PART 4: BALANCING LIGHT, WATER, AND NUTRIENTS

Are you unwittingly compromising your plants?

In a controlled environment many variables affect production. But if any one of those variables gets out of balance, it can undermine your whole operation. For example, if you apply enough nutrients for high production but only enough light for low production, you’ll increase costs and limit yield. To get the most out of your crop, you’ll need to measure and balance environmental inputs correctly to get the most efficient use out of them. If you’re not measuring the right variables, fixing problems that keep you from your goals will be a shot in the dark, because you won’t know what the real problem is.

Amplify your production and efficiency

In part 4 of our popular controlled-environment webinar series, world-renowned soil physicist, Dr. Gaylon Campbell, teaches what is required to ensure all environmental variables remain balanced for the highest possible efficiency and production. Discover:

• How to model biomass production from light, water, and nutrient resources
• Relationships between biomass production, light, and CO₂
• Relationships between  biomass production and water use
• Relationships between biomass production and nutrient uptake
• Limiting factors in the balance equations
• Examples and monitoring applications

Next steps

• See part 1, part 2, and part 3 in this webinar series
• Access slideshare
• 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

Transcript

BRAD NEWBOLD 0:08
Hello, everyone, and welcome to part four of our webinar series, Irrigation of Controlled Environment Crops: Balancing Light, Water and Nutrients. Today’s presentation will be about 30 minutes, followed by about 10 minutes of Q&A with our presenter, Dr. Gaylon Campbell, who I will introduce in just a moment. But before we start, we’ve got a couple of housekeeping items. First, we want this webinar to be interactive. So we encourage you to submit any and all questions in the questions pane. And we will be keeping track of these for the Q&A session for the end. Second, if you want us to go back or repeat something you missed, don’t worry. We will be sending around a recording of the webinar via email within the next three to five business days.

BRAD NEWBOLD 0:49
And 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 get the most efficiency and production out of your greenhouse crop. 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 understand plant water status. Dr. Campbell is one of the world’s foremost authorities on physical measurements in the soil plant atmospheric 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 1:42
Well, thank you. Thanks for being with us today to talk about balancing light water and nutrients in controlled environment crops. Now when you start a production cycle, you start with a mostly empty room like the one that you see here. Eight or nine weeks later, the room is mostly full with biomass, like you see in this picture. Where did all that biomass come from? Typical amount might be four kilograms per square meter, we supplied a lot of water through irrigation for the plants, we assume the vegetation is around 70% water, that’d be around 2.8 kilograms. Now that leaves 1.2 kilograms per square meter of dry matter. We put some fertilizer in the water and the plants consumed much of that the dry matter is around 10% nutrients and so we could strip drank that too. And we come out with something like 1.1 kilograms per square meter, still at that point, now to go into this in a little more damped. Let’s return to this picture of a plant that we used in an earlier seminar. It showed the nine environmental variables with parameters that represent main connections between the plant and its environment. We said it’s useful to think in some of these environmental parameters such as light, carbon dioxide, water, nutrients as resources. And consider the fact that bankers that enable the plant to capture those resources. Assimilation is the process by which carbon dioxide, water, and light from a lab environment are captured and converted to carbohydrate. To grow, the plant captures nutrients and water and binds those with the carbohydrate produced by assimilation, and makes those into stems, leaves, roots, flowers and fruits that make up the biomass of the plant. We also noted the third set of processes listed here occurs simultaneously with the other two that doesn’t capture resources.

GAYLON CAMPBELL 4:20
Development is the progression of a plant with recognizable phases. The rate of development can be influenced by availability of resources, but it’s most strongly influenced by temperature and daylength. On the other hand development can strongly influence the plant’s resource capture, since it controls the time in each of the phases. In controlled environment crop production, all the variables listed here are under the control of the grower. It’s the job of the grower to choose the values of each of those variables to attain the goals of the production facility. To properly control those variables, the grower needs to know what values they have, and how they influence the outcome of the production process. The AROYA production platform is capable of monitoring those variables, and reporting their values to the grower. So the 1.1 kilogram per square meter comes from the light and the carbon dioxide kind of like magic converting light and air to biomass.

GAYLON CAMPBELL 5:38
Let’s look in more detail at how that happens. But first, I’m gonna consider the idea of limiting resources in crop production. About 150 years ago, Justus von Liebig gave us Liebigs law that states that growth is dictated not by total resource availability, but by the scarcest resource. It’s often illustrated using Liebigs barrel. Here the staves represent the various resources. mineral nutrients are typically the resources that are illustrated by the barrel, but I added the environmental ones that we just talked about in the previous slide, in nature, any of these resources could be limiting.

