dam Refer to tillage, dam.

dammer-diker Refer to tillage, reservoir tillage.

Darcy's law (a) A law describing the rate of flow of water through saturated porous media. (Named for Henry Darcy of Paris, who formulated it in 1856 from extensive work on the flow of water through sand filter beds.) As formulated by Darcy, the law is Q = KS(H+ e)/e, where Q is the volume of water passed in unit time, S is the area of the bed, e is the thickness of the bed, H is the depth of water on top of the bed, and “K is a coefficient dependent on the nature of the sand,” and for cases “when the pressure under the filter is equal to the weight of the atmosphere.” (b) Generalization for three dimensions: The rate of viscous flow of water in isotropic porous media is proportional to, and in the direction of, the hydraulic gradient. (c) Generalization for other fluids: The rate of viscous flow of homogenous fluids through isotropic porous media is proportional to, and in the direction of, the driving force.

Dark Gray Gleysolic soil [Canadian soil classification] An intrazonal group of imperfectly to poorly drained forested soils having dark-gray A horizons, moderately high in organic matter, underlain by mottled gray or brownish gleyed mineral horizons. They have a low degree of textural differentiation. (Not used in current U.S. soil taxonomy; USDA, 1999.)

dead-ice (not recommended) Use stagnant ice.

dead-ice moraine (not recommended) Use disintegration moraine.

deadfurrow Refer to tillage, deadfurrow.

debris Any surficial accumulation of loose material detached from rock masses by chemical and mechanical means, as by decay and disintegration. It consists of rock clastic material of any size and sometimes organic matter ( Jackson, 1997).

debris avalanche [mass movement] The process, associated sediments (debris avalanche deposit) or resultant landform characterized by a very rapid to extremely rapid type of flow dominated by the sudden downslope movement of incoherent, unsorted mixtures of soil and weathered bedrock which, although comparatively dry, behave much as a viscous fluid when moving. Compare debris flow, rock fragment flow, debris avalanche, earthflow, landslide (Schoeneberger and Wysocki, personal communication, 2013).

debris fall [mass movement] The process, associated sediments (debris fall deposit) or resultant landform characterized by a rapid type of fall involving the relatively free, downslope movement or collapse of detached, unconsolidated material which falls freely through the air (lacks an underlying slip face); sediments have substantial proportions of both fine earth and coarse fragments; common along undercut stream banks. Compare rock fall, soil fall, landslide (Schoeneberger and Wysocki, personal communication, 2013).

debris flow [mass movement] The process, associated sediments (debris flow deposit) or resultant landform characterized by a very rapid type of flow dominated by a sudden downslope movement of a mass of rock, soil, and mud (more than 50% of the particles are> 2 mm), and whether saturated or comparatively dry, behaves much as a viscous fluid when moving. Compare lahar, mudflow, landslide (Schoeneberger and Wysocki, personal communication, 2013).

debris slide [mass movement] The process, associated sediments (debris slide deposit) or resultant landform characterized by a rapid type of slide, composed of comparatively dry and largely unconsolidated earthy material which slides or rolls downslope (does not exhibit backward rotation) and resulting in an irregular, hummocky deposit somewhat resembling a moraine. Compare rotational landslide, block glide, lateral spread, landslide (Schoeneberger and Wysocki, personal communication, 2013; Jackson, 1997).

debris spread [mass movement] The process, associated sediments (debris spread deposit) or resultant landforms characterized by a very rapid type of spread dominated by lateral movement in a soil and rock mass resulting from liquefaction or plastic flow of underlying materials that may be extruded out between intact units; sediments have substantial proportions of both fine earth and coarse fragments. Compare earth spread, rock spread, landslide (Schoeneberger and Wysocki, personal communication, 2013; Cruden and Varnes, 1996).

debris topple [mass movement] The process, associated sediments (debris topple deposit) or resultant landform characterized by a localized, very rapid type of topple in which large blocks of soil and rock material literally fall over, rotating outward over a low pivot point; sediments have substantial proportions of both fine earth and coarse fragments. Portions of the original material may remain intact, although reoriented, within

decalcification (no longer used in SSSA publications) The removal of calcium carbonate or calcium ions from the soil by leaching.

deep percolation The downward movement of water at the bottom of the soil profile, which represents a loss of water from the root zone.

