When you irrigate in a greenhouse or growth chamber, you need to get the most out of your substrate so you can maximize the yield and quality of your product.
But if you’re lifting a pot to gauge how much water is in the substrate, it’s going to be difficult—if not impossible—to achieve your goals. To complicate matters, soil substrates and potting mixes are some of the most challenging media in which to get the water exactly right.
Without accurate measurements or the right measurements, you’ll be blind to what your plants are really experiencing. And that’s a problem, because irrigating incorrectly will reduce yield, derail the quality of your product, deprive the roots of oxygen, and increase risk of disease.
Supercharge yield, quality—and profit
At METER, we’ve been measuring soil moisture for over 40 years. Join Dr. Gaylon Campbell, founder, soil physicist, and one of the world’s foremost authorities on soil, plant, and atmospheric measurements, for a series of irrigation webinars designed to help you correctly control your crop environment to achieve maximum results. In this 30-minute webinar, learn:
Why substrates hold water differently than normal soil
How the properties of different substrates and potting mixes compare
Why it’s difficult if not impossible to irrigate correctly without accurately measuring the amount of water in the substrate
The fundamentals of measuring soil moisture: specifically water content and electrical conductivity
How measuring soil moisture helps you get the most out of the substrate you choose, so you can improve your product
Easy tools you can use to measure soil water in a greenhouse or growth chamber to maximize yields and minimize inputs
Irrigation management: Why it’s easier than you think
Years ago, we received an irrigation management call from a couple of scientists, Drs. Bryan Hopkins and Neil Hansen, about the sports turfgrass they were growing in cooperation with the Certified Sports Field Managers at Brigham Young University (BYU) and their turfgrass research and education programs. They wanted to optimize performance through challenging situations, such as irrigation controller failure and more. Together, we began intensively examining the water in the root zone.
As we gathered irrigation and performance data over time, we discovered new critical best practices for managing irrigation in turfgrass and other crops, including measuring “soil water potential”. We combined soil water potential sensors with traditional soil water content sensors to reduce the effort it took to keep the grass performance high, while saving water costs and reducing disease potential and poor aeration. We also reduced fertilization costs by minimizing leaching losses out of the root zone due to overwatering.
Supercharge yield, quality and profit in any crop with soil moisture-led irrigation management
This article uses turfgrass and potatoes to show how to irrigate using both water potential and water content sensors, but these best practices apply to any type of crop grown by irrigation scientists, agronomists, crop consultants, outdoor growers, or greenhouse growers. By adding water potential sensors to his water content sensors, one Idaho potato grower cut his water use by 38%. This reduced his cost of water (pumping costs) per 100 lbs. of potatoes, saving him $13,000 in one year.But that’s not even the best part. His yield increased by 8% and he improved his crop quality—the rot he typically sees virtually disappeared.
What is soil water potential?
In simple terms, soil water potential is a measure of the energy state of water in the soil. It has a complicated scientific definition, but you don’t have to understand what soil water potential is to use it effectively. Think of it as a type of plant thermometer that indicates “plant comfort”—just as a human thermometer indicates human comfort (and health). Here’s an analogy that explains the concept of soil water potential in terms of optimizing irrigation.
The environment plays a large role in any plant study. Ensuring you’re capturing weather and other environmental parameters in the best way allows you to draw better conclusions. To accurately assess plant stress tolerance, you must first characterize all environmental stressors. And you can’t do that if you’re only looking at above-ground weather data.
For example, drought studies are notoriously difficult to replicate and quantify. Knowing what kind of soil moisture data to capture can help you quantify drought, allowing you to accurately compare data from different years and sites.
Get better, more accurate conclusions
It’s important for your environmental data to accurately represent the environment of your site. That means not only capturing the right parameters but choosing the right tools to capture them. In this 30-minute webinar, application expert Holly Lane discusses how to improve your current data and what data you may not be collecting that will optimize and improve the quality of your plant study. Find out:
How to know if you’re asking the right questions
Are you using the right atmospheric measurements? And are you measuring weather in the right location?
