Glossary of Soil Science Terms - Browse
tableland A general term for a broad upland masswith nearly level or undulating summit area of large extent and steep side slopes descending to surrounding lowlands (e.g., a large plateau). Compare plateau, mesa, cuesta (Hawley and Parsons, 1980).
tactoid The colloidal-sized aggregates of phyllosilicate clay particles that can form under certain conditions of exchangeable cations and ionic strength.
tailwater Refer to irrigation, tailwater.
tailwater recovery Refer to irrigation, tailwater recovery.
talc Si4Mg3O10(OH)2 A trioctahedral magnesium silicate mineral with a 2:1 type layer structure but without isomorphous substitution. May occur in soils as an inherited mineral. Compare Appendix I, Table A3.
talf [geomorphology] A geomorphic component of flat plains (e.g., lake plain, low coastal plain, low-gradient till plain) consisting of an essentially flat (e.g., 0-1% slopes) and broad area dominated by closed depressions and a non-integrated or poorly integrated drainage system. Precipitation tends to pond locally and lateral transport is slow both above and below ground, which favors the accumulation of soil organic matter and a retention of fine earth sediments; better drained soils are commonly adjacent to drainageways(Schoeneberger and Wysocki, personal communication, 2013). Compare rise.
talud A short, steep slope formed gradually at the downslope margin of a field by deposition against a hedge, a stone wall, or other similar barrier.
talus Rock fragments of any size or shape (usually coarse and angular) derived from and lying at the base of a cliff or very steep rock slope. The accumulated mass of such loose, broken rock formed chiefly by falling, rolling, or sliding ( Jackson, 1997). Compare talus slope, colluvium, mass movement, scree.
talus cone A steep (e.g., 30-40°), cone-shaped landform at the base of a cliff or escarpment, that heads in a relatively small declivity or ravine, and composed of poorly sorted rock and soil debris that has accumulated primarily by episodic rockfall or, to a lesser degree, by slope wash. Finest material tends to be concentrated at the apex of the cone. Not to be confused with an alluvial cone; a similar feature but of fluvial origin, composed of better stratified and more sorted material, and that tapers up into a more extensive drainageway (Schoeneberger and Wysocki, personal communication, 2013). Compare alluvial cone, beveled base, talus slope.
talus slope A portion of a hillslope or mountainslope mantled by talus and lying below a rockfall source (e.g., cliff; Schoeneberger and Wysocki, personal communication, 2013. Compare scree slope, scree, talus, beveled base.
tank (colloquial: southwestern USA) A natural depression or cavity in impervious rocks in which water collects and remains for the greater part of the year ( Jackson, 1997).
tarn A relatively deep, steep-banked lake or pool occupying an ice-gouged rock basin amid glaciated mountains. A cirque lake ( Jackson, 1997).
taxadjunct [soil taxonomy] A soil that is correlated as a recognized, existing soil series for the purpose of expediency. They are so like the soils of the defined series in morphology, composition, and behavior that little or nothing is gained by adding a new series (USDA, 1999).
taxon [soil survey] A class at any categorical level in the U.S. system of soil taxonomy (USDA, 1999).
taxonomic unit Refer to taxon.
TDR Refer to time-domain reflectometry.
temperature A potential term that describes the warmth or coldness of an object.
tensile strength The load per unit area at which an unconfined cylindrical specimen will fail in a simple tension test.
tensiometer A device for measuring the soil-water matric potential in situ; a porous, permeable ceramic cup connected through a water-filled tube to a manometer, vacuum gauge, pressure transducer, or other pressure measuring device.
tephra A collective term for all clastic volcanic materials that are ejected from a vent during an eruption and transported through the air, including ash (volcanic; <2 mm), blocks (volcanic; >64 mm), cinders (2-64 mm), lapilli (2-76 mm and specific gravity > 2.0), pumice (> 2 mm and specific gravity < 1.0), and, scoria (> 2 mm and specific gravity < 2.0), Tephra is a general term that, unlike many volcaniclastic terms, does not denote properties of composition, visicularity, or grain size.
terminal moraine An end moraine that marks the farthest advance of a glacier and usually has the form of a massive arcuate or concentric ridge, or complex of ridges, underlain by till and other drift types (Hawley and Parsons, 1980; Jackson, 1997). Compare end moraine, ground moraine, recessional moraine.
