O horizon Refer to soil horizon and Appendix II.

Oa horizon (H layer) A layer occurring in mor humus consisting of well-decomposed organic matter of unrecognizable origin (sapric material). Compare soil horizon and Appendix II.

oc The distribution coefficient, K_d, calculated on thebasis of organic carbon content. K_oc = K_d/f_oc where f_oc is the fraction of organic carbon.

Occam's Razor A name for the philosophical principle of parsimony: the simplest explanation of natural phenomena (or the use of the minimum number of assumptions) until new information requires otherwise, is most likely the correct one ( Jackson, 1997). Also spelled Ockham’s.

Occam’s Razor A name for the philosophical principle of parsimony: the simplest explanation of natural phenomena (or the use of the minimum number of assumptions) until new information requires otherwise, is most likely the correct one ( Jackson, 1997). Also spelled Ockham’s.

ocean The continuous salt-water body that surrounds the continents and fills the Earth’s great depressions; also, one of its major geographic divisions ( Jackson, 1997). Compare sea.

Ochrepts [soil taxonomy] (obsolete; not used in current soil taxonomy) Prior to 1999, this was a suborder of Inceptisols formed in cold or temperate climates and that commonly have an ochric epipedon and a cambic horizon. They may have an umbric or mollic epipedon<25-cm thick or a fragipan or duripan under certain conditions. These soils are not dominated by amorphous materials and are not saturated with water for periods long enough to limit their use for most crops (USDA, 1999).

ochric epipedon [soil taxonomy] A surface horizon of mineral soil that is too light in color, too high in chroma, too low in organic carbon, or too thin to be a plaggen, mollic, umbric, anthropic, or histic epipedon, or that is both hard and massive when dry (USDA, 1999).

octahedral coordination Term describing a cation surrounded by six equidistant anions.

Oe horizon (F layer) A layer of partially decomposed litter with portions of plant structures still recognizable (hemic material). Occurs below the L layer on the forest floor in forest soils. It is the fermentation layer. Compare soil horizon and Appendix II.

offshore bar (not recommended) use barrier beach.

Ohm's law The law describing the movement of electricity as caused by a gradient in electrical potential.

Oi horizon (L layer [litter]) A layer of organic material having undergone little or no decomposition (fibric material). On the forest floor, this layer consists of freshly fallen leaves, needles, twigs, stems, bark, and fruits. This layer may be very thin or absent during the growing season. Compare soil horizon and Appendix II.

oil wasteland [soil survey] A miscellaneous area (map unit) applied to barren land where oily wastes, principally saltwater and oil, have accumulated; includes slush pits and adjacent areas affected by liquid wastes (USDA, 1993).

Oligocene An epoch of the Tertiary Period of geologic time (from 23.0 to 33.9 million years ago), which follows the Eocene Epoch and precedes the Miocene Epoch; also the corresponding (time-stratigraphic) “series” of earthy materials (Schoeneberger and Wysocki, personal communication, 2013).

oligotrophic Environments in which the concentration of nutrients available for growth is limited. Nutrient poor habitats.

oligotrophs Organisms able to grow in environments with low nutrient concentrations.

one-third-atmosphere percentage (no longer used in SSSA publications) The percentage of water contained in a soil that has been saturated, subjected to, and is in equilibrium with an applied pressure of one-third atmosphere. Approximately the same as one-third-bar percentage. Refer to soil water, soil water potential.

one-third-bar percentage (no longer used in SSSA publications) The percentage of water contained in a soil that has been saturated, subjected to, and is in equilibrium with, an applied pressure of one-third bar. Approximately the same as one-third-atmosphere percentage. Refer to soil water, soil water potential.

open depression A generic name for any enclosed or low area that has a surface drainage outlet whereby surface water can leave the enclosure; an area of lower ground indicated on a topographic map by contour lines forming an incomplete loop or basin indicating at least one surface exit (Schoeneberger and Wysocki, personal communication, 2013). Compare closed basin.

openpit mine A relatively large depression resulting from the excavation of material and redistribution of overburden associated with surficial mining operations (Schoeneberger and Wysocki, personal communication, 2013; Jackson, 1997). Compare quarry, surface mine.

organan A cutan composed of a concentration of organic matter.

organic farming Crop production system that reduces, avoids, or largely excludes the use of synthetically compound fertilizers, pesticides, growth regulators, and livestock feed additives.

organic fertilizer Byproduct from the processing of animals or vegetable substances that contain sufficient plant nutrients to be of value as fertilizers.

organic soil A soil in which the sum of the thicknesses of layers containing organic soil materials is generally greater than the sum of the thicknesses of mineral layers.

organotroph Refer to heterotroph.

