Explore the processes involved in soil formation, including weathering of rocks, organic matter decomposition, and the role of living organisms in soil development.
Soil Formation – Key Concepts & Processes
| Aspect | Description |
|---|---|
| Definition | A natural, dynamic mixture of mineral particles, organic matter, water, air, and living organisms that supports plant growth. |
| Functions | • Provides a medium for root anchorage • Stores and supplies water & nutrients • Regulates climate (heat, moisture) • Filters pollutants • Habitat for countless organisms |
| Depth | Typically 0–2 m (active zone); deeper layers (subsoil, parent material) influence properties but are less biologically active. |
| Factor | Symbol | Main Influence on Soil |
|---|---|---|
| Climate | C | Controls temperature & precipitation → weathering rates, organic‑matter decomposition, leaching, and moisture regime. |
| Organisms | L (Live) | Plants, animals, microbes → add organic matter, bioturbation, nutrient cycling, and influence pH. |
| Relief (Topography) | R | Slope, aspect, elevation → drainage, erosion, solar radiation, and micro‑climate variations. |
| Parent Material | P | Source rock or sediment → mineral composition, texture, and chemical supply. |
| Time | T | Duration of exposure to the above factors → development of horizons, accumulation of stable minerals, and pedogenic processes. |
All five interact; the dominant factor may differ from site to site.
| Process | Description | Typical Result |
|---|---|---|
| Physical Weathering | Freeze‑thaw, thermal expansion, abrasion, pressure release. | Breakdown of parent material into sand, silt, clay fractions. |
| Chemical Weathering | Hydrolysis, oxidation, carbonation, solution, hydration. | Formation of secondary minerals (e.g., clays, oxides), leaching of soluble ions. |
| Biological Activity | Root growth, burrowing, litter deposition, microbial metabolism. | Incorporation of organic matter, formation of aggregates, bioturbation. |
| Translocation (Illuviation) | Downward movement of water‑soluble substances (clay, iron, organic compounds). | Development of B‑horizon enrichment (Bt, Bf, Bs). |
| Erosion & Deposition | Removal or addition of material by water, wind, gravity. | Creation of thin soils on slopes, thick soils in depressions. |
| Carbonate Accumulation | Precipitation of calcium carbonate in arid/semiarid climates. | Formation of caliche or carbonate horizons (Ck). |
| Humification | Decomposition of organic residues into humus. | Dark, nutrient‑rich A‑horizon (A‑h). |
O – Organic layer (litter, humus) – most common in forested soils
A – Topsoil; mineral + organic matter; high biological activity
E – Eluviation horizon; leached of silicate clays, iron, or organic matter
B – Subsoil; accumulation (illuviation) of clays, iron, aluminum, carbonates
C – Weathered parent material; little alteration
R – Unweathered bedrock
Not every soil has all horizons; their thickness and characteristics reflect the CLORPT influences.