Managing salts: Why you should care more

Mismanagement of salt applied during irrigation ultimately reduces production—drastically in many cases. Irrigating incorrectly also increases water cost and the energy used to apply it. Understanding the salt balance in the soil and knowing the leaching fraction, or the amount of extra irrigation water that must be applied to maintain acceptable root zone salinity is critical to every irrigation manager’s success. Yet monitoring soil salinity is often poorly understood.

Measure EC for consistently high crop yields

In this webinar, world-renowned soil physicist Dr. Gaylon Campbell teaches the fundamentals of measuring soil electrical conductivity (EC) and how to use a tool that few people think about—but is absolutely essential for maintaining crop yield and profit. Learn:

• The sources of salt in irrigated agriculture
• How and why salt affects plants
• How salt in soil is measured
• How common measurements are related to the amount of salt in soil
• How salt affects various plant species
• How to perform the calculations needed to know how much water to apply for a given water quality

Next steps

• See SlideShare
• Access the Complete guide to soil electrical conductivity
• Explore the TEROS 12 soil moisture, electrical conductivity and temperature sensor
• View past webinar: Using Electrical Conductivity Measurements to Optimize Irrigation
• Request a ZENTRA Cloud live demo

Questions?

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

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• Request a quote

Presenter

Dr. Gaylon S. Campbell has been a research scientist and engineer at METER for over 20 years, following nearly 30 years on faculty at Washington State University. Dr. Campbell’s first experience with environmental measurement came in the lab of Sterling Taylor at Utah State University making water potential measurements to understand plant water status.

Dr. Campbell is one of the world’s foremost authorities on physical measurements in the soil-plant-atmosphere continuum. His book written with Dr. John Norman on Environmental Biophysics provides a critical foundation for anyone interested in understanding the physics of the natural world. Dr. Campbell has written three books, over 100 refereed journal articles and book chapters, and has several patents.

Transcript

BRAD NEWBOLD 0:09
Hello, everyone, and welcome to Soil Electrical Conductivity Managing Salts for Sustained High Yields. Today’s presentation will be about 30 minutes, followed by about 10 minutes of Q&A with our presenter, Dr. Gaylon Campbell, who I’ll introduce in just a moment. But before we start, we’ve got 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 want us to go back or repeat something you missed, no worries. We’re recording the webinar, and we’ll send around a link to the recording via email within the next three to five business days. All right, let’s get started. Today we’ll hear from Dr. Gaylon Campbell, who will discuss the fundamentals of measuring soil electrical conductivity to manage salt and soil. Dr. Campbell has been a research scientist and engineer at METER for over 20 years, following nearly 30 years on faculty at Washington State University. His first experience with environmental measurements came in the lab of Sterling Taylor at Utah State University, making water potential measurements to understand plant water status. Dr. Campbell is one of the world’s foremost authorities on physical measurements in the soil plant atmosphere continuum. His book written with Dr. John Norman on environmental biophysics provides a critical foundation for anyone interested in understanding the physics of the natural world. He has written three books, over 100 refereed journal articles and book chapters, and has several patents. So without further ado, I’ll hand it over to Dr. Campbell to get us started.

GAYLON CAMPBELL 1:43
Hey, thank you. Thank you for being with us today. I visited the city of Yinchuan in the north of China in the early 1980s. Yinchuan is on the Yellow River where irrigation or irrigated agriculture has been practiced for 1000s of years. This was just before Christmas, and I saw a lot of light there. But it wasn’t snow. Everything was covered with salt. Unfortunately, that’s the fate of too many places where irrigated agriculture has been practiced for a number of years. The earliest irrigation systems of those were established in the Fertile Crescent, at the dawn of civilization, eventually succumb to poor drainage and accumulated accumulations of salt. Many later ones have seen a similar fate, as we’ll see salt is in the water that we apply for irrigation. And if the salt and the water are not managed properly, the salt will build up in the soil, decreasing yields, and finally ruining the land for agricultural production. The costs of this mismanagement are enormous. A recent calculation that I saw just for the Central Valley in California was a billion dollars. So it’s worth it to know what’s going on and to manage for sustainable production. But how do we do that?

