abiotic factor: A physical, meteorological, geological, or chemical aspect of the environment.

ablation till: A general term for loose, relatively permeable material, either contained within or accumulated on the surface of a glacier deposited during the downwasting of nearly static glacial ice.

absorptance: The ratio of the radiant flux absorbed by a body to that incident upon it. Also called absorption factor. absorption Uptake of matter or energy by a substance.

absorption: (i) (active) Movement of ions and water into the plant root because of metabolic processes by the root, frequently against an electrochemical potential gradient. (ii) (passive) Movement of ions and water into the plant root from diffusion along a chemical potential gradient.

access tube: Small diameter tube (typically about 50 millimeters) inserted through the soil root zone to provide passage of a neutron probe to determine the water content of soil at various depths.

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.

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.

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.

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.

aerobic: (i) Having molecular oxygen as a part of the environment. (ii) Growing only in the presence of molecular oxygen, such as aerobic organisms. (iii) 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.

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.

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.

agronomic rate: The rate at which fertilizers, organic wastes or other amendments can be added to soils for optimum plant growth.

air dry: (i) 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. (ii) To allow to reach equilibrium in water content with the surrounding atmosphere.

air entry value: The value of water content or potential at which air first enters a porous media.

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 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.

Alfisols: Mineral soils that have umbric or ochric epipedons, 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 have a mean annual soil temperature of <8°C or a base saturation in the lower part of the argillic horizon of 35% or more when measured at pH 8.2.

alkaline soil: Soil with a pH value >7.0.

alkalinity soil: The degree or intensity of alkalinity in a soil, expressed by a value >7.0 for the soil pH.

alluvium: 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.

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 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.

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.

amorphous material: Noncrystalline constituents that either do not fit the definition of allophane or it is not certain if the constituent meets allophane criteria.

anaerobic: (i) The absence of molecular oxygen. (ii) Growing in the absence of molecular oxygen (such as anaerobic bacteria). (iii) 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.

andic: Soil properties related to volcanic origin of materials. The properties include organic carbon content, bulk density, phosphate retention, and iron and aluminum extractable with ammonium oxalate.

Andisols: Mineral soils that are dominated by andic soil properties in 60 percent or more of their thickness.

angle of repose: The maximum angle of slope (measured from a horizontal plane) at which loose, cohesionless material will come to rest.

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 changes with direction of measurement.

antagonism: Production of a substance by one organism that inhibits one or more other organisms. The terms antibiosis and allelopathy have also been used to describe such cases of chemical inhibition.

anthraquic conditions: A special kind of aquic conditions which occurs in soils that are cultivated and irrigated.

anthric saturation: A variation of episaturation associated with controlled flooding, which causes reduction in a soil layer and oxidation of mobilized iron and manganese in a lower unsaturated subsoil.

anthropic epipedon: A surface layer of mineral soil that has the same requirements as the mollic epipedon with respect to color, thickness, organic carbon content, consistence and base saturation, but that has >110 mg P kg-1 soluble 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.

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.

antigen: A substance that incites specific antibody production.

apedal soil material: Soil materials without peds, i.e. structureless.

apparent cohesion: Cohesion in granular soils due to capillary forces associated with water. 

application rate: (i) (irrigation) Rate at which water is applied per unit area; usually in mm per hour, (ii) weight or volume of a fertilizer, soil amendment, or pesticide applied per unit area.

aquic: A mostly reducing soil moisture regime nearly free of dissolved oxygen due to saturation by groundwater or its capillary fringe and occurring at periods when the soil temperature at 50 cm below the surface is >5°C.

arable land: Land so located that production of cultivated crops is economical and practical.

arbuscule: Specialized dendritic (highly branched) structure formed within root cortical cells by endomycorrhizal fungus.

argillan: A cutan composed dominantly of clay minerals.

argillic horizon: A 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 has coatings of oriented clay on the surface of pores or peds or bridging sand grains.

aridic: 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.

Aridisols: Mineral soils that have an aridic moisture regime, an ochric epipedon, and other pedogenic horizons but no oxic horizon.

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.

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.

