The natural process by which weathered rock (regolith) combines with organic matter, water, air, and living organisms to form soil, a layered body that supports plant life.
- The classic five soil-forming factors are parent material, climate, relief (topography), living organisms (biota), and time.
- A vertical section through soil is the soil profile, made of horizons: the O and A horizons (topsoil, rich in humus), the B horizon (subsoil, where leached material accumulates), the C horizon (weathered parent rock), and the R horizon (bedrock).
- Humus, the decayed organic matter in the topsoil, gives fertility and dark colour; leaching is the downward washing of soluble nutrients in heavy rainfall.
- Climate strongly controls type: laterite soil forms by intense leaching in hot wet regions; chernozem (black-earth) soils form in temperate grasslands.
- The Indian Council of Agricultural Research classifies Indian soils into eight major groups, including alluvial, black, red, and laterite.
The five factors of soil formation, the horizon sequence (A, B, C), the role of humus and leaching, and the climate control link directly to the Indian soils syllabus.
Weathering (breaks rock) is one input; soil formation also needs organic matter and organisms; A horizon (topsoil with humus) versus B horizon (subsoil where leached material gathers); leaching versus erosion.
Soil forms from weathered rock plus humus, water, air, and organisms over time; its profile runs A (topsoil), B (subsoil), C (parent rock).