Soil supports the food we eat, stores water, carbon and nutrients, and provides habitat for an enormous share of life on Earth. Scientists estimate that more than half of global biodiversity exists below the surface. But soil biodiversity is declining around the world, threatening agriculture, food security and the ability of ecosystems to function properly.
To investigate this hidden world, researchers from the University of Zurich (UZH) and Agroscope, the Swiss federal center of excellence for agricultural research, launched the citizen science project Proof by Underpants (Beweisstück Unterhose) in 2021. The project was designed both to raise awareness of soil ecosystems and to generate scientific data.
Following the same standardized method, 1,000 volunteers buried more than 2,000 pairs of cotton underwear and 12,000 tea bags at roughly 1,000 sites across Switzerland. After two months, participants dug them up, photographed them, and sent the material to the laboratory along with soil samples for further study.
Decomposing Underwear Reveals Soil Activity
The findings, published in Plants, People, Planet, produced a broad map of biological activity in Swiss soils. Researchers found that the cotton underwear broke down at very different speeds depending on where it was buried.
That decomposition rate served as a simple indicator of how much biological activity was taking place underground. When soil organisms were more active, they broke down the cotton more quickly.
Private gardens showed the fastest decomposition. Researchers also found that these soils contained the highest amounts of organic matter, creating favorable conditions for earthworms, fungi, bacteria, and other organisms that live in soil.
Lawns showed the lowest level of soil organism activity. Agricultural fields and meadows ranked between gardens and lawns.
Land Use Strongly Shapes Soil Health
“Our results show that how soil is managed can significantly affect both soil life and soil quality,” says co-study leader Marcel van der Heijden, UZH professor of agroecology. “Healthy, biologically active soil is crucial for fertility, nutrient cycling and many other ecosystem services.”
The researchers point to several practices that can encourage soil life. These include keeping soil permanently covered, applying compost, using more diverse crop rotations and reducing the use of mineral fertilizers and pesticides.
Among the factors studied, land use had the strongest influence on how quickly the underwear decomposed. Whether a location was a garden, meadow, cultivated field or lawn explained differences in decomposition better than most of the soil characteristics the researchers measured.
Chemical properties also mattered. Nutrient levels, for example, had a significant effect on how active the soil was.
Faster Decomposition Is Not Always Better
Rapid breakdown of the underwear was often associated with fertile soil and a plentiful supply of nutrients. For agricultural land, those conditions can be beneficial because they support plant production.
“However, maximum decomposition is not desirable in every ecosystem,” says co-project leader Franz Bender, head of the agroecological assessments team at Agroscope.
“In near-natural habitats such as forests, very high nutrient levels can indicate disruptions to the natural balance of nutrients. In gardens, too, rapid decomposition of the underwear may point to an oversupply of nutrients,” says Bender.
When that happens, gardeners may want to consider whether they are using too much fertilizer. Biological activity in soil is also strongly influenced by temperature and moisture. If the ground becomes too cold or too dry, soil organisms become far less active.
An “Underwear Index” for Soil Health
The experiment shows that burying cotton underwear can offer a simple but useful way to compare biological activity and fertility in different soils.
The researchers suggest creating an “underwear index” that could make soil processes easier for the public to understand while drawing attention to the importance of protecting healthy soils.
Citizen Scientists Build a National Soil Dataset
The “Proof by Underpants” project also highlights the value of citizen science. A study spanning about 1,000 locations across Switzerland would have been difficult to carry out without the participation of so many volunteers.
Because samples came from every region of the country, the project produced one of Switzerland’s most comprehensive datasets on soil life.
The scale of public involvement is also reflected in the study’s authorship. Around 240 citizen scientists are listed as co-authors of the publication.
In return for their participation, researchers gave volunteers personalized feedback, including soil analysis results, evaluation tools and advice for managing soil more sustainably.