Why archaic methods are killing your accuracy

If you’re still spending months generating a handful of points to produce only a partial soil characteristic curve—old-school methods are holding you back. What if you could create a soil moisture release curve in just 48 hours? And not just a curve with a few points, but a detailed absorption and desorption curve composed of hundreds of points that show exactly what happens as your soil absorbs and desorbs water throughout the entire range of water potentials?

Change the way you understand your soil

Partial curves made with older methods don’t give you enough data for a complete picture of what’s happening in your soil. Hundreds of studies show that faster, high-precision modern methods are more accurate—so you can reach better conclusions that stand up to rigorous scientific scrutiny. In this 20-minute webinar, METER scientist Leo Rivera compares available methods and teaches how to combine the latest technology to generate full, accurate curves with hundreds of points in only a couple of days—instead of a couple of months. Learn:

• The science behind current available methods
• The pros and cons of each method
• Advances in soil moisture release curve technology
• Best practices

Next steps

• See SlideShare
• Watch Soil Moisture 201: Moisture Release Curves—Revealed
• Check out Lab vs. In Situ Soil Water Characteristic Curves—A Comparison
• Download the Complete Guide to Soil Moisture Release Curves
• Take our master class on soil moisture
• Read our related article on defining water potential
• Follow us on LinkedIn: https://www.linkedin.com/company/meter-group

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

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.

Transcript

BRAD NEWBOLD 0:12
Hello everyone, and welcome to Five Reasons You’re Getting Less Accurate Soil Moisture Release Curves. Today’s presentation will be about 30 minutes, followed by about 10 minutes of Q&A with our presenter Leo Rivera, whom I’ll introduce in just a moment. But before we start, we’ve got a couple of housekeeping items. First, we want this webinar to be interactive. So we encourage you to submit any and all questions in the Questions pane. And we’ll be keeping track of these for the Q&A session towards the end. Second, if you want us to go back or repeat something you missed, don’t worry. We’ll be sending around a recording of the webinar via email within the next three to five business days. All right, with all of that out of the way, let’s get started. Today we’ll hear from METER research scientist Leo Rivera, who will discuss how to get better, more accurate soil texture information in less time. Leo operates as a research scientist and Hydrology Product Manager at METER Group. He earned his undergraduate degree in agricultural system 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 the HYPROP and WP4C. He also works in R&D to explore new instrumentation for water and nutrient movement in soil. So without further ado, I’ll hand it over to Leo to get us started.

LEO RIVERA 1:36
Thanks, Brad. Hi, everyone. Thanks for joining today’s webinar. As Brad said, my name is Leo Rivera. My background is in soil physics and pedology. And throughout my career, I have spent much of the last 14 years measuring soil hydraulic properties and soil moisture release curves. And I’ve learned a lot of the things that are good to do and a lot of things that are bad to do. And so today, I’m hoping to share some of the things that I’ve learned over that time and some of the best practices that I have adopted and picked up over that time. I would also like to cover a little bit about the history of the measurement, how far we’ve come, and where we hope to keep pushing the measurement to get better.

LEO RIVERA 2:20
So before we get into that, though, I think it is important that we first briefly cover what a soil moisture release curve is, especially for those that are new to the topic. I will also add some additional resources at the end of the webinar that I think are also helpful if you’re trying to learn more or understand this topic a little bit better. So we’ll add those at the end as well. Also, I get the feeling that I’m going to say soil moisture release curve a lot today. So if you wanted to play a little game, you could keep track of how many times I say it, that might make listening to the webinar just a little more fun. So let’s keep going.

LEO RIVERA 2:59
So when thinking about what a soil moisture release curve is, it’s important to consider the two variables that are necessary to describe the state of matter or energy in the environment. Those variables are the extensive variable, which describes the extent or amount of matter or energy, and then the intensive variable, which describes the intensity or quality of matter or energy. In that light, a soil moisture release curve describes the relationship between volumetric water content, which is the extensive property, and water potential, which is the intensive property of water in soil. Typical factors that impact that relationship include soil texture, bulk density, organic matter, structure, and various other factors. But we don’t need to dive too deep into that. So essentially, when you think about it, a soil moisture release curve is a lot like a fingerprint for soil. So that’s great. So we now we know what a soil moisture release curve is. But why are they important for us to understand and measure? Understanding the soil moisture release curve and the intensive and extensive state of water in soil are really critical in unlocking our understanding of many things, including how water is stored in soil, how available that water is for primary productivity of crops and microorganisms and knowing how water and solutes will move in soil. So understanding this really is key if you’re interested in optimizing water use for crops or modeling soil hydrology.

