A horizon Refer to soil horizon and Appendix II.
A line The line on the plasticity chart that separates the clays that, by definition, lie above it from the silts and organic soils that lie below it.
abiontic enzymes Enzymes (exclusive of live cells) that are (a) excreted by live cells during growth and division; (b) attached to cell debris and dead cells; (c) leaked into soil solution from extant or lyzed cells but whose original functional location was on or within the cell. Synonym exoenzymes.
abiotic factor A physical, meteorological, geological, or chemical aspect of the environment.
ablation till (not preferred; use supraglacial till) A general term for loose, relatively permeable earthy material, either contained within or accumulated on the surface of a glacier deposited during the downwasting of nearly static glacial ice. Compare flow till, melt-out till, ground moraine.
absorptance The ratio of the radiant flux absorbed by a body to that incident upon it. Synonym absorption factor.
absorption Uptake of matter or energy by a substance.
absorption, active Movement of ions and water into the plant root because of metabolic processes by the root, frequently against an electrochemical potential gradient.
absorption, passive Movement of ions and water into the plant root from diffusion along a chemical potential gradient.
accelerated erosion Refer to erosion, accelerated erosion.
acceleration The time rate of change in velocity.
access tube Small-diameter tube (typically about 50 mm) inserted through the soil root zone to provide passage of a neutron probe to determine the water content of soil at various depths.
accretion [sedimentology] The gradual increase or extension of land by natural forces acting over a long period of time, as on a beach by the washing up of sand from the sea or on a flood plain by the accumulation of sediment deposited by a stream ( Jackson, 1997). Synonym aggradation.
acetylene-block assay
A technique for demonstrating or estimating denitrification by measuring nitrous oxide (N2O) released from acetylene-treated soil. Acetylene inhibits nitrous oxide reduction to dinitrogen (N2) by denitrifying bacteria.
acetylene-reduction assay
A technique for demonstrating or estimating nitrogenase activity by measuring the rate of acetylene (C2H2) reduction to ethylene (C2H4).
acid precipitation Atmospheric precipitation that is below pH 7 and is often composed of the hydrolyzed by-products from oxidized halogen, nitrogen, and sulfur substances.
acid soil Soil with a pH value <7.0. Compare alkaline soil.
acidic cations
Cations that, on being added to water, undergo hydrolysis resulting in an acidic solution. Hydrated acidic cations donate protons to water to form hydronium ions (H3O+) and thus in aqueous solutions are acids according to the definition given by Bronsted. Examples in soils are Al3+and Fe3+.
acidity, active (no longer used in SSSA publications) The activity of hydrogen ion in the aqueous phase of a soil expressed as a pH value.
acidity, exchange (no longer used in SSSA publications) The acidity of a soil that can be neutralized by lime or a solution buffered in the range of 7 to 8. Compare acidity, total.
acidity, exchangeable Refer to acidity, salt-replaceable.
acidity, free (no longer used in SSSA publications) The titratable acidity in the aqueous phase of a soil.
acidity, reserve Refer to acidity, residual.
acidity, residual Soil acidity that is neutralized by lime or a buffered salt solution to raise the pH to a specified value (usually 7.0 or 8.0) but which cannot be replaced by an unbuffered salt solution. It can be calculated by subtraction of salt-replaceable acidity from total acidity. Compare acidity, salt-replaceable and acidity, total.
acidity, salt-replaceable The aluminum and hydrogen that can be replaced from an acid soil by an unbuffered salt solution such as KCl or NaCl.
acidity, total
The total acidity including residual and exchangeable acidity. Often it is calculated by subtraction of exchangeable bases from the cation exchange capacity determined by ammonium exchange at pH 7.0. It can be determined directly using pH buffer-salt mixtures (e.g., BaCl2 plus triethanolamine, pH 8.0 or 8.2) and titrating the basicity neutralized after reaction with a soil.
acidulation The process of treating a fertilizer source with an acid. The most common process is treatment of phosphate rock with an acid (or mixture of acids) such as sulfuric, nitric, or phosphoric acid.
actinorhizal Nitrogen-fixing symbiosis formed with shrubs and tree species by an actinomycete of the genus Frankia.
activation energy A term used in kinetics to indicate the amount of energy required to bring all molecules in one mole of a substance to their reactive state at a given temperature. Conceptually, this energy barrier must be overcome to get a reaction to go forward. At higher activation energies, reactions are slower if temperature and composition are constant. It is usually determined from an Arrhenius plot of the inverse of the absolute temperature vs. rates of reaction at different temperatures.
active layer The top layer of ground subject to annual thawing and freezing in areas underlain by permafrost (National Research Council of Canada, 1988).
active slope (not recommended; obsolete) A mountain or hill slope that is responding to valley incision with erosion (either geologic or accelerated) exceeding regolith weathering, and that has detritus accumulated behind obstructions indicating contemporary transport of slope alluvium. Slope gradients usually exceed 45% (Schoeneberger and Wysocki, personal communication, 2013). Compare metastable slope.
