Topic 4.2 Notes – Soil Formation and Erosion

What Soil Is and How It Forms

Soil is a living, active mixture of:

• weathered mineral material from rock
• organic matter
• water
• air
• living organisms

This is why soil sits at the meeting point of the geosphere, atmosphere, hydrosphere, and biosphere.

Parent material is the mineral source of soil. It can be bedrock underneath the soil, or sediment that was moved in from somewhere else. Soil formation begins when that parent material is weathered, and sometimes the material is then transported and deposited before a full soil profile develops.

A common APES mix-up is weathering vs. erosion:

• Weathering breaks rock down.
• Erosion removes and transports that material.

Weathering types

• Physical weathering breaks rock into smaller pieces without changing its chemistry.
  • freeze-thaw
  • abrasion
  • heating and cooling
  • pressure release or exfoliation
  • root wedging
• Chemical weathering changes minerals through reactions.
  • dissolution
  • oxidation
  • carbonic acid
  • organic acids
  It happens faster in warm, wet climates.
• Biological weathering comes from living things.
  • roots crack rock
  • burrowing animals mix soil
  • lichens, fungi, and microorganisms release chemicals
  It also adds organic matter.

Humus is partly decomposed, stable organic matter. It helps make soil dark and fertile.

Over time, these processes turn parent material into a more developed soil profile with distinct horizons.

Stages of soil profile development

Weathered material can be moved by:

• water
• wind
• ice
• gravity

Transported parent materials you should know:

• alluvium = water-deposited sediment
• loess = wind-deposited silt
• glacial deposits = material left by glaciers
• colluvium = gravity-moved downslope material

What Controls Soil Development

Five factors work together to shape soil:

Time matters, but older soil does not always mean more fertile soil. Long weathering can also leach nutrients away.

Four soil-forming processes

1. Additions

   Material enters, like litter, dust, or sediment.

2. Losses

   Material leaves, like through erosion or leaching.

3. Transformations

   Material changes, like rock weathering or decomposition into humus.

4. Translocations

   Material moves within the soil, often downward with water.

Soils form slowly. Because topsoil can be lost much faster than it forms, APES treats it as nonrenewable or very slowly renewable on a human time scale.

Soil Horizons and the Soil Profile

A soil profile is a vertical section of soil layers, called horizons. The ideal order is:

O → A → E → B → C → R

Not every soil has every horizon. This kind of labeled profile is what that sequence looks like in cross-section.

Soil profile with major horizons

Horizon meanings

• O horizon

  Surface organic layer with litter, dead organisms, decomposing material, and humus

• A horizon

  Topsoil. Minerals mixed with organic matter. High biological activity and major plant-growth zone

• E horizon

  Eluviation zone. Pale, leached layer that loses clay, iron, aluminum, and organics

• B horizon

  Subsoil. Illuviation zone where clay and minerals from above accumulate

• C horizon

  Partly weathered parent material with little organic matter

• R horizon

  Bedrock

When reading a profile, use position plus composition, not just color. A classic sequence is dark O, dark A, pale E, reddish-brown B, broken-rock C, then R.

Soil Erosion and Its Effects

Erosion is the detachment, transport, and deposition of soil by wind or water. It becomes a problem when soil is lost faster than it forms.

Vegetation protects soil because it:

• intercepts rainfall
• slows runoff and wind
• binds soil with roots

Erosion gets worse with:

• intense precipitation
• strong wind
• steep slopes
• bare or disturbed soil
• compaction
• very dry, loose, or saturated soil

Water erosion

• Splash = raindrops knock particles loose
• Sheet = thin layer removed across surface
• Rill = small channels form
• Gully = large channels form

Wind erosion

This is worst in dry, loose, sparsely vegetated soil. The Dust Bowl is the key example. Drought + plowing + removal of grass cover led to massive soil loss.

Consequences include:

• loss of nutrient-rich O and A horizons
• lower fertility and less organic matter
• shallower root zone
• sediment buildup elsewhere
• turbidity, habitat burial, filled reservoirs/channels
• transport of nutrients, pesticides, metals, and other pollutants

Soil Protection and Water Quality

Healthy soil protects water because water moving through soil gets slowed and filtered. Soil:

• traps particles
• adsorbs some dissolved substances
• allows root uptake
• supports microbial breakdown
• reduces runoff into streams

When erosion removes soil, that filtering service gets weaker, and the sediment itself becomes a pollutant.

Erosion-control methods

• vegetation and cover crops
• reduced tillage and leaving crop residue
• contour plowing
• terracing
• windbreaks
• riparian buffers
• avoiding overgrazing and deforestation

Erosion investigation

A common APES experiment compares bare vs. vegetated plots under equal rainfall, slope, soil, and area.

• Independent variable = vegetation cover
• Dependent variable = dry sediment mass, turbidity, runoff volume, or erosion rate
• Constants = rainfall intensity and duration, slope, soil amount, plot area, preparation
• Use replicates

$$\text{erosion rate}=\frac{\text{dry mass of sediment removed}}{\text{plot area}\times\text{time}}$$

Bare plots should usually show more runoff and sediment loss.