GAYLON CAMPBELL 6:28
But in controlled environments, all of them can be optimized to get maximum production or maximum yield with minimal waste, and many production facilities, the measurements can be difficult to difficult to get a clear picture of what the limitations are. But in controlled environments, the AROYA production platform provides all the information that we need. A lot is going on in the plan to take it from the small size you saw on the first slide to the full room that you saw on the second, we can lump some of those things into three main sets of processes. And we can model those processes. Studying these models in detail will give us insight into how we can balance resources for maximum production. The three processes that will consider the conversion of photons to biomass, the conversion of water to biomass, and the conversion of nutrients to biomass.

GAYLON CAMPBELL 7:33
This is a light response curve for a leaf. The rate of uptake of carbon dioxide by the leaf is plotted here as a function of the photosynthetic photon flux density, or the light level on the leaf is zero PPFD, the assimilation rate is negative. So the leaf is not carrying on photosynthesis, it’s turning carbohydrate that it had into energy that it can use for metabolic processes. As the light increases, the assimilation increases. These low light levels, each molecule of co2 takes about 12 photons to convert it to carbohydrate.The theoretical minimum is nine. But in real life leaves there are so many efficiencies. The horizontal axis goes to 2000 micromoles per square meter per second. And that’s around the PPFD and full sun. Some indoor facilities approach that value, the most are quite a bit lower than that, maybe around half in a good facility. Note that the higher the light level the lower the slope. Therefore the efficiency the light use that these highlight levels, something else is becoming the limiting resource.

GAYLON CAMPBELL 9:08
Now if we increase the carbon dioxide concentration, the assimilation rate also increases. The blue curve here shows the response from the previous slide, then that’s for ambient co2 around 400 parts per million. The gray curve is for 1000 parts per million. We think about Liebigs barrel doesn’t do much good to supply more light if co2 is limiting our yield. To get the good out of the high light levels you need higher co2 levels too.

GAYLON CAMPBELL 9:48
Now a useful way to think about the amount of light available for photosynthesis is the daily light integral or DLI. An outdoor environment where lights always changing, computing this number can be a challenge. But in the growth room, it’s easy. If you had a growth room with a light density of 1000 micromoles per square meter per second, and the light were on for 12 hours per day, the daily light integral would be 43.2 moles per day as shown in the calculation here. Now, if you want to know how much biomass we could produce with that many photons, we’d multiply the daily light integral by a light use efficiency and the total time the crop was growing. The light use efficiencies vary depending on co2 levels and other variables. But one gram per mole can be thought of as a kind of an upper limit, we multiply these together we get 2.6 kilograms per square meter of dry matter produced, we can compare that with the 1.1 kilogram per square meter value that we talked about a little earlier.

GAYLON CAMPBELL 11:12
Now, why might this one be lower? I’ve listed several possible reasons here. First, part of the energy we fix in photosynthesis goes to carrying on metabolic processes within the plant. We call that respiration. These processes are temperature dependent so more assimilate is lost to respiration at high temperatures than low. It’s therefore good to decrease nighttime temperatures to reduce respiration losses. The next factor is fractional light interception. The canopy can’t capture 100% of the incident radiation, and photons that aren’t captured by leaves can’t produce dry matter.

GAYLON CAMPBELL 11:59
And finally, we saw in the light response curves that higher light levels fix less co2 per mole of light than low light levels do. Now let’s consider our second model, how we convert water to biomass. Let’s start by looking at the table in the right side of the slide. Water participates in biomass production in several ways. The photosynthesis equation we saw a few slides back showed water co2 and light combining to form carbohydrate, takes point six grams of water for each gram of dry matter that we get. We also talked about something like 70% of the plant structure being watered, and this takes an additional 2.4 grams of water per gram of dry matter. But the transpiration process shown in the diagram on the left, so thoroughly out does the other two processes that we don’t even worry about the water used in the first to consider that diagram. No, the carbon and the plant fixes must come from the air. But the metabolic processes that fix the co2 must go on in a highly hydrated environment. We talked about that in the first seminar showing that inside the leaf, the relative humidity is very near 100%. While the humidity outside the leaf may be half that.

