Office Hours 1: Soil moisture

Office Hours 1: Soil moisture

Learn soil moisture measurement methods, best practices, applications, and more in this Q&A session with our science and product experts.

Better data—better understanding

Without an accurate understanding of soil moisture and water potential, you could unwittingly undermine your experiment, or worse, obtain poor quality data which lead to wrong decisions or conclusions, causing publication issues. Whether a lab tech, research scientist, or grad student, a strong understanding of soil moisture can help you make better decisions and measurements, ensuring your data are publishable.

In this first-ever METER Environment Office Hours Live Q & A session, join research scientist Dr. Colin Campbell and application specialist Chris Chambers as they tackle your most pressing soil moisture questions about:

  • Experimental design
  • Measurement science and methodology
  • Best practices
  • Installation issues
  • Data interpretation
  • And more

Dr. Colin Campbell has been a research scientist at METER for 20 years following his PH.D. at Texas A&M University in Soil Physics. He is currently serving as Vice President of METER Environment. He is also adjunct faculty with the Dept. of Crop and Soil Sciences at Washington State University, where he co-teaches Environmental Biophysics, a class he took over from his father, Gaylon, nearly 20 years ago. Dr. Campbell’s early research focused on field-scale measurements of CO2 and water vapor flux but has shifted toward moisture and heat flow instrumentation for the soil-plant-atmosphere continuum.

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.

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Hello, everyone, and welcome to Office Hours Live with the METER Environment Team. Today’s session will focus on soil moisture. And we’re shooting for about an hour of live Q&A with our experts, Dr. Colin Campbell and Chris Chambers, whom I will introduce in just a moment. But before we start, we’ve got a couple of housekeeping items. So we first want this session to be interactive. So we encourage you to submit any and all questions in the questions pane. And we’ll try to get all to all of your questions. I’m just gonna say here, right up front, we will not get to all of your questions. More than likely, only a few of them will will be answered during this hour. But we will have someone from our science and support team get back to you with an answer via email, or the email that you registered with. So second, if you want us to go back or repeat something you missed, don’t worry. We will be emailing you a recording of the session via your email within the next three to five business days. All right, with all of that out of the way. Let’s get started. Today our panelists, our research scientist, Dr. Colin Campbell, and application specialist Chris Chambers. Colin Campbell has been a research scientist at METER for 20 years following his PhD at Texas A&M University in soil physics. He’s currently serving as Vice President of METER Environment. And he’s also adjunct faculty with the Department of Crop and Soil Sciences at Washington State University, where he co teaches environmental biophysics. Dr. Campbell’s early research focused on field scale measurements of co2 and water vapor flux, but has shifted toward moisture and heat flow instrumentation for the soil plant atmosphere continuum. Chris Chambers operates as the Support Manager at METER Group. And 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. And let’s get started taking some questions. Thank you to everybody who’s sent in questions. We sent out an email to register for this with an invite to submit questions. We’ve got a ton of questions. So like I said, we’re going to try to hit some of the questions that came in via email. And we will also try to get some of your live questions. So feel free to submit any and all questions there in the Questions pane. And we’ll try to get to as many as we can, during this hour.

So our first one here, is we’re going to start here. And this this questioner asks, In my experience, field capacity varies in terms of kPa among different soils. Would you agree? Thanks, Fred.

Yeah, thanks.

Good morning, Colin.

Hey, good to see you, Chris.

Good to see you, too. So field capacity? Yeah, this is a question that we actually get all the time. And it’s appropriate to hit permanent wilting point talking about this. Let’s start by just a quick definition of field capacity. For those of you who are new to the topic, it’s basically the point after a soil hits saturation, where all of the gravitational water drains out. And so if you have a bucket full of saturated soil, punch holes in the bottom, you hit field capacity after there’s no more water draining out. Just a quick definition, right?

Yeah. And some people call it the drained upper limit. Other people say it’s the time when, when no longer does this vertical flow dominate. So lateral flow and vertical flow are about the same and depends on your level of wanting to talk about it. But in my soil physics class, or even other classes, they’ve all said Oh, it’s negative 33 kPa.

Right. And we see that quite often. And that’s really attractive from one point of view, right? Because it’s, it’s always the same, you know, in the definition I just gave you. That’s super subjective.

But I didn’t know that when we first learned about it, right?That it was like, hey, here’s the number you can depend on negative 33 kPa. And the interesting thing was that, you know, we’ve started using our TEROS 21 water potential sensor. It’s always known about water potential, in fact, I grew up really knowing about it because my father was so heavily in it, and suddenly, now we measure that in the field, and it’s not

always negative 33, is it? That’s the answer to your question is yeah, that’s pretty much right. The field capacity can have a different matric potential or a different potential, depending on soil type.

So what what would you I mean, you’ve seen this before in support experiences. I think I remember one of funniest moments. We sell a WP4C, right? Yep. Somebody wrote me an email. And they were like, well, I know your instrument doesn’t work because I put some well drained soil in the WP4C and it didn’t read negative 33 kPa. One of the problems was the instrument simply doesn’t really work in that really wet region.

That’s pretty wet.

