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  • 1. Z03_BRAD9383_14_SE_GLOS.QXD 8/14/07 2:42 AM Page 926 GLOSSARY OF SOIL SCIENCE TERMS1 A horizon The surface horizon of a mineral soil having acidity, total The total acidity in a soil. It is approxi- maximum organic matter accumulation, maximum mated by the sum of the salt-replaceable acidity plus biological activity, and/or eluviation of materials such the residual acidity. as iron and aluminum oxides and silicate clays. Actinomycetes A group of bacteria that form abiotic Nonliving basic elements of the environment, branched mycelia that are thinner, but somewhat simi- such as rainfall, temperature, wind, and minerals. lar in appearance to fungal hyphae. Includes many members of the order Actinomycetales. accelerated erosion Erosion much more rapid than normal, natural, geological erosion; primarily as a result activated sludge Sludge that has been aerated and of the activities of humans or, in some cases, of animals. subjected to bacterial action. acid cations Cations, principally Al3+, and Fe3+, H+, active layer The upper portion of a Gelisol that is that contribute to H+ ion activity either directly or subject to freezing and thawing and is underlain by through hydrolysis reactions with water. See also permafrost. nonacid cations. active organic matter A portion of the soil organic acid rain Atmospheric precipitation with pH values less matter that is relatively easily metabolized by microor- than about 5.6, the acidity being due to inorganic acids ganisms and cycles with a half-life in the soil of a few (such as nitric and sulfuric) that are formed when oxides days to a few years. of nitrogen and sulfur are emitted into the atmosphere. adhesion Molecular attraction that holds the surfaces of acid saturation The proportion or percentage of a two substances (e.g., water and sand particles) in contact. cation-exchange site occupied by acid cations. adsorption The attraction of ions or compounds acid soil A soil with a pH value <7.0. Usually applied to the surface of a solid. Soil colloids adsorb large to surface layer or root zone, but may be used to char- amounts of ions and water. acterize any horizon. See also reaction, soil. adsorption complex The group of organic and inor- acid sulfate soils Soils that are potentially extremely ganic substances in soil capable of adsorbing ions and acid (pH < 3.5) because of the presence of large amounts molecules. of reduced forms of sulfur that are oxidized to sulfuric aerate To impregnate with gas, usually air. acid if the soils are exposed to oxygen when they are drained or excavated. A sulfuric horizon containing the aeration, soil The process by which air in the soil is yellow mineral jarosite is often present. See also cat clays. replaced by air from the atmosphere. In a well-aerated soil, the soil air is similar in composition to the atmo- acidity, active The activity of hydrogen ions in the sphere above the soil. Poorly aerated soils usually con- aqueous phase of a soil. It is measured and expressed as tain more carbon dioxide and correspondingly less a pH value. oxygen than the atmosphere above the soil. acidity, residual Soil acidity that can be neutralized by aerobic (1) Having molecular oxygen as a part of the lime or other alkaline materials but cannot be replaced environment. (2) Growing only in the presence of mo- by an unbuffered salt solution. lecular oxygen, as aerobic organisms. (3) Occurring acidity, salt replaceable Exchangeable hydrogen and only in the presence of molecular oxygen (said of cer- aluminum that can be replaced from an acid soil by an tain chemical or biochemical processes, such as aerobic unbuffered salt solution such as KCl or NaCl. decomposition). 1 This glossary was compiled and modified from several sources, including Glossary of Soil Science Terms [Madison, Wis.: Soil Sci. Soc. Amer. (1997)] Resource Conservation Glossary [Anheny, Iowa: Soil Cons. Soc. Amer. (1982)], and Soil Taxonomy [Washington, D.C.: U.S. Department of Agriculture (1999)]. 926
  • 2. Z03_BRAD9383_14_SE_GLOS.QXD 8/14/07 2:42 AM Page 927 aerosolic dust A type of eolian material that is very allophane A poorly defined aluminosilicate mineral fine (about 1 to 10 µm) and may remain suspended in whose structural framework consists of short runs of the air over distances of thousands of kilometers. Finer three-dimensional crystals interspersed with amorphous than most loess. noncrystalline materials. Along with its more weathered companion, it is prevalent in volcanic ash materials. aggregate (soil) Many soil particles held in a single mass or cluster, such as a clod, crumb, block, or prism. alluvial fan Fan-shaped alluvium deposited at the mouth of a canyon or ravine where debris-laden waters agric horizon A diagnostic subsurface horizon in fan out, slow down, and deposit their burden. which clay, silt, and humus derived from an overlying cultivated and fertilized layer have accumulated. alluvium A general term for all detrital material Wormholes and illuvial clay, silt, and humus occupy at deposited or in transit by streams, including gravel, least 5% of the horizon by volume. sand, silt, clay, and all variations and mixtures of these. agroforestry Any type of multiple cropping land-use Unless otherwise noted, alluvium is unconsolidated. that entails complementary relations between trees alpha particle A positively charged particle (consisting and agricultural crops. of two protons and two neutrons) that is emitted by agronomy A specialization of agriculture concerned with certain radioactive compounds. the theory and practice of field-crop production and soil aluminosilicates Compounds containing aluminum, management. The scientific management of land. silicon, and oxygen as main constituents. An example air-dry (1) The state of dryness (of a soil) at equilibrium is microcline, KAlSi3O8. with the moisture content in the surrounding atmo- amendment, soil Any substance other than fertilizers, sphere. The actual moisture content will depend upon such as lime, sulfur, gypsum, and sawdust, used to alter the relative humidity and the temperature of the sur- the chemical or physical properties of a soil, generally rounding atmosphere. (2) To allow to reach equilibrium to make it more productive. in moisture content with the surrounding atmosphere. amino acids Nitrogen-containing organic acids that air porosity The proportion of the bulk volume of soil couple together to form proteins. Each acid molecule that is filled with air at any given time or under a given contains one or more amino groups (—NH2) and at condition, such as a specified moisture potential; usu- least one carboxyl group (—COOH). In addition, some ally the large pores. amino acids contain sulfur. albic horizon A diagnostic subsurface horizon from ammonification The biochemical process whereby which clay and free iron oxides have been removed or ammoniacal nitrogen is released from nitrogen- in which the oxides have been segregated to the extent containing organic compounds. that the color of the horizon is determined primarily by the color of the primary sand and silt particles ammonium fixation The entrapment of ammonium rather than by coatings on these particles. ions by the mineral or organic fractions of the soil in forms that are insoluble in water and are at least tem- Alfisols An order in Soil Taxonomy. Soils with gray to porarily nonexchangeable. brown surface horizons, medium to high supply of bases, and B horizons of illuvial clay accumulation. amorphous material Noncrystalline constituents of soils. These soils form mostly under forest or savanna vegeta- anaerobic (i) The absence of molecular oxygen. tion in climates with slight to pronounced seasonal (ii) Growing or occurring in the absence of molecular moisture deficit. oxygen (e.g., anaerobic bacteria or biochemical reduction algal bloom A population explosion of algae in surface reaction). waters, such as lakes and streams, often resulting in anaerobic respiration The metabolic process whereby high turbidity and green- or red-colored water, and electrons are transferred from a reduced compound commonly stimulated by nutrient enrichment with (usually organic) to an inorganic acceptor molecule phosphorus and nitrogen. other than oxygen. alkaline soil Any soil that has pH > 7. Usually applied andic properties Soil properties related to volcanic ori- to the surface layer or root zone but may be used to gin of materials, including high organic carbon con- characterize any horizon or a sample thereof. See also tent, low bulk density, high phosphate retention, and reaction, soil. extractable iron and aluminum. allelochemical An organic chemical by which one Andisols An order in Soil Taxonomy. Soils developed plant can influence another. See allelopathy. from volcanic ejecta. The colloidal fraction is domi- allelopathy The process by which one plant may nated by allophane and/or Al-humus compounds. affect other plants by biologically active chemicals angle of repose The maximum slope steepness at introduced into the soil, either directly by leaching or which loose, cohesionless material will come to rest. exudation from the source plant, or as a result of the decay of the plant residues. The effects, though usually anion Negatively charged ion; during electrolysis it is negative, may also be positive. attracted to the positively charged anode. GLOSSARY 927