GAYLON CAMPBELL 3:22
Dr. Richard Stirzaker, a CSIRO soil physicist in Australia, published a book in 2010 called out of the scientists garden. He calls it a story of water and food. In it, he talks about a Goldilocks principle that relates to his own career path in water management research, he started out focusing on instruments for monitoring routes on water content, in order to manage irrigation. Now, I explained the use of these kinds of instruments to you in a seminar that I gave a couple of months ago here. The correct measurement of water content and especially of soil water potential are important for proper day to day irrigation management. But they don’t give much insight into sustainability. They’re too fast. He then studied salt and groundwater in rivers. And again, this is important information for an irrigation project, but it’s too slow for management decisions. By the time you see those responses you’ve already lost the battle. Dr. Stirzaker finally started focusing on salts in and just below the root zone. And this was just right to provide the information needed for proper sustainable management of irrigation. So today we want to talk about how to make those measurements.

GAYLON CAMPBELL 5:18
The problem as I said is that salts are in the irrigation water. When we irrigate, those salts enter the soil with the water. The water leaves the soil mostly by evapotranspiration, the water can evaporate, but the salts can’t, and so they stay in the soil.

GAYLON CAMPBELL 5:45
Now, there are other important sources of salts in the soil. We apply chemical fertilizers to crops, the crops take up some hopefully most of those but some usually remained. Groundwater also contains salt. If the water table is sufficiently close to the surface, evaporation will bring the salts to the surface over irrigation that brings the water table to the surface more rapidly and therefore is one of the main causes of salinization in irrigation projects. Now this was the source of the salt that I saw in Yinchuan. Drainage systems can drop the water table and mitigate this problem. But it’s better and cheaper when possible to just avoid that by proper irrigation in the first place.

GAYLON CAMPBELL 6:42
Why do we care if salts build up in the soil? In the last lecture, we talked about water potential, and the fact that gradients and water potential are the driving force for water movement from the soil through the plant to the atmosphere. The water potential can be expressed as the sum of several components. The most important for plants are the osmotic and the matric components. We talked about it the matric component, and its measurement in the last seminar depends on how wet the soil is and the makeup of the soil and its texture. The osmotic component depends on how much salt there is in the soil. As the soil dries, water availability to the plant decreases, both because the matric potential decreases, but also because the salts are becoming more concentrated, and the water more difficult to remove from the soil. Crops vary in their sensitivity to osmotic stress.

GAYLON CAMPBELL 8:04
Here’s a list of crops organized according to their sensitivity to that kind of stress. To those of you who are experienced in irrigated agriculture, a list like this isn’t any surprise, you know where the salty places are in the field, and you know what will grow and won’t grow there. We have records of crop production in those early irrigated fields of Mesopotamia, they were kept on clay tablets. Early records show that lot of wheat was produced in those areas. And you notice that wheat is in the moderately tolerant column in our table, but that later on, only barley would grow. And you can see that that’s in the tolerant crop column.

GAYLON CAMPBELL 9:06
Now to quantify how salty the soil is, we measure its electrical conductivity. The old units for electrical conductivity were millimoles per centimeter. Conductivity is a reciprocal resistance, Mole is lome spelled backwards. And so it was used as the unit of conductance that has now been replaced by the unit siemens to keep the numbers the same size as they were in that old system. We use units of decisiemens per meter and data on crop production. For each of the four groups of crops that we saw on the last slide are organized on this graph. We can see for a particular crop and a particular electrical conductivity, we can see which group would thrive or survive there, and what the yield reduction might be for a particular value.

GAYLON CAMPBELL 10:17
It might be obvious, but it’s really important to understand that the more salt you have in the water, the higher its electrical conductivity. In this graph, I showed that relationship for sodium chloride. Now, of course, there are other salts in the soil. And the relation between those salts and the electrical conductivity will be a little bit different than this. But since sodium chloride is the most important one, this gives a pretty good idea for the host. The other thing that I want to show here is that the relationship between electrical conductivity and osmotic potential. You see the electrical conductivity on the left axis, osmotic magnetic potential on the right, and a direct correspondence between the two. They’re about minus 40 kilopascals per decisiemen per meter. Now, you might remember that in our last seminar, maximum potato production was achieved by keeping the soil wetter than minus 100 kilopascals. 100 kilopascals is around two and a half decisiemens per meter, as you can see on the graph here, and so if we have concentrations, osmotic potentials that are more negative then that we could expect, at least in sensitive plants to see a reduction in yield or in production. And that gives us a good kind of benchmark to understand the effects on production.