deflation The sorting out, lifting, and removal of loose, dry, fine grained soil particles (clay, silts, fine sands) by the turbulent, eddy action of the wind; a form of wind erosion ( Jackson, 1997).

deflation basin A topographic basin excavated and maintained by wind erosion which removes unconsolidated material and commonly leaves a rim of resistant material surrounding the depression. Unlike a blowout, a deflation basin does not include adjacent deposits derived from the basin. Compare blowout. ( Jackson, 1997).

deflation flat (colloquial – U.S. Gulf Coast) A series of low ridges and troughs on an essentially flat surface on barrier islands formed by dune field migration during alternating wet and dry periods; a type of interdune. Troughs are eroded down to the wet sand level during drought periods (dune slack), while the ridges are stabilized by vegetation that invades the edge of dune fields during wet periods. Compare blowout, deflation basin (Fisk, 1959).

deflocculate The inverse of flocculation. When soil solutions are at low ionic strength and dominated by alkali metal cations, especially at higher pH values, soil colloidal particles can be dispersed throughout the solution. Compare dispersion.

degradation (a) The process whereby a compound is transformed into simpler compounds. (b) (no longer used in SSSA publications) The changing of a soil to a more highly leached and a more highly weathered condition; usually accompanied by morphological changes such as development of an A2 horizon.

Degraded Chernozem [soil classification] (obsolete; not used in soil taxonomy) A zonal great soil group consisting of soils with a very dark brown or black A1(A) horizon underlain by a dark gray, weakly expressed A2 (E) horizon and a brown B (?) horizon; formed in the forest-prairie transition of cool climates (Baldwin et al., 1938).

dehydration Loss of adsorbed water molecules on

dehydroxylation Loss of structural hydroxyl ions as water molecules on heating

deleterious rhizosphere microorganisms Root-colonizing bacteria and fungi that aggressively colonize roots and are detrimental to plant growth but are not parasitic.

delineation [soil survey] An individual polygon shown by a closed boundary on a soil map that defines the area, shape, and location of a map unit within a landscape.

Delmarva Bay Use Carolina Bay.

delta A body of alluvium, nearly flat and fan-shaped, deposited at or near the mouth of a river or stream where it enters a body of relatively quiet water, usually a sea or lake (Hawley and Parsons, 1980).

delta plain The level or nearly level surface composing the land-ward part of a large delta; strictly, a flood plain characterized by repeated channel bifurcation and divergence, multiple distributary channels, and interdistributary flood basinsn ( Jackson, 1997).

demand system of irrigation Refer to irrigation, demand system of irrigation.

dendritic drainage pattern A common drainage pattern in which the tributaries join the gently curving mainstream at acute angles, resembling in plan view the branching habit of an oak or chestnut tree; it is produced where a consequent stream receives several tributaries which in turn are fed by smaller tributaries. It indicates streams flowing across horizontal rock strata and homogenous soil typified by the landforms of soft sedimentary rocks, volcanic tuff, old dissected coastal plains, or complex crystalline rocks offering uniform resistance to erosion (Schoeneberger and Wysocki, personal communication, 2013; Jackson, 1997; Way, 1973).

denitrification Reduction of nitrogen oxides (usually nitrate and nitrite) to molecular nitrogen or nitrogen oxides with a lower oxidation state of nitrogen by bacterial activity (denitrification) or by chemical reactions involving nitrite (chemodenitrification). Nitrogen oxides are used by bacteria as terminal electron acceptors in place of oxygen in anaerobic or microaerophilic respiratory metabolism.

deposit Either consolidated or unconsolidated material left in a new position that has accumulated by natural processes or by human activity (Schoeneberger and Wysocki, personal communication, 2013; Jackson, 1997; Way, 1973). ple, landslide (Schoeneberger and Wysocki, personalcommunication, 2013).

deposition The laying down of any material by any agent such as wind, water, ice or by other natural processes (Hawley and Parsons, 1980).

depression Any relatively sunken part of the Earth’s surface; especially a low-lying area surrounded by higher ground. A closed depression has no natural outlet for surface drainage (e.g., a sinkhole). An open depression has a natural outlet for surface drainage. Compare closed depression, open depression ( Jackson, 1997).

