Harness the power of soil moisture

Researchers measure evapotranspiration and precipitation to understand the fate of water—how much moisture is deposited, used, and leaving the system. But if you only measure withdrawals and deposits, you’re missing out on water that is (or is not) available in the soil moisture savings account. Soil moisture is a powerful tool you can use to predict how much water is available to plants, if water will move, and where it’s going to go.

What you need to know

Soil moisture is more than just knowing the amount of water in soil. Learn basic principles you need to know before deciding how to measure it. In this 20-minute webinar, discover:

• Why soil moisture is more than just an amount
• Water content: what it is, how it’s measured, and why you need it
• Water potential: what it is, how it’s different from water content, and why you need it
• Whether you should measure water content, water potential, or both
• Which sensors measure each type of parameter

Next steps:

• Need a transcript? Get it here.
• Slide presentation
• Watch Soil Moisture 201: Moisture Release Curves—Revealed
• Read When to water: Dual measurements solve the mystery
• Request a ZENTRA Cloud live demo

Questions?

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

• Talk to an expert
• Request a quote

Presenter

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.

Transcript

BRAD NEWBOLD
Hello, everyone and welcome to Soil Moisture 101—Need-to-Know Basics. Today’s presentation will be 20 minutes followed by 10 minutes of Q&A with our presenter Chris Chambers, whom I’ll introduce in just a moment. But before we start, a couple of housekeeping items. First, we want this to be interactive. So we encourage you to submit any and all questions in the questions pane. We’ll be keeping track of these for the Q&A session toward the end. Second, if you’re wanting us to go back and repeat something you might have missed, no problem, we’re recording the webinar, and we’ll send around the recording via email within the next three to five business days. Alright, let’s get started.

BRAD NEWBOLD
Today we’ll hear from Chris Chambers, who will introduce soil moisture, and how it can be used as a powerful tool to predict how much water is available to plants, if water will move, and where it’s going to go. 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. So without further ado, I’ll hand it over to Chris to get us started.

CHRIS CHAMBERS
Thank you, and thank you all for joining us this morning. And I’d like to start with a scenario that many of you are probably familiar with, whether you’re a graduate student that has a measurement campaign coming up, or an experienced researcher that’s been doing environmental measurements, or a grower that has crops to grow. At some point, you’ve probably either realized or had someone tell you that you need to measure soil moisture. And that’s because water availability is one of the main drivers of ecosystem productivity, and the soil moisture is the immediate source for most plants. So after that conversation or that realization, you may have had the question, what does soil moisture even mean?

CHRIS CHAMBERS
You hear that term, that phrase, thrown around a lot and I’m going to start with some questions that may help you narrow in on what you’re actually trying to find out. So are you interested in the water stored in the soil? Do you care more about water available for primary productivity to maximize production, or to understand the production on your site? Are you studying water and solute movement in soils? Do you aim to optimize water use of crops? Or are you modeling soil hydrology? Depending on which of these questions you’re interested in, soil moisture might mean something very different.

CHRIS CHAMBERS
So our goals for today are to know which variables you need to measure to get your project started, and to define some of the terms that you will encounter to describe the water state of your soil, and to know what questions you need to ask to be successful in this field season measurement campaign. Now, this is not a substitution for a soils class. We’re just here to help you get started, and take your soils classes, they’re really fun.

CHRIS CHAMBERS
So two types of variables are required to describe the state of matter or energy. And this is true, it’s not necessarily exclusive to water, it’s true for heat and temperature, matter like carbon dioxide diffusion, it basically the state of pretty much anything. And so for water, we’re looking at water content and water potential. And water content is defined as the amount of water per total unit volume or mass. That’s how much is there. And then the water potential is the potential energy per mole of water with reference to pure water at zero potential. And you can look at water potential as the energy state of the water in your system.

