Madison Reporter

University of Wisconsin–Madison engineers have developed low-cost sensors that enable real-time, continuous monitoring of nitrate in soil types common in Wisconsin. These printed electrochemical sensors could help farmers make better-informed nutrient management decisions and achieve economic benefits.

"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," said Joseph Andrews, an assistant professor of mechanical engineering at UW–Madison who led the research. "If they can buy less fertilizer, the cost savings could be quite significant at large-acreage farms."

While nitrate is essential for growing crops, excess amounts can leach out of soil into groundwater, posing risks to both human health and the environment. The new sensors may also serve as agricultural research tools to monitor nitrate leaching and guide best practices for mitigating its harmful effects.

Current methods for monitoring soil nitrate are laborious, expensive, and lack real-time data capabilities. To address this issue, Andrews and his team created a more efficient solution using an inkjet printing process to fabricate potentiometric sensors—thin-film electrochemical devices typically used to measure nitrate in liquid solutions but not suitable for harsh 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," Andrews explained.

The team's solution involved adding a polyvinylidene fluoride layer over the sensor. This material features tiny pores that allow nitrate ions through while blocking soil particles and attracts water like a sponge.

"So any nitrate-laden water gets preferentially soaked into our sensor," Andrews said. "These features enable us to extract the nitrate-laden water, get it to the surface of our sensor, and accurately sense nitrate."

The researchers detailed their advance in a paper published in March 2024 in Advanced Material Technologies. They tested their sensors in sandy soils common in north-central Wisconsin and silt loam soils found in southwestern Wisconsin with accurate results.

The team is now integrating their nitrate sensors into multifunctional sensing systems called "sensing stickers." These include moisture and temperature sensors mounted on flexible plastic surfaces with adhesive backs. Several stickers are attached at different heights on rods buried in soil for multi-depth measurements.

"By measuring the nitrate, moisture, and temperature at different depths, we can now quantify the process of nitrate leaching and capture how it moves through the soil," Andrews noted.

Further testing will occur during summer 2024 with 30 sensing rods deployed at UW–Madison's Hancock Agricultural Research Station and Arlington Agricultural Research Station. The researchers are patenting their technology through the Wisconsin Alumni Research Foundation.

UW–Madison co-authors on the paper include Kuan-Yu Chen, Aatresha Biswas, Shuohao Cai, and Jingyi Huang from the Department of Soil Science. This research was supported by grants from USDA Agriculture & Food Research Initiative Foundational Program (project no. WIS04075), National Science Foundation Signals in Soil grant 2226568, and University of Wisconsin–Madison Dairy Innovation Hub.