Depression Podzol [Canadian soil classification] Poorly drained depressional soils of the grassland and parkland regions of Canada with bleached A2 (E) horizons and finer-textured B horizons. (Not used U.S. soil taxonomy.)

deranged drainage pattern A distinctively disordered drainage pattern of nonintegrated streams which indicates a complete lack of underlying structural and bedrock control, resulting from a relatively young landscape having a flat or undulating topographic surface and a high water table. It is characterized by relatively few, irregular streams with few, short tributaries, that flow into and out of depressions containing swamps, bogs, marshes, ponds, or lakes; interstream areas are swampy. Regional streams may meander through the area but do not influence its drainage. These drainage patterns commonly occur on young, thick till plains, end moraines, flood plains, and coastal plains (Schoeneberger and Wysocki, personal communication, 2013; Way, 1973).

desert crust A hard layer, containing calcium carbonate, gypsum, or other binding material, exposed at the surface in a desert region.

desert pavement A natural, residual concentration of wind-polished, closely packed pebbles, boulders, and other rock fragments, mantling a desert surface. It is formed where wind action and sheetwash have removed all smaller particles, or where coarse fragments have migrated upward through sediments to the surface. It usually protects the underlying, finer-grained material from further deflation. Compare erosion pavement, stone line (Schoeneberger and Wysocki, personal communication, 2013; Jackson, 1997).

Desert soil [soil classification] (obsolete; not used in U.S. soil taxonomy) A zonal great soil group consisting of soils with a very thin, light-colored surface horizon, which may be vesicular and is ordinarily underlain by calcareous material; formed in arid regions under sparse shrub vegetation (Baldwin et al., 1938).

desert varnish (not preferred) Use rock varnish.

desorption The migration of adsorbed entities off of the adsorption sites. The inverse of adsorption.

destructional [geomorphology] (adjective) Said of a landform that owes its origin, form, position, or general character to the removal of material by erosion and weathering (degradation) processes resulting from the wearing-down or away of the land surface. Compare constructional ( Jackson, 1997).

detachment Refer to erosion, detachment.

detoxification Conversion of a toxic molecule or ion into a nontoxic form.

detritus [geology] A collective term for rock and mineral coarse fragments occurring in sediments, that are detached or removed by mechanical means (e.g., disintegration, abrasion) and derived from pre-existing rocks and moved from their place of origin. Compare clastic, epiclastic, pyroclastic ( Jackson, 1997).

diagnostic horizons [soil taxonomy] Combinations of specific soil characteristics that are indicative of certain classes of soils. Those which occur at the soil surface are called epipedons, those below the surface, diagnostic subsurface horizons (USDA, 1999).

diamict (not preferred; refer to diamicton) A general term that includes both diamictite (coherent rock) and diamicton (unconsolidated sediments) ( Jackson, 1997).

diamictite A general term for any nonsorted or poorly sorted, noncalcareous, terrigenous sedimentary rock (e.g., pebbly mudstone) containing a wide range of particle sizes. Compare diamicton ( Jackson, 1997).

diamicton A generic term for any nonlithified, nonsorted or poorly sorted sediment that contains a wide range of particle sizes, such as coarse fragments contained within a fine earth matrix (e.g., till) and used when the genetic context of the sediment is uncertain. Compare diamictite (Schoeneberger and Wysocki, personal communication, 2013; Jackson, 1997).

diapir A dome or anticlinal fold in which the overlying rocks or sediments have been ruptured by the squeezing-out of plastic core material. Diapirs in sedimentary strata usually contain cores of salt or shale; igneous intrusions may also show diapiric structure ( Jackson, 1997).

diatomaceous earth A geologic deposit of fine, grayish siliceous material composed chiefly or wholly of the remains of diatoms. It may occur as a powder or as a porous, rigid material. Also called diatomaceous materials.

diatomite A light-colored, soft, siliceous sedimentary rock deposit (generally consolidated) consisting chiefly of opaline diatom frustules deposited in a lacustrine or marine environment. Diatomite has a number of uses owing to its high surface area, absorptive capacity, and relative chemical stability but the term is generally reserved for deposits of actual or potential commercial value. Compare diatomaceous earth ( Jackson, 1997).

diatoms Algae having siliceous cell walls that persist as a skeleton after death. Any of the microscopic unicellular or colonial algae constituting the class Bacillariaceae. They are abundant in fresh and salt waters, and their remains are widely distributed in soils.