activity (chemical) (a) A dimensionless measure of the deviation of the chemical potential of a substance from its value at a standard state. It is defined by the equation: µ = µ°+ RT ln a, where µ is the chemical potential at activity = a, µ° is the chemical potential in the standard state (where a = 1.0), R is the molar gas constant, and T is the absolute temperature. In solution a = molal concentration at infinite dilution (molal concentration = molar concentration at low concentrations), and in gases a = partial pressure in atmospheres. (b) Informally, in solution, it may be taken as the effective concentration of a substance. Compare activity coefficient.
activity coefficient The ratio between the activity (chemical) and the concentration of a substance in solution. Activity of component n is usually indicated by (n) and concentration by [n].
adenylate energy charge ratio (EC)
A measure of the metabolic and growth state of microorganisms and microbial communities. The energy charge ratio is calculated using the formula: EC = (ATP + 1/2ADP)/(ATP + ADP + AMP). The denominator represents the total adenylate pool; the numerator, the portion charged with high energy phosphate bonds.
adhesion Forces of attraction between unlike molecules, e.g., water and solid.
adsorption The process by which atoms, molecules, or ions are taken up from the soil solution or soil atmosphere and retained on the surfaces of solids by chemical or physical binding.
adsorption complex Collection of various organic and inorganic substances in soil that are capable of adsorbing ions and molecules.
adsorption isotherm A graph of the quantity of a given chemical species bound to an adsorption complex, at fixed temperature, as a function of the concentration of the species in a solution that is in equilibrium with the complex. Called an isotherm only because adsorption experiments are done at constant temperature.
advance time Refer to irrigation, advance time.
advection Refer to convection.
aeolian (not recommended; obsolete) Use eolian.
aerate To allow or promote exchange of soil gases with atmospheric gases.
aeration porosity Refer to air(-filled) porosity.
aeration, soil
The process by which air in the soil is replaced by air from the atmosphere. In a well-aerated soil, the soil air is very similar in composition to the atmosphere above the soil. Poorly aerated soils usually contain a much higher content of CO2 and a lower content of O2 than the atmosphere above the soil. The rate of aeration depends largely on the volume and continuity of air-filled pores within the soil.
aeration, soil [turf] The introduction of voids into the rooting zone to increase gas exchange and improving soil drainage; e.g., the mechanical removal of small soil plugs in lawns.
aerobic (a) Having molecular oxygen as a part of the environment. (b) Growing only in the presence of molecular oxygen, such as aerobic organisms. (c) Occurring only in the presence of molecular oxygen (said of chemical or biochemical processes such as aerobic decomposition).
aerobic digestion The partial biological decomposition of suspended organic matter in waste water or sewage in aerated conditions.
aerotolerant anaerobes Microorganisms that grow under both aerobic and anaerobic conditions but do not shift from one mode of metabolism to another as conditions change; energy is obtained exclusively via fermentation.
aggradation [sedimentology] The building-up of the Earth’s surface by deposition; specifically, the accumulation of material by any process in order to establish or maintain uniformity of grade or slope; also called accretion ( Jackson, 1997). Compare degradation.
aggregate A group of primary soil particles that cohere to each other more strongly than to other surrounding particles.
aggregate stability [unclear, obeselete term] A measure of the proportion of the aggregates in a soil that do not easily slake, crumble, or disintegrate.
aggregation The process whereby primary soil particles (sand, silt, clay) are bound together, usually by natural forces and substances derived from root exudates and microbial activity.
agric horizon A mineral soil horizon in which clay, silt, and humus derived from an overlying cultivated and fertilized layer have accumulated. The wormholes and illuvial clay, silt, and humus occupy at least 5% of the horizon by volume. The illuvial clay and humus occur as horizontal lamellae or fibers, or as coatings on ped surfaces or in wormholes.
agrichemicals Chemical materials used in agriculture.
agroforestry Any type of multiple cropping land-use that entails complementary relations between tree and agricultural crops and produces some combination of food, fruit, fodder, fuel, wood, mulches, or other products.
agrohydrology Refer to hydrology.
agronomic rate The rate at which fertilizers, organic wastes, or other amendments can be added to soils for optimum plant growth.
agronomy The theory and practice of crop production and soil management.
air permeability Ability of soil to conduct air by convective flow of molecules in response to a pressure gradient.
air dry (a) The state of dryness at equilibrium with the water content in the surrounding atmosphere. The actual water content will depend upon the relative humidity and temperature of the surrounding atmosphere. (b) To allow to reach equilibrium in water content with the surrounding atmosphere.
air(-filled) porosity The fraction of the bulk volume of soil that is filled with air at any given time or under a given condition, such as a specified soil-water content or soil-water matric potential.
air-entry value The value of water content or potential at which air first enters a porous medium.