GAYLON CAMPBELL 12:22
Now I’ve been talking about electrical conductivity, without being very specific about what I mean by that term, and I want to get a lot more specific now. I will refer to three kinds of electrical conductivity. And it matters a lot which is which so it’s important that you understand which is which. The first is the bulk electrical conductivity EC sub b. If I stick one of our TEROS 12 sensors into the soil and measure the electrical conductivity of the soil, that measurement will be the bulk EC of the soil. Now, that doesn’t mean much by itself, but it along with other data can be converted into much more useful numbers. However, it has somehow squeezed the water out of the soil and measure the electrical conductivity of the water that I get out. We would call that the pore water or soil solution EC sub w. That’s the electrical conductivity or the equivalent osmotic potential that the plant sees and it determines how stressed the plant is. The final of the three electrical conductivities is called the saturation extract EC sub e. You get that by saturating a soil sample with distilled water, squeezing some of the water out of the soil and measuring the electrical conductivity of the water. When someone talks about the EC of soil, that number is the one that they mean. The EC values we associate with the sensitive, moderately sensitive, moderately tolerant and tolerant lists of crops. We saw on the previous slide. The EC that we mean is the saturation extract EC. That value has been used to classify soils for many years. It may seem a completely arbitrary way to get a number, but it turns out to be brilliant. The exact value we need for Strizaker’s Goldilock measurement.

GAYLON CAMPBELL 14:54
I hope with this simple dimensional analysis to convince you that the saturation extract EC is a fundamental property of soil. We start with the kilograms of salt per cubic meter of water in the soil. So that’s the pore water. And if we multiply that by the water content of the soil under cubic meters of water per cubic meter of soil that comes out to have units of kilograms of salt per cubic meter of soil. So that’s the salt content of the soil, fundamental property of the soil. And the electrical conductivity of the pore water is a measure of the amount of salt in the water. And if we multiply that by the volumetric water content of the soil, we’ll get an electrical conductivity that’s proportional to the amount of salt in the soil. The ECW, the pore water EC becomes saturation extract EC, when the soil is saturated. And since the electrical conductivity, the pore water conductivity, and the water content vary reciprocally, the value on the right hand side of this equation will be constant for soil as long as the salt content doesn’t change. You can get the same number by taking that same number by taking a soil sample, saturating it with distilled water and squeezing out some of that water and measuring its electrical conductivity. That’s the way that has been done for years and years. But that’s an awful lot of work, especially if you have to cover several thousand acres, it’d be a lot better and a lot easier if you could just put a sensor in the soil and that sensor would tell you what that saturation extract EC is.

GAYLON CAMPBELL 17:11
Now to help fix some of these ideas in your mind I want to go through a couple of thought experiments with you. Let’s consider what happens to the salt in the soil during two processes that occur all the time in the soil. One is the redistribution of water following initial infiltration and the other is evapotranspiration. In redistribution, water and salt move to deeper depths in the soil. So the water content of that initially wetted zone gradually decreases and the salt content also decreases. With evapotranspiration, the water content decreases, but the salt stays in the soil so the water content changes but the salt content stays constant. Let’s for this thought experiment assume that the soil has 50% pore space and the saturation extract EC is one decisiemen per meter. Now here we’re showing the electrical conductivity versus water content.

GAYLON CAMPBELL 18:25
For three values the pore water electrical conductivity EC sub w, the bulk electrical conductivity is the blue line at Ecb and then the circle is the saturation extract EC. We can see as the water moves deeper in to the soil both the salt and water are lost from the initially wetted level at the soil. And so the pore water EC stays constant. But both water content and solid content decrease. If we took a soil sample, added distilled water to determine the saturation extract EC, it would be lower than the initial one that we see here because the salt content is decreasing in the layer. Since both water and the solid are decreasing, the bulk nitric conductivity decreases pretty rapidly to a very low value as the water content decreases.

