Understanding the amount of drainage that comes out of the bottom of the root zone and infiltrates into groundwater recharge is a very difficult measurement to do well. Drain gauges do a good job of it but on a small scale. Large lysimeters do an even better job, but are extremely expensive and complex. There is an economical alternative, however, called the salt balance approach to measuring drainage.
Soil profile underneath canola
The Salt Balance Approach
Since the majority of non-fertilizer salts in the soil solution don’t get taken up by plants, this salt can be used in soil as a conservative tracer. This means that whatever salt is applied to the soil through rainfall or irrigation water is either stored in the soil or leaches through the profile with the soil water, enabling us to use conservation of mass in our salt balance analysis. The electrical conductivity of water (ECw) is directly proportional to the salt concentration, so ECw can be used in place of salt concentration in this analysis. If you measure the EC of the water that’s applied to the soil, either through irrigation or precipitation, as well as the EC of the water that’s coming out of the bottom of your profile, then you can calculate what fraction of the applied water is being transpired by the plants, and what fraction is draining out of the bottom. This method is useful for measuring water balance at field sites.
To illustrate this concept, let’s work through a simple example. A particular field received 40 cm of water through precipitation and irrigation. The average ECw of the precipitation and irrigation water is 0.5 dS/m. Measurements of ECw draining from the soil profile below the root zone indicate an ECw of 2.0 dS/m. The drainage or leaching fraction can be easily calculated as :
The amount of water drained can also be easily calculated as:
Leaching fraction * applied water = 0.25 * 40 cm = 10 cm
Measuring Pore Water EC (ECw)
One challenge to this approach is the measurement of water electrical conductivity itself. Bulk EC is a relatively simple measurement, and several types of soil water content sensors measure it as a basic sensor output. However, the electrical conductivity of water, called pore water EC (ECw), is more complex. Pore water EC requires that it be either estimated from the bulk EC and soil water content or that a sample of pore water be pulled from the soil matrix and measured. When estimated, pore water EC can contain considerable error. In addition, removing a water sample and measuring the pore water EC is not easy.
During a recent semester at Washington State University, a film crew recorded all of the lectures given in the Environmental Biophysics course. The videos from each Environmental Biophysics lecture are posted here for your viewing and educational pleasure.
Dr. Khot and his postdoc, Dr. Jianfeng Zhou, are using leaf wetness sensors to determine if and how long water is present on cherry tree canopies after a rain event. Dr. Khot hopes that data from these sensors will help growers decide whether or not it makes sense to fly helicopters in order to dry the canopies.
Dr. John Selker, hydrologist at Oregon State University and one of the scientists behind the Trans African Hydro and Meteorological Observatory (TAHMO) project, gives his perspective on the future of sensor technology.
Michelle Newcomer, a PhD candidate at UC Berkeley, (previously at San Francisco State University), recently published research using rain gauges, soil moisture, and water potential sensors to determine if low impact design (LID) structures such as rain gardens and infiltration trenches are an effective means of infiltrating and storing rainwater in dry climates instead of letting it run off into the ocean.
Looking up at a tree canopy
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In the conclusion of our three part series on the reforestation of Banguet province in the Philippines, we asked Dr. Anthony S. Davis, Tom Alberg and Judi Beck Chair in Natural Resources at the University of Idaho, Loreca Stauber, one of the visionaries behind the project, and Kea Woodruff, former U of I Nursery Production and Logistics Associate, now at Harvard University, to explain some challenges associated with teaching reforestation to different cultures.
Even with increased environmental awareness, we’re still losing almost thirty million acres of forest globally every year.
What are some of the cultural challenges?
Anthony: As I spend more and more time looking at international forests, I realize that we’re losing forests at a phenomenal rate. Even with all of our awareness about where we get supplies, where trees come from, where wood comes from, and where paper comes from, we’re still losing almost thirty million acres of forest globally every year. That’s terrifying to me. What’s even worse is that most of it comes from countries that don’t have environmental controls. They don’t have systems in place that keep them from cutting down all the trees. Often, when we cut trees down for forestry, we replant. But, when you start to work in countries where that’s not valued or not part of the culture or the system, then a huge problem emerges.
