by The sound of an earthworm is a distinct screeching and screeching sound. Ants sound like the soothing pattern of rain. Tunneling volts make a noise like a squeaky dog toy being chewed over and over.
On a spring day at Rothamsted Research, an agricultural research institute in Herefordshire, soaring skyscrapers and the M1 motorway compete for airspace. But the focus here is on the suppressed soundscapes: a rich ecosystem with its own exotic sounds. More than half of the planet’s species live in soil, and we’re only just beginning to get to grips with what they’re up to. Beetle larvae, millipedes, centipedes and lice have other sound signatures, and scientists are trying to figure out which sounds come from the creatures.
In a field divided into test strips, Carlos Abrahams pushes a knitting needle length sensor into the soil. Wearing a pair of headphones, he listens to the “poor man’s rainforest”: a dark landscape of tiny caves, tunnels and decaying material jutting out beneath our feet.
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“A few ticks and clicks on,” says Abrahams, an eco-acoustic specialist from Baker Consultants, as he listens in.
Abrahams and scientists from the University of Warwick are building libraries of underground sounds. The soil makes different sounds depending on the season and whether it is night or day. Even in the afternoon when the soil has warmed, sounds become richer, according to research.
“Soil is a mystery,” says Dr Jacqueline Stroud, from the University of Warwick’s Crops Centre. “This is like opening the door and seeing what’s going on underground. It’s a different way of exploring the world.”
Until recently, soil has been a relative blank spot for monitoring species abundance. Farmers and gardeners hoping to find out how healthy their soils were had to dig up soil and perform labor tests.
Last year, a study found that soil was the most species-rich habitat in the world, with more than half of all species living in it. But only a fraction has been identified, and most are too small to see. Soundscapes are increasingly popular for monitoring wildlife abundance, above ground, below ground and underwater.
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Loamy soil is generally healthier because there is a greater range of bugs and worms to roam around. Soil organisms change and improve soil structure by transferring nutrients between them and by creating a well-aerated and varied environment. These webs provide people with food, fiber and clean water – topsoil is where 95% of the planet’s food is grown.
Soils with little biodiversity are more fragile: they have lost the structure and bonds that hold particles together. This means they are more likely to be submerged by floods or blown away by strong winds. An estimated 24bn tonnes of fertile soil is lost each year through intensive farming, according to a UN-backed study, the Global Land Outlook.
Related: ‘Crucial to look after the soil’: fears as UK earthworm population declines
Farmers have repeatedly called for more effective ways to measure the abundance of earthworms, which are a good indicator of soil heath, according to researchers.
Baker Consultants and the University of Warwick have funded a two-year research project to develop a prototype recording unit. The aim is to record soil sounds at “big data” scales.
On the land Abrahams is testing, scientists are experimenting with more ecological ways of farming, including crop rotation with legumes and a higher proportion of oats. A total of 70 scientists are working on this piece of land, marked out in 66 plots 24m by 24m, discovering new things about soil structure, viruses, microbes and fungi – making it one of the most studied soils in the world. “It’s a unique outdoor laboratory,” says Kim Hammond-Kosack of Rothamsted Research, which set up the experiments.
Abrahams and Stroud’s teams began their sampling at Rothamsted in October last year. Every month they make two recordings on each of the plots, measuring how activity above ground affects what is happening in the soil.
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Dr Simon Butler from the University of East Anglia was listening to the soil before and after the application of zebra dung in Eswatini (formerly known as Swaziland). The soil fizzed with post-application activity. “I’ve never looked at the sound of soils, so it was really interesting to hear how the acoustic properties change in response to the presence of fresh manure,” he says.
The sounds being produced are within the lowest range of human hearing, so there may be sounds in the soil that we haven’t heard yet. Early research from Switzerland shows that soils produced the most complex sounds in spring and summer, which decreased in autumn and winter. Previous research by Abrahams has shown that soils in restored forests in the UK appear to have a greater diversity of sounds than soils from deforested plots. He says: “As a general rule, the more diversity there is above ground, the more there is going on in the soil.”
In January, researchers published what they believe is the first paper looking at tropical forest soils, which are among the most biodiverse habitats in the world. Like others, they documented numerous mysterious sounds. The next task is to create a library of soil sounds so they can work out what they are listening to.
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