We don’t give soil too much thought. We walk on it, we grow our plants in it, we build on it, but otherwise, we rarely pay it much consideration. But for most species on Earth, soil is home.

Two-thirds of all known species live in the soil, making it the most species-rich habitat on Earth, according to a new study. This more than doubles previous estimates of soil biodiversity and shows how important soils are for all of us, the researchers said, calling for further conservation efforts to protect them from their multiple threats.

The researchers from the Swiss Federal Institute for Forest, Snow and Landscape Research found 59% of all species depend on the soil for their survival — including 90% of fungi, 86% of plants and 40% of bacteria. A previous study in 2006 estimated only 26% of species lived in the soil, but only considered certain types of organisms.

Mark Anthony, a mycologist and the study’s co-author, says the previous number of soil species just felt too low. So he set out to refine the estimate.

Along with colleagues, Anthony reevaluated all the data they could find on species that have been identified in soils. The researchers defined a species as living in the soil if it lives on it, within it, or spent at least a part of its life cycle on it. They then compared the numbers they came up with with the creatures that live in any other habitats.

Anthony said this is the first time a study has attempted to estimate the diversity of all the organisms living in the soil, including the smallest ones, such as bacteria, viruses, archaea, fungi, and protists. These play key functions, such as recycling nutrients and storing carbon, and are also important as pathogens and partners of trees, Anthony added.

The findings, however, come with uncertainties. Data on soil diversity is very incomplete, especially in the Global South, so the researchers found big ranges in some cases. For example, for bacteria, the average value is 40% of species living in the soil, but the range extends from 25% to 88%. Uncertainties are also big for viruses, mainly studied as human pathogens.

Stefan Geisen, a soil ecologist at Wageningen University who wasn’t involved in the study, told The Messenger that the global estimate for soil biodiversity is very abstract. However, “you need a number to appreciate what you have and what you might lose and need to protect. It’s not just earthworms and a few moles and voles,” Geisen added.

Soil, the ecosystem powerhouse

Soil, the top layer of Earth’s crust, is formed from a mix of water, gases, minerals, and organic matter, as The Guardian explains. While it plays a key role in food production, it has been largely left forgotten in the wider environmental debates. Soil organisms have unique functions on which we rely for food, fiber, and human and planetary health.

When managed sustainably, the quality of the soil is maintained or even increases over time. If not, soils easily degrade and are negatively affected. Human activities such as overgrazing, unsustainable land use practices, and inappropriate clearing techniques have led to a severe nutrient decline in soils. Water and wind erosion and salinization have also played a role.

The findings of the new study, the researchers said, provide the basis for decisions to protect soils and their organisms. “Soils are under enormous pressure, whether from agricultural intensification, climate change, invasive species, and much more. Our study shows that the diversity in soils is great and correspondingly important,” Anthony said in a press release.

The study was published in the journal PNAS.