The researchers at Washington State University have offered an unusual method for assessing soil health, using electrical signals created by the microorganisms living in it. Their work shows that microorganisms forming the so-called electrochemically active biofilms on the surface of electrodes generate characteristic electrical signals we can use to distinguish nutrient-rich, healthy soil from the depleted one. This discovery could lead to the development of inexpensive sensors capable of real-time assessing of soil health without costly and time-consuming laboratory analyses.
Soil health is a cornerstone in agriculture. Crop yields, moisture retention capacity, fertilizer efficiency, and plant stress tolerance are dependable on it. However, assessing soil health remains a challenging task. Traditionally, this involves conduct of chemical analyses, nutrient content determination, and the study of the composition of microbial communities and activity of soil enzymes. Such studies requiring time and specialized equipment provide only a snapshot of soil condition.
The American scientists decided to use natural electrical activity of soil microorganisms for these purposes. Soil constantly contains substances capable of donating and accepting electrons (the so-called redox compounds), as well as bacteria able to transfer electrons to the external surfaces. After placing a carbon electrode in soil and applying a small voltage to it, the researchers noticed that some microorganisms begin to use this voltage in their metabolic processes. A biofilm forms on the electrode’s surface, and the resulting electrical current reflects microbial community activity. The higher the voltage, the stronger and more characteristic the signal.
To test whether these signals are indeed related to soil condition, the scientists sampled two types of soil from the same plot in Washington state. The first was regularly fertilized with compost for over 15 years and used to grow tomatoes. The second one located nearby did not receive organic fertilizers for many years. The analysis confirmed that the first soil not only contained significantly more organic matter, phosphorus, potassium, sulfates, and trace elements, but also had higher microbial activity.
Then the researchers inserted carbon electrodes into both soil samples and began to monitor the resulting signals with several electrochemical methods. The applied methods allowed them to measure the current, the electrical potential, and the nature of redox processes on the electrode surfaces.
Then carbon electrodes measuring 4, 16, and 64 cm² were placed in both soil samples for 16 days. Within a week, the electrical signals from the healthy and depleted soils began to differ significantly. The healthy soil produced characteristic electrochemical signals indicating the development of active microbial biofilms, while the less fertile soil showed stronger signals, presumably related to other redox processes. By the 16th day, the electrical current density in the healthy soil was significantly higher for all the electrode sizes.
Even on electrodes without the applied voltage, the scientists observed differences in the form of electrochemical signals. This shows that the method allows capturing natural characteristics of soil microbial communities, not just the microorganisms’ responses to the experimental conditions.
However, the researchers have discovered an important limitation of the technology. The larger the electrode, the weaker the signal per unit area. On electrodes measuring 64 cm², the current density was almost three times lower than on electrodes measuring 16 cm². In addition, larger electrodes required more time for formation of a stable biofilm. According to the authors, this is due to heterogeneity of the soil itself and the limited nutrient transport within its pores.
Nevertheless, even the largest electrodes reliably distinguished between healthier and less healthy soils, which means that electrical signals from soil microorganisms can serve as a reliable indicator of the soil health, regardless of a sensor size. In the future, such systems could become part of the precision farming technologies allowing farmers to quickly monitor changes in the soil microbiome and make timely decisions about fertilization, irrigation, and fertility restoration.