Which type of soil moisture data is right for the goals of your research or variety trial
How to improve your drought study, why precipitation data is not enough, and why you don’t need to be a soil scientist to leverage soil data
How to use soil water potential
How accurate your equipment should be for good estimates
Key concepts to keep in mind when designing a plant study in the field
What ancillary data you should be collecting to achieve your goals
Holly Lane has a BS in agricultural biotechnology from Washington State University and an MS in plant breeding from Texas A&M, where she focused on phenomics work in maize. She has a broad range of experience with both fundamental and applied research in agriculture and worked in both the public and private sectors on sustainability and science advocacy projects. Through the tri-societies, she advocated for agricultural research funding in DC. Currently, Holly is an application expert and inside sales consultant with METER Environment.
If you’re an astronaut on an exploration Mission to Mars, lack of breathable oxygen isn’t the only challenge you’re facing. There’s another issue that can heavily impact your performance: lack of veggies.
It turns out the astronaut food system must be supplemented with fresh crops during long-term exploration missions. NASA has identified that vitamins and antioxidants degrade over time and is now funding research on the best way to grow fresh vegetables in space to supplement astronaut nutrition.
How well do plants grow in space?
Researchers including Dr. Oscar Monje, research scientist at NASA’s Kennedy Space Center in Florida, are studying the best materials and methods for growing vegetables in space. Monje says, “Growing plants in a space station is challenging because both space and power are limited. All the plant chambers built in the past 40 years focused on enabling space biology studies that centered on how to grow plants in space. They wanted to know the effects of growing in zero gravity. Can plants grow normally? Are they stressed? Can they produce seeds? But now we want to focus on space crop production. We want to supplement astronauts’ diets with essential minerals and vitamins during long duration missions.”
Early growing experiments in the BPS
Monje was a student of Dr. Bruce Bugbee at Utah State University who studies plants in space for bioregenerative life support systems. After graduating and doing a postdoc at the Space Dynamics lab, he started at Kennedy in 1998 researching how to grow wheat in space in the PESTO (Photosynthesis Experiment and System Testing Operations) Experiment. Dr. Gary Stutte, the principal investigator and Dr. Monje grew wheat in the Biomass Production System (BPS), a four chamber system that consumed 280 W of power. The BPS was used to measure photosynthesis, demonstrating plant harvests, priming pre-planted root modules, pollinating plants, as well as collecting gas and liquid samples.
Monje says, “Back then, all experiments were shuttle experiments (7-11 days at a time). The PESTO Experiment flew for 73 days in space and was essentially several shuttle missions conducted back to back. We measured root zone moisture with a pressure sensor monitoring root module matric potential. We learned that as long as you provide plants with adequate root zone aeration, good soil moisture, and the right light and CO2, they grow normally with no visible plant stress—just like on earth. The BPS was a precursor of the Advanced Plant Habitat (APH) facility on the International Space Station plant, an environmentally controlled growth chamber designed for conducting both fundamental and applied plant research for experiments lasting as long as 135 days.
