Soil Formation
Soil formation (pedogenesis) is the process by which parent rock material is transformed into soil through the combined effects of weathering, biological activity, organic matter accumulation, and the movement of water and dissolved substances through the soil profile. The CLORPT model identifies five key soil-forming factors: climate, organisms, relief (topography), parent material, and time. The result is a layered soil profile with distinct horizons—O, A, B, C, and R—each reflecting the degree of weathering, organic content, and mineral alteration at different depths.
Soil Profile Horizons and Their Characteristics
| Horizon | Name | Composition | Color | Key Feature |
|---|---|---|---|---|
| O | Organic horizon | Decomposing plant/animal matter | Dark brown/black | Humus-rich; leaf litter |
| A | Topsoil | Mineral + organic mix | Dark brown | High biological activity; leaching zone |
| B | Subsoil | Accumulated minerals (Fe, Al, clay) | Red/orange/yellow | Illuviation zone; less organic matter |
| C | Parent material | Partially weathered rock | Gray/tan | Minimal biological activity |
| R | Bedrock | Unweathered solid rock | Variable | Foundation material; no soil processes |
Interactive Tools
USDA Web Soil Survey
Interactive map and data portal for soil types, properties, and land capability across the USA.
Open ToolKhan Academy — Soil Formation
Video lesson on CLORPT factors, soil horizons, and the timescales of pedogenesis.
Open ToolBrilliant.org — Earth Science
Problem-based lessons on soil science and geological processes with interactive exercises.
Open Tool
Wikimedia Commons, CC BY-SA
Related Terms
Weathering (Geology)
Weathering is the in-situ breakdown of rocks and minerals at or near Earth's surface through physical disintegration, chemical decomposition, and biological activity, without significant transport of the resulting material. Physical (mechanical) weathering fragments rock into smaller pieces without changing its mineral composition, while chemical weathering alters mineral chemistry through reactions with water, oxygen, and acids. Biological weathering involves living organisms such as plant roots, lichens, and bacteria accelerating both physical and chemical breakdown processes.
Erosion
Erosion is the geological process by which rock, soil, and sediment are loosened and transported away from their original location by agents such as water, wind, ice, and gravity. It is a key component of the rock cycle that continuously reshapes Earth's surface by removing material from one location and depositing it elsewhere as sediment. Erosion rates are significantly influenced by climate, vegetation cover, rock type, and slope gradient, and accelerated erosion caused by deforestation or poor land management poses major environmental challenges.
Mineral
A mineral is a naturally occurring, inorganic, crystalline solid with a definite chemical composition and characteristic physical properties. Minerals are the building blocks of rocks and are classified by their crystal structure, hardness, luster, cleavage, and color. They are essential to industry, construction, and biological processes, with over 5,000 known mineral species recognized by the International Mineralogical Association.
The term "pedogenesis" comes from Greek "pedon" (ground, soil) and "genesis" (origin, creation). The modern scientific study of soil formation was pioneered by Russian geologist Vasily Dokuchaev in the 1880s, who first recognized that soil is a natural body shaped by environmental factors rather than mere weathered rock.