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.

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: 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. and The contents of legally designated "available" nutrients in fertilizers determined by specified laboratory procedures which 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.

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/hr.

badland: In Soil Survey a map-unit that is a type of miscellaneous area, which is generally devoid of vegetation , is intricately dissected by a fine, drainage network with a high drainage density and has short, steep slopes with narrow interfluves resulting from erosion of soft geologic materials. Most common in arid or semiarid regions.

basal till: Unconsolidated material deposited and compacted beneath a glacier and having a relatively high bulk density.

base level: The theoretical limit or lowest level toward which erosion of the Earth's surface constantly progresses but seldom, if ever, reaches; especially the level below which a stream cannot erode its bed. The general or ultimate base level for the land surface is sea level, but temporary base levels may exist locally.

base saturation: The ratio of the quantity of exchangeable bases to the cation exchange capacity. The value of the base saturation varies according to whether the cation exchange capacity includes only the salt extractable acidity (see cation exchange capacity) or the total acidity determined at pH 7 or 8. Often expressed as a percent.

bay: (i) Any terrestrial formation resembling a bay of the sea, as a recess or extension of lowland along a river valley or within a curve in a range of hills, or an arm of a prairie extending into, or partly surrounded by, a forest. (ii) A Carolina Bay.

beach: A gently sloping area adjacent to a lake or ocean that lies between the low and high water marks, which is devoid of vegetation, and is composed of unconsolidated material, typically sand or gravel, deposited by waves or tides.

bed: (i) Geologic - The smallest, formal lithostratigraphic unit of sedimentary rocks. The designation of a bed or a unit of beds as a formally named lithostratigraphic unit generally should be limited to certain distinctive beds whose recognition is particularly useful. Coal beds, oil sands, and other layers of economic importance commonly are named, but such units and their names usually are not a part of formal stratigraphic nomenclature. (ii) Agronomic - A raised (usually) cultivated area between furrows or wheel tracks of tractors specially prepared, managed and/or irrigated to promote the production of a planted crop.

bedrock: A general term for the solid rock that underlies the soil and other unconsolidated material or that is exposed at the surface.

bentonite: A relatively soft rock formed by chemical alteration of glassy, high silica content volcanic ash. This material shows extensive swelling in water and has a high specific surface area. The principal mineral constituent is clay size smectite.

biodegradable: A substance able to be decomposed by biological processes.

biological availability: That portion of a chemical compound or element that can be taken up readily by living organisms.

biological interchange: The interchange of elements between organic and inorganic states in a soil or other substrate through the action of living organisms. It results from the biological decomposition of organic compounds with the liberation of inorganic materials (mineralization) and the utilization of inorganic materials with synthesis of microbial tissue (immobilization).

biomass: The total mass of living organisms in a given volume or mass of soil. The total weight of all organisms in a particular environment.

biosequence: A group of related soils that differ, one from the other, primarily because of differences in kinds and numbers of plants and soil organisms as a soil-forming factor.

birefringence: The numerical difference between the highest and lowest refractive index of a mineral. Minerals with birefringence exhibit interference colors in thin section when viewed with crossed-polarized light.

bisect: A profile of plants and soil showing the vertical and lateral distribution of roots and tops in their natural position.

bisequal: Soils in which two sequa have formed, one above the other, in the same deposit.

blocky soil structure: A shape of soil structure.

blown-out land: In soil survey a map-unit which is a type of miscellaneous area from which most of the soil has been removed by wind erosion. The areas are generally shallow depressions with flat, irregular floors, which in some instances have a layer of pebbles or cobbles.

blowout: A hollow or depression of the land surface, which is generally saucer or trough-shaped, formed by wind erosion especially in an area of shifting sand, loose soil, or where vegetation is disturbed or destroyed.

bog: A peat-accumulating wetland that has no significant inflows or outflows and supports acidophilic mosses, particularly Sphagnum.

boulders: Rock or mineral fragments >600 mm in diameter.

bradyrhizobia: Collective common name for the genus Bradyhizobium. (See also rhizobia.)