CHRIS CHAMBERS
So let’s start with water content. And we’re going to break it down further into two types of water content, gravimetric water content, which is the mass of water per mass of soil, so grams of water per gram of soil or basically your units of choice, and volumetric water content, which is the volume of water per total volume. With gravimetric water content, it’s the primary measurement for water content. We’re actually measuring the amount of water directly in the soil by measuring the mass. I’m used to grams of water per gram of soil, it’s pretty straightforward expression of units. And basically you weigh the wet soil sample from the field, dry it an oven, weigh the dry soil, and then your gravimetric water content equals your wet soil weight minus your dry soil weight divided by the dry soil weight. So the mass of your water divided by the mass of your soil. And then volumetric water content is basically the same thing except expressing it on a volume basis. And so here we have the components of your soil. And so these are the 100% and your volumetric water content is the volume of the water divided by the total soil volume. In this case, it’s going to be 35% VWC. You can see it in centimeters cubed per centimeter cubed, or inches per foot, but we’re looking at it on a volume basis here. So most of your volumetric water content measurements that you get these days are taken by some kind of sensor.

CHRIS CHAMBERS
So I’m gonna talk just a little bit about where that measurements coming from because you have a sensor in the soil and it’s giving you a number, or you’re remotely sensing it. Basically, you’re taking advantage of one property of water and that’s the polarity of water to take this measurement. So here we have our good friend, the water molecule, and we’ve got a negative pole up on the side with an oxygen, and positive pole on the side with your hydrogen atoms. So bear in mind, when this is in an electromagnetic field, here we have our water molecules kind of just scattered willy nilly, and when we apply an electromagnetic field, these water molecules are gonna jump to attention, right. And then if we change that field, say we reverse it, they’re gonna dance the other way. So by introducing this electromagnetic field, we can measure the effect of the water on that electromagnetic field. If you have more water in the soil, you’re going to get a larger effect. And so it doesn’t, most of the measurement technology these days, uses something like this, not necessarily electromagnetic fields, but we’re taking advantage of this polarity of the water molecule.

CHRIS CHAMBERS
And this, having a sensor opens up the possibility for a time series, which is extremely powerful. We looked at gravimetric water content, if you’re going to measure water, that way, you have to take a sample, take it back to the lab, or take a series of samples. And if you want to get at a time series, then you’re basically out there sampling all the time, it’s really impractical. But with this we can measure the timing of changes in water content, we can compare depths and a profile, and the shape of these of these curves actually gives you a lot of information about what’s happening to the water in your soil. So gravimetric water content, it’s a good first principles measurement, but it’s time consuming, destructive, and it gives you a snapshot in time. Where your water content sensors, its time series enables profile sensing over time, it’s less intrusive than gravimetric water content in that you’re not destructive sampling, but you do still have to get a sensor into the soil. And then the remote sensing gives you time series at a limited scale, but it’s extremely powerful for spatial sampling which is important for measuring water content.

CHRIS CHAMBERS
So we have two ways to measure it, and we can link them by the bulk density of the soil. So if you have, just for an example, a soil that’s 20% gravimetric water content, point two grams of water per gram of soil, and you know the bulk density is 1.6 grams of soil per centimeter cubed, then you can set it up to convert it to the volumetric water content just by multiplying the gravimetric by the book density. Cancel your units out, and we can see that we get our gravimetric into centimeters cubed of water per centimeters cubed soil. So in this case, that 20% gravimetric water content is actually 32% volumetric water content. Now, something that is really not a trade secret, but is kind of at the heart of where the volumetric water content readings come from, is that the gravimetric water content is used to develop the calibrations and validate readings of almost all the volumetric water content readings you’re gonna see. If you have a sensor, you have some relationship that converts what you’re reading in your electromagnetic field into a water content.

CHRIS CHAMBERS
And so if you’re not sure that your volumetric water content is correct that you’re sensing, you can sample some soil, measure the gravimetric water content, take a bulk density sample and check for yourself. Because the gravimetric water content is the first principles way that we know how much water is in soil. I’m going to spend a minute on the water content scale. So for volumetric water content, oven dry soil is 0% by by definition, that’s one of our defined end points. And then pure water is the is the other end of the scale. And this gets confused quite often, I see this a lot in customer interactions and conversations about water content, where many people think that 100% VWC, isn’t that fully saturated soil? No, your soil is going to saturate at different water contents for different soil types.

