drying curve.

CHRIS CHAMBERS 23:16
But the resolution and temporal variability are— there’s a lot of power that’s previously been untapped with that, and you know, the TEROS 21 really just makes the matric potential measurements so much more accessible, with a lot more confidence in the data that you’re getting from them.

LEO RIVERA 23:34
Yeah, exactly. So it’s fun to see. And we’re seeing more and more people starting to do this, too. So I think as we get deeper into it, we’re gonna see more come out in the literature of what we can learn from doing this.

CHRIS CHAMBERS 23:45
Yep. Absolutely.

BRAD NEWBOLD 23:48
Okay, this next one is a general question asking, what is osmotic potential?

CHRIS CHAMBERS 23:55
Ah, yes. So I suppose we should break down just the basic definition first.

LEO RIVERA 24:02
Yeah, I think that’s a good place to start.

CHRIS CHAMBERS 24:04
All right. Basically, it’s the potential. Gosh, I should have looked in the textbook before we got here. I’ll take a stab at it, though. It’s the… Maybe I won’t take a stab at it. It’s the potential difference across a membrane from solute concentrations differing on both sides. And so the key here is that there’s a membrane or some barrier to solute transport. Right?

LEO RIVERA 24:35
Exactly. Yeah.

CHRIS CHAMBERS 24:36
Because if water can flow freely, then the salt water does flow freely. And that’s the point of osmotic potential. Or at least it moves. But the solute concentration difference being maintained is kind of the key factor there.

LEO RIVERA 24:55
Yeah, exactly. Yeah, osmotic potential for it to really be a factor where you have to have that semi permeable membrane

CHRIS CHAMBERS 25:01
Roots as an example, microbial membranes, fungi.

LEO RIVERA 25:07
Yeah, and those salts act as a dilution and they bind the water up when you have that semi permeable membrane. And typically osmotic potential is—not typically, it’s always expressed as a negative value, just like major potential. And so I actually have a nice little graph here, that kind of just describes the two. And let’s just jump into that really quick. We’re gonna share Chrome tab. Okay, let’s jump over one, right here. Okay. So this is another soil moisture release curve, it’s expressed a little bit differently. This is in terms of water potential on the x axis, and then the y axis is essentially expressed in terms of the maximum water holding capacity. So if we were saturated, we would be at 1 here and as it dries, so it’s the amount of water that’s there divided by the total amount of water that the soil can hold. But the important part here is to show how matric and osmotic potential differ in soils, and there needs to be an appreciable amount of solutes in the soil for it to really matter. Which you do see in the environment, we don’t always see it, but in some cases, osmotic potential is negligible. But in other cases it’s not. And what we see as we get towards higher water potentials, the osmotic component becomes more and more dominant. And so it actually will shift the nature of the soul must release curve. Over more, what it means is that the water is less available at these higher water contents than it would be if you were just looking at the matric potential alone. And then as it gets drier, the osmotic potential becomes less and less dominant relevant, matric potential becomes more dominant and

CHRIS CHAMBERS 27:30
So it’s important to understand the method that you’re using to get your soil moisture release curve, then, isn’t it? Yes, your WP4C is going to include the osmotic potential where the HYPROP, the TEROS 21, those methods, the TEROS 32. Those methods just give you matric potential.

LEO RIVERA 27:50
Exactly. And that’s where this you know, information comes from. So this is actually from one of our application notes, where we talk about how to make this correction, how to remove the osmotic component from the WP4C measurement, which is , it’s the WP4C is based on a dew point measurement. And what we’re doing is using a really precise relative humidity measurement, and then using the Kelvin equation to determine what the water potential is. And we’ve talked about this a lot in other virtual seminars. So I highly recommend watching them if you want more about this. But it is important to take that into account. And if you do know that the osmotic, if there’s a certain amount of solutes, I don’t know what a good threshold is there. The best thing I find is to take your measurements and and combine your WP4C and HYPROP measurements. And if you see they don’t line up, then okay, you question Okay, there are five solutes I should do saturated extract EC, right, figure out how many solids there are and then I can easily calculate the osmotic component, which is great. So, but yeah, it’s something that we can easily account for. But then you can actually what this means is it’s actually pretty easy to determine what your osmotic potential, is just take your saturated extract EC. And then there’s an equation that you can apply at different water contents to determine what the osmotic potential should be at that content. And so you can correct for that pretty easily, and account for that. And if it’s something that you need to understand, it’s pretty easy to figure it out. Great. I’ll let you jump into our next question, Brad.

