Soil particle analysis is more complicated than it looks

Accurate soil texture information is critical for understanding experimental results or modeling—and if you’re just guessing—you’ll be in trouble when it comes time for publication. Soil particle analysis is hard. You need to know what to watch out for, or your accuracy can be off by orders of magnitude. And that’s a problem—get it wrong, and your models and assumptions will be incorrect and ultimately you’ll reach bad conclusions.

What you need to know

Measuring soil texture can be tedious, complex, and prone to human error. In this 30-minute webinar, researcher and application expert Leo Rivera teaches best practices for higher accuracy and how to choose the right method for your unique application. Learn:

• How soil texture measurement has evolved over time
• Fundamentals behind the measurement
• Comparison of different measurement methods (including Stokes law-based and optics-based)
• Pros and cons of each method
• Best practices: making an accurate measurement regardless of the methodology

Next steps

• Read Lab vs. field instruments: Why you should use both
• See the PARIO Soil Texture Analyzer
• 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

Transcript

BRAD NEWBOLD 0:09
Hello, everyone, and welcome to Five Reasons Your Soil Texture Analysis Isn’t Accurate Enough. Today’s presentation will be about 30 minutes, followed by about 10 minutes of Q&A with our presenter Leo Rivera, whom I will introduce in just a moment. But before we start, we do have 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, do not worry. We are recording this and we will be sending you a recording via email within the next three to five business days.

