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Soil Formation
and Erosion

Explore how parent material transforms into soil over time and how natural and human forces break it down through erosion.

AP Environmental Science

What Is Soil?

Soil is a dynamic mixture of minerals, organic matter, water, air, and living organisms. It forms at the interface between the lithosphere, atmosphere, hydrosphere, and biosphere: a thin layer that sustains nearly all terrestrial life.

45% Minerals
Sand, silt, and clay particles weathered from rock
5% Organic Matter
Humus from decomposed plants, animals, and microbes
25% Water
Soil solution carrying dissolved nutrients for roots
25% Air
Pore spaces providing oxygen for root respiration

How Soil Forms: Pedogenesis

Soil forms through pedogenesis: the slow transformation of parent material by physical, chemical, and biological weathering over hundreds to thousands of years. Five key factors control the process.

The Five Soil-Forming Factors

1
Climate: Temperature and precipitation drive the rate of chemical weathering and decomposition. Warm, wet climates form soil fastest.
2
Organisms: Plants, fungi, bacteria, and burrowing animals mix organic matter into mineral material and create structure.
3
Relief (Topography): Steep slopes lose soil to erosion; flat areas and depressions accumulate material and moisture.
4
Parent Material: The underlying rock or sediment determines mineral composition, texture, and initial chemistry.
5
Time: Soil development is slow: mature profiles take thousands of years; young soils are thin and weakly layered.

Soil horizons reflect additions,translocation and weathering

A forest soil cutaway with roots and litter accompanies authored O,A,E,B,C horizon bands and a downward translocation arrow.Compare surfaces of different relative ages while holding climate, organisms, relief and parent material similar. Horizon differentiation can change with development, while erosion can remove an A horizon and expose underlying material.

Relative age and surface loss are different influences on soil horizons.

Relative age and surface loss are different influences on soil horizons.. Compare surfaces of different relative ages while holding climate, organisms, relief and parent material similar. Horizon differentiation can change with development, while erosion can remove an A horizon and expose underlying material.Compare surfaces of different relative ages while holding climate, organisms, relief and parent material similar. Horizon differentiation can change with development, while erosion can remove an A horizon and expose underlying material.

Soil Horizons: The Soil Profile

O Organic litter (leaves, humus) A Topsoil: dark, mineral + organic mix (Zone of eluviation) E Leached layer: light-colored, sandy B Subsoil: accumulation of clay, iron, aluminum (zone of illuviation) C Weathered parent material (regolith) R Bedrock (unweathered) Increasing depth

Soil Texture: Sand, Silt, and Clay

Soil texture describes the relative proportions of sand, silt, and clay particles. It determines water retention, drainage, aeration, and nutrient-holding capacity. The USDA Soil Texture Triangle classifies soils by these proportions.

Sand
\(0.05\text{--}2.0 \text{ mm}\) diameter. Feels gritty. Drains fast, low nutrient retention.
Silt
\(0.002\text{--}0.05 \text{ mm}\) diameter. Feels smooth. Moderate drainage and fertility.
Clay
\(< 0.002 \text{ mm}\) diameter. Feels sticky. Holds water and nutrients tightly.

Weathering: Physical vs. Chemical

Physical Weathering Chemical Weathering
What happens Rock breaks into smaller pieces without changing composition Minerals are chemically altered into new substances
Key agents Frost wedging, root growth, abrasion, thermal expansion Water, acids, oxidation, hydrolysis, carbonation
Climate link Dominant in cold, dry climates with freeze-thaw cycles Dominant in warm, wet climates
Example Ice splits granite along cracks Acid rain dissolves limestone (\(\text{CaCO}_3\))
Soil result Coarse, sandy, mineral-rich parent material Fine clay particles, leached minerals

What Is Erosion?

Erosion is the removal and transport of soil or rock by water, wind, ice, or gravity. While weathering breaks material down in place, erosion moves it. Natural erosion shapes landscapes over geologic time, but human activities can accelerate it by 10 to 100 times the natural rate.

Surface runoff Wind Topsoil Subsoil Rainfall Splash

Types of Erosion

Water Erosion
Sheet, rill, and gully erosion caused by rainfall and surface runoff. The most widespread form globally.
Wind Erosion
Deflation and abrasion remove fine particles from dry, exposed surfaces. Major cause of the 1930s Dust Bowl.
Mass Wasting
Gravity-driven movement: landslides, mudflows, and creep. Triggered by steep slopes, water saturation, or earthquakes.