terminal velocity The equilibrium rate at which a particle falls in water or air.
terrace [geomorphology] A collective term for a steplike surface, bordering a stream or shoreline, that represents the former position of a flood plain, lake, or sea shore. The term is usually applied to both the relatively flat summit surface (tread), cut or built by stream or wave action, and the steeper descending slope (scarp, riser), graded to a lower base level of erosion (Schoeneberger and Wysocki, personal communication, 2013). Compare stream terrace, flood-plain step.
terrace slope (not recommended) use riser.
terracettes Small, irregular step-like forms on steep hillslopes, especially in pasture, formed by creep or erosion of surficial materials that may be induced or enhanced by trampling of livestock such as sheep or cattle. Synonyms (not preferred) catstep, sheep or cattle track (Hawley and Parsons, 1980).
terrain A generic name for a tract or region of the Earth’s surface considered as a physical feature, an ecological environment, or a site of some planned human activity ( Jackson, 1997).
Tertiary A period of the Cenozoic Era of geologic time (approximately from 65.5 to 2.6 million years ago). The Tertiary Epoch/series subdivisions comprise, by increasing age, the Pliocene, Miocene, Oligocene, Eocene, and Paleocene (Schoeneberger and Wysocki, personal communication, 2013).
tetrahedral coordination Term describing a cation surrounded by four anions.
textural classification Refer to soil texture.
textural triangle A classification of earth materials with equivalent particle diameters less than 2.0 mm based solely on particle-size distribution.
texture Refer to soil texture.
thalweg [geomorphology] The line of continuous, maximum descent from any point on a land surface; for example, the line connecting the lowest points along the bed of a stream, or the line crossing all contour lines at right angles. ( Jackson, 1997).
thaw-sensitive permafrost Perennially frozen ground which, upon thawing, will experience significant thaw settlement and suffer loss of strength to a value significantly lower than that for similar material in an unfrozen condition (National Research Council of Canada, 1988). Compare thaw-stable permafrost.
thaw-stable permafrost Perennially frozen ground which, upon thawing, will not experience either significant thaw settlement or loss of strength (National Research Council of Canada, 1988). Compare thaw-sensitive permafrost.
thermal band A general term for middle-infrared wavelengths that are transmitted through the atmosphere window at 8 to 13 µm. Occasionally also used for the windows around 3 to 6 µm.
thermal analysis Measurement of changes in physical or chemical properties of materials as a function of temperature, usually heating or cooling at a uniform rate. (a) Differential thermal analysis (DTA) measures temperature difference (ΔT) between a sample and reference material. (b) Differential scanning calorimetry (DSC) measures the differential heat flow between a sample and reference material. (c) Thermogravimetric analysis (TGA) measures weight loss or gain. (d) Various designs of heat pulse probes determine thermal diffusivity, heat capacity, and thermal conductivity based on temperature changes over time at one location from a pulse of heat applied at another location.
thermal conductivity The proportionality factor in Fourier’s law that represents the ability of soil to conduct heat and is equivalent to the thermal flux per unit temperature gradient.
thermal diffusivity The ratio of the thermal conductivity to the volumetric heat capacity.
thermal gravimetric analysis (TGA) Method used in the analysis of minerals to detect weight (mass) loss on heating.
thermal properties Properties of a medium (soil) relative to heat content and heat transfer, such as thermal conductivity, specific heat capacity, and thermal diffusivity.
thermic [soil taxonomy] A soil temperature regime that has mean annual soil temperatures of 15°C or more but <22°C, and >5°C difference between mean summer and mean winter soil temperatures at 50 cm below the surface. Isothermic is the same except the summer and winter temperatures differ by <5°C.
thermocouple A device consisting of two junctions of dissimilar metals that responds to temperature differences between the two junctions.
thermogenic soils Soils with properties that have been influenced primarily by high temperature as the dominant soil-formation factor; developed in subtropical and equatorial regions.
thermokarst Karst-like topographic features produced in a permafrost region by local melting of ground ice and subsequent settling of the ground ( Jackson, 1997).
thermokarst depression A hollow in the ground resulting from subsidence following the local melting of ground ice in a permafrost region ( Jackson, 1997).