How do you teach people to grow trees that can survive in their native terrain?
Anthony: There isn’t a lot of knowledge globally about how to grow high-quality tree seedlings. I’ve gotten really interested in the question of how to take a tree seedling which is grown in a nursery, where it essentially has all of the water and all of the nutrients it could possibly ask for, and get it into a condition where it’s likely to survive somewhere extremely harsh: with limited nutrients and water. How do you get it to the point where it’s able to overcome those challenges?
There are two ways to look at that. One is to get more water to that seedling after it’s planted. The other is to make sure that the seedling you’re planting has its best possible chance of developing a root system that can access water that might not normally be available in those six inches where healthy roots are located when it’s first planted. Based on work that’s be done here at the University of Idaho in graduate student projects over the years, we found that if you can grow a seedling in a healthy manner in the nursery, it’s more likely to grow roots or access water that previously they might not have been able to access.
Working on one of the water tanks that will supply water to the Benguet nursery in the Philippines. The project is proceeding nicely after a series of setbacks: a destructive typhoon, slides that had to be cleared, 2 deaths, 1 funeral, and electrical power interruptions.
What challenges the plants after they leave the nursery?
Anthony: If that seedling can get roots down and access water, it starts to grow. The beauty of reforestation, in general, is that it’s very simple; it can be very easy to get trees to grow. However, what often happens is you have a social element that overlaps the biological element. Some of it could be a lack of education, where people don’t understand that a large amount of foliage or leaves on a tree means that you need more water. You think about that image of success: people want to plant the biggest tree possible. That might work in a yard, but it really doesn’t work in a reforestation situation.
What are the challenges of establishing a nursery in a place like the Philippines?
Kea: In the place like the Philippines where resources aren’t necessarily as available, it becomes a huge challenge just finding the right kind of media or container. Also, there’s a decentralization of the knowledge resource itself. While we were there, we had the opportunity to meet with different government agencies, and there are definitely people who know a lot about the species that are available and how to grow them, but in terms of that information being disseminated and widely available to the public, that’s a challenge. The techniques that will be needed to actually produce a seedling resource need to be addressed.
Loreca: The basic thing is a good nursery. That has been a problem. In the past, the government, in an effort to green the Philippines, has given seedlings, but oftentimes, these seedlings are so poor in quality that they don’t survive in out planting.
Coffee beans will thrive in the tropical Philippines.
How can you help other cultures to succeed at reforestation?
Anthony: During some work I was doing in the Middle East, in Lebanon, we found that communicating to people what a high-quality seedling became really important. You teach them about quality, defining it in terms of how much water a plant needs to survive, or how a plant has to grow in order to colonize a site. We had a lot of success with the project there, getting people to understand that there was a problem in only looking at above ground information in terms of what makes a high-quality seedling. Really, when the roots are what’s driving survival, they’re looking at the wrong part of the picture.
How do you teach people to think beyond the nursery?
Anthony: Our work in Lebanon coincided with a project in Haiti. In Haiti, we had a former student who had been here at the University of Idaho who asked for help starting a nursery. These same conversations occurred: what is a healthy seedling, what is likely to survive, where do you get your seed, how long do you grow it for, when do you plant it? We were able to have conversations around all of the elements that go into growing trees.
I remember clearly the “aha” moment where this young woman said, “We’ve been doing it wrong! We’ve always focused on growing as many seedlings as possible, and we haven’t worried about quality.”
See it live
Watch a video where Anthony talks about his work.
You can learn more about the reforestation programs that the University of Idaho nursery is involved with here.
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In one of the first agroforestry efforts in mountainous terrain, Moscow, Idaho community leader Loreca Stauber, Dr. Anthony S. Davis, Tom Alberg and Judi Beck Chair in Natural Resources at the University of Idaho, and their partners have initiated a program where U of I students travel overseas to work with farmers of Banguet province in the Philippines to develop the skills needed to grow high quality tree seedlings. Local vegetable farmers have historically terraced the mountains that have been forested so they could grow monoculture crops, causing serious erosion (read about it here). The land has degraded so much that the Philippine government has stepped in: warning farmers to begin conservation techniques, or they will take away the land and manage it themselves.
