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How Healthy Is a Farm's Soil? Check How Active Its Microbes Are

Doug Palen runs a family farm in north-central Kansas, a place he’s called home all his life. Since he started farming for himself in the early 1990s, he’s tried any number of tests that measure soil health, like basic nutrient profiles and fatty acid assays. These tests give farmers an idea of how fertile their soil is, or estimate how tiny microorganisms in the soil are responding to changing environmental conditions. “I’m not sure any of them by themselves is the final answer or the Holy Grail,” Palen says.But researchers at Washington State University are hoping to change this. In August, a team published a proof-of-concept study in the Journal of the Electrochemical Society demonstrating that they could use electrodes to measure electric currents produced by those microbes. Detecting a current, they concluded, meant that the soil was healthy, because those microbes were conducting metabolic activity—doing things like recycling nutrients and creating compounds that help crops weather environmental stressors.Biochemical engineers Abdelrhman Mohamed and Haluk Beyenal, two of the study’s authors, had previously worked together to create instruments to study microbes in wastewater treatment systems, remote hypersaline lakes, and hot springs in Yellowstone National Park. They used their prior work as the basis for studying the microbiome of soil. “A lot of what people currently do is look at what the structure of soil is, but not what it does—the microbial activity,” Mohamed says.In general, soil microbes include anything from helpful bacteria to symbiotic fungi; up to 10,000 bacterial species can be found in a single gram of soil. The tiny organisms decompose material, hold onto carbon, transform nitrogen, and enhance the availability of important nutrients. To do these jobs, microbes need nutrients to live and reproduce, and their process of consuming energy produces activity that can be read by a sensor. While other tests can paint a portrait of the soil’s chemical and physical composition, measuring microbial activity provides a better picture of the system’s active properties. "The electrochemical sensor is going to be the voice of the soil,” says Beyenal. “That’s my sci-fi vision.”

David Huggins and Maren Friesen collecting soil from the Cook Agronomy Farm Long Term Agroecosystem Research site. Photograph: Maren Friesen
Some of the other soil tests can also be expensive, or take multiple seasons to produce usable results. Others are not good predictors of yield. “Measuring the microbial activities gets us closer,” Mohamed says. “Hopefully, we not only give a simpler answer, but a faster answer.”