Printed Soil Sensors Could Help Farmers Boost Crop Yields and Cut Costs

Engineers at the University of Wisconsin–Madison have created affordable sensors to monitor soil nitrate levels in real-time for soil types common in Wisconsin. These printed electrochemical sensors can help farmers make smarter decisions about fertilization, potentially saving them money.

“Our sensors can give farmers a clearer picture of their soil’s nutrient levels and how much nitrate is available for crops. This information allows them to make precise decisions on how much fertilizer is needed,” says Joseph Andrews, a UW–Madison assistant professor of mechanical engineering and lead researcher. “Reducing fertilizer use could mean significant cost savings, especially for large farms.”

Nitrate is essential for crop growth, but too much of it can seep into groundwater, polluting drinking water and harming the environment. These new sensors can also serve as tools in agricultural research, tracking nitrate runoff and guiding better practices to reduce pollution.

Traditional methods to monitor soil nitrate are time-consuming, costly, and don’t offer immediate results. To address this, Andrews, an expert in printed electronics, and his team designed these sensors as a simpler and more economical alternative.

For this project, the researchers used an inkjet printing method to make potentiometric sensors, which are a kind of thin-film sensor that uses electrochemical reactions. These sensors are typically used to measure nitrate levels in liquid solutions accurately. However, they usually don’t work well in soil because rough soil particles can scratch the sensors and affect accurate readings.

Andrews explains, “Our main goal was to make these electrochemical sensors work effectively in challenging soil conditions and accurately detect nitrate ions.”

To solve this, the team added a protective layer over the sensor using a material called polyvinylidene fluoride. According to Andrews, this material has two important qualities. First, it has extremely small pores, around 400 nanometers, which let nitrate ions pass through but keep soil particles out. Second, it’s hydrophilic, meaning it attracts water like a sponge.

Andrews says, “This means any water containing nitrates will be absorbed by our sensor, which is crucial because soil also absorbs water. Without this, it would be hard for the sensor to get enough moisture, but since our material matches soil’s water absorption, it helps draw nitrate-rich water to the sensor’s surface for accurate readings.”

The researchers shared their progress in a paper published in March 2024 in Advanced Material Technologies.

The team tested its sensors in two types of soil found in Wisconsin: sandy soil, which is common in the north-central area, and silt loam soil, which is found in southwestern Wisconsin. They found that the sensors gave accurate results in both types.

Now, the researchers are adding their nitrate sensors to a system they call a “sensing sticker.” This system combines three different sensors — for nitrates, moisture, and temperature — on a flexible plastic sheet with adhesive on the back.

They plan to place several of these sensing stickers on a rod at different heights, then bury the rod in the soil. This setup will allow them to measure conditions at different depths in the soil.

Andrews explains, “By measuring nitrate, moisture, and temperature at various soil depths, we can now track the process of nitrate leaching and observe how nitrate moves through the soil, something we couldn’t do before.”

In the summer of 2024, the researchers will continue testing their sensors by placing 30 sensor rods in the soil at UW–Madison’s Hancock and Arlington Agricultural Research Stations.

The team is working to patent this technology through the Wisconsin Alumni Research Foundation.

Co-authors from UW–Madison include Kuan-Yu Chen, Aatresha Biswas, Shuohao Cai, and Professor Jingyi Huang from the Soil Science Department.

This research was funded by the USDA Agriculture and Food Research Initiative Foundational Program (project no. WIS04075), the National Science Foundation’s Signals in the Soil grant 2226568, and the University of Wisconsin–Madison Dairy Innovation Hub.