BRAD NEWBOLD 0:46
Alright, 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 a more accurate soil, excuse me, to get a more accurate soil particle analysis. Leo operates as the Director of Scientific outreach at METER Group, he earned his undergraduate and master’s degree from Texas A&M University in soil science. And 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 SATURO. 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 1:32
All right. Thank you, Brad. And thank you to everybody who’s attending today. And I was really amazed to see the type of engagement that we got over a topic like this. I believe we had a little over 2000 registrants for this webinar, which just amazes me that there’s that many of you interested in measuring soil texture and soil particle size analysis. So that’s great, because I think it’s a really important topic. But let’s go ahead and jump into this and talk about some of these different tools out there that are available. But you know, before we talk about the tools, I think it’s really important to understand what particle size is and what soil texture is. So the measurement of particle size distribution is a topic that I have a love hate relationship with. And I’ll have to explain this a little bit further as we go through the presentation. But if some of you will probably relate to this, but as most of you are likely aware, when we refer to the particle size distribution, we’re referring to the mineral fraction of soil that is made up of soil particles ranging in size from stones and rocks down to submicron clays. Soil particles smaller than two millimeters or 2000 microns are generally divided into three classes, you have sand, silt, and clay, which you can see makes up the iconic soil triangle many of us are familiar with today, especially if you’re familiar with US soil taxonomy. So measuring soil texture is critical in many applications, whether it’s to understand soil water retention, and hydraulic conductivity, which as we all know are important hydraulic properties of soil. Texture also plays a role in the leaching of water through the soil profile. Erosion potential of soil, plant nutrients, storage, organic matter dynamics, and carbon sequestration capacity. As we all know, these are all very important properties of soil, and these are all impacted by soil texture in one way or another. So, you know, how have we traditionally measured soil texture? Well, there’s a lot of tools out there. So the way we measure this important property has evolved over the years. Many of us have learned how to texture soils by hand, through ribboning and building little pyramids, which is always a lot of fun. Especially those of us that have been soil judgers, we all are very familiar with doing this and practicing this, because as you know, it takes a lot of practice to get good at hand texturing. It’s a really useful way to characterize soils in the field. But as you can imagine, it’s not very accurate, and it’s very subjective. And one day you might be accurate at it. And another day you might not. It depends on how good you are and just how practiced you are and how familiar you are with those different types of soils. Now when we collect soils and bring them back to the lab for analysis, some of the traditional methods that we have used include sieving analysis, and sedimentation methods like the hydrometer and pipette methods. And as technology has developed, has advanced, we’ve developed new methods and instruments that have been adopted. Most of these new methods have been in the form of optical methods like X-ray attenuation, and laser diffraction. I’m also going to talk about a new method that focuses on the sedimentation methods as well today. But all of these methods have their advantages and disadvantages. All of them. They each have pros and cons. I’m not going to say there’s a perfect method for measuring soil texture and it’s important to understand that as we have this discussion, and just keep that in mind as you’re trying to decide what method is right for you. So in this presentation, we’re really going to focus primarily on the sedimentation methods and a couple of the optical methods. But you know, before we get to deep into it, I really think it’s important to think about some of the fundamentals. So as I mentioned earlier, soil particles span a large size range, varying from stones and rocks, stones exceeding up to .25 meters in size, and down to those submicron clays that can be less than one micron in size. The measurements we will discuss today focus on particles smaller than two millimeters, or 2000 microns. But all of these components are a part of the classification and need to be accounted for. So you need to make sure you’re taking into account all the gravels, the rocks, and all that when you’re actually doing your full classification. But it’s also important to understand there are various systems of size classification that had been used to define some of these arbitrary limits, and ranges for soil particle size. This chart here shows some of those classification systems like the USDA and the Unified Soil Classification System (USCS). There are other classification systems around and these may depend on your field and your location. For example, Germany has a different classification system. There are many, many other areas that have their own classification system. In the US, we primarily use the USDA system for agricultural and environmental purposes. And for engineering applications, we typically use the Unified Soil Classification System. Now, when reporting your results, it’s really important that you identify which system is being used for your classification, so people know how to know how to interpret the results that you’re reporting. Now let’s talk about how that might look when you’re reporting results. So on top of the various classification systems, particle size analysis data can be presented and use in several ways. One of the common ways of presenting particle size analysis is through the cumulative particle size distribution curve. You can see an example of that here. Now this method is really nice because it gives you a lot of detail and the total breakdown of the soil texture throughout that full range. So it gives you a little more detail and understanding Okay, really how different is this soil, especially when you see things like the stairstep curve, it’s important that you have some of that detail when you’re trying to understand how these properties might impact what you’re looking at. But it can also be reported as a mass base percentage of the different size classes. So the mass of the sand, the silt, and clay fraction, we typically see that in percentages of those three particles, but it can be even broken down even further into like fine silts, the middle fine and coarse sands things like that. Or it can be simply reported as textural class like clay loam, sandy loam. And you’re typically used to seeing that when you are looking at the textural triangle, and you’re trying to determine what your soil texture is. Cumulative distribution curves contain more detail. But as you can imagine, they’re not as easy to interpret. You look at this, like this gives you a lot of detail in the soil. But if I’m just trying to say, Okay, I have, you know, X amount of sand, silt, and clay, then it takes a little more time to figure that out. So sometimes, you might want to break it down more simply depending on your audience. So, some other really important pieces that I think you also need to take into account when we’re talking about soil particle size analysis is that soil handling and prep for the measurement are super critical. For nearly all methods of doing soil particle size analysis, the soil will need to go through a pre treatment process, which includes dispersion of the sample to separate the soil particles to actually physically separate them from each other because they’re often bound together. This typically involves both physical and chemical methods. So typically, for the physical method, we’re going to use something like a shaking table, or a physical like a milkshake mixer. They make ASTM standard milkshake mixers to physically disperse the samples. But we’re also going to use a chemical dispersant. Those particles want to