thermokarst drainage pattern Drainage patterns that form polygonal and hexagonal shapes with streams that may connect rounded depressions, exhibiting a beaded appearance; developed in poorly drained, finegrained sediments and in organic materials in regions of permafrost. Freezing causes many cracks to develop; thawing causes slumping, settlement, and depressions. This type of drainage pattern with its associated hexagons and beaded ponds indicates the existence or previous presence of permafrost conditions (Schoeneberger and Wysocki, personal communication, 2013, Way, 1973).
thermokarst lake Lake or pond produced in a permafrost region by melting of ground ice (Hawley and Parsons, 1980).
thermophile Refer to thermophilic organism.
thermophilic organism An organism whose optimum temperature for growth is above 45�C.
thermosequence A group of related soils that differ, one from the other, primarily as a result of differences in temperature as a soil-formation factor.
threshold moisture content [biology] The minimum moisture condition, measured either in terms of moisture content or moisture stress, at which biological activity just becomes measurable.
throughfall That portion of precipitation that falls through or drips off of a plant canopy.
throughflow Water that infiltrates and moves laterally through the upper soil horizons and emerges downslope as seepage before entering a stream. Refer to interflow.
throughflow wetland [soil hydrology] A wet area which moves subsurface water recharge from adjacent recharge areas onward to discharge areas landscape; a type of wetland. Compare discharge wetland, recharge wetland.
throw Refer to tillage, throw.
tidal flat An extensive, nearly horizontal, barren or sparsely vegetated tract of land that is alternately covered and uncovered by the tide, and consists of unconsolidated sediment mostly clays, silts and/or sands and organic materials). Compare tidal marsh, wind-tidal flat ( Jackson, 1997).
tidal marsh An extensive, nearly level marsh bordering a coast (as in a shallow lagoon, sheltered bay or estuary) and regularly inundated by high tides; formed mostly of unconsolidated sediments (e.g., clays, silts, and/or sands and organic materials), and the resistant root mat of salt tolerant plants; a marshy tidal flat (Schoeneberger and Wysocki, personal communication, 2013; Jackson, 1997). Compare tidal flat.
tidal flats A miscellaneous map unit area of nearly flat, barren mud periodically covered by tidal waters. Normally these materials have an excess of soluble salt.
tidal inlet Any inlet through which water alternately floods landward with the rising tide and ebbs seaward with the falling tide. Compare inlet, relict tidal inlet ( Jackson, 1997).
tie-ridging Refer to tillage, tie-ridging.
tile drain Concrete, ceramic, plastic etc., pipe, or related structure, placed at suitable depths and spacings in the soil or subsoil to enhance and/or accelerate drainage of water from the soil profile.
till [agronomy] To prepare the soil for seeding; to seed or cultivate the soil.
till plain An extensive flat to undulating surface underlain by till and bounded on the distal end by subordinate recessional or end moraines (Schoeneberger and Wysocki, personal communication, 2013). Compare till, ground moraine.
till-floored lake plain A glaciated land area that has characteristics of a till plain, but that was also inundated by a glacial lake. The area possesses a gently undulating till-topography, rather than a distinctive, low-relief lake plain surface, and has thin (e.g., ≈1-3 m), continuous or discontinuous lacustrine sediments atop the till. Topographic highs that once existed as islands may exhibit shore features (e.g., wave-cut scarps, strandlines, beach deposits; Schoeneberger and Wysocki, personal communication, 2013).
tillability Refer to tillage, tillability.
tillage
The mechanical manipulation of the soil profile for any purpose; but in agriculture it is usually restricted to modifying soil conditions and/or managing crop residues and/or weeds and/or incorporating chemicals for crop production.anchor Partially burying foreign materials such as plant residues or paper mulches.backfurrow The resulting ridge of soil turned up when the first furrow slice is lapped over the previous soil surface when starting the plowing operation. basin listing Refer to tillage, reservoir tillage.bed planting A method of planting in which the seed is planted on slightly raised areas between furrows with two or more seed rows sometimes planted on each bed. Compare tillage, ridge planting. bed shaper A soil-handling implement that forms uniform ridges of soil to predetermined shapes. bedding The process of preparing a series of parallel ridges, usually no wider than two crop rows, separated by shallow furrows. The resulting structures are beds.block (thinning, checking) To remove plants from a row with hoes or other cutting devices as a means of reducing and uniformly spacing plants. broadcast planting A uniform planting of seeds dis-tributed over the entire planted area.