But the other problem is, that’s a fallacy, it shouldn’t have right? So sand, clay, what do you expect to see when you go?

Ah, so, for me, and this number is really important, right? But it’s kind of a physical construct that it’s an important conceptual point in the soil, that it’s good to understand the soil physics and how water moves through it. But I find that less interesting now that we can measure matric potential a lot better, especially with the TEROS 21. And with water content and matric potential together, I just think that we’re kind of stuck in the rut of field, what water content or what matric potential field capacity is, because now we can measure it on a continuous scale in the field.

I get what you’re saying. And that’s a concept I hadn’t thought about too deeply. At one time, we were trying to understand field capacity, because we were measuring water content often, right? And so if we could say, Okay, after two days, that’s kind of field capacity. And that’s maybe around where the plants grow optimally. We want to nail that value each time. But if you’re using a TEROS 21, or even a TEROS 32 tensiometer maybe for more accuracy up in the wet range, you don’t really care because you know, the range. That’s right.

No, I don’t think so. Because, you know, as we said, it is an important, physical concept, but in the end, the definition of it is kind of subjective. And we don’t need to rely on that subjectivity to try to find out the soil moisture state.

But what if you really want to know the field capacity of a sand? Oh, my gosh, I mean, in my experience, the maybe negative 10kPa. So if we did find a point, you’re right.

It is important, because that’s basically where your refill point is.

Yeah. And that’s where we’re gonna, I mean, anywhere above that, if we put more water in we’re essentially donating to the groundwater right, and we’re doing that with our nutrients, with whatever’s in the soil, we’re sending it by sometimes that’s important if we got salt affected soil, maybe we’ll get to a question like that later on. But here’s the interesting thing, and in the turfgrass we’re working in, this is a pretty sandy soil, obviously, the whole point is to drain, negative 10 kPa is about what I expect to see. I looked at some of the data this week. When we installed in a potato field down in southern Idaho, what we saw there was negative 40 to negative 60. And that was after you know a nice winter good snowpack on this high elevation. And it’s all melted. It’s just sitting there and nothing’s growing on it. They’re getting ready to plant negative 40 to 60 kPa. And that was their field capacity. Essentially, what moisture’s left there after its drained right.

So okay, so this question didn’t ask this, but what about permanent wilting point, those go together? Right? Yeah they are. And the permanent wilting point is something we even know maybe even better than our field capacity. I mean, in terms of numbers.

And this comes down to you know another important physical parameter of the soil. And just, you know, it was originally determined. Gosh, I can’t even remember.

That’s intentional. Yeah, Briggs and Shots, I think right. You can go look up the paper but essentially, the permanent wilting point is defined as the matric potential at which sunflowers permanently wilt.

So they fail to recover after the night of, you know, coming across,

right, and that’s that minus 15 bar, that minus 1500 kPa number has just kind of been the standard ever since. It’s objective, it’s super easy. But it’s, going to be different from plant to plant. And

Why is that? I mean, you talked about that earlier, and I loved kind of what you were saying.

So you think about a sunflower and they grow extremely fast, right? High assimilation rates, and they use a lot of water during that. But different plants have different strategies to avoid plant stress or to protect themselves from plant stress. Say a conifer or ponderosa pine or something. They might get down to minus 2.2 megapascals, before they really enter a stress zone, and something like that with woody tissues really glorified. But I mean, leaf needles that are really hard, wilting isn’t really the right terminology for that either.

And that is interesting. So, the needles, I mean, they exhibited different physiology, that maybe my potato plants, I mean terms in

facing stress. So, but then again, you know, as we get the actual matric potential, then we can not just kind of benchmark this one point. But, you know, look at the data as a continuous variable.

Yeah. So, we’ve done some of this work, you know, what is wilting point anyway? Why are we interested in wilting point? You know, the answer to that is wilting point is something we want to keep our plants in an optimal range and wilting point is something we never want to approach. Interestingly, you know, when I work in wheat when I work in, you know, other just vacious plants. When I work in potatoes humorously and potatoes, they’re the ones that are like, no, there’s no water here we’re giving up. Yeah, they run that white flag, they’re like we’re done. You know, and wilting point it’s just something you can never, never approach because, you know, for something we’re trying to produce, yield out of their wilting point would be crazy to reach it, right. But in wheat, you know, you give wheat a challenge, like, Hey, you’ve run out of water, they just send their roots deeper, you know, and when we did some measurements here, locally, we had water being taken up at 150 centimeters, just like that, toward the end of the summer, they’re like, well, we’ll just grow roots down and grab it. And so wilting point important parameter: not negative 1500 kPa? Right,

Right. Or maybe it is. But you know, it does give us kind of an objective. An objective way to define the water tank.

Yeah. So we’ll go there. But don’t send me any email about how the WP4C doesn’t measure for wilting point. That same person actually said that on the wilting point side, too. Okay, Brad. Another question.

This individual is asking does sensor orientation matter? And how many would you recommend installing in a field scale study?

Yes, it matters and lots.