Water and wind erode soil through distinct transport modes

Water panel distinguishes detachment, runoff and incision; wind strip shows near-surface particles and airborne dust. Illustrated grain sizes and trajectories are not a wind-speed measurement.Splash, sheet flow, rills and gullies describe different forms of water erosion. Wind creep, saltation and suspension differ in how particles move.

Different erosion transport modes

Different erosion transport modes. Splash, sheet flow, rills and gullies describe different forms of water erosion. Wind creep, saltation and suspension differ in how particles move.Splash, sheet flow, rills and gullies describe different forms of water erosion. Wind creep, saltation and suspension differ in how particles move.

Formation vs. Erosion: A Balance

Soil Formation Soil Erosion
Process Weathering builds soil from rock Transport removes soil from land
Rate Very slow: ~1 cm per 200-1000 years Can be rapid: cm per year on bare land
Driving forces Climate, organisms, time Water, wind, gravity, human activity
Result when dominant Deep, fertile soil profile develops Thin or absent topsoil; degraded land
Human influence Composting, no-till farming Deforestation, overgrazing, construction

Human Activities That Accelerate Erosion

1
Deforestation: Removes root networks that bind soil. Exposed ground is vulnerable to splash and sheet erosion.
2
Overgrazing: Livestock strip vegetation faster than it regrows, compacting soil and reducing infiltration.
3
Poor agricultural practices: Monoculture, plowing along slopes, and leaving fields bare between seasons expose soil to water and wind.
4
Urban development: Impervious surfaces increase runoff velocity and volume, scouring downstream channels.

Soil Conservation Strategies

1
Contour plowing: Plow along the natural contour of the slope, not up-and-down. This creates ridges that slow runoff.
2
Terracing: Cut flat steps into steep hillsides. Each terrace catches water and soil, preventing gully formation.
3
No-till farming: Leave crop residue on the field surface. Protects soil from splash erosion and maintains structure.
4
Cover crops and windbreaks: Plant vegetation between seasons or along field edges to hold soil, reduce wind speed, and add organic matter.
5
Riparian buffers: Maintain strips of native vegetation along waterways to filter sediment and reduce stream-bank erosion.

Worked Example: The USLE

A farmer's field has the following USLE factor values. Estimate the annual soil loss \(A\).

\( R = 200 \)
Rainfall erosivity index
\( K = 0.30 \)
Soil erodibility (silt loam)
\( LS = 1.5 \)
Slope length/steepness
\( C = 0.40 \)
Row crop, conventional till

Reducing Loss with Conservation

Now the farmer adopts contour plowing (\(P = 0.50\)) and switches to no-till with cover crop (\(C = 0.10\)). Same field, same climate.

Erosion Pathway: Cause to Impact

Deforestation Overgrazing Poor Farming Urbanization Exposed, unprotected soil Water Erosion Wind Erosion Mass Wasting Loss of fertile topsoil Reduced crop yields Sedimentation Clogs waterways, harms aquatic life

Common Mistakes

✕ Confusing Weathering and Erosion
Weathering breaks rock in place. Erosion transports the material. They are sequential but distinct processes.
✕ Mixing Up Horizons
The A horizon is topsoil (eluviation zone). The B horizon is subsoil (illuviation zone). Don't reverse them.
✕ Thinking Clay Equals Poor Soil
Clay has the highest nutrient-holding capacity (CEC). It only becomes problematic when drainage is poor. Loam includes clay.

Question 1 of 5

Which soil horizon is characterized by the accumulation of leached minerals (illuviation) such as clay, iron oxides, and aluminum?

Question 2 of 5

A farmer switches from conventional tillage (\(C = 0.40\)) to no-till with cover crops (\(C = 0.10\)) while keeping all other USLE factors the same. By what percentage does soil loss decrease?

Question 3 of 5

Which of the five soil-forming factors (CLOrPT) is most difficult for humans to modify?

Question 4 of 5

Contour plowing reduces erosion by modifying which factor in the Universal Soil Loss Equation?

Question 5 of 5

A soil sample contains 40% sand, 40% silt, and 20% clay. This texture class is closest to which description?