alas A type of thermokarst depression with steep sides and a flat, grass-covered floor, found in thermokarst terrain, produced by thawing of extensive areas of very thick and exceedingly ice-rich permafrost (National Research Council of Canada, 1988; Jackson, 1997). Compare thermokarst depression.
alban A cutan that is light colored in thin section because of the reduction and translocation of iron.
albedo The ratio of the amount of solar radiation reflected by a body to the amount incident upon it, often expressed as a percentage, as, the albedo of the earth is 34%.
albic horizon A diagnostic mineral soil horizon from which clay and free iron oxides have been removed or in which the oxides have been segregated to the extent that the color of the horizon is determined primarily by the color of the primary sand and silt particles rather than by coatings on these particles (USDA, 1999).
albite A plagioclase feldspar containing sodium (90– 100%) and calcium (0–10%).
Albolls [soil taxonomy] A suborder of Mollisols that have an albic horizon and an argillic or natric horizon immediately below the mollic epipedon. These soils also have redoximorphic features such as low-chroma mottles, iron-manganese concretions, or both, within the albic, argillic, or natric horizon. Most of these soils are saturated with water at or near the soil surface at some time during winter or spring in normal years. Ground water is commonly not within a depth of 200 cm during the summer (USDA, 1999; Appendix I).
Alfisols [soil taxonomy] An order of mineral soils that have an ochric (or rarely an umbric) epipedon, argillic horizons, and that hold water at <1.5 MPa tension during at least 90 days when the soil is warm enough for plants to grow outdoors. Alfisols may also have a fragipan, duripan, kandic, natric, or petrocalcic horizon, plinthite or other features. Most Alfisols have a udic, ustic, or xeric soil moisture regime, and may have wet (aquic) conditions, Alfisols have a mean annual soil temperature of <8°C or a moderate to high base saturation in the lower part of the argillic horizon (35% or more when measured at pH 8.2; USDA, 1999; Appendix I).
alkali soil (no longer used in SSSA publications) (a) A soil with a pH of 8.5 or higher or with a exchangeable sodium ratio >0.15. (b) A soil that contains sufficient sodium to interfere with the growth of most crop plants. Compare saline-sodic soil and sodic soil.
alkaline soil Soil with a pH value >7.0. Compare acidic soil.
alkalinity, soil The degree or intensity of alkalinity in a soil, expressed by a value >7.0 for the soil pH.
alkalophile Microorganism that grows best under alkaline soil conditions (up to pH 10.5).
allelopathy Refer to antagonism.
allochthonous A term that connotes that something (an allochthon) is derived from someplace else, or is not indigenous to a site or area. For example, the allochthonous parent material of an alluvial soil, or an allochthonous community of organisms that invaded an area (i.e., an “allochthonous flora”). Antonym, autochthonous.
allophane An aluminosilicate with primarily shortrange structural order. Occurs as exceedingly small spherical particles especially in soils formed from volcanic ash.
alluvial (adjective) Pertaining to processes or materials associated with transportation or deposition by concentrated running water. Refer to colluvium.
alluvial cone A semi-conical type of alluvial fan with very steep slopes; it is higher, narrower, and steeper (e.g., > 40% slopes) than a fan, and composed of coarser, and thicker layers of material deposited by a combination of alluvial episodes and to a much lesser degree, landslides (e.g., debris flow). Coarsest materials tend to concentrate at the cone apex. Compare alluvial fan, talus cone (Schoeneberger and Wysocki, personal communication, 2013).
alluvial fan A low, outspread mass of loose materials and/or rock material, commonly with gentle slopes, shaped like an open fan or a segment of a cone, deposited by a stream (best expressed in semiarid regions) at the place where it issues from a narrow mountain or upland valley; or where a tributary stream is near or at its junction with the main stream. It is steepest near its apex which points upstream and slopes gently and convexly outward (downstream) with a gradual decrease in gradient ( Jackson, 1997).
alluvial flat (a) (colloquial: western USA) A nearly level, graded, alluvial surface in bolsons and semi-bolsons that lacks distinct, traceable channels. Compare floodplain step, terrace, valley flat. (b) (not preferred) A general term for a small flood plain bordering a river, on which alluvium is deposited during floods. ( Jackson, 1997).
alluvial plain (a) A large assemblage of fluvial landforms (braided streams, terraces, etc.,) that form low gradient, regional ramps along the flanks of mountains and extend great distances from their sources (e.g., High Plains of North America; Schoeneberger and Wysocki, personal communication, 2013); (b) (not recommended, use flood plain.) A general, informal term for a broad flood plain or a low-gradient delta. Compare alluvial flat (Peterson, 1981).
alluvial plain remnant An erosional remnant of an alluvial plain which retains the surface form and alluvial deposits of its origin but was not emplaced by, and commonly does not grade to a present-day stream or drainage network (Schoeneberger and Wysocki, personal communication, 2013). Compare alluvial plain, erosional remnant, paleoterrace.
alluvial terrace (not preferred) Refer stream terrace.
