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Posts tagged ‘Irrigation’

Degradation of soil-applied herbicides under limited irrigation

Soil-applied herbicides are important for controlling weeds in many crops because they offer a broadened control spectrum and chemical diversity. But if soil-applied herbicides persist in the soil too long, there is risk for damage to susceptible rotational crops in succeeding years. Since herbicide degradation in the soil is highly dependent on water, imminent needs to reduce agricultural water use in the future could lead to limited herbicide degradation and a greater risk for carryover.

Image of a sunflower in a sunflower field facing the sun
Some crops don’t have a wide variety of post-emergent herbicide options, so growers are dependent on soil-applied herbicides for weed control.

Recently Daniel Adamson and a research team at the University of Wyoming, under the guidance of Dr. Gustavo Sbatella, investigated the effects of soil-applied herbicides under limited irrigation conditions. They wanted to understand how limited irrigation affects the efficacy and carryover of soil-applied herbicides in Wyoming’s irrigated crop rotations. A two-part field study was undertaken by applying four soil-applied herbicides to dry beans and four soil-applied herbicides to corn. 

Soil microbe activity matters

Describing his research site, Adamson says, “Wyoming is not a huge farming state but there’s a pocket of farm ground near the Powell/Cody area with a unique rotation. The main crop is sugar beets, and they also grow dry, edible beans, sunflowers and malt barley. Some of these crops don’t have a wide variety of post-emergent herbicide options, so growers are dependent on soil-applied herbicides for weed control. However, they need to balance weed control with timely dissipation so sensitive rotational crops won’t be injured.

Adamson says that soil-applied herbicides tend to be fairly long-lived in the soil, which is advantageous for weed control. Importantly, the herbicides dissipate through degradation by soil microbes, and soil microbes are highly influenced by how much water is in soil. When the soil is moist and warm, microbes are more active, and they degrade the herbicides faster. Thus, his team hypothesized that if future climate change effects led to limited availability of surface water for irrigation, these herbicides may not degrade as quickly and possibly injure crops planted successionally.

Assessing herbicide damage

During the first year, the research team applied three irrigation treatments to each crop: 100%, 85%, and 70% of crop evapotranspiration. Both crops and soil moisture were monitored using METER data loggers and soil moisture sensors. Adamson recalls, “The sensors were our means of tracking what was happening in the soil in terms of volumetric water content. Some of the areas were chronically dry, so the sensors enabled us to confirm that the treatments were applied correctly and should theoretically affect how the herbicides were performing. The volumetric soil water content of the three irrigation treatments averaged 24%, 18%, and 16% throughout the growing season, and crop yield decreased as irrigation was reduced.” 

Over the course of the second year, the team collected soil samples at regular intervals following herbicide application. They analyzed the samples for herbicide level and used them to perform a greenhouse bioassay to determine crop response to residual herbicide. Also during the second year, crop response was evaluated in the field when sugar beet, sunflower, and dry bean or corn was planted over the original plots and assessed for herbicide damage.

Crops planted in a field assessed for herbicide damage
The results of the experiment were surprising.

Hurdles and challenges

Adamson said timing was the major difficulty in terms of applying irrigation treatments. He said, “There were no differences in irrigation timing for the various treatments. The way we irrigated was not representative of a typical deficit irrigation strategy because we were tied to a sprinkler with other projects on it. So we irrigated based on when the full water treatment would normally be irrigated. Other treatments had smaller nozzles so the amount of water was physically reduced.”

Adamson said they also weren’t prepared to track how some of the herbicides would behave in the soil. “Some of the herbicides degrade into metabolites that are phytotoxic in the soil, and it was hard to analyze for all molecules that were plant active. So that was challenging.”

Surprising results

Adamson said the results of the experiment were surprising. He says, “It was a good result for growers because we found there were no differences in the fields, statistically or visually, between how the herbicides carried over in the really dry soil versus the normally irrigated soil. So that was surprising, but from a practical standpoint for farmers, it was important information. They now know if they do have to start applying less water, it isn’t something to be overly concerned about.”

More research is needed

Adamson says more work is needed in this area of research. He adds, “There’s a tremendous amount of information within the weed science community about what herbicides do in the soil and things that influence that. But relatively few studies look at changing irrigation rates in a practical sense. A lot of the current studies are done in rain-fed systems where the amount of rain changes (i.e., a normal year vs. a drought year). In irrigated systems, you might reduce the amount of water, but it’s not a drastic reduction like a rain-fed system might experience. There’s not a huge amount of research looking at how different irrigation rates affect herbicide management, so I do think it would be worth exploring in the future.”

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Irrigation Curves—A Novel Irrigation Scheduling Technique

This week, guest author Dr. Michael Forster, of Edaphic Scientific Pty Ltd & The University of Queensland, writes about new research using irrigation curves as a novel technique for irrigation scheduling.

Corn field with a blue cloudy sky background

Growers do not have the time or resources to investigate optimal hydration for their crop. Thus, a new, rapid assessment is needed.