LEO RIVERA 4:43
And let’s dive a little deeper into one of those key areas and how we can use that to make irrigation decisions. On the right you’ll see a chart that shows the optimal water potential ranges for various types of plants. What you see is different plants have different optimal ranges for growth. For example, we know potatoes’ optimal range is minus 30 to minus 50 kilopascals. And grapes’ optimal range during the early season is minus 40 to minus 50 kilopascals. And then during maturity, that changes to being below minus 100 kilopascals. And if we’re only looking at water content, it is hard to tell if we’re maintaining conditions within that optimal range. This will change across different soil types. But if we combine the information that we can gain from a soil moisture release curve, we can find that optimal range and determine how much water we need to apply during our irrigation cycles to stay within that range. So soil moisture release curves are powerful tools if we’re trying to optimize irrigation strategies and improve our decision making with a better understanding of our soils.

LEO RIVERA 5:54
There are other applications of soil moisture release curves that go beyond irrigation decisions. An example of this is shown in the graph on the right. Researchers have found that we can use the slope of the soil moisture release curve to predict the shrink swell capacity of soil. Here we have different soil types with a range of shrink swell capacities or COLE values or coefficient of linear extensibility, and the slope correlates with how expansive the different soils are. We’ve also found that the dry end of the soil moisture release curve can also help us predict cat ion exchange capacity and solve specific surface area. There’s still a lot of research happening in this area and more publications are coming out, especially in the geotechnical engineering realm, so I’m really excited to see where this research continues to go and how much information we can gain from a soil moisture release curve.

LEO RIVERA 6:48
So how are soil moisture release curves measured? I think when reviewing the different types of measurement methods, it’s important to understand the history behind some of these methods and how they’ve evolved over time. So in the early 1900s, the Bureau of Soils recruited pure physicists like Edgar Buckingham to tackle perplexing problems in agriculture. His experience in thermodynamics really actually helped form the beginning of our understanding of unsaturated waterflow in soils. Using the fundamentals in thermodynamics, he determined that the driving force for water movement in soil was driven by what he quote unquote, called the time capillary conductivity, which we now will typically refer to as water potential and unsaturated hydraulic conductivity, so those are some two different factors that help govern that. But the earliest tools for measuring soil moisture release curves didn’t really come around until the 1920s. In 1920, one of those early methods, which is now known as the filter paper technique, evolved into a tool for measuring water potential, and then was brought to the US in 1937 by Robert Gardner. In the 1930s, L. A. Richards built on that and developed the pressure plate, which was one of the first instruments which was capable of effectively measuring what was referred to as capillary conductivity or water potential. But knowing that pressure plates were not a perfect tool, in the 1940s, L. A. Richards and John Monteith published papers describing how thermocouple psychrometers could be used to measure the water potential of soil samples. That wasn’t really proven until 1951 when D. C. Spanner was the first to successfully demonstrate that a thermocouple psychrometer could be used to measure water potential in soil. And then again, it wasn’t till 1983 that the first thermocouple psychrometer was actually commercialized. In 1960, Wind introduced the evaporation method, which was using tensiometers and a scale to log the change in water potential and water content, as water evaporated from a saturated core sample. And many of these methods that we’ve discussed have continued to be used today and have been adopted to standards in many areas.