tillage erosion Refer to erosion, tillage erosion.
tillage, wheel track planting
A practice of planting in which the seed is planted in tracks formed by wheels (usually tractor wheels) rolling immediately ahead of the planter.
tilted fault block A fault block that has become tilted, perhaps by rotation on a hinge line (fault; Jackson, 1997). Compare fault-line scarp.
tilth Refer to tillage, tilth.
time-domain reflectometry A method that uses the timing of wave reflections to determine the properties of various materials, such as the dielectric constant of soil as an indication of water content; commonly spoken of and abbreviated as TDR.
todorokite [(Na, Ca, K, Ba, Mn2+)2 Mn4O12
toe [mass movement] The lowest, usually curved margin of displaced material of a landslide, most distant from the main scarp. Commonly it has an irregular surface that has ripples and may be breached by radial cracks or gaps (Cruden and Varnes, 1996; Schoeneberger and Wysocki, personal communication, 2013). Compare main scarp, minor scarp.
toeslope The hillslope position that forms a gently inclined surface at the base of a slope. Toeslopes in profile are commonly gentle and linear, and are constructional surfaces forming the lower part of a slope continuum that grades to a valley or closed depression (Hawley and Parsons, 1980). Compare summit, shoulder, backslope, footslope, valley floor.
tombolo A sand or gravel bar or barrier that connects an island with the mainland or with another island ( Jackson, 1997).
top dressing An application of fertilizer to a soil surface, without incorporation, after the crop is established.
topography The relative position and elevations of the natural or manmade features of an area that describe the configuration of its surface (Hawley and Parsons, 1980).
toposequence A sequence of related soils that differ, one from the other, primarily because of topography as a soil-formation factor.
topple [mass movement] A category of processes, associated sediments (topple deposit) or resultant landforms characterized by a localized, very rapid type of fall in which large blocks of soil or rock literally fall over, rotating outward over a low pivot point. Portions of the original material may remain intact, although reoriented, within the resulting debris pile. Types of topples can be specified based on the dominant particle size of sediments (i.e., debris topple, soil topple, rock topple( Schoeneberger and Wysocki, personal communication, 2013; Varnes, 1978). Compare fall, flow, slide, spread, complex landslide, landslide.
topsoil (a) The layer of soil moved in cultivation. Frequently designated as the Ap layer or Ap horizon. Compare surface soil. (b) Presumably fertile soil material used to topdress roadbanks, gardens, and lawns.
tor A high, isolated pinnacle, or rocky peak; or a pile of rocks, much-jointed and usually granitic, exposed to intense weathering, and often assuming peculiar or fantastic shapes ( Jackson, 1997).
Toreva block A slump block consisting of a single large mass of unjostled material which, during descent, has undergone a backward rotation toward the parent cliff along a horizontal axis that roughly parallels it; a type of rotational landslide. The unit forms a crude, elongated rectangular block rather than a bowl shape or chaotic mass; typically associated with horizontal to gently dipping, interbedded bedrock (e.g., Black Mesa area, NM; Schoeneberge and Wysocki, 2013; Jackson, 1997; Fairbridge, 1968).
Torrands [soil taxonomy] A suborder of more or less well drained Andisols of dry regions. They have an ochric or mollic epipedon and a cambic horizon, and may have a duripan or a petrocalcic horizon. Torrands that have an aridic soil moisture regime (USDA, 1999; Appendix 1).
Torrerts [soil taxonomy] A suborder of Vertisols of arid regions. If not irrigated during the year, they have cracks that commonly stay open for most of the year and are closed for less than 60 days during the growing season (USDA, 1999; Appendix 1).
torric A soil moisture regime defined like aridic soil moisture regime but used in a different category of theU.S. soil taxonomy (USDA, 1999).
