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Humus has a characteristic black or dark brown color and is organic due to an accumulation of organic carbon. Soil scientists use the capital letters O, A, B, C, and E to identify the master soil horizons, and lowercase letters for distinctions of these horizons. Most soils have three major horizons: the surface horizon (A), the subsoil (B ...
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 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.
Highly oxidized red soil in Tirunelveli District, India. Red colors often indicate iron accumulation or oxidation in oxygen-rich, well-aerated soils. [4] Iron concentrations caused by redox reactions because of diffusion of iron in crystalline and metermorphic rock, in periodically saturated soils may also present red colors, particularly along root channels or pores.
They also leave behind organic residues which contribute to humus formation. [3] Plant roots with their symbiotic mycorrhizal fungi are also able to extract nutrients from rocks. [4] New soils increase in depth by a combination of weathering and further deposition. The soil production rate due to weathering is approximately 1/10 mm per year. [5]
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Moder humus form may include only the Ah horizon or ectorganic horizons below end- organic horizons. [2] Fine humus elements have permeated the mineral soil on this horizon. Although soil animals may carry organic matter into the upper region of this horizon, this will only happen over a short distance. [2]