BRAD NEWBOLD 29:33
All right. So this next one is actually these next two flow right from that discussion of osmotic potential, they’re gonna be asking about salinity. And so this first one is asking how does irrigating with saline water impact the soil moisture release curve?

LEO RIVERA 29:49
Good question. So, typically, Chris, correct me if I’m wrong. What we see when you irrigate with saline water is that you’re going to start getting a higher and higher concentration of salts in the soil.

CHRIS CHAMBERS 30:01
It’s depending on how much water you’re evaporating or getting, or taking up through the plant roots. You know, if you have saline water coming in, and then you’re removing the water and leaving the salt, that salt’s going to accumulate. And it’s unless you’re either rinsing it through or dealing with it in some other way, salt can accumulate and become a problem.

LEO RIVERA 30:30
Exactly. And so it’s essentially what’s going to happen is, as you irrigate more, and the salt accumulates more, if you’re not flushing them out, then that osmotic component is going to grow and become a bigger factor. And so you’re gonna need to understand that.

CHRIS CHAMBERS 30:43
And so monitoring EC in that situation is always going to be valuable.

LEO RIVERA 30:46
Yeah. What’s the best tool to do that in the field?

CHRIS CHAMBERS 30:49
TEROS 12.

LEO RIVERA 30:51
Yeah, exactly. So it’s important to understand that, especially if you already are irrigating with saline water, you want to monitor that because you’re going to create salt stresses on the plant, and potentially toxicity. If you get to a really high level, I believe, depending on the type of plants you’re growing.

CHRIS CHAMBERS 31:09
That’s right.

LEO RIVERA 31:10
So it’s definitely something you want to monitor and keep at a lower level, if possible.

CHRIS CHAMBERS 31:16
And if you get a high enough level of salt, it can interfere with the water content reading as well. So it is important to consider how much salt you might be accumulating in the soils. And think about your water content sensor choice as well.

LEO RIVERA 31:30
Yeah.

BRAD NEWBOLD 31:33
All right. And this next one is asking, is there a correction for matric potential measured with saline soil samples using WP4C? And you’ve already mentioned this previously.

LEO RIVERA 31:43
Yeah, yeah. So I mean, the best thing, like I said, is the best way to make that correction and I don’t, you know, correction, maybe isn’t the best way to separate out those two components because we’re not correcting, it’s not an error. It’s yeah, it’s just something that exists. It’s what it is. And but we want to do is understand, okay, what of this, let’s say I have a total water potential measurement here. How much of that is osmotic and how much of that is matric potential? And the best way to do that is to take your saturated extract EC of a sample, and then we have a guide. On the website, I highly recommend checking it going into the Resources page for both the HYPROP and WP4C and looking for the guide on how to combine HYPROP and WP4C measurements. And in that guide, we have the equations on how to correct for the how to calculate the osmotic potential based on that saturated extract EC and the water content

CHRIS CHAMBERS 32:42
So that you’re comparing apples to apples.

LEO RIVERA 32:44
Yep, exactly.

BRAD NEWBOLD 32:47
Awesome. All right, this next one. Do you have any suggestions on sample and data collection timing? And how can these impact your results?