New growth chambers introduced
The BPS was very complex, but the chambers were small and the light level was moderate. Ten years ago, NASA developed two large area (0.2 square meter) crop production systems to grow fresh salad crops in substrate-based media for the astronauts: the “Veggie” and the “APH”. Monje says, “Veggie is open to the cabin so there is no environmental control of CO2 or temperature, and it is watered by the crew. The light level provided by red, blue and green LEDs is moderate and the environment is not monitored. However, it grows lettuce crops that are safe to eat by the crew. The Advanced Plant Habitat on the other hand, is a Cadillac compared to the Veggie. With the APH you can load experiment profiles from the ground that control the light level (up to 1000 umol/m2s, half-full sunlight), the spectral quality, the CO2 concentration (up to 5000 ppm), photoperiod of light, and root zone moisture. The APH can be monitored in near-real time with minimal crew intervention for weeks at a time. To date, it has been used to grow wheat, Arabidopsis, and radish crops during space biology and crop production experiments”
Measuring root zone moisture in space
Monje says that the 5-cm tall APH root zone is divided into four independently controlled root modules, called quadrants. In each quadrant, media moisture is controlled based on matric potential using a pressure sensor. However, the matric potential measured by the pressure sensor does not capture vertical variation in volumetric moisture. He says, “Each quadrant is watered with a porous tube system that distributes water throughout the porous media (arcillite) that is mixed with slow release fertilizer. In the 5-cm tall root zone at one g, most of the water is ponded at the bottom, and the top layer of media where the plants are germinating can become too dry. For these reasons, two small, rugged volumetric EC-5 moisture sensors were added to each quadrant to monitor moisture redistribution phenomena in microgravity. These sensors are insensitive to salinity and temperature effects. Thus, APH uses eight volumetric moisture sensors, two in each quadrant (one high, one low) to monitor root zone moisture. When watering in space, moisture redistribution occurs because capillary forces in microgravity distribute water evenly across the substrate and affect aeration. Even though the sensors are at different heights, they can read the same, as opposed to one flooded and the other one drier.”
Monje adds that, “Balancing the mix of aeration with enough water in a microgravity environment is the crux of the problem. If you don’t water plants enough, they don’t grow fast enough, but if you give them too much water, then you inhibit O2 supply to roots and nutrient uptake. So we’re using volumetric water content sensors in the APH root module at different levels to control the moisture. The sensors are like our ‘fingers in the soil’.”
Monje says the next step is to develop novel watering systems that do not use granular media. He says, “Each APH root module holds about six kilograms of arcillite media, which is used only once per crop. Similarly, Veggie uses nearly two kilograms of media distributed into six independently watered root modules. Although these root modules grow normal plants in space biology experiments, this approach is not sustainable for crop production as transporting this much mass all the way to Mars is not going to be feasible.”
Monje believes that getting these crop growth systems ready for a trip to Mars may take a while. He says right now, the Moon is a new proving ground for these technologies as part of the Artemis program, and deploying experiments on the Moon brings new challenges too. He adds, “An interesting thing about the Moon is you have partial gravity. It’s not one g. It’s 1/6 g. Plus, you have the issue of space radiation, and temperature must be controlled, unless you deploy your experiment in a manned habitat. From a biology point of view, we want to understand how plants will respond to growing in high radiation and partial gravity environments.”
Monje says it’s been an incredible journey to work on this type of research for many years. He laughs, “Since 1998, I’ve been pinching myself every day because this work is so challenging and yet so much fun. It’s pretty amazing.”
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
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.
Charles Bauers has been a hydroponic snapdragon grower for 17 years. He knows—in detail—how to produce a good snap. But five years ago, he needed a better way to measure water.
“We had no quantitative way to measure water. That was the limiting factor for me,” he explains. Other inputs, like fertilizer, were quantifiable, but Bauers still depended on “gut feel” for watering, and no matter how quickly he reacted to changes in the crop, he couldn’t consistently produce grade-one snapdragons.
He wanted a scale, a “recipe of numbers” that would let him produce a good crop all the time in all sections of the greenhouse.
“There are always areas that seem to produce good quality flowers, and then there are areas that are a bit more of a challenge. I installed METER soil moisture sensors in the good areas and the stressed areas and compared the two. Then I worked my stressed areas up to the same numbers.”
Snapdragons are very sensitive to moisture stress. “It’s a ten-week crop. If you don’t get the moisture right in the first two weeks, you can compromise that crop.”
Identifying irrigation set points
The soil moisture sensors made a huge difference in Bauers’s ability to get the moisture right. “They give me, targeted set points that I can shoot for all the time, and if I hit the targeted set point, I know I’m going to have good quality snaps, barring any other type of stress.
