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Nov 19

Killing Cheatgrass and Shooting for the Moon

My grandfather, Grant A. Harris, wrote his Ph.D. thesis about the detrimental effects of cheatgrass (bromus tectorum) on rangeland ecology, so I’ve been taught since birth to hate this invasive plant species. So it didn’t surprise me to read that cheatgrass has become the equivalent of an eco-supervillain, wreaking havoc in farmer’s fields, rapidly spreading, and reducing wheat yield—sometimes by fifty percent.  

Microscope focusing on glass panel with sample on it

Washington State University scientist, Dr. Ann Kennedy, has successfully worked with a naturally-occurring soil bacteria that limits the depth of root growth in cheatgrass.

Cheatgrass increases the spread of wildfire, aiding the jump from plant to plant, and it afflicts livestock: lodging in the eyes and mouths of grazing cattle, not to mention having little nutritional value. For years, it’s been tenacious and incredibly prolific, out-competing native grasses and essentially “taking over” the eco-world.  Until now.  This New York Times article spotlights Washington State University scientist, Dr. Ann Kennedy, and her successful work with a naturally-occurring soil bacteria that limits the depth of root growth in cheatgrass, reducing its competitive advantage on the prairie.  

As a scientist, I was intrigued by this article because of what it didn’t say.  Dr. Kennedy, a good friend of mine and a great scientist, once told me that her bacteria experiment was the one she thought least likely to work. She’d looked at it as a kind of “shoot the moon” idea, riddled with “unknowns,” making it risky to spend too much time on.  In fact, she’d only had time to pursue this interesting and challenging experiment because she’d made time for it.

A concentrated solution of bacteria being sprayed on cheatgrass

A concentrated solution of bacteria is sprayed on fields, and over time, the organisms colonize the roots of the cheatgrass.

In a seminar she gave years ago about her work with cheatgrass, Dr. Kennedy shared her simple 60-30-10 prioritization method.  Sixty percent of her research effort was put into core projects she knew would yield publishable papers and keep her lab running.  Thirty percent of her time was spent on challenging projects that were more impactful but less likely to succeed. Finally, she put ten percent of her effort into “shoot the moon” type projects: research that was unlikely to come to fruition, but if successful, would have a dramatic impact in the world.  

In science, it’s easy to get stuck in the purely practical, only spending time on the experiments we know will work. It’s safer and won’t expose us to ridicule when things don’t go the way we hope. But, Dr. Kennedy has proven that there is value in trying things that might fail.  

It’s been more than a decade since I’ve listened to her lecture, but it still impacts the way we do research at METER. Although we spend a lot of time on projects we know will turn into finished instruments, we continue to dream up ways to produce frozen soil moisture release curves or measure leaf water potential. These ideas may not succeed, but if they do, they could have a big impact on the way we make measurements.

As I think about my team’s research priorities and the possibilities of success, I always first consider core projects: What are we really good at?  What will be a sure bet for success?  But because of Dr. Kennedy, I’ll always devote some of my time to more risky endeavors, speculating on what could happen and what might possibly change the world.  

(Read about our most spectacular example of risky research:  the collaboration with NASA’s Jet Propulsion Laboratories to send one of our sensors to Mars.)

Download the “Researcher’s complete guide to water potential”—>

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Nov 14

Sensor Data Improves Cherry Production

In July of 2013, Lav Khot and his team were in the field looking at how cherries were picked, weighed, and transported, when suddenly a helicopter began circling around a nearby orchard block.  When Dr. Khot asked the grower about it he said, “There was a rain last night, and we are trying to dry the tree canopies.” The grower told Khot that cherries are susceptible to cracking if moisture stays on the fruit too long, so they hire helicopters to fly over their orchards to remove water from the fruit and leaves, hoping to prevent fruit loss.

Bright red cherries on a tree

The economic impact of solving the cherry cracking problem could be huge as growers now suffer heavy losses each year.

Fresh market cherries are a lucrative business. That’s how the growers can afford the approximately $25K it costs to rent the helicopters every season. They try to do everything that they can to stop any cracking or splitting, but interestingly, Dr. Khot says grower decisions are influenced completely by emotion. “If there is a rain event, the farmer will become anxious, and they will hire pilots to fly the helicopter.”  

Dr. Khot wondered if he could help the cherry growers make their decisions based on real data instead. He and his postdoc, Dr. Jianfeng Zhou, are using leaf wetness sensors to determine if and how long water is present on the tree canopies after a rain event. Dr. Khot hopes that the data from these sensors will help growers decide whether or not it makes sense to fly the helicopter.