diatreme A breccia-filled volcanic pipe that was formed by a gaseous explosion (e.g., hydrovolcanic eruption); commonly, but not exclusively associated with exposed throat or neck of maar, as in the Hopi Buttes area of Arizona. Compare neck [volcanic] (Schoeneberger and Wysocki, personal communication, 2013; Jackson, 1997).

diazotroph A microorganism or association of microorganisms that can reduce molecular nitrogen (N ) to

dielectric constant Also known as the relative (dielectric) permitivity. The magnitude of the displacement of constrained charges in response to an electric field. A property of soil constituents representing their magnitude of non-conductance of electricity.

differential thermal analysis (DTA) Method used to detect energy changes in a substance on heating; useful for identification of clay minerals.

differential water capacity Refer to soil water, differential water capacity.

diffuse double layer A conceptual model of a heterogeneous system that consists of a solid surface (e.g., clay or oxide surface) having a net electrical charge together with an ionic swarm in solution containing ions of opposite charge, neutralizing the surface charge.

diffusion (nutrient) A random movement of ions or molecules due to thermal agitation, which tends to move because of their chemical activity gradient from areas of higher to lower concentration.

diffusion coefficient Proportionality constant that indicates the ability of a material to allow gases and ions to flow under a partial pressure or concentration gradient.

dig Refer to tillage, dig.

digestibility [organic wastes] The potential degree to which organic matter in waste water or sewage can be broken down into simpler and/or more biologically stable products.

dike [intrusive rocks] A tabular igneous intrusion that cuts across the bedding or foliation of the country rock. Compare sill ( Jackson, 1997).

dinitrogen fixation Conversion of molecular nitrogen (N₂) to ammonia and subsequently to organic nitrogen utilizable in biological processes.

dioctahedral An octahedral sheet or a mineral containing such a sheet that has two-thirds of the octahedral sites filled by trivalent ions such as aluminum or ferric iron. Compare phyllosilicate mineral terminology, trioctahedral and Appendix I, Table A3.

dip [soil survey] A geomorphic component (characteristic piece) of flat plains (e.g., lake plain, low coastal plain, low-relief till plain) consisting of a shallow and typically closed depression that tends to be an area of focused groundwater recharge but not a permanent water body and that lies slightly lower and is wetter than the adjacent talf, and favors the accumulation of fine sediments and organic materials (Schoeneberger and Wysocki, personal communication, 2013; Jackson, 1997).

dip slope [geomorphology] A slope of the land surface, roughly determined by and approximately conforming to the dip of underlying bedded rocks (i.e., the long, gently inclined surface of a cuesta). Compare scarp slope (Hawley and Parsons, 1980).

direct counts [soil microbiology] A method of estiammonia. 2

direct problem The predicting of the behavior of a system given its inherent properties.

direct shear test A shear test in which soil under an applied normal load is stressed to failure by moving one section of the sample or sample container relative to the other section.

discharge The volume of water flow through a stream or open channel past a point in a given time period.

discharge curve (a) Rating curve showing the relation between stage and rate of flow of a stream. (b) Curve showing the relation of discharge of a pump and the speed, power, and head.

discharge wetland [soil hydrology] A wet area which focuses subsurface water recharge from the surrounding landscape; a type of wetland. Compare recharge wetland, throughflow wetland.

discontinuity [stratigraphy] Any interruption in sedimentation, whatever its cause or length, usually a manifestation of nondeposition and accompanying erosion, an unconformity ( Jackson, 1997).

discontinuous permafrost Permafrost occurring in some areas beneath the exposed land surface throughout a geographic region where other areas are free of permafrost. Compare continuous permafrost.

disease-suppressive soils Soils in which pathogens do not establish or persist, pathogens establish but cause little or no damage, or pathogens cause disease for a while, but the disease becomes less important even though the pathogens persist in soil.

disintegration Refer to mechanical weathering.

disintegration moraine A drift topography characterized by chaotic mounds and pits, generally randomly oriented, developed in supraglacial drift by collapse and flow as the underlying stagnant ice melted. Slopes may be steep and unstable and there will be used and unused stream courses and lake depressions interspersed with the morainic ridges. Characteristically, there are numerous abrupt, lateral and vertical changes between unconsolidated materials of differing lithology (Sugden and Johns, 1976; Schoeneberger and Wysocki, personal communication, 2013).