Torrox [soil taxonomy] A suborder of Oxisols that are arid and commonly have a higher base saturation than other Oxisols. Torrox have an aridic (torric) soil moisture regime (USDA, 1999).
tortuosity The nonstraight nature of soil pores.
tortuosity factor Reciprocal of the tortuaosity or the ratio of the straight path length to the actual flow-path length.
total head (total potential) The sum of the separate energy components acting upon soil water expressed as energy per weight (head) or energy per volume (potential).
tourmaline A ring or cyclo-silicate mineral that contains boron.
tower karst (a) A type of tropical karst topography characterized by isolated, steep-sided, residual limestone hills or ridges with vertical or near-vertical walls, and may be relatively flat-topped; commonly surrounded by a flat alluvial plain or lagoons. (Also called fenglin). (b) A cluster of peaks or ridges with vertical or near-vertical walls, and convex upper side slopes where towers rise from a common base and are separated by deep, rugged ravines or large sinkholes. (Also called fengcong, turmkarst). Compare karst tower, cockpit karst, cone karst, fluviokarst, kegel karst, sinkhole karst (Schoeneberger and Wysocki, personal communication, 2013; Jackson, 1997).
toxicity Quality, state, or degree of the harmful effect from alteration of an environmental factor.
trace elements (a) No longer used in SSSA publications in reference to plant nutrition. Compare micronutrient. (b) In environmental applications it is those elements exclusive of the eight abundant rock-forming elements: oxygen, aluminum, silicon, iron, calcium, sodium, potassium, and magnesium.
traffic pan Refer to pan (ii).
transfer The movement of mass at a point in the soil from one region to another region, e.g., solute movement from adsorbed to solution phases or solute movement from a macropore flow region to a micropore flow region. Refer to cation exchange or mass tranfer.
transitional soil (intergrades) A soil that possesses properties and distinguishing characteristics of two or more separate soils.
translational earth slide [mass movement] The process, associated sediments (translational earth slide deposit) or resultant landform characterized by an extremely slow to moderately rapid type of slide, composed of comparatively dry and largely unconsolidated earthy material, portions or blocks of which remain largely intact and in which movement occurs along a well-defined, planar slip face roughly parallel to the ground surface and resulting in lateral displacement but no rotation of the displaced mass; sediments predominantly fine earth (< 2 mm). The landform may be single, successive (repeated up and down slope), or multiple (as the number of slide components increase; Schoeneberger and Wysocki, personal communication, 2013; Varnes, 1978). Compare translational debris slide, translational rock slide, rotational slide, lateral spread, landslide.
translational debris slide [mass movement] The process, associated sediments (translational debris slide deposit) or resultant landform characterized by an extremely slow to moderately rapid type of slide, composed of comparatively dry and largely unconsolidated earthy material, portions or blocks of which remain largely intact and in which movement occurs along a well-defined, planar slip face roughly parallel to the ground surface and resulting in lateral displacement but no rotation of the displaced mass; sediments have substantial proportions of both fine earth and coarse fragments. The landform may be single, successive (repeated up and down slope), or multiple (as the number of slide components increase; Schoeneberger and Wysocki, personal communication, 2013; Varnes, 1978). Compare translational earth slide, translational rock slide, rotational slide lateral spread, landslide.
translational rock slide [mass movement] The process, associated sediments (translational rock slide deposit) or resultant landform characterized by an extremely slow to moderately rapid type of slide, composed of comparatively dry and largely consolidated rock bodies, portions or blocks of which remain largely intact and in which movement occurs along a well-defined, planar slip face roughly parallel to the ground surface and resulting in lateral displacement but no rotation of the displaced mass; sediments predominantly fine earth (< 2 mm). The landform may be single, successive (repeated up and down slope), or multiple (as the number of slide components increase; Schoeneberger and Wysocki, personal communication, 2013; Varnes, 1978). Compare translational debris slide, translational earth slide, rotational slide, lateral spread, landslide.
translational slide A category of mass movement processes, associated sediments (translational slide deposit) or resultant landforms characterized by the extremely slow to moderately rapid downslope displacement of comparatively dry soil-rock material on a surface (slip face) that is roughly parallel to the general ground surface, in contrast to falls, topples, and rotational slides. The term includes such diverse slide types as translational debris slides, translational earth slide, translational rock slide, block glides, and slab or flake slides (Schoeneberger and Wysocki, personal communication, 2013; Varnes, 1978; Jackson, 1997). Compare rotational slide, slide, landslide.
transmission zone That part of a soil profile that is behind the wetting front during infiltration and does not increase in water content during further infiltration.