LEO RIVERA 9:06
So, how have we tried to improve on these methods since the 1960s? Well, in between the introduction of the methods discussed, we’ve continued to search for a better tool for measuring water potential and ways to generate soil moisture release curves. Advancements in these measurements have been slow, and a lot of that has been dependent on evolving technology, really. A good example of this is the introduction of the dewpoint potentiometer in the late 1990s as an improved method for measuring water potential over thermocouple psychrometers. This method was an improvement in terms of measurement speed and accuracy, but it was still limited to the dry end of the soil moisture release curve. The Wind method was simplified in the early 2000s by Uwe Schindler with the simplified evapotranspiration method, leading to the development of easy-to-use tools like the HYPROP for measuring the wet end of the soil moisture release curve. But even if we were to combine measurements from these two devices, there would still be a gap in the soil moisture release curve that we couldn’t measure. But as micro electronic technologies advanced, and we had improved analog to digital converters, this has opened up more opportunities for advancing these measurements with things like an improvement in the resolution of the temperature measurement for the dewpoint method. So with advanced ADCs, we’ve been able to improve the resolution of the temperature measurement from a hundredth of a degree C to a thousandth of a degree C. Now, this seems minor, but it actually improved the accuracy of the water potential measurements enough to push the capabilities of the dewpoint method into the tensiometer range, making it where we could characterize the full soil moisture release curve with these two devices.

LEO RIVERA 11:01
I would say we still have a long way to go in our search for the perfect water potential measurement. Here we see a chart that shows some of the methods we discussed, along with some other field and laboratory methods that are commonly used. The reason I bring this up is that it shows us that there still isn’t one device that can measure the full range of the soil moisture release curve. So the search will continue on.

LEO RIVERA 11:29
So now that we’ve covered some of the different methods for measuring soil moisture release curves, let’s talk about some of the factors that impact accuracy. And to stick with what we’ve already been covering, let’s focus on how choices in the measurement methods used can impact accuracy. I think one of the most important things, just like in many other aspects of life, it’s important to understand the limits of the different measurement methods. One of these limits applies to methods like pressure plates and the filter paper technique. Researchers like Gee, et al. in 2002, and Batali and Flory in 2009 have found that there are equilibration issues that can result in large errors in our measurements. These errors especially occur at lower water potentials, and are caused by things like clogged pores in the ceramic of the pressure plate, flow restrictions within the sample, and loss of hydraulic contact between the plate and the soil due to soil shrinkage. These issues can cause the measurement to take weeks for equilibration. And in some cases, equilibration is never achieved. Another limitation which is primarily caused by how long measurements can take is the lack of measurement points to cover the entire curve. I’ll show an example of this here in just a moment. And I bring this up because I think it’s important to remember that no method covers the entire range, and most methods have have ranges that they will perform optimally. So it’s important to choose the right tool for the range you need to measure. And one last limitation that we might overlook is how do we account for spatial variability of soils? Most methods are limited to small sample sizes. So we either have to take a lot of samples and measurements or hope that our measurements are representative enough of our site.

LEO RIVERA 13:27
There are other factors as well to consider that can impact the accuracy of our measurements. Some of those factors are related to the physical and chemical traits of soil. One of those factors is hysteresis, which is a physical phenomena, which results in a difference in the relationship between water content and water potential under wetting and drying processes. This graph on the right shows how complex this can actually be. Depending on where you start your wetting or drying curve, you can have a different relationship and this can be challenging to represent with most of the methods available for measuring soil moisture release curves. Another factor is accounting for the different components that make up water potential. So as we know there are four components that are a part of the total water potential. So we have matric, gravitational, osmotic, and pressure potential. And we typically only look at matric and osmotic potential in most of our measurements. Some methods measure both matric and osmotic potential combined, while other methods may only measure matric potential. So this is important to remember and account for if you’re trying to combine multiple methods to generate a soil moisture release curve. And we’ll dive into that a little bit deeper here in just a moment as well.