  • 3. Z03_BRAD9383_14_SE_GLOS.QXD 8/14/07 2:42 AM Page 928 anion exchange Exchange of anions in the soil solu- regions annual precipitation is usually less than 25 cm. tion for anions adsorbed on the surface of clay and It may be as high as 50 cm in tropical regions. Natural humus particles. vegetation is desert shrubs. anion exchange capacity The sum total of exchange- Aridisols An order in Soil Taxonomy. Soils of dry cli- able anions that a soil can adsorb. Expressed as centi- mates. They have pedogenic horizons, low in organic moles of charge per kilogram (cmolc/kg) of soil (or of matter, that are never moist for as long as three consec- other adsorbing material, such as clay). utive months. They have an ochric epipedon and one or more of the following diagnostic horizons: argillic, anoxic See anaerobic. natric, cambic, calcic, petrocalcic, gypsic, petrogypsic, anthropic epipedon A diagnostic surface horizon of salic, or a duripan. mineral soil that has the same requirements as the mol- lic epipedon but that has more than 250 mg/kg of P2O5 aspect (of slopes) The direction (e.g., south or north) that a slope faces with respect to the sun. soluble in 1% citric acid, or is dry more than 10 months (cumulative) during the period when not irrigated. association, soil See soil association. The anthropic epipedon forms under long-continued cultivation and fertilization. Atterberg limits Water contents of fine-grained soils at different states of consistency. antibiotic A substance produced by one species of liquid limit (LL) The water content corresponding organism that, in low concentrations, will kill or to the arbitrary limit between the liquid and plastic inhibit growth of certain other organisms. states of consistency of a soil. Ap The surface layer of a soil disturbed by cultivation plastic limit (PL) The water content corresponding or pasturing. to an arbitrary limit between the plastic and semi- solid states of consistency of a soil. apatite A naturally occurring complex calcium phos- phate that is the original source of most of the phosphate autochthonous organisms Those microorganisms thought fertilizers. Formulas such as [3Ca3(PO4)2] · CaF2 illustrate to subsist on the more resistant soil organic matter and lit- the complex compounds that make up apatite. tle affected by the addition of fresh organic materials. Contrast with zymogenous organisms. See also k-strategist. aquic conditions Continuous or periodic saturation (with water) and reduction, commonly indicated by autotroph An organism capable of utilizing carbon redoximorphic features. dioxide or carbonates as the sole source of carbon and obtaining energy for life processes from the oxidation aquiclude A saturated body of rock or sediment that is of inorganic elements or compounds such as iron, sul- incapable of transmitting significant quantities of fur, hydrogen, ammonium, and nitrites, or from radi- water under ordinary water pressures. ant energy. Contrast with heterotroph. aquifer A saturated, permeable layer of sediment or available nutrient That portion of any element or rock that can transmit significant quantities of water compound in the soil that can be readily absorbed and under normal pressure conditions. assimilated by growing plants. (“Available” should not arbuscular mycorrhiza A common endomycorrhizal be confused with “exchangeable.”) association produced by phycomycetous fungi and available water The portion of water in a soil that can characterized by the development, within root cells, of be readily absorbed by plant roots. The amount of small structures known as arbuscules. Some also form, water released between the field capacity and the per- between root cells, storage organs known as vesicles. manent wilting point. Host range includes many agricultural and horticultural crops. Formerly called vesicular arbuscular mycorrhiza B horizon A soil horizon, usually beneath the A or E (VAM). See also endotrophic mycorrhiza. horizon, that is characterized by one or more of the following: (1) a concentration of soluble salts, silicate arbuscule Specialized branched structure formed within clays, iron and aluminum oxides, and humus, alone a root cortical cell by endotrophic mycorrhizal fungi. or in combination; (2) a blocky or prismatic structure; Archaea One of the two domains of single-celled and (3) coatings of iron and aluminum oxides that prokaryote microorganisms. Includes organisms adapted give darker, stronger, or redder color. to extremes of salinity and heat, and those that subsist Bacteria One of two domains of single-celled on methane. Similar appearing, but evolutionarily dis- prokaryote microorganisms. Includes all that are not tinct from bacteria. Archaea. argillan A thin coating of well-oriented clay particles on bar A unit of pressure equal to 1 million dynes per the surface of a soil aggregate, particle, or pore. A clay film. square centimeter (106 dynes/cm2). It approximates the argillic horizon A diagnostic subsurface horizon char- pressure of a standard atmosphere. acterized by the illuvial accumulation of layer-lattice base-forming cations (Obsolete) Those cations that silicate clays. form strong (strongly dissociated) bases by reaction arid climate Climate in regions that lack sufficient with hydroxyl; e.g., K+ forms potassium hydroxide (K+ + moisture for crop production without irrigation. In cool OH). See nonacid cations. 928 Glossary
  • 4. Z03_BRAD9383_14_SE_GLOS.QXD 8/14/07 2:42 AM Page 929 base saturation percentage The extent to which the broadcast Scatter seed or fertilizer on the surface of adsorption complex of a soil is saturated with the soil. exchangeable cations other than hydrogen and alu- brownfields Abandoned, idled, or underused industrial minum. It is expressed as a percentage of the total and commercial facilities where expansion or redevelop- cation exchange capacity. See nonacid saturation. ment is complicated by real or perceived environmental bedding (Engineering) Arranging the surface of fields contamination. by plowing and grading into a series of elevated beds buffering capacity The ability of a soil to resist separated by shallow depressions or ditches for drainage. changes in pH. Commonly determined by presence of bedrock The solid rock underlying soils and the clay, humus, and other colloidal materials. regolith in depths ranging from zero (where exposed bulk blended fertilizers Solid fertilizer materials blended by erosion) to several hundred feet. together in small blending plants, delivered to the farm bench terrace An embankment constructed across in bulk, and usually spread directly on the fields by truck sloping fields with a steep drop on the downslope side. or other special applicator. beta particle A high-speed electron emitted in bulk blending Mixing dry individual granulated fertil- radioactive decay. izer materials to form a mixed fertilizer that is applied promptly to the soil. bioaccumulation A buildup within an organism of spe- cific compounds due to biological processes. Commonly bulk density, soil The mass of dry soil per unit of applied to heavy metals, pesticides, or metabolites. bulk volume, including the air space. The bulk vol- ume is determined before drying to constant weight bioaugmentation The cleanup of contaminated soils at 105 °C. by adding exotic microorganisms that are especially efficient at breaking down an organic contaminant. A buried soil Soil covered by an alluvial, loessal, or other form of bioremediation. deposit, usually to a depth greater than the thickness of the solum. biodegradable Subject to degradation by biochemical processes. by-pass flow See preferential flow. biological nitrogen fixation Occurs at ordinary temper- C horizon A mineral horizon, generally beneath the atures and pressures. It is commonly carried out by cer- solum, that is relatively unaffected by biological activ- tain bacteria, algae, and actinomycetes, which may or ity and pedogenesis and is lacking properties