disperse away from each other and one of the most common chemical dispersants used is sodium hexametaphosphate. That’s the one that I use and I’ve used throughout most of my career, but there are other options out there as well. Depending on the amount of organic matter, iron oxide, and carbonates present in the soil, you may also have to do some pretreatment to remove these components because organic matter and high amounts of iron oxide and high amounts of carbonate can impact your measurement. So really, I just want you to take away that how important soil handling and prep is because it can have a significant impact on the accuracy of your measurement. There are many methods available that are recommended out there. I often recommend referring to the Methods of Soil Analysis part four, Physical Methods. This is a book that’s put out by the Soil Science Society of America. Often, I refer to it as the soil Bible. It’s what I’ve spent a lot of time in, looking at all different types of methods for measuring and for measurements in soil. So that’s a good reference to utilize. So now that we’ve covered all of that, let’s dive into the methods. And today we’re going to start with some of the different sedimentation methods. Sedimentation analysis relies on the relationships that exists between settling velocity and particle diameter. This relationship was first defined by George Gabriel Stokes in 1851. He was an Irish English physicist from the University of Cambridge. Now this is commonly referred to as Stokes’ law. So anytime we’re talking about sedimentation methods, we always refer to a Stokes’ law based method. Now, to make these measurements, we’re going to take an aqueous suspension of soil and place it in a sedimentation cylinder. That suspension is then going to be homogenized or mixed, to get the particles all in suspension. That way, they’re all floating in suspension at the same time. And then as soon as the mixing is done, the measurement time begins. And once that begins, the large particles are going to start falling out faster, and then the smaller particles are going to stay in suspension for longer. So that those particles that remain in suspension impact the density of the solution, which many of the measurement methods that we talked about, rely on that density of the solution to characterize which particles are falling out when. Now, the settling velocity is really going to depend on the size of the soil particles according to Stokes’ law. Now, I really think it should be stated that there are some basic assumptions in Stokes’ law based methods. Now, one of those assumptions is that terminal velocity is attained as soon as settling begins. Resistance to settling is entirely due to the viscosity of the fluid, and not other things within the cylinder. It also assumes that particles are smooth and spherical. Now, we all know that soil particles are not perfectly smooth and are not perfectly spherical. But overall these methods are still good methods, even with that assumption. And then there is no interaction between the individual particles and solution. Now, if you have the right amount of dispersion in there that helps keep that from happening. But we can imagine that there’s probably still some interaction. But overall, these methods are still primarily the gold standard for making these measurements. So for sedimentation methods, we’ve talked about you have to treat the samples and get everything in suspension. Now we’re trying to measure that change in density as the particles are falling out. And there’s three common techniques for making these measurements. We have the hydrometer method, which you can see an example of here in the middle, in the middle image, where we take a hydrometer and measure the density, you can see what that sampling zone looks like in the cylinder. We have the pipette method where we’re going to take a small subsample, and you can see where that measurement zone would be for the pipette method. And then we have a newer method, which we’ll talk a little bit more about as we get into it. And that’s the integral suspension pressure method. And we’ll dive into the details of that a little bit as we get further into this. But let’s go ahead and start with the hydrometer method. The hydrometer method is by far one of the most commonly utilized measurement methods, because well, it’s cheap. And you can easily buy ASTM approved hydrometer for typically less than $100, oftentimes way less than that. And it makes it easy to run a lot of samples. This is one of the methods that I personally am way more familiar with. Probably more familiar with than I would prefer to be. But there are probably quite a few of you out there that are familiar with this in the same way that I am. I’ve probably made somewhere around 500 measurements using the hydrometer method. And oftentimes we would be running around 30 samples at one time, running around trying to take multiple measurements at a time with a single hydrometer, which was very challenging, especially because of the amount of measurements that we have to make. So let’s talk a little bit about how this works. So as the particles begin to fall out of suspension, the density of the solution is gonna change, just like we talked about. The hydrometer method looks at the settling depth, theta. And you can see the equation on the right there, what that looks like, over time t to calculate the particles that have fallen out of suspension at a given time interval. Now, we’re going to make multiple measurements at different time steps to quantify the different particles that are falling out over that time. This method does require a, what we refer to as a blank cylinder to correct for the water that we’re using, and the temperature of that water, and the dispersant density. So when you make your blank cylinder, you’re going to put the same amount of dispersant into that cylinder as you would have put it into your soil sample. And then we measure the temperature as well again to make that correction for temperature. The sand fraction is typically best measured separately, by sieving analysis. You can try and quantify the sand fraction with the hydrometer method, and many people do. But the sands fall out so quickly, it is really hard to really properly characterize the sand fraction with just a hydrometer on its own. So I find that to be inaccurate, and I prefer to use the sieving analysis to quantify our sand fraction. And usually that just involves wet sitting the sand out after the measurement is complete, which isn’t too hard to do. So a typical measurement duration for this approach is going to be around 24 hours depending on how accurate you want to be with your clay fraction. So what are the challenges with the hydrometer method? Well, manual readings are error prone. You can see an image here of a person, you can imagine myself in the lab running around trying to measure 30 samples all at one time and do it accurately, especially when we’re trying to measure at 30 seconds or one minute time intervals. And then at six hours or four hours or three hours, whatever your intervals are, you’re trying to measure all of these samples at one time and be accurate with those measurements. So you can imagine there’s a lot of error involved with that. There are fixed measurement times that we need to try and catch. And so that involves a lot of just being back in the lab to make these measurements. Disturbance of the sedimentation process when inserting the hydrometer can also induce error. So as you can imagine, if we’re running around measuring multiple samples, we usually don’t have 30 hydrometers go with each sample. That is an approach you could take and that will help reduce the error from the every time if you reintroduce that hydrometer into the sample. Again, it’s going to disturb that so it is one way to reduce that source of error. But as you can imagine, if we’re trying to use one hydrometer and making all these measurements, we’re going to introduce error just every time we disturb the sample. Now, the best accuracy that we can typically get with the hydrometer method is going to be around plus or minus 3%. And that’s best accuracy assuming that you’ve completely reduced the manual, the error, you know, the human component of this measurement.