Why the cherries split

Not all varieties of cherry crack, but high sugar content varieties do as the skin is thin during maturation.  There are two hypotheses associated with fruit splitting or cracking:

Irrigation:  High water availability in the soil as the fruit is maturing (a few weeks before harvest) encourages trees to take up more water and causing the fruit to split.

Rainwater:  Rain collects in the cherry stem bowl or hangs off the bottom and is slowly absorbed into the fruit along the osmotic potential gradient. The fruit will start to split due to increased pressure inside the skin.

Dr. Khot and his team will use soil moisture sensors to investigate the first hypothesis with the object of improving irrigation management, especially as harvest approaches.  And he’s getting some support:  “Dr. Matt Whiting (colleague at the Center for Precision and Automated Agricultural Systems, Washington State University) is helping us understand this cracking phenomenon from the soil perspective. He is doing work on deficit irrigation (reducing the rate of irrigation below optimal) towards harvest time and seeing how that relates to cracking.  Also, the WSU CAHNRS ERI (Emerging Research Issues grant), which supports high-risk research, has funded us $75,000, along with Decagon who is supporting us with their sensors.”

Last Year’s Research

There are two approaches to drying canopies. One uses a sprayer that produces a cross-wind that moves sideways through the canopies, while the other uses the downwash from helicopter blades.  Last year, Dr. Khot and his research assistant experimented with crosswind velocities to see how much wind was being generated and how much water was really being dispersed.   Dr. Khot commented, “Last season we went out to the WSU orchard and ran the sprayer at two settings in order to see how water was removed and how much wind was coming through the canopies for a given amount of time.” They had good success at both removing the water from the trees and measuring it with the leaf wetness sensors.  But, they started the measurements after the cherries had matured, so weren’t able to tie it to cracking.

This Year’s Experiment

One issue with using helicopters is that they are extremely dangerous.  Accidents are not uncommon, and unfortunately pilots have died.  This year the team will also evaluate the efficacy of a mid-size, unmanned helicopter in order to test if it can produce enough downwash to dry the cherries and compare it with manned helicopters.  Dr. Khot says, “The helicopters are large and difficult to fly close to the canopies, but we can program the unmanned drone to fly close to the canopy and get rid of the water safely.” Digital Harvest and Yamaha, who are supporting this aspect of the research, have received an exemption from the FAA so they can test their unmanned helicopter.

Differing Tree Architectures

Dr. Khot’s team did their first experiments on traditional cherry tree architectures (imagine a typical tree), but this year they will perform their experiments on trees that are trained into a “Y” shape, or completely vertical.

Cherry production trees

These trees represent traditional tree architecture, but this year researchers will perform their experiments on trees that are trained into a “Y” shape, or completely vertical.

Researchers have developed these new architectures for ease of harvesting and management, but Dr. Zhou says that there will be less canopy variability and thus more interpretable results compared to the traditional tree architecture where wind velocity is more heterogeneous throughout the canopy.

Economic Impact  

Dr. Khot says the economic impact of solving the cherry cracking problem could be huge as growers now suffer heavy losses each year.  One former grower underscored this when he noted they lost one crop in every four. But, there could be other benefits as well. The implications of this research could lead to solving other grower problems such as disease and pest management.   “WSU already has a good AgWeatherNet program where we monitor the weather outside the trees at different locations, but not inside the canopies. If we had some smart sensing equipment like the leaf wetness sensor sitting in the canopy monitoring the wetness level over a 24-hour cycle, then we could develop some models based on the wetness and relate them to the number of pests at different locations in the orchard.  That is something every grower can benefit from.”

Download the “Researcher’s complete guide to water potential”—> 

Download the “Researcher’s complete guide to soil moisture”—>

Get more info on applied environmental research in our

Nov 13

Will Sample Disturbance Lead to Lower Accuracy?

Sampling soil for laboratory analysis of water potential is done for two basic reasons.  The simplest is to determine the current water potential of the soil.  The other is to determine the moisture release curve of the soil.  Regardless of the reason for measurement, the question of sample disturbance is important to ensure an accurate result.  Dr. Colin Campbell explains why:

researcher hand holding soil

Soil is disturbed when it’s removed from its natural structure.

Water Potential and Pore Size:

In soil samples, the void spaces (pores) in between soil particles can be simplistically thought of as a system of capillary tubes, with a diameter determined by the size of the associated particles and their spatial association.  The smaller the size of those tubes, the more tightly water is going to be held because of the surface association.  

