Novel Sensors Track Soil Nutrients to Benefit Farmers and the Environment

Researchers from the University of Wisconsin–Madison have created inexpensive sensors that enable continuous, real-time nitrate monitoring in a variety of Wisconsin soil types. With the use of these printed electrochemical sensors, farmers may be able to manage their nutrient input more profitably and with greater knowledge. The study was published in the journal Advanced Material Technologies.

Soil science PhD Student Shuohao Cai places a sensing rod, which has multifunctional sensing stickers positioned to enable multi-depth measurements, in the soil at UW–Madison’s Hancock Agricultural Research Station to test the team’s technology. Image Credit: Kuan-Yu Chen

Our sensors could give farmers a greater understanding of the nutrient profile of their soil and how much nitrate is available for the plants, helping them to make more precise decisions on how much fertilizer they really need. If they can buy less fertilizer, the cost savings could be quite significant at large-acreage farms.

Joseph Andrews, Assistant Professor and Study Lead, Department of Mechanical Engineering, University of Wisconsin–Madison

Although nitrate is necessary for crop growth, too much can seep into groundwater from the soil. This kind of contamination is bad for the ecology and poses a risk to anyone who consumes tainted well water.

The researchers' new sensors may also be employed in agricultural studies to track nitrate leaching and inform optimal techniques for reducing its detrimental impacts.

The current techniques for tracking soil nitrate are time-consuming, costly, and lack real-time data; because of this, printed electronics specialist Andrews and his group set out to develop a more effective and affordable remedy.

For this effort, the researchers created potentiometric sensors, a kind of thin-film electrochemical sensor, using the inkjet printing technique. Potentiometric sensors are widely employed in liquid solution chemistry for precise nitrate measurement.

However, because coarse soil particles will scrape these sensors and obstruct the acquisition of precise readings, they are generally not appropriate for use in soil environments.

The main challenge we were trying to solve is figuring out a way to enable these electrochemical sensors to work well in the harsh environment of soil and accurately sense nitrate ions.

Joseph Andrews, Assistant Professor and Study Lead, Department of Mechanical Engineering, University of Wisconsin–Madison

The group's idea was to cover the sensor with a polyvinylidene fluoride coating. According to Andrews, this content contains two important aspects. First, nitrate ions can travel through its 400 nm-sized pores, which simultaneously obstruct soil particles. Second, it draws water to it and absorbs it like a sponge because it is hydrophilic.

So, any nitrate-laden water gets preferentially soaked into our sensor, and this is really important because soil also acts like a sponge, and you are going to have a losing battle for getting moisture to come to your sensor unless you can match the water absorption potential of soil. These features of the polyvinylidene fluoride layer enable us to extract the nitrate-laden water, get it to the surface of our sensor, and accurately sense nitrate.

Joseph Andrews, Assistant Professor and Study Lead, Department of Mechanical Engineering, University of Wisconsin–Madison

The researchers tested their sensors in two distinct soil types that are representative of Wisconsin: silt loam soil, which is typical in southwest Wisconsin, and sandy soil, which is typical in the state's north-central region. The results of the tests were accurate.

The nitrate sensors that the researchers are now using are part of a multipurpose sensing device that they are calling a “sensing sticker.” Three different types of sensors are attached to a flexible plastic surface using an adhesive on the back of the surface. Temperature and moisture sensors are also included in these stickers.

The researchers attach multiple sensing stickers to a rod at various heights and then bury the rod in the ground. This arrangement allows them to measure the soil at various depths.

Andrews said, “By measuring the nitrate, moisture, and temperature at different depths, we can now quantify the process of nitrate leaching and capture how nitrate is moving through the soil, which hasn’t been possible before.”

The Hancock Agricultural Research Station at UW–Madison and the Arlington Agricultural Research Station will host 30 sensing rods in the soil for the researchers to test their sensors further in the summer of 2024.

The Wisconsin Alumni Research Foundation assists researchers in patenting their inventions.

Kuan-Yu Chen, Aatresha Biswas, Shuohao Cai, and Jingyi Huang, Professor of soil science are the Co-authors of this study.

The study was funded by the USDA Agriculture and Food Research Initiative Foundational Program, the National Science Foundation Signals in the Soil grant, and the University of Wisconsin–Madison Dairy Innovation Hub.

Journal Reference:

Chen, K., et al. (2024) Inkjet Printed Potentiometric Sensors for Nitrate Detection Directly in Soil enabled by a Hydrophilic Passivation Layer. Advanced Materials Technologies.

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