braided stream: A channel or stream with multiple channels that interweave as a result of repeated bifurcation and convergence of flow around interchannel bars, resembling (in plan view) the strands of a complex braid. Braiding is generally confined to broad, shallow streams of low sinuosity, high bedload, non-cohesive bank material, and a steep gradient.

breakthrough curve: The relative solute concentration in the outflow from a column of soil or porous medium after a step change in solute concentration has been applied to the inlet end of the column, plotted against the volume of outflow (often in number of pore volumes).

breccia: A coarse grained, clastic rock composed of angular fragments (>2mm) bonded by a mineral cement or in a finer-grained matrix of varying composition and origin.

bulk area: The total area, including solid particles and pores, of a cross-section through an arbitrary quantity of soil; the area counterpart of bulk volume.

bulk volume: The volume, including the solids and the pores, of an arbitrary soil mass. The bulk volume is determined before drying to constant weight at 105°C.

buried soil: Soil covered by an alluvial, loessal, or other surface mantle of more recent depositional material, usually to a depth greater than 50 cm.

capillary fringe: A zone in the soil just above the plane of zero gauge pressure that remains saturated or almost saturated with water. The extent can be inferred from the retentivity profile and depends upon the size-distribution of pores.

catabolism: The breakdown of organic compounds within an organism.

chambers: Vesicles or vughs connected by a channel or channels.

chemical potential: (i) The rate of change of Gibbs free energy, G, with respect to the number of moles of one component in a mixed chemical system at fixed temperature, pressure and number of moles of other components. (ii) The chemical potential of a component increases with increasing concentration or partial pressure.

chemical weathering: The breakdown of rocks and minerals due to the presence of water and other components in the soil solution or changes in redox potential.

chemostat: A device for the continuous culture of microorganisms in which growth rate and population size are regulated by the concentration of a limiting nutrient in incoming medium.

chemotaxis: The oriented movement of a motile organism with reference to a chemical agent. May be positive (toward) or negative (away) with respect to the chemical gradient.

class, soil: A group of soils defined as having a specific range in one or more particular property(ies) such as acidity, degree of slope, texture, structure, land-use capability, degree of erosion, or drainage.

clay: (i) A soil separate consisting of particles <0.002 mm in equivalent diameter. (See also soil separates.) (ii) A textural class. (iii) (In reference to clay mineralogy) A naturally occurring material composed primarily of fine-grained minerals, which is generally plastic at appropriate water contents and will harden when dried or fired. Although clay usually contains phyllosilicates, it may contain other materials that impart plasticity and harden when dried or fired. Associated phases in clay may include materials that do not impart plasticity and organic matter.

climatic index: A simple, single numerical value that expresses climatic relationships; for example, the numerical value obtained in Transeau's precipitation-evaporation ratio.

clod: A compact, coherent mass of soil varying in size, usually produced by plowing, digging, etc., especially when these operations are performed on soils that are either too wet or too dry and usually formed by compression, or breaking off from a larger unit, as opposed to a building-up action as in aggregation.

coating: A layer of a substance completely or partly covering a surface of soil material. Coatings include clay coatings, calcite coatings, gypsum coatings, organic coatings, salt coatings, etc.

cobblestones: Rounded or partially rounded rock or mineral fragments between 75 and 250 mm in diameter.

compaction: (i) To unite firmly; the act or process of becoming compact. (ii) (geology) The changing of loose sediment into hard, firm rock. (iii) (soil engineering) The process by which the soil grains are rearranged to decrease void space and bring them into closer contact with one another, thereby increasing the bulk density. (iv) (solid waste disposal) The reducing of the bulk of solid waste by rolling and tamping.

compost: Organic residues, or a mixture of organic residues and soil, that have been mixed, piled, and moistened, with or without addition of fertilizer and lime, and generally allowed to undergo thermophilic decomposition until the original organic materials have been substantially altered or decomposed. Sometimes called "artificial manure" or "synthetic manure." In Europe, the term may refer to a potting mix for container-grown plants.

composting: A controlled biological process which converts organic constituents, usually wastes, into humus-like material suitable for use as a soil amendment or organic fertilizer.