GAYLON CAMPBELL 19:45
Now the picture is quite different for the amount of transpiration. Here the water content decreases while the salt stays constant. So the pore water EC increases dramatically from that saturation value.Bulk EC decreases less rapidly than it did in the previous example because only the water is being lost and the salt is staying there and being concentrated. If we were to stop at any point along the black line, we could add distilled water until the soil saturated again and we would end up back at that saturation extract circle. The black line is therefore a function that we know and can always follow to get to the saturation extract EC from the pore water EC. So if we can somehow find a way to go from bulk to pore water EC then we can go to the saturation extract EC.

GAYLON CAMPBELL 20:54
I want to use these pictures to help you understand why the bulk EC is always less than or equal to the pore water EC. In the left hand picture we have just water with salt in it. Nothing in that impedes the flow of electricity when we make the measurement and so the bulk electrical conductivity is equal to the electrical conductivity to the poor water EC. Now if we add soil particles, the cross section for flow of electricity is decreased by the presence of the soil particles and the length of the path that the electricity has to flow is increased. In a typical saturated soil leaves the fact reduce the bulk electrical conductivity to about a third the electrical conductivity of the pore water. Now if the soil desaturates, the panel on the right then airspaces appear in the soil and so the cross section is even further reduced in the distance that the electricity has to travel is increased even further. And so that ratio is even bigger, I show it as a value of 10 here, but of course the drier the soil gets the bigger that gets.

GAYLON CAMPBELL 22:33
So this graph shows the multiplier. An example of multipliers that you can use to multiply the bulk EC to get the pore water EC as a function of the water content of the soil. And for wet soil that number as I said is something around three, you can see that here. And then as the soil dries out the value increases rapidly. And the uncertainty also increases rapidly. So besides the if you want to go from bulk EC to pore water EC, you can see from this graph that you need to know both the bulk EC and the water content of the soil to do the calculation. The TEROS 12 sensor gives both measurements and so it’s easy to do that calculation here. But you can see that you wouldn’t want to do the calculation for very dry soil or the calculation becomes pretty uncertain. Because of the large value the multiplier field capacity is typically around half of saturation. Saturation here we show is 50% and so probably below about 25% water you would not want to address the calculation you’d get from this.

GAYLON CAMPBELL 24:15
I want to say just a few words about leaching fraction. It’s the ratio of the amount of water draining out the bottom of the soil profile to the amount of water that we apply. If you go through the calculations in some detail, you’ll see that that definition is equivalent to the ratio of the electrical conductivity of the irrigation water to the electrical conductivity of the drainage water. But the electrical conductivity of the drainage water is the EC of the saturation extract because the water drains the saturation. This calculation is normally used to compute the amount of water we need to supply in excess of crop requirements to maintain some desired electrical conductivity in the root zone. So if we were supplying irrigation water that was .3 decisiemens per meter, we wanted to maintain a root zone saturation extract EC of three decisiemens per meter, we’d need to apply 10% more water than the crop uses. But we could turn that calculation around and apply it the other way, we could measure the EC of the irrigation and the EC of the saturation extract EC of the soil below the root zone and know how much water we’re to be losing too deep drainage. I think that gives you a little bit of insight into why Dr. Stirzaker said knowing the EC and below the root zone is that Goldilocks principle.

GAYLON CAMPBELL 26:10
To give a feeling for electrical conductivity of irrigation waters, some of them have this table. It gives I think some valuable insights; you could irrigate with Columbia or Sacramento River water for quite a long time without much concern for salt problems. But if you were irrigating from the Pecos, you’d have to be pretty careful and probably grow barley or sugar beets.

GAYLON CAMPBELL 26:49
Now at this point, you should have a good understanding of the principles involved in managing salt and water in irrigated agriculture. But how do you apply those principles? You can’t see the salt in the irrigation water. And by the time you can taste it, water wouldn’t be much good for irrigation, you won’t know the state of salts in the soil until the crops start to fail. And then it’s too late. You need a way to measure the salt. And here are some excellent tools to help you do that. The ES-2 measures the electrical conductivity of water, and it does it very accurately. The TEROS 12 measures the water content and bulk electrical conductivity of soil. These are plugged into the ZL6 logger that connects to ZENTRA Cloud, the data flow to the cloud through a cellular connection. And the computations are made in ZENTRA Cloud to give you pore water EC and the electrical conductivity of the irrigation water.

