horizon

/heuh ruy"zeuhn/, n.

1. the line or circle that forms the apparent boundary between earth and sky.

2. Astron.

a. the small circle of the celestial sphere whose plane is tangent to the earth at the position of a given observer, or the plane of such a circle (sensible horizon).

b. the great circle of the celestial sphere whose plane passes through the center of the earth and is parallel to the sensible horizon of a given position, or the plane of such a circle (celestial horizon).

3. the limit or range of perception, knowledge, or the like.

4. Usually, horizons. the scope of a person's interest, education, understanding, etc.: His horizons were narrow.

5. Geol. a thin, distinctive stratum useful for stratigraphic correlation.

6. any of the series of distinctive layers found in a vertical cross section of any well-developed soil.

[1540-50; < L horizon < Gk horízon (kýklos) bounding (circle), equiv. to horíz(ein) to bound, limit + -on prp. suffix (nom. sing.); r. ME orizonte < MF < L horizontem, acc. of horizon]

Syn. 4. world, perspective, domain, viewpoint.

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In pedology, a distinct layer of soil forming part of the vertical sequence in a soil profile.

Each horizon differs from the one above or below it in colour, chemical composition, texture, and structure. The horizons become differentiated during soil development because conditions vary with depth. There are generally three major layers within any given soil profile, and they are designated, from surface downward, as A, B, and C horizons. The A horizon generally contains more organic matter than the others; it is also the most weathered and leached. The B horizon tends to be a zone of accumulation, since all or part of the mineral matter removed from the A horizon in solution may be deposited in it. The C horizon consists chiefly of the materials from which the A and B layers were derived; called parent materials, these are only slightly altered, because they are in general not subjected to soil-forming processes.

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▪ astronomy

      in astronomy, boundary where the sky seems to meet the ground or sea. (In astronomy it is defined as the intersection on the celestial sphere of a plane perpendicular to a plumb line.) The higher the observer, the lower and more distant is his visible horizon. To one 5 feet (1.5 m) above the surface, the horizon is about 2.8 statute miles (4.5 km) away; and for one at 10,000 feet (3,048 m) altitude, it is about 126 miles (203 km). The distance in statute miles equals 1.224 times the square root of the height, in feet, above the surface. On bodies of different radius from that of the Earth, the horizon's distance is also different; e.g., when the eye is 5 feet (1.5 m) above a level lunar plain, the horizon is only 1.4 miles (2.3 km) away.

▪ soil

 a distinct layer of soil, approximately parallel with the land surface, whose properties develop from the combined actions of living organisms and percolating water. Because these actions can vary in their effects with increasing depth, it is often the case that more than one horizon exists beneath the surface of any soil area, at depths ranging from only a few centimetres to several metres. One or more horizons make up what is known as the soil profile, the vertical sequence of distinct layers that is unique to each soil type.

      For general descriptive purposes, soil horizons are commonly given abbreviated designations based on their location in the soil profile and on their composition. From the surface downward, they are named O if they form at the land surface and are almost entirely constituted of litter and humus, A if they form at or near the land surface and show some humus accumulation, E if they lie below the land surface and exhibit a significant loss of clay and humus that gives them a bleached appearance, B if they are subsurface and show accumulations of material from upper layers, C if they are largely unweathered, unconsolidated subsurface mineral layers (frequently the parent material from which the upper layers have formed), and R if they are consolidated bedrock. A mature soil profile will show A, E, B, and C horizons in descending order. Special features of each horizon are delineated by lowercase suffixes, such as h (accumulated humus), k (carbonates), n (sodium), o (iron and aluminum oxides), q (silica), s (mixtures of metal oxides and humus), t (silicate clay), v (iron), y (gypsum), and z (soluble salts). For example, Bh designates a B horizon in which significant humus has accumulated by translocation.

      For the further classification of soils more precise technical concepts are necessary than the simple layer designations given above. One important concept is the epipedon, which is the uppermost horizon used to classify a soil within a designated area. Epipedons are characterized by their colour, texture, structure, and content of organic matter and certain plant nutrients (e.g., calcium, phosphate). Another important concept is that of subsurface diagnostic horizons. These are characterized by the type of accumulated weathering products that they contain (e.g., clay, mixtures of iron oxides and humus, or soluble salts) or by the possible presence of a hard, impermeable layer (e.g., an indurated calcium carbonate or iron-rich layer).

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