Part 2: Nutrients and Osmotic Stress

Rev up your productivity

If you’re crop steering to optimize quality and productivity, understanding nutrient concentration is critical to stressing your plants correctly. If nutrient concentrations get too low, you won’t get the production you’re paying for with the rest of your infrastructure. If the concentrations are too high—you’ll risk killing your plants.

Measure. Don’t guess.

You can’t quantify nutrient concentration just by looking at your plants or tasting them. The only way to know the nutrient concentration is to measure it. Crop steering can only be done if you know the electrical conductivity (EC) of the nutrient solution in the growth substrate. In this 30-minute webinar, world-renowned soil physics expert, Dr. Gaylon Campbell discusses how to measure EC and osmotic stress to optimize crop steering for maximum yield. He’ll cover:

• Environmental control of growth and development in plants
• Electrical conductivity as a measure of nutrient concentration in the growth medium
• Techniques for measuring pore water electrical conductivity in unsaturated media
• How to relate pore water electrical conductivity to the bulk conductivity being measured
• Crop steering using osmotic stress and how to monitor that stress

Next steps

• See SlideShare
• See part 1 and part 3 and part 4 in this webinar series
• Access the Complete Guide to Soil Moisture
• Explore the TEROS 12 soil moisture, electrical conductivity and temperature sensor
• View past webinar: Soil Electrical Conductivity: Managing Salts for Sustained High Yields
• Request a ZENTRA Cloud live demo

Questions?

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

• Talk to an expert
• Request a quote

Presenter

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 0:11
Hello, everyone, and welcome to part two of our webinar series, Irrigation of Controlled Environment Crops for Increased Quality and Yield. Today’s presentation will be about 30 minutes, followed by about 10 minutes of Q&A with our presenter, Dr. Gaylon Campbell, whom I’ll introduce in just a moment. But before we start, we’ve got a couple of housekeeping items. First, we wanted 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 toward 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. Alright, with all of that out of the way, let’s get started. Today we’ll hear from Dr. Gaylon Campbell, who will discuss how to measure EC and osmotic stress to optimize crop steering for maximum yield. 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 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. 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 Gaylon to get us started.