CHRIS CHAMBERS
So one way to look at it is as a percent saturation, and percent saturation equals the volumetric water content divided by the porosity times 100. So if you know your porosity, you can get at, for any given soil type, you can get at approximately what your water content at saturation is going to be. But you can seldom reach that point in the field. Here we’ve got, this is just a cross sectional diagram of a soil. And here we’ve got our water film, alright, the soil’s absorbing water, creating this film, and then we have these pore spaces with some air in it. In field conditions, it’s hard to get rid of these air spaces, so you will seldom actually see your percent saturation equal to your theoretical saturation maximum for that any given all the time, because you’re going to get air entrapment in there.

CHRIS CHAMBERS
Okay, let’s move on to water potential. And this is the energy state of soil. So the physical definition is the potential energy per mole, or per mole of water, with reference to pure water at zero potential. So what does that mean? Basically, it’s the energy that you need to overcome to move water one direction or another. And there are several components in the soil water, and the total soil water potential equals the gravitational plus the matric, plus the pressure potential, plus the osmotic potential. And in reality, you’ll seldom see this pressure potential unless you have ponded water or something. This, in unsaturated soils, you just won’t worry about this very much. The matric potential is the really significant component as far as soil is concerned, because it’s the water that’s adhering to soil surfaces.

CHRIS CHAMBERS
And I’m going to go back to our little diagram here. See here, we’ve got some air spaces in here so this is unsaturated. The matric potential is what’s creating this water film here. As water adheres to these particles, as you get less water, this pore space gets bigger, water gets more tightly held to the soil particles, and your matric potential decreases. And let’s look at what that means exactly. So water moves from a higher energy state to a lower energy state. And just to go back to our definition of water potential, say if we’ve got some water at zero kPa, you need to exert energy to move that water. And so water is going to move from a higher energy state 100 kilopascals, I’m using kilopascals, it’s my preferred unit, you also see bars or centibars, centimeters of water. But water is going to move from this higher energy potential to a lower energy potential and here we have a positive pressure difference, and water is gonna move in this direction. And water is going to continue to move in this direction towards the lower energy state. So from zero to minus 100, we call this a suction or a tension. And the main take home is that water is going to keep moving in this direction because that’s our energy gradient.

CHRIS CHAMBERS
Now let’s shift gears to a slightly different system, where we have minus 50 kPa here, minus 1000 kPa in the middle, minus 100,000 kPa and water is going to move, it’s gonna, it’s still gonna move from your higher energy state to a lower energy state so it’s going to move towards this more negative water potential. And this actually closely approximates what happens in our plant soil atmosphere continuum. So here we’ve got our soil. And this can change, these are just example numbers of what our energy gradient looks like. But here we have minus 1.5 megapascals, it’s just an order of magnitude, or just a different, still a unit of pascal. But then, so for this example, we have minus 1.5, here, and then the roots will be slightly more negative, and they’re going to pull water from the soil into the roots, up through the xylem, out through the leaves across this potential gradient, and the atmosphere at minus 100 megapascals minus 100,000 kilopascals, is what’s driving this gradient. So the water potential is going to define which direction your water is actually going to move in the system.

CHRIS CHAMBERS
And so we’ve got our two variables out of the way and let’s talk about some points in the soil just for orientation. You’ll see field capacity come up all the time, and field water capacity is the content of water on a mass or volume basis, remaining in the soil two or three days after having been wetted with water and after free drainage is negligible. Now I remember my first soils class, we won’t say how long ago it was, Soils 101. And Professor gave this definition and I was like, well, that’s remarkably subjective, is it two or three days after having been wetted, which gets at one of the one of the obstacles with this field capacity definition, is that these measurements can be can be wonderfully subjective. By wonderful, I mean really difficult to interpret. The field capacity turns out to be at about minus 33 kPa in most soils. But if you’re looking at some field capacity data, it’s good to know how that point was arrived at. But it is an extremely important point in soil because it’s where your matric potential is greater than the gravitational potential.

