Crunching worms, squeaking voles, drumming ants: how scientists are learning to eavesdrop on the sounds of soil
The sound of an earthworm is a distinctive rasping and scrunching. Ants sound like the soothing patter of rain. A passing, tunnelling vole makes a noise like a squeaky dog’s toy repeatedly being chewed.
On a spring day at Rothamsted Research, an agricultural research institution in Hertfordshire, singing skylarks and the M1 motorway are competing for the airways. But the attention here is on the soundscapes underfoot: a rich ecosystem with its own alien sounds. More than half of the planet’s species live in the soil, and we are just starting to tune into what they are up to. Beetle larvae, millipedes, centipedes and woodlice have other sound signatures, and scientists are trying to decipher which sounds come from which creatures.
In a field divided up into test strips, Carlos Abrahams pushes a sensor the length of a knitting needle into the soil.
With a pair of headphones on, he listens to the “poor man’s rainforest”: a dark landscape of miniature caves, tunnels and decomposing matter stewing away under our feet.
“A few ticks and clicks going on,” says Abrahams, an ecoacoustics specialist from Baker Consultants, as he listens in.
Abrahams and scientists from the University of Warwick are building up libraries of subterranean sounds. The soil makes different noises depending on the season and whether it’s night or day. Even in the afternoon when the soil has warmed up, sounds get richer, research suggests.
“The soil is such a mystery,” says Dr Jacqueline Stroud, from the University of Warwick’s Crop Centre. “This is like opening the door and seeing what is going on below ground. It’s a different way of exploring the world.”
Until recently, soil had 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 spadefuls and carry out laborious tests.
Last year, a study found soil was the single most species-rich habitat on Earth, with more than half of all species living in it. But only a fraction have been identified, and most are too small to see. Soundscapes are becoming an increasingly popular way of monitoring wildlife abundance, above ground, beneath the earth and underwater.
Noisy soil is generally healthier because it contains a greater range of bugs and worms busying around. Soil organisms alter and improve the structure of soil by passing nutrients between one another and creating an environment that is well ventilated and diverse. These webs provide food, fibre and clean water for people – topsoil is where 95% of the planet’s food is grown.
Soils that have little biodiversity are more fragile: they have lost the structure and connections that keep particles together. This means they are more likely to be washed away by