Lawns ranked lowest. Agricultural fields and meadows sat in between.

Land use — whether a site was a garden, meadow, cultivated field or lawn — explained differences in decomposition better than most of the soil's measured chemical and physical properties. Nutrient levels also mattered: richer soils tended to break down cotton more quickly.

Is faster decomposition always better?

Not necessarily. In forests and other near-natural habitats, high nutrient levels may signal disruption of the natural balance. In gardens, rapid decomposition can flag an oversupply of fertilizer.

"However, maximum decomposition is not desirable in every ecosystem," said co-project leader Franz Bender, head of the agroecological assessments team at Agroscope.

Rapid breakdown on farmland, however, usually reflects fertile, productive soil — the kind most growers want.

What can farmers and gardeners do with this?

Co-study leader Marcel van der Heijden, a UZH professor of agroecology, pointed to practices that support soil life.

"Our results show that how soil is managed can significantly affect both soil life and soil quality," van der Heijden said. "Healthy, biologically active soil is crucial for fertility, nutrient cycling and many other ecosystem services."

The team highlighted four practices:

• Keep soil permanently covered
• Apply compost
• Use more diverse crop rotations
• Reduce mineral fertilizers and pesticides

Biological activity also depends heavily on temperature and moisture. When the ground becomes too cold or too dry, soil organisms slow down sharply.

Could cotton underwear become a standard soil test?

The researchers propose an "underwear index" — a simple, visual indicator that lets non-specialists gauge biological activity and fertility in their own soil. Participants received personalized feedback, including soil analysis results and advice for more sustainable management.

Around 240 of the volunteers appear as co-authors on the published paper (DOI: 10.1002/ppp3.70259), a striking example of how citizen science can scale up fieldwork.

The study has limits. The two-month window captures activity in a single season, and the climate, soils and land-use mix of Switzerland may not translate directly to other regions. Still, the authors argue buried cotton offers a cheap, engaging way to draw attention to a resource most people never see.

Scientists estimate that more than half of global biodiversity lives below the surface. Soil biodiversity is declining worldwide, with consequences for food security and ecosystem function. A pair of cotton briefs rotting in a backyard may not solve that problem, but the Swiss dataset shows communities can help measure it.