LEO RIVERA 32:58
Yeah, well, it’s a fairly open ended question, which is good. There’s a couple of things I think when I think about sample, and data collection timing, I mean, I think about your measurement intervals, but there’s also time seasoning, seasonal timing. So let’s just say I’m taking a lab sample. Or a sample from the field to bring back to the lab. And I’m going out to take core samples. If I’m working in expansive soils, and I go out and take a core sample, during dry period, actually working on the dry period here, and then you saturate it, that sample is going to swell up. So you want to time your sample collection when the soil is going to be more so that it’s more in an expanded state. But there’s also other seasonal differences, right? I mean, there’s root—

CHRIS CHAMBERS 33:50
Microbial communities vary, you know, your population for osmotic potential that can vary.

LEO RIVERA 33:56
Yeah, yeah. So that can all play a role. And then the other thing I think about is just like your time interval. Things are usually somewhat slow changing in the field. But they might not be especially depending, like a sandy soil, for example. There is a point where as you get further along the curve, a small change in water content results in a huge change in water potential. And so knowing when you’re getting to those points, for example, on the HYPROP if we really wanted to characterize when you hit that point, as its drying, where the water potential is going to start changing more rapidly, you could increase your measurement interval to capture that. Still, even then, I think 10 minute measurement interval, which is what the HYPROP does, typically is enough, but it’s something that you could adjust if you wanted to. So I don’t know if you have any other thoughts on that Chris, but.

CHRIS CHAMBERS 34:54
Nope.

BRAD NEWBOLD 34:56
All right. This next one, in a laboratory setting, can we use a TEROS 21 in lab for measuring water potential?

CHRIS CHAMBERS 35:07
Theoretically, yes. It just kind of depends on, it depends a lot on your experiment. Yeah, that TEROS 21. You know, if you’ve got a small column, the TEROS 31 might be better. It depends on how precisely you’re trying to control the water levels. The resolution and the measurement, tensiometers are generally I think, a little more wieldy in the in the lab.

LEO RIVERA 35:32
Yeah, as long as you’re staying within that wet range.

CHRIS CHAMBERS 35:34
As long as you’re in the wet range. But then if you need to get into the dry range, then you know the TEROS 21 is really the way to go.

LEO RIVERA 35:41
Yep. Yeah, for sure. And I think what soil really plays a factor here too, right? Soilless media is not the way to go.

CHRIS CHAMBERS 35:49
That’s a great point. It’s hard to get contact with the soilless media with TEROS 21 and soilless media and that steep part of the curve can be problematic sometimes.

LEO RIVERA 36:03
Yeah. And I think what you find with soilless media is it stays in 0 to minus 10 kPa range anyways for a long time. And if you’re getting beyond that, I think the plants are going to start being unhappy as it gets that dry anyways, and most soilless medias. So yeah, but the TEROS 21 can be powerful for some of these. But the TEROS 31 might be a better option, depending on,

CHRIS CHAMBERS 36:24
Depending on what your goals are and the apparatus that you’re using.

LEO RIVERA 36:27
Yep.

BRAD NEWBOLD 36:31
All right. Next question. How can we get data from the near saturated range? In other words from 0 to 60, hectopascals.

LEO RIVERA 36:41
I think it’s helpful here to clarify what a hectopascal is. We haven’t talked about that.

CHRIS CHAMBERS 36:47
It’s a mini kilopascal, right?

LEO RIVERA 36:49
Yeah. So there are 10 hectopascals in a kilopascal. So this is 0 to minus 6 kilopascals, which is pretty dang wet.

CHRIS CHAMBERS 36:57
That’s pretty wet, tensiometer all the way, right?

LEO RIVERA 37:00
Exactly. Yeah, the tensiometers are the best way to do it. And if you’re trying to get the curve in this range, again, this is where the HYPROP is just, it’s hitting its stride. And it’s one of the most powerful tools for characterizing anything from 0 to minus 100 kilopascals.

CHRIS CHAMBERS 37:18
That’s right. For the water content, you get the resolution and matric potential. Yeah,

LEO RIVERA 37:23
I know traditionally, people have used tools like hanging water columns, and killing sandboxes, those types of things. I still, I mean, the precision and the the ease of use that you get out of the HYPROP, there’s really no other reason to use tools like that. I mean, those are tools that existed when we didn’t have a good way to get this measurement. Now, with the HYPROP, I think we can really hone in on that and get good data in that range.

