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The A horizon is the top layer of the mineral soil horizons, often referred to as 'topsoil'. This layer contains dark decomposed organic matter, which is called "humus". The technical definition of an A horizon may vary between the systems, but it is most commonly described in terms relative to deeper layers.
Most soils have three major horizons: the surface horizon (A), the subsoil (B), and the substratum (C). Some soils have an organic horizon (O) on the surface, but this horizon can also be buried. [44] The master horizon (E) is used for subsurface horizons that have significantly lost minerals . Bedrock, which is not soil, uses the letter R.
The umbric horizon (Latin: umbra, shade) 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 ...
The term humus form is not the same as the term humus.Forest humus form describes the various arrangement of organic and mineral horizons at the top of soil profiles. [1] It can be composed entirely of organic horizons, meaning an absence of the mineral horizon. [2]
The mineral material from which a soil forms is called parent material. Rock, whether its origin is igneous, sedimentary, or metamorphic, is the source of all soil mineral materials and the origin of all plant nutrients with the exceptions of nitrogen, hydrogen and carbon.
Fossil soil horizons from paleosols can be found within sedimentary rock sequences, allowing the study of past environments. [193] The exposure of parent material to favourable conditions produces mineral soils that are marginally suitable for plant growth, as is the case in eroded soils. [194]
Most soils have a dry bulk density between 1.0 and 1.6 g/cm 3 but organic soil and some porous clays may have a dry bulk density well below 1 g/cm 3. Core samples are taken by pushing a metallic cutting edge into the soil at the desired depth or soil horizon. The soil samples are then oven dried (often at 105 °C) until constant weight.
Most soils have three major horizons—the surface horizon (A), the subsoil (B), and the substratum (C). Some soils have an organic horizon (O) on the surface, but this horizon can also be buried. The master horizon, E, is used for subsurface horizons that have a significant loss of minerals (eluviation).