Introduction to Soil Science
Key concepts, terminology, and the role of soil in natural and managed ecosystems.
1. What Is Soil Science?
| Term |
Definition |
| Soil Science |
The interdisciplinary study of soil as a natural resource, covering its formation, classification, physical and chemical properties, biology, and management. |
| Pedology |
The branch that focuses on soil genesis, morphology, and classification. |
| Edaphology |
The branch that examines the influence of soil on plants, animals, and human activities (e.g., agriculture, engineering). |
2. Why Soil Matters
- Food Production – Provides the medium for crop growth and supplies essential nutrients.
- Water Regulation – Stores, filters, and transmits water; mitigates flood and drought impacts.
- Carbon Sequestration – Holds more carbon than the atmosphere and vegetation combined.
- Habitat – Supports a vast diversity of organisms (microbes, insects, earthworms).
- Infrastructure – Influences foundation stability, road building, and landfill design.
3. Soil Formation (Pedogenesis)
3.1 The Five Parent Factors
| Factor |
Influence on Soil Development |
| Climate |
Temperature & precipitation control weathering rates and organic matter decomposition. |
| Organisms |
Plant roots, microbes, and fauna contribute organic matter and bioturbation. |
| Parent Material |
Rock type (igneous, sedimentary, metamorphic) provides mineral base. |
| Topography |
Slope, aspect, and drainage affect erosion, moisture, and deposition. |
| Time |
Longer periods allow more extensive weathering and horizon development. |
3.2 Soil Development Stages
- Weathering of Parent Material – Physical (freeze‑thaw, abrasion) and chemical (hydrolysis, oxidation).
- Organic Matter Accumulation – Litter → humus; enriches C, N, and nutrients.
- Horizon Formation – Distinct layers (O, A, E, B, C, R) develop with characteristic colors, textures, and structures.
4. Soil Horizons (Typical Profile)
| Horizon |
Symbol |
Main Features |
Typical Processes |
| O |
O |
Organic layer (leaf litter, decomposing material) |
Decomposition, humus formation |
| A |
A |
Topsoil; mix of mineral particles & organic matter |
Root growth, nutrient cycling |
| E |
E |
Leached (eluviated) layer; lighter color |
Removal of silicate clays, iron, aluminum |
| B |
B |
Subsoil; accumulation (illuviation) of clays, iron, carbonates |
Clay translocation, carbonate precipitation |
| C |
C |
Partially weathered parent material |
Minimal biological alteration |
| R |
R |
Unweathered bedrock |
Basal material |
Not all soils contain every horizon; the sequence varies with environment.
5. Fundamental Soil Properties
| Property |
What It Describes |
Measurement / Unit |
| Texture |
Proportion of sand, silt, clay particles |
% by weight (feel test, laser diffraction) |
| Structure |
Arrangement of peds (granular, blocky, platy) |
Visual classification, aggregate stability tests |
| Bulk Density |
Mass of dry soil per unit volume |
g cm⁻³ (core method) |