4 Soil Erosion and Conservation

How soil erosion occurs

Soil erosion is the detachment and movement of soil by water, wind, or—in cultivated land—tillage. It occurs naturally, but land use that leaves soil exposed can accelerate it. When soil is removed faster than it can form, land productivity may decline over time.

Conditions that increase erosion

Water and wind are the main agents of erosion. Water erosion is encouraged by intense rainfall, runoff, long or steep slopes, and soil that absorbs water slowly. Wind erosion is more likely on dry, loose, bare soil, especially across long, open fields during strong winds.

Soil texture and structure also affect its resistance to movement. Stable aggregates and good infiltration help soil resist erosion. Human activities can increase risk by removing protective cover or disturbing soil structure; examples include clearing vegetation, overgrazing, frequent or intensive tillage, and construction.

A freshly tilled slope with little crop residue is more vulnerable to heavy rain than a slope protected by living plants and plant litter.

Forms of water erosion

Water erosion takes several forms, from raindrop impact to concentrated flow and bank loss.

Raindrop impact and surface runoff

Splash erosion occurs when raindrops strike bare soil and dislodge particles that may then be moved by runoff. Sheet erosion removes a thin, relatively even layer of soil across a broad area. Its loss can be hard to see and may go unnoticed for years.

Concentrated flow

Rill erosion occurs when flowing water concentrates into small channels. Ordinary field operations can often smooth rills, but unchecked rills may deepen or join together. Gully erosion occurs when concentrated runoff cuts larger, deeper channels that normal tillage cannot readily remove. Gullies can expand and carry substantial sediment downhill.

Streambanks

Streambank erosion is the wearing away of stream and channel banks by flowing water. It is particularly likely where banks lack stabilizing vegetation.

Forms of wind erosion

Wind moves soil particles in three ways:

• Suspension: Fine particles are carried in the air.

• Saltation: Particles bounce along the surface.

• Surface creep: Larger particles roll or slide along the surface.

Blowing soil may form dust clouds or collect along fences and roads.

Effects on land, plants, and water

Erosion removes topsoil, which commonly contains much of a soil’s organic matter and nutrients. Soil loss can reduce soil depth, water-holding capacity, fertility, and crop productivity.

Erosion can also damage plants. Windblown particles may abrade seedlings, while erosion or deposition may expose roots or bury them under soil.

Eroded soil can cause problems beyond the field. Sediment can cloud water, fill channels and reservoirs, and carry nutrients or other materials into waterways. Windblown soil can reduce air quality. Severe erosion can contribute to land degradation and increase risks such as flooding or slope failure.

Protect soil and manage runoff

Effective conservation reduces soil detachment, slows soil movement, or both. Practices should suit local soil, slope, climate, and land use. Combining methods is often more effective than relying on just one.

Keep the surface covered

Living vegetation, cover crops, grasses, and retained crop residues shield soil from raindrop impact and wind. Roots help bind soil and support its structure. Leaving crop residue after harvest, for example, protects a field during the period before the next crop grows.

Slow runoff and shorten effective slope length

• Contour farming follows the natural contours of a slope, slowing water as it moves downhill.

• Terraces divide a long slope into shorter sections, helping reduce runoff speed and soil loss.

• Grassed waterways provide a stable, vegetated route for concentrated runoff.

• Riparian vegetation along streams can help stabilize banks and filter sediment from runoff.

Reduce disturbance and wind exposure

Conservation tillage, including reduced-till or no-till systems, leaves more residue on the surface and disturbs soil less. Crop rotations and cover crops can maintain plant cover and support soil structure. Avoiding excessive compaction can help water enter the soil rather than run off.

Wind exposure can be reduced with windbreaks—rows of trees or shrubs—and herbaceous barriers, which slow wind near the ground. Strip cropping alternates bands of crops or protective vegetation to help interrupt wind across a field. Maintaining residue or growing plants between crop seasons further protects exposed soil.