Grade-one snapdragons are worth 40% more than grade twos, and the difference between the two is created by “incipient stress—water stress that you can’t measure with your fingers. You can’t see it, you can’t feel it, it’s stress at the root. There’s a difference between a 28% vwc [volumetric water content] and a 23% vwc. It’s only 5%, but one produces grade ones and one produces grade twos.”
Empowered with real-time information
Moisture sensors gave Bauers real-time information that helped him get the watering right in every part of the greenhouse. “I became more consistent because I had a number to go at. Because we’re a hydroponic crop, we see the effects real quick, and I’d say ‘I just have to add a little more water here.’ But [before the sensors,] invariably we had areas that were stressed because you really never knew when you had enough water on that crop. With sensors, you can consistently put the right amount of water on all the time.”
Bauers quickly became adept at using sensors to address his irrigation challenges. The sensors showed him where his irrigation system was broken or underperforming, helped him identify problems like a root growing into a drip tube, or an unplugged dripper. But as the sensors became part of his routine, he was surprised to discover a new opportunity.
“Besides giving me the real-time information, the sensors gave me the ability to look at trends…over a week or a month and be proactive if we started moving away from our set point. We could add more water, set shorter run times, or just make some changes in the irrigation system to get more in line with the set points. That was one of my biggest surprises, how well we were able to be proactive toward environmental changes using the trending of the charts. That was a bonus.”
Reducing production and labor costs
After five years of daily monitoring, Bauers is now ready to go to an even higher level. “The next huge area we see sensors in is as big, or bigger, than the actual growing of the plant itself. We’re going to use these sensors to guide us as we strip out all excess production costs, and that’s happening today. As an example, over the next five months we’ll be trimming our substrate use by 85%. Not only do we save on materials, but if you have 85% less substrate to work with or move, you reduce labor costs.”
In fact, the sensors have become an integral part of how Bauers does business. I asked him how he would feel if he lost them. “My gosh,” he said, “It would be like going back ten years. It would be like trying to measure the temperature in a room without a thermometer. We are totally dependent on them.”
Everybody measures soil water content because it’s easy. But if you’re only measuring water content, you may be blind to what your plants are really experiencing.
Soil moisture is more complex than estimating how much water is used by vegetation and how much needs to be replaced. If you’re thinking about it that way, you’re only seeing half the picture. You’re assuming you know what the right level of water should be—and that’s extremely difficult using only a water content sensor.
Get it right every time
Water content is only one side of a critical two-sided coin. To understand when to water or plant water stress, you need to measure both water content and water potential.
In this 30-minute webinar, METER soil physicist, Dr. Colin Campbell, discusses how and why scientists combine both types of sensors for more accurate insights. Discover:
Why the “right water level” is different for every soil type
Why soil surveys aren’t sufficient to type your soil for full and refill points
Why you can’t know what a water content “percentage” means to growing plants
How assumptions made when only measuring water content can reduce crop yield and quality
Water potential fundamentals
How water potential sensors measure “plant comfort” like a thermometer
Why water potential is the only accurate way to measure drought stress
Why visual cues happen too late to prevent plant-water problems
Case studies that show why both water content and water potential are necessary to understand the condition of soil water in your experiment or crop
Dr. Colin Campbell has been a research scientist at METER for 20 years following his Ph.D. at Texas A&M University in Soil Physics. He is currently serving as Vice President of METER Environment. He is also adjunct faculty with the Dept. of Crop and Soil Sciences at Washington State University where he co-teaches Environmental Biophysics, a class he took over from his father, Gaylon, nearly 20 years ago. Dr. Campbell’s early research focused on field-scale measurements of CO2 and water vapor flux but has shifted toward moisture and heat flow instrumentation for the soil-plant-atmosphere continuum.
