4.4 Soil Formation and Erosion

How Soil Forms

Soil formation results from weathering — the physical and chemical breakdown of parent rock — combined with organic matter decay. Factors affecting formation include climate, parent material, topography, organisms, and time. Physical weathering breaks rock into smaller pieces (freeze-thaw, abrasion), while chemical weathering alters mineral composition (oxidation, dissolution). Over long periods, distinct soil horizons (layers) develop, forming a soil profile.

Soil Horizons & Properties

A typical soil profile has layers: O horizon (organic litter), A horizon (topsoil, rich in humus and nutrients), B horizon (subsoil, mineral accumulation), C horizon (weathered parent rock), and bedrock. Soil properties like texture, porosity, permeability, and pH affect fertility and water retention. Loam soils, with balanced sand/silt/clay, are ideal for agriculture.

Soil Erosion & Conservation

Erosion is the removal of topsoil by wind, water, or human activity, often worsened by deforestation, overgrazing, and poor farming practices. Erosion depletes fertile A horizon soil, reduces crop yields, and causes sedimentation in waterways. Conservation methods — contour plowing, terracing, no-till farming, windbreaks, and crop rotation — reduce erosion and preserve soil health.

Key terms

Weathering

Physical or chemical breakdown of rock into smaller particles, contributing to soil formation.

Soil horizon

A distinct layer in a soil profile (O, A, B, C) differing in composition and properties.

A horizon

Topsoil layer rich in organic matter (humus) and nutrients important for plant growth.

Soil erosion

The removal of topsoil by wind, water, or human activity, often worsened by poor land use.

Terracing

A conservation method creating level platforms on slopes to reduce water runoff and erosion.

Try an AP-style question

How does soil form?

1. Plants produce new soil from sunlight
2. Parent rock weathers and organic matter builds up
3. Soil settles down from the atmosphere
4. Rain turns directly into soil over time