compressibility: The property of a soil pertaining to its susceptibility to decrease in bulk volume when subjected to a load.

concentration: The amount of suspended or dissolved particles, or elements in a unit volume or unit mass as specified at a given temperature and pressure.

concretion: (i) A cemented concentration of a chemical compound, such as calcium carbonate or iron oxide, that can be removed from the soil intact and that has crude internal symmetry organized around a point, line, or plane. (ii) (micromorphological) A glaebule with a generally concentric fabric about a center which may be a point, line, or a plane.

consistence: The attributes of soil material as expressed in degree of cohesion and adhesion or in resistance to deformation or rupture.

consistency: The manifestations of the forces of cohesion and adhesion acting within the soil at various water contents, as expressed by the relative ease with which a soil can be deformed or ruptured. Engineering descriptions include: (i) the designation of five inplace categories (soft, firm or medium, stiff, very stiff, and hard) as assessed by thumb and thumbnail penetrability and indentability; and (ii) characterization by the Atterberg limits (i.e., liquid limit, plastic limit, and plasticity number).

constant-charge surface: A mineral surface carrying a net electrical charge whose magnitude depends only on the structure and chemical composition of the mineral itself. constant charge surfaces in soils usually arise from isomorphous substitution in phyllosilicate clay structures.

constant-potential surface: Variable charge surfaces are called constant potential surfaces because at constant activity of the potential determining ion (e.g. constant pH) the electrical potential difference between the solid surface and the bulk solution is constant.

constructional surface: A land surface owing its origin and form to depositional processes, with little or no modification by erosion.

contrasting soil: A soil that does not share diagnostic criteria and does not behave or perform similar to the soil being compared.

convection: A process by which heat, solutes, or particles are transferred from one part of a fluid to another by movement of the fluid itself; also called advection.

creep: Slow mass movement of soil and soil material down slopes driven primarily by gravity, but facilitated by saturation with water and by alternate freezing and thawing.

crop rotation: A planned sequence of crops growing in a regularly recurring succession on the same area of land, as contrasted to continuous culture of one crop or growing a variable sequence of crops.

crumb structure: A structural condition in which most of the peds are crumbs.

crust: A transient soil-surface layer, ranging in thickness from a few millimeters to a few centimeters, that is either denser, structurally different or more cemented than the material immediately beneath it, resulting in greater soil strength when dry as measured by penetration resistance or other indices of soil strength.

cryic: A soil temperature regime that has mean annual soil temperatures of >0°C but <8°C, >5°C difference between mean summer and mean winter soil temperatures at 50 cm, and cold summer temperatures.

cumulative infiltration: Total volume of water infiltrated per unit area of soil surface during a specified time period. Contrast with infiltration flux (or rate).

cutan: A modification of the texture, structure, or fabric at natural surfaces in soil materials due to concentration of particular soil constituents or in situ modification of the plasma.

cyclic salt: Salt lifted by wind from the spray of sea water or salt lakes, blown inland, and returned to the originating water body via drainage.

degradation: (i) The process whereby a compound is transformed into simpler compounds.

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.

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

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

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

digestibility: (as applied to 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.

dispersion: (i) A term used in relation to solute movement. (ii) The break-down of soil aggregates into individual component particles.

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

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

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

divide: The line of separation, or the summit area, or narrow tract of higher ground that constitutes the watershed boundary between two adjacent drainage basins; it divides the surface waters that flow naturally in one direction from those that flow in the opposite direction.

drag: The force retarding the flow of a fluid over the surface of a solid body.

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

drumlin: A low, smooth, elongated oval hill, mound, or ridge of compact till that may or may not have a core of bedrock or stratified drift. 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

dryland farming: Crop production without irrigation (rainfed agriculture).

dumps: Areas of smooth or uneven accumulations or piles of waste rock or general refuse that without major reclamation are incapable of supporting plants.

dune: A low mound, ridge, bank or hill of loose, windblown, granular material (generally sand), either bare or covered with vegetation, capable of movement from place to place but always retaining its characteristic shape.

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.

↑ go to top ↑