Orthels [soil taxonomy] A suborder of Gellisols that show little or no evidence of cryoturbation. These soils occur primarily within the zone of widespread permafrost or in areas of coarse textured materials in the continuous zone of permafrost. Orthels are generally drier than Turbels and Histels (USDA, 1999; Appendix 1).

Orthents [soil taxonomy] A suborder of Entisols found primarily on recent erosional surfaces. Any former soil that was on the landscape has been completely removed or so truncated that the diagnostic horizons for all other orders do not occur. A few Orthents are formed in recent eolian deposits, in areas of solifluction or glacial deposits, or in areas of debris from recent landslides and mudflows. Orthents are not saturated with water for periods long enough to limit their use for most crops (USDA, 1999; Appendix 1).

Orthids [soil taxonomy] (obsolete; not used in current soil taxonomy) Prior to 1994, this term was used to indicate Aridisols that have a cambic, calcic, petrocalcic, gypsic, or salic horizon or a duripan but that lack an argillic or natric horizon. (USDA, 1994; Keys to Soil Ttaxonomy, 6th ed).

orthoclase feldspar A potassium anhydrous alumnosilicate with a framework silicate structure; KAlSi₃O₈.

Orthods [soil taxonomy] A suborder of Spodosols that are relatively freely drained and have a horizon of accumulation containing aluminum, or aluminum and iron, and organic carbon. Undisturbed Orthods normally have an O horizon, an albic horizon, and a spodic horizon and may have a fragipan. Orthods that have been mixed by the roots of falling trees or by animals have a very thin albic horizon or no albic horizon. Cultivated Ortods commonly have the albic horizon mixed with part of the spodic horizon. The soil temperature regimes range from frigid to hyperthermic. In the United States, the soil moisture regime of Orthods is predominantly udic, but a few are xeric. Orthods are not saturated with water for periods long enough to limit their use for most crops (USDA, 1999; Appendix 1).

orthophosphate A salt of orthophosphoric acid such as (NH₄)₂HPO₄, CaHPO₄, or K₂HPO₄.

Orthox [soil taxonomy] (obsolete; not used in current soil taxonomy). Prior to 1987 this was a suborder of Oxisols that are moist all or most of the time, and that have a low to moderate content of organic carbon within the upper 1 m or a mean annual soil temperature of 22°C or more (USDA, 1975).

ortstein A cemented spodic horizon; [soil taxonomy] a diagnostic, subsurface layer of the same (USDA, 1999).

osmotic potential, pressure The potential energy acting upon soil water due to the effect of solutes. Solution in contact with pure water will draw water from the reservoir of pure water due to the decrease in potential energy, that is, osmotic poential, of the solution relative to the pure water. Osmotic potential equals the product of the universal gas constant, R, the temperature, T, and the total molar concentration of solutes, C(=RTC).

outcrop (a) That part of a geologic formation or structure that appears at the surface of the earth. (b) [soil survey] An actual exposure of bedrock at or above the ground surface (Schoeneberger and Wysocki, personal communication, 2013; Jackson, 1997). Compare cliff.

outer sphere adsorption Adsorption of ions that occurs with the retention of waters of hydration between the surface and the adsorbed ion where the force that retains the ion is only electrostatic attraction. Ions that are retained by outer sphere adsorption are readily exchangeable. Compare exchangeable cation and exchangeable anion.

outlet A location that maintains a hydraulic head lower than that of the drainage devices in a soil.

outwash Stratified and sorted detritus (chiefly sand and gravel) removed or “washed out” from a glacier by melt-water streams and deposited in front of or beyond the end moraine or the margin of an active glacier. The coarser material is deposited nearer to the ice (Schoeneberger and Wysocki, personal communication, 2013). Compare pitted outwash, drift, esker, kame, till.

outwash plain An extensive lowland area of coarse textured, glaciofluvial material. An outwash plain is commonly smooth; where pitted, due to melt-out of incorporated ice masses (pitted outwash plain), it is generally low in relief and largely retains it’s original gradient (Schoeneberger and Wysocki, personal communication, 2013; Hawley and Parsons, 1980). Compare outwash, pitted outwash plain, collapsed outwash plain, kettles; also called sandur.

outwash delta A relict (inactive) delta composed of glaciofluvial sediments formed where a sediment laden outwash river emptied into an open lake, commonly a proglacial lake. Sediment attributes include very gently dipping topset beds (coarser textures) and steeply dipping foreset beds (finer textures; Schoeneberger and Wysocki, personal communication, 2013; Goldthwaite and Matsch, 1988).

outwash fan A fan-shaped accumulation of outwash deposited by meltwater streams in front of the end or recessional moraine of a glacier. Coalescing outwash fans form an outwash plain ( Jackson, 1997).