CHRIS CHAMBERS
So think about what’s happening here, we’ve had started from a saturated soil, and all of the water has drained out that can drain out, so your gravitational potential has done its work and you no longer have water draining out of the soil. So the matric potential and osmotic potential are now the dominant forces in the soil and water is being held to the soil. And kind of opposite, on the opposite end of the scale from field capacity is permanent wilting point. And permanent wilting point is, it was experimentally determined in sunflowers, and it’s defined as minus 15 bars or minus 1500 kPa, and it’s the soil potential at which sunflowers wilt. So it’s theoretically the empty tank. Right, we’ve got happy, I think these are squash plants, happy squash plants over here and this one has had a complete loss of turgor and has wilted by definition. Now, this 1500 kPa is going to not necessarily be the wilting point for all plants. Many plants will have different points and many plants will start to protect themselves from permanent damage much sooner than minus 1500 kPa or well after minus 1500 kPa. So it’s a useful reference point in the soil, but just be aware that this cactus probably doesn’t care about minus 1500 kPa and these Ponderosa pines are certainly not going to shut down at minus 1500 kPa. So it could mean different things for different plants and different crops.

CHRIS CHAMBERS
Now we’re going to move into soil type, because if you’re going to draw meaningful conclusions about water content, you must know something about your soil type. And here we have just a chart of our 12 most common texture classes, sand all the way down to clay. And so these are different particle size distributions. And you can see that here we have field capacity about minus 33 kPa and we have permanent wilting point, about minus 1500 kPa, and you have very different water contents at these points. So let’s pick just the sandy clay loam that can have a field capacity of 32% volumetric water content, which so basically, you know, it’s really, really well hydrated soil, has lots of water in it, but for a clay that is at the permanent wilting point for that water content. So if you’re measuring, if you get nothing else out of this presentation, take a soil sample when you’re installing sensors. Make sure you know what’s happening in your soil and especially where you get changes in soil type, whether it’s a profile, or whether it’s spatial variability from site to site. Note that the water potential doesn’t care for all the soil types minus 33 kPa is minus 33 kPa, whether it’s clay or sand, and the same thing with minus 1500 kPa.

CHRIS CHAMBERS
So we’re going to take this a step further and take a quick look at this relationship between water potential and volumetric water content. And this is a plot for three soils and here we have water potential on a logarithmic scale on the x axis, and volumetric water content on the y. And there’s a relationship, and it’s different for every soil, of what a given water content will be at a water potential. Here, we have our good old Palouse silt loam. And for reference, so, this is kind of how important these measurements are, you know, I kind of talked about how subjective and how not exactly accurate field capacity and permanent wilting point are, but I needed some reference point for my eye for these. So I threw field capacity and permanent wilting point in here, here’s field capacity, permanent wilting point, and with this relationship, you can find out how different soils will behave anywhere along the curve. So I’m going to leave this point here, we’re actually going to come back and talk about soil water retention curves more in Soil Moisture 201.

CHRIS CHAMBERS
But right now, I’d like to kind of wrap up where we started, tie it all together, and if you are embarking on a soil moisture measurement campaign, this field season coming up really quick for the northern hemisphere, the south is probably well on its way already, here’s some questions to ask, before you start your installation or getting things going. Do I need to know how much water is stored in soil? If the answer to that is yes, then you probably need water content. It’s where you should focus? Do I need to know which way water is going to move in the soil? If the answer to that is yes, then you’re going to need water potential. Do I need to know if my plants can get water? I think you guys can answer this one, I think you see where this is going, you’re going to need water potential. Do I need to know how much water is in the soil for my plants? That actually gets a little bit tricky, I would say you probably need both because you need to know that point, you need to know how much water is in there physically, and you need to know at what point your plants are not going to be able to get it, you’re going to need to know the size of your tank. So I would really like to thank you guys a lot for joining us. Tune in next time for Soil Moisture 201, as we flesh out this idea of soil water retention curves, not so much how to make them but what do you do with the data.