Measuring the hydration level of plants is a significant challenge for growers. Hydration is directly quantified via plant water potential or indirectly inferred via soil water potential. However, there is no universal point of dehydration with species and crop varieties showing varying tolerance to dryness. What is tolerable to one plant can be detrimental to another. Therefore, growers will benefit from any simple and rapid technique that can determine the dehydration point of their crop.

New research by scientists at Edaphic Scientific, an Australian-based scientific instrumentation company, and the University of Queensland, Australia, has found a technique that can simply and rapidly determine when a plant requires irrigation. The technique builds on the strong correlation between transpiration and plant water potential that is found across all plant species. However, new research applied this knowledge into a technique that is simple, rapid, and cost-effective, for growers to implement.

Current textbook knowledge of plant dehydration

The classic textbook values of plant hydration are field capacity and permanent wilting point, defined as -33 kPa (1/3 Bar) and -1500 kPa (15 Bar) respectively. It is widely recognized that there are considerable limitations with these general values. For example, the dehydration point for many crops is significantly less than 15 Bar.

Furthermore, values are only available for a limited number of widely planted crops. New crop varieties are constantly developed, and these may have varying dehydration points. There are also many crops that have no, or limited, research into their optimal hydration level. Lastly, textbook values are generated following years of intensive scientific research. Growers do not have the time, or resources, to completely investigate optimal hydration for their crop. Therefore, a new technique that provides a rapid assessment is required.

How stomatal conductance varies with water potential

There is a strong correlation between stomatal conductance and plant water potential: as plant water potential becomes more negative, stomatal conductance decreases. Some species are sensitive and show a rapid decrease in stomatal conductance; other species exhibit a slower decrease.

Plant physiologist refer to P50 as a value that clearly defines a species’ tolerance to dehydration. One definition of P50 is the plant water potential value at which stomatal conductance is 50% of its maximum rate. P50 is also defined as the point at which hydraulic conductance is 50% of its maximum rate. Klein (2014) summarized the relationship between stomatal conductance and plant water potential for 70 plant species (Figure 1). Klein’s research found that there is not a single P50 for all species, rather there is a broad spectrum of P50 values (Figure 1).

Leaf water potential chart

Figure 1. The relationship between stomatal conductance and leaf water potential for 70 plant species. The dashed red lines indicate the P80 and P50 values. The irrigation refill point can be determined where the dashed red lines intersect with the data on the graph. Image has been adapted from Klein (2014), Figure 1b.

Taking advantage of P50

The strong, and universal, relationship between stomatal conductance and water potential is vital information for growers. A stomatal conductance versus water potential relationship can be quickly, and easily, established by any grower for their specific crop. However, as growers need to maintain optimum plant hydration levels for growth and yield, the P50 value should not be used as this is too dry. Rather, research has shown a more appropriate value is possibly the P80 value. That is, the water potential value at the point that stomatal conductance is 80% of its maximum.

Irrigation Curves – a rapid assessment of plant hydration

Research by Edaphic Scientific and University of Queensland has established a technique that can rapidly determine the P80 value for plants. This is called an “Irrigation Curve” which is the relationship between stomatal conductance and hydration that indicates an optimal hydration point for a specific species or variety.

Once P80 is known, this becomes the set point at which plant hydration should not go beyond. For example, a P80 for leaf water potential may be -250 kPa. Therefore, when a plant approaches, or reaches, -250 kPa, then irrigation should commence.

P80 is also strongly correlated with soil water potential and, even, soil volumetric water content. Soil water potential and/or content sensors are affordable, easy to install and maintain, and can connect to automated irrigation systems. Therefore, establishing an Irrigation Curve with soil hydration levels, rather than plant water potential, may be more practical for growers.

Example irrigation curves

Irrigation curves were created for a citrus (Citrus sinensis) and macadamia (Macadamia integrifolia). Approximately 1.5m tall saplings were grown in pots with a potting mixture substrate. Stomatal conductance was measured daily, between 11am and 12pm, with an SC-1 Leaf Porometer. Soil water potential was measured by combining data from an MPS-6 (now called TEROS 21) Matric Potential Sensor and WP4 Dewpoint Potentiometer. Soil water content was measured with a GS3 Water Content, Temperature and EC Sensor. Data from the GS3 and MPS-6 sensors were recorded continuously at 15-minute intervals on an Em50 Data Logger. When stomatal conductance was measured, soil water content and potential were noted. At the start of the measurement period, plants were watered beyond field capacity. No further irrigation was applied, and the plants were left to reach wilting point over subsequent days.

Irrigation curves for citrus and macadamia based on soil water potential measurements

Figure 2. Irrigation Curves for citrus and macadamia based on soil water potential measurements. The dashed red line indicates P80 value for citrus (-386 kPa) and macadamia (-58 kPa).

Figure 2 displays the soil water potential Irrigation Curves, with a fitted regression line, for citrus and macadamia. The P80 values are highlighted in Figure 2 by a dashed red line. P80 was -386 kPa and -58 kPa for citrus and macadamia, respectively. Figure 3 shows the results for the soil water content Irrigation Curves where P80 was 13.2 % and 21.7 % for citrus and macadamia, respectively.