And no, it doesn’t. The sensor doesn’t care. You can install it up or down vertical or horizontal, it doesn’t care. Oh, that’s true. But it does matter on you know, how water moves through the soil around the sensor, how you’re installed, what you’re doing with your cables. Cable management is always important in installation. And installation is if you pay attention to nothing else when you’re installing soil moisture sensors, or when you’re doing a soil moisture experiment, how you install the sensors is the single most important thing you can do to get good data on.

So what can we give as a principle as people go out in the field? Hey, if you remember nothing, remember this. And if it was me, after all the many installations, what I would say is, water gets everywhere and really take care of that. Don’t impede the water flow by laying your sensor flat, right? Don’t aid the water flow by having your sensor cables have a channel for water strait to your stressor. Right.

And, and one of our good friends, Gareth Vinod, talked about how he installed his tensiometer at 45 degrees, to try to limit the preferential flow. And I think, you know, in every learning I’ve done with water content sensors over the years, or anything installed in the soil, again, its water will get to the most bizarre places, it will get inside your sensor if you’re not careful. It will get down. It will flow down your install hole if you’re not careful, it will do everything you don’t want it.

So what about the second part of this question? You want to tackle that one?

Yes. So I think you should, like 80 or 90 sensors across the field.

If you’re not installing 80 sensors. You’re doing it wrong. Yeah, I

think so. I think you should spend all your sensor money on sensors. Your budget. Now, in all honesty, that this question is really challenging. It does relate to budget, right? I mean, there’s always a finite budget. And we’ve done this a lot of different ways. I mentioned the study in the week, a little earlier, we installed 42 sites in a 35 hectare field outside of town. So that’s a really dense sensor network. And we discovered some really interesting things. In fact, one of my data science friends who got a PhD in geography said, hey, that’s one of the seminal datasets people are using right now, to do some of the data analysis out there. And so you can do that. But I don’t think a lot of budgets are going to be able to handle that. And it takes a lot of time, took bunch of time. Heavy analysis too, you had a good idea on this,

I fall back to just your basic statistical sampling principles. One, you want to be representative of your overall population that you’re sampling, right. If you’re placing a sensor in a field, you want to be representative of the entire field. And part of that is going to be dependent on the soil variability in your field. And then second, you want to get a handle on the variability by having kind of at least a minimum sampling number. So per soil type, you want to try and have at least at least three sensors in there, which is the minimum, you need to calculate a variance. So you know, if you have one soil type, and it’s consistent throughout the entire field, you can, which I think is fairly rare And

I’ve never actually seen that happen. But we have installed that way before because

that was just I mean, sometimes that’s the best that you can do. So finding the spot that’s representative and most representative of your field is tricky.

It is and we’ve been trying to deal with that some, we’ve put together a little app that will take satellite data and try to create statistically representative locations in the field and give you a suggestion of where you might put that. We look at NDWI Normalized Difference Water Index and some of these things and overlay it and say, Hey, this might be a great spot for sensors. And so that’s so far, our best guess. I think the future looks like we’re going to spend a lot more time trying to connect all these scales. I mean, that’s what everybody’s talking about. Where we’re going, okay, Brad, let’s try another one.

Along with orientation. This one’s asking where does the sensor have to be, in the most active route section, in the center, at 50% effective route depth? And also, where are you going to do that in a drip irrigation situation?

Threw in that curveball, right, yeah.

This is really related to the question that we just had, and it depends a lot on the questions that you’re answering. And what other resources that you want to use. Certain soil moisture networks, SNOTEL sites, a lot of the MISA nets that provide soil moisture data as a product, they have set depths. So if you want to compare and interact with those, if that’s one of your questions, then you should use the same depths.

But they don’t always agree right, then. I mean, I’ve been in meetings where, ya know, it’s 27. But no,

I think it’s 25. Yeah, exactly. And then, you know, but if you’re really interested in what water is available to your plants, then you want your sensor to be in root zone.

Yeah, I mean, it’s something we talked about, like the potato example I used. So we’ve been going kind of a mid level, upper to mid and lower, you know, toward the bottom of the root zone. And then I’ve been sticking a sensor out the bottom of the root zone to see, are we through? Exactly how deep roots are reaching? And then the question is, well, what sensors you’re putting out there? Right now I’m colocating water potential water and content sensors, I want to see, potentially trying to create this envelope, right, right, Of upper and lower limits for water potential, what that looks like and water content, and therefore how much water to put on.

Well, then that gives you information about your soil too and what’s happening. You know, what soil type you have, what your soil water retention is, and

it’s a fingerprint for the soil. That’s what I’ve been excited about doing this, putting these sensors,

you get the mass and the energy state at the same time. Yeah.

So that’s where I’ve been installing. I like your answer, though, about these networks, because there are kind of standardized ways to look at this. As we worked with growers, though. I have my own thing. I was like, Okay, we’re going to put them in 15, 45 centimeters. And the grower basically said we’re putting them at 30, and you know, what, guess where we put them. At 30. Yeah, he didn’t care. So. Okay, so drip irrigation.

Oh, my gosh. Okay. I think you get information from again, you probably want it in the root zone. Yep. And you probably want to know the extremes. You want one near the drip line and you want one away from the drip line, especially if you have plants that aren’t right on the drip line. Yep. And if you have the drip line running right down the center of a row or something like that, you know, then you might not care what’s going on in the inter-row.