LEO RIVERA 18:22
Now, one other part, and I wanted to bring this up because I really found this funny, as I was kind of doing some research for this presentation, I found this post and it’s that dreaded 24 hour reading, I found this post on Twitter from a soil scientist. And he’s posting about having to take the Saturday morning hydrometer method, his 24 hour reading on Saturday morning. And that’s the thing is, you really are, you have to be there to make those measurements. So whenever you start it, you’ve got to be back in 24 hours to make that measurement if you want to be accurate with your clay fraction. And I always find that to be really challenging at times, and just you know, it’s just the way it is. I had to do this and spent a lot of time in the lab making these measurements. But that’s one of the downfalls of this method is just you’re really you know, you’ve got to be there. So, we’ve talked about the hydrometer method. Let’s go now into the pipette method. And the pipette method really is typically referred to as the gold standard method for measuring soil particle size analysis. The pipette method differs a little from the hydrometer method where it is a direct sampling procedure instead of just measuring the density, now you’re taking a subsample and drying that subsample in the oven to get the weights of the soil that are in suspension at those times. And so after the sample is put in suspension, small subsamples are taken with a pipette similar to the one in the image here. And these subsamples are taken at specific time intervals to represent a specific particle sizes, that should be in the suspension. So typically, we’re looking at 2, 5, and 20 microns. And so they’re specific time intervals, where you’re going to take subsamples to get those fractions. Just like with the hydrometer method, the sand fraction is best quantified separately, and actually, with this method it needs to be quantified separately, again, using sieving analysis or something along those lines. But the nice thing is the typical measurement duration for the pipette method is usually around six hours. Now, you can use the pipette method as well to get really fine clay estimates, and that does require a longer measurement interval. But if you’re trying to get fine clays, you know, like, say half a micron instead of one micron, or two microns, you can do that with a pipette method, which is great. So that’s really one advantage of the pipette method is getting to those fine clay fraction estimates. Now one of the challenges with the pipette method, again, these are manual readings, manual readings can be error prone. Now, as you get more skilled and more practiced with this procedure, you’re likely will reduce those manual reading errors. But it’s still, we’re humans. And we can make mistakes. It again, they have fixed measurement times that we need to take subsamples. And timing is pretty critical with these measurements. And just like with the hydrometer method, we have a disturbance of the sedimentation price process from inserting the pipette and actually pulling that sample out from the solution. The disturbance is probably a little less with this method than with the others with the pipette method or with the hydrometer method. But there’s still some disturbance that occurs. And even at that the typical accuracy is still around plus or minus 3%. Now, we’ve talked about all of these manual methods that have been around for, well, decades, and have been used in the literature and use the standards for many years. But you know, new technologies come along, and one of those is the integral suspension pressure method. We often refer to it as the ISP method. And the way that works is we have a high precision pressure transducer that’s measuring the density changes as a particle settle over time. Instead of doing those manual measurements, we use a really precise sensor to do that. That particle size distribution is then determined by inverse modeling of that pressure measurement to quantify the particles that have fallen out and what the total particle size distribution is. And you can see what that measurement zone typically would look like with a tool like this. In here you see the vise we call the PARIO. And it has that pressure transducer that is measuring the density change over time. And then plugging that into some software. Now, again, just like with the other methods, this hand fraction is best quantified separately, by using sieve analysis. With the ISP method, typical measurement times were around 8 to 12 hours. Here’s an example of what that measurement actually looks like. So the graph on the left actually shows the really precise pressure measurement that we’re making. And if you look at the scale, we’re measuring pressure to the pascal level of pressures, which if you’re familiar with pressure measurements, that is incredibly precise, and requires a really well calibrated pressure transducer to make that measurement. And you can see what that change, that pressure change, looks like over time. For a sample, as the particles are falling out of suspension, we’re then able to take that measurement and generate a curve like what we see on the right, which gives us our cumulative particle size distribution curve. Now we are using sieve analysis to get the sand fractions and you can see some of that on the right of the graph, where we’re putting in our sand fractions, then the ISP method with the PARIO is giving us the remainder of the curve all the way down to the clay fraction. And you can see this example was actually set up to compare with the pipette method. And you can see what that, how those match up and and how well those methods actually match up. Now, what were the challenges with, what are challenges with the ISP method? Well, it’s really dependent on precision electronics. So despite cutting edge pressure sensor technology, with a resolution of point one pascals, the accuracy of the ISP method implemented in the PARIO was still less than expected from numerical analysis. There are several reasons for some of the reduced performance. Some of this includes user error in the dry mass calculation of the soil sample, poor temperature equilibrium of the sensor head with the sample, and errors in the sand fraction measurement. Any of these little errors that come in can propagate in the measurement. So, as an example of this in a sandy soil, say with 50% sand and 5% clay, a relative error, say we were to have a relative error of 2.5% in the sand fraction, this will cause a relative error of 25% in the clay fraction. So, again, a small error in the sand fraction propagates to a much larger error in our estimate of the clay fraction. So, when we ran into this and when we found these problems, this was motivation to search for an improvement of the methodology that was convenient to implement and does not affect the overall practicality of the method. And that is where we were able to develop what is called the integral suspension plus method or the ISP plus method. This is an extension of the original ISP method. The ISP plus method is based on is an extension of the experimental ISP protocol. After a certain time of measurement, part of the suspension is then drained laterally from the sedimentation cylinder through an outlet and collected into a beaker and then oven dried. So, it’s in a way kind of similar, it’s a mix of the hydrometer concepts and some of the concepts of the pipette method where we’re actually taking a sub sample to get those clay fractions. And you can see what that looks like here. We can have an example of a cylinder with the drain valve. And let’s just say at two and a half hours, we’re going to drain that sub sample. And you can see what that looks like with the pressure measurement, we have the pressure measurement coming down, and then at a certain time, we drain off that sub sample. That drain suspension is composed of all of the concentrations that are aligned above the outlet depth. So thus it contains some of the subsamples. Some of the sorry, so thus it contains typically, your finer particles, the clays and some of the silts depending on the time that you make that. So once we’ve taken that subsample, we can then actually bookend the sand fraction with our sieve analysis and use that drainage sample to quantify our clay fraction more accurately. And then that completely bookends both areas and really reduces our overall error, with that drain suspension. And what we found with this is that the ISP plus method decreased the measurement time to two and a half hours and increased the accuracy of the clay fraction estimate to plus or minus 0.5%. So a really significant improvement in the clay fraction analysis— or in the total analysis of the particle size distribution.

