Erosion: why it happens and what to do about it

Erosion is a major problem worldwide, affecting both rich and poor countries.

Soil erodes from cropland following a rain of 60 mm (2.5 inches) in 1 hour. This fertile grain-growing land lies 20 km north of Coolah in New South Wales, Australia.

The United States' biggest export, in tons and dollar value, is eroding topsoil.

North American topsoil pours into the ocean at the mouth of the Mississippi River, here seen from space. As topsoil disappears, farmers need more fertilizer to keep crop yields high. Fertilizer runoff is a major cause of the oxygen-depleted "dead zone" in the Gulf of Mexico, which claimed 22,000 km² (8,500 mi²) in 2002 -- an area the size of Massachusetts.

Under natural conditions, most ecosystems grow soil. The world's topsoils formed thanks to these natural soil-building processes.

Soil layers. Subsoil (light bottom section) is mostly mineral particles. Topsoil (darker top section) is high in nutrients, and contains organic matter from plants and animal dung. Organic matter and fungi glue soil particles together to resist erosion.

Soil loss over wide areas is usually not natural. Most areas suffering soil loss once grew the soil that is now eroding.

The Dust Bowl years of the 1930s saw unprecedented erosion as drought hit newly plowed land across North America's Great Plains. Blowing dust buried roads, farms, and buildings, sometimes darkening the skies for days. Today the U.S. loses even more soil per year, largely to water erosion.

Erosion reduces crop yields and causes problems for hundreds of millions of people.

Su Rongxi next to his house in Hebei province, China, after dust storms nearly buried it. "We have no money to move and, besides, who would have us?" says Su. "There's nothing to do but dig away the sand and wait to see what happens. Sometimes I dream of the sand falling around me faster than I can dig away. The sand chokes me. I worry that in real life, the sand will win."

Erosion happens easily where the soil surface is bare.

Summer fallowing in eastern Washington, U.S.A., attempts to conserve moisture for a monoculture of wheat by eliminating all other plants. Dust storms, a very high rate of erosion, and the decline of rural communities are unintended side effects.

Erosion often happens when plant roots are too shallow to hold soil. On grasslands, short roots occur mainly due to

• Overgrazing -- plants are grazed too often.
• Overrest -- grazing-dependent plants are grazed and trampled too seldom. Overrest is a problem mainly in brittle (seasonally dry) environments.
• A shift from perennials to annuals when management favors annuals. Perennials generally have much deeper roots.

This brittle grassland was damaged by overrest. It has had no livestock or significant concentrations of grazing animals on it since a nearby freeway interchange was built in the 1970s. Near Pendleton in northeastern Oregon, U.S.A.

More about brittle environments.

Overgrazing keeps grass roots short by keeping the leaves short. Short leaves can't generate enough energy to feed deep roots.

Canadian researchers kept the tops of a common bunchgrass clipped at different levels. Shallow-rooted plants are vulnerable to drought and supply little material for soil-building.

Overrest shortens grass roots by killing part or all of the top of the plant.

Grass dying from prolonged rest on a destocked range in Nevada, U.S.A. Accumulated dead leaves shaded and killed the center of the plant, leaving a typical donut shape. Only living leaves can feed roots.

Both overgrazing and overrest produce bare soil by killing plants and keeping new plants from starting.

Overgrazing exposed virtually all soil (red area) on a pasture east of Cobar, on the western plains of New South Wales, Australia.

Outside the pasture (gray area), overrest killed most plants, left the remainder with shallow roots, and bared large areas of soil.

The results of thousands of hectares of exposed soil surface in New South Wales, Australia: the town of Griffith is engulfed by a dust storm on 12 November 2002. Says Judi Earl, "We hadn't seen anything like November's windstorms for close to 100 years."

Shallow roots and bare soil make hills vulnerable to landslides.

Unsound grazing management allowed shallow-rooted annuals to replace deep-rooted native perennials on virtually all of California's grasslands. Landslides are now a problem statewide. Slumps (sunken areas) at left and center right are small landslides. Near San Juan Bautista, south of San Francisco Bay, U.S.A.