transmissivity The rate at which water moves through a unit cross-sectional area of an aquifer of specified thickness under a unit hydraulic gradient. It is equal to the hydraulic conductivity times the thickness of the aquifer.
transport The movement of mass from one point in the soil to another point in the soil.
transverse dune A very asymmetric sand dune elongated perpendicular to the prevailing wind direction, having a gentle windward slope and a steep leeward slope standing at or near the angle of repose of sand; it generally forms in areas of sparse vegetation ( Jackson, 1997). Compare longitudinal dune.
trash farming Refer to tillage, mulch tillage; no-tillage (zero tillage) system; minimum tillage; plowless farming.
tread [geomorphology] A geomorphic component of terraces, flood-plain steps, and other stepped landforms consisting of the flat to gently sloping, topmost and laterally extensive slope. Commonly a recurring part of a series of natural, step-like landforms such as successive stream terraces. It’s characteristic shape and alluvial sediment composition is derived from the cut and fill processes of a fluvial system (Schoeneberger and Wysocki, personal communication, 2013). Compare riser.
tree-throw (not preferred) use tree-tip, tree-tip mound, tree-tip pit.
tree-tip The process of uprooting and tipping over of trees by strong winds, commonly resulting in a small depression from which the root-ball is displaced and an adjacent mound from the sediments subsequently sloughed from the root ball. Most prevalent in shallow forested soils over a restrictive layer (e.g., bedrock); also called “tree-throw”, “windthrow” (Schoeneberger and Wysocki, personal communication, 2013). Compare tree-tip mound, tree-tip pit.
tree-tip mound The small mound of debris sloughed from the root plate (ball) of a tipped-over tree. Local soil horizons are commonly obliterated, which results in a heterogeneous mass of soil material; sometimes called a “cradle knoll” (not recommended; Schoeneberger and Wysocki, personal communication, 2013; Buol et al., 1997). Compare tree-tip pit.
tree-tip pit The small pit or depression resulting from the area vacated by the root plate (ball) from tree-tip. Such pits are commonly adjacent to small mounds composed of the displaced material. Subsequent infilling usually produces a heterogeneous soil matrix that may or may not include a stone line that lines the depression; also called “tree throw” (Schoeneberger and Wysocki, personal communication, 2013; Buol et al., 1997). Compare tree-tip mound.
tree-tip pit and mound topography A local-scale topography composed of irregularly spaced, small, closed depressions and adjacent mounds caused by the displacement of root balls from trees knocked down by wind (i.e., tree-tip; also called tree-throw). The result is a subdued, irregularly pock-marked or undulating surface; most common in forested areas overlying shallow rooting conditions (e.g., lithic contact, water table, etc). Micro-elevational differences generally range from0.5 to < 2 m. Sometimes also referred to as (not preferred): cradle and knoll, or pit and mound topography (Schoeneberger and Wysocki, personal communication, 2013). Compare tree-tip mound, tree-tip pit.
trellis drainage pattern A drainage pattern characterized by parallel main streams intersected at, or nearly at, right angles by their tributaries, which in turn are fed by elongated secondary tributaries and short gullies parallel to the main streams, resembling, in plan view, the stems of a vine on a trellis. This pattern indicates marked bedrock structural control rather than a type of bedrock and usually indicates in which the main parallel channels follow the strike of the beds. It is commonly developed where the beveled edges of alternating hard and soft rocks outcrop in parallel belts, as in titled, interbedded sedimentary rocks in a rejuvenated folded-mountain region or in a maturely dissected belted coastal plain of tilted strata (Schoeneberger and Wysocki, personal communication, 2013; Jackson, 1997; Way, 1973).
triaxial shear test A test in which a cylindrical specimen of soil umbric epipedon encased in an impervious membrane is subjected to a confining pressure and then loaded.
trickle irrigation Refer to irrigation, trickle.
trioctahedral An octahedral sheet or a mineral containing such a sheet that has all of the sites filled, usually by divalent ions such as magnesium or ferrous iron. Compare dioctahedral and phyllosilicate mineral terminology.
tripartite symbiosis An association involving three different organisms; e.g., soybean-Bradyrhizobium-arbuscular mycorrhizae.
tripoli A light-colored, porous, friable, siliceous (largely chalcedonic) sedimentary rock, which occurs in powdery or earthy masses that result from the weathering of siliceous limestone. It has a harsh, rough feel and is used to polish metals and stones ( Jackson, 1997).
