Umbric horizon
Encyclopedia
The umbric horizon is a thick, dark coloured, surface Soil horizon
rich in organic matter. It is identified by its dark colour and structure.
Normally it has a pH of less than 5.5 representing a base saturation of less than 50 percent. An indication for Soil acidity
is a rooting pattern whereby the roots tend to be horizontal.
Similar thick, dark coloured, organic-rich, base-desaturated surface horizons occur in anthropedogenic horizons from human activities such as deep cultivation, kitchen middens and the addition of organic manures etc.
These horizons can usually be differentiated by the presence of artifacts, spade marks or by checking the agricultural history of the area
The World Reference Base for Soil Resources
diagnostic criteria states that an umbric horizon must have:
Soil horizon
A soil horizon is a specific layer in the land area that is parallel to the soil surface and possesses physical characteristics which differ from the layers above and beneath. Horizon formation is a function of a range of geological, chemical, and biological processes and occurs over long time...
rich in organic matter. It is identified by its dark colour and structure.
Normally it has a pH of less than 5.5 representing a base saturation of less than 50 percent. An indication for Soil acidity
Soil acidification
Soil acidification is the buildup of hydrogen cations, also called protons, reducing the soil pH. This happens when a proton donor is added to the soil. The donor can be an acid, such as nitric acid and sulfuric acid . It can also be a compound such as aluminium sulfate, which reacts in the soil to...
is a rooting pattern whereby the roots tend to be horizontal.
Similar thick, dark coloured, organic-rich, base-desaturated surface horizons occur in anthropedogenic horizons from human activities such as deep cultivation, kitchen middens and the addition of organic manures etc.
These horizons can usually be differentiated by the presence of artifacts, spade marks or by checking the agricultural history of the area
The World Reference Base for Soil Resources
World Reference Base for Soil Resources
The World Reference Base for Soil Resources is the international standard taxonomic soil classification system endorsed by the International Union of Soil Sciences . It was developed by an international collaboration coordinated by the International Soil Reference and Information Centre and...
diagnostic criteria states that an umbric horizon must have:
- Soil structure sufficiently strong that the horizon is not both massiveCrystal habitCrystal habit is an overall description of the visible external shape of a mineral. This description can apply to an individual crystal or an assembly of crystals or aggregates....
and hard or very hard when dry.
- Munsell colours with a chroma value darker than 3.5 when moist and 5.5 when dry, both on broken and crushed samples.
- It is described as base-desaturatedCation exchange capacityIn soil science, cation-exchange capacity is the maximum quantity of total cations, of any class, that a soil is capable of holding, at a given pH value, for exchanging with the soil solution. CEC is used as a measure of fertility, nutrient retention capacity, and the capacity to protect...
in that it has a base saturation of less than 50 percent on a weighted average throughout the depth of the horizon; and
- Organic carbon content of 0.6 percent or more throughout the thickness of mixed horizon
- A thickness of 100 mm if resting directly on hard rock; at least 200 mm/ more than one-third of the solumSolumThe solum in soil science consists of the surface and subsoil layers that have undergone the same soil forming conditions. The base of the solum is the relatively unweathered parent material, termed substratum....
where the solum is less than 750 mm thick; or more than 250 mm where the solum is more than 750 mm thick.