GAYLON CAMPBELL 13:41
The diagram that I show here is a cross section of a plant leaf. We see a pore stomate in the leaf through which the co2 diffuses to be fixed by photosynthesis. And the purpose of stomates and the leaf epidermis is to minimize water loss from the leaf. But as co2 diffuses in, water escapes. This simultaneous loss of water and uptake of co2 means that there is a relationship between the amount of water that’s lost and the amount of co2 that’s fixed into dry matter. The transpiration rate depends on things like leaf temperature, wind speed vapor deficit of the air. Notice the leaf effect the amount of dry matter that we get for each kilogram of water we transpire. The range for controlled environment crops there’s something like three to six grams of dry matter per kilogram of water. When co2 is high and the vapor deficit is low, we get something like six grams of dry matter per kilogram of water. In ambient co2 and larger vapor deficits typical of greenhouse environments, it’s closer to three.

GAYLON CAMPBELL 15:05
We’ll use this equation in a previous presentation or in this series to compute transpiration. It’s the product of a vapor conductance for the leaf and the boundary layer. And the difference between the saturation vapor pressure at leaf temperature and the vapor pressure the air.So if we knew the wind speed, the model conductance, the canopy temperature, the air vapor pressure, we can compute a transpiration rate. Here’s a graph showing transpiration rate versus difference between the saturation vapor pressure at the leaf temperature and the vapor pressure of the air. Now before we get into it, I want to say a few words about the horizontal axis. It’s the vapor pressure deficit of the leaf or at leaf temperature. Vapor pressures may not be very familiar to you, but relative humidity probably is. A vapor pressure deficit of zero corresponds to 100% relative humidity in the air.

GAYLON CAMPBELL 16:19
At typical growth room temperatures and vapor pressure deficit of two like maximum extend of the horizontal axis corresponds to a humidity of about 50%. A vapor pressure deficit of one would be about 75% relative humidity. So you can think of the horizontal axis as going from 100% humidity, the left end. Then this is referred to the leaf surface to about 50% on the dry end on the right end.

GAYLON CAMPBELL 16:58
Now, if we looked at the graph, the equation is the vapor conductance of the leaf were constant, our equation would predict that that blue line would be straight starting at zero, and it would slope upward. But instead the blue line goes up and then back down. So that must mean that the vapor conductance isn’t constant. The reason is that large vapor pressure differences or low humidities close the stomates. That stands to reason that guard sells to the soulmates lose water too and it’s their turgor pressure that keeps the soulmates open. So if they lose water too rapidly, they will lose turgor and close. We want the stomates to be as wide open as possible to live in as much co2 as we can get. So we want to maintain the humidity high enough to do that.

GAYLON CAMPBELL 18:02
The exact shape and location of this the model response function depends on the species and possibly cultivar. The one shown here is probably typical. The AROYA QuickStart Guide recommends a vapor pressure deficit of around one kilopascal for cannabis. This would be around 75% humidity. The vapor pressure difference shown here is probably a bit larger than the vapor pressure deficit of the air because it leaves in lighter, warmer than the air. But if the response function for cannabis is similar to the one shown here, looks like we would get stomata closure if we went much beyond a kilopascal and that’s consistent with the information that is in that quickstart guide.

GAYLON CAMPBELL 18:56
So we’ll come back to calculating dry matter using these two models in a moment. But let’s now look for a minute at the nutrient requirements for biomass production. As we have said in earlier lectures and controlled environments, the substrates typically contain no stored nutrients. So all the nutrients are supplied in the water. Bruce Bugbee a Utah State University professor published a wonderful and helpful paper almost 20 years ago, and this table is from his paper shows the fraction of the dry biomass made up of each of the nutrients shown. Other nutrients are required beyond those shown here, but their mass fraction is so much smaller than these that for our purposes are negligible. The amounts and the leaf stems and fruits and roots differ. But if we look just at the leaves, we see that about 10% of the dry mass is nutrients that we supply.

GAYLON CAMPBELL 20:02
Now let’s go through the calculations with each of these models and compare them. Starting with the light, we assume 1000 micromoles per square meter per second 12 hours of light. So that daily light integral is 43 moles per square meter per day, we assume 70% of the radiation is intercepted, and the light use efficiency of .6 grams of dry matter per mole of light. This would give 18 grams per square meter per day, over 60 days, this would give 1.08 kilograms per square meter. And that’s about the value that we mentioned at the start of the lecture.