alluvium Uncosolidated sediments deposited by running water of streams and rivers. It may occur on terraces well above present streams, on the present flood plains or deltas, or as a fan at the base of a slope. Compare colluvium, slope alluvium (Hawley and Parsons, 1980).
alpha-alpha-dipyridyl Refer to α,α-dipyridyl.
alpine (a) [geomorphology] (adjective) Characteristic of, or resembling the European Alps, or any lofty mountain or mountain system, especially one so modified by intense glacial erosion as to contain cirques, horns, etc. (e.g., alpine lake); ( Jackson, 1997) (b) [landform not recommended] An ecological community term for high-elevation plant communities.
alpine glacier (a) Any glacier in a mountain range except an ice cap or ice sheet. It usually originates in a cirque and may flow down into a valley previously carved by a stream. Refer to continental glacier. ( Jackson, 1997) (b) (not preferred – Refer to U-shaped valley): (relict) landforms or sediments formed, modified or deposited by a glacier in or on mountains or high hills that has since melted away (Schoeneberger and Wysocki, personal communication, 2013). Compare glacial-valley floor, glacial-valley wall.
Alpine Meadow soils [soil classification] (obsolete; not used in soil taxonomy) A great soil group of the intrazonal order, comprised of dark soils of grassy meadows at altitudes above the timberline (Baldwin, et al., 1938).
amensalism An interaction between two organisms in which one organism is suppressed by the other (such as suppression of one organism by toxins produced by the second).
ammonia volatilization Mass transfer of nitrogen as ammonia gas from soil, plant, or liquid systems to the atmosphere.
ammoniation The process of introducing various ammonium sources into other fertilizer sources forming ammoniated compounds. Ammonium polyphosphates and ammoniated superphosphate are ammoniated compounds.
ammonification The biological process leading to ammoniacal nitrogen formation from nitrogen-containing organic compounds.
ammonium phosphate A generic class of compounds used as phosphorus fertilizers. Manufactured by the reaction of anhydrous ammonia with orthophosphoric acid or superphosphoric acid to produce either solid or liquid products.
ammonium fixation
The process of entrapment of ammonium ions in interlayer spaces of phyllosilicates, in sites similar to K+in micas. Smectites, illites, and vermiculites all can fix ammonium, but vermiculite has the greatest capacity. The fixation may occur spontaneously in aqueous suspensions or as a result of heating to remove interlayer water. Ammonium ions in collapsed interlayer spaces are exchangeable only after expansion of the interlayer. Compare potassium fixation.
amorphous material Noncrystalline constituents that either do not fit the definition of allophane or it is not certain if the constituent meets allophane criteria.
amphiboles Ferromagnesian mineral group containing silica as double chain units and OH as an essential constituent.
amplitude Maximum deviation from the mean for periodic wave motion.
anaerobic (a) The absence of molecular oxygen. (b) Growing in the absence of molecular oxygen (such as anaerobic bacteria). (c) Occurring in the absence of molecular oxygen (as a biochemical process).
anaerobic respiration The metabolic process whereby electrons are transferred from a reduced compound (usually organic) to an inorganic acceptor molecule other than oxygen. The most common acceptors are carbonate, sulfate, and nitrate. Compare denitrification.
anchor Refer to tillage, anchor.
Andepts [soil taxonomy] (obsolete; not used in current soil saxonomy) Prior to 1994, this term was used to indicate Inceptisols that have formed either in vitric pyroclastic materials, or had low bulk density and large amounts of amorphous materials, or both (USDA, 1999).
andesitic lahar deposit A lahar dominated by andesitic volcaniclastics (Schoeneberger and Wysocki, personal communication, 2013).
Andic [soil taxonomy] Soil properties commonly related to volcanic origin of materials. The properties include organic carbon content, bulk density, phosphate retention, and iron and aluminum extractable with ammonium oxalate. [Note: weathering of primary alluminosilicates from non-volcanic parent materials may also lead to the formation of andic soil properties (e.g., spodic materials), USDA, 1999].
Andisols [soil taxonomy] An order of mineral soils that are dominated by andic soil properties in 60% of a layer in the upper part of the soil. Andisols commonly develop in volcanic ejecta (such as volcanic ash, pumice, cinders, and lava) and/or in volcaniclastic materials, the colloidal fraction of which is dominated by short-range-order minerals or Al-humus complexes. Under some environmental conditions, weathering of primary aluminosilicates in nonvolcanic parent materials may also lead to the formation of short-range-order minerals. (Some of these soils also are included in Andisols). Andisols may have any diagnostic epipedon, provided that the minimum requirements for the order are met in and/or below the epipedon. Andisols may also have any soil temperature regime and any soil moisture regime (USDA, 1999; Appendix I).