Soil Water Content Charts

Figure 3. Irrigation Curves for citrus and macadamia based on soil volumetric water content measurements. The dashed red line indicates P80 value for citrus (13.2 %) and macadamia (21.7 %).

From these results, a grower should consider maintaining soil moisture (i.e. hydration) above these values as they can be considered the refill points for irrigation scheduling.

Further research is required

Preliminary research has shown that an Irrigation Curve can be successfully established for any plant species with soil water content and water potential sensors. Ongoing research is currently determining the variability of generating an Irrigation Curve with soil water potential or content. Other ongoing research includes determining the effect of using a P80 value on growth and yield versus other methods of establishing a refill point. At this stage, it is unclear whether there is a single P80 value for the entire growing season, or whether P80 shifts depending on growth or fruiting stage. Further research is also required to determine how P80 affects plants during extreme weather events such as heatwaves. Other ideas are also being investigated.

For more information on Irrigation Curves, or to become involved, please contact Dr. Michael Forster: [email protected]

Reference

Klein, T. (2014). The variability of stomatal sensitivity to leaf water potential across tree species indicates a continuum between isohydric and anisohydric behaviours. Functional Ecology, 28, 1313-1320. doi: 10.1111/1365-2435.12289

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How to Protect your Soil Moisture Sensors from Lightning Surge

We occasionally see soil moisture sensors damaged by lightning.  Here’s what to do to protect them.

The secondary products of a lightning strike include electromagnetic pulses, electrostatic pulses, and earth current transients.

Lighting striking

Surge suppression components typically perform their suppression function by temporarily short circuiting the voltage between two wires, several devices, or ground.

Electromagnetic pulses are created by the strong magnetic field that is formed by the short term current flow taking place in the lightning strike. With current flows as high as 510kA per microsecond, these currents create very large magnetic fields. These short-term magnetic fields then induce voltages onto wires and cables.

Electrostatic pulses are created by electrostatic fields that accompany a thunderstorm. Any cable suspended above the earth during a thunderstorm is immersed in the electrostatic field and will be electrically charged. Quick changes in the charges stored in both the clouds and earth take place whenever there is a lightning strike. The charge on the cable must now be discharged or neutralized. Unable to find a path to ground (earth), it breaks down insulation and component in its efforts to return to earth.

Earth current transients are the direct result of the neutralization process that immediately follows the end of lightning strike. Neutralization is accomplished by the movement or redistribution of charge along or near the earth’s surface from all the points where the charge had been initially induced to the point where the lightning strike has just terminated. Earth current transients create a shift in potential across a ground plan, often called a “ground bounce”.

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Stem Water Content Changes Our Understanding of Tree Water Use (Part 2)

This week, we continue highlighting the second of two current research projects (see part one) which use soil moisture sensors to measure volumetric water content in tree stems and why this previously difficult to obtain measurement will change how we look at tree water use.

Image of Tamarisk tree in Sudan

Tamarisk tree: an invasive species dominant in Sudan and arid parts of the United States. (Photo credit: biolib.cz)

Determining Tree Stem Water Content in Drought Tolerant Species

Tadaomi Saito and his research team were interested in using dielectric soil moisture sensors to measure the tree stem volumetric water content of mesquite trees and tamarisk, two invasive species dominant in Sudan and arid parts of the United States. Mesquite is a species that can access deep groundwater sources using their taproots which is how they compete with native species. Tamarisk, on the other hand, uses shallow, saline groundwater to survive.  The team wanted to see if dielectric probes were useful for real-time measurement of plant water stress in these drought-tolerant species and if these measurements could illuminate differing tree water-use patterns.  These sensors could then potentially be used for precision irrigation strategies to assist in agricultural water management.  

Temperature Calibration Was Essential

After calibrating the soil moisture sensors to the wood types in a lab, the team inserted probes into the stems of both trees.  They also monitored groundwater and soil moisture content to try and infer whether or not the trees were plugged into a deep source of water.  Interestingly, Saito found that, unlike soil, where temperature fluctuation is buffered, tree stems are subject to large variations in temperature throughout the course of the day.  This temperature fluctuation interfered with the soil moisture probes’ ability to accurately measure VWC.   The team came up with a simple method for accounting for temperature variability and were then able to obtain accurate VWC measurements.  

Image of a Mesquite tree on a desert mountain slope

Photo credit: desertusa.com

Water Use Depended on Landscape Position

Saito’s results were similar to Ashley Matheny’s study (see part 1), in that they found a lot of different patterns, even in trees of the same species.  Water-use depended on where the trees were on the landscape.  Some of them were tapped into groundwater, and the stem water storage didn’t change no matter how dry the soil became.  Whereas others, depending on their position in the landscape, were very dependent on soil moisture conditions.  

You can read the full study details here.