Building a local nursery in Benguet.
Inspiring Students to Look at the Big Picture
One of the steps in helping local farmers to solve this problem is to create a local nursery where they can start growing native plants and trees. Fortunately, the University of Idaho has operated a tree nursery for over one hundred years, and they understand how to grow trees. Dr. Davis specializes in setting up native nurseries for growing native plants all over the world. He says, “I want our students to be exposed to this because we’re graduating students who should be problem solvers, who should be able to look at the biggest challenges and contribute their own ideas towards resolving those challenges.”
Loreca Stauber adds, “We are part of the world and the world is part of us. The students can do more than just get their degree and find a job. Anthony and Kea, when they do this, inspire students to look at a bigger world than they are currently living in.”
Training Students to Understand Native Terrain and Resources
Davis says a good plan needs to take local conditions into account: “The principles of growing trees are actually universal. It doesn’t matter whether you’re in Haiti, Lebanon, Idaho, or in the Philippines. Those principles are the same and they’re readily transferable. It’s how you adapt them to unique local situations that makes a difference.”
“It’s not really about the best way to grow a plant in a greenhouse environment; It’s about the best way to grow a plant that will also survive on its outplanting site.”
Kea Woodruff, former U of I Nursery Production and Logistics Associate, now at Harvard University, says they train the students who go overseas on the “target plant” concept: designing a growing regime based on what the plant is going to need in its future home. She says, “It’s not really about the best way to grow a plant in a greenhouse environment; It’s about the best way to grow a plant that will also survive on its outplanting site. Determining what the outplanting site is and what each species will need to survive on that outplanting site is what determines greenhouse operations.”
Dr. Davis says you need to consider native resources when doing these types of projects. “There could be plumbing there, but there’s no guarantee that when you turn the system on, the tap water will come out. That depends on the seasonality of the rains. It’s part of why we wanted the project partners (the farmers) to have data loggers: so we could look at the data together and get a better feel for when water is most abundant and when it’s most scarce, so it can be stored for later use.”
Overcoming Native Challenges with Remote Data
Decagon (now METER) donated data loggers to the program so that Dr. Davis and other people on the team could look at data with the farmers in the Philippines and advise them when to irrigate. Davis says, “One of the things that’s most important in trying to set up a very remote nursery and manage the production in that nursery from approximately four flights, twelve hours, and twelve time zones away, is knowing what’s going on. There are things that are really easy to ask, like could you send me a picture every Wednesday and Saturday of the nursery, or could you measure the height and the diameter of the seedlings? What’s much harder to tell is how much water is coming in, or what the temperature was during the day or night, because those require people to be monitoring things at a greater frequency than is often possible. If we know how much water is coming into the nursery from rainfall, we can build collection systems so that we can manage where that water goes later on.”
Managing data for both the short and long term is critical, says Davis, because it’s often whether there was rainfall in the predicted amount, and at the right time, that determines whether a seedling establishes or not.
Acknowledgements: The SEAGAA agroforestry project in Benguet is agro and forest; the farmers received a grant from the Rufford Foundation based in the UK to build a greenhouse and much of the water catchment system and auxiliary structure that go with a nursery facility. They also received a sizable grant from the Philippine government to launch mushroom growing as a necessary complement to help support long-term agroforestry. The project is beyond reforestation – it is the growing of trees, shrubs, ground cover, the restoring of watersheds, creating livelihoods, the rebuilding of soil fertility and integrity, the revival of springs which have vanished with the removal of perennial flora, and the restoring biodiversity to bring back the natural checks and balances of a natural ecosystem.
In the mountainous Benguet province of the Philippines, farmers grow up to three crops of vegetables a year. Their mountain vegetable farms exist at the expense of original forest cover, causing tremendous erosion difficulties. To counteract erosion and preserve the watershed as well as promote reforestation, the Philippine government issued a mandate: farmers must find alternatives that restore the watershed or lose their land.
