We walk on it every day. Get it under our fingernails. Track it into the house. But do we really appreciate the vital role soil plays—not just in the environment, but in human health?

The United Nations Food and Agriculture Organization is giving soil its due. Friday, December 5, is World Soil Day, and 2015, the FAO has declared, will be the International Year of Soils.

"The minerals, the nutrients that make up our muscles and bones almost entirely come from soil," says Jerry Glover, a National Geographic Emerging Explorer and agroecologist at the U.S. Agency for International Development.

"This is, of course, very critical because we're supposed to be increasing agricultural production to feed and nourish some of the ten billion people, but it's at the same time that our soils are the thinnest and most nutrient depleted."

Here are five things you should know about soil.

1. Soil, like oil, is a finite resource.

Poor farming practices deplete soil nutrients faster than they are able to form, leading to loss of soil fertility and degraded lands. Glover compares it to the concerns that surround oil depletion.

"We still continue to harvest more nutrients than we replace in soil," he says. If a country is extracting oil, Glover points out, people worry about what will happen if the oil runs out. But they don't seem to worry about what will happen if we run out of soil. (Read "Our Good Earth" in National Geographic magazine.)

By 1991, an area bigger than the United States and Canada combined was lost to soil erosion—and it shows no signs of stopping. In fact, says Glover, native forests and vegetation are being cleared and converted to agricultural land at a rate greater than any other period in history. To restore soil in the United States to its pre-Columbian levels would take about 200 years.

View Images Cletus Reed, 80, and his grandson Sam stand at a cross-section of exposed ground on their Iowa corn farm. Photographer Jim Richardson created the cross-section to illustrate the potential impact of erosion. The Reeds, who lost more than a foot of topsoil to erosion, subsequently altered their farming practices. Photograph by Jim Richardson, National Geographic

2. Misusing soil can topple civilizations.

Modern examples of the impact of soil erosion are well-known: the Dust Bowl in the American and Canadian prairies, the erosion of China's Loess Plateau, the famine in Africa's Sahel. Ancient societies also reaped what they sowed when it came to their farming practices.

"The Romans still plowed themselves out of business, as did the Greeks, and Easter Islanders," says David Montgomery, who studies topography at the University of Washington in Seattle and is the author of Dirt: The Erosion of Civilizations.

On the flip side, few societies have actually taken care of their soil, he says. Inca terracing practices and agroforestry on the Polynesian island of Tikopia are on the short list of exceptions.

3. Good soil usage helps prevent droughts.

During recent droughts in the western U.S., farmers who used no-till practices—for example, not disturbing the soil through plowing—produced healthier crops, according to Montgomery. Soil conservation goes hand in hand with water conservation, he says; healthier soils retain more water.

The impact of poor soil use, meanwhile, goes beyond food production. Wind can carry thinned topsoil off fields and onto large bodies of water. Through a process known as eutrophication, the excess nutrients hasten plant growth and algae bloom, sucking up oxygen in the water and killing fish and other marine fauna.

"In the Gulf of Mexico, dead zones have developed during certain parts of the year," says Glover. "These are often prime fishing areas and prime biodiversity areas, now dead because of soil carried thousands of miles downstream."

4. High-tech makes a difference.

There has been improvement in soil conservation in the United States since the introduction of no-till and low-till farming, but reducing soil disturbance is not enough. The restoration of soil health, experts say, will require new practices and old-fashioned "soil husbandry."

Experts agree that healthy soil requires a marriage of ecology and technology, such as planting perennial strains of grain crops. Another approach: sophisticated farming systems that integrate crop production with native vegetation and livestock—a system that has successfully restored soils in northern Ethiopia, says Glover. (Related: "Why Tiny Microbes Mean Big Things for Farming.")

View Images Date trees line a terrace in the Loess Plateau region of China. Restoration projects like this one mix forestry with agriculture to improve soil fertility. Photograph by Jim Richardson, National Geographic

5. Soil is alive.

Chemical fertilizers, which replace three or four nutrients, are simply not enough to replace the complex system that is soil. They're "not a full health package," says Glover.

That's because soil is crawling with microbes and bugs, which nourish the soil. They help cycle nutrients in exchange for plant sugars. It's a symbiotic relationship that is the root of life, but we don't fully understand it, according to Montgomery.

"This is brand-new science. Over the past 30 years, there's been a big shift in our understanding of microbial connections and the community dynamics under the ground," he says. "It's the hidden half of nature."