You know, we had this experience last summer. So we’re in a silt loam, we have the drip line going down through. Okay. And we have a big inter-row. So there’s two meter spacing. And so the question was where to put it in. And so my thought was, you know what we could do? We could go out and just look online. You know, wow, yeah, terrible. In fact, I was talking to our friend Doug Cobos. And he was like, hey, that’s going to be simple. Where to put this underneath the drip line we’ll just go look in the literature. Now, I’m not going to say I did an exhaustive study, I asked, you know, Captain Google, or whatever, right? And so I went to Google Scholar, went around to these things. And was like, Okay, where do I put my sensor? There was almost nothing, the best I did was like, hey, put it 10 centimeters from the dripper, you know, laterally and 10 centimeters horizontally from the dripper. Go there. And that’s what I used and I can’t tell you if it worked. Because I mean, we measured you know, under the drip line, we got the onion. And it was certainly interesting. But was that the right place? I don’t know. How far is it go? Yeah. So there’s a potential study. Maybe I’ve just missed it, you know, you just spend more time. But that’s, what I got.

We’ve got one that says, when measuring soil moisture at varying depths, should the sensors be vertically stacked for different depths? Or is horizontal spacing also recommended? And if so at what distances?

They’re both important. Both you want some laterally to capture the soil variability. We’ve already touched on that a little bit. But water moves vertically, and there’s a lot of spatial variability for water laterally and vertically. Sure. Closing fingers. I mean, people don’t really realize that, but it’s not like a big block coming down to the soil. That’s right. And again, this partly depends on the questions that you’re asking. Sometimes knowing what water, is A if it’s below your root zone, and that water isn’t being accessed. If you have a reservoir down there that can be helpful, or what kind of drainage you’re getting to have that deeper sensor can really give you a lot more information.

But you know, I don’t think I’d go laterally. And here’s why. Borehole installation tool. Yeah. And I have to admit, I have put in, I don’t know, at least several 100, probably more than 1000 sensors in the soil in various installations over the last many years. And when I got a hold of the borehole installation, tool, life change, not for the water potential, yet, we’re working on that. But for the water content, you know, digging that that hole, even in rocky soil, and then using the borehole installation tool to just shoot those sensors into the side of that borehole changed my life. That sounds Homer, right? Because we build that but I love it.

Well, we had that trial too where it’s like, we’re gonna get our whole METER environment staff out, and we’re gonna dig pits. Yeah, we tested it. We were digging, I was the control group. I led our team, of interns to dig the traditional pit. And you and Leo were done. Had your sensors installed and your holes cleared up in 30 minutes. That’s right. And we’re still digging. Yeah, digging down to get to

What did we do with that video?

I don’t know. That’d be fun, wouldn’t it anyway. We’re digressing. Yeah, essentially what I’d say we’ve done this different ways before, and we did in that, actually, in that field, that 42 site. One remember, we did poke holes kind of laterally, just makes me a little nervous based on soil water flow that we may miss. You know, if you’re stacked vertically, at least you got some idea that the water flowing down through a regular pattern. That’s right.

I still think the lateral especially if you have a lot of soil variability, because you can have like a pretty significant soil texture change and your soil moisture can be, they can both be right, they can be 15% off just can’t be. Absolutely. So I still think the lateral is important. Hopefully we gave you guys something to think about on this one.

That’s a hard one. Okay, Brad, what do we got?

How does salinity, temperature, and capillary rise affect the data from the sensors? Is there a threshold for the noise it creates in the data?

Brad’s throwing us some pretty hard ones.

This is why we’re here though. Right? Okay. Yeah.

Okay, Chris, while you’re here. I just want an easy morning. Salinity.

Let’s start with salinity. Yeah, great.

So we’ve spent a long time worrying about salinity measurements from the sensors. And one of the challenges with measuring the charged storing capacity of the soil through a sensor like the TEROS 12, TEROS 10, 11, 12 Is that the charge is affected by the salinity. So we’ve spent a lot of time testing the sensors and within a reasonable range of salinities. For most soils, there’s not a lot of effect of those things of the salinity. But you know, if we get higher salinity, maybe above 10 decisiemens per meter,

definitely about 10. I’m pretty sketched out saturation extract EC Yeah, so if you saturate the soil, collect the EC that’s coming off the bottom of it, and it’s about 10 decisiemens per meter, then like the TEROS 12, you have to, you’ll probably see if it’s affecting your data, it’s a pretty clear response. And especially if you’re measuring EC, then you can have an idea of when to just maybe not completely trust the data that you’re getting.

So how do you know what the EC is, salinity is?

So that’s right, because I mean, a soil sample, maybe a soil sample, generally, you can tell from in the soil. If your bulk EC is above three to four warning time,

you got problems.