GAYLON CAMPBELL 28:02
So here’s our scenario, the TEROS 12 measures water content and bulk EC, which goes to the ZL6 and ZENTRA Cloud and is converted to pore water EC. The pore water EC and the water content are used to get saturation extract EC. The value at the bottom of the root zone is Strizaker’s Goldilocks measurement. We can use it for crop suitability, crop loss calculations, or to estimate drainage losses. For example, let’s say that the saturation extract value that we got was five decisiemens per meter right here. And let’s say that we wanted to grow strawberries, which are a sensitive crop. And so we’re interested in this line here. So we would go up from the five decisiemens per meter. And we’d see that the relative yield that we would expect for that salty soil for a sensitive crop is something like 60%, about a 40% reduction in yield. Now that information would be useful for us, we would determine whether maybe strawberries isn’t the best choice of crop to grow here, or if it is, if it’s the one that we want to do, we’d know what to expect for a yield. We might decide that we need to reduce the electrical conductivity of the soil, some by leaching some water through it so that we could get increased production. And then the last point I show on the slide here is that leaching fraction calculation done backwards, they emphasize the idea that we can use those calculations to determine the fraction of the water that we apply that’s going up the bottom of the profile.

GAYLON CAMPBELL 30:18
At the last seminar I gave, I showed some data obtained from experiments that were a collaboration of METER scientists with faculty at Brigham Young University in Provo, Utah, and a very progressive farmer in southern Idaho. These data are from that same farm. But here we’re computing the saturation extract electrical conductivity values under irrigated wheat. I’m gonna show three levels here 15, 45, and 65 centimeters or six inches, a foot and a half, and little over two feet. Remember this picture is the salt content of the soil at the level of measurement. The shallowest level is in blue, a couple of spikes are shown here. Those likely are the result of nutrient additions from fertigation. The middle damps that’s shown in orange shows a couple of those bumps but a lot smaller. But in general, the electrical conductivity at this level is decreasing. And that likely is because of plant uptake of nutrients. The lowest level shows a slight increase over time, possibly from downward movement of salts. The irrigation water had an electrical conductivity of 1.1 decisiemens per meter. If that were the only source of water, the leaching fraction that we would calculate here would be 25%. But for this farm, probably about half of the water that is used by the crop is precipitation. And precipitation has essentially no salt. And so we probably would divide that irrigation electrical conductivity in half, meaning that the deep drainage is maybe between 10 and 15%. Now, this record is too short to make any grand predictions about general trends and sustainability. But if we had a record that went over 10or 20 growing seasons, we’d have plenty good idea of what our practices were doing to the sustainability and what we need to do to make adjustments to be more sustainable. So I’d conclude with these thoughts. First, the obvious one that mismanagement of salt in irrigated agriculture is costly.

GAYLON CAMPBELL 33:15
Goes without saying but monitoring the saturation extract EC at the bottom of the root zone is the adjuster height measurement for long term management of irrigated agriculture. And the final point I’d like to make is that METER has the tools to provide the right measurements for modern irrigators to choose the right crops and to manage water sustainably. Thank you. All right.

BRAD NEWBOLD 33:49
Thank you, Dr. Campbell. And thank you, everybody, for participating for joining us today. We’ll take we’ve got some time for some questions. And we’ve got several questions that have come in already. And we appreciate you for all those questions. If you again, there’s still time to enter your questions into the questions pane. And we will see then, we’ll try to get to as many as we can. Depending on the time constraints that we have, we’ll see how many we get to, there are quite a few that are coming in and have come in already. If we do not get to your question, I just want to put this out there. If we do not get to your question, we do have them recorded. And one of our experts here at METER will be able to get back to you via email to answer your question directly. So first question, Dr. Campbell. This question they’re asking about salinity stress. So their question is, are there also positive effects of salinity stress like being a trigger for defense mechanisms to biotic stresses? And I think maybe, on the other side, or along with that, are there any crop plants or plants in general that do better or thrive in more saline conditions?

GAYLON CAMPBELL 35:10
Certainly, that’s a really good question. One of the most interesting areas in plant water relations to me at least, that the crops can be steered by adjusting stress and we talked about this in earlier seminars, but in for example, in wine grapes and in fruit crops, by managing stress, you can control the vegetative versus reproductive growth, and that’s done typically and with matric stress in soils, but in soilless media, in greenhouse production, why you can’t use matric stress, you have to use osmotic stress. And so all of that staring in greenhouse production is done by manipulating the osmotic stress.