Implications

Saito’s study illustrates that we see everything about a tree that’s above ground, but we may have no sense of what’s going on below ground.   We can put a soil moisture sensor in the ground and decide there’s plenty of moisture available.  Or if conditions are dry, we may decide the tree is under drought stress, but we don’t know if that tree is tapped into a more permanent source of groundwater.   

Other researchers have put soil moisture sensors in orchards looking at stem water storage from a practical standpoint for irrigation management.  Their data didn’t work out so well because of cable sensitivity where water on the cable created false readings.  However, the data they were able to obtain showed that some of the trees were plugged into water sources that were independent of the soil.  Those trees were able to withstand drought and needed less irrigation, whereas other trees were much more sensitive to soil moisture.  

If we had an inexpensive, easy to deploy measurement device plugged into every tree in an orchard, we could irrigate tree by tree, give them precisely what they needed, and account for their unique situation.

What Does it All Mean?

The interesting thing about using soil moisture sensors in a tree is that stem water content is a difficult-to-obtain piece of information that has now been made easier.  Historically, we’ve focused on measuring sap flow, but that’s just how much water is flowing past the sensor. We’ve measured what’s in the soil: a pool of moisture that’s available to the tree. But some trees are huge in size, such as ones along the coast of California. They’re able to store vast amounts of water above-ground in their tissue.  Understanding how a tree can use that water to buffer or get through periods of drought is a unique research topic that has had very little attention. With these kinds of sensors, we can start to investigate those questions.

Reference: Saito T., H. Yasuda, M. Sakurai, K. Acharya, S. Sueki, K. Inosako, K. Yoda, H. Fujimaki, M. Abd Elbasit, A. Eldoma and H. Nawata , Monitoring of stem water content of native/invasive trees in arid environments using GS3 soil moisture sensor , Vadose Zone Journal , vol.15 (0) (p.1 – 9) , 2016.03

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Stem Water Content Changes Our Understanding of Tree Water Use

In an update to our previous blog, “Soil Moisture Sensors in a Tree?”, we highlight two current research projects using soil moisture sensors to measure volumetric water content (VWC) in tree stems and share why this previously difficult-to-obtain measurement will change how we look at tree water usage.

Image of green leafs with sunlight streaming through them

Researchers explore the feasibility of inserting capacitance soil sensors in tree stems as a real-time measurement.

Soil Moisture Sensors in Tree Stems?

In a recent research project, Ph.D. candidate Ashley Matheny of the University of Michigan used soil sensors to measure volumetric water content in the stems of two species of hardwood trees in a northern Michigan forest: mature red oak and red maple.  Though both tree types are classified as deciduous, they have different strategies for how they use water. Oak is anisohydric, meaning the species doesn’t control their stomata to reduce transpiration, even in drought conditions.  Isohydric maples are more conservative. If the soil starts to dry out, maple trees will maintain their leaf water potential by closing their stomata to conserve water.  Ashley and her research team wanted to understand the different ways these two types of trees use stem water in various soil moisture scenarios.

Historically, tree water storage has been measured using dendrometers and sap flow data, but Ashley’s team wanted to explore the feasibility of inserting a capacitance-type soil sensor in the tree stems as a real-time measurement.  They hoped for a practical way to make this measurement to provide more accurate estimations of transpiration for use in global models.  

Image of a Hardwood tree in northern Michigan in Autumn

Scientists measured volumetric water content in the stems of two species of hardwood trees in a northern Michigan forest: mature red oak and red maple.

Measurements used

Ashley and her team used meteorological, sap flux, and stem water content measurements to test the effectiveness of capacitance sensors for measuring tree water storage and water use dynamics in one red maple and one red oak tree of similar size, height, canopy position and proximity to one another (Matheny et al. 2015). They installed both long and short soil moisture probes in the top and the bottom of the maple and oak tree stems, taking continuous measurements for two months. They calibrated the sensors to the density of the maple and oak woods and then inserted the sensors into drilled pilot holes.  They also measured soil moisture and temperature for reference, eventually converting soil moisture measurements to water potential values.

Results Varied According to Species

The research team found that the VWC measurements in the stems described tree storage dynamics which correlated well with average sap flux dynamics.  They observed exactly what they assumed would be the anisohydric and isohydric characteristics in both trees.  When soil water decreased, they saw that red oak used up everything that was stored in the stem, even though there wasn’t much available soil moisture.  Whereas in maple, the water in the stem was more closely tied to the amount of soil water. After precipitation, maple trees used the water stored in their stem and replaced it with more soil water.  But, when soil moisture declined, they held onto that water and used it at a slower rate.

Red, yellow, green leafs in Autumn

Researchers want to figure out the appropriate level of detail for tree water-use strategy in a global model.

Trees use different strategies at the species level

The ability to make a stem water content measurement was important to these researchers because much of their work deals with global models representing forests in the broadest sense possible.  They want to figure out the appropriate level of detail for tree water-use strategy in a global model. Both oak and the maple are classified as broadleaf deciduous, and in a global model, they’re lumped into the same category. But this study illustrates that if you’re interested in hydrodynamics (the way that trees use water), deciduous trees use different strategies at the species level.  Thus, there is a need to treat them differently to produce accurate models.

