udic: A soil moisture regime that is neither dry for as long as 90 cumulative days nor for as long as 60 consecutive days in the 90 days following the summer solstice at periods when the soil temperature at 50 cm below the surface is above 5°C.
Ultisols: Mineral soils that have an argillic horizon with a base saturation of <35% when measured at pH 8.2. Ultisols have a mean annual soil temperature of 8°C or higher.
umbric epipedon: A surface layer of mineral soil that has the same requirements as the mollic epipedon with respect to color, thickness, organic carbon content, consistence, structure, and phosphorus content, but that has a base saturation <50% when measured at pH 7.
unaccommodated: Applied to peds. Virtually none of the faces of adjoining peds are molds of each other.
unconformity: A substantial break or gap in the geologic record where a unit is overlain by another that is not in stratigraphic succession.
underfit stream: A stream that appears to be too small to have eroded the valley in which it flows; a stream whose volume is greatly reduced or whose meanders show a pronounced shrinkage in radius. It is a common result of drainage changes effected by capture, glaciers, or climatic variations.
underground runoff (seepage): Water that seeps toward stream channels after infiltration into the ground.
undifferentiated group: A kind of map unit used in soil surveys comprised of two or more taxa components that are not consistently associated geographically. Delineations show the size, shape, and location of a landscape unit composed of one or the others, or all of two or more component soils that have the same or very similar use and management for specified common uses. Inclusions may occur up to some allowable limit.
unsaturated flow: The movement of water in soil in which the pores are not filled to capacity with water.
urban land: Areas so altered or obstructed by urban works or structures that identification of soils is not feasible.
ustic: A soil moisture regime that is intermediate between the aridic and udic regimes and common in temperate subhumid or semiarid regions, or in tropical and subtropical regions with a monsoon climate. A limited amount of water is available for plants but occurs at times when the soil temperature is optimum for plant growth.
vapor flow: The gaseous flow of water vapor in soils from a moist or warm zone of higher potential to a drier or colder zone of lower potential.
variable charge: A solid surface carrying a net electrical charge which may be positive, negative, or zero, depending on the activity of one or more species of a potential-determining ions in the solution phase contacting the surface. For minerals and other materials common in soils(e.g. soil organic matter, and oxides), the potential-determining ion usually is H+ or OH-, but any ion that forms a complex with the surface may be potential-determining.
varve: A sedimentary layer, lamina, or sequence of laminae, deposited in a body of still water within 1 year; specifically, a thin pair of graded glaciolacustrine layers seasonally deposited, usually by meltwater streams, in a glacial lake or other body of still water in front of a glacier.
ventifact: A stone or pebble that has been shaped, worn, faceted, or polished by the abrasive action of windblown sand, usually under arid conditions. When the pebble is at the ground surface, as in a desert pavement, the upper part is polished while the lower or below ground part is angular or subangular.
Vertisols: Mineral soils that have 30% or more clay, deep wide cracks when dry, and either gilgai microrelief, intersecting slickensides, or wedge-shaped structural aggregates tilted at an angle from the horizon.
vesicles: (i) Unconnected voids with smooth walls. (ii) Spherical structures, formed intracellularly, by vesicular arbuscular endomycorrhizal fungi.
void ratio: The ratio of the volume of soil pore (or void) space to the solid-particle volume.
volcaniclastic: Pertaining to the entire spectrum of fragmental materials with a preponderance of clasts of volcanic origin. The term includes not only pyroclastic materials but also epiclastic deposits derived from volcanic source areas by normal processes of mass movement and stream erosion. Examples: welded tuff, volcanic breccia.
vughs: Relatively large voids, usually irregular and not normally interconnected with other voids of comparable size; at the magnifications at which they are recognized they appear as discrete entities.wasteland: Land not suitable for, or capable of, producing materials or services of value.
weathering: The breakdown and changes in rocks and sediments at or near the Earth's surface produced by biological, chemical, and physical agents or combinations of them.
wetland: A transitional area between aquatic and terrestrial ecosystems that is inundated or saturated for long enough periods to produce hydric soils and support hydrophytic vegetation.
wetting front: The boundary between the wetted region and the dry region of soil during infiltration.xeric: A soil moisture regime common to Mediterranean climates that have moist cool winters and warm dry summers. A limited amount of water is present but does not occur at optimum periods for plant growth. Irrigation or summer-fallow is commonly necessary for crop production.
yield: The amount of a specified substance produced (e.g., grain, straw, total dry matter) per unit area.
zonal soil: A soil characteristic of a large area or zone.
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