In this chalk talk, METER Group research scientist, Dr. Colin Campbell, extends his discussion on humidity by discussing how to calculate vapor pressure from wet bulb temperature. Today’s researchers usually measure vapor pressure or relative humidity from a capacitance-based relative humidity sensor.
However, scientists still talk in terms of wet bulb and dew point temperature. Thus, it’s important to understand how to calculate vapor pressure from those variables.
Hello, my name is Dr. Colin Campbell. I’m a research scientist here at METER group, and also an adjunct professor at Washington State University where I teach a class on environmental biophysics. And today we’re going to be extending our discussion on humidity by talking about how using a couple of common terms related to humidity, we can calculate vapor pressure. The first term we’re going to talk about is dew point temperature. I’ve drawn a couple of figures below that illustrate a test I performed when I was a graduate student in a class related to biophysics.
The professor had us take a beaker of water and a thermometer and put ice in the beaker and start to stir it. The thermometers were rotating around in the glass, and our job was to look carefully and find out when a thin film of dew began to form around on the glass. So we watched the temperature go down, and at some point, we observed a thin film form onto that glass. At the point the film began to form, we looked at the temperature to get the dew point temperature, which means exactly what it says: the point at which dew begins to form.
This experiment wasn’t perfect because there is certainly a temperature difference between the inside of our glass where we’re stirring with the thermometer and the outer surface of the glass. But it was a good approximation and a great way to demonstrate what dew point temperature is. So we can say that the dew point temperature is the point at which the air is saturated and water begins to condense out. We call this Td or dew point temperature. The beautiful thing about dew point temperature is that if you know this value, you can easily calculate vapor pressure and even go on to calculate relative humidity, as I talked about in another lecture.
To calculate vapor pressure from our dew point temperature, we’ll call vapor pressure of the air, ea which is equal to the saturation vapor pressure (es) at the dew point temperature (Td) (Equation 1).
And as I discussed in my other lecture, the saturation vapor pressure is a function of the temperature (not multiplied by the temperature). It’s pretty simple to get the saturation vapor pressure at the dew point temperature. We simply use Tetons formula (Equation 2 discussed here), which says that the saturation vapor pressure at the dew point is equal to 0.611 kilopascals times the exponential of b Td over C plus Td (Td being the dewpoint temperature).
So let’s assume our dew point temperature is five degrees C. This is something you can find in many weather reports. If you look down the list of measurements carefully, it’s usually there. So the vapor pressure of the air (ea) is calculated by the formula I showed (Equation 1). Our first constant b is 17.502 and our second constant C, is just 240.97 degrees C. If we plug all the values into that equation, it ends up that our vapor pressure is 0.87 kilopascals.
Now there might be a variety of reasons we want this value. We might want to use it to calculate the relative humidity. If so, we’d simply divide that by the saturation vapor pressure at the air temperature. Then we’d have our relative humidity. More commonly we use the ea and the saturation vapor pressure at the air temperature to calculate the vapor deficit. So possibly in some agronomic application that might be interesting to us. So that is dew point temperature.
Now we’ll talk about another common measurement, our wet bulb temperature. This was much more common in past years where there weren’t electronic means to measure things like dew point or humidity sensors. And we used to have to make a measurement of humidity by hand. And what they did was to collect a dry bulb temperature or a standard air temperature. And that dry bulb temperature (or the temperature of the air) was compared to what we call a wet bulb temperature.
Researchers made this wet bulb temperature by putting a cotton wick around the bulb of the thermometer. This was just a fabric with water dripped onto it. Once that wick is saturated with water, the water begins to evaporate, and they would use wind to enhance that evaporation. For example, some instruments had a small fan inside that would blow water across this wick, or more commonly, two temperature sensors were attached on a rotating handle, so they could spin them in the air at about one meter per second (or two miles an hour). I don’t know how you’d ever estimate that speed, but that was the goal. This would help the water evaporate at an optimum level.
