How soils are formed

Soil is formed by the weathering (breakdown) of parent rock by physical, chemical and biological agents. The rock breaks into smaller particles that are either left in place or carried away and deposited elsewhere.

1. Physical (mechanical) weathering

Rock is broken into smaller pieces without change in its mineral composition. Causes:

• Temperature changes (expansion and contraction, daily heating and cooling).
• Freezing of water in cracks (frost wedging).
• Abrasion by wind, running water, glaciers and waves.
• Plant roots growing in cracks, and unloading (release of pressure).

This produces coarse-grained soils such as gravel, sand and silt.

2. Chemical weathering

Minerals of the rock are altered into new minerals by chemical reactions: oxidation, hydration, carbonation, hydrolysis and leaching. Feldspar, for example, changes to clay minerals (kaolinite, illite, montmorillonite). This produces fine-grained clayey soils.

3. Biological action

Lichens, bacteria, burrowing animals and roots add organic acids and break rock, adding organic matter to the soil.

Fate of the weathered material

• Residual soils stay at the place of formation over the parent rock.
• Transported soils are carried by water (alluvial), wind (aeolian), ice (glacial) or gravity (colluvial) and deposited elsewhere.

Parent rock -> Weathering -> Soil particles
                 |-> stays in place: residual soil
                 |-> moved by agents: transported soil