The new Method opens a window for science, says Or: “We can learn when roots grow, for example." (Photo: Colourbox)
What cannot be seen might perhaps be heard: with this in mind, researchers from ETH Zurich and the French National Institute for Agricultural Research (INRA) have been using piezoelectric sensors to investigate various soils for acoustic emissions.
The result: when roots grow or earthworms burrow tunnels, they make noises that can plainly be traced back to root growth and worm activity. This has been demonstrated by a study conducted by Marine Lacoste from INRA, Siul Ruiz and ETH-professor Dani Or from the Institute of Biogeochemistry and Pollutant Dynamics at ETH Zurich.
Time-lapse images recorded root-growth visually (a,b), which gave the estimated total root length, while two sensors registered acoustic emissions (c). (Graphic: Marine Lacoste, Scientific Reports, CC BY 4.0)
To test this hypothesis, the researchers compared visual observations with acoustic measurements. They demonstrated that in a sandy soil- filled glass cells with corn plants and in similar cells filled with loamy soil allowing earthworms to burrow tunnels.
Piezoelectric sensors registered the acoustic emissions, which take the form of elastic waves at a frequency of 1-100 kilohertz. Sound waves of this nature occur, for example, when small grains move or rub against each other – or when tiny cracks form in the soil. Humans cannot hear them.
Dani Or has already successfully investigated terrain for early signs of landslides. The work taught him to distinguish and interpret a range of acoustic emissions. He says: “The noises in the soil are of a very low magnitude, but the fact that they exhibit specific signatures means they can be attributed to specific sources. Worms, for instance, move much faster than roots, while their acoustic emissions are much more irregular.”
The study has allowed the researchers to demonstrate that soil biological activity can be measured acoustically – under laboratory conditions, at least. The next step involves looking at the situations that suit the method and how it plays out in the field. A preliminary test has already proved successful: the researchers let plants grow in an soil column – which allowed them to measure root growth acoustically. Or can well see his method being used by farmers in the future alongside other means of soil analysis.