In agricultural regions, such as in the Midwest, up to 80 percent of soils considered rare have been reduced to less than half of their original extent. That is, more than half of the soil has been converted to agriculture or urban uses.
"Over the past two centuries, we have reconfigured part of a continent to the point where today's landscape is almost unrecognizable from its natural state," said Ronald Amundson, professor of ecosystem sciences at UC Berkeley's College of Natural Resources and lead author of the paper. "The Great Plains used to be characterized by tall grasses and prairies. They have now been replaced by crops and housing tracts."
Like their plant and animal counterparts, soils have their own taxonomy. In the United States, there are 11 soil orders that are ultimately divided into 13,129 series. A soil series is comparable to a plant or animal species. Soils that comprise less than 25,000 acres are considered rare. What the report calls "rare-unique" soils exist only in one state and comprise less than 25,000 acres. The researchers considered a rare or rare-unique soil endangered if more than half of its area was tilled, excavated or otherwise disturbed.
The researchers found 508 endangered soil series in the United States. Six states have more than half of their rare soil series in an endangered state, with Indiana leading the group at 82 percent, followed closely by Iowa at 81 percent. Most of the soil danger hotspots reside in the country's agricultural heartland.
The researchers also found that 31 soils are effectively extinct because they have been nearly completely converted to agricultural or land use.
Why the concern over undisturbed, virgin soil? As the foundation of terrestrial ecosystems, soils form an intimate bond with the plants and animals they support, said Amundson. Rare plants have evolved to inhabit rare soils, such as those that are highly acidic or low in nutrients. An area of very ancient and nutrient-poor soils near the town of Ione, Calif., for example, provides the habitat for four species of endemic plants, including the Amador Rock Rose and the Irish Hill buckwheat. The plants are listed in the "Inventory of Rare and Endangered Vascular Plants of California," and are not found naturally anywhere else in the country.
In essence, soil diversity is tied to biological diversity, said Amundson.
But tilling the soil changes its biogeochemistry by stimulating microorganisms to quickly metabolize the soil's organic matter for food. The disturbance of the soil impacts the plants and animals that depend upon it, the researchers said.
"Soil that has been cultivated is like an animal that has been domesticated," said Amundson. "It retains some resemblance to its wild or native ancestor, but there are enormous and profound changes in its characteristics."
Research has also shown that the process of digging up soil produces carbon dioxide, which contributes to the greenhouse gases in the atmosphere. "Soil has more carbon in the form of organic matter than all the plants in the world," said Amundson.
Cultivating the soil breaks up the organic matter, making it available as food for microorganisms such as bacteria and fungi. The process of breaking down the organic matter releases a significant amount of carbon dioxide in the atmosphere. Twenty percent of the carbon dioxide added to the atmosphere by human activity is related to land-use activities such as burning forests and farming, said Amundson.
To conduct this study, Amundon and the other researchers combined data from digitized maps on soil types compiled by the U.S. National Resource Conservation Service with information from maps of agricultural and urban growth provided by the U.S. Geological Survey and the Environmental Protection Agency.
"Never before has soil in the United States been analyzed in such a way," said Peng Gong, UC Berkeley professor of ecosystem sciences and co-author of the paper. "Our study is the country's first quantitative analysis of soil diversity."
Standing at the forefront of soil activism, the researchers argue for the preservation of rare and unique soils. "Soil might harbor microbial life that has benefits unknown to us today," said Amundson.
The research of the late soil scientist Selman Waksman may be one of the best examples of the contributions possible from soil research. Observing that soils do not become contaminated when diseased bodies are buried in the ground, Waksman set out to isolate soil microorganisms that produced natural antibiotics. His research led to the discovery of streptomycin, the first antibiotic that was effective in treating tuberculosis, for which he was awarded the Nobel Prize in Physiology or Medicine in 1952.
"We certainly need land to farm and develop. I'm not advocating the discontinuation of agricultural expansion," said Amundson. "But I think it'd be fair to set aside modest areas of these remaining natural landscapes for study and contemplation."
"Some of these soils developed over thousands to millions of years," added Gong. "We can destroy that in a few hours. It's a preservation issue. We need to save it for future generations."
The study was also co-authored by Yin Yang Guo, a former post-doctoral researcher at UC Berkeley's Department of Environmental Science, Policy and Management. The Kearney Foundation of Soil Science funded the research.