Ando soils [soil classification] (obsolete; not used in soil taxonomy; Thorp and Smith, 1949).
angle of repose The maximum angle of slope (measured from a horizontal plane) at which loose, cohesionless material will come to rest ( Jackson, 1997).
anhydrite calcium sulfate
A mineral consisting of anhydrous calcium sulfate; usually occurs as a sedimentary deposit or in association with gypsum and halite in evaporates, as in mineral soil crusts.
anion An atom or atomic group that is negatively charged because of a gain in electrons.
anion exchange capacity The sum of exchangeable anions that a soil can adsorb. Usually expressed as centimoles, or millimoles, of charge per kilogram of soil (or of other adsorbing material such as clay).
anion exclusion The exclusion or repulsion of anions from the vicinity of negatively charged soil particle surfaces.
anisotropic soils Soils not having the same physical properties when the direction of measurement is changed. Commonly used in reference to permeability ion of measurement.
annular drainage pattern A drainage pattern in which subsequent streams follow a roughly circular or concentric path along a belt of weak rocks, resembling in plan view, a ring-like pattern where the bedrock joints or fracturing control the parallel tributaries. It is best displayed in streams draining a maturely dissected granitic or sedimentary structural dome or basin where erosion has exposed rimming sedimentary strata of greatly varying degrees of hardness, as in the Red Valley which nearly encircles the domal structure of the Black Hills, SD (Schoeneberger and Wysocki, personal communication, 2013; Jackson, 1997; Way, 1973).
anorthite A plagioclase feldspar containing calcium (90–100%) and sodium (0–10%).
antagonism Production of a substance by one organism that chemically inhibits one or more other organisms. Synonyms antibiosis and allelopathy.
antecedent soil water Amount of water in the soil, generally prior to measurement of another hydrologic process such as infiltration or runoff (also called initial soil water content).
anthraquic conditions A special kind of aquic condition that occurs in soils that are cultivated and irrigated. (i.e., rice paddies).
Anthrepts A suborder of Inceptisols that are more or less freely drained. They have either an anthropic (elevated phosphorus and calcium) or plaggen (appreciably thickened Ap horizon from prolonged manuring) epipedon, and commonly associated with cropland or human occupation for many years. (USDA, 1999; Appendix I).
anthric saturation A variation of episaturation associated with human-controlled flooding, which causes reduction in a soil layer and oxidation of mobilized iron and manganese in a lower unsaturated subsoil.
anthropic epipedon
[soil taxonomy] A surface layer of mineral soil that has been modified by human activity and has the same requirements as the mollic epipedon with respect to color, thickness, organic carbon content, consistence, and base saturation but that has>110 mg P kg-1soluble in 0.05 M citric acid, or is dry>300 days (cumulative) during the period when not irrigated. The anthropic epipedon forms under long continued cultivation and fertilization (USDA, 1999).
anthropogenic feature An artificial feature on the earth’s surface (including those in shallow water), having a characteristic shape and range in composition, composed of unconsolidated earthy or organic materials, artificial materials, or rock, that is the direct result of human manipulation or activities; can be either constructional (e.g., artificial levee) or destructional (e.g., quarry; Schoeneberger and Wysocki, personal communication, 2013).
anthroscape (a) A human-modified “landscape” of substantial and permanent alterations (removal, additions, or reorganization) of the physical shape and/ or internal stratigraphy of the land, associated with management for habitation, commerce, food or fiber production, recreation, or other human activities that have substantively altered water flow and sediment transport across or within the regolith (Schoeneberger and Wysocki, personal communication, 2013). (b) A category in NASIS and the Geomorphic Description System for large, human-modified areas (Schoeneberger and Wysocki, personal communication, 2013).
antibiosis Refer to antagonism.
antibiotic An organic substance produced by one organism that in low concentrations will kill or inhibit growth of other organisms.
antibody A protein produced by the body in response to the presence of an antigen to which it can specifically combine.
anticline [landform] A unit of folded strata that is convex upward and whose core contains the stratigraphically oldest rocks, and occurs at the earth’s surface. In a single anticline, beds forming the opposing limbs of the fold dip away from its axial plane (Schoeneberger and Wysocki, personal communication, 2013; Hawley and Parsons, 1980). Compare monocline, syncline, fold.
antigen A substance that incites specific antibody production.
apatite A mineral containing mainly calcium and phophate ions; Ca5(PO4)3 (OH, Cl, F).
apedal soil material Soil materials without peds, i.e., structureless. This structureless condition can occur either as “single grain” material (e.g., loose sand), or as a massive material (e.g., weathered volcanic tuff).
apparent cohesion Cohesion in granular soils due to capillary forces associated with water.
apparent density (no longer used in SSSA publications) A term formerly used to designate the mass of dry soil (105°C) per unit volume. Compare bulk density, soil.
apparent specific gravity (no longer used in SSSA publications) A term formerly used to designate the ratio of the mass per unit bulk volume of soil and water.
application rate (a) (irrigation) Rate at which water is applied per unit area; usually in millimeter per hour,(b) weight or volume of a fertilizer, soil amendment, orpesticide applied per unit area.
