A Dirty Word

By Brad Scriber and Emily Krieger, National Geographic staff

First things first: Soil is more than just dirt. While "dirt" suggests something that's dead, soil is very much alive: a complex stew of living organisms and decaying organic matter mixed with pebbles, sand, clay, silt, air, water, and nutrients that supports life literally from the ground up. Essential organisms digest dead matter and transform the nutrients into a form plants can use. These nutrients keep plants healthy enough to perform photosynthesis—a process that converts the sun's radiant energy into a form that all plant, animal, and fungal life depends on. Without plants to make this transformation, we would starve. And just as we depend on plants, plants depend on good soil.

The mix and amount of nutrients determines what will grow in any given soil. Carnivorous plants evolved in part because flora growing in nutrient-poor environments needed a way to supplement their diets. Humans enhance soil quality with fertilizers, both organic and synthetic. Nitrogen, one of the most essential nutrients and a basic component of protein, must be "fixed" (chemically combined with another element) or released from decaying matter by organisms before plants can use it. Symbiotic bacteria that live on the root nodules of legumes—soybeans, peas, alfalfa—fix nitrogen gas, producing compounds plants can use. Planting legumes in crop fields in alternate years boosts available nitrogen levels and varies the demand on soil nutrients. The pH of soil is also important. Plants generally prefer earth that is alkaline; too much acidity stunts the growth of microbes, including those that fix and release nitrogen.

Mann, Charles C. "Our Good Earth." National Geographic (September 2008), 80-107.

"Plant Nutrients." North Carolina Department of Agriculture.

Frantz, J. M., and others. "Towards a Healthy Planet."

"The Nitrogen Cycle."

Name That Soil

By Brad Scriber and Emily Krieger, National Geographic staff

Not all soil is created equal, but all soil creation takes time. It starts with the weathering of parent material such as exposed bedrock, which leaves a mineral signature in the soil as it breaks down. Later, plant growth and other organic activity contribute to the unique soil profile of a given place. The amount of precipitation and heat largely determines the speed of these processes, but topography also has some influence. A mature soil has a number of distinct layers called soil horizons that are clearly visible when the ground is cut away.

Every year at the National Soils Judging Contest teams of aspiring pedologists, or soil scientists, compete to get to the bottom of a set of soil profiles, figuring out what they are and describing how suitable each is for agriculture, urban development, or other uses. Nonexperts who don't know an arid gypsid from a permafrost-laden turbel can start learning about soil by getting to know the official state soils –emblematic varieties akin to state birds or flowers. Another good introduction is the Smithsonian exhibition "Dig It! The Secrets of Soil", which opened in the summer of 2008 in Washington, D.C., and showcases samples from across the nation.

U.S. Department of Agriculture Natural Resources Conservation Service. Soil Survey Manual.

U.S. Department of Agriculture Natural Resources Conservation Service. Soil Taxonomy: A Basic System of Soil Classification for Making and Interpreting Soil Surveys.

"State Soils." U.S. Department of Agriculture Natural Resources Conservation Service.

"Dig It! The Secrets of Soil." Smithsonian National Museum of Natural History.

Blighting the Land That Feeds Us

By Brad Scriber and Emily Krieger, National Geographic staff

Terra firma, always described as solid, is still vulnerable: Human activity has already degraded more than 7.5 million square miles of land, an area the size of the United States and Canada combined. This degradation falls into a few broad categories: wind erosion, water erosion, chemical changes, and physical changes.

Felling trees or allowing livestock to overgraze makes it easier for wind and water to carry nutrient-rich topsoil away, rendering the land less productive. The erosion also exposes organic material in the soil, which then releases carbon dioxide into the atmosphere as it decomposes. Certain farming techniques can change the chemical and physical qualities of the soil as well. Too little fertilizer can lead plants to leach nutrients from the soil; too much fertilizer or pesticide can acidify or otherwise pollute the ground. Poor irrigation systems—those that deliver water to crops inefficiently or use low quality water—can leave harmful salt behind when the water evaporates. Driving heavy equipment over wet fields crushes air out of the soil—a process called compaction—which makes it harder for plants and water to reach deep into the ground. Water that can't infiltrate the surface runs off the land instead, exacerbating erosion.

Only about 11 percent of Earth's land surface is suitable for farming, and demands on that land will increase dramatically in the coming decades. The UN's Food and Agriculture Organization estimates that by 2030 the world's growing population will require 30 percent more grain than we now produce. To meet that need, healthy soil is a must.