GAYLON CAMPBELL 1:46
Okay, thank you. And thanks for being with us today for this second installment. The picture on the right, borrowed from Dr. Bruce Bugbee at Utah State University, shows the nine environmental variables or parameters that represent the main connections between the plant and its environment. It’s useful to think of some of these environmental parameters, such as light, carbon dioxide, water, and nutrients, as resources, and to consider the factors that enable the plant to capture those resources. Assimilation is the process by which carbon dioxide, water, and light from the plant environment are captured and converted to carbohydrate. To grow, the plant captures nutrients and water and combines those with carbohydrate produced by assimilation to make stems, leaves, roots, flowers, and all that makes up the biomass of the plant. Now the third process that I’ve listed here occurs simultaneously with the other two, but doesn’t directly capture resources. Development is the progression of the plant through recognizable phases. The rate of development can be influenced by the availability of resources, but it’s most strongly influenced by temperature and day length. On the other hand, development can strongly influence the plant’s resource capture, since it controls the time in each of these phases. Now in controlled environment crop production, all of these variables that I’ve listed here are under the control of the grower. It’s the job of the grower to choose the values of each of these variables to attain the goals of the production facility. To properly control these variables, the grower needs to know what they are and how they influence the outcome of the production process. The AROYA production platform is capable of monitoring these variables and reporting their values to the grower. Now in our last session, we talked about substrates and how to monitor the water, and this time, we want to focus more specifically on nutrients and stress. As we said last time, in controlled environment production, the nutrients are supplied with the water. The nutrient solution is carefully designed, is a mixture of the macro and micronutrients that are needed for plant growth. To determine the actual concentration of any given nutrient element would be a big job. But fortunately, we don’t need to do that to know that the nutrient needs of the plant are being met. The graph here shows the relationship between nutrient concentration, in grams per liter on the x axis, and the electrical conductivity of the solution, on the y axis. For two different nutrient mixes, one that’s optimized for vegetative production and the other optimized for flower, the relationship is nearly linear. So if we know the electrical conductivity of this solution, and we know what nutrients went into it, we can know the concentration of the nutrients. If the nutrient solution is properly designed, the plant will take up the nutrients in the proportion that they exist in the solution. So the ratios should remain approximately constant. The slope of the line does depend a little bit on the mix that we use. But you can see that something around three decisiemens per meter is equivalent to a concentration about three grams per liter. So roughly a one to one relationship between those two. That relationship means that we can use electrical conductivity as a surrogate for the nutrient concentration. Now in our previous webinar, we talked about the TEROS 12 probe that measures water content, electrical conductivity, and temperature, so three of the four below ground environmental variables that we talked about in the first slide. Now even the fourth of those variables, oxygen, relates closely to the water content and temperature that are measured with the TEROS 12. And so that one probe gives us essentially all of the environmental variables for the below ground plant portion. We said thay dielectric, from which we get water content, is measured between the first and second tine of the probe, the electrical conductivity is measured between the second and third tine, temperature is measured in the center tine. The electrical conductivity that we measure is the bulk electrical conductivity. The electrical conductivity that we want to know is the conductivity of the water in the substrate pores. That’s the pore water electrical conductivity, we use the symbol EC sub W, that’s what the plant sees. And that’s the one that we graphed in the previous slide. We might ask, Why aren’t they the same? Now electrical conductivity is a measure of how well a conductor conducts electricity. Salty water is a conductor. In the left diagram we have just salt or water with salt in it. Nothing there impedes the flow of electricity. And so, the bulk conductivity is the same as the pore water conductivity or ECW. But if we add soil or substrate particles to the water, like we have in the middle picture, the cross section for flow of electricity is decreased by the solids in the water, and the length of the flow path is increased. So the conductivity is decreased. Now in a typical saturated soil, those effects make it so that the bulk electrical conductivity is about one third of the pore water electrical conductivity. In saturated horticultural substrates, there’s a lot less solids present and so that ratio was larger. If the soil, if we desaturate the soil or the substrate, so that their air pockets then the cross section for flow of electricity is further reduced, the distance for the electricity to travel further increased. So that ratio of the bulk EC to the pore water EC is further increased. We show a value of 10 here, but in drier soils, of course, even much higher than that. Now the pore water electrical conductivity is always greater than or equal to the bulk electrical conductivity as we just discussed. This graph shows a typical multiplier to get from bulk electrical conductivity to pore water electrical conductivity. Of course, this relationship will depend on the particular medium that you’re working in. But for any given medium, it can be found. You can see that for dry substrate or dry soil, that multiplier gets pretty big. And you might also infer also pretty unreliable, you get below water content say of about point three or 30%, you’re multiplying by numbers like 15 or 20. And so probably you wouldn’t count on those numbers very much. They can be fairly unreliable. Use this relationship and the water content that you measure with the dielectric to do the conversion from pore— or from bulk EC to pore water EC. Once you’ve done that conversion, then you would— found that multiplier, you’d multiply that by the bulk EC to get the pore water EC. And finally, you’d apply the temperature correction to it to correct to a standard temperature to get the nutrient concentration. Now this all looks simple enough, but getting this relationship is not a trivial matter. So one of our strengths at METER, we have soil physicists on staff who know the theory and can make the measurements to obtain reliable relationships like this for whatever substrate you might be using. Both the water and the salt in the soil is dynamic. To make sense of the changes that we see, we need to have some feeling for the processes that change the concentrations of water and salt or nutrients in the substrate. Infiltration occurs when water and nutrients enter the substrate. Redistribution occurs as matric and gravitational forces move the water and nutrients deeper into the substrate following infiltration. And in both of those cases, infiltration and redistribution, the salts move with the water. Evaporation occurs when liquid water turns to vapor at the substrate surface. Transpiration is the uptake of water by the plant, its transport to the evaporating surfaces of the leaf, and its evaporation to the atmosphere. In both of those cases, the water leaves but the salts stay behind, leaving the remaining water more concentrated. The final process that I list here is the uptake of nutrients. The plant removes the nutrients from the substrate solution. The nutrients of course move to the root surface with the water and are taken up but nutrient uptake isn’t a passive process. There are membranes at the endodermis and the root, and the plant determines which ions are taken up and which are left in solution. Now let’s look at some examples of these processes to further illustrate what I’m talking about here. This is a single day’s record from AROYA. The water content from a TEROS 12 is the blue line shown on the right axis. The pore water electrical conductivity that we calculated, the way we explained previously, is the red line and it’s shown on the left axis. The yellow spikes are irrigation shots. There are 10 of them. They’re spaced about 15 minutes apart. There were 240 milliliters in each shot. All three processes that we just mentioned can be seen in this graph. But for our purposes right now, we want to focus on the time just as irrigation starts. So, look where the the yellow lines are and where the irrigation starts. Irrigation and redistribution are occurring during this time. I’m sorry— infiltration and redistribution are occurring. And as the water content rapidly increases at the beginning of irrigation electrical conductivity, and therefore, the nutrient concentration in the substrate also rapidly increase, the nutrients move with the water to replenish the substrate.