LEO RIVERA 14:47
So, like I said earlier, was going to talk about an example of limited number of measurements. So here we have one of those examples of a limited number of measurement points. These points on the curve would represent typical points that we would try and measure with the pressure plate method. And the question is, are these measurements giving us enough information to understand what is happening with this soil? And I would say the answer is no. And we’ll see why here in just a second. So this particular example is a soilless media that was being used in a nursery. One of the issues that the growers were experiencing was that the plants were beginning to show signs of stress, even though the water potential was still around minus 10 kPa, so what you would think would be well within the range that we would expect plants to be happy. So we took this sample and measured it using the HYPROP. And here is the curve that we found. What you see, when you have a higher resolution measurement, is a completely different picture from before. It is clear that this soil has what is called a bimodal relationship. You can also see that the second air entry point of this curve is right around minus 10 kilopascals, the same point that the plants were beginning to show signs of stress. So now that we have a better picture and we’ve combined this with our understanding of hydraulic conductivity, and we found that as we approached minus 10 kPa, the water was not actually able to redistribute in the soil, causing drier zones around the roots than what we actually thought the plants were experiencing. And this was ultimately resulting in stressed plants.

LEO RIVERA 16:30
So now that we’ve talked about all of the potential issues and things to look out for, how can we go about getting the best measurements? So let’s go over some of the best practices that we have found for getting accurate measurements. One of the first things I would focus on is knowing which properties are critical. What is the range of water that we actually need to measure to be successful? I’d say this is especially important when looking at different soil types. The graph on the right shows two different soil moisture release curves, one for a fine textured silt loam soil, and another for a coarse textured loamy fine sand. And what you see with the coarse textured soil is that the majority of the water is held in the soil between zero and minus 100 kilopascals. Knowing this, we could focus our measurements on the wet range and stick to one method like the HYPROP. The finer textured soil, however, is better served with using two methods to fully characterize a curve, like the HYPROP and WP4C because with one method, we’re still missing a large portion of the soil moisture release curve, so we need to make sure we characterize that whole curve. This same concept is true when looking at soilless media versus mineral soils. In most cases with soilless media, the majority of the water is held in the wet range, and we can focus our measurements there.

LEO RIVERA 18:00
Moving on from that, let’s focus on how we can account for hysteresis in our measurements. First, it is important to know whether a method you are using is measuring on the wetting or drying curve, or is it capable of measuring both. So for example, the evaporation method, as you probably guessed by its name, is only capable of measuring on the drying curve. And if you were trying to combine evaporation based measurements from a tool like the HYPROP with a vapor pressure method like the WP4C, which is capable of measuring on both a wetting or drying curve, then we would want to make sure when preparing our samples for the WP4C, we are doing this based on a drying curve. Because if you don’t, if you were to prep those samples on a wetting curve, you could wind up with an example like we see in the graph here on the right, where those curves don’t actually match up because of the measurements on a wetting versus a drying curve. So just something to take into account.

LEO RIVERA 19:14
Other issues that can cause curves to not line up is osmotic potential. The graph on the right shows how osmotic and matric potential impact total water potential. What it shows is that the osmotic potential becomes more dominant as we approach the wet range of the soil moisture release curve. And this becomes a bigger factor when soils that have higher amount of salts— this becomes a bigger factor with soils that have a higher amount of salts present. Some methods will only measure the matric component while other methods are measuring the osmotic and matric potential components. So, when we’re trying to combine these measurements from these different methods, it can be hard if again if these soils have a higher amount of salts present, and this is something we need to take into account. And the best way to deal with this issue is to correct for osmotic potential by measuring the saturated extract EC of the soil sample. We can then use some simple equations to separate the matric and osmotic potential components. And once we’ve separated these components, we can then combine the matric potential data for the two different methods, allowing us to have a continuous curve. I’m not going to spend a lot of time on the exact details of how to make some of these corrections, because we have better writeups that we can send out. And so instead, we’ll send out a guide following this webinar that has more details on how to make these corrections.