Rice terraces in the Philippines
An Agroforestry Alternative
Loreca Stauber is no scientist, but she loves Benguet, and a letter from her friend, a scientist living in the Philippines, inspired her with the vision of teaching farmers to reforest the mountains and grow vegetables amongst the trees.
Her friend writes, “We envision mountain farms as forest ecosystems whose primary social responsibility to the communities around and below is to be part of responsible watersheds that court, catch, store and gradually share water. We see mountain farms that are not prone to soil erosion or leaching: cultivated with minimal chemical inputs and tillage that will allow the natural buildup of biomass, organic matter, helpful organisms and fauna. We think of forest ecosystems that may not make millionaires of its farmers for one generation and heavy debtors even before the next. Rather, we envision forest farm ecosystems that are self-sufficient and self-sustaining. We are working on demonstrating forest ecosystems that can substitute for monocrop vegetable farms that deplete and leach the soil, pollute watersheds and are self-destructing.”
Realizing the problem in the Philippines could be solved by reforestation, Loreca emailed Dr. Anthony S. Davis, Tom Alberg and Judi Beck Chair in Natural Resources in the University of Idaho’s Department of Forest, Rangeland, and Fire Sciences. The U of I operates a 100-year-old nursery specializing in growing hardy tree seedlings. Dr. Davis recalls, “The email she sent me said, “I think you should do something about this,” and I thought, “Actually I agree. I think we should do something about this. So we began to screen the idea, asking: are there partners? Is it a good idea? Does it fit with this little thing that we do really well, which is essentially teaching people how to grow tree seedlings, and is there an educational component that’s valuable for our students? When those check boxes lined up, then it was a matter of taking advantage of that opportunity and seeing where it could go.”
Forested mountains in the Philippines
Determining What Already Works
Together, they and other partners started a program in which U of I students went overseas to teach the people of Benguet how to grow trees, with the goal of moving the land toward agroforestry. They wanted to grow a forest ecosystem (trees, shrubs, and ground cover) along with annual crops. Kea Woodruff, former U of I Nursery Production and Logistics Associate, now at Harvard University, traveled to the Philippines with an interdisciplinary team of undergraduate and graduate students to look at what agroforestry projects were already working and to conduct a needs assessment. She says, “I saw a wide variety of landscapes in the areas that we were. One woman decided on her own that she was going to practice agroforestry, and people come and view her land as a demonstration site. It has mature bamboo, coffee trees, and mature Benguet pine. It really looks like what you would expect the native forest to look in an area like the Philippines.”
Kea said there were also intermediate sites where there are Benguet pines and some coffee with row crops blended in, such as strawberries and squash. She adds, “There’s clearly great potential to grow different species on these lands if we can help figure out the best way to use the resources that are available.”
Next week: Learn how partners in the project have been able to use native resources in the quest to reforest erosion-plagued Benguet.
Take our Soil Moisture Master Class
Six short videos teach you everything you need to know about soil water content and soil water potential—and why you should measure them together. Plus, master the basics of soil hydraulic conductivity.
Screening for drought tolerance in wheat species is harder than it seems. Many greenhouse drought screenings suffer from confounding issues such as soil type and the resulting soil moisture content, bulk density, and genetic differences for traits like root mass, rooting depth, and plant size. In addition, because it’s so hard to isolate drought stress, some scientists think finding a repeatable screening method is next to impossible. However, a recent pilot study done by researcher Andrew Green may prove them wrong.
Automatic Irrigation Setup
The Quest for Repeatability
Green says, “There have been attempts before of intensively studying drought stress, but it’s hard to isolate drought stress from heat, diseases, and other things.”Green and his advisors, Dr. Gerard Kluitenberg and Dr. Allan Fritz, think monitoring water potential in the soil is the only quantifiable way to impose a consistent and repeatable treatment. With the development of a soil-moisture retention curve for a homogeneous growth media, they feel the moisture treatment could be maintained in order to isolate drought stress. Green says, “Our goal is to develop a repeatable screening system that will allow us to be confident that what we’re seeing is an actual drought response before the work of integrating those genes takes place, since that’s a very long and tedious process.”