GAYLON CAMPBELL 15:56
Now, the colors and the axes are the same here as in the previous slide. And here we see two days, we don’t see the irrigations. The yellow lines are gone. But you can tell when irrigation is occurring by the water content record when the water content rapidly increases. The part I want you to notice is the increase in pore water electrical conductivity, as the water content is depleted by transpiration. So the electrical conductivity starts at about nine decisiemens per meter. And by the time it gets to the next irrigation, it’s up to almost 20. The water content during that time changed from about 50% down to 30%. Now if we took half the water out of a substrate and left all of the salt there, the salt concentration would double. And that’s what’s going on here. The dry down after irrigation we call the dry back. And that’s the tool that we use to apply controlled stress in controlled environment crops. Now the colors and the axes here again are similar to the previous ones. We’re looking at a 24 hour period of time, as before, but we’ve shifted the axis— shifted the irrigation time so that it starts toward the left end side and you can see a whole day of dry down. The water content decreases rapidly until about the middle of the graph. You can see the that, and that’s when the lights are on. And then it decreases more slowly for the rest of the day as the lights are off. The electrical conductivity increases rapidly as irrigation is applied and fresh nutrients come in. It stays fairly constant while the lights are on and then at night it drops more rapidly, presumably from nutrient uptake by the plant. Since both water content and nutrient and electrical conductivity or water electrical conductivity are changing, how can we know what’s happening to the nutrients in the substrate? Well, it turns out that the measure can be computed from these numbers that we already have, that will tell us what the amount of nutrients is still in the substrate. If we go through this dimensional analysis, we start with kilograms of salt per cubic meter of water in the soil, that’s proportional to the pore water electrical conductivity, and when multiply that by the water content and cubic meters of water per cubic meter of soil, that comes out to be kilograms of salt per cubic meter of soil. Or in other words the salt content or nutrient content of the soil. So multiplying water content by pore water EC, gives us the amount of nutrient per unit volume of soil. I’ve plotted that quantity here as the grey line. You can easily see now by looking at it that nutrients are added to the substrate when we irrigate, and they’re taken up at a more or less constant rate for the rest of the day. Now this is a pretty powerful tool that can give us some good insights about nutrient uptake.

GAYLON CAMPBELL 20:31
So let’s turn our attention now to crop steering. AROYA publishes a crop steering guide on the internet. And if you look in that, you will see the following statement. “Crop steering is a plant growth management practice that manipulates the environment (light, climate, irrigation) to encourage plants to grow a certain way. Next to light intensity, it’s the most important tactic you can use to manipulate yield.” And then they have a couple of recommendations: irrigation for optimum vegetative growth—no water stress, and irrigation for optimum generative growth—simulated water stress.