BRAD NEWBOLD 37:57
All right. Okay, it’s time to get deep. What is the real physical meaning of water potential, and for soil what importance is revealed from that?

LEO RIVERA 38:08
This is a philosophical question on water potential.

CHRIS CHAMBERS 38:13
Yeah. Should I take a stab at this one?

LEO RIVERA 38:15
Yeah, you go ahead.

CHRIS CHAMBERS 38:16
I’m definition guy today.

LEO RIVERA 38:17
Yes, do it.

CHRIS CHAMBERS 38:18
I’m taking on that role for myself. So it’s the energy needed to move the water, essentially. If you have, and really, it’s relevant to the difference in potential around it. Because that’s the key with the potential is that you are, there’s a gradient, you know, if there is no gradient, then you’re at equilibrium, water isn’t going to move one way or another. And then if you do have two different energy states, basically, then water will move from the high potential to the low.

LEO RIVERA 38:53
Yep, exactly. I, you know, we use these terms, a lot to talk about this intensive versus extensive properties.

CHRIS CHAMBERS 39:03
You know, here you go again, yeah.

LEO RIVERA 39:06
My favorite thing, but the easier way to look at this is and I think we’ve talked about this numerous times before too, but measuring water content is like measuring heat content and air what does it mean? Would you even know that? Yeah.

CHRIS CHAMBERS 39:23
It’s gonna be completely you’re missing so many contextual variables. If you’re just looking at that. Whereas in I think I can guess where you’re going with this then the water potential is the temperature right? Is what you’re telling me?

LEO RIVERA 39:38
Yeah, exactly. And it just gives us more information on how we feel. And what that means. It’s the same for plants and soil right, water potential actually tells us how the plants are feeling.

CHRIS CHAMBERS 39:51
And how quickly something is going to dry out, which is how many plants actually control their stomata and the water loss drives a lot of the conservation of water and plants.

LEO RIVERA 40:06
Exactly. So that’s why it’s important. I mean, it tells us exactly how the plant is going to behave at a given point

CHRIS CHAMBERS 40:16
Or lacking plants, which way the water is going to move.

LEO RIVERA 40:19
Exactly, exactly.

BRAD NEWBOLD 40:23
All right. And this next question, they were asking pretty much the same thing about, asking about water potential. Their question was, Is it existing water content? Or is it the potential to absorb water even if the water is not there? Anything you know, anything you want to add to that?

CHRIS CHAMBERS 40:40
It’s a bit tricky. Try to think I mean, you think about absorbing water. You know, if the— getting off into the weeds here. I can feel it. Where’s my bailout?

LEO RIVERA 40:56
Right? Yeah, I, this is an interesting one. I mean, I think, yes. If you know other factors, right? Then you can determine yes, this is how much water it can absorb, based on these other factors like

CHRIS CHAMBERS 41:13
Basically your soil water retention curve, right. That’s basically what you’re looking at, is, you know, what is that gap between— In soils, it’s permanent wilting point, and field capacity. And the ability to absorb more water is basically going to depend on that relationship.

LEO RIVERA 41:33
Exactly.

BRAD NEWBOLD 41:37
All right. Next question is a natural drying process of a soil a better way to measure soil hydraulic properties?

LEO RIVERA 41:47
So I think what this question is referring to is the process that we use to do the drying curve with the HYPROP is an evaporation method, we call it, it’s a natural drying process. And the reason I want to say yes, is because, one, that’s how things happen in the field, typically, the soil is evaporating, and those properties are changing. But the more important thing, I personally think, you can tell me what you think, is we’re not trying to control anything, we’re just letting it dry. And we’re measuring as it changes. When you try to control things, say for example with pressure plates, you assume that you’ve controlled it to a set point. Right. But is it actually at that?