Most native landscapes protect soil.

Aboriginal management maintained significant tracts of rain forest in northwestern Australia's Kimberley until recent times. This rain forest remnant lies on Kachana Station, where managers are preserving and expanding such areas.

When the processes that grow and protect soil get damaged, erosion becomes a problem.

Poor or no management destroyed most of the Kimberley's forests. Now this lone rain forest tree struggles to survive in an unrestored area of Kachana Station.

Exposed eucalyptus roots tell of heavy soil loss. Despite restoration efforts, Kachana still loses over one million tons of soil to erosion every year.

Worldwide, damaged landscapes erode billions of tons of soil per year.

The Dunham River (right) carries millions of tons of soil to the ocean each year. It contrasts with clear waters from the Ord River (left) downstream from the dam at Lake Kununurra, which is rapidly filling with trapped silt. The Dunham catchment lies between Kachana Station and this spot.

Dunham River report on westnet.com.

Damaged landscapes absorb water poorly. Water runs off instead of soaking into soils, causing flooding and erosion downstream. Dry soils reduce crop yields.

Topsoil and water pour into Tamworth Creek following heavy rain near Coolah, Australia.

As land condition deteriorates, flooding and droughts become common.

As the Mississippi River's catchment area got converted from prairie and forest to agricultural land, major floods* increased from three in 100 years (1844-1943), to four in 35 years (1944-1978), to four in 15 years (1979-1993). The river flooded these grain silos at MacGregor, Iowa (central U.S.A.) in April 2003.

* Measuring 9 meters (30 feet) or more at St. Louis, Missouri

Erosion can remove huge amounts of soil.

A bunchgrass plant (Idaho fescue) on a soil pedestal. Continued soil loss around the plant has exposed roots and left an erosion pavement. Brittle grassland near Elgin, Oregon, U.S.A.

Here over 1 meter (3 feet) of soil has eroded over a wide area, as demonstrated by exposed tree roots. Western U.S.A.

Most erosion control methods fail because they only address symptoms, not causes.

Riprap, gambions, and recontouring failed to heal this stream in northern California, which washed out with every winter's rains. Conditions were wrong for establishing and encouraging deep-rooted plants that could hold the banks. Big Bluff Ranch outside Red Bluff, California.

Addressing root causes of erosion allows solutions.

Under sound grazing management, livestock establish and maintain desirable plants. The stream's banks have healed, even though poor land conditions upstream still cause it to flood every winter. This is the same place as the previous photo (note same fence posts and tree at right).

Article about this stream restoration.

Soils are protected from erosion when

• The soil surface is covered by living plants or litter (dead plant material on the soil surface)
• Plant root are deep and hold soil
• Soil contains plenty of organic matter to glue particles together.

Soil surface in the dormant season on a highly productive tropical savanna. Deep-rooted perennials grow enormous amounts of forage, which when trampled by grazers provides excellent soil cover.

See our grassland productivity slide show for examples of soil cover in different climates.

Soil kept bare by conventional farming is vulnerable to erosion.

Eroded soil piled 600 mm (2 feet) deep along 140 m (150 yards) of fence. This grain-producing area is known for its rich soils, now rapidly eroding. Near Coolah, New South Wales, Australia.

Several newer farming systems seek to minimize cropland erosion by keeping soil covered and organic matter high. Methods include no-till, Permaculture, and Natural Systems Agriculture.

After 15 years of no-till, Tim Melville's irrigated ground in eastern Oregon absorbs 12 times more water than under conventional management. Crop residue protects the soil surface from erosion and evaporation.

In the early 1980s, Tim plowed one field and direct-seeded an adjacent field without tilling it. In June, a thunderstorm dropped 130-150 mm (5-6") of rain and hail in 30 minutes. "It washed nearly all the topsoil off the plowed field, and down the canyon," says Tim. "We lost six inches of topsoil." There was no damage to the field that had not been tilled.

Article about Tim Melville's no-till experiences, on Wallowa.net website.