dispersion (a) A term used in relation to solute movement. Compare hydrodynamic dispersion. (b) The breakdown of soil aggregates into individual component particles. Compare deflocculate.

dispersivity The ratio of the hydrodynamic dispersion coefficient (d) divided by the pore water velocity (v); thus D = d/v.

dissected breaklands Very steep slopes flanking major rivers and streams in mountainous terrain and dominated by deeply incised, sub-parallel to dendritic, chutelike drainageways that occupy > 50% of the ground surface. Dissected breakland slopes are dominated by hillslope elements that grade to secondary drainageways, rather than directly to the axial stream; a type of breakland (Schoeneberger and Wysocki, personal communication, 2013; Holdorf and Donahue, 1990).

dissected plateau A land area (landscape) produced by significant stream erosion and incision of a plateau such that only a small part of the plateau surface is at or near the original summit level. Much of the area occurs as hillslopes, or if incision is sufficient and relief is > 1000 feet, as mountain slopes. Compare plateau (Schoeneberger and Wysocki, personal communication, 2013).

dissection Fluvial erosion of a land surface or landform by the cutting of gullies, arroyos, canyons, or valleys leaving ridges, hills, mountains, or flat-topped remnants separated by drainageways.

dissimilation The release from cells of inorganic or organic substances formed by metabolism.

distal [sedimentology] (adjective) Said of a sedimentary deposit consisting of fine clastics and deposited farthest from the source area ( Jackson, 1997).

distributary [streams] (a) A divergent stream flowing away from the main stream and not returning to it, as in a delta or on a flood plain. It may be produced by stream deposition choking the original channel. (b) One of the channels of a braided stream; a channel carrying the water of a stream distributary ( Jackson, 1997).

distribution coefficient (K_d)The distribution of a chemical between soil and water.

ditch An open and usually unpaved (unlined), channel or trench excavated to convey water for drainage (removal) or irrigation (addition) to or from a landscape; smaller than a canal; some ditches are modified natural waterways ( Jackson, 1997).

diversion dam A structure or barrier built to divert part or all of the water of a stream to a different course.

divide A summit area, or tract of higher ground which can vary from broad to narrow, or a line of separation that constitutes a watershed boundary between two adjacent drainage basins; divides separate the surface waters that flow naturally in one direction from those that flow in a different or opposite direction (Schoeneberger and Wysocki, personal communication, 2013; Jackson, 1997). Compare interfluve.

doline (not preferred) Synonym sinkhole.

doline karst (not preferred) Synonym sinkhole karst.

dolomitic lime A naturally occurring liming material composed chiefly of carbonates of Mg and Ca in approximately equimolar proportions.

dolostone (not recommended; use dolomite [rock]) An obsolete term proposed for the sedimentary rock called dolomite, to avoid confusion with the mineral of the same name. Compare dolomite [rock] ( Jackson, 1997).

dome [structural geology] An uplift or anticlinal structure, either circular or elliptical in outline, in which the rocks dip gently away in all directions; it may or may not be visible at the land surface. A dome may be small (e.g., a salt dome) or many kilometers in diameter ( Jackson, 1997).

double chain The silica arrangement characteristic of amphiboles where two long chains of linked silica tetrahedra act as a unit.

double layer The name given to the system involving negative charges associate.

double-bedding mound Raised, linear mounds with subdued, convex slope cross-sections constructed by mounding and shaping spoil material dredged from adjacent drainage ditches and placed over natural soil. The mounds serve as preferred, better-drained bedding areas for managed timber plantations; common in the lower coastal plains of the Atlantic and Gulf coasts, USA (Schoeneberger and Wysocki, personal communication, 2013).

drag (a) The force retarding the flow of a fluid over the surface of a solid body. (b) Refer to tillage, drag.

drain tile Concrete, ceramic, plastic, or other rigid pipe or similar buried structure used to collect and conduct profile drain-water from the soil in a field.

drain, to (a) To provide channels, such as open ditches or drain tile, so that excess water can be removed by surface or by internal flow. (b) To lose water (from the soil) by percolation.

drainage Movement of water out of the soil profile.

drainage basin A general term for a region or area bounded by a drainage divide and occupied by a drainage system.