GAYLON CAMPBELL 20:49
We won’t go through the transpiration calculation in detail. But again, we assume 70% of the light is intercepted, we get 3.2 kilograms of water per square meter per day. We use the dry matter water ratio of five grams of dry matter per kilogram of water, we get 16 grams of dry matter per square meter per day. So that’s coming out about the same as it is for light. If we take the 16 grams per square meter per day that we calculated for water, and assume that the nutrient requirements are those for leaves, that would take 1.6 grams of nutrients per square meter per day for the dry matter that we’re producing. But if you take the 3.2 kilograms of water per square meter per day, and multiply by a typical concentration of fertilizer in the irrigation water of two and a half grams of fertilizer per kilogram of water, now less than half of that mass is the actual nutrients that we need. So we multiply by point four to correct for that, if we do that, we get 3.2 grams of nutrients per square meter per day in the water that was taken up.

GAYLON CAMPBELL 22:21
So now are these balanced? The light and water seem pretty well balanced. If we were to cut the light to 500 micromoles per square meter per second, that would cut the photons to biomass value to nine grams per square meter per day. But that change would have very little effect on the transpiration. So those wouldn’t be balanced. In that case, the nutrients in the water taken up for transpiration appear to be more than can be used for plant growth. But remember in the second lecture, we talked about using osmotic potential for stressing the crop to do crop steering. Concentrations like those shown here are required to stress the crop. So I think it’s clear from that that the concentrations of nutrients that we need to do the crop steering are in excess will be in excess of those that we need to grow the crop.

GAYLON CAMPBELL 23:26
So what does this all mean? If we know the daily light, integral, fractional interception, and light conversion efficiency, we can estimate the light limited dry matter production. If we know or can compute the transpiration rate, fractional cover the water use efficiency we can estimate water limited dry matter production. Since the nutrients are supplied with the water, all factors that affect transpiration rate will also affect plant nutrient. Finally, the nutrient levels that are high enough to allow crop steering are likely in excess of those that we would need for the biomass that we’re producing.

GAYLON CAMPBELL 24:13
Now five years ago our company in Pullman was called Decagon and it combined with UMS an enviornmental instrumentation company in Munich, Germany, to form the company we have now called METER. The tagline for UMS was measure to know that’s good advice for controlled environment growers. And here are the sensors needed for those measurements. Starting at the upper left picture the AROYA Nose as a solar panel that charges its battery. The solar panel also works as a radiation sensor. Since noses are installed both above and below the canopy, the ratio of the below to above canopy measurement gives an estimate of fractional light interception. The lower left picture shows METER’s SC-1 leaf barometer clip for measuring stomatal conductance of a leaf. This is a manual measurement typical day and night values can be established from these measurements and used for the transpiration calculation. The upper center picture is the TEROS 12 water content, electrical conductivity, and temperature sensor for monitoring the plant environment, the root environment. The bottom center picture is the AROYA climate station that gives co2 concentration and accurate air temperature and vapor pressure measurements suitable for the transpiration calculations. The upper right is an apogee quantum sensor connected to the AROYA system. This provides the light measurements needed for the daily light integral and also for the transpiration calculations. Average wind is probably fairly constant in a growth facility over time, and it could be measured with a handheld anemometer. But if one wanted the record of it, the lower right picture is a METER ATMOS 22 Sonic anemometer that could be used for continuous monitoring.

GAYLON CAMPBELL 26:30
Those measurements are displayed by the AROYA software to provide the guidance the grower needs to balance the control inputs and monitor resources. We’ve seen these in previous lectures. This view shows the substrate water content and pore water electrical conductivity for a complete growth cycle. That information is conveniently summarized in dashboards so that you can know the status of all the systems and here’s a kiosk view. And then here’s the summary view so you can see the conditions for the whole growth facility. So these are the points that we clarified in today’s presentation. Starting with Liebigs law, we tried to make clear that your crop yields are determined by your most limiting resource. We then talked about resource capture by plants in a growth facility in terms of converting photons to biomass, converting water to biomass, and converting nutrients to biomass.

GAYLON CAMPBELL 27:45
The potential conversion rate for photons to biomass is determined by the photosynthetic photon flux density, the co2 concentration, the light use efficiency. The potential conversion rate for water to biomass determined by the radiation, the vapor deficit, water use efficiency and model conductance. Nutrients are supplied by fertigation. And those need to be balanced with each other, and with the light and water use. But they’re also used for the crop steering

GAYLON CAMPBELL 28:22
AROYA tools are designed to help growers maximize production, minimize waste, and to continuously improve. Thank you for being with us today.