CHRIS CHAMBERS 42:34
That’s right, I think, I don’t know if it’s necessarily a question of better or worse. Yeah, but a question of, I mean, there’s going to be consequences to the method that you choose here. The VSA very much has artificial, I don’t want to say artificial drying process, right? You grow, you control the you can control the relative humidity in chamber, you can control all of those variables that determine how quickly you’re going to arrive at that.

LEO RIVERA 43:05
Yeah, but I think something that’s different between, say, like the VSA and pressure plates, the VSA, we’re still measuring, okay, we’re drying it, but then we measure. Okay, what is the actual water potential right now? We don’t assume it’s right at any given water potential. Whereas with like the pressure plates, you set it to Okay, I’m sending it to minus 10 kilopascals. This is not, yeah, you can do minus 10. And you’re assuming that the soil then stabilizes at that point,

CHRIS CHAMBERS 43:32
Right. But that’s not necessarily, so the set point in controlling it isn’t necessarily the problem with that method, it’s that over time, it’s been shown that you just never are sure if you hit equilibrium.

LEO RIVERA 43:45
Exactly, exactly. So you’re assuming that you’re at equilibrium. And there oftentimes, as we’ve seen in the literature, there are cases that we’re not, especially as we get towards lower water potentials. And as we deal with finer textured soils, you lose contact with the ceramic, there’s a lot of things that happen and never actually hit equilibrium. So that’s the issue. That’s why I like at least the natural drying process, or actually measuring the water potential and not assuming it’s at a certain point.

BRAD NEWBOLD 44:14
Okay, this next question, As I work with soil surfactants, is it better to test water potential and the effect of surfactants on that instead of the water content?

LEO RIVERA 44:30
I think it’s gonna affect both, right?

CHRIS CHAMBERS 44:34
I don’t know. Yeah. I mean, the water potential is what it’s actually affecting, right?

LEO RIVERA 44:38
Right. Is it surface tension? Yeah.

CHRIS CHAMBERS 44:40
Yeah. So that I think would be key. I’m not sure what.

LEO RIVERA 44:45
Yeah, I don’t know. I think maybe water potential is the more important factor here. Because like you said, it’s impacting the surface tension of the soil. And it’s going to change those properties. So that is yeah, that’s an interesting one. And I think, I don’t know how much work has been done in this area. I haven’t seen a ton of work. I mean, I know there’s a lot people working with surfactants, but characterizing the actual impacts of surfactants on the hydraulic properties of soil?

CHRIS CHAMBERS 45:12
I’m curious to see where that goes.

LEO RIVERA 45:14
Yeah, yeah, it’ll be interesting to see, but I imagine it’s going to impact the water potential more than anything else.

BRAD NEWBOLD 45:25
Okay. Two more questions here. This second to last question, Can the TEROS 21 be used in a purely sandy soil?

LEO RIVERA 45:36
Chambers, you’re the expert here.

CHRIS CHAMBERS 45:37
Yeah. Yes, I’m going to put an asterick in there, asterisk. So sandy soils, it’s easy to run into problems with the TEROS 21. And the TEROS 21 relies so much on that contact between the ceramic, not the metal plates, the actual ceramic, and the soil around it. And if you have a coarse textured sand— a fine sand, no problem, install it, no problem. But if you have a coarse textured sand, and, you know you try to get the substrate rubbed right into that ceramic in between the discs, and then you look at it, and it just kind of all crumbles out of it. So that’s the real tricky part. If you can get good contact between the sand particles and the ceramic, then yes, you can get some, it’ll work and you’ll get good data. But if it, you know, and, you know, when you don’t get good data with time, and it just stays right at the water potential doesn’t change at all, it just looks saturated the whole time.

LEO RIVERA 46:43
What are ways to improve that contact?

CHRIS CHAMBERS 46:47
Well, and tell me what you think about this idea because I think it works. I mean, the TEROS 21, I mean, its whole method is based on equilibrium with the soil, right? And so you can actually use a finer textured substrate right next to it, use that to get good contact with the sensor. And then as long as you’re in equilibrium with the sand around it, then you should be able to arrive at the matric potential of the sand.

