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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.
As a result of this, the E horizon (or Ae horizon in the Canadian system of soil classification) is left bleached and ash-grey in colour, while the B horizon becomes enriched with relocated organo-mineral complexes. The colour of B horizon is consequently red, brown or black, depending on the dominance of metal ions or organic matter.
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.
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.
Parent material is the underlying geological material (generally bedrock or a superficial or drift deposit) in which soil horizons form. Soils typically inherit a great deal of structure and minerals from their parent material, and, as such, are often classified based upon their contents of consolidated or unconsolidated mineral material that has undergone some degree of physical or chemical ...
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]
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]
Influenced primarily by soil forming processes over time, it commonly forms below an albic horizon. It has less clay content than an argillic horizon but is still a visible layer. [3] In the pedogenesis process under a chronosequence, once the soil develops into an alfisol, the Cambic horizon will develop into an argillic (Bt) horizon.