And so if you’re measuring EC, you generally have indication when your water content is going to be affected. But that’s a pretty high EC level at 10. decisiemens per meter.

it is, I mean, you know, I’ve dug into the salinity labs in Riverside, produced a handbook many years ago. Handbook 60 maybe? They have some where if that’s the sensor, yes. So but they’ve showed showed that, you know, certain crops grow I mean, a saturated extract, you see a 10, that’s getting into only a few crops are going to be able to grow in there. So it is a concern. Okay, so capillary rise.

That’s just fine. Hopefully, you’ve got matric potential sensors in there to depths because then you know what’s happening. Yeah, right. Coming up from the bottom. Yeah, I’ve never and we see it occasionally. And have you dealt with that? I don’t think I have. Yep. And so in the older sensors, this partly depends on the sensors, too. And the MPS 6s had a little more temperature, a little more temperature effects in them. And it was sometimes kind of hard to see the difference between the diurnal temperature effect and hydraulic distribution in the soil. But I don’t really see it very often, or we don’t see it clearly, very often. There’s a handful of cases where Oh, yeah, this looks like

yeah, that’s pretty rare to see some of these things, I mean, the temperature at very, very dry in the TEROS 21 and water potential, we can see temperature effects, but then again, temperature does affect also the water potential. That’s right. And in fact, I was just kind of bemoaning that fact in a TEROS 21 I was looking at and also had a tensiometer, which I trust is kind of the golden kind of thing, the standard you look at, and it’s the same thing. There was temperature effects almost identical in that sensor, which really suggests it was something related to physics, the physics of the soil, so Okay, let’s try another one. Brad.

Is there a way to calibrate the moisture sensor for soils of different textures and different bulk density? Can you correct a moisture sensor reading?

Yes, Next question.

We should spend a minute on calibrations. Okay, that’s true because it does come up, it is relevant with our factory calibrations in a mineral soil, and we’ll focus mostly on mineral here. Generally, they’ll get within plus or minus .03 meters cubed, four meters cubed. And for a lot of applications, a custom calibration isn’t worth it. It is an investment in time and effort to do a custom calibration but if you have the soil sample, and your soil moisture readings, you’ve got a season worth of data that just doesn’t look quite right. Then you can go back and correct your water content by doing a custom calibration there.

So too regularly people ask me, you know, how do you do this calibration? And we have an application.

Yep, our procedures online. It is pretty straightforward. It does take some labor though.

It does take some work. We have lab technicians who scrubbed the little clay balls, you know, wet clay balls shredded homogenizes soil. I used to do this a lot. Yeah, but I, you know, one of the good things is it creates really good calibrations. And I’ve seen a lot of people like you know what we’re going to do, we’re going to skip all that work, we’re going to make a little tray, we’re going to put the sensor in the tray, we’ll put the tray on the scale, and we’ll just let it evaporate. Right, and it creates terrible cut.

That’s right. It is better to just use the factory calibration than to not do the calibration correctly.

Yeah. I’m glad you said that, because I hadn’t really thought about that. If you’re gonna do it badly, just use what we did. Because I can tell you that we spent time. But you know, if you’re going to take your time definitely calibrate if you want to.

You can get a 1 to 2% improvement, and some soils do really need a calibration. Yeah, some come in with crazy, bulk densities.

We were talking about this earlier, talking about the bulk density, what range do we need to worry about?

Generally I mean, what is the typical bulk density? Sensors pretty robust to bulk density. And that’s from like, 1.25 to 1.7. Yep, anywhere in that range is probably not a big enough difference in bulk density to require a calibration. It’s kind of grim. And you probably have soil type differences in between there that are, you know, across that range as well, where we really seen crazy data is I don’t even know; the soil came from somewhere in California, but it came in with like a bulk density of point nine grams per centimeter cubed. Could you tell? And I mean, just I couldn’t tell, but the water content readings were crazy. And so that definitely needed custom calibration for that one.

And so I mean, we also in that calibration sheet, if you just want to take the bulk density, there’s ways to calculate that in there too. Yep, exactly. So that’s what I’d suggest to, you know, volcanic soils, for example. Yeah. Okay, Brad,

Asking about their application here. So I’m just gonna paraphrase because it’s a long question, but they’re saying that they’re growing one year old cork oak seedlings in five liter pots. They’re saturated, dried down, they did an experiment measuring the predawn leaf water potential and matric potential in the root zone and did not get a very good agreement between the two measurements. Can you explain why they would see a difference?

A predawn water potential is a pretty common tool for physiologists to understand the… basically it’s kind of a hack to get at the soil matric potential by measuring the water potential. Because he had to get up at this time of year, it’s like 3:30 in the morning. Yeah, but sometimes there must be valuable if you have to get up that or it’s hard. If you don’t have to do it very often, it’s fine. Right?

I’ve never done like, I’m going to

Watching the sunrise at the top of Smoot Hill. Okay, because you’ve been up all night taking readings, not all night. But anyway, the point is that it’s in theory, you can get the matric potential of the soil by by measuring the leaf potential. And you can do that because when the stomata close, the plant and soil come in equilibrium, and that’s the key assumption that’s happening. It is really hard. I think it’s really hard to validate that measurement to know whether you reach that equilibrium. It’s also really hard to tell where in the soil that leaf is it’s connected with right. So this is the traditional wisdom just as you say like in class you learned that, I learned that overnight predawn should kind of relax the transpiration stream and we should all be in agreement. And this particular question asked about five liter pots and planting trees and saturated soil and just let them dry down. What would you think would be the ideal condition to test this? Right? I like a small soil volume, a personal canopy. Yep. There’s only only so many places you can connect it.

