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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.
Soil formation, also known as pedogenesis, is the process of soil genesis as regulated by the effects of place, environment, and history. Biogeochemical processes act to both create and destroy order within soils.
Soil profiles contain multiple layers, known as horizons, that are generally parallel to the soil surface. These horizons are distinguishable from adjacent layers by their changes in morphological properties as the soil naturally forms. The same soil horizons may be named and labeled differently in various soil classification systems around the ...
In soil classification systems, topsoil is known as the O Horizon or A Horizon. [8] [9] Soil horizons are layers parallel to the soil surface whose physical, chemical and biological characteristics differ from the layers above and beneath.
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.
This is a somewhat arbitrary definition as mixtures of sand, silt, clay and humus will support biological and agricultural activity before that time. [47] These constituents are moved from one level to another by water and animal activity. As a result, layers (horizons) form in the soil profile.
Due to the presence of soil organisms and high biological activity, the disappearance of plant litter is fast, and there are no distinct layers because thick organic horizons do not accumulate. [1] Conversely, the Ah horizon is well developed, resulting from the assemblage of organic matter with mineral particles.
Soil scientists use the capital letters O, A, B, C, and E to identify the master horizons, and lowercase letters for distinctions of these horizons. 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.