Soil Formation and Soil Horizons

Soil is made, slowly

Soil forms where parent rock is broken down and mixed with organic matter. Five factors control the result — climate, organisms, topography, parent material and time. Physical weathering (freeze-thaw, abrasion) breaks rock into smaller pieces without changing its chemistry; chemical weathering (acid rain, hydrolysis, oxidation) changes the minerals themselves. Warm and wet climates weather rock fastest, which is why tropical soils are deep but often nutrient-poor: the same rain that makes soil also leaches nutrients out of it.

Reading a soil profile

• O horizon — surface litter, leaves and partly decomposed organic matter.
• A horizon — topsoil, humus mixed with mineral particles. Most roots and soil organisms live here; this is the layer that erosion removes first.
• E horizon — the zone of eluviation, pale because clay, iron and aluminium have been washed out of it.
• B horizon — subsoil, the zone of illuviation where material leached from above accumulates.
• C horizon — weathered fragments of the parent material.
• R — unweathered bedrock.

Soil forms at roughly $1$ cm per $200$ to $1000$ years, so on a human timescale topsoil is a non-renewable resource.