How can you tell if soil is healthy? Just listen to it.

Think back to the last concert you went to. Now replace the music that rang through the venue with an erratic series of pops, muffled staccatos, distorted taps, and sudden clicks. Not one sound is quite distinguishable from the other, all blending together in a medley of unsynchronized noise. 

Except, instead of musicians, what you’re hearing is a mass of underground invertebrates. And they’re putting on an unorthodox show for the handful of humans who know where, and how, to tune in — a complex symphony of vibrations and pulses that relay the state of the very soils these organisms are moving within.

“You can actually use sound to listen to the soil, and get an indication of soil health, based on the little critters moving around,” said Jake Robinson, a microbial ecologist at Flinders University in Australia. He’s the lead author of a study just published in the Journal of Applied Ecology that found that ecoacoustics, or the study of environmental sounds, can not only be used to detect organisms in the soil, but also mined to identify the difference between restored and degraded land. 

Although the practice of recording the sounds of nature has existed for over a century, using those recordings to analyze ecosystem health is a newer discipline. Scientists have, in recent years, started experimenting with using ecoacoustic tools to capture the full range of sounds in healthy ecosystems — such as in coral reefs, caves, and oyster beds — and applying those recordings to restoration efforts in damaged and degraded areas. 

And yet, soils, and the many hidden species and organisms that subsist underground, haven’t been considered for such techniques. Until now. 

“People in the past have thought ecoacoustics probably can’t be used for that, because there’s no vocalization or echolocation. We’ve shown that it actually can be used,” said Robinson. The trick, he noted, is deploying sensitive-enough microphones that allow you to detect the most minuscule of movements. “Things like millipedes, the little tappy legs, you can compare that to a worm, [which has] more of a slidey action. So actually, you can tell slight differences between the acoustic profiles of these little critters.”

From millipedes to nematodes, soils across the world teem with billions of living organisms that make up Earth’s biosphere and contribute to the global food supply. All told, the ground beneath our feet houses the most biodiverse habitat on the planet. “The more invertebrates that are in the soil, the more active that they are, the more different sounds and vibrations they are emitting,” said Robinson. 

His team used a belowground sampling device and sound chamber to record and collect 240 soil acoustic samples from deforested plots, locations undergoing restoration, and those with at least some of their original vegetation in a corridor of grassy woodlands in Mount Bold, South Australia. After listening to the acoustic recordings first onsite, and then removing soil samples to analyze them in controlled conditions in the field, they discovered a pattern: The acoustic complexity and diversity of the soundscapes were significantly lower in the deforested plots.

Lack of diversity in a soundscape indicates that life belowground is missing, so the soils are likely to be in a state of degradation, meaning they are experiencing physical, biological and chemical losses in quality. Degradation causes significant biodiversity decline, hampering soil’s vital ecosystem services, such as water cycling, and results in colossal consequences for the world’s agricultural productivity, curbing crop yields and livelihoods. It is a problem impacting more than three-quarters of land on Earth. “That could rise to 90% by 2050 unless we intervene,” said Robinson, echoing a warning issued by the United Nations Food and Agriculture Organization in 2022. “We need to do something.”

Unsustainable land management, intensive farming practices, urbanization, and anthropogenic climate change can disturb and damage soil, triggering degradation; which, in turn, affects how much carbon soils have the capacity to store. 

Degradation also creates a negative feedback loop for many soil species, reducing their chances of survival, and further deteriorating the soil. That’s because these soil organisms are themselves key to preserving soil health, so their presence is not only an indicator of soil viability, but also helps create that viability. Earthworms in particular augment global food production, contributing to the growth of more than 140 million metric tons of food every year by increasing plant growth and enriching soils. (If earthworms were a country, they would be the fourth-largest grain producer.) 

In fact, in many parts of the world, counting earthworms is commonly deployed by farmers to measure soil health, but those measurements aren’t always accurate. “Everything has its place in ecosystem ecology. So if you’re only measuring one thing, you miss other parts,” said Victoria Burton, a postdoctoral researcher who studies soil biodiversity at the British Natural History Museum. One example is that in areas where earthworms are invasive, you’re likely to find plenty of the invertebrates in degraded habitats, which by that nature would make counting their abundance a poor benchmark for healthy land. This bolsters the case for widely applying soil acoustics for more accurate readings in agricultural and conservation contexts, she said. 

Burton added that this research is the first-of-its-kind to show how ecoacoustics can be used to identify soil health specific to the grassy woodland ecosystem — a study released last year, also helmed by Robinson, found similar results when measuring soil biodiversity in Britain’s temperate forests — but she has questions regarding how this technique would perform in other ecosystems. She’s exhilarated by the prospect, however: Her team is installing soil acoustic sensors in the museum’s gardens to better monitor changes to urban wildlife in the space. 

Scientists aren’t only listening to underground realms for classification purposes. They’re also using acoustics to see if they can speed up the restoration process, according to Robinson. “The vision, I think, is to see if degraded soils can be played sounds that help them recover,” he said. 

In another forthcoming study, his team discovered that, when they played certain sounds to a type of fungus called Trichoderma, widely used in agriculture to protect crops against diseases and improve plant health and revegetation, it effectively stimulated organism growth. Of course, this result is preliminary, but offers up major implications for how ecoacoustics could be applied to deteriorated soils on farmland. Monitoring soil health by way of soundscapes could be used by farmers and growers to figure out where they need to preemptively intervene. And playing back healthy soil recordings in eroded swaths of land could, in theory, encourage that beneficial fungi growth, jump-starting restoration. 

As with any body of research in its infancy, just how this technology will be applied, and what degree of impact the use of ecoacoustics could wield in efforts to restore deteriorating soils, remains up in the air. The biggest uncertainties revolve around how different soil types and properties may affect sound transmission belowground, in addition to how this technique might perform in other ecosystems and geographies. 

While researchers work to figure out those missing pieces, the natural symphonies found within soil are beginning to attract attention from unlikely sources. Last year, a composer in Norway reached out to Robinson with an unusual request: She wanted to incorporate sounds emitted by earthworms into one of her orchestral productions. 

“It’s quite interesting to hear a millipede, who’s got tiny legs. It’s like a little hairy, high-frequency tapping sound. Then, for instance, a snail, which is like a slow, slimy, glide-y sound, and the worm is something in between,” said Robinson. “That’s like listening to Mother Earth, isn’t it?”

This story was originally published by Grist with the headline How can you tell if soil is healthy? Just listen to it. on Aug 20, 2024.