Read the full study in Ecosphere.

Reference: Matheny, A. M., G. Bohrer, S. R. Garrity, T. H. Morin, C. J. Howard, and C. S. Vogel. 2015. Observations of stem water storage in trees of opposing hydraulic strategies. Ecosphere 6(9):165. http://dx.doi.org/10.1890/ES15-00170.1

Next week: Part 2 of this article showcases more research being done using soil moisture sensors to measure volumetric water content in tree stems.

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Electrical Conductivity of Soil as a Predictor of Plant Response

Plants require nutrients to grow, and if we fail to supply the proper nutrients in the proper concentrations, plant function is affected. Fertilizer in too high concentration can also affect plant function, and sometimes is fatal.

Grass with dew droplets covering them

Plant function is affected by nutrient concentration.

Most of us have had the experience of fertilizing some part of a lawn too heavily, perhaps by accident, and killing grass in that part of the lawn. Generally, it isn’t the nutrients themselves that cause the damage, it is their effect on the water. Salt in the water reduces its water potential making it less available to the plant. The salt therefore causes water stress in the plant.

Salt in soil comes from the fertilizer we apply, but also from irrigation water and dissolving soil minerals. Relatively small amounts are removed with the plants that are harvested, but most leaches with the water out of the bottom of the soil profile. When water evaporates at the soil surface, or from leaves, it is pure, containing no salt, so evapotranspiration concentrates the salts in the soil. If more salt is applied in the irrigation water than is leached or taken off in harvested plants, the soil becomes more saline and eventually will cease to support agricultural production. Thousands of acres have been lost from production in this way, and production has been drastically reduced on tens of thousands of additional acres.

Super green bamboo stalks

Thousands of acres have been lost from over-fertilization.

Soil Salinity and Electrical Conductivity

Soil salinity has been measured using electrical conductivity for more than 100 years. It is common knowledge that salty water conducts electricity. Whitney and Means (1897) made use of that fact to measure the concentration of salt in soil. Early methods made measurements directly on a soil paste, but the influence of the soil in the paste on the measurement was not fully understood until recently, leading to uncertainty in the measurements. By about 1940 the accepted method for determining soil salinity was to make a saturated paste by a specified procedure, extract solution from the paste, and measure the electrical conductivity of the solution (Richards, 1954). The measurement is referred to as the electrical conductivity of the saturation extract. These values were then correlated with crop response.

Richards (1954) defined 4 soil salinity classes, as shown in Table 1. Crops suitable for these classes are also listed by Richards, but a much more extensive list is given by Rhoades and Lovejoy (1990). For example, bean is listed as a sensitive crop. It can only be grown without yield damage in soils with EC below 2 dS/m. Barley is a tolerant crop. It can be grown without much yield reduction in any soil up to EC of 16 dS/m.

Salinity classes for soils chart

Table 1: Salinity classes for soils

Two other columns are shown in the table. The “salt in soil” column shows how much salt is required to salinize a soil. In terms of the total soil mass, only a small percentage change is needed to make a big difference in salinity, but this would still represent a large addition of fertilizer. A 200 kg/ha addition of fertilizer would represent a fairly high rate. If this were incorporated into the top 15 cm of soil, it would represent

The salt in soil equation

This wouldn’t cause much change in soil salt percentage.

The other column shows osmotic potential of the saturation extract. To give some reference for this number, remember that the nominal permanent wilt water potential of soil is -1500 kPa. Osmotic potentials of plant leaves vary widely depending on species, but -1500 kPa is a kind of median value. The values in the table may seem small compared to the permanent wilt (PW) value, but remember that these are values at saturation. When a soil is saturated, water quickly drains to a “drained upper limit” (UL) water content which is around half the saturation value. The useful water storage of the soil is between the UL and the PW or lower limit water content, which, again, is about half the UL. The concentration of salts at the UL is about the same as at saturation because the water drained away, but the water loss between the UL and PW is typically by evapotranspiration, so little or no salts are lost. The concentration at the lower limit is therefore twice that shown in Table 1, which is significant compared to the permanent wilt water potential. Likewise the osmotic potential of the soil solution after fertilizing with 200 kg/ka and mixing wouldn’t change much, but the same amount of fertilizer concentrated in a band near seed would have a much larger effect.

Understand EC sensor readings

Understanding the difference between electrical conductivity readings in water and in soil can help you make better use of your EC readings. Watch the video to answer questions such as “Why does water that’s 1.9 dS/m not read 1.9 dS/m when it’s in the soil?

 

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Next Week: Read part 2 of Electrical Conductivity as a Predictor of Soil Response.

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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.