Why Hasn’t This Been Done Before?
Andrew Green, as a plant breeder, thinks the problem lies in the fact that most geneticists aren’t soil scientists. He says, “In past experiments, the most sophisticated drought screening was to grow the plants up to a certain point, stop watering them, and see which ones lived the longest. There’s never been a collaborative approach where physiologists and soil scientists have been involved. So researchers have imposed this harsh, biologically irrelevant stress where it’s basically been an attrition study.” Green says he hopes in his research to use the soil as a feedback mechanism to maintain a stress level that mimics what exists in nature.
Green used volumetric water contentsensors, matric potential sensors, as well as column tensiometers to monitor soil moisture conditions in a greenhouse experiment using 182 cm tall polyvinyl chloride (PVC) growth tubes and homogenous growth media. Measurements were taken four times a day to determine volumetric water content, soil water potential, senescence, biomass, shoot, root ratio, rooting traits, yield components, leaf water potential, leaf relative water content, and other physiological observations between moisture limited and control treatments.
Soil Media: Advantages and Disadvantages
To solve the problem of differing soil types, Andrew and his team chose a homogeneous soil amendment media called Profile Greens Grade, which has been extensively studied for use in space and other applications. Green says, “It’s a very porous material with a large particle size. It’s a great growth media because at the end of the experiment you can separate the roots of the plant from the soil media, and those roots can be measured, imaged, and studied in conjunction with the data that is collected.” Green adds, however, that working with soil media isn’t perfect: there have been hydraulic conductivity issues, and the media must be closely monitored.
What’s Unique About this Study?
Green believes that because the substrate was very specific and his water content and water potential sensors were co-located, it allowed him to determine if all of his moisture release curves were consistent. He says, “We try to pack these columns to a uniform bulk density and keep an eye on things when we’re watering, hoping it’s going to stay consistent at every depth. So far it’s been working pretty well: the water content and the water potential are repeatable in the different columns.”
Entire Irrigation setup for the expanded study.
Plans for the Future
Green’s pilot study was completed in the spring, and he’s getting ready for the expanded version of the project: a replicated trial with wild relatives of wheat. He’s hoping to use soil moisture sensors to make automatic irrigation decisions: i.e. the water potential of the columns will activate twelve solenoid valves which will disperse water to keep the materials in their target stress zone, or ideal water potential.
The Ultimate Goal
The ultimate goal of Green’s research is to breed wild species of wheat into productive forms that can be used as farmer-grown varieties. He is optimistic about the results of his pilot study. He says, “Based on the very small unreplicated data that we have so far, I think it is going to be possible to develop a repeatable method to screen these materials. With the data that we’re seeing now, and the information that we’re capturing about what’s going on below ground, I think being able to hold these things in a biologically relevant stress zone is going to be possible.”
Take our Soil Moisture Master Class
Six short videos teach you everything you need to know about soil water content and soil water potential—and why you should measure them together. Plus, master the basics of soil hydraulic conductivity.
In a previous post, we discussed water potential as a better indicator of plant stress than water content. However, in most situations, it’s useful to take dual measurements and measure both water content and water potential. In a recent email, one of our scientist colleagues explains why: “The earlier article on water potential was excellent. But what should be added is an explanation that the intensity measurement doesn’t translate directly into the quantity of water stored or needed. That information is also required when managing water through irrigation. This is why I really like the dual measurement approach. I am excited about the possibilities of information that can be gleaned from the combined set of water content, water potential, and spectral reflectance data.”
Potato field irrigation
The value of combined data can be illustrated by what’s been happening at the Brigham Young University Turf Farm, where we’ve been trying to optimize irrigation of turfgrass (read about it here). As we were thinking about how to control irrigation, we decided the best way was to measure water potential. However, because we were in a sandy soil where water was freely available, we also guessed we might need water content. Figure 1 illustrates why.
