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Cutting-Edge Soil Moisture Sensor

08:45AM Feb 07, 2015

Neutron rover helps optimize seeding rates, variety selection, irrigation timing and more

Trenton Franz is patiently weaving across Nebraska farmland in a most unlikely agriculture vehicle—a spectacle of odd loops to curious bystanders. In the bed of Franz’s gray truck, which eases along in a snake-like pattern, the devise sucks up subatomic particles. Franz is at the helm of the cosmic-ray neutron rover, and by day’s end, he’ll cover 4,000 acres, revealing what lies beneath a vast expanse of farmland.

Franz is no hopeful water witch. His neutron rover uses a cutting-edge soil moisture sensor. Physicists have long known the amount of water in soil affects neutron counts in the air. As neutrons travel to Earth from the sun and exploding stars, they are highly sensitive to hydrogen and water in the soil. The cosmic-ray sensor measures how many neutrons are in the air, and that’s correlated to how much water is in the soil. 

A measurement with a low amount of neutrons indicates a high amount of water is present in the soil; a high neutron suggests a low amount of moisture. Cosmic-ray sensors are sold commercially by Hydroinnova. In 2010, Franz, a University of Nebraska–Lincoln (UNL) hydrogeophysicist, began using them for academic research. 

The 150-lb. cosmic-ray sensor is tailor-made for spatial mapping. The sensors are attached to a vehicle to create a map of how much water is in the soil, and the data is used for variable-rate irrigation systems. 

“Beyond spatial mapping, we can determine how much water is in the topsoil and adjust seeding rates or 
variety based on germination potential,” he says. “You can use the sensor for irrigation scheduling and for practical applications of farm management.”

Part of the sensor’s beauty is a plug-and-play aspect with simple operation: Connect the cables, flip the switch and the sensor begins taking neutron measurements immediately. Data comes back via smartphone or satellite to Franz, who sets up equations to turn the data into useful information. 

As the sensor moves across a field, it takes a measurement every minute and sees a 1,000' radius circle, shooting about 1' into the ground. Essentially, it takes a CAT scan of a field to measure neutrons and soil moisture. The rate of coverage depends on resolution, but on a 2.5-acre grid, the cosmic-ray neutron rover scans 4,000 acres per day.

The cosmic-ray technology has a remarkably broad footprint, says Chase Johnson, a York, Neb., agronomist with Wilbur-Ellis. “The technology offered by the cosmic-ray neutron rover is unique, and there’s nothing else out there that offers a spatially-based map of soil moisture.”

When Franz began working at UNL, he met with U.S. military personnel interested in real-time mapping of soil moisture. Pared down, the military wanted to determine how long it takes to drive heavy machinery through different environments.

The sensor is a new tool for any physical science, but Franz is most excited about the agricultural applications. “Just think about Nebraska, where we’re No. 1 in irrigated acreage. Water is critical to all aspects of agriculture, and this technology helps optimize application of water. It can help with irrigation timing, preseason planting, winter recharge and determining cover crop water usage.”