BRAD NEWBOLD
And at that point, I’m going to hop in for some of you guys’ questions. Okay, so first up, okay, a minus 100 kPa atmospheric is equivalent to what air temperature and relative humidity?

CHRIS CHAMBERS
That is actually a great question. And gets into the actual physics a lot more because your pressure and vapor pressure of the air are going to be affected by this. And the actual relationship, I’m pretty sure the guy who asked us knows a lot more about this topic than I do, but it’s gonna have some relationship to the air pressure and temperature. So if you’re actually calculating your gradient, look more into, it’s going to depend on your elevation, it’s going to depend on your atmospheric pressure, and dig more into the model for the energy of water vapor in the atmosphere than what I’ve presented here. And if you’d like some help, I can help you dig into that a little bit more with digging into the research a little bit. And I referenced an introduction to environmental biophysics pretty heavily here by Campbell and Norman, that’s a great resource for any of these calculations.

BRAD NEWBOLD
Next question. How can you measure kPa or MPa? Or what tools can you use for container production?

CHRIS CHAMBERS
kPa or MPa, that’s really just a preference, you can go, it’s just basically moving the decimal point three places I believe. So there are several methods for measuring these, they’re in the soil, you can use tensiometers, highly accurate at the wet end, but they’re not very good at the dry end. So in containers, a tensiometer is probably what you’re looking for. There’s some other major potential sensors out there, you can look at the TEROS 21 from, it’s on our website, it’s not going to be as accurate in the wet end as a tensiometer, but it’s going to give you a better range and less maintenance.

BRAD NEWBOLD
What will measuring electrical conductivity give you a metric of versus VWC, or volumetric water content?

CHRIS CHAMBERS
So well, electrical conductivity, can also be really helpful. It’s a function of your water content and your solute concentration in the soil. It’s trickier to use than water content in many ways, because you have to tease out that part of it. Is this change in electrical conductivity due to my solutes, or is it due to the water content? It’s pretty easy to measure in most cases and there’s a couple of models out there that can help you extrapolate what your water potential is or what you’re, sorry, lets you extrapolate what your electrical conductivity of your pore water is, versus your bulk electrical conductivity. But that’s the real trick is teasing out that difference there.

BRAD NEWBOLD
Okay, I’m being signaled, but I only have time for one more question and there’s a ton coming in here. So I will try to get back to you guys offline and reply to your questions. Some, we do have some resources on our website for some of these. So I will try to pick one that I can send the link to. Bear with me one moment. So here is, oh, what are the important considerations when we are thinking about measuring water content and water potential in peatlands with organic soils?

CHRIS CHAMBERS
So this is, as you could tell my presentation is mostly geared around mineral soils. And organic soils like peat, they present some challenges that are difficult to, that can be difficult to overcome but aren’t necessarily impossible. You can generate a relationship on soil water potential curve, or a soil water retention curve in soilless substrates, we’ve done that before, it works really well. So that’s something I would definitely look at. If you were sensing your sensor to substrate contact is going to be one of the important factors in that. So spend a lot of time on your installation and then just realize that this field capacity and permanent wilting point are going to have slightly different concepts in your organic soils, that isn’t necessarily easy to predict from one organic soil to another, so watch out for your variability.

BRAD NEWBOLD
With that, I’m gonna have to shut it down. I will respond to the rest of the questions offline. And you can watch this presentation at any time. You’ll receive a link in a couple of days. And I really appreciate you guys taking the time to visit with us today. Thanks a lot, Chris. That’s gonna wrap it up for us here. Thanks for joining us today. We hope you enjoyed the discussion. And thanks again for all your great questions. Like Chris said, he will be getting back to you if your question was not answered here on the webinar, he will get back to you with that. Also, please consider answering the short survey that will appear after this webinar is finished once you close out of it. Tell us what types of webinars you’d like to see in the future. Look for the recording of today’s presentation in your email, and stay tuned for Soil Moisture 201 as well as other future METER webinars. Thanks again and have a great day.