outwash terrace A flat-topped bank of outwash with an abrupt outer face (scarp or riser) extending along a valley downstream from an outwash plain or terminal moraine; a valley train deposit (Schoeneberger and Wysocki, personal communication, 2013). Compare kame terrace, valley train.

oven-dry soil Soil that has been dried at 105°C until it reaches constant mass.

overbank deposit Fine-grained sediments (silt and clay) deposited from suspension on a flood plain by floodwaters that cannot be contained within the stream channel ( Jackson, 1997).

overburden (a) Recently transported and deposited material that occurs immediately superjacent to the surface horizon of a contemporaneous soil. (b) The upper part of a sedimentary deposit, compressing and consolidating the materials below. (c) A term used to designate disturbed or undisturbed material of any nature, consolidated or unconsolidated, that overlies a deposit of useful materials, ores, lignites, or coals, especially those deposits mined from the surface by open cuts.

overconsolidated soil deposit A soil deposit that has been subjected to an effective pressure greater than the present overburden pressure.

overflow stream channel A watercourse that is generally dry but conducts flood waters that have overflowed the banks of a river, commonly from large storms, annual meltwater, or glacial meltwaters (Schoeneberger and Wysocki, personal communication, 2013).

overlay (a) A transparent sheet giving information to supplement that shown on maps. When the overlay is laid over the map on which it is based, its details will supplement the map. (b) A tracing of selected details on a photograph, mosaic, or map to present the interpreted features and the pertinent detail, or to facilitate plotting (Schoeneberger and Wysocki, personal communication, 2013).

overprinted soil A soil in which new soil morphology has developed and is superimposed upon that of a pre-existing soil due to a shift in pedogenic conditions such as a change in climate or hydrology; the composite morphology retains some relict features that would not form under present day conditions. Sometimes called welded soil (Schoeneberger and Wysocki, personal communication, 2013).

overprinting The process of superimposing a new set of features over a preexisting set due to a shift in environmental conditions such as a change in climate or local hydrology. The resulting composite morphology retains features that would not form under present conditions (Schoeneberger and Wyscoki, 2013). Compare overprinted soil.

overthrust A low angle thrust fault of large scale, with displacement generally measured in kilometers ( Jackson, 1997).

overthrust belt (not preferred; use fold-thrust hills).

ow The octanol-water partition coefficient. The ratio of the concentration of an organic compound in octanol and in water after equilibration of the two phases. Can be used to estimate the value of K_oc for some organic compounds.

oxbow A closely looping stream meander having an extreme curvature such that only a neck of land is left between the two parts of the stream.; (colloquial: northeastern USA) the land enclosed, or partly enclosed, within an oxbow ( Jackson, 1997). Compare meander belt, oxbow lake, bayou.

oxbow lake The crescent-shaped, often ephemeral body of standing water situated by the side of a stream in the abandoned channel (oxbow) of a meander after the stream formed a neck cutoff and the ends of the original bend were silted up ( Jackson, 1997).

oxic horizon [soil taxonomy] A diagnostic, subsurface mineral soil horizon that is at least 30 cm thick and characterized by the virtual absence of weatherable primary minerals or 2:1 layer silicate clays, the presence of 1:1 layer silicate clays and highly insoluble minerals such as quartz sand, the presence of hydrated oxides of iron and aluminum, the absence of water-dispersible clay, and the presence of low cation exchange capacity and small amounts of exchangeable bases (USDA, 1999).

oxidation The loss of one or more electrons by an ion or molecule.

oxidation ditch An artificial open channel for partial digestion of liquid organic wastes in which the wastes are circulated and aerated by a mechanical device.

oxidation state The number of electrons to be added (or subtracted) from an atom in a combined state to convert it to the elemental form.

oxidation-reduction potential Refer to EH and pe.

oxidative phosphorylation Conversion of inorganic phosphate into the energy-rich phosphate of adenosine 5’-triphosphate.

Oxisols [soil taxonomy] An order of low fertility mineral soils that have an oxic and no kandic horizon within 150 cm of the surface and that do not have a spodic or argillic horizon above the oxic horizon. Oxisols characteristically have very low cation-exchange capacity and very few weatherable minerals. Their profiles are distinctive because of a lack of obvious horizons. Their surface horizons are normally somewhat darker than the subsoil, but the transition of subsoil features is gradual. Oxisols occur in every soil moisture regime from aridic to perudic and aquic (USDA, 1999; Appendix 1).

oxyaquic conditions Pertaining to soils that are saturated but are not reduced and do not contain redoximorphic features.

oxytropic The response of a biological organism to the presence of oxygen.