GAYLON CAMPBELL 21:27
But how do we simulate water stress in a controlled environment? Let’s review a couple of points from the last lecture to remind you about measuring water availability to plants. Water potential is a measure of the work required to remove water from a substrate. The two most important components of the water potential, with respect to plant water availability are the matric and the osmotic potential. The water potential is the sum of the two. Matric forces come from the attraction of the matrix or substrate for the water. Osmotic forces come from the dilution of the water by salts when a semi permeable membrane is present. Now, in the last lecture, we showed that, at least over the water content range normally used in horticultural media, the matric potential change is too small to be of significance to the plant. So, the important point I want to make here is if we want to simulate water stress, it has to be by manipulating the osmotic potential.

GAYLON CAMPBELL 22:53
Now, we showed earlier that electrical conductivity and salt concentration are linearly related. Here we show that osmotic potential and electrical conductivity are also linearly related. There is little scatter in this data measurements of water potential this close to zero, are kind of difficult to do. But again METER builds and sells the equipment that make these measurements. The number for you to remember, is minus 40 kilopascals per decisiemen per meter. That’ll be a useful number whenever you want to convert back and forth between electrical conductivity and osmotic potential. So a two and a half decisiemen per meter solution would have an osmotic potential around minus 100 kilopascals. This is the same graph we saw earlier to illustrate electrical conductivity changes in the substrate as the plant takes up water, but now the left vertical axis shows osmotic potential rather than electrical conductivity. Osmotic potentials are negative numbers, but it’s easier to see what’s going on if we show them as positive, so I’ve just shown the magnitude here. The osmotic potential recovers to around minus 400 kilopascals after irrigation, and then it decreases to around minus 100 kilopascals, just before irrigation. Now, what does this mean? There’s a lot of literature on the effect of matric stress on plants growing in soil. There’s also a lot of literature on effective soil salinity on crop production, but I’m not aware of much literature showing effects of nutrient solution induced water stress on growth in assimilation. A soil nitric potential of minus 100 kilopascals would start to reduce growth in many species, and a soil matric potential of minus 1000 kilopascals would stop the growth and would reduce assimilation. The nominal permanent wilting point for soil is minus 1500 kilopascals. And so this might give some rough ideas of what this range of osmotic potentials would do. I think you can see that the stress during the dry back that we’re applying here would be significant. Species and even cultivars differ substantially in their response to stress. So, I don’t think any general guideline is possible. The grower has to determine the appropriate levels by experimentation. It’s important, though, that, however, or it’s important, though, that it’s the decrease in osmotic potential or the increase in pore water EC that’s important, not the change in water content at the substrate. The dry back is important for concentrating the nutrient solution. But if the solution ECs are so low that the plant takes up the nutrients and there’s little or no increase in pore water EC, then no crop steering will occur, even with a large dry back.