Aqualfs [soil taxonomy]A suborder of Alfisols that are saturated with water at or near the soil surface for periods long enough to limit their use for most crops other than pasture or woodland unless they are artificially drained. Aqualfs have aquic conditions (e.g., gray redox depletions and iron-manganese redox concretionsimmediately below the A1 or Ap horizons and within 50 cm of the soil surface (USDA, 1999; Appendix IA).
Aquands [soil taxonomy] A suborder of Andisols that are saturated with water at or near the surface for periods long enough to limit their use for most crops other than pasture unless they are artificially drained. Commonly, Aquands have dark colored surface horizons with a histic, umbric, or mollic epipedon. Aquands have aquic conditions (e.g., prolonged saturation, low chromas in redox depletions or on ped faces) within 50 cm of the surface. (USDA, 1999; Appendix 1)
Aquents [soil taxonomy] A suborder of Entisols that are saturated with water at or near the surface for periods long enough to limit their use for most crops other than pasture unless they are artificially drained. Aquents have aquic conditions (e.g., prolonged saturation, bluish or gray colors, low chromas or distinct mottles) within 50 cm of the surface, or are permanently water saturated (USDA, 1999; Appendix I).
Aquepts [soil taxonomy] A suborder of Inceptisols that are saturated with water at or near the soil surface for periods long enough to limit their use for most crops other than pasture or woodland unless they are artificially drained. Aquepts commonly have either a histic or umbric epipedon, with a gray to black surface horizon. Aquepts have aquic conditions (e.g., a gray subsurface horizon with redox concentrations) within 50 cm of the surface (USDA, 1999; Appendix I).
Aquerts [soil taxonomy] A suborder of Vertisols that are saturated with water at or near the surface for periods long enough to limit their use for most crops other than pasture and woodland unless they are artificially drained. Aquerts have aquic conditions (e.g., prolonged saturation, gray colors, low chromas or distinct redox concentrations) within 50 cm of the surface for extended periods during the year, but they also are dry for long enough periods in normal years for cracks to open (USDA, 1999; Appendix I).
aquic (adjective) A mostly water saturated and reduced (lacking dissolved oxygen) soil condition. Refer to aquic moisture regime.
aquic conditions [soil classification] Continuous or periodic saturation and reduction. The presence of aquic conditions is indicated by redoximorphic features and can be verified by measurement of saturation and reduction. (USDA, 1999).
aquic moisture regime [soil taxonomy] A reducing soil moisture regime that is virtually free of dissolved oxygen because it is saturated by groundwater or by water of the capillary fringe, and occurring at periods when the soil temperature at 50 cm below the surface is >5°C (5°C is considered to be “biological zero” in soil taxonomy). Very commonly, the level of ground water fluctuates with the seasons; it is highest in the rainy season or in fall, winter, or spring if cold weather virtually stops evapotranspiration. Compare anthraquic, peraquic (USDA, 1999).
aquiclude A layer of soil, or rock sediment that may or may not be saturated, that is incapable of transmitting significant quantities of water under ordinary hydraulic gradients. Refer to aquitard (Freeze, et al., 1979).
aquifer A saturated, permeable unit of soil, sediment or bedrock that can transmit significant quantities of water under ordinary hydraulic gradients to wells or springs. Refer to aquiclude (Freeze, et al., 1979).
aquitard A body of soil, sediment, or bedrock that retards but does not prevent the flow of water to or from an adjacent aquifer. It does not readily yield water to wells or springs but may serve as a storage unit for groundwater ( Jackson, 1997). Refer to aquiclude.
Aquods [soil taxonomy] A suborder of Spodosols that are saturated with water at or near the surface for periods long enough to limit their use for most crops other than pasture or woodland unless they are artificially drained. Aquods may have a histic epipedon, a placic horizon, a duripan, or be cemented by an amorphous mixture of sesquioxides. Aquods have aquic conditions (e.g., redox depletions or concentrations in an albic or spodic horizon) within 50 cm of the surface (USDA, 1999; Appendix I).
Aquolls [soil taxonomy] A suborder of Mollisols that are saturated with water at or near the surface for periods long enough to limit their use for most crops other than pasture unless they are artificially drained. Aquolls have a mollic epipedon and may have a histic epipedon Aquolls also have aquic conditions (e.g., redox depletions or concentrations within or immediately below the mollic epipedon and above a densic, lithic, or paralithic contact or in a soil layer, within 40 and 50 cm from the mineral soil surface (USDA, 1999; Appendix I).
Aquox [soil taxonomy] A suborder of Oxisols that have continuous plinthite near the surface or that are saturated with water at or near the surface sometime during the year if not artificially drained. Aquox have either a histic epipedon, or redox depletions and concentrations (e.g., plinthite) indicative of poor drainage within the oxic horizon, or both. Aquox have aquic conditions within 50 cm of the surface (USDA, 1999; Appendix I).
