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Soil Features: Formation Sequence and Horizons

Soil Formation Sequence:

Soil Horizons:

Soil Formation Sequence Diagram
Next time you pass a road cutting see if you can recognise horizons in the soil and note how colour and structure change with depth and between cuttings.
Any discussion about soil or soils typically refers to that zone of biologically active and altered material that extends down to about one metre below the ground surface.

Soils may form from mineral material such as volcanic ash, weathered rock or alluvium (river or stream deposits), or from  organic material, most often peat formed from swamp or bog vegetation.
With mineral soils, the topsoil is often dark in colour due to an accumulation of decayed material from plant and animal remains known as humus.

Below the topsoil  is the subsoil, which may be various colours such as red, yellow, white, blue, brown or orange.

The upper half of the subsoil often differs in colour from that of the lower half.

Subsoil may also have mottles or 'blotches' of another colour through it.
A soil may have formed from one or more materials, indicated by varying texture (sand, silt and clay content) with depth.

Contrasts in texture are common in soils formed from alluvium where watercourses have deposited layers of sand and silt. 

There are other variations in properties between soils:
   - the ease with which soils break into natural aggregates
   - the structure or size and shape of soil aggregates
   - the consistency or looseness/firmness of soils.

The Soil Description Handbook (Milne et al, 1995) explains the methods of describing soils in much greater detail.
Soil colour, texture, consistency and structure usually vary together with depth, appearing in a vertical face of exposed soil as approximately horizontal layers.

These layers or horizons together constitute a soil profile.

It is this profile from the ground surface to the lower subsoil that is referred to as a soil.