LEO RIVERA 20:40
I actually added this slide in response to some of the questions that we received from some of you last week. And some of the questions— this is this correlates with some of the questions that we get often, which is around models for representing soil moisture release curves. So just like the measurement methods, the models available for representing soil moisture release curves have evolved a lot as well. And so how do we choose the right model? There’s a lot of options out there. Well, that really depends on what model best fits your data because I would say that not one model will fit all types of data. So a good example of this is shown on the right. This is a soil that we showed earlier, which has those bimodal characteristics. So we can’t use one of our standard, like, for example, Van Genuchten equations to fit this set of data, so we need to choose an equation that is meant for a bimodal soil, which there are options out there. So I also think it’s important to consider whether or not theta r, or residual water content is still relevant. And you can kind of see what this might look like in the graph on the right, because we don’t have dry end data to fit that. But this model uses theta r, which so theta r is based on the concept that the soil can only dry to a minimum water content. This is a common term used in many models, including the Van Genuchten equations like the one shown here. But what we have found through the advancement of technology, and through our improved understanding of soil physics, is that this is not true. Soil can dry to a true zero water content, typically around, if you’re talking in terms of pF, around a pF of seven, and we need models that represent that fact. Models like the Fredlund Zing and the PDI variants, the Peters and Durner variants of the Van Genuchten equation actually better represent this concept. So we might want to choose those to have have a better interpretation of the drying data in our model. And another factor to keep in mind is how complex of a model are you actually okay with using? As models have been optimized to better fit some of the complex shapes of these curves, the models have become more complex as well. So it can make it harder to implement and interpret some of these models. Depending on how you plan to use them, you may not need to add some of the complexity. So you might choose one of the simpler models if you don’t need it to fit your data as well. These are just all things, some of the things that you need to think about when you’re looking at models.

LEO RIVERA 23:16
So, to close things out, I’d like to summarize all of this with a real world example. This past summer, we were working with a research group running some plant breeding studies outdoors. These plants were being grown on a clay loam soil. And the typical reference evapotranspiration during the summer is six to seven millimeters per day. Last year, they irrigated these plants with the following irrigation scheme. They irrigated with a flow rate of about 0.5 liters per hour, cycling at 30 minutes on and 30 minutes off, and running that for about two to three cycles. So this comes out to really less than one millimeter of water per day being added. And we already saw that we have 6 to 7 millimeters per day of reference evapotranspiration. But they had really good results last year. So they ran the same irrigation scheme this year and had completely different results. The plants did not grow nearly as well this year and measurements showed that they were severely stressed. So what went wrong? Well, they didn’t really take into account the soil properties. And the fact that last year the soil was able to retain a lot of water was a strong spring recharge. This year, there was not nearly as strong of a recharge during the spring. And we had a really dry, hot summer which resulted in this irrigation scheme not being sufficient for the soil type and for these plants. They could have better designed their irrigation scheme if they had better taking into account the soil properties and some of the environmental factors that came into play this year.

LEO RIVERA 24:53
So I hope you found the information in today’s webinar helpful. If you’re looking for additional resources on soil moisture release curves or water potential and water content in general, we have our soil moisture masterclass series of webinars available on demand to watch. Two in particular that I think are helpful are Soil Moisture 201 and Soil Moisture 202, which are good webinars to watch if you’re looking for more information, specifically on the topics we discussed today, which is soil moisture release curves and choosing the right water potential device. Thanks again. And now let’s try and address some of your questions.

BRAD NEWBOLD 25:32
All right. Thanks, Leo. And yeah, we’d like to use the next 10 minutes or so to take some questions from the audience. Thank you to those who have already submitted questions. There’s plenty of time to submit your questions. We’ll try to take as many as we can before we need to finish. If we do not get to your question, Leo or somebody else from our METER Environment team will be able to get back to via email, the email that you registered with, to answer your question directly. So feel free to submit any and all questions. And again we’ll try to get to as many as we can. Leo, really quick, I wanted to pull this up here. It seems that we have an audience that is pretty varied on their understanding of soil physics in general, could you do just a quick definition of the four subsets of potential? So the matric, osmotic, gravitational, pressure? Just to give a good foundation before we move on in the questions.

LEO RIVERA 26:32
Yeah, so the components we talked about gravitational potential, matric, osmotic, and pressure potential. So pressure potential actually only comes into play during saturated conditions. And so in most states, it’s something that we really need to measure — it’s the pressure that’s being created by the water table. And then the gravitational potential is actually based on your position in a gravitational field. So say, for example, I’m trying to extract water at one meter below the surface. Not only do I have to overcome the energy that the water is being retained by the soil, I actually also have to make up for how far I have to pull that sample up in relation to the gravitational field. And then matric potential is primarily governed by the structure of the soil and the actual physical bonding of how the water is retained in the pores, whereas the osmotic potential is mostly due to the chemistry and the amount of salts in the soil. So those are just some of the main factors that that impact water potential.