BRAD NEWBOLD 36:17
This next question is a combination looks like we’ve got about three different questions squished into one so I’ll try to break it down to make it a little bit easier. So in evapotranspiration example, where you’re plotting water content versus EC, you show the bulk EC decreasing slightly. Is that because of the trade off between increasing pore fluid EC during evaporation, versus decreasing water content? That’s the first question.

GAYLON CAMPBELL 36:44
Yeah, as the water content decreases, why that if the salt content stayed constant, that would decrease the bulk EC pretty rapidly but since the salt concentration is increasing as the water content decreases, it doesn’t decrease as rapidly.

BRAD NEWBOLD 37:07
And so in practice, does this depend on the particular field site and soil such that in some cases, you may see a stronger decrease or even an increase in that relationship?

GAYLON CAMPBELL 37:19
You would never see an increase. And that relationship does depend to some extent on the soil but not strongly on the soil.

BRAD NEWBOLD 37:34
Okay. Next question here, what management practices can be implemented to balance the needs that plants have for irrigation water and keeping the saturation EC low enough for sensitive crops?

GAYLON CAMPBELL 37:49
Well, that’s where the leaching fraction comes in. Depending on the quality of irrigation water that you can apply, or that you’re applying, you have to apply enough water to leach the salts out the bottom of the soil profile, or at least out of the root zone. And then you have to usually provide for drainage to get that waterway so the water table won’t increase.

BRAD NEWBOLD 38:24
Another question here, this one. How might somebody who’s studying coastal agriculture go about studying EC where you have the effects of seawater, and then maybe along the same lines, where you might have crops that are at or near the water table just in regular settings as well? Any insights for there?

GAYLON CAMPBELL 38:51
I’d go to Israel to study, by far the leading place in the world for understanding how to deal with irrigation with high EC waters.

BRAD NEWBOLD 39:11
So in speaking of that Goldilocks zone about trying to find and measure EC, right at the base of the root zone, where might people find references for root zones of specific plants?

GAYLON CAMPBELL 39:34
I can’t say off the top of my head, there’s, I mean, there. I’d start on the internet. There’s a lot of information out there about that, but I can’t tell you specifically.

BRAD NEWBOLD 39:49
And do you have any insight on any remediation tools that are available to reduce solidity and soils?

GAYLON CAMPBELL 39:55
Well, the only way that I know of that is done is through drainage and leaching. And you need good measurements in order to know what’s happening and what you’re doing with that.

BRAD NEWBOLD 40:15
All right. I think we’ve got time for maybe one more question. We also have a number of questions regarding potted plants. We have several experts who can answer those questions directly. So we probably won’t have time to get to any of those questions here. Just wanted to let you know for those of you who have asked questions about dealing with EC and potted situations. Let’s see, we’ll find one last question here. Not only might well, you stated that depending on the plant type, some plants are more tolerant, more sensitive to salts, some do better or do worse in salty conditions and soils. Do we know, is there any impact on seed quality when it comes to that where the plants might be able to they might seem like they’re doing okay, but the seeds themselves might do better or worse.

GAYLON CAMPBELL 41:16
Well you know it’s a good question, but a complicated one. Depending on when the plant is stressed, and how much is stressed that all has a big effect on how assimilates are partitioned. And eventually what effect that would have not only on the whole plant, but on the seed that’s produced. Okay.

BRAD NEWBOLD 41:49
Well, that’s gonna do it for us today. We’re going to need to wrap up there, we still have a ton of questions that we did not get to. So again, I just want to let you know that that we do have your questions recorded and somebody from our METER team will be able to get back to you directly via email, and answer your questions. We hope you enjoyed this discussion as much as we did here. Thank you again for your questions, and please consider answering the short survey that will appear after this webinar is finished. Just let us know what types of webinars you’d like to see in the future. For more information on what you’ve seen today, also visit us at metergroup.com. And finally, look for a link to the recording of today’s presentation in your email. And stay tuned for future METER webinars. Stay safe and have a great day.