Tropepts [soil taxonomy] (obsolete; not currently used in soil taxonomy) Proir to 1999, this was a suborder of Inceptisols that have a mean annual soil temperature of 8°C or more, and <5°C difference between mean summer and mean winter temperatures at a depth of 50 cm below the surface. Tropepts may have an ochric epipedon and a cambic horizon, or an umbric epipedon, or a mollic epipedon under certain conditions but no plaggen epipedon, and are not saturated with water for periods long enough to limit their use for most crops (USDA, 1999).
Trophic level The presence of nutrients and energy within a stage, represented by a group of organisms, of the food chain, ranging from primary nutrient assimilating autotrophs to the predatory organotrophs.
trough [geomorphology] (a) Any long, narrow depression in the earth’s surface, such as one between hills or with no surface outlet for drainage. (b) (not preferred; refer to U-shaped valley, mountain valley) A broad, elongate U-shaped valley, such as a glacial trough. Compare U-shaped valley ( Jackson, 1997).
trough end (not recommended refer to cirque, cove). The steep, semicircular rock wall forming the abrupt head or end of a U-shaped valley ( Jackson, 1997). Compare headwall.
trough bottom (not preferred) Use glacial-valley floor.
trough valley (not preferred) use U-shaped valley.
trough wall (not preferred) Refer to glacial-valley wall.
truncated (adjective) Having lost all or part of the upper soil horizon or horizons by soil removal (erosion, excavation, etc.).
truncated soil Soil that has had part or all of the upper soil horizon(s) removed by accelerated erosion, excavation, etc., but retains some portion of the original subsoil horizons intact (Schoeneberger and Wysocki, personal communication, 2013). Compare scalped area.
tuff A compacted deposit that is ≥50% (<2 mm) volcanic ash and dust. Various types of tuff can be recognized based on composition: acidic tuff is predominantly composed of acidic particles; basic tuff is predominantly composed of basic particles (Schoeneberger and Wysocki, personal communication, 2013).
tumulus (pl. tumuli) A small dome or mound on the surface of a lava flow formed by the buckling of the congealing crust near the edge of a flow caused by differences in flow rates of the cooler crust above and the hotter, more fluid lava below. Dimensions commonly range from <1 to 5 m in height, 3 to 10 m in width and 30 to 40 m in length. Some tumuli are hollow(Schoeneberger and Wysocki, personal communication, 2013; Jackson, 1997; Green and Short, 1971). Compare pressure ridge [volcanic].
Tundra soils (a) Soils characteristic of tundra regions.(b) [soil classification] (obsolete; not used in soil taxonomy) A zonal great soil group consisting of soils with dark-brown peaty layers over grayish horizons mottled with rust and having continually frozen substrata; formed under frigid, humid climates, with poor drainage, and native vegetation of lichens, moss, flowering plants, and shrubs (Baldwin et al., 1938).
tunnel valley A relatively shallow trench or depression cut into drift and other loose material, or in bedrock, by a subglacial stream not loaded with coarse sediment that may or may not be part of the present day drainage pattern ( Jackson, 1997).
tunnel-valley lake A glacial lake occupying a portion of a tunnel valley ( Jackson, 1997).
Turbels [soil taxonomy] A suborder of Gelisols that have one or more horizons with evidence of cryoturbation in the form of irregular, broken, or distorted horizon boundaries, involutions, the accumulation of organic matter on top of the permafrost, ice or sand wedges, and oriented rock fragments (USDA, 1999; Appendix I).
turbulent flow Movement of water molecules at sufficiently high energy levels that they do not slide over one another in parallel paths but cause eddie currents such that the mean velocity is not proportional to the pressure drop.
turf hummock A hummock consisting of vegetation and organic matter with or without a core of mineral soil or stones (typically 10-50 cm height; 20-90 cm diameter). Groups of hummocks can form a type of patterned ground common to tundra or wet areas (e.g., marsh; National Research Council of Canada, 1988; Schoeneberger and Wysocki, personal communication, 2013). Compare earth hummock, non-sorted circle, patterned ground.
turn strip Refer to tillage, turnrow.
turnrow Refer to tillage, turnrow.