GAYLON CAMPBELL 26:48
We talked last time about the equipment needed to get the data for managing nutrients and water. The AROYA nose provides the communication. The TEROS 12 makes the measurements of water content EC and temperature. The AROYA software processes, stores, and displays the data so that you can know what’s going on to make decisions. I showed this graph too in the previous webinar, but hopefully it’ll make more sense now with your additional knowledge. It’s for the whole nine weeks of a grove. Light below the canopy is shown in green, so you can see the day night cycle and that the canopy starts out sparse, letting a lot of light through, and then it quickly closes and the light below the canopy goes to a low value and stays there. Temperature is in purple, water content is blue, pore water EC is in red. Now in week two, the water content is reduced to eliminate drainage and concentrate the nutrients. Here the electrical conductivity starts to increase. Through week three, the water content is manipulated to get quite high electrical conductivity values, some of them higher than 20 decisiemens per meter. The end of week three the water content has increased sufficiently to initiate drainage and bring the electrical conductivity down to about seven decisiemens per meter where they maintain it for a couple of weeks. During week five and six, the drainage has increased even more to bring the electrical conductivity down to around five. Finally, at the end of the grow, the dry back and the electrical conductivity are increased again by dropping the average water content. Now the point in showing this is not to say that this is how every crop should be grown. The recipe for this is probably different for each species and each cultivar that you would grow. The point is to illustrate how you can do crop steering if you monitor and know what’s going on in the roots owning your crop. This steering would be impossible without good sensors, good calibrations, and a good understanding of what you’re measuring and how to manipulate it. So to conclude, we can make these points that electrical conductivity directly measures the concentration of nutrients in the irrigation water. But sensors measure the bulk electrical conductivity and what we need to know is the solution electrical conductivity. So we have to have a way of converting between the two. The water and nutrients vary over time for a number of different reasons. And we need to monitor both of those to know what’s going on in the root zone of the crop. We showed that osmotic potential and electrical conductivity are directly related to each other in that you can use electrical conductivity as a surrogate for osmotic potential. And that we can manipulate the osmotic potential to simulate stress for crop steering. Finally, that careful monitoring and control of water and nutrients is essential for optimum production. So thanks for being with us today. We hope these ideas will be useful to you in managing the irrigation of your controlled environment crops.

BRAD NEWBOLD 31:01
All right, thank you, Gaylon. And we’d like to, let’s see, use the next 10 minutes or so to take some questions from the audience. And thanks again, to everyone who sent in questions already. There’s still time to submit questions now if you’d like. And we’ll try to get to as many as we can before we finish. Just want to give you a heads up that if we do not get your question before we finish here during this live webinar, we do have them recorded and Gaylon or somebody else from our METER team will be able to get back to via email and answer your question directly. So don’t worry, and you can submit any and all questions. All right. So let’s see here. Gaylon, this first question was asking about measuring nitrogen content in real time. Is it possible to measure nitrogen content in real time both in soil and in the crops?

GAYLON CAMPBELL 32:02
Well, with electrical conductivity all that you can measure is the total nutrient content and so, nitrogen typically is a pretty large part of the total nutrients that are being taken up and so, in a sense, you can measure that by following the electrical conductivity, and that often works in soil too. It depends. Some irrigated soils have pretty high salinity, background salinity, and that tends to mask the signal that you can get because all you’re measuring is electrical conductivity, but in soils that have relatively low background salinity, you can do a pretty good job of monitoring the the nutrients that are there by measuring electrical conductivity.

BRAD NEWBOLD 33:09
All right. And along those same lines, this other individual is asking what is the general relationship between EC and plant nutrients such as nitrogen, phosphorus, potassium, etc.?

GAYLON CAMPBELL 33:26
Well, that relationship is the one I showed in the second slide, I think it was, and again, it differs some depending on the makeup of the nutrient solution, but for a controlled environment situations, for hydroponic situations, where we mix up the nutrient solution to meet the needs the plant, typically the plants will take up the nutrients in proportion in the right proportions. And so, the overall concentration that you get from the electrical conductivity will tell you the level of nutrients available in the substrate.

BRAD NEWBOLD 34:19
Alright. This next one is, I’d like to know more about the methods used for determination of field moisture capacity and how it relates to plant osmosis.

GAYLON CAMPBELL 34:32
Okay, that’s something that we didn’t cover in this lecture, but we intend to in the next one, and so I’ll try to go into that in more detail, determining the field capacity, or I don’t know what you call it in controlled environments situations, but we’ll talk about the water balance and field capacity in the next lecture.

BRAD NEWBOLD 35:04
Great. Okay, so stay tuned for that one. There’s a little plug for the next the next webinar. Let’s see here. This next one, could you please return to the short point made about matric potential, I think, would you ever try to account for the sum of matric plus osmotic potential, or create low level drought stress by drying back water content more significantly?