LEO RIVERA 28:11
So we’ve talked about a few of the sedimentation based methods, but what are other methods out there? And some of the most common methods used are optical methods. We have x ray attenuation, laser light scattering and diffraction, and visible near infrared spectroscopy. Today, we’re going to focus primarily on the laser light scattering and the diffraction method and the visible near infrared spectroscopy approach. So, with the laser diffraction method, this is based on the principle that particles of the given size diffract light at a certain angle and that increases as a particle size decreases. So here you can see a schematic of what that measurement light might look like. And what we do is we have a parallel beam of monochromatic light that is passed through a suspension. The diffracted light is then focused on to a photosensitive ring and then the intensity that is measured at the detector as a function of the angle is used to estimate the particle size distribution based on what is known as the Mie theory. Typical range of this measurement is .04 to 2000 microns. So pretty good range overall. But the measurement volume is limited by the width of the laser beam typically 10 to 25 millimeters. So there’s some of that. The really nice thing about this approach is we can run a lot of samples through and process a lot of samples. But just like any other method It does have its challenges. So because of the strong dependency on the particle shape and orientation, several authors have argued that the laser diffraction method underestimates the amount of clay particles by 20 to 70%, relative to the pipette method. And this has to do with the fact that clay particles are not perfect spheres. And depending on what part of the clay particle you’re measuring, it may look bigger than what it actually is, depending if it’s the flat side or the short side of the clay particle. As we all know, clay particles are typically flat plate like particles. And so that can induce error. There’s also the high cost of the instrumentation along with the uncertainties in the correction factors that make this method less attractive. But when you do have a large number of samples that you need to run through, this can be a really nice approach then if you can have the budget to get a more expensive tool like this. Another commonly used optical method is visible near infrared spectroscopy. Now, there are many laboratory and portable spectrometers available that allow for fast measurements. You can see some examples of those on the right. And the way this works is, you’re going to have a light source that’s shining light in the visible and near infrared spectrum. So visible is 350 to 760 nanometers, and the near infrared light is 760 to 2500 nanometers. So it’s passing that light, and it’s reflected onto the sample. And then that reflectance off the sample is measured. And we call that the spectrum that gets reflected off that sample. And you can measure the peaks in that spectrum to correlate them with different things, for example, clay content being one of them. But you can also use it to calculate things like organic matter, organic carbon, soil moisture, mineralogy also plays an impact in the spectrum that gets reflected off the sample. And that spectral response is then analyzed using multivariate calibration models. Now, as you can imagine, that means you need to have a strong calibration model to support that measurement. So what are some of the challenges with visible near infrared spectroscopy? Well, the performance is highly dependent on the model that’s built into it. And typically, it takes a really big library to build that model out. And there are other factors that can impact performance, i.e. soil moisture, carbonates, I mean, there’s a lot of things that play a role there. So as long as you have a really strong model built around that then you can work around those factors. Oftentimes, I think we find that you can be more accurate with this method, if you prep the sample in the lab, because it reduces removes some of those sources of variability by air drying the sample and keeping that more consistent. And so sample prep may still be needed. It’s not the same type of sample prep that you would have with some of the other methods, you don’t have to pretreat the samples. But so it does allow for a higher throughput as well of samples to get the clay content and to get the soil particle size analysis. Now you’re not going to get high detail, like you would with the other methods of the fine clays for example, you’re just going to be able to get things like total clay content. And silt and sand fractions. So in summary, you know, there are many methods available to measure particle size distribution, all of them with their advantages and disadvantages. And really, it’s important that you just take into account what are your goals, and what works best for you, in your research, to pick a method that’s gonna work best for characterizing your samples. And also, again, you can present soil texture data in many ways. So it’s really important that you take into account your audience and who you’re presenting this to, to make sure that it’s easily digestible and understood by them. And with that, thank you, everybody, and take some questions.

