For as long as humans have been farming, they’ve been trying to figure out what’s going on below ground. Soil is incredibly complex—full of organisms, microbes and chemicals that move and change constantly—and it all feeds into crop health and the Earth’s nutrient cycles in ways that aren’t fully understood. But getting data was a problem, since this generally required taking soil samples and then analyzing them in the lab, which is slow and often expensive.
Their project, called the Thoreau sensor network
, buried more than 30 sensing boxes in a variety of different locations around the UChicago campus. Each one is a cube about five inches square, containing four sensors that measure the soil’s water content, salt, temperature and water potential, the measure of how readily the soil holds or drains moisture. Twice an hour, a tiny radio transmitter and antenna—fully buried underground—sends a burst of data to the receiver, located atop the William Eckhardt Research Center.
For the hardware, they used commercial sensors that already exist. But there were a lot of questions about how the sensors might behave underground: Can the signals make it to the receivers above ground? Does the battery die faster? What happens to the machinery during freeze-thaw cycles?
“This test run provides us extremely helpful real-world data on how one could actually run a sensor network like this,” Ghosh said. “For example, the Chicago winter gave us some very helpful information.” (A few of the sensors didn’t survive last winter.)
There were also questions about whether the radio signals would be able to be transmitted from below the ground. The research team found that they could successfully transmit over distances of one and a half miles, even though the antennas were buried six to eight inches below the surface of the ground.
Wet soil appears to inhibit the signal, they said, but the biggest issue so far is battery life. The group of undergraduate students working on the project have been a big help, Ghosh said. “They’ve come up with some really excellent ideas for saving battery life,” including a low-power timer that puts the sensor to sleep in between its 30-minute wake-up calls.
Seasons of change
The land that is now the University of Chicago campus was once sand, marsh and prairie at the edge of Lake Michigan. Now it’s home to a network of streets, century-old buildings, quadrangles, athletic fields, flowerbeds and libraries. Each use has different impacts on the soil below—and these differences show up in the data.
The study is just underway, but Ghosh said they’ve seen some interesting trends and questions in their data. “For example, we believe we’re seeing patterns in how water leaves different types of soil after a rainfall,” she said, as well as moisture differences in the growing season versus the winter.
As a materials scientist, Guha is interested in the sensor hardware. He heads the Center for Nanoscale Materials at Argonne National Laboratory, where research on developing new capabilities for sensors is underway.
“For example, what we would really love to do is to make a sensor that can measure soil nitrates,” he said. This would provide a way to measure how much of the fertilizer that farmers apply to their soil gets to the plants. It’s thought that less than half of the nitrogen goes to plants; the rest of it likely washes off and pollutes rivers, lakes and oceans.
“There’s a lot we’d like to do,” Ghosh said. “Can we hook sprinklers up to receive input from our sensors? How far down is the right depth for the best data? How much can we extend the range of how far a sensor can be from the receiver? Can we boost the signal to reach from beneath paved areas or sidewalks?”
A complementary project is ongoing in India, testing the water quality of the Godavari River in southern India and how it reacts to weather, pollution, fishing and general use. In this case, a boat carries a mobile sensing platform equipped with GPS along the river every few days, enabling scientists to map the river chemistry.
“Those results have been spectacular,” Guha said. “We’re seeing that dynamic mapping of river water quality can accurately help pinpoint and assess pollution sources.”