soil horizon

A soil horizon is a specific layer in the soil which parallels the land 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 periods. Identification and description of the horizons present at a given site is the first step in classifying a soil at higher levels, through the use of systems such as the USDA soil taxonomy or the Australian Soil Classification.
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Noun
1. a layer in a soil profile
(hypernym) horizon
(hyponym) A-horizon, A horizon
(part-holonym) profile

A layer of soil that can be distinguished from the surrounding soil by such features as chemical composition, color, and texture.