LEO RIVERA 47:19
Yeah, no, I love that. And I agree with that. I mean, that’s the beautiful thing about water potential, right? Is it’s always trying to be at equilibrium.

CHRIS CHAMBERS 47:26
That’s right. And if it is, then you know, it’s going to be the same value. Yep.

LEO RIVERA 47:33
Yep. No, I like that approach.

CHRIS CHAMBERS 47:35
But keep an eye on and people do ask this quite often. It’s like, well, the sensor’s working, we take it out, it dries out in air, but you know, my sand is super dry. Shouldn’t my sensor dry out anyway, even if it’s not in good contact with the soil? But if you think about it, even minus two megapascals is 98% relative humidity? So if you’re trying to dry out a sensor in the vapor phase, you’re gonna be waiting a long time.

LEO RIVERA 48:04
Yeah. That’s a slow process.

CHRIS CHAMBERS 48:06
It’s slow.

LEO RIVERA 48:07
Yep. So contact is King.

CHRIS CHAMBERS 48:10
Contact.

LEO RIVERA 48:13
Yeah.

BRAD NEWBOLD 48:15
All right, we are up to the end of our time. So this is going to be our last and final question here. They’re asking, Is it possible to have in practice a soil with a one giga pascal matrix, suction value?

LEO RIVERA 48:30
One gigapascal, which is

CHRIS CHAMBERS 48:34
Holding on to your last water molecule.

LEO RIVERA 48:36
Yeah. So I think it’s a minus 1000 megapascals is one gigapascal.

CHRIS CHAMBERS 48:43
That’s I mean, it happens, right?

LEO RIVERA 48:45
I’m just trying to think of where you would see that right. Because it’s

CHRIS CHAMBERS 48:47
Not anywhere with life, or at least not active life.

LEO RIVERA 48:51
Do you think we’d see that in the extreme deserts, would we ever even get close to that? I mean, I’m trying to think what relative humidity you’d have to be at to get to that.

CHRIS CHAMBERS 49:03
Yeah, because they’re a minus 100 megapascals in the air, right, in general?

LEO RIVERA 49:09
Yeah. Typically, I think it’s minus 10 megapascal.

CHRIS CHAMBERS 49:11
So we’re going an order of magnitude beyond that.

LEO RIVERA 49:13
Yeah. I’m just trying to think I can’t think of a place where we’d ever see something like.

CHRIS CHAMBERS 49:17
What are the matric potentials in like the Atacama in the middle of the summer?

LEO RIVERA 49:20
I don’t know. Maybe somebody else who’s done these measurements can tell us. I’ve never done Death Valley. Yes. Yeah. This, we could probably look it up. Yeah, we probably could look it up. But I think in practice, it’s highly unlikely, except maybe in these high extremes. But in most environments, like here, we’re never going to hit a relative humidity like that, where it’s going to be drawing the slowdown that much.

CHRIS CHAMBERS 49:48
That’s right, because if you get dry enough, it will pull water out of the air.

LEO RIVERA 49:53
Exactly. Yeah.

CHRIS CHAMBERS 49:54
And so we’re up to the soil surface.

CHRIS CHAMBERS 49:56
Yeah. Again, it’s all about equilibrium. And so the soil is going to equilibrate with whatever the relative humidity. Or the yeah, whatever the air matric potential or matric potential of this or air water potential is so. Yeah. In theory possible,

CHRIS CHAMBERS 50:13
I’ve never tried it.

LEO RIVERA 50:14
in practice unlikely. Yeah, so I don’t know if anybody’s ever seen these, I’d love to see that.

BRAD NEWBOLD 50:24
All right, fun question to end on. But that’s gonna wrap it up for us today. Thank you again for joining us and we hope that you enjoyed this discussion. Thanks again for all of those great questions that you submitted. And if you have any other questions that we didn’t answer, please contact us via our website 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. Thanks again. Stay safe, and have a great day.