drainage class (natural) A group of soils defined as having a specific range in relative wetness under natural conditions as it pertains to wetness due to a water table under conditions similar to those under which the soil developed.

drainage curves Design curves giving prescribed rates of surface runoff for different levels of crop production, and which may vary according to size of drainage area.

drainage network (not preferred) Use drainage pattern.

drainage pattern The configuration of or arrangement, in plan view, of the natural stream courses in an area, including gullies or first-order channelized flow areas, higher order tributaries, and main streams. It Drainage pattern is related to local geologic materials and structure, geomorphologic features, and geomorphic history of an area. Major drainage patterns include dendritic, trellis, artificial, etc. Also called drainage network (Schoeneberger and Wysocki, personal communication, 2013; Jackson, 1997; Way, 1973).

drainage terrace Refer to terrace.

drainage, excessive Too much or too rapid loss of water from soils, either by percolation or by surface flow. The occurrence of internal free water is very rare or very deep.

drainage, surface Used to refer to surface movement of excess water; includes such terms as ponded, flooded, slow, and rapid.

drainageway A general term for a course or channel along which water moves in draining an area ( Jackson, 1997).

draw A small, natural watercourse cut in unconsolidated materials, generally more open with a broader floor and more gently sloping sides than an arroyo, ravine or gulch, and whose present stream channel may appear inadequate to have cut the drainageway that it occupies.

dredge spoil bank A subaerial mound or ridge that permanently stands above the water composed of dredge spoils; randomly mixed sediments deposited during dredging and dumping. Compare dredged channel, dredge-deposit shoal, filled marshland (Schoeneberger and Wysocki, personal communication, 2013).

dredge spoils Unconsolidated, randomly mixed sediments composed of rock, soil, and/or shell materials extracted and deposited during dredging and dumping activities. Dredge spoils lie unconformably upon natural, undisturbed soil or regolith and can form anthropogenic landforms (e.g., dredge spoil bank; Schoeneberger and Wysocki, personal communication, 2013).

dredge-deposit shoal A subaqueous area, substantially shallower than the surrounding area that resulted from the deposition of materials from dredging and dumping (modified from Demas 1998; Subaquesous Soils Subecommittee, 2005). Compare dredged channel, shoal.

dredged channel A roughly linear, deep water area formed by a dredging operation for navigation purposes (after Wells et al., 1994; dredged hole; Subaqueous Soils Subcommittee, 2005). Refer to dredge-deposit shoal.

drift [glacial geology] A general term applied to all mineral material transported by a glacier and deposited directly by or from the ice, or by running water emanating from a glacier. Drift includes unstratified material (till) that forms moraines, and stratified glaciofluvial deposits that form outwash plains, eskers, kames, varves, and glaciolacustrine sediments ( Jackson, 1997). Also redundantly called glacial drift.

drip irrigation Refer to irrigation.

dropstone An oversized stone (compared to the matrix sediments) in laminated sediment that depresses the underlying laminae and can be covered by “draped laminae”. Most dropstones originate through ice-rafting; another source is floating tree roots ( Jackson, 1997). Compare erratic, ice-rafting.

drumlin A low, smooth, elongated oval hill, mound, or ridge of compact till has a core of bedrock or stratified drift. It usually has a blunt nose facing the direction from which the ice approached and a gentler slope tapering in the other direction. The longer axis is parallel to the general direction of glacier flow. Drumlins are products of streamline (laminar) flow of glaciers, which molded the subglacial floor through a combination of erosion and deposition. Compare drumlinoid ridge (Schoeneberger and Wysocki, personal communication, 2013; Hawley and Parsons, 1980; Jackson, 1997).

drumlin field Groups or clusters of closely spaced drumlins or drumlinoid ridges, distributed more or less en echelon, and commonly separated by small, marshy tracts or depressions (i.e., interdrumlins; Schoeneberger and Wysocki, personal communication, 2013).

drumlinoid ridge A rock drumlin or drift deposit whose form approaches but does not fully attain that of a classic drumlin, even though it seemingly results from similar processes of moving ice. Compare drumlin, interdrumlin (Schoeneberger and Wysocki, personal communication, 2013; Jackson, 1997).