You can imagine what happens during this evaporation by thinking about climbing out of the pool. You feel some cooling on your skin as water begins to evaporate when you climb out of a pool on a dry, warm summer day. That’s water as it changes from liquid into water vapor, and it actually takes energy for this to happen (44 kilojoules per mole). That’s actually quite a bit of energy used for changing liquid water into water vapor. When that happens, it decreases the temperature of this bulb. If we wait till we’ve reached that maximum temperature decrease, we can take that as our wet bulb temperature, or Tw.
This wet bulb temperature is not quite as simple as our dew point temperature to use in a calculation. Here’s the calculation we need to estimate vapor pressure from the wet bulb temperature.
We take the saturation vapor pressure (es) at the wet bulb temperature (Tw) and subtract, the gamma (Ɣ), which is the psychrometer constant 6.66 times 10-4 ℃-1 times the pressure of the air (Pa), multiplied by the difference between the air temperature (Ta) or that dry bulb that I mentioned earlier, and the wet bulb temperature (Tw).
Gamma is an interesting number. It’s actually the specific heat of air divided by the latent heat of vaporization, or that 44 kilojoules per mole that I mentioned before. We can simply take it as a constant for our purposes here as 6.66 times 10-4 ℃-1. So let’s actually put it into a calculation. Our example problem says find the vapor pressure of the air. If air temperature (Ta) is 20 degrees Celsius, the wet bulb temperature (Tw) is 11 degrees Celsius, and air pressure (Pa) is 100 kilopascals (basically at sea level). And just to remind us, this is the constant gamma (6.66 times 10-4 ℃-1). Air pressure is 100 kilopascals. We take this standard equation (Equation 4) and insert all these numbers.
So our vapor pressure is going to be this calculation from Tetons formula (Equation 2) and if you plug all those numbers into your calculator (notice our degrees C will cancel) we’re left with kilopascals. So our vapor pressure is about 0.71 kilopascals. So that is how we calculate the vapor pressure from the wet bulb temperature.
I hope this has been interesting. These are values that you may hear about. It’s less common today since we usually get our relative humidity from a capacitance-based relative humidity sensor, but still scientists talk in terms of wet bulb and dew point temperature. So it’s important to understand how we actually calculate our vapor pressure from those variables. If you’d like to know more about this, please visit our website, metergroup.com, and look at some of the instruments that are there to make measurements. Or you can email me if you want to know more at [email protected]. I hope you have a great day.
If you rely on soil moisture data to make decisions, understand treatment effects, or make predictions, then you need that data to be accurate and reliable. But even one small oversight, such as poor installation, can compromise accuracy by up to +/-10%. How can you ensure your data represent what’s really happening at your site?
Best practices you need to know
Over the past 10 years, METER soil moisture expert Chris Chambers has pretty much seen it all. In this 30-minute webinar, he’ll discuss 6 common ways people unknowingly compromise their data and important best practices for higher-quality data that won’t cause you future headaches. Learn:
Are you choosing the right type of sensor or measurement for your particular needs?
Are you sampling in the right place?
Why you must understand your soil type
How to choose the right number of sensors to deal with variability
At what depths you should install sensors
Common installation mistakes and best practices
Soil-specific calibration considerations
How cable management can make or break a study
Factors impacting soil moisture you should always record as metadata
Choosing the right data management platform for your unique application
Hello, my name is Colin Campbell. I’m a senior research scientist here at METER Group. And today we’ll talk about how to estimate the transpiration from a single leaf. Occasionally we get this question: Can I estimate the transpiration from a leaf by measuring its stomatal conductance? Unfortunately, you can’t. And I want to show you why that’s true and what you’ll need to do to estimate the total conductance, and therefore, the evaporation of a leaf.
The calculation of transpiration (E) from a leaf is given by Equation 1
where gv is the total conductance of vapor from inside the leaf into the air, Cvs is the concentration of vapor inside the leaf and Cva is the concentration of vapor in the air.