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ST. LOUIS — Our soil is key to our ability to grow food — and it serves as a protective layer between the sources of human waste and the water we drink.

It also has the potential to be a toxic bath, keeping us awash in ever-stronger populations of disease-causing pathogens.

Those were two of several views of soils presented Monday during the 172nd annual meeting of the American Association for the Advancement of Science. Researchers from the University of Arizona, the Idaho National Laboratory and Kansas State University who spoke in the session said soils could both help and hurt us as the climate changes and the human population grows.

"Life without soils wouldn't exist," said Ian Pepper, a researcher with the UA's Environmental Research Lab. Besides the basic functions of supporting trees and food crops, soils have yielded important cures, he said — including the antibiotics penicillin and streptomycin.

He called the bacteria that live in soil an "ultimate example of evolution in action and a public health savior."

UA researcher Charles Gerba takes a different view. He said 80 percent of all infectious diseases are transmitted through the environment. And if temperatures increase through global warming, disease organisms in soils are likely to increase.

Infectious diseases have risen in recent years to become the third-leading cause of death in the United States, after heart disease and cancer. Gerba thinks the risk is going up.

"Since 1990, one new waterborne pathogen has been recognized every year," he said. "I have no doubt that … they'll (infectious diseases) be number one on that list."

In Arizona, a soil-dwelling fungus causes a respiratory ailment, called valley fever, that sickens people and their pets. It's commonly caught when people and dogs scatter soil by their footsteps and then inhale the spores.

The fungus lives in soils throughout the Southwest and could become more easily released from desert soils made dustier by the ongoing drought, Gerba said.

The bacteria that cause Legionnaire's disease are considered waterborne, but there have been outbreaks in soils, especially near composting bins. That respiratory disease kills a fifth of its victims, and people over age 55 are particularly vulnerable.

And in Phoenix, two 5-year-old boys died in 2002 because of an amoeba called Naegleria fowleri, which causes a deadly brain disease. It's most common in groundwater but can be caught by swimming in surface water. Gerba said science isn't sure yet how common the risks are from pathogens in soils — but he advocates getting a handle on those risks because he believes a changing climate threatens to make them worse.

The UA's Pepper added that soils are a great laboratory for studying how genes jump between the plentiful bacteria that live there. But in some cases, the genes that jump can code for resistance to antibiotics or the ability to infect humans.

"Resistance genes can find their way into groundwater, which is certainly another route of exposure," he said. At this point, Pepper added, soil isn't as much of a health threat from that possibility as are other sources — including our food.

"In terms of exposure, soil is less of a public health threat than lettuce," he said.

Deborah Newby, a researcher at the Idaho National Laboratory, said soils are still the ultimate repository for our wastes. She said some soil microbes are effective at neutralizing poisons released from industry, like TCE — which is blamed for causing a rash of birth defects in Tucson several decades ago and is the subject on ongoing local cleanup efforts throughout the area.

"Our soils really are doing some amazing things for us," she said. But as the population grows, she said, we could start to see the limits of its protective values.

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