Just 3% of the world's land is suited for growing crops. The rest is too dry, wet, steep, or cold, or has erratic rainfall, or poor or erosion-prone soil. Land unsuitable for growing crops often deteriorates very quickly once the the soil surface is exposed.

Only sand remains beneath this center-pivot irrigator near Lambert's Bay in western South Africa. South Africa loses an average of 20 tons of soil for every ton of agricultural crop (wheat, sugar, etc.) produced.

Currently touted as a solution to soil erosion on seasonally dry rangeland, biological soil crusts are not good protectors of soil because they

• Are fragile
• Take a long time to regenerate
• Grow almost no food to fuel ecosystem processes or feed wildlife
• Reduce soil's ability to absorb water
• Keep the soil from breathing
• Inhibit seed germination, and may keep plant communities from advancing to more productive forms.

Mature soil crusts (ridged areas) show that this landscape in the western U.S.A. has been undisturbed by wild grazers or livestock for decades. Properly managed, this land could support grassland.

Compare our soil crust basics to a conventional view of soil crusts at www.soilcrust.org.

The only way to economically protect eroding rangelands worldwide is to do what nature does and have the land grow its own protection.

To do this, nature's forces must be able to

• Plant seeds and establish young plants to cover bare ground and replace old plants that die
• Grow enough plant material to protect the soil surface
• Knock down standing growth and place it on the soil as litter (mulch)
• Build soil.

Tallgrass prairie does an excellent job of protecting soil. The combination of grasslands, grazers, and predators to kept herds bunched and moving, built soils up to 2.7 meters (9 feet) deep over vast areas of North America.

Once grazed by bison and elk, this prairie outside Salina, Kansas, U.S.A. is now maintained by longhorn cattle, which are managed to imitate wild grazers.

In seasonally dry areas (about 70% of Earth's land area), nature depends on herding grazers to

• Break soil crusts and plant seeds
• Mulch the soil surface
• Compost forage into fertilizer
• Prune plants to keep them productive
• Incorporate soil-building materials into the soil.

A traffic jam of wildebeest maintains grassland health in Africa's Serengeti.

Where insufficient wild grazers and predators remain, or where landscapes are too damaged to support native wildlife, properly managed livestock can maintain and restore land.

Cattle maintain a tropical savanna where most wildlife is gone. This type of landscape desertifies very rapidly when grazers are removed or mismanaged. Mimicking wild herds, these cattle get moved 5 times daily. Marius Nel farm near Vryburg, South Africa.

Livestock can establish plants and cover soil even in areas with no forage and zero carrying capacity.

Joe Morris explains how he healed this former landslide by broadcasting seed, then spreading hay and feeding it to cattle. By stamping in a mix of hay and manure, the cattle planted seeds and kick-started the soil-building process. San Juan Bautista, California, U.S.A.

The same process can revegetate and grow soil over large areas.

The Teels Marsh basin in Nevada, U.S.A., is the proposed site for a 250,000 acre (100,000 ha) ecological restoration. Once open water with ducks and reeds, embraced by grassland, the marsh is now a vast alkali flat surrounded by desert.

Article about Teels Marsh project.

By working with nature, even "impossible" sites can be restored.

Cockatoo Creek, a former rain forest wetland on Kachana Station, in the rugged Kimberley region of northwest Australia. Badly damaged by feral cattle, donkeys, and frequent wildfires, by 1992 the area could not even sustain a fire.

Once plants get established, periodic grazing that imitates the actions of wild herds keeps ecosystem processes functioning and aids the soil-building process. Up to 30 mm (1 inch) of new soil can be grown in a year using good grazing management. Even on arid sites, significant amounts of soil can be grown in 3 years.

November 1998: Same area revegetated, using managed grazing by cattle as the main restoration tool.

Restoring land with livestock is inexpensive and can often be made to make money. Millions of acres of eroding land can now be reclaimed and lost topsoil replaced with new.

June 1999: the herd restoring the area grazes during the middle of the dry season. Soil is protected from erosion, and soil-building processes are active. With plenty to eat, wildlife is coming back.

Article about this restoration.