SIDNEY, Mont. – Using soil moisture sensors can be a boost to irrigation management, according to USDA-ARS Northern Plains Agricultural Research Lab (NPARL) scientists in Sidney.
Due to COVID-19 restrictions, scientists at NPARL, along with others at Montana State University and North Dakota State University, talked about some of their research virtually for the 2021 MonDak Ag Research Summit.
Bart Stevens, NPARL-ARS research agronomist and research leader, utilizes soil moisture sensors at his ARS lab location in Sidney and at Nesson Valley in Williston, N.D.
Soil moisture sensors are used in a conservation program for irrigation water management plans, for irrigation scheduling to optimize yield and minimize energy and labor costs, and to conserve water and keep nutrients from leaching.
Stevens and Jay Jabro, NPARL-ARS research soil scientist, use soil moisture probes in many of their irrigation studies, including the sugarbeet tillage study under irrigation, ongoing each spring/summer through 2023.
In particular, the ARS team uses the IRROmesh Relay Node and Watermark sensor system for the sugarbeet tillage study. The system is connected with the internet and can be read in real-time on a cellphone/tablet at remote locations.
The user can then make irrigation decisions based on the sensor readings.
Stevens explained why he uses soil moisture sensors.
“The motivating factor for me is economics, optimizing yields and managing our outputs efficiently,” he said.
Sensors include the electromagnetic (EM) type, which measure water content by emitting an EM signal, the time-domain reflectometry (TDR) types, the capacitance types, and the Watermark.
“We like using Watermark soil moisture sensors because they are low cost and simple. They measure how tightly the soil holds water and they work the best in finer-textured soils, like clay soils,” he said.
But NPARL has tried and used most of the soil moisture sensors on the market.
Regardless of the sensor a producer uses, it is important that the sensors are installed correctly.
NPARL scientists use a soil slurry to ensure direct contact with the soil to install the Watermark.
“These soil sensors don’t measure a large volume of soil, so having close contact with the soil is important. Air pockets could have an effect (on the reading),” he said.
Jabro compared four sensor types, including the Watermark, the Campbell TDR, the HydraProbe, and the Neutron Probe, which is primarily used as a research probe.
They were installed close to each other at the Nesson Valley location in conjunction with NDSU and at the MSU Eastern Ag Research Center location. Nesson Valley had sandy loam soils, while EARC had clay loam soils.
Jabro evaluated the soil moisture sensors for their ability to estimate water content based on calibrated neutron probe measurements.
What Jabro found was that all sensors in each location and the soil at that location showed similar patterns and trends, but the sensors, however, performed differently depending on soil type.
“Results showed that the Watermark, the Campbell TDR, and the HydraProbe provided different estimates of soil moisture contents in both soils,” Jabro said.
Watermark overestimates the moisture in sand, demonstrating this sensor works better in clay soils.
In the EARC soil, the Watermark was most similar to the Neutron Probe.
The EM sensors were not as accurate as the Watermark in the EARC site, and readings varied widely.
Results showed that soil moisture probes work best alongside other tools, such as observing the crop growing.
In general, Stevens and Jabro found that each soil moisture sensor type has some drawbacks.
Sensors are only indicative of conditions at the point of installation.
“Keep in mind the limitations of soil sensors. Most measure a very small soil volume, especially the capacitance types,” Stevens said.
There is no universal calibration that works for all soil types. Factory calibrations can deviate from actual calibrations, which need to be site-specific.
The only way to quantitatively estimate water use by sensors is to do a site-specific calibration.
“We find soil sensors to be useful, especially as an indication of when to irrigate,” he said.
Soil sensors can also show rooting depth and wetting depth.
The technology, a combination of art and science, is continually improving.
For more information about the continuing webinar series, see the NPARL site at https:/www.ars.usda.gov/plains-area/Sidney-mt/nc. Webinars air on alternating Thursdays.