Figure 1: Turf farm data: water potential only
Early water potential data looks uninteresting; it tells us there’s plenty of water most of the time, but doesn’t indicate if we’re applying too much. In addition, if we zoom in to times when water potential begins to change, we see that it reaches a stress condition quickly. Within a couple of days, it is into the stress region and in danger of causing our grass to go into dormancy. Water potential data is critical to be able to understand when we absolutely need to water again, but because the data doesn’t change until it’s almost too late, we don’t have everything we need.
Figure 2: Turf farm data, volumetric water content only
Unlike water potential, the water content data (Figure 2) are much more dynamic. The sensors not only show the subtle changes due to daily water uptake but also indicate how much water needs to be applied to maintain the root zone at an optimal level. However, with water content data alone, we don’t know where that optimal level is. For example, early in the season, we observe large changes in water content over four or five days and may assume, based upon onsite observations, that it’s time to irrigate. But, in reality, we know little about the availability of water to the plant. Thus, we need to put the two graphs together (Figure 3).
Figure 3: Turfgrass data: both water potential and volumetric water content together.
In Figure 3, we have the total picture of what’s going on in the soil at the BYU turf farm. We see the water content going down and can tell at what percentage the plants begin to stress. We also see when we’ve got too much water: when the water content is well above where our water potential sensors start to sense plant stress. With this information, we can tell that the turfgrass has an optimal range of 12% to 17% volumetric water content. Anything below or above that range will be too little or too much water.
Figure 4: Turfgrass soil moisture release curve (black). Other colors are examples of moisture release curves for different types of soil.
Dual measurements will also allow you to make in situsoil moisture release curves like the one above (Figure 4), which detail the relationship between water potential and water content. Scientists can evaluate these curves and understand many things about the soil, such as hydraulic conductivity and total water availability.
Trevor Dragon, a former METER Research and Development Engineer, was pouring concrete at his Beeville, Texas, farm one day and wondered if he could measure moisture in concrete with a matric potential sensor instead of the more traditionally used volumetric water contentsensor (VWC) to get more accurate readings. Dragon says, “We had about five concrete trucks come in that day, and we poured five different slabs. Every truck had a different amount of water added. One particular batch of concrete was really wet and soupy, and I became curious to measure it and compare it to the other slabs.”
Concrete slab drying down at Trevor’s Texas farm.
Why Measure Moisture in Concrete?
As concrete hardens, portland cement reacts with water to form new bonds between the components of the concrete. This chemical process, known as hydration, gives concrete its characteristic rock-like structure. Too much or too little water can reduce the strength of the concrete. Adding excess water can lead to excessive voids in concrete while providing too little water can inhibit the cement hydration reaction. Thus, when you pour a slab in south Texas, where it’s exposed to high wind and intense heat, sufficient water must be added, and precautions must be taken to minimize evaporation of water from the slab surface as the concrete hardens.
Dragon chose the matric potential sensor because he wondered if it would be more accurate than a VWC measurement. He says, “I knew that VWC sensors were calibrated for soil, and because of that they would lack accuracy. But the water potential sensor is calibrated for the ceramic it contains. I figured it would be closer to the real thing without having to do a custom calibration.”
Moisture in concrete has been difficult to measure because the high electrical conductivity early in the hydration process throws off water content sensor calibration. So, Dragon was surprised when his data turned out to be really good. He comments, “The dry down curve of the matric potential sensor was a perfect curve. There was a nice knee (drop from saturation) after about 200 minutes, and it just went down from there. We’re kind of stumped because we are trying to understand why the data came out so well and why the curve looks so good.”
Water Potential in Concrete
The scientists at METER sent the drydown curve to Dr. Spencer Guthrie, a civil engineering professor, to see what he thought. He explains, “I suspect that the concrete is experiencing initial set at around 200 minutes. This is a very normal time frame by which finishing operations need to be complete. At this stage in cement hydration, the concrete becomes no longer moldable. A rigid capillary structure is forming, and individual pores are taking shape. As hydration continues, the pores become smaller and smaller, which may explain the decrease in matric potential.”