There are a couple of questions I have on this that I didn’t see in the question. And maybe I didn’t read quite carefully enough but one was what soil they actually were using. Now, our friend Leo Rivera did some studies on some of the coarse potting soil mix and found that hydraulic conductivity does make a big difference in that. I think that’s what you’re getting at. It’s like where is the plant taking that water that’s then in this leaf, and if we have a coarser mix, some sunshine mixes all the universities like to use, the hydraulic conductivity gets sore. So we might be measuring very high water potentials at one location meaning close to zero, very wet. And yet right next to the root, the water flow is poor. And so the root can’t get up to that one. So that was one possible reason.

Yeah, from my point of view, this is a really hard question to answer. It’s just so tricky to validate what’s happening.

And I did some work last summer, I actually had the same idea, but I didn’t have this great idea of doing it with oak in pots, I went out to an apple tree and a pear tree. And I measured the stem water potential continuously with a new sensor that’s out there. And then some soil water potential. They didn’t agree either. And it felt maybe the heart of this question like what is going on? There is a gradient, though. I mean, so certainly during the daytime, we assume the gradient right, we assume there wouldn’t be so much of a gradient. But those assumptions are made because domains close, we think everything kind of relaxes in there. For sure. The daytime water potential and those leaves in the plants I was measuring for sure that was tied to atmospheric math, I could match ET up with leaf water potential or stem water potential. And those were highly correlated. The predawn water potentials I tried to make the technicians to go measure, but they told me no. I don’t even know what was going on there. So the answer to that is, you know, so what’s the answer? I actually looked there were an order of magnitude different negative, like 100 to 300, where the soil water potential’s a negative, let’s say 1100, to 1500, where the leaf water. Yeah. Any other reason why that might be?

You know, I’m not sure; it’s easy to say, well, trust the sensor, because the sensors sensor right, it’s calibrated, it’s normalized, you can get that out and test it.

I’m confident in that reading.

So that reading is good. But again, there’s things that we can’t see coming through. It sounds like you’re looking for me to just definitively answer the question, which I’m not going to do. Chris. Nope. You’re gonna shy away. I think this one, we can have a lively discussion, but in the end I don’t have the answer. I think more research, you know, that. I’m confident, you know, I’ve used the sholander pressure bomb, pressure chamber for years and years. I imagine even if we had been testing with some of these new devices, we would have seen the same thing. Maybe not as large a difference? I don’t know. But, I think it has to do with the physiology. I also think there’s unsaturated hydraulic conductivity in the soil potential issue there. If it was native soil, then that will take it away somewhat. But that’s what I answered so far. Okay, pick an easy one, Brad, we’re all tapped out. I’m just kidding.

How about let’s go back to field capacity. All right. So they’re asking does field capacity in one particular texture change during a cropping season? I guess this could be, you know, throughout any season as well, right?

I don’t know the answer to that.

So last summer, I was out working in potatoes. And I gave, you know, these growers, I’m like, Okay, here’s the range, we need to be about, let’s say negative 20, negative 60. And these one, growers were at like negative 120. And I’m like, Man, you guys are super low. And he’s like, You need to recalibrate your sensors. No, I mean, yeah, it’s possible that a single sensor could be for whatever reason, not behaving. But we had multiple sensors all telling us the same thing. And in the end, and like, No, we can’t read it, like you need to recalibrate your water potential scale. And like, you know, I didn’t make the water potential scale. Plants made the water potential scale, you know we can’t do that.

That’s sometimes a jump to move from a subjective method of doing things and then to get this objective. Yeah, exactly. This objective thing is going to skip the biases and other things and it might not provide the complete picture. And it has some uncertainty around the measurement. But it’s, going to give you objective information to work into your decision making.

That I mean, that’s the take home, right, I’m trying to say, look, here are some standard syllabi, if you think your plant, you know is happier negative 120 kPa, which I actually don’t think but if you think that’s true, that’s fine. We can say, oh, this happens to be a variety of potatoes. Super good at negative 120 Maybe it’s because you know, we have a problem with this, you know, center pivot sprinkler system. But I guess that to get back to this question is field capacity. Does it change? I would say no, I would say maybe a little bit like in it. Here’s one case where it might. Yeah. So if we have roots growing like in a turf grass, in a sand, if we had a change in structure, quote, unquote, where we have a lot of roots now filling those small gaps to find roots or filling the gaps in the sand, yeah, then we could change our moisture release curve, our field capacity, that kind of stuff. All things being equal and what I’m more used to, you know, even a sandy loam or a silt loam or something, I would say, it’s generally a soil property. It’s just a soil property we can count on. Although I will say that when we look at in situ moisture release curves from TEROS 10, 11, 12 and a TEROS 21, we do see a little bit of differences over the season that may change a little bit. And how much of that do you think is from installation? I wonder that. I didn’t know that they often in a lot of these irrigated soils, we have to take sensors out of the season. And some of that might be settling. We’re just starting on, for example, our second year and some of these turf grass applications where we have let them over winter did really interesting this year. They, you know, changed almost on their head from what we did last year. So maybe one of these day I’ll try to answer that question. Great. Okay, Brad

Similar, asking about field capacity and permanent wilting point. So given a specific topic, how useful or relevant is soil water content tool or model, in your opinion, to use a reference for field capacity and permanent wilting point?