BRAD NEWBOLD 34:36
All right. Thank you, Leo. So yeah, we’d like to use the next 10 minutes or so and we’ll see how far we get to take some questions from the audience. Thank you to everybody who sent in questions already, and there’s still plenty of time to submit your questions, if you’d like, and we’ll try to get to as many as we can before we finish. Also, if we do not get to your question during this live webinar, we do have them recorded, and we will be able to get back to you via email to answer your question directly, either Leo or somebody else from our METER Environment team. All right. So let’s see, this first question here is asking about I guess there’s a general theme in a lot of the questions about value of these methods and instruments. I don’t know, if you have a personal ranking, or something along those lines, but what are some of these methods that might be the best value when it comes to, you know, cost, numbers of instruments you have to do, the time that goes into it, the ease of use, and all that kind of stuff?

LEO RIVERA 35:49
Yeah, you know, one of the most commonly used methods we talked about is, again, the hydrometer method, because it’s so cheap, but there’s a lot of potential error in it. And we, you know, when we talk about cost, we don’t always take into account the labor costs behind making those measurements. So the hydrometer message is what we teach in your intro to soils class and when you’re learning how to do soil particle size analysis. But there are other also cost effective ways out there. And I think, you know, both pipette takes a little more skill. And then tools, like the ISP method, that I think are all really cost effective approaches, and they all rely on the same principles. And so it really just depends on what your goals are. I typically like to have something that’s going to give me as much detail as possible. That’s what I’ve loved about working with some of the tools that I’ve gotten to work with over time is, as I get more detail in what I’m trying to characterize, whether it’s soil particle size analysis, or soil moisture release curves, whatever, I learned that there’s more information in that detail on the curve. And so, you know, having tools like the PARIO are costs that have been cost effective, and also help us characterize things a little bit deeper and have a better understanding of what it is that we’re trying to understand about soil properties. So yeah, so things like that I’ve, you know, most of the sedimentation methods are probably the most effective or cost effective way to make these measurements, especially because they don’t require any models behind them to be more accurate.