One theory Dragon and his colleague Dr. Colin Campbell came up with was that perhaps Dragon’s unique method of inserting the sensors made a difference in the measurements. He explains, “The first thing I did was look for the rebar in the concrete, and I placed the sensors in the exact center of one of the squares to avoid the influence of metal on the sensor electromagnetic field. Also, I didn’t insert the sensors the same way you would insert them into soil. In soil, you put the sensors in vertically; I placed the water potential sensor horizontally because in this case, I was not interested in how water was moving in the slab but how it was being used over time.
What Does It Mean for the Future?
The behavior of the water potential sensor embedded in the concrete clearly indicated a drying process where water becomes less available over time. However, the implications are still unknown. Can the quality of the concrete be determined from the speed or extent of water becoming less available? Hopefully, this opportunistic experiment by Dragon will lead to more tests to show whether this approach is useful to others.
Dr. Guthrie agrees the idea should be explored further and comments, “The matric potential measurements were not redundant with the water content measurements. Instead, they offered additional, interesting information about the early hydration characteristics of the concrete. In the context of construction operations, the water potential data indicated what is normally determined by observing the impression left in the concrete surface from the touch of a finger. In the context of research, however, the use of a water potential sensor may yield helpful information about how certain admixtures, for example, influence the development of hydration products in concrete over time.”
In China recently, a fellow scientist asked Dr. Colin Campbell if matric potential sensors work in frozen soils. His answer? Sort of. In this blog, he explains what he meant by his enigmatic reply: When water freezes in the soil, most matric potential sensors won’t work accurately because frozen water essentially disappears to the measurement. For example, in a dielectric measurement circuit, most of the water that was polarized in the electromagnetic field solidifies in the ice matrix. Thus, because dielectric measurements determine the charge that is stored when water is polarized, ice is not measured. But, many matric potential sensors contain a component that will measure frozen water potential: the temperature sensor.
Horseshoe prints in frozen soil.
How Does Temperature Measure Water Potential?
The temperature of a frozen matrix like soil has a fundamental thermodynamic relationship to the energy state of that water. For every one degree C below freezing, the water potential decreases by 1.2 MPa. For example, if the soil drops down to -4 C, the soil water potential will be -4.8 MPa. However, one thing many people don’t understand is that there is still liquid water in frozen soils.
Where is the Liquid Water in a Frozen Soil?
Some liquid water will always be associated with soil surfaces because water, as a polar molecule, is attracted by opposite surface charges. Ice is a collection of water molecules that have slowed enough that they are arranged in a crystal-like structure. When ice arranges in that structure, it will attract and use all those water molecules that are available but will have difficulty stealing away water bound to soil surfaces. That water will remain liquid. As soil temperature drops, water layers closer and closer to soil particle surfaces will slow and join the ice structure.
Why Worry about Frozen Water Potential?
Previously, we’ve discussed the importance of water potential in determining the availability of water for plant growth. But below freezing, plants are either dormant or expired, so why measure frozen water potential?
There are a couple of reasons frozen soil water potential may be interesting to scientists. Liquid water in frozen soil still has the possibility to move. So, knowing soil temperature will allow models to predict water flow.
Even more interesting is what could be done with a temperature sensor and a measurement of water content using dielectric permittivity. As we mentioned earlier, ice essentially disappears to a dielectric measurement. Thus, a dielectric sensor water content measurement should provide the amount of liquid water in the soil. Using the temperature sensor to infer water potential (assuming the soil begins wet enough that its pre-frozen state has not reduced WP significantly), we can combine the WP and VWC measurements over a range of temperatures to generate an in situ moisture release curve. This idea was developed into a prototype instrument that appeared to have promise as a new laboratory technique to obtain moisture release curves.
Many athletes don’t like artificial turf. They say it’s hot, uncomfortable to run on, causes burns when you slide or fall on it, and changes the way a ball moves. Professional women’s soccer players even started a lawsuit over FIFA’s decision to use artificial turf in the 2015 Women’s World Cup.
Soccer players on natural turf.