Aquults [soil taxonomy] A suborder of Ultisols that are saturated with water at or near the surface for periods long enough to limit their use for most crops other than pasture or woodland unless they are artificially drained. Aquults have an ochric or umbric epipedon and an argillic or kandic horizon. Some have a fragipan, and others have plinthite in or below the argillic or kandic horizon. Aquults have aquic conditions (e.g., redox depletions and concentrations) immediately below the A1 or Ap horizons and within 50 cm of the surface (USDA, 1999; Appendix I).
arable land Land on which the production of cultivated crops is economical and practical.
arbuscular mycorrhizae (AM) Mycorrhizal type that forms highly branched arbuscles within root cortical cells of host or compatible plants.
arbuscule Specialized dendritic (highly branched) structure formed within root cortical cells by endomycorrhizal fungus.
archaebacteria (a) Prokaryotes with cell walls that lack murein, having ether bonds in their membrane phospholipids, that are characterized by growth in extreme environments. (b) A primary biological kingdom distinct from both eubacteria and eukaryotes.
Arents [soil taxonomy] A suborder of Entisols that do not have natural horizons and contain recognizable fragments of pedogenic horizons that have been mixed by mechanical disturbance (e.g., deeply mixed by plowing, spading, strip-mine restoration, or munitions bombardment, etc.). Arents are not saturated with water for periods long enough to limit their use for most crops (USDA, 1999; Appendix I).
arete A narrow, jagged mountain crest, often above the snowline, sculptured by alpine glaciers and formed by backward erosion of adjoining cirque walls (Hawley and Parsons, 1980).
Argids [soil taxonomy] A suborder of Aridisols that have an argillic or a natric horizon and lack a duripan or a gypsic, petrocalcic, petrogypsic, or salic horizon within 100 cm of the soil surface. Argids have an aridic soil moisture regime (USDA, 1999; Appendix I).
argillan A cutan composed dominantly of clay minerals. Synonym clay film.
argillic horizon [soil classification] A diagnostic mineral soil horizon that is characterized by the illuvial accumulation of phyllosilicate clays. The argillic horizon has a certain minimum thickness depending on the thickness of the solum, a minimum quantity of clay in comparison with an overlying eluvial horizon depending on the clay content of the eluvial horizon, and usually coatings of oriented clay on the surface of pores or peds or bridging sand grains (USDA, 1999).
aridic soil moisture regime
[soil taxonomy] A soil moisture regime that has no water available for plants for more than half the cumulative time that the soil temperature at 50 cm below the surface is >5°C, and has no period as long as 90 consecutive days when there is water for plants while the soil temperature at 50 cm is continuously >8°C (USDA, 1999).
Aridisols [soil taxonomy] An order of mineral soils in which water is not available to mesophytic plants for long periods (less than 90 consecutive days). Aridisols have an aridic moisture regime, an ochric epipedon, and other pedogenic horizons but no oxic horizon. Moisture is continuously available for plant growth for less than 90 consecutive days (USDA, 1999; Appendix I).
arroyo (colloquial: southwest USA) The channel of a flat-floored, ephemeral stream, commonly with very steep to vertical banks cut in unconsolidated material; sometimes called a wash. It is usually dry but can be transformed into a temporary watercourse or shortlived torrent after heavy rain within the watershed. Where arroyos intersect zones of ground-water discharge, they are more properly classed as intermittent stream channels (Hawley and Parsons, 1980).
artesian well (condition) The occurrence of the water level in a well rising above the top of the confined aquifer or, in special occasions, above the soil surface.
artifact An artificial (human-derived) object or material (e.g., brick, concrete, metal, plastic, lumber, etc.), commonly larger than 2 mm in diameter, made and deposited in association with habitation, manufacturing, excavation or construction activities (Schoeneberger and Wysocki, personal communication, 2013).
artificial collapsed depression A collapse basin, commonly a closed depression, which is the direct result of surficial subsidence associated with subsurface mining (e.g., long-wall mining; Schoeneberger and Wysocki, personal communication, 2013).
artificial drainage pattern Human-made networks of drainage structures (ditches, canals, etc.) built primarily to lower or control the local water table in low lying, flat topography such as glacial lakebeds, broad flood plains, low coastal plains, or marshes most commonly in humid climates. (Irrigation ditches found in arid and semiarid climates, which bring water into the fields, should not be confused with drainage structures; Schoeneberger and Wysocki, personal communication, 2013; Way, 1973).
artificial levee An artificial embankment constructed along the bank of a watercourse or an arm of the sea, to protect land from inundation or to confine streamflow to its channel ( Jackson, 1997).
artificial manure (no longer used in SSSA publications) In European usage denotes commercial fertilizers.
aseptic Free from pathogenic or contaminating organisms.
ash (volcanic) Unconsolidated, pyroclastic material less than 2 mm in all dimensions. Commonly called “volcanic ash.” Compare cinders, lapilli, tephra (Schoeneberger and Wysocki, personal communication, 2013; Soil Survey Staff, 1994).