BRAD NEWBOLD 27:37
Alright. Hopefully that helps for those of you that were asking about a general overview of potential and its various components. Here’s a question, do measuring instruments drift in any way if you’re using different substrates? So it could be soilless media or various types of soils. And then, do these measuring tools need to be calibrated, at what frequency, if you’re changing different substrates?

LEO RIVERA 28:02
Yeah. So if you’re measuring water potential in particular, they shouldn’t. So it should be independent of substrate. There are some factors that can be more important, especially if they’re instruments that need good contact, like tensiometers, that can be a substrate issue. But the beautiful thing about water potential is it, in theory, should be substrate agnostic. And it’s just an energy state measurement. But instruments can drift and depending on the type of device may need calibration. But we have, for example, dewpoint methods have salt standards that we can use to calibrate the device. And some of the measures, things like tensiometers are a pressure based measurement, and we can calibrate the pressure measurement to make sure it’s still accurate.

BRAD NEWBOLD 28:50
Alright. Another question. They’re wanting to know how best to wet samples. So in this case of a dominantly sandy soil to obtain good range for the soil characteristic curve. But I guess this could be for any type of soil, not just sandy soil.

LEO RIVERA 29:09
Well, one of the biggest things you want to take into account when you’re trying to wet your samples is not having trapped air in the sample. So you always want to wet from the bottom up of the sample. And it’s often recommended to use deaerated water. And also, you want to make sure you’re using water that has an appropriate chemistry because if you’re using water that doesn’t have an appropriate chemistry, you may impact your measurement. But yeah, those are things that you want to take into account. And then the best thing to do that I found is to bring the water level up just to the bottom of the sample and allow it to start wetting the sample. And depending on the type of soil, you might let it sit for 10 minutes and for finer textured soils, you might let it sit like that for an hour. And then we’ll bring the water level all the way up just to right at the bottom of the sample to saturate it that way. And with some really fine textured soils, it also might help to put the whole system under vacuum to help saturate the sample a little bit faster. But it all depends on how fast the sample can take up water.

BRAD NEWBOLD 30:12
All right. You also talked early on about issues with sampling your soils. So if you’re out on site or something like that, to try to get a representative sample of that site. But will there be differences in those soil moisture retention curves or release curves of a particular soil? So you’ve got same soil, but from different portions or different spots in your plot.

LEO RIVERA 30:44
Yeah, you can absolutely see that. And it really depends on— there are some things about the site that you need to take into account. You know, especially if there’s a significant change in soil types, even if it is, well, you want to be careful with the variability and the change in soil type, because we see some sites that can have a really drastic change, even with small changes in distance. But other factors, and this is a challenge with laboratory measurements in general is getting a big enough sample to represent what’s happening in the field. Say, for example, we get one spot that has a lot of active wormholes, whereas another spot that may not have as much, that can impact our measurements. There’s a lot of factors that can come into play there. And it’s tough to, I don’t think there’s a perfect solution. Typically, the best way to go is to at least have enough replicates, at least three in most cases, to help account for some of that. But also, just you’ve got to make sure your sampling scheme is designed well around the site. There’s a lot that comes into play there.

BRAD NEWBOLD 31:53
And so if they’re also concerned with change over time, or from season to season, is that something then that you’d suggest they are taking a sample in the same spot, each season or each year?

LEO RIVERA 32:05
Yeah. And there are some factors that we’ve seen, actually not relevant to water potential, but hydraulic conductivity, for example, we know it has seasonal changes. And so it is important to measure with those seasonal changes. And I would say it’s probably the same for these, especially if you’re seeing changes in degrading root channels or whatever. There’s a lot of those things that you might want to try and take into account.

BRAD NEWBOLD 32:31
All right. Somebody was asking for a recommendation of sensors that combine osmotic and matric potential. And will cost be an issue with it?