Some universities—including Brigham Young University—have responded to athlete concerns by using natural turf fields for practice and in their stadiums. But the challenge is to develop plants and management practices for natural turf that help it stand up to frequent use and allow it to perform well even during the difficult fall months. It’s a perfect research opportunity.
BYU turf professor and manager of BYU sports turf, Bryan Hopkins and his colleagues in the Plant and Wildlife Department, have been able to set up a new state-of-the-art facility to study plants and soil in both greenhouse and natural conditions. The facility includes a large section of residential and stadium turfgrass.
Before Soil Sensors
Initially, BYU maintained the turf farm grass on a standard, timer-based irrigation control system, but over time they realized that understanding the performance of their turf relative to moisture content and nutrient load is crucial. Last year during Memorial Day weekend their turf farm irrigation system stopped working when no one was around to notice. During those four days temperatures rose to 40 C (100 F), and the grass in the field slipped into dormancy due to heat stress. In response, Dr. Hopkins began imagining a system of soil moisture sensors to constantly monitor the performance of the turf grass. He wanted not only to make sure the turf never died but also to really understand the elements of stress so they could do a better job growing healthy turf.
Sensors Give a Clear Picture
Soon afterward, a team of scientists, including fellow professor Dr. Neil Hansen, installed volumetric water content (VWC) and matric potential sensors at two different sites: one in the sports turf and one in a residential turf plot. Each plot had two installations of sensors at 6 cm and 15 cm, along with VWC only at 25 cm, to measure water moving beyond the root zone. Combining these measurements, they could clearly see when the grass was reaching stress conditions and how quickly the turf went from the beginning of stress (in terms of water content and time) to permanent wilting point. In addition, ancillary measurements of temperature and electrical conductivity provide an opportunity for modeling surface and root zone temperature as well as fertilizer concentration dynamics.
Installing water content sensors at the BYU turf farm.
What the researchers learned was that they were using too much water. Dr. Colin Campbell, a METER research scientist who worked with BYU on sensor installation, comments, “We found in the first year that the plants never got stressed at all. So this year, the researchers allowed the water potential (WP) at 6 cm to drop into the stress range (~ -500 kPa) while observing WP at 15 cm (-50 kPa to -60 kPa). We hope this approach will reduce irrigation inputs while creating some stress in the grass in order to push the roots deeper.”
What’s happening with the water?
Dr. Campbell’s favorite part of the sensor data was the detailed picture it gave of what was happening with the water in the sandy soil (Figure 1). He says, “Most people believe that they have an intuitive feel for water availability in soil. If we were only using water content sensors, seeing a typical value of 20% would lead us to believe we were comfortably in the middle of the plant available range (A). But in this study, using our colocated soil water content and soil water potential sensors, the data showed readings over 15% VWC were too wet to affect the WP (B). However, once WP visibly changed, it quickly moved toward critical stress levels (C, -1500 kPa is permanent wilting point); it only took two days for the water potential to change from -8 kPa to -1000 kPa. A subsequent dry period (D) shows similar behavior, but this time the 15 cm WP drops to near -1000 kPa.”
The plant stress levels were reached surprisingly quickly in this soil because its sand composition has a lot of large pores and not very many small ones (Figure 2). Campbell explains, “The large pores store water that is not held tightly due to low surface area, so the water is freely available. But at around 10% VWC all the water from the large pores is used up. As the soil dries beyond that, the water is held tightly in small pores and becomes increasingly unavailable. This is clear in the moisture release curve. We see almost no change in water potential as the soil dried to 16% VWC, but from 10% down to 7%, the water potential reached permanent wilting point, and it happened in just over a day.”
What the Future Holds:
The researchers wanted to make sure that if they went down to certain stress levels, they wouldn’t cause harm to the plants, so this year, they installed a weather station to monitor evapotranspiration and calculate irrigation application rates. They also began measuring spectral reflectance to monitor changes in leaf area (NDVI) and photosynthesis (PRI). This will enable them to see the impact on the plants as the turf is drying down. “In the future,” says Campbell, “we hope that both commercial and residential turf growers will be able to more effectively control their irrigation and nutrients based on what we find in this study.”