Soil Formation

From bedrock to topsoil — the five factors of soil development

Soil formation (pedogenesis) transforms bedrock and sediment into layered soil profiles through physical, chemical, and biological weathering. The soil profile consists of distinct horizons: O (organic litter), A (topsoil, rich in humus), B (subsoil, accumulated minerals from leaching), C (weathered parent material), and R (bedrock). Five factors control soil development: (1) Parent material — the original rock or sediment determines mineral composition. (2) Climate — temperature and precipitation drive chemical weathering rates; tropical soils are deeply weathered while desert soils are thin. (3) Organisms — roots break rock, earthworms mix organic matter, bacteria decompose material. (4) Topography — steep slopes lose soil to erosion while valleys accumulate it. (5) Time — mature soils take thousands of years to develop distinct horizons. Soil is a non-renewable resource on human timescales — it takes ~500 years to form 2 cm of topsoil.