BRAD NEWBOLD 37:41
Do you have do you have any idea of like price range when it comes to these different methods?

LEO RIVERA 37:47
Yeah, for sure. So you know, like we said, the hydrometer method, typically a hydrometer is less than $100. And then you have to buy a sedimentation cylinder, which also it’s been a while since I bought one, it’s probably around $100 to $150. And then there’s also some of the pretreatment that goes into it. So there’s some investment there. Typically, if you’re looking anywhere from $300 to $500, in total investment. Then there’s tools like the PARIO, which typically are around $1,500, to get going with that. If I’m remembering that correctly. And sorry, sales team, if I’m misquoting that. But then when you look at tools like the optical methods, you’re getting up into the $30,000, $40,000, $50,000 range for some of those optical methods.

BRAD NEWBOLD 38:37
All right. Okay, there are some questions in here about pretreatment of samples. Could you go into a little bit more detail about the different methods and some are asking about pretreating, any advice for organic matter, as well?

LEO RIVERA 38:55
Great question. And yeah, I didn’t dive super deep into that. So, one of the most common— pretreatment again involves three things, dispersing, and then removing things like organic matter, iron oxides, and carbonates. Iron oxides and carbonates are a little trickier. Organic matter is fairly easily removed using hydrochloric acid. Is that right? Hydrochloric acid? Yes. And then or a strong hydrogen peroxide, I believe, is one of the most common ways. I like to refer to the methods of soil analysis, I’ve found that those methods for removing organic matter have been the best. And it just takes a little bit patience. When you add the acid, you see the organic matter starting to burn off. And yeah, so it just takes time. I like to try and determine, do I need to do that because it is a pain if I have to remove organic matter. And usually you can do that by looking at maybe a, If you have information from web soil survey or something on what the typical organic matter of that soil is, but it is nice if we can avoid having to do that. But then after that, yeah, the most important ones are the physical and chemical dispersion, beyond that. It’s not as common that you have to remove iron oxides and carbonates, depending on where your sample is coming from. But again, you can get a little more— if you have solid characterization data, you can determine what you need to do there.

BRAD NEWBOLD 40:30
A follow up to that organic matter one, they’re asking about volcanic areas with very clayey soils, they’re saying up to what amount can it be no longer a good idea to? Or when should they stop applying peroxide?

LEO RIVERA 40:45
Yeah, so volcanic soils are a little trickier. And I have personally never had to make measurements of volcanic soils, but the minerals that are contained in volcanic soils make things a little bit harder. And so, I don’t know the answer to that, to be honest. I think volcanic soils oftentimes provide many other challenges. And I’d have to look a little bit more into the some of literature on people that have made those measurements in volcanic soils.

BRAD NEWBOLD 41:19
Alright, I know we often get questions about the clay portion, clay fraction, this individual is asking about, are the instrumentation being worked on to quickly characterize the sand portion?