BRAD NEWBOLD 28:36
All right. Thank you very much, Gaylon. So we’d like to use the next 10 minutes or so to take some questions from the audience. Thank you to those who have submitted your questions already. And there’s still plenty of time to submit your questions. So feel free to ask as many as you’d like. And we’ll get to as many as we can. Before we finish here, we do want to let you know that if we do not get to your question, either Gaylon or somebody from our METER team will be able to get back to you via email to answer your question directly. All right. So for the first question here, so is there a way to train plants to collect more light and improve the light use efficiency you were talking about?

GAYLON CAMPBELL 29:26
I don’t know if there’s a way to train them to do that. I think there’s a way to train growers to do that in a controlled environment facility. If it’s understood that photons that aren’t captured aren’t going to be useful, there’ll be adjustments to the canopy to improve the fraction of radiation that’s intercepted and a lot of techniques, I mean, there are a lot of techniques that are in use right now to do that to maximize the light interception by the canopy.

BRAD NEWBOLD 30:09
Okay, Question number two here. Is it practical to try to measure the evaporation in my greenhouse? Can I assume a stomotal conductance? Or do I need to measure it? And if so, how many locations? So a combination question there.

GAYLON CAMPBELL 30:28
Okay that is practical to compute the transpiration in a greenhouse or in a growth facility, and we’ve talked about the equations to use to do that, and the measurements that are required for it, stomotal conductance is one of the most important of those measurements and that needs to be measured quite often in a growth facility or greenhouse so that plants in those environments don’t close their stomates at night. And then so you need to make sure both nighttime and daytime conductance but if the plant isn’t water stressed why we can expect the stomatal conductance to stay fairly constant. So it’s reasonable to do. Make some measurements by hand, and use those for doing those calculations. And how many measurements you need, you get an idea as you make some measurements of the variability that exists. And so you can do statistics on that to come up with a reliable number that you can use for the calculation. We’d expect those to change over time as the plant grows and matures. So we need to have some representative of early stages of growth and some representative of later stages.

BRAD NEWBOLD 32:09
All right. Okay, do you have any thoughts on how air exchange, airflow, and HPS versus LED affect transpiration rates?

GAYLON CAMPBELL 32:22
Yeah, the airflow of course affected a lot especially in controlled environment facilities where the boundary layer conductance is a pretty big fraction of the total conductance for water vapor and heat and in their equations for that and if you know the wind speed, you can know that boundary layer conductance, the wind speed, and the leaf size. The radiant energy also has a substantial effect on that and not just the visible radiation or the radiation that’s useful for photosynthesis, but also the thermal radiation and tend to get quite a bit more heat with the high pressure sodium lamps and so, the transpiration that you get with those lamps tends to be higher than it is with LEDs.

BRAD NEWBOLD 33:38
Okay. This next individual is asking what is the best way to model stomatal conductance and leaf area index? Is there a best way first off?

GAYLON CAMPBELL 33:53
Well I wouldn’t try to model stomotal conductance. I would just go and measure that with the parameter. And we do a lot of modeling of light interception by plant canopies under outdoor conditions where the plants decide what canopy structure they will choose and then that works to model that but in a controlled environment where the canopy structure is determined mainly by the grower I’m not sure that’s practical to try to model that. I think probably again, that’s something we just need to measure. So easy to measure.

BRAD NEWBOLD 34:47
All right. Let’s see, I think we’ll take another question or two here. It looks like you could add more co2 and light at the same time and it would help with Liebigs Law of the Minimum, are there practical limitations to this approach, more light and more co2 until what limit?

GAYLON CAMPBELL 35:12
Obviously, we’re looking at a pretty small part of the total picture here. Plants certainly have limitations in I mean once the stimulate is produced in the plant, then lots of other stuff has to go on, has to be transported and stored. Those kinds of things in their limits there that we haven’t considered at all here. But you can see from that light response curve that we saw that even at high co2, we were starting to limit stimulation. And those limitations come elsewhere in the plant. Those are things that we can address with genetics, with looking for cultivars that have increased capability to do that. Stimulate transport and storage. And I’m sure there’s huge variations that exist in that. Those are things that we beyond my ability to explain.

BRAD NEWBOLD 36:30
All right. Well, I think that’s going to finish it for us here today. We’re going to wrap up with that last question there. Again, for those that did not have questions answered, Gaylon, or somebody else from our METER Group team will be able to get back to you and answer your questions. Thank you again for joining us today. We hope that you enjoyed this discussion as much as we did here. And thank you again for your great questions. Also, 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 about what you’ve seen today, 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.