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Avocado Growers in Kenya Fight Drought with Recycled Water Bottle Irrigation (Part 2)

Dr. Brent Clothier, Dr. Steve Green, Roberta Gentile and their research team from Plant and Food Research in New Zealand are working in Kenya to alleviate the poverty of the many small-holder farmers who grow avocados in the Central Highlands of Kenya (see part 1). This week, read about an inexpensive irrigation solution for these farmers and how the researchers are developing a plan to manage nutrients.

Flowering avocado plant

The period of water stress in October is at the time of main flowering.

Recycled Water Bottles Provide a Solution

When the team was visited Kenya in early March, the Long Rains had not arrived, and the trees were under water stress. The researchers sought to reduce the impact of drought by using a prototype of a portable drip-irrigation system they developed. They used ‘old’ 20-liter drinking water bottles to deliver water to the trees at 4 L/hr.

Researcher standing with 20 L water bottles used for tree irrigation

20 L water bottles used for tree irrigation.

The bottles can be refilled and moved from tree to tree. By measuring water content in the soil, the team found that the 20 L of drip irrigated water lasted in the soil about 2 days. When the period was increased to 4 days, the root water uptake was reduced over days 3 and 4 after wetting. Thus they recommended the bottle be recharged and reapplied every two days. This enables the bottle to be used on another tree on the intervening day and should help the farmers to reduce the worst impacts of the drought while waiting for the Long Rains to arrive.

People refilling the water bottles in town

Refilling the water bottles.

Replacing Low Soil Nutrients

In another phase of the experiment, Dr. Clothier’s team surveyed soil and plant nutrient contents in the main avocado production regions to assess the current fertility status of the farms. Soils in this region are classified as Nitisols, deep red soils with a nut-shaped structure and high iron content (Jones et al. 2013). These soils have low levels of organic matter and low pH. Soil sampling revealed a decrease in pH and increase in organic matter with altitude in the Kandara valley. This observed gradient is likely attributable to the higher amounts rainfall received in the higher altitudes of the valley, which can increase organic matter production and leach base cations from the soil. Soil and leaf nutrient analyses of the monitoring farms showed similar trends in nutrient availability. There are also low levels of the macronutrients nitrogen and phosphorus and the micronutrient boron in these soils. These nutrients are essential for avocado growth and production. One challenge to improve avocado productivity is finding ways to improve soil nutrient availability and tree nutrition.

Cow resting underneath the shade of a tree

An example of the benefits of a secure revenue-stream: One farmer purchased a new cow, which enables him to meet the nutrient requirements of more avocado trees.

A Plan for Managing Nutrients

The majority of the small-holder farms supplying avocados to Olivado use organic production methods. This means organic amendments such as plant residues, composts and animal manures are required to replenish the nutrients that are exported from the farms and improve soil fertility. Livestock have the potential to provide nutrient amendments for a considerable number of avocado trees. Even better, the input of organic materials will build-up soil organic matter levels, which benefit soil conservation, water holding capacity, pH buffering, and soil biological activity.

The researchers are developing simple nutrient budgets for these avocado trees using yield and fruit nutrient concentration data to assess the quantity of nutrients being exported off-farm in the harvested crop. Using the nutrient concentrations of locally available organic amendments, they will provide recommendations on the amount of organic material needed to sustain soil fertility.

Nutrient balances will be incorporated into a decision support tool to assist small-holder farmers in enhancing their soil and plant nutrition. These budgets will be enhanced by further characterizing the nutrient composition and quantities of available organic matter amendments in the region. The researchers are working to improve these nutrient budget estimates with data specific to the avocado farms in the region. They will also set up demonstration farms to evaluate the production responses to recommended nutrient management practices.

To find out more about Kenyan avocado research contact Brent Clothier: [email protected] .

(This article is a summary/compilation of several articles first printed in WISPAS newsletter)

References:

Jones, A., Breuning-Madsen, H., Brossard, M., Dampha, A., Deckers, J., Dewitte, O., Gallali, T., Hallett, S., Jones, R., Kilasara, M., Le Roux, P., Micheli, E., Montanarella, L., Spaargaren, O., Thiombiano, L., Van Ranst, E., Yemefack, M., Zougmore, R., (eds.) 2013. Soil Atlas of Africa. European Commission, Publications Office of the European Union, Luxembourg. 176 pp.

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Avocado Growers in Kenya Fight Drought with Recycled Water Bottle Irrigation

Dr. Brent Clothier, Dr. Steve Green, Roberta Gentile and their research team from Plant and Food Research in New Zealand are working in Kenya to alleviate the poverty of the many small-holder farmers who grow avocados in the Central Highlands of Kenya. These farmers have old and very large avocado trees. The fruit from these trees are purchased by the company Olivado EPZ who presses over 1300 small-holders’ avocados for oil. Dr. Clothier and his team are investigating how to increase the productivity of the farmers’ avocado trees and increase the quality of the fruit so they yield more oil.

Avocados on an avocado tree

Small-holder farmers grow avocados in the Central Highlands of Kenya.