dry wash (not preferred refer to wash). A dry, ephemeral stream channel, especially in semiarid regions which only moves water in response to intense, infrequent precipitation (Schoeneberger and Wysocki, personal communication, 2013). Compare arroyo.

dry-mass content or ratio The ratio of the mass of any component (of a soil) to the oven-dry mass of the soil. Compare oven-dry soil.

dry-weight percentage Refer to dry-mass content or ratio.

dryland farming Crop production without irrigation (rainfed agriculture).

duff A generally firm organic layer on the surface of mineral soils. It consists of fallen plant material that is in the process of decomposition and includes everything from the litter on the surface to underlying pure humus. Also refer to litter. Duff is an organic soil material that is also one of the USDA textures of muck (sapric soil material), mucky peat (hemic soil material), or peat (fibric soil material).

duff mull A forest humus type, transitional between mull and mor, characterized by an accumulation or organic matter on the soil surface in friable Oe horizons, reflecting the dominant zoogenous decomposers. They are similar to mors in that they generally feature an accumulation of partially to well-humified organic materials resting on the mineral soil. They are similar to mulls in that they are zoologically active. Duff mulls usually have four horizons: Oi (L), Oe (F), Oa (H), and A. Oe horizons have a USDA texture of mucky peat (hemic soil material are Oi horizons. Sometimes differentiated into the following Groups: Mormoder, Leptomoder, Mullmoder, Lignomoder, Hydromoder, and Saprimoder.

dump An area of smooth or uneven accumulations or piles of waste rock or general refuse that without major reclamation are incapable of supporting plants. Compare sanitary landfill, fill.

dumps [soil survey] A miscellaneous area (map unit) applied to areas of accumulation of waste rock and general refuse (USDA, 1993).

dune A low mound, ridge, bank, or hill of loose, windblown, subaerially deposited granular material (generally sand), either bare or covered with vegetation and, capable of movement from place to place but always retaining its characteristic shape. Compare barchan dune, parabolic dune, parna dune, shrub-coppice dune, seif dune, transverse dune; Schoeneberger and Wysocki, personal communication, 2013; Jackson, 1997).

dune field An assemblage of moving and/or stabilized dunes, together with sand plains, interdune areas, and the ponds, lakes, or swamps produced by the blocking of steams by the sand. Compare dune lake (Schoeneberger and Wysocki, personal communication, 2013).

dune lake (a) A lake occupying a deflation basin as in a blowout on a dune. (b) A lake occupying a basin formed by the blocking of a stream by sand dunes migrating along a shore (e.g., Moses lake, WA; Jackson, 1997).

dune land [soil survey] A miscellaneous area (map unit) applied to active sand dunes and intervening troughs that shift with the wind (USDA, 1993).

dune slack A damp depression or trough between dunes in a dune field or dune ridges on a shore, caused by intersecting the capillary fringe of the local water table; a moist type of interdune. Compare interdune, dune lake (Schoeneberger and Wysocki, personal communication, 2013; Jackson, 1997).

dune traces A series of linear to semi-concentric micro-ridges and intervening troughs, on the floor of a dune slack or interdune that were exposed by deflation or dune migration. The ridges are remnant bases of slip face lamina held together by soil moisture and /or cemented by evaporites (Schoeneberger and Wysocki, personal communication, 2013).

Durids [soil taxonomy] A suborder of Aridisols that have a duripan with its upper boundary within 100 cm of the soil surface. They may also have an argillic or natric horizon above the duripan. The duripan is cemented partly with opal or chalcedony, and the soils commonly contain calcium carbonate. Durids have an aridic soil moisture regime (USDA, 1999; Appendix I).

duripan [soil taxonomy] A diagnostic subsurface soil horizon that is cemented by illuvial silica, usually opal or microcrystalline forms of silica, to the degree that<50% of the volume of air-dry fragments will slake in water or HCl.

dust mulch A very loose, finely granular, or powdery condition on the soil surface.

dy Colloidal humic substances that accumulate in peat soils at the transition zone between the peat and the underlying mineral material.

dynamic head Refer to irrigation, dynamic head.

dynamic nonequilibrium Flow rate dependent hydraulic properties or local nonequilibrium not due to traditional hysterese (Diamantopoulos and Durner, 2012).

dynamic penetrometer A penetrometer which is driven into the soil by a hammer or falling weight.

dysic Low level of bases in soil material, specified at family level of classification.