In a clay, water will be held more tightly than in a sand at the same water content because clay contains smaller pores and thus more surface area for the water to bind to. But, even sand can eventually dry to a point where there is only a thin film of water on its surfaces and water will be bound tightly.  In principle, the closer water is to a surface, the tighter it will be bound.

Sample Disturbance

Sample disturbance (disturbing soil pores when you remove a sample from the ground) becomes an issue depending on the water potential of your sample. Typically, the less negative (wetter) the water potential, the larger impact sample disturbance will have on the measurement.  We can do a calculation that shows there are specific pore sizes associated with specific water potentials (see table 1).

Water potential units diagram

If you disturb a sample with low water potential, permanent wilting point (-1.5 MPa) for example, the pores that are still filled with water would be approximately 0.2 um in diameter, far too small to be broken apart by scooping up a sample.  Thus, we could reasonably assume that your WP readings won’t be affected much.  But if you disturb soil with higher water potential, say field capacity (-0.033 MPa), it’s much more likely that water will be disturbed, as it fills pores to approximately 9 um.    

Hygrometers

Still, this is only an issue if you are attempting to measure in a high WP range.  If your chilled-mirror hygrometer only measures up to -1000 kPa, sample disturbance will not be an issue because those pores that will have broken will likely be larger than the sub-micrometer that are holding water, which is beyond the accuracy of your instrument.   However, some hygrometers can now measure to an upper limit of -100 kPa, which approaches the point where sample disturbance will make a difference.  

Tensiometers

If you are sampling to measure with a tensiometer (measures 0 kPa to -80kPa), it’s extremely important to keep your samples intact because tensiometers cover the emptying range of the largest pores found in soil.  A soil collar (sample ring) pounded into the ground will yield the most intact soil core.  It’s the best method to use if you need make sure soil pores remain undisturbed to yield an accurate water potential measurement.

For a more in-depth examination of the magnitude of the effects of sample disturbance, read this chilled-mirror hygrometer App Note detailing the subject.

Download the “Researcher’s complete guide to water potential”—>

Download the “Researcher’s complete guide to soil moisture”—>

Get more information on applied environmental research in our

Nov 9

Screening for Drought Tolerance

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.

An automatic irrigation setup with green plants sticking out

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.

Data acquisition a cabinet setup for green's expanded experiment

Data Acquisition Cabinet setup for Green’s expanded experiment.

The Pilot Study

Green used volumetric water content sensors, 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.”

Irrigation setup for the expanded study with research data cabinet

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.

Watch it now—>

Download the “Researcher’s complete guide to soil moisture”—>

Download the “Researcher’s complete guide to water potential”—>

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Nov 5

Accurate Field Saturated Hydraulic Conductivity—Why is it so difficult?

Inaccurate saturated hydraulic conductivity (Kfs) measurements are common due to errors in soil specific alpha estimation and inadequate 3D-flow buffering.  Leo Rivera, METER research scientist, explains why getting an accurate saturated hydraulic conductivity (Kfs) measurement is so difficult.

Farming driving tractor spraying his field

Water infiltrates the soil in three dimensions; it spreads laterally, as well as downward.

“Sorptivity, or the ability of soil to absorb water, has traditionally been a complex measurement for scientists to make.  This is because water infiltrates the soil in three dimensions; it spreads laterally, as well as downward.  The problem is, the value which represents sorptivity, Kfs, is a one-dimensional value.  Scientists use Kfs in modeling as the basis of their decision-making, but they have to remove the effects of the three-dimensional flow to get that value.  

“The traditional method for removing those effects is to look at a table of alphas or the soil macroscopic capillary length.  But since alpha is an estimate of the sorptivity effect, or how much the soil is going to pull the water laterally, if you use the wrong value, your estimate is going to be significantly off.

“The other problem with making this measurement is that most researchers have found the double ring infiltrometer does not buffer three-dimensional flow perfectly. Thus, if you are operating on the assumption that you’re getting one-dimensional flow in the center ring, you will overestimate your field saturated conductivity (Kfs) values.  This can be disastrous, particularly if you’re working with a soil that has been engineered to have a very low permeability.  If you overestimate Kfs, you could incorrectly assume your cover is ineffective (Ks is over 10-5 cm s-1).  But really, you’ve overestimated Kfs, and the cover may actually be compliant.”

Leo discusses solutions to these and other infiltrometer difficulties the webinar “Advances in Lysimeter Technology“. 

Watch the webinar

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Download the “Researcher’s complete guide to soil moisture”—>