By Tina Hilding, Voiland College of Engineering and Architecture
Washington State University researchers have developed a method for assessing soil health by measuring the electrical current produced by the smallest of microbes.
The team used a probe originally designed to measure the electrochemical signal from microbes in aquatic environments and tested it on healthy and unhealthy soil samples to measure microbial metabolism and other indicators of soil health. This feasibility study, published in the Journal of Electrochemical Society, could one day lead to a simple real-time test for farmers to determine if the soil is productive.
“Soil is the foundation of all the food we eat, and most of it is broken down worldwide,” said Maren Friesen, associate professor in the Plant Pathology and Crop and Soil Sciences departments and co-author of the study. “One of the biggest barriers to soil improvement is the inability to take real-time, rapid measurements to develop appropriate management strategies for them. This sensor has the potential to measure not only the structure of the soil in real time, but also how it actually works. It would be a huge step forward in this area. “
“I believe this is one of our most significant pieces of work and will have a major impact on determining soil health,” said Haluk Beyenal, professor at the Gene and Linda Voiland School of Chemical Engineering and Bioengineering and corresponding author of the study.
Further co-authors of the study are the postdoc Abdelrhman Mohamed and the PhD students Eduardo Sanchez and Natalie Sanchez.
Soil health is critical to agriculture and crop success around the world, but it is not easy to measure it. Farmers and researchers use soil chemistry, nutrient analysis, texture and pH measurements to understand the physical and chemical properties of the soil. While this information can be valuable, it doesn’t always reflect how productive the soil actually is.
Because one key to soil productivity is how microbes work, said Friesen. Billions of bacteria, fungi and other organisms play a crucial role in the mobilization and supply of nutrients, the defense against pathogens and plant growth. However, until now there has not been an easy way to measure microbial activity in real time.
“What makes a soil beneficial to a plant is that it is alive and has all of these bacteria and fungi in it,” she said.
In the new work, the WSU research team was able to measure the flow through the soil in order to determine the microbial activity and differentiate between healthy and unhealthy soils.
The researchers used a probe developed a few years ago to measure the electrochemical signal from microbes in aquatic environments. Similar to how humans eat and breathe, microorganisms ingest food and then use the electrons released during metabolism for their energy. Eventually, microbes donate these electrons to an acceptor molecule such as oxygen. The probe developed by the team replaces these acceptor molecules with an electrode. With this electrode they can then measure the electrical current and get an idea of the magnitude of the microbial activity.
“We are able to measure the metabolic rate of the microbes by capturing electrons that are released as part of the metabolism,” said Mohamed, a postdoctoral fellow at Voiland School. “We watch how the microbes breathe in the soil.”
The two soil samples used by the researchers came from the RJ Cook Agronomy Farm and looked almost identical in their soil composition. They were both collected from uncultivated plots that were relatively high in organic matter and had the same pH and soil type. However, the researchers had data showing that one of the soils was significantly more productive than the other in terms of its wheat yield.
The researchers found that the more productive soil produced an electrical current, while the less productive soil produced almost no electricity – about 1% of the more productive soil.
“There was a really dramatic difference in the amount of electricity generated,” said Friesen.
They also found another difference between the two floors in the open circuit potential measured in the floor. When adding sugar to stimulate metabolic activity, the researchers also observed that the electrochemical signals converged in the healthy and unhealthy soil samples, suggesting that the addition of sugar stimulated microbial activity in both types of soil.
“We could see that the microbes in the soil began to breathe after a few days,” said Mohamed.
With only the two initially compared soil samples, the researchers say that their idea is still just a proof of concept. You have many additional questions, such as what the creatures do to generate electricity and what specific microorganisms might be in the samples to create productive soil.
“We have two different signals, but what do they really say about the basic parameters of the soil?” Said Mohamed. “Both parameters say slightly different things and we have to work on their interpretation.”
You also want to test a lot more soils, including in real agricultural fields and not in the controlled environment of a laboratory. They hope to eventually develop a portable probe that could be inserted directly into the ground to provide real-time information.
“In terms of working for a just society with sustainable global food production, I have the potential to be a breakthrough technology,” said Friesen.