You know, we’re there.

No, we’re there. Here’s what I would say, here’s what I actually thought you were going on this one, let me just say, I just read an article about some of the stuff; it was actually an article on the information gap and water potential in the field. And they were talking about pedo-transfer functions and saying, well, look, a lot of you guys use these models. These pedo transfer functions, put them in your models and are estimating the, you know, the field scale, or even larger scale, catchment scale, movement of water, the water balance, etc. But these pedo transfer functions, because we don’t have a lot of information on the water potential they’re in big error. And so what I would say is, if you just go out and use a reference field capacity, permanent wilting point, if you drive some of your understanding, just by, you know, going and get the starting data, get your soil type, generate a pedo transfer function, and you think you’re good to go. I would say no, I think that the data would suggest that understanding what’s going on in the soil better, improving our measurements, filling the gaps is the kind of the clarion call right now fill the gaps. You know, we just don’t have enough field measurements. I think that’s important.

And they are important benchmarks, but you know, they’re, maybe a little bit more helpful when they were the best tools we have. And now we have some tools to help move beyond them. Yeah, I love that concept. That’s the train of thought; we’re back on the rails here.

No we’re heading down the track. No, I agree, but I get stuck in that trap too. Sometimes, honestly. I mean, I’ve been teaching environmental biophysics for 22 years now. Right. You know, we do it out of a book that was written, you know, and released in 1998. There is new knowledge out there. That’s one of the reasons I love to bring students into the class is because they often teach me things. And one thing to think about for me is that I sometimes reject their new knowledge, shall we say, just because of my kind of age bias, you know, I come in, and I’m like, Hey, I already learned this, you know, my teachers taught me this back in the 90s. When I was taking classes, we’re good to go. And, really we’ve got to embrace the tools, the new steps that we have, it’s hard for me, but there’s a lot of learning we can do. So I would be careful with that. That’s, I guess, my endpoint.

This next one, we’re gonna go back to talking about salinity. So what is the relationship between salinity and soil and available water?

Looking at this from a sensor perspective, which is kind of my default position, right? I’m taking the measurements. We measure the water content, right? We measure the EC. And those two things, the water content, the EC is dependent on the water content to a certain extent. Right, you have the water content and the solute concentration of your solution that are going to be the main components of the EC of the soil. And the bulk electrical conductivity just isn’t always that helpful, right? Because unless you can control for the water content, you have two variables in the bulk conductivity that you’re trying to, you know, and you can see one of them, the water content independently. And so we use some EC models to kind of try and help us get at this. And so for the water content, the available water is generally independent. Right. But the EC is kind of part of the heart.

Let me ask you a question though. Go ahead. In terms of salinity, have you ever experienced a salinity that actually caused a reduction in available water?

Oh, well, that’s from the plant perspective. Yeah. Once you start hitting plant stress.

This it’s interesting. That’s where I went with this question. You went in, like the the limitation of water content, which is also a key piece, but I got it running down this road of about, the matric potential versus osmotic potential. Right. And, you know, we have been doing quite a bit of work on the ecosystems in horticulture right now talking about the osmotic water potential. It’s something I didn’t even know. I didn’t even see. There were years ago, you were a young employee, just gotten into METER Group. You were like a year older than I am.

You’re just recently returned from your vagabond. Okay. And you walk in, and we were testing this new MPS with our original water water potential sensor. Oh, yeah. So do you remember that. That’s right. And we put it in a potted plant. And we’re like, we’ll be able to stress our potted plant and make it do all these great things. And nothing happened. Mostly because of this, saturated, unsaturated hydraulic conductivity deal. But there was also another thing that I didn’t even know. And I don’t know if you knew that, that really to pull water all the way down in this potted plant to the point that it would be stressed, couldn’t get the water anyway. And the only way to stress this would be osmotically. And it turns out, I’ve always thought that osmotic stress in plants would produce a salt toxicity. And so far in the work we’ve done, which is not on a lot of plants, right, ah, the osmotic stress that we can create with this actually doesn’t cause soft toxicity. And it actually does cause the plants to flower more and put more energy into their fruit. Nice.

So that’s where I thought those go. That’s a better direction than I was heading.

No, but I liked your direction. I liked your direction, because I think there’s an interplay here that’s really important, where we get salinity, how it affects sensors and water content and how a toxicity question for the plants in the field certainly this question of salt affected soils where we need a leaching thrash right. And then this other concept that there could be enough salt in the soil to actually create stress a different way from matric and we can’t even measure that on the matric. We can with the water content sensors with these. We could estimate a salt stress, but we can’t do it with the TEROS 21s. That was a fun one. I like that.