Reducing Leaf Area to Avoid Water Stress

Because of the age and size of these trees, harvesting of the avocados is difficult and time consuming, and through dropped fruit, the quality of the avocados can be comprised. In addition, any dry season water-stress negatively impacts fruit filling. The research team performed some initial remedial pruning of these trees to develop a more manageable and productive tree form. They sought to assess whether the reduced leaf area would enable the trees to avoid water stress during the dry season of January through March between the short and long rainy seasons. They removed 30-40% of the central limbs of the avocado tree to create a more open canopy form.

The team instrumented two trees with heat-pulse sap-flow probes. One tree was left unpruned and the tree in the photo above was pruned. The tree that was pruned was using between 300-400 liters per day, as expected for a tree of that large size. The unpruned tree was smaller in size, and it was using between 150-250 liters per day during May and June. The selective limb pruning resulted in the rate of water-use dropping to 200-300 liters per day, a drop of 100 liters per day.

Pruned avocado tree

The more open canopy form of the pruned avocado tree.

Determining Tree Water Use During Rainy and Dry Seasons

The team also measured the water-use of four trees of different sizes during the entire season using the compensation heat-pulse method and soil water content. They found the trees’ water-use doubled with the arrival of the Short Rains and then began to decline in early January after the rains ended. The trees were under a degree of water stress prior to the arrival of the (short) Short Rains, and as the weak Short Rains ended early, the trees again went into water stress with only occasional respite due to isolated rainstorms in January and February.

This pattern of water stress presents a challenge for sustaining high levels of avocado production. The period of water stress in October is at the time of main flowering, and researchers who were there noted a carpet of aborted flowers on the orchard floor. They also noticed that the fruit were smaller at one farm than those higher up in the Central Highlands where rainfall is higher and more frequent. Thus, to improve production it is imperative to mitigate the impacts of drought, and this needs to be done without reference to any infrastructure for irrigation.

Next week: Read about an inexpensive irrigation solution for these farmers and how the researchers are developing a plan to manage nutrients.

(This article is a summary/compilation of several articles first printed in WISPAS newsletter)

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Soil Moisture Sensors: Why TDR VS. Capacitance May Be Missing the Point (Part 2)

Dr. Colin S. Campbell discusses whether TDR vs. capacitance (see part 1) is the right question, the challenges facing soil moisture sensor technology, and the correct questions to ask before investing in a sensor system.

Image of plants Growing in a Field

It’s easy to overlook the obvious question: what is being measured?

What are You Trying to Measure?

When considering which soil water content sensor will work best for any application, it’s easy to overlook the obvious question: what is being measured?  Time Domain Reflectometry (TDR) vs. capacitance is the right question for a researcher who is looking at the dielectric permittivity across a wide measurement frequency spectrum (called dielectric spectroscopy). There is important information in these data, like the ability to measure bulk density along with water content and electrical conductivity. If this is the desired measurement, currently only one technology will do: TDR. The reflectance of the electrical pulse that moves down the conducting rods contains a wide range of frequencies.  When digitized, these frequencies can be separated by fast fourier transform and analyzed for additional information.

The objective for the majority of scientists, however, is to simply monitor soil water content instantaneously or over time, with good accuracy. There are more options if this is the goal, yet there are still pitfalls to consider.

Soil moisture sensor close-up

Considerable research has been devoted to determining which soil moisture sensors meet expectation.

Each Technology Has Challenges

Why would a scientist pay $100+ for a soil volumetric water content (VWC) sensor, when there are hundreds of soil moisture sensors online costing between $5 and $15? This is where knowing HOW water content is measured by a sensor is critical.

Most sensors on home and garden websites work based on electrical resistivity or conductivity. The principle is simple: more water will allow more electrons to flow. So conductivity will change with soil water content. But, while it’s possible to determine whether water content has changed with this method, absolute calibration is impossible to achieve as salts in the soil water will change as the water content changes. A careful reading of sensor specs will sometimes uncover the measurement method, but sometimes, price is the only indication.

Somewhere between dielectric spectroscopy and electrical resistance are the sensors that provide simple, accurate water content measurement. Considerable research has been devoted to determining which of these meet expectation, and the results suggest that Campbell Scientific, Delta-T, Stevens, Acclima, Sentek, and METER (formerly Decagon Devices), provide accurate sensors vetted by soil scientists. The real challenge is installing the sensors correctly and connecting them to a system that meets data-collection and analysis needs.

Installation Techniques Affect Accuracy

Studies show there is a difference between mid-priced sensor accuracy when tested in laboratory conditions. But, in the field, sensor accuracy is shown to be similar for all good quality probes, and all sensors benefit from site-specific soil calibration. Why? The reason is associated with the principle upon which they function. The electromagnetic field these sensors produce falls off exponentially with distance from the sensor surface because the majority of the field is near the electrodes. So, in the lab, where test solutions form easily around sensor rods, there are differences in probe performance.  In a natural medium like soil, air gaps, rocks, and other detritus reduce the electrode-to-soil contact and tend to reduce sensor to sensor differences. Thus, picking an accurate sensor is important, but a high-quality installation is even more critical.