LEO RIVERA 41:32
That is a great question. They’re really still reliant on the tried and true sieve analysis when it comes to the sand fraction. Sadly, there hasn’t really been any advances in the technology to quickly characterize that. Although I found that the approach that I like to use that works that I found has to be the easiest, if I’m just interested in the total sand fraction. When I’m done making doing my sedimentation based measurement, I will then just drain that sample into a 53 micron sieve, and wet wash the fines away and that leaves behind just your sand fraction. And then you just take that and put that into a beaker, put it into an oven and get your sand fraction that way. I found that to be the quickest approach. If I’m just interested in total fraction, then if from there, if I want to break it down, I’ll take that dried sample and run it through a series of sieves to break it down into the, say the fine sands, the middle sands, and the core sands or however you want to break that down with your different size sieves.

BRAD NEWBOLD 42:40
Alright, let’s see. I think we have time for a couple more questions. This individual’s asking about repeatability. Any thoughts on methods with best repeatability?

LEO RIVERA 42:53
Yeah, that’s a great question. Repeatability is always a challenge. Especially, there’s a couple things that can impact. Obviously, the way you process the sample can impact that. And then the way that you’re making your measurements. So the one thing I think that helps improve repeatability is automating the measurement. That’s why, you know, we’ve developed tools like the PARIO, because it removes that human error component of it. And with the improvement with ISP plus method, we’ve really seen that that’s really increased the repeatability of the measurement, as long as your pretreatment process is consistent. But with manual methods, like hydrometer and pipette, you can see a slight decrease in the repeatability because, again, you’re introducing that human error component of it.

BRAD NEWBOLD 43:49
All right. I think we will do two really quick here. This one here is asking if I have a PARIO can I upgrade it to run the ISP method?

LEO RIVERA 44:02
Yeah. To run ISP plus method? Yeah, you can. So the nice thing about what we did there with ISP plus method is all it was was a modification to the cylinder and not to the center itself. So if you have a PARIO with that is built on the original ISP method, all you need is a new cylinder, and with the drain valve in it, and then the software update, and you can run the ISP plus method. So it’s a pretty simple change.

BRAD NEWBOLD 44:39
All right, I think this is going to be our last question here. So, and Leo, you’re gonna have to put on your futurist hat with this one, but first of all, they’re asking, Can we use satellite derived products or VisNIR spectroscopy with the same level of confidence as a pipette method? And then second, Which method is going to be more popular in the coming future?

LEO RIVERA 45:04
Yeah. So that’s a great question on the satellite derived products. I would say, it’s probably going to be challenging because I don’t think you get down to the scale that you need to look just at the soil, when looking at VisNIR data from satellites, and there’s a lot of things that impact that. But depending on the scale, if you are looking at the soil on its own, there maybe is a way that you can do that. I think you’d have to look at the spectrum that’s getting, you know, reflected, and then have a model. Again, it takes, you need a good model to build that multivariate analysis behind the VisNIR data. So I don’t know, I highly doubt that you can get to the same level of confidence as a pipette method with that type of method approach. I don’t even think you can get to that with a hand scanner, where you’re scanning the soils, you’re not going to be as accurate as you are with a pipette method using VisNIR. I think as we move towards the future of these measurements, I do think we’ll see advances in the VisNIR approach, I think it’ll get better and better, especially because you can scan so many samples. But I think if you’re trying to really characterize the actual particle size distribution curve, we may see more lower cost optical methods that are built around the sedimentation approach, which we’re seeing some research being done on that now. And hopefully, we’ll continue to see improvements with tools like the PARIO, where we can continue to make those measurements better as well. But yeah, that’s kind of where I see things going. I don’t know if we’ll ever see laser diffraction get better than what it is now, because it’s really dependent on the orientation of the particle. And that’s going to be a challenge. So yeah, we’ll see where it goes.

BRAD NEWBOLD 47:01
Awesome. Thanks, Leo. That’s going to wrap it up for us today. Thank you, again, everybody for joining us. We hope that you enjoyed this discussion here. Thanks again, for all those great questions that you submitted as well. 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 for more information on what you’ve seen today, please visit us at metergroup.com. 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.