ash field A land area covered by a relatively thick or distinctive, surficial deposit of volcanic ash (air fall) that can be traced to a specific source and has well defined boundaries. An ash field can be distinguished from adjacent landforms or land areas based on ash thickness, mineral composition, and physical characteristics. Soils within an ash field form solely or predominantly within the ash deposit (Schoeneberger and Wysocki, personal communication, 2013; Jackson, 1997).
ash flow (not preferred–refer to pyroclastic flow, pyroclastic surge) A highly heated mixture of volcanic gases and ash, traveling down the flank of a volcano or along the surface of the ground; produced by the explosive disintegration of viscous lava in a volcanic crater, or by the explosive emission of gas-charged ash from a fissure or group of fissures. The solid materials contained in a typical ash flow are generally unsorted and ordinarily include volcanic dust, pumice, scoria, and blocks in addition to ash ( Jackson, 1997). Compare nueé ardente, lahar, pyroclastic.
aspect The direction toward which a slope faces with respect to the compass or to the rays of the sun. Also called slope aspect.
associative dinitrogen fixation A close interaction between a free-living diazotrophic organism and a higher plant that results in enhanced dinitrogen fixation rates.
associative symbiosis A close but relatively casual interaction between two dissimilar organisms or biological systems. The association may be mutually beneficial but is not required to accomplish specific functions. Compare commensalism, symbiosis.
atoll A coral reef appearing in plan view as roughly circular, and surmounted by a chain of closely spaced, low coral islets that encircle or nearly encircle a shallow lagoon in which there is no land or islands of non-coral origin; the reef is surrounded by open sea ( Jackson, 1997).
attapulgite clay Refer to palygorskite.
Atterberg limits The collective designation of seven so-called limits of consistency of fine-grained soils, suggested by Albert Atterberg, 1911–1912, but with current usage usually retaining only the liquid limit, the plastic limit, and the plasticity number (or index). Compare consistency, liquid limit, plastic limit, and plasticity number.
augite A dark-colored, ferromagnesium mineral representative of the pyroxene group.
autochthonous Microorganisms and/or substances indigenous to a given ecosystem; the true inhabitants of an ecosystem; referring to the common microbiota of the body of soil microorganisms that tend to remain constant despite fluctuations in the quantity of fermentable organic matter.
autochthonous flora (a) That portion of the microflora presumed to subsist on the more resistant soil organic matter and little affected by the addition of fresh organic materials. (b) Microorganisms indigenous to a given ecosystem; the true inhabitants of an ecosystem; referring to the common microbiota of the body of soil microorganisms that tend to remain constant despite constant fluctuations in the quantity of fermentable organic matter. Compare zymogenous flora. Synonym oligotrophs.
autotroph
An organism capable of utilizing CO2 or carbonates as a sole source of carbon and obtaining energy for carbon reduction and biosynthetic processes from radiant energy (photoautotroph or photolithotroph) or oxidation of inorganic substances (chemoautotroph or chemolithotroph).
autotrophic nitrification Oxidation of ammonium to nitrate through the combined action of two chemoautotrophic bacteria, one forming nitrite from ammonium and the other oxidizing nitrite to nitrate.
available nutrients (a) The amount of soil nutrient in chemical forms accessible to plant roots or compounds likely to be convertible to such forms during the growing season. (b) The contents of legally designated “available” nutrients in fertilizers determined by specified laboratory procedures that in most states constitute the legal basis for guarantees.
available water (capacity) The amount of water released between in situ field capacity and the permanent wilting point (usually estimated by water content at soil matric potential of –1.5 MPa). It is not the portion of water that can be absorbed by plant roots, which is plant specific. Compare nonlimiting water range.
avalanche
A large mass of snow, ice, soil, or rock, or mixtures of these materials, falling, sliding, or flowing very rapidly under the force of gravity. Velocities may sometimes exceed 500 km h–1( Jackson, 1997).
avalanche chute The central, channel-like corridor, scar, or depression along which an avalanche has moved. An eroded surface marked by pits, scratches, and grooves. ( Jackson, 1997).
avalanche track (not recommended as a landform term Refer to avalanche chute). The path formed by an avalanche. It may take the form of an open path in a forest, with bent and broken trees, or an eroded surface marked by pits, scratches, and grooves. Compare avalanche chute ( Jackson, 1997).
avulsion A sudden cutting off or separation of land by a flood or by abrupt change in the course of a stream, as by a stream breaking through a meander or by a sudden change in current whereby the stream deserts its old channel for a new one. Compare crevasse, flood-plain splay ( Jackson, 1997).
axial stream (a) The main stream of an intermontane valley, flowing in the deepest part of the valley and parallel to its longest dimension. (b) A stream that follows the axis of a syncline or anticline ( Jackson, 1997).
azonal soils [soil classification] (obsolete; not used in soil taxonomy.) Soils without distinct genetic horizons (Baldwin, et al., 1938).