Crops with a blue sky background

Improper installation is the largest barrier to accuracy.

Which Capacitance Sensor Works Best?

Sensor choice should be based on how sensors will be installed, the nature of the research site, and the intended collection method. Some researchers prefer a profile sensor, which allows instruments to be placed at multiple depths in a single hole. This may facilitate fast installation, but air gaps in the auger pilot hole can occur, especially in rocky soils. Fixing this problem requires filling the hole with a slurry, resulting in disturbed soil measurements. Still, profile sensor installation must be evaluated against the typical method of digging a pit and installing sensors into a sidewall. This method is time consuming and makes it more difficult to retrieve sensors.

New technology that allows sensor installation in the side of a 10 cm borehole may give the best of both worlds, but still requires backfill and has the challenge of probe removal at the end of the experiment.

The research site must also be a consideration. If the installation is close to main power or easily reached with batteries and solar panels, your options are open: all sensors will work. But, if the site is remote, picking a sensor and logging system with low power requirements will save time hauling in solar panels or the frustration of data loggers running out of batteries.

ZL6 Data Logger

Often times it comes down to convenience.

Data Loggers Can Be a Limitation

Many manufacturers design data loggers that only connect to the sensors they make. This can cause problems if the logging system doesn’t meet site needs. All manufacturers mentioned above have sensors that will connect to general data loggers such as Campbell Scientific’s CR series. It often comes down to convenience: the types of sensor needed to monitor a site, the resources needed to collect and analyze the data, and site maintenance. Cost is an issue too, as sensors range from $100 to more than $3000.

Successfully Measure Water Content

The challenge of setting up and monitoring soil water content is not trivial, with many choices and little explanation of how each type of sensor will affect the final results. There are a wealth of papers that review the critical performance aspects of all the sensors discussed, and we encourage you to read them. But, if soil water content is the goal, using one of the sensors from the manufacturers named above, a careful installation, and a soil-specific calibration, will ensure a successful, accurate water content measurement.

For an in-depth comparison of TDR versus capacitance technology, read: Dielectric Probes Vs. Time Domain Reflectometers

Watch the webinar

In this webinar, Dr. Colin Campbell discusses the details regarding different ways to measure soil moisture and the theory behind the measurements.  In addition, he provides examples of field research and what technology might apply in each situation. The measurement methods covered are gravimetric sampling, dielectric methods including TDR and FDR/capacitance, neutron probe, and dual needle heat pulse.

 

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”—>

Top Five Blog Posts in 2016

In case you missed them the first time around, here are the most popular Environmental Biophysics.org blog posts in 2016.

Lysimeters Determine if Human Waste Composting can be More Efficient

Waste in the water canals

In Haiti, untreated human waste contaminating urban areas and water sources has led to widespread waterborne illness.  Sustainable Organic Integrated Livelihoods (SOIL) has been working to turn human waste into a resource for nutrient management by turning solid waste into compost.  Read more

Estimating Relative Humidity in Soil: How to Stop Doing it Wrong

Image of a researchers hand holding soil

Estimating the relative humidity in soil?  Most people do it wrong…every time.  Dr. Gaylon S. Campbell shares a lesson on how to correctly estimate soil relative humidity from his new book, Soil Physics with Python, which he recently co-authored with Dr. Marco Bittelli.  Read more.

How Many Soil Moisture Sensors Do You Need?

Road winding through a mountain pass

“How many soil moisture sensors do I need?” is a question that we get from time to time. Fortunately, this is a topic that has received substantial attention by the research community over the past several years. So, we decided to consult the recent literature for insights. Here is what we learned.

Data loggers: To Bury, or Not To Bury

Data Logger in an orange bury-able box sitting on next to installation site

Globally, the number one reason for data loggers to fail is flooding. Yet, scientists continue to try to find ways to bury their data loggers to avoid constantly removing them for cultivation, spraying, and harvest.  Chris Chambers, head of Sales and Support at Decagon Devices always advises against it. Read more

Founders of Environmental Biophysics:  Champ Tanner

Image of Champ Tanner

Image: http://soils.wisc.edu/people/history/champ-tanner/

We interviewed Gaylon Campbell, Ph.D. about his association with one of the founders of environmental biophysics, Champ Tanner.  Read more

And our three most popular blogs of all time:

Do the Standards for Field Capacity and Permanent Wilting Point Need to Be Reexamined?

Image of green wheat and a bright blue sky

We asked scientist, Dr. Gaylon S. Campbell, which scientific idea he thinks impedes progress.  Here’s what he had to say about the standards for field capacity and permanent wilting point.  Read more

Environmental Biophysics Lectures

Close up of a leaf on a tree

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.  Read more

Soil Moisture Sensors In a Tree?

Close up image of tree bark

Soil moisture sensors belong in the soil. Unless, of course, you are feeling creative, curious, or bored. Then maybe the crazy idea strikes you that if soil moisture sensors measure water content in the soil, why couldn’t they be used to measure water content in a tree?  Read more

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