LEO RIVERA 32:44
Well, I’m wondering if you’re probably referring to field sensors for doing this. And there’s really not many good options for the field, for making this measurement in the field. The one approach that a lot of people are taking actually is using matric potential sensors to get the matric component. And then if you take a water content measurement that has an EC measurement as well, you can determine what the pore water EC is. And if we can measure the pore water EC, you can use that to determine what your osmotic potential is and your potential for osmotic stress. So.

BRAD NEWBOLD 33:25
All right. Okay, here’s one, it’s kind of more of a application specific, but I think it might be interesting to kind of discuss this one. So this individual is concerned, they’re watering in pots, and they’re trying to keep a certain moisture content, 40 or 50% moisture content, but they’re also wanting to get some runoff so they can build up nutrients — or sorry to reduce nutrient buildup, but they’re not getting as much runoff, even though it’s 50% water content. So there’s some water potential might be able to answer some of those questions. Can you help him understand how to achieve this without oversaturating?

LEO RIVERA 34:07
Yeah, you know, pots are challenging because you have a capillary barrier, essentially. So it actually increases the water holding capacity of the material. So actually, I would say one of the best things to do is to measure your soil moisture release curve and know what that curve looks like, because then you can determine what your total water holding capacity is for the pot. And then, when it comes to irrigating to cause flushing, it just has to, I’m not an exact expert on this exact process, but when you’re talking about trying to do that, you have to know how much water you need to add to hit that. And then how much time you need to have dry back so it’s not staying saturated. So it’ll dry back and you’re not running the risk of keeping the soil too wet. And the best thing probably to do is to do this intermittently. So have your normal irrigation scheme where you’re keeping it in that optimal range. And then occasional, you know, higher intensity irrigation, where you’re trying to flush out some of the nutrients.

BRAD NEWBOLD 35:15
All right. Do you have any suggestions for how to get data from the near saturated range of soils?

LEO RIVERA 35:25
Yeah, I guess this depends a little bit on if you’re trying to do this in the field, or in the lab. The best tool in the field for getting water potential data and doing this type of measurement is a tensiometer. There’s nothing more accurate than a tensiometer for the field based measurements. In the lab, some of the better tools would include something like the HYPROP, because that’s going to give you better data at near saturation. But there are also other tools like hanging water columns, which we didn’t discuss today, and sandboxes, which are harder to find, but I know those also work for that as well. It really depends on how much you’re trying to control it and what you have available. But yeah, there are some options out there.

BRAD NEWBOLD 36:15
All right. Looks like we’ve got time for maybe one or two more questions here. This person is working with soil surfactants. Is it better to test water potential and the effect of the surfactant on that instead of water content?

LEO RIVERA 36:33
Whoo, that’s a good question. I’m gonna say water potential probably is going to be more helpful because, again, you’re measuring the actual energy state, and it’s hard to tell how the surfactant might impact that if you’re not measuring it. And I’m not an expert on surfactants. But yeah, if you can directly measure the energy state of water, then you know always what’s happening, and regardless of the impact of the surfactant, you’re going to know what the matric potential is. Hopefully, that’s helpful.

BRAD NEWBOLD 37:11
If not, that we can get back to you, again. Let’s see, I think that’s going to do it for us today. Again, please feel free to reach out to METER Group. We have quite a few resources and instrumentation to be able to help you in working with these soil moisture release curves, and water potential, both in the field and in the lab. So again, feel free to reach out. Leo’s offering his personal email

LEO RIVERA 37:42
I might regret that.

BRAD NEWBOLD 37:44
with his LinkedIn and Twitter handle as well. So feel free to reach out. And you can ask, I will volunteer, you can ask him any and all questions about that. We hope you have appreciated this discussion. We’ve enjoyed it here. There were a ton of questions that we did not get to. So again, Leo or another specialist from our METER Environment team will be able to get back to you via email to answer your questions. We’ve got dozens here that we did not get to. Please consider answering the short survey that will appear after this webinar is finished, just to let us know what types of webinars you’d like to see in the future. And again, for more information on what you’ve seen today, visit us at metergroup.com. And then finally, look for the recording of today’s presentation in your email. And stay tuned for future METER webinars. Thanks again, stay safe, and have a great day.