diagnos- may not be associated with higher plants. tic of an A or B horizon. It may or may not be like the material from which the A and B have formed. biomass The total mass of living material of a speci- fied type (e.g., microbial biomass) in a given environ- calcareous soil Soil containing sufficient calcium car- ment (e.g., in a cubic meter of soil). bonate (often with magnesium carbonate) to effervesce visibly when treated with cold 0.1 N hydrochloric acid. biopores Soil pores, usually of relatively large diame- ter, created by plant roots, earthworms, or other soil calcic horizon A diagnostic subsurface horizon of sec- organisms. ondary carbonate enrichment that is more than 15 cm thick, has a calcium carbonate equivalent of more than bioremediation The decontamination or restoration of 15%, and has at least 5% more calcium carbonate polluted or degraded soils by means of enhancing the equivalent than the underlying C horizon. chemical degradation or other activities of soil organisms. biosequence A group of related soils that differ, one caliche A layer near the surface, more or less cemented by secondary carbonates of calcium or mag- from the other, primarily because of differences in nesium precipitated from the soil solution. It may kinds and numbers of plants and soil organisms as a occur as a soft, thin soil horizon; as a hard, thick bed soil-forming factor. just beneath the solum; or as a surface layer exposed by biosolids Sewage sludge that meets certain regulatory erosion. standards, making it suitable for land application. See sewage sludge. cambic horizon A diagnostic subsurface horizon that has a texture of loamy very fine sand or finer, contains biostimulation The cleanup of contaminated soils some weatherable minerals, and is characterized by the through the manipulation of nutrients or other soil alteration or removal of mineral material. The cambic environmental factors to enhance the activity of nat- horizon lacks cementation or induration and has too urally occurring soil microorganisms. A form of few evidences of illuviation to meet the requirements bioremediation. of the argillic or spodic horizon. blocky soil structure Soil aggregates with blocklike capillary conductivity (Obsolete) See hydraulic shapes; common in B horizons of soils in humid regions. conductivity. broad-base terrace A low embankment with such capillary fringe A zone in the soil just above the plane gentle slopes that it can be farmed, constructed across of zero water pressure (water table) that remains satu- sloping fields to reduce erosion and runoff. rated or almost saturated with water. GLOSSARY 929
  • 5. Z03_BRAD9383_14_SE_GLOS.QXD 8/14/07 2:42 AM Page 930 capillary water The water held in the capillary or chlorosis A condition in plants relating to the failure small pores of a soil, usually with a tension >60 cm of of chlorophyll (the green coloring matter) to develop. water. See also soil water potential. Chlorotic leaves range from light green through yellow to almost white. carbon cycle The sequence of transformations whereby carbon dioxide is fixed in living organisms by photosyn- chroma (color) See Munsell color system. thesis or by chemosynthesis, liberated by respiration chronosequence A sequence of related soils that differ, and by the death and decomposition of the fixing one from the other, in certain properties primarily as a organism, used by heterotrophic species, and ultimately result of time as a soil-forming factor. returned to its original state. classification, soil See soil classification. carbon/nitrogen ratio The ratio of the weight of organic carbon (C) to the weight of total nitrogen (N) clay (1) A soil separate consisting of particles <0.002 mm in a soil or in organic material. in equivalent diameter. (2) A soil textural class containing >40% clay, <45% sand, and <40% silt. carnivore An organism that feeds on animals. clay mineral Naturally occurring inorganic material casts, earthworm Rounded, water-stable aggregates of (usually crystalline) found in soils and other earthy soil that have passed through the gut of an earthworm. deposits, the particles being of clay size, that is, cat clays Wet clay soils high in reduced forms of sul- <0.002 mm in diameter. fur that, upon being drained, become extremely acid claypan A dense, compact, slowly permeable layer in because of the oxidation of the sulfur compounds and the subsoil having a much higher clay content than the formation of sulfuric acid. Usually found in tidal the overlying material, from which it is separated by a marshes. See acid sulfate soils. sharply defined boundary. Claypans are usually hard catena A group of soils that commonly occur together when dry and plastic and sticky when wet. See also in a landscape, each characterized by a different slope hardpan. position and resulting set of drainage-related propri- climosequence A group of related soils that differ, one eties. See also toposequence. from another, primarily because of differences in cli- cation A positively charged ion; during electrolysis it mate as a soil-forming factor. is attracted to the negatively charged cathode. clod A compact, coherent mass of soil produced artifi- cation exchange The interchange between a cation cially, usually by such human activities as plowing and in solution and another cation on the surface of any digging, especially when these operations are per- surface-active material, such as clay or organic matter. formed on soils that are either too wet or too dry for normal tillage operations. cation exchange capacity The sum total of exchange- able cations that a soil can adsorb. Sometimes called coarse fragments Mineral (rock) soil particles larger total-exchange capacity, base-exchange capacity, or cation- than 2 mm in diameter. Compare to fine earth fraction. adsorption capacity. Expressed in centimoles of charge coarse texture The texture exhibited by sands, loamy per kilogram (cmolc/kg) of soil (or of other adsorbing sands, and sandy loams (except very fine sandy loam). material, such as clay). cobblestone Rounded or partially rounded rock or cemented Indurated; having a hard, brittle consis- mineral fragments 7.5 to 25 cm (3 to 10 in.) in diameter. tency because the particles are held together by cementing substances, such as humus, calcium carbon- co-composting A method of composting in which two ate, or the oxides of silicon, iron, and aluminum. materials of differing but complementary nature are mingled together and enhance each other’s decompo- channery Thin, flat fragments of limestone, sand- sition in a compost system. stone, or schist up to 15 cm (6 in.) in major diameter. cohesion Holding together: force holding a solid or chelate (Greek, claw) A type of chemical compound in liquid together, owing to attraction between like mole- which a metallic ion is firmly combined with an cules. Decreases with rise in temperature. organic molecule by means of multiple chemical bonds. collapsible soil Certain soil that may undergo a sud- chert A structureless form of silica, closely related to den loss in strength when wetted. flint, that breaks into angular fragments. colloid, soil (Greek, gluelike) Organic and inorganic chisel, subsoil A tillage implement with one or more matter with very small particle size and a correspond- cultivator-type feet to which are attached strong knife- ingly large surface area per unit of mass. like units used to shatter or loosen hard, compact lay- ers, usually in the subsoil, to depths below normal colluvium A deposit of rock fragments and soil mate- plow depth. See also subsoiling. rial accumulated at the base of steep slopes as a result of gravitational action. chlorite A 2:1:1-type layer-structured silicate mineral having 2:1 layers alternating with a magnesium- color The property of an object that depends on the dominated octahedral sheet. wavelength of light it reflects or emits. 930 Glossary