GAYLON CAMPBELL 35:32
This gets back to the things that we covered in the first lecture. And if you want to go back and review that some, I think we can, I mean, that makes the point a lot better than we can here. But for example, for a rock wool, you typically would never dry that down more than to about 30%. And by the time you get to 30% water in rock wool, the matric potential on it is almost the same as it is when it’s 60% or 70%. That hardly changes and in other substrates it changes a little more than that. But I would say that the matric component is never significant when you keep the water contents within the range you typically would for horticultural substrates.

BRAD NEWBOLD 36:35
And, let’s see, there’s a couple of questions. I think, kind of under a similar vein. This one is asking, Is it possible to have a real relationship between pore water EC, and extract saturation soil EC? There’s also another question about using a pour through method and then taking or measuring the EC of the drainage water.

GAYLON CAMPBELL 37:04
Well the pore water EC should be the same as the EC of if you took the substrate and squeezed the water out of it and measured its electrical conductivity. That should be the same as the pore water EC. Now the pour through method, that method is not one that I’ve used actually but where you pour water through and then measure the electrical conductivity of the water that comes out and I think that would be more similar to what they call in soils the saturation extract electrical conductivity. Or I suppose somewhere between pore water and saturation extract, you’re adding water to the substrate, so you’re diluting salts to some extent, maybe not as much as you would for a saturation extract.

BRAD NEWBOLD 38:12
Okay, this next one, I think if I can read this right and get this passed on correctly, alright. So in one of the graphs you showed that you used, EC equals 20 decisiemens per meter for soil water, which crop has been planted in the soil? Is there any restriction for the crop as well as for the instrument?

GAYLON CAMPBELL 38:35
You know, the crop was cannabis. And there certainly are restrictions that I mean, just like with matric stress on crops, there’s a limit to the osmotic stress that a crop can stand. I’m not sure what it is for cannabis. Well, I don’t know what it is for any other crop either. I haven’t had experience. And you certainly are right to question whether there’s instrument limitations there, too. So I mentioned that that multiplier that we use becomes pretty uncertain when you get down to low water contents and up to high electrical conductivities. So there’s a lot more uncertainty in that 20 decisiemen number than then there would be in five decisiemen or something like that, but it’s still a good indication of how stressed the crop is likely to be.

BRAD NEWBOLD 39:45
All right. And it looks like I think we’ve got time for one more question. Again, feel free to submit any any more questions that you have. We’ll get back to you via email. This last question then is along similar lines about using osmotic stress to mimic matric stress. Is there any concern, then, that, as you’re introducing, or working with a nutrient level, that you might create salt toxicity or situations where where you might actually begin to damage the plant?

GAYLON CAMPBELL 40:23
Certainly, and this is, again, something that the grower would need to work with. And I mean, you have all of the measurements, and then you see the response of the plant. The idea of crop steering is not to kill the plant or even to maim it, but just to give it a signal that it’s time to switch over to a different way of partitioning. It’s assimilate, to put it into flowers instead of vegetative growth. And so what you would like to do is to give that signal as benignly as you can, without damaging the crop. The point is not to damage the crop, but to give a steering signal.

BRAD NEWBOLD 41:20
All right. Thank you again, Gaylon. That’s gonna wrap it up for us today. Thanks for joining us. We hope you enjoyed this discussion as much as we did here. And thank you, again, for all the great questions. We didn’t have a chance to get to all of them, but again, like I mentioned before, we will be able to get back to you via email and answer your questions directly if we did not get to them here during the live webinar. Also, please consider answering the short survey that will appear after the webinar is finished, just to let us know what types of webinars you’d like to see in the future. And also, as we plugged earlier, stay tuned for a future webinar, about water balance and fuel capacity and all of that. Also, for more information on what you have seen today, please visit us at metergroup.com. And finally, look for the recording of today’s presentation in your email. And again, stay tuned for future METER webinars. Thanks again, stay safe, and have a great day.