"Global Assessment of Human-induced Soil Degradation (GLASOD)." ISRIC World Soil Information.

"Assessment of CO2 Emissions From Drained Peatlands in Southeast Asia." Delft Hydraulics and Wetlands International, 2006.

Cardon, G.E., and others. "Salt-Affected Soils." Colorado State Extension.

"Soil Quality Information Sheet: Rangeland Soil Quality—Compaction." U.S. Department of Agriculture, Natural Resources Conservation Service, May 2001.

"World Agriculture: Towards 2015/2030." Food and Agriculture Organization of the United Nations, 2002.

Restoration Methods

By Brad Scriber and Emily Krieger, National Geographic staff

Though people have spoiled huge tracts of the world's soil, there's good news: It can be restored, and many proven methods are surprisingly simple.

In the African Sahel, a zone between the Sahara and tropical forests that has been hard hit by drought and famine in recent decades, much of the land has a hard time absorbing rainwater. In this region, what happens to the water after it falls is as important as how frequently it falls. Farmers and development experts have revived and improved traditional techniques to help the land get the most from every drop. In one method, rows of stones called cordons pierreux are placed along the contours of the land to slow runoff. Gradually, silt and seeds are deposited along these strategically placed barriers, and small plants that grow in these deposits are even more effective at slowing and holding later rains. This benevolent cycle has helped farmers grow not just small plants but healthy rows of trees in the dry land. Another technique that has helped farmers in Burkina Faso, also in the Sahel, bring life to barren lands is the use of zaï, small planting holes about a foot deep that are filled with compost. Termites drawn to the organic material dig channels in the soil that help rainwater penetrate crusted soil even farther. The water collected in these and other micro-catchment systems has helped restore lands that were severely threatened by desertification, turning them green again.

In Haiti, a nation that is almost completely deforested, much of the soil has washed away. One method used there to put more organic material into the soil is composting toilets. When properly treated in a closed-loop system, human waste can become a cheap fertilizer that can preserve or restore soil fertility and reduce pollution from centralized treatment centers. One organization, Sustainable Organic Integrated Livelihoods, has built dozens of composting toilets across Haiti. A recent assessment by the health-advocacy group Zanmi Lasante Paris, notes other positive steps already being taken by some Haitian farmers. These include building rock walls and residue bunds (physical barriers that slow the erosion of soil on steep slopes) as well as intercropping (planting a mix of crops in the same field to help maintain soil fertility and ensure that the farmer has something to harvest even if one crop fails). ZLP recommends that farmers be encouraged to use a mix of sustainable agricultural techniques so that they can reap both short-term profits and long-term soil improvements.

No-till farming refers to the increasingly popular practice of not tilling, or turning over, the earth. Tilling releases stored carbon into the atmosphere, though it has the benefit of aerating (oxygenating) soil and killing meddlesome weeds. No-till farmers keep weeds at bay in part by leaving crop residue on the ground after harvesting. Critics are concerned that these farmers also often use herbicides to control weeds, but proponents have found that well-managed cover crops can be used in place of herbicides, and they argue no-till farming produces thicker, healthier topsoil. On the other hand, a recent long-term study by the USDA in Maryland reported that organic farming produced higher quality organic matter in the soil than no-till farming.

Cofie, Olufunke, Boubacar Barry, and Deborah Bossio. "Human Resources as a Driver of Bright Spots: The Case of Rainwater Harvesting in West Africa." November, 2004.

Werner, Christine, and others. "Reasons for and Principles of Ecological Sanitation." April 2003.

Sustainable Organic Integrated Livelihoods.

Lal, Rattan, and others. "Managing Soil Carbon." Science (April 2004).

Comis, Don. "Organic Farming Beats No-Till?" U.S. Department of Agriculture Agricultural Research Service. July 10, 2007.

McClintock, Nathan. "Regenerative Agriculture for Haiti's Central Plateau–A Sustainable Foundation for Nutrition Security." Zanmi Lasante Paris, September 2004.

Other Resources
Montgomery, David R. Dirt: The Erosion of Civilizations. University of California Press, 2007.

"Black Is the New Green." Nature (August 2006).

Wood, William. "Amazonian Dark Earth."

Other National Geographic Resources
Salopek, Paul. "Lost in the Sahel." National Geographic (April 2008).

"A World Transformed." National Geographic map supplement (September 2002).

Norris, Scott. "Human Activities Triggering 'Global Soil Change.'" National Geographic News, February 5, 2008.

Glasener, Erica. "Pay Dirt for Gardeners." The Green Guide.

Last updated: August 6, 2008