So here’s another fun one, then, we haven’t had many questions come in about soilless media, so we’re going to hit this one. So can you discuss how soilless organic media such as biochar, or coco coir effects dielectric sensor accuracy, does the physical shape and size of the pores holding the water being measured affects the measurement, since it affects the electrical path between the anode and cathode?

So every year when I teach this biophysics class, we have students who have to do class projects and she did her class project on biochar, which is really interesting in different amounts of amendment to soil. You’d never want to fully biochar something. It’s only an additive to something else, right, turns into like this horrific mud.

It depends. You know, it depends on what it’s made from. Biochar is equal.

Oh, that’s a good point. Hers did. She wrote me apologetic emails. No she’s awesome. So how does it affect the diagrams? You know, this one’s easy to overthink it. Yeah, that’s what I think because you know, we’re emitting an electromagnetic field in the soil, and it’s going to kind of take, it’s gonna measure the charge time over the entire value, right? So if you don’t have anything conductive in there, and we know kind of where that limit is based off of our salt solutions,

and by the way, as far as anything else weird, but almost nobody has, can’t remember, one of our friends had some really kind of strange soil from somewhere that had.

Oh, you get like higher iron content or something in there. Exactly. Exactly. Yeah, it was really weird. But it’s extremely rare to get enough in there. I’ve heard of one in more than 20 years, right. So don’t worry, everybody. That’s not a typical situation and so basically organic matter is going to change your calibration. Not so much. Because of the electrical properties of what you’re adding. But because of density, your interactive, your surface exchange.

Exactly. So as you mentioned, it would just be a function of both density right below. I’m sure this is below one, probably gram per centimeter.

So you might need a custom calibration for something like that, which is why we have soilless calibrations as well.

Yeah. And so physical shape and size of the pores, I don’t think so. We’re doing this electromagnetic measurements going over a finer resolution,

than we can detect.

We got time for one, maybe two more, we’ll see. We’ll see how this goes. All right, what would be the best size container when measuring water content with different soils with the procheck? The procheck doesn’t really care. It’s just going to give you a readout. But you should for every sensor, and this isn’t just METER sensors, you should take a look at what their measurement volumes are, how big of a soil volume, and then you’ll want to at least get the minimum soil value. And I usually recommend a buffer. Because if you make a mistake there, then you can read air which will bring your reading down. Or things like that.

I agree. Just be conservative on your volume, go a little higher. I like that. Number 28. Brad, see, that may be a quick one.

That’s what water content percent can you trust that EC reading from the sensor?

Yeah. The bulk EC is the bulk EC so you can always trust there.

But when’s the last time you used bulk EC?

Actually well, from a sensor like I’m super sensitive.

I mean, you’ve looked at it. Yes. But what did you do with it? It’s a great diagnostic tool. I know, so you’re looking at water content is the corroborator for things that are happening in your water content if you’re trying to do some problem solving. And but when can it be like oh point three bulk EC? Is this tap water? Or is this right? Like you’re converting it right? You’re converting it to pore water, your model is generally going to give you a better.

So the question here is can you find a pore water EC at a water content at 10%? And the answer is no. That’s right,

because that is too low for the pore water EC. For the bulk EC is still going to be good. But as Colin mentioned, that’s not always that helpful.

So let me be clear on why this is true. We use a model called Hill Horst. Max Hill Horst. Real nice guy met him years ago, just in a chance meeting in the Netherlands. And, we talked over his model a little bit, the big thing is that it’s in the denominator is your measurement of water content, it’s a dielectric value. And as that gets lower and lower, if you’re pushing below 20% water content, that essentially blows us up, it makes it huge. And so because it’s in the denominator because it actually subtracts a sum value just becomes really large, the denominator becomes small, the overall becomes really large. And there’s big errors so below 25% I’d be really little careful. Maybe 20%. What’s your experience?

Below 15%? Our software limit is 15%. Is it 15? We won’t even give you a reading below 15% Because it’s not.

Yeah, if you’re in ZENTRA cloud, you’ll think oh, the water cloud, your EC is broken. Yeah. And then, we’ll tell you, No, you’re an EC is not broken.

And that’s when you go to the bulk EC like okay, the readings fine. Yeah, our water content conditions are just too low for this model. Exactly.

That’s gonna wrap it up for us today. So thank you all for joining us. We’ve had a really good crowd this whole time. This is our first time doing this. And it seems to have gone pretty well. I know that we enjoyed the discussion over here. Hopefully you did as well. Thank you again also for all of your great questions. We had a ton that came in via email before the session today. We had several come in, a bunch come in. During this live Q&A, we tried to get to a bunch of them. Again, if we did not get to your question, we will be getting back to you, Colin or Chris or somebody else from our METER Environment Team to answer your questions directly via email. Also, please consider answering the short survey that will appear after we finish here just to tell us what types of Q&A themes you’d like to see in the future. Also, for more information on what you’ve heard from us today, please visit us at METER Finally, look for the recording of today’s presentation in your email. And stay tuned for future METER Office Hours events. Thanks again. Stay safe. Have a great day.

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