  • 6. Z03_BRAD9383_14_SE_GLOS.QXD 8/14/07 2:42 AM Page 931 columnar soil structure See soil structure types. repeated compaction of the soil does not occur outside the selected paths. companion planting The practice of growing certain species of plants in close proximity because one species convection The transfer of heat through a gas or solu- has the effect of improving the growth of the other, tion because of molecular movement. sometimes by positive allelopathic effects. cover crop A close-growing crop grown primarily for compost Organic residues, or a mixture of organic the purpose of protecting and improving soil between residues and soil, that have been piled, moistened, and periods of regular crop production or between trees allowed to undergo biological decomposition. Mineral and vines in orchards and vineyards. fertilizers are sometimes added. Often called artificial creep Slow mass movement of soil and soil material manure or synthetic manure if produced primarily from down relatively steep slopes, primarily under the influ- plant residues. ence of gravity, but facilitated by saturation with water concretion A local concentration of a chemical com- and by alternate freezing and thawing. pound, such as calcium carbonate or iron oxide, in the crop rotation A planned sequence of crops growing in form of grains or nodules of varying size, shape, hard- a regularly recurring succession on the same area of ness, and color. land, as contrasted to continuous culture of one crop conduction The transfer of heat by physical contact or growing different crops in haphazard order. between two or more objects. crotovina A former animal burrow in one soil horizon that has been filled with organic matter or material conductivity, hydraulic See hydraulic conductivity. from another horizon (also spelled krotovina). conservation tillage See tillage, conservation. crumb A soft, porous, more or less rounded natural consistence The combination of properties of soil unit of structure from 1 to 5 mm in diameter. See also material that determine its resistance to crushing and soil structure types. its ability to be molded or changed in shape. Such crushing strength The force required to crush a mass terms as loose, friable, firm, soft, plastic, and sticky of dry soil or, conversely, the resistance of the dry soil describe soil consistence. mass to crushing. Expressed in units of force per unit consistency The interaction of adhesive and cohesive area (pressure). forces within a soil at various moisture contents as crust (soil) (i) physical A surface layer on soils, ranging expressed by the relative ease with which the soil can in thickness from a few millimeters to as much as 3 cm, be deformed or ruptured. that physical-chemical processes have caused to be consociation See soil consociation. much more compact, hard, and brittle when dry than the material immediately beneath it. consolidation test A laboratory test in which a soil mass (ii) microbiotic An assemblage of cyanobacteria, is laterally confined within a ring and is compressed with algae, lichens, liverworts, and mosses that commonly a known force between two porous plates. forms an irregular crust on the soil surface, especially constant charge The net surface charge of mineral on otherwise barren, arid-region soils. Also referred to particles, the magnitude of which depends only on the as cryptogamic, cryptobiotic, or biological crusts. chemical and structural composition of the mineral. cryophilic Pertaining to low temperatures in the range The charge arises from isomorphous substitution and is of 5 to 15°C, the range in which cryophilic organisms not affected by soil pH. grow best. consumptive use The water used by plants in transpi- cryoturbation Physical disruption and displace- ration and growth, plus water vapor loss from adjacent ment of soil material within the profile by the forces of soil or snow, or from intercepted precipitation in any freezing and thawing. Sometimes called frost churning, specified time. Usually expressed as equivalent depth it results in irregular, broken horizons, involutions, of free water per unit of time. oriented rock fragments, and accumulation of organic contour An imaginary line connecting points of matter on the permafrost table. equal elevation on the surface of the soil. A contour cryptogam See crust (ii) microbiotic. terrace is laid out on a sloping soil at right angles to the direction of the slope and nearly level throughout crystal A homogeneous inorganic substance of defi- its course. nite chemical composition bounded by planar surfaces that form definite angles with each other, thus giving contour strip-cropping Layout of crops in compara- the substance a regular geometrical form. tively narrow strips in which the farming operations are performed approximately on the contour. Usually crystal structure The orderly arrangement of atoms in strips of grass, close-growing crops, or fallow are a crystalline material. alternated with those of cultivated crops. cultivation A tillage operation used in preparing land controlled traffic A farming system in which all for seeding or transplanting or later for weed control wheeled traffic is confined to fixed paths so that and for loosening the soil. GLOSSARY 931
  • 7. Z03_BRAD9383_14_SE_GLOS.QXD 8/15/07 4:59 PM Page 932 cutans A modification of the texture, structure, or diatomaceous earth A geologic deposit of fine, gray- fabric at natural surfaces in soil materials due to ish, siliceous material composed chiefly or wholly of concentration of particular soil constituents; e.g. “clay the remains of diatoms. It may occur as a powder or as skins.” a porous, rigid material. cyanobacteria Chlorophyll-containing bacteria that diatoms Algae having siliceous cell walls that persist as accommodate both photosynthesis and nitrogen fixa- a skeleton after death; any of the microscopic unicellular tion. Formerly called blue-green algae. or colonial algae constituting the class Bacillariaceae. They occur abundantly in fresh and salt waters and their deciduous plant A plant that sheds all its leaves every remains are widely distributed in soils. year at a certain season. diffusion The movement of atoms in a gaseous mix- decomposition Chemical breakdown of a compound ture or of ions in a solution, primarily as a result of (e.g., a mineral or organic compound) into simpler their own random motion. compounds, often accomplished with the aid of microorganisms. dioctahedral sheet An octahedral sheet of silicate clays in which the sites for the six-coordinated metallic atoms deflocculate (1) To separate the individual compo- are mostly filled with trivalent atoms, such as A13+. nents of compound particles by chemical and/or phys- ical means. (2) To cause the particles of the disperse disintegration Physical or mechanical breakup or sep- phase of a colloidal system to become suspended in the aration of a substance into its component parts (e.g., a dispersion medium. rock breaking into its mineral components). delineation An individual polygon shown by a closed disperse (1) To break up compound particles, such as boundary on a soil map that defines the area, shape, aggregates, into the individual component particles. and location of a map unit within a landscape. (2) To distribute or suspend fine particles, such as clay, in or throughout a dispersion medium, such as water. delivery ratio The ratio of eroded sediment carried out of a drainage basin to the total amount of sediment dissolution Process by which molecules of a gas, solid, moved within the basin by erosion processes. or another liquid dissolve in a liquid, thereby becom- ing completely and uniformly dispersed throughout delta An alluvial deposit formed where a stream or the liquid’s volume. river drops its sediment load upon entering a quieter body of water. distribution coefficient (Kd) The distribution of a chemical between soil and water. denitrification The biochemical reduction of nitrate or nitrite to gaseous nitrogen, either as molecular nitro- diversion terrace See terrace. gen or as an oxide of nitrogen. drain (1) To provide channels, such as open ditches or density See particle density; bulk density. drain tile, so that excess water can be removed by sur- face or by internal flow. (2) To lose water (from the soil) desalinization Removal of salts from saline soil, usu- by percolation. ally by leaching. drain field, septic tank An area of soil into which the desert crust A hard layer, containing calcium carbon- effluent from a septic tank is piped so that it will drain ate, gypsum, or other binding material, exposed at the through the lower part of the soil profile for disposal surface in desert regions. and purification. desert pavement A natural residual concentration of drainage, soil The frequency and duration of periods closely packed pebbles, boulders, and other rock frag- when the soil is free from saturation with water. ments on a desert surface where wind and water action has removed all smaller particles. drift Material of any sort deposited by geological processes in one place after having been removed from desert varnish A thin, dark, shiny film or coating of another. Glacial drift includes material moved by the iron oxide and lesser amounts of manganese oxide and glaciers and by the streams and lakes associated with silica formed on the surfaces of pebbles, boulders, rock them. fragments, and rock outcrops in arid regions. drumlin Long, smooth, cigar-shaped low hills of desorption The removal of sorbed material from glacial till, with their long axes parallel to the direction surfaces. of ice movement. detritivore An organism that subsists on detritus. dryland farming The practice of crop production in detritus Debris from dead plants and animals. low-rainfall areas without irrigation. diagnostic horizons (As used in Soil Taxonomy): duff The matted, partly decomposed organic surface layer of forest soils. Horizons having specific soil characteristics that are indicative of certain classes of soils. Horizons that duripan A diagnostic subsurface horizon that is occur at the soil surface are called epipedons; those cemented by silica, to the point that air-dry fragments below the surface, diagnostic subsurface horizons. will not slake in water or HCL. Hardpan. 932 Glossary
  • 8. Z03_BRAD9383_14_SE_GLOS.QXD 8/15/07 4:59 PM Page 933 dust mulch A loose, finely granular or powdery condi- endotrophic mycorrhiza (endomycorrhiza) A symbiotic tion on the surface of the soil, usually produced by association of the mycelium of fungi and roots of a shallow cultivation. variety of plants in which the fungal hyphae penetrate directly into root hairs, other epidermal cells, and occa- E horizon Horizon characterized by maximum illuvia- sionally into cortical cells. Individual hyphae also tion (washing out) of silicate clays and iron and alu- extend from the root surface outward into the sur- minum oxides; commonly occurs above the B horizon rounding soil. See also arbuscular mycorrhiza. and below the A horizon. enrichment ratio The concentration of a substance earthworms Animals of the Lumbricidae family that (e.g., phosphorus) in eroded sediment divided by burrow into and live in the soil. They mix plant its concentration in the source soil prior to being residues into the soil and improve soil aeration. eroded. ecosystem A dynamic and interacting combination of Entisols An order in Soil Taxonomy. Soils that have all the living organisms and nonliving elements (mat- no diagnostic pedogenic horizons. They may be found ter and energy) of an area. in virtually any climate on very recent geomorphic ecosystem services Products of natural ecosystems that surfaces. support and fulfill the needs of human beings. Provision eolian soil material Soil material accumulated through of clean water and unpolluted air are examples. wind action. The most extensive areas in the United ectotrophic mycorrhiza (ectomycorrhiza) A symbiotic States are silty deposits (loess), but large areas of sandy association of the mycelium of fungi and the roots of deposits also occur. certain plants in which the fungal hyphae form a com- epiaquic (episaturation) A condition in which the soil pact mantle on the surface of the roots and extend into is saturated with water due to a perched water table in the surrounding soil and inward between cortical cells, one or more layers within 200 cm of the mineral soil but not into these cells. Associated primarily with cer- surface, implying that there are also one or more unsat- tain trees. See also endotrophic mycorrhiza. urated layers within 200 cm below the saturate layer. edaphology The science that deals with the influence See also endoaquic. of soils on living things, particularly plants, including epipedon A diagnostic surface horizon that includes human use of land for plant growth. the upper part of the soil that is darkened by organic effective cation exchange capacity The amount of matter, or the upper eluvial horizons, or both. (Soil cation charges that a material (usually soil or soil col- Taxonomy.) loids) can hold at the pH of the material, measured as equilibrium phosphorus concentration The concentra- the sum of the exchangeable Al3+, Ca2+, Mg2+, K+, and tion of phosphorus in a solution in equilibrium with a Na+, and expressed as moles or cmol of charge per kg of soil, the EPC0 being the concentration of phosphorus material. See cation exchange capacity. achieved by desorption of phosphorus from a soil to effective precipitation That portion of the total pre- phosphorus-free distilled water. cipitation that becomes available for plant growth or erosion (1) The wearing away of the land surface by for the promotion of soil formation. running water, wind, ice, or other geological agents, Eh In soils, it is the potential created by oxidation- including such processes as gravitational creep. reduction reactions that take place on the surface of (2) Detachment and movement of soil or rock by a platinum electrode measured against a reference water, wind, ice, or gravity. electrode, minus the Eh of the reference electrode. esker A narrow ridge of gravelly or sandy glacial This is a measure of the oxidation-reduction poten- material deposited by a stream in an ice-walled valley tial of electrode-reactive components in the soil. See or tunnel in a receding glacier. also pe. essential element A chemical element required for the electrical conductivity (EC) The capacity of a sub- normal growth of plants. stance to conduct or transmit electrical current. In soils or water, measured in siemens/meter (or often dS/m), eukaryote An organism whose cells each have a vis- and related to dissolved solutes. ibly evident nucleus. eluviation The removal of soil material in suspension eutrophic Having concentrations of nutrients optimal (or nearly so) for plant or animal growth. (Said of algal- (or in solution) from a layer or layers of a soil. Usually, enriched bodies of water) the loss of material in solution is described by the term “leaching.” See also illuviation and leaching. eutrophication Nutrient enrichment of lakes, ponds, and other such waters that stimulates the growth of endoaquic (endosaturation) A condition or moisture aquatic organisms, which leads to a deficiency of oxy- regime in which the soil is saturated with water in all gen in the water body. layers from the upper boundary of saturation (water table) to a depth of 200 cm or more from the mineral evapotranspiration The combined loss of water from a soil surface. See also epiaquic. given area, and during a specified period of time, by GLOSSARY 933
  • 9. Z03_BRAD9383_14_SE_GLOS.QXD 8/14/07 2:42 AM Page 934 evaporation from the soil surface and by transpiration fertilizer Any organic or inorganic material of natural from plants. or synthetic origin added to a soil to supply certain ele- ments essential to the growth of plants. exchange capacity The total ionic charge of the adsorption complex active in the adsorption of ions. fibric materials See organic soil materials. See also anion exchange capacity; cation exchange capacity. field capacity (field moisture capacity) The percentage of water remaining in a soil two or three days after its exchangeable ions Positively or negatively charged having been saturated and after free drainage has prac- atoms or groups of atoms that are held on or near the tically ceased. surface of a solid particle by attraction to charges of the opposite sign, and which may be replaced by other fine earth fraction That portion of the soil that passes like-charged ions in the soil solution. through a 2 mm diameter sieve opening. Compare to coarse fragments. exchangeable sodium percentage The extent to which the adsorption complex of a soil is occupied by fine texture Consisting of or containing large quanti- sodium. It is expressed as follows: ties of the fine fractions, particularly of silt and clay. exchangeable sodium 1cmolc>kg soil2 (Includes clay loam, sandy clay loam, silty clay loam, sandy clay, silty clay, and clay textural classes.) cation exchange capacity 1cmolc>kg soil2 ESP = * 100 fine-grained mica A silicate clay having a 2:1-type lat- tice structure with much of the silicon in the tetrahe- exfoliation Peeling away of layers of a rock from dral sheet having been replaced by aluminum and with the surface inward, usually as the result of expan- considerable interlayer potassium, which binds the sion and contraction that accompany changes in layers together, prevents interlayer expansion and temperature. swelling, and limits interlayer cation exchange capacity. expansive soil Soil that undergoes significant volume fixation (1) For other than elemental nitrogen: the change upon wetting and drying, usually because of a process or processes in a soil by which certain chemical high content of swelling-type clay minerals. elements are converted from a soluble or exchangeable external surface The area of surface exposed on the form to a much less soluble or to a nonexchangeable top, bottom, and sides of a clay crystal. form; for example, potassium, ammonium, and phos- phorus fixation. (2) For elemental nitrogen: process by facultative organism An organism capable of both aer- which gaseous elemental nitrogen is chemically com- obic and anaerobic metabolism. bined with hydrogen to form ammonia. See biological fallow Cropland left idle in order to restore produc- nitrogen fixation. tivity, mainly through accumulation of nutrients, flagstone A relatively thin rock or mineral fragment water, and/or organic matter. Preceding a cereal grain 15 to 38 cm in length commonly composed of shale, crop in semiarid regions, land may be left in summer slate, limestone, or sandstone. fallow for a period during which weeds are controlled by chemicals or tillage and water is allowed to accu- flocculate To aggregate or clump together individual, mulate in the soil profile. In humid regions, fallow tiny soil particles, especially fine clay, into small land may be allowed to grow up in natural vegetation clumps or floccules. Opposite of deflocculate or disperse. for a period ranging from a few months to many years. floodplain The land bordering a stream, built up of Improved fallow involves the purposeful establishment sediments from overflow of the stream and subject to of plant species capable of restoring soil productivity inundation when the stream is at flood stage. Sometimes more rapidly than a natural plant succession. called bottomland. family, soil In Soil Taxonomy, one of the categories inter- flora The sum total of the kinds of plants in an area at mediate between the great group and the soil series. one time. The organisms loosely considered to be of Families are defined largely on the basis of physical and the plant kingdom. mineralogical properties of importance to plant growth. fluorapatite A member of the apatite group of miner- fauna The animal life of a region or ecosystem. als containing fluorine. Most common mineral in fen A calcium-rich, peat-accumulating wetland with phosphate rock. relatively stagnant water. fluvial deposits Deposits of parent materials laid down ferrihydrite, Fe5HO8 # 4H2O A dark reddish brown by rivers or streams. poorly crystalline iron oxide that forms in wet soils. fluvioglacial See glaciofluvial deposits. fertigation The application of fertilizers in irrigation foliar diagnosis An estimation of mineral nutrient waters, commonly through sprinkler systems. deficiencies (excesses) of plants based on examination fertility, soil The quality of a soil that enables it to of the chemical composition of selected plant parts, provide essential chemical elements in quantities and and the color and growth characteristics of the foliage proportions for the growth of specified plants. of the plants. 934 Glossary
  • 10. Z03_BRAD9383_14_SE_GLOS.QXD 8/15/07 4:59 PM Page 935 food web The community of organisms that relate to genesis, soil The mode of origin of the soil, with special one another by sharing and passing on food substances. reference to the processes responsible for the develop- They are organized into trophic levels such as producers ment of the solum, or true soil, from the unconsolidated that create organic substances from sunlight and inor- parent material. ganic matter, to consumers and predators that eat the pro- genetic horizon Soil layers that resulted from soil- ducers, dead organisms, waste products and each other. forming (pedogenic) processes, as opposed to sedimen- forest floor The forest soil O horizons, including litter tation or other geologic processes. and unincorporated humus, on the mineral soil surface. geographic information system (GIS) A method of fraction A portion of a larger store of a substance overlaying, statistically analyzing, and integrating operationally defined by a particular analysis or separa- large volumes of spatial data of different kinds. The tion method. For example, the fulvic acid fraction of data are referenced to geographical coordinates and soil organic matter is defined by a series of laboratory encoded in a form suitable for handling by computer. procedures by which it is solubilized. Compare to pool. geological erosion Wearing away of the Earth’s surface fragipan Dense and brittle pan or subsurface layer in by water, ice, or other natural agents under natural soils that owes its hardness mainly to extreme density environmental conditions of climate, vegetation, and or compactness rather than high clay content or so on, undisturbed by man. Synonymous with natural cementation. Removed fragments are friable, but the erosion. material in place is so dense that roots penetrate and gibbsite, Al(OH)3 An aluminum trihydroxide mineral water moves through it very slowly. most common in highly weathered soils, such as Oxisols. friable A soil consistency term pertaining to soils that gilgai The microrelief of soils produced by expansion crumble with ease. and contraction with changes in moisture. Found in frigid A soil temperature class with mean annual tem- soils that contain large amounts of clay that swells and perature below 8° C. shrinks considerably with wetting and drying. Usually a succession of microbasins and microknolls in nearly fritted micronutrients Sintered silicates having total level areas or of microvalleys and microridges parallel guaranteed analyses of micronutrients with controlled to the direction of the slope. (relatively slow) release characteristics. glacial drift Rock debris that has been transported by fulvic acid A term of varied usage but usually referring to glaciers and deposited, either directly from the ice or the mixture of organic substances remaining in solution from the meltwater. The debris may or may not be het- upon acidification of a dilute alkali extract from the soil. erogeneous. functional diversity The characteristic of an ecosystem glacial till See till. exemplified by the capacity to carry out a large number of biochemical transformations and other functions. glaciofluvial deposits Material moved by glaciers and subsequently sorted and deposited by streams flowing functional group An atom, or group of atoms, from the melting ice. The deposits are stratified and attached to a large molecule. Each functional group may occur in the form of outwash plains, deltas, (e.g., —OH, —CH3, —COOH, etc.) has a characteristic kames, eskers, and kame terraces. chemical reactivity. gleyed A soil condition resulting from prolonged sat- fungi Eukaryote microorganisms with a rigid cell wall. uration with water and reducing conditions that mani- Some form long filaments of cells called hyphae that fest themselves in greenish or bluish colors throughout may grow together to form a visible body. the soil mass or in mottles. furrow slice The uppermost layer of an arable soil to the depth of primary tillage; the layer of soil sliced away from glomalin A protein-sugar group of molecules secreted by certain fungi resulting in a sticky hyphal surface the rest of the profile and inverted by a moldboard plow. thought to contribute to aggregate stability. gabion Partitioned, wire fabric containers, filled with stone at the site of use, to form flexible, permeable, and goethite, FeOOH A yellow-brown iron oxide mineral that accounts for the brown color in many soils. monolithic structures for earth retention. gamma ray A high-energy ray (photon) emitted dur- granular structure Soil structure in which the indi- ing radioactive decay of certain elements. vidual grains are grouped into spherical aggregates with indistinct sides. Highly porous granules are com- Gelisols An order in Soil Taxonomy. Soils that have monly called crumbs. A well-granulated soil has the permafrost within the upper 1 m, or upper 2 m if cry- best structure for most ordinary crop plants. See also oturbation is also present. They may have an ochric, soil structure types. histic, mollic, or other epipedon. granulation The process of producing granular mate- gellic materials Mineral or organic soil materials that rials. Commonly used to refer to the formation of soil have cryoturbation and/or ice in the form of lenses, structural granules, but also used to refer to the pro- veins, or wedges and the like. cessing of powdery fertilizer materials into granules. GLOSSARY 935
  • 11. Z03_BRAD9383_14_SE_GLOS.QXD 8/14/07 2:42 AM Page 936 grassed waterway Broad and shallow channel, content and pieces of the hard layer do not slake in planted with grass (usually perennial species) that is water. See also caliche; claypan. designed to move surface water downslope without harrowing A secondary broadcast tillage operation causing soil erosion. that pulverizes, smooths, and firms the soil in seedbed gravitational potential That portion of the total soil preparation, controls weeds, or incorporates material water potential due to differences in elevation of the ref- spread on the surface. erence pool of pure water and that of the soil water. heaving The partial lifting of plants, buildings, road- Since the soil water elevation is usually chosen to be ways, fenceposts, etc., out of the ground, as a result of higher than that of the reference pool, the gravita- freezing and thawing of the surface soil during the winter. tional potential is usually positive. heavy metals Those metals that have densities of 5.0 gravitational water Water that moves into, through, Mg/m or greater. Elements in soils include Cd, Co, Cr, or out of the soil under the influence of gravity. Cu, Fe, Hg, Mn, Mo, Pb, and Zn. great group A category in Soil Taxonomy. The classes heavy soil (Obsolete in scientific use) A soil with a in this category contain soils that have the same kind high content of clay, and a high drawbar pull, hence of horizons in the same sequence and have similar difficult to cultivate. moisture and temperature regimes. hematite, Fe2O3 A red iron oxide mineral that con- green manure Plant material incorporated with the tributes red color to many soils. soil while green, or soon after maturity, for improving the soil. hemic material See organic materials. greenhouse effect The entrapment of heat by upper herbicide A chemical that kills plants or inhibits their atmosphere gases, such as carbon dioxide, water vapor, growth; intended for weed control. and methane, just as glass traps heat for a greenhouse. herbivore A plant-eating animal. Increases in the quantities of these gases in the atmo- sphere will likely result in global warming that may heterotroph An organism capable of deriving energy have serious consequences for humankind. for life processes only from the decomposition of organic compounds and incapable of using inorganic groundwater Subsurface water in the zone of satura- compounds as sole sources of energy or for organic tion that is free to move under the influence of gravity, synthesis. Contrast with autotroph. often horizontally to stream channels. histic epipedon A diagnostic surface horizon consisting grus A sediment or soil material comprised of loose of a thin layer of organic soil material that is saturated grains of coarse sand and fine gravel size composed of with water at some period of the year unless artificially quartz, feldspar and rock fragments. Produced from drained and that is at or near the surface of a mineral soil. rocks by physical weathering or selectively transported by borrowing insects. Histosols An order in Soil Taxonomy. Soils formed from materials high in organic matter. Histosols with essen- gully erosion The erosion process whereby water accu- tially no clay must have at least 20% organic matter by mulates in narrow channels and, over short periods, weight (about 78% by volume). This minimum organic removes the soil from this narrow area to considerable matter content rises with increasing clay content to depths, ranging from 1 to 2 ft to as much as 23 to 30 m 30% (85% by volume) in soils with at least 60% clay. (75 to 100 ft). horizon, soil A layer of soil, approximately parallel to gypsic horizon A diagnostic subsurface horizon of sec- the soil surface, differing in properties and characteris- ondary calcium sulfate enrichment that is more than tics from adjacent layers below or above it. See also 15 cm thick. diagnostic horizons. gypsum requirement The quantity of gypsum required horticulture The art and science of growing fruits, veg- to reduce the exchangeable sodium percentage in a soil etables, and ornamental plants. to an acceptable level. hue (color) See Munsell color system. halophyte A plant that requires or tolerates a saline (high salt) environment. humic acid A mixture of variable or indefinite compo- sition of dark organic substances, precipitated upon hard armor Pertains to the use of hard materials (such acidification of a dilute alkali extract from soil. as large stones or concrete) to prevent soil and stream bank erosion by reducing the erosive force of flowing humic substances A series of complex, relatively high water. See soft armor. molecular weight, brown- to black-colored organic sub- stances that make up 60 to 80% of the soil organic matter hardpan A hardened soil layer, in the lower A or and are generally quite resistant to ready microbial attack. in the B horizon, caused by cementation of soil parti- cles with organic matter or with such materials as humid climate Climate in regions where moisture, silica, sesquioxides, or calcium carbonate. The hardness when distributed normally throughout the year, does not change appreciably with changes in moisture should not limit crop production. In cool climates 936 Glossary
  • 12. Z03_BRAD9383_14_SE_GLOS.QXD 8/14/07 2:42 AM Page 937 annual precipitation may be as little as 25 cm; in hot plant’s tissues. Often pertaining to concentrations of climates, 150 cm or even more. Natural vegetation in heavy metals to 1% or more of the tissue dry matter. uncultivated areas is forests. hyperthermic A soil temperature class with mean humification The processes involved in the decompo- annual temperatures >22°C. sition of organic matter and leading to the formation of humus. hypha (pl. hyphae) Filament of fungal cells. Actinomycetes also produce similar, but thinner, filaments of cells. humin The fraction of the soil organic matter that is not dissolved upon extraction of the soil with dilute alkali. hypoxia State of oxygen deficiency in an environment so low as to restrict biological respiration (in water, typ- humus That more or less stable fraction of the soil ically less than 2 to 3 mg O2/L). organic matter remaining after the major portions of added plant and animal residues have decomposed. hysteresis A relationship between two variables that changes depending on the sequences or starting point. An Usually it is dark in color. example is the relationship between soil water content hydration Chemical union between an ion or com- and water potential, for which different curves describe pound and one or more water molecules, the reaction the relationship when a soil is gaining water or losing it. being stimulated by the attraction of the ion or com- igneous rock Rock formed from the cooling and solid- pound for either the hydrogen or the unshared electrons ification of magma that has not been changed appre- of the oxygen in the water. ciably since its formation. hydraulic conductivity An expression of the readiness illite See fine-grained mica. with which a liquid, such as water, flows through a solid, such as soil, in response to a given potential gradient. illuvial horizon A soil layer or horizon in which mate- rial carried from an overlying layer has been precipi- hydric soils Soils that are water-saturated for long tated from solution or deposited from suspension. The enough periods to produce reduced conditions and layer of accumulation. affect the growth of plants. illuviation The process of deposition of soil material hydrogen bonding Relatively low energy bonding exhib- removed from one horizon to another in the soil; ited by a hydrogen atom located between two highly usually from an upper to a lower horizon in the soil electronegative atoms, such as nitrogen or oxygen. profile. See also eluviation. hydrologic cycle The circuit of water movement from immature soil A soil with indistinct or only slightly the atmosphere to the Earth and back to the atmo- developed horizons because of the relatively short time sphere through various stages or processes, as precipita- it has been subjected to the various soil-forming tion, interception, runoff, infiltration, percolation, processes. A soil that has not reached equilibrium with storage, evaporation, and transpiration. its environment. hydrolysis A reaction with water that splits the water immobilization The conversion of an element from molecule into H+ and OH- ions. Molecules or atoms the inorganic to the organic form in microbial tissues participating in such reactions are said to hydrolyze. or in plant tissues, thus rendering the element not hydronium A hydrated hydrogen ion (H3O+), the form readily available to other organisms or to plants. of the hydrogen ion usually found in an aqueous system. imogolite A poorly crystalline aluminosilicate mineral hydroperiod The duration of the presence of surface with an approximate formula SiO2Al2O3 • 2.5H2O; water in seasonal wetlands. occurs mostly in soils formed from volcanic ash. hydroponics Plant-production systems that use nutri- impervious Resistant to penetration by fluids or by roots. ent solutions and no solid medium to grow plants. improved fallow See fallow. hydrostatic potential See submergence potential. Inceptisols An order in Soil Taxonomy. Soils that are hydrous mica See fine-grained mica. usually moist with pedogenic horizons of alteration of parent materials but not of illuviation. Generally, the hydroxyapatite A member of the apatite group of direction of soil development is not yet evident from minerals rich in hydroxyl groups. A nearly insoluble the marks left by various soil-forming processes or the calcium phosphate. marks are too weak to classify in another order. hygroscopic coefficient The amount of moisture in a induced systemic resistance Plant defense mecha- dry soil when it is in equilibrium with some standard nisms activated by a chemical signal produced by a rhi- relative humidity near a saturated atmosphere (about zosphere bacteria. Although the process begins in the 98%), expressed in terms of percentage on the basis of soil, it may confer disease resistance to leaves or other oven-dry soil. aboveground tissues. hyperaccumulator A plant with unusually high capac- indurated (soil) Soil material cemented into a hard ity to take up certain elements from soil resulting in mass that will not soften on wetting. See also consis- very high concentrations of these elements in the tence; hardpan. GLOSSARY 937
  • 13. Z03_BRAD9383_14_SE_GLOS.QXD 8/15/07 9:14 PM Page 938 infiltration The downward entry of water into the soil. Koc The distribution coefficient, Kd, calculated based on organic carbon content. Koc ϭ Kd/foc where foc is infiltration capacity A soil characteristic determining the fraction of organic carbon. or describing the maximum rate at which water can enter the soil under specified conditions, including the kame A conical hill or ridge of sand or gravel presence of an excess of water. deposited in contact with glacial ice. inner-sphere complex A relatively strong (not easily kandic horizon A subsurface diagnostic horizon hav- reversed) chemical association or bonding directly ing a sharp clay increase relative to overlying horizons between a specific ion and specific atoms or groups of and having low-activity clays. atoms in the surface structure of a soil colloid. kaolinite An aluminosilicate mineral of the 1:1 crys- inoculation The process of introducing pure or mixed tal lattice group; that is, consisting of single silicon cultures of microorganisms into natural or artificial tetrahedral sheets alternating with single aluminum culture media. octahedral sheets. inorganic compounds All chemical compounds in Ksat Hydraulic conductivity when the soil is water sat- nature except compounds of carbon other than carbon urated. See also hydraulic conductivity. monoxide, carbon dioxide, and carbonates. k-strategist An organism that maintains a relatively insecticide A chemical that kills insects. stable population by specializing in metabolism of resistant compounds that most other organisms can- intergrade A soil that possesses moderately well- not utilize. Contrast with r-strategist. See also autochtho- developed distinguishing characteristics of two or more nous organisms. genetically related great soil groups. interlayer (mineralogy) Materials between layers labile A substance that is readily transformed by micro- within a given crystal, including cations, hydrated organisms or is readily available for uptake by plants. cations, organic molecules, and hydroxide groups or lacustrine deposit Material deposited in lake water sheets. and later exposed either by lowering of the water level internal surface The area of surface exposed within a or by the elevation of the land. clay crystal between the individual crystal layers. land A broad term embodying the total natural envi- Compare with external surface. ronment of the areas of the Earth not covered by interstratification Mixing of silicate layers within the water. In addition to soil, its attributes include other structural framework of a given silicate clay. physical conditions, such as mineral deposits and water supply; location in relation to centers of com- ionic double layer The distribution of cations in the merce, populations, and other land; the size of the soil solution resulting from the simultaneous attrac- individual tracts or holdings; and existing plant cover, tion toward colloid particles by the particle’s negative works of improvement, and the like. charge and the tendency of diffusion and thermal forces to move the cations away from the colloid sur- land capability classification A grouping of kinds of soil faces. Also described as a diffuse double layer or a dif- into special units, subclasses, and classes according to their fuse electrical double layer. capability for intensive use and the treatments required for sustained use. One such system has been prepared ions Atoms, groups of atoms, or compounds that are by the USDA Natural Resources Conservation Service. electrically charged as a result of the loss of electrons (cations) or the gain of electrons (anions). land classification The arrangement of land units into various categories based upon the properties of iron-pan An indurated soil horizon in which iron the land or its suitability for some particular purpose. oxide is the principal cementing agent. land forming Shaping the surface of the land by scrap- irrigation efficiency The ratio of the water actually ing off the high spots and filling in the low spots with consumed by crops on an irrigated area to the amount precision grading machinery to create a uniform, of water diverted from the source onto the area. smooth slope, often for irrigation purposes. Also called isomorphous substitution The replacement of one land smoothing. atom by another of similar size in a crystal lattice land-use planning The development of plans for the without disrupting or changing the crystal structure uses of land that, over long periods, will best serve the of the mineral. general welfare, together with the formulation of ways isotopes Two or more atoms of the same element that and means for achieving such uses. have different atomic masses because of different num- laterite An iron-rich subsoil layer found in some bers of neutrons in the nucleus. highly weathered humid tropical soils that, when joule The SI energy unit defined as a force of 1 newton exposed and allowed to dry, becomes very hard and applied over a distance of 1 meter; 1 joule ϭ 0.239 calorie. will not soften when rewetted. When erosion removes the overlying layers, the laterite is exposed and a vir- Kd See distribution coefficient, Kd. tual pavement results. See also plinthite. 938 Glossary
  • 14. Z03_BRAD9383_14_SE_GLOS.QXD 8/14/07 2:42 AM Page 939 layer (Clay mineralogy) A combination in silicate loamy Intermediate in texture and properties between clays of (tetrahedral and octahedral) sheets in a 1:1, fine-textured and coarse-textured soils. Includes all tex- 2:1, or 2:1:1 combination. tural classes with the words loam or loamy as a part of the class name, such as clay loam or loamy sand. See leaching The removal of materials in solution from also loam; soil texture. the soil by percolating waters. See also eluviation. lodging Falling over of plants, either by uprooting or leaching requirement The leaching fraction of irriga- stem breakage. tion water necessary to keep soil salinity from exceed- ing a tolerance level of the crop to be grown. loess Material transported and deposited by wind and consisting of predominantly silt-sized particles. leaf area index The ratio of the area of the total upper leaf surface of a plant canopy and the unit area on luxury consumption The intake by a plant of an essen- which the canopy is grown. tial nutrient in amounts exceeding what it needs. For example, if potassium is abundant in the soil, alfalfa legume A pod-bearing member of the Leguminosae may take in more than it requires. family, one of the most important and widely distributed plant families. Includes many valuable food and forage lysimeter A device for measuring percolation (leach- species, such as peas, beans, peanuts, clovers, alfalfas, ing) and evapotranspiration losses from a column of sweet clovers, lespedezas, vetches, and kudzu. Nearly all soil under controlled conditions. legumes are associated with nitrogen-fixing organisms. macronutrient A chemical element necessary in large lichen A symbiotic relationship between fungi and amounts (usually 50 mg/kg in the plant) for the growth cyanobacteria (blue-green algae) that enhances colo- of plants. Includes C, H, O, N, P, K, Ca, Mg, and S. nization of bare minerals and rocks. The fungi supply (Macro refers to quantity and not to the essentiality of water and nutrients, the cyanobacteria the fixed nitro- the element.) See also micronutrient. gen and carbohydrates from photosynthesis. macropