Geography · Ch 11 — Geomorphic Processes
Soil-forming Factors
Soil-forming Factors
Soil forms because five factors work together, and no single one acts alone. These factors are parent material, topography, climate, biological activity, and time. They operate in union, each affecting the action of the others. So when you study soil formation, think of these five as interacting forces, not isolated causes.
Parent Material
Parent material is a passive control factor — it does not actively drive soil formation but sets the raw ingredients. It can be either:
- In-situ or on-site weathered rock debris, which gives rise to residual soils
- Transported deposits, which give rise to transported soils
What matters about the parent material is its texture (the sizes of the debris particles), its structure (how the individual grains or particles are arranged), and its mineral and chemical composition. Along with these, the nature and rate of weathering, and the depth of the weathering mantle, are important considerations.
A key point: soils over similar bedrock can differ, and dissimilar bedrocks can produce similar soils above them. This happens because other factors, especially climate and biological activity, often override the influence of the parent rock. However, there are two situations where the link with parent rock is strong:
- When soils are very young and have not matured, they show a clear connection to the type of parent rock
- In some limestone areas, where weathering processes are specific and peculiar, the soil shows a clear relation with the parent rock
Topography
Topography is the second passive control factor. Its influence works through two things:
- The amount of exposure of the surface covered by parent materials to sunlight
- The amount of surface and sub-surface drainage over and through the parent materials
The effects are direct and observable. Soils are thin on steep slopes because erosion removes material faster than it can form, and thick over flat upland areas where material accumulates. On gentle slopes, where erosion is slow and the percolation of water is good, soil formation proceeds well.
The textbook text here trails off mid-sentence, mentioning that soil formation on gentle slopes is discussed further in the next chapter on landforms and their evolution. The core idea stands: slope steepness controls soil thickness through the balance of erosion and water movement.