Soil Horizon

A soil horizon is a layer parallel to the soil surface, whose physical characteristics differ from the layers above and beneath. Each soil type has at least one, usually three or four horizons. Horizons are defined in most cases by obvious physical features, chiefly colour and texture. These may be described both in absolute terms (particle size distribution for texture, for instance) and in terms relative to the surrounding material, i.e. ‘coarser’ or ‘sandier’ than the horizons above and below. water dissolves and removes nutrients as it passes through the soil.

Identification and description of the horizons present at a given site is the first step in soil classification at higher levels, through the use of systems such as the USDA soil taxonomy or the Australian Soil Classification. The World Reference Base for Soil Resources lists 40 diagnostic horizons.

Many soils, especially temperate soils, conform to a similar general pattern of horizons, often represented as an ‘ideal’ soil in diagrams. Each main horizon is denoted by a capital letter, which may then be followed by several alphanumerical modifiers highlighting particular outstanding features of the horizon. While the general O-A-B-C-R sequence seems fairly universal, some variation exists between the classification systems in different parts of the world. In addition, the exact definition of each main horizon may differ slightly – for instance, the US system uses the thickness of a horizon as a distinguishing feature, while the Australian system does not. It should be emphasised that no one system is more correct – as artificial constructs, their utility lies in their ability to accurately describe local conditions in a consistent manner. Also, many subtropical and tropical areas have soils such as oxisols or aridisols that have very different horizons from an "ideal" soil, or no horizons at all.

Read more about Soil Horizon:  Main Horizons, Layers, Sub-horizon Suffixes and Labels, Buried Soils

Famous quotes containing the words soil and/or horizon:

    Let the soil squeal I am the biting man
    And the velvet dead inch out.
    Dylan Thomas (1914–1953)

    “Dark times” is what they call it in Norway when the sun remains below the horizon all day long: the temperature falls slowly but surely at such times.—A nice metaphor for all those thinkers for whom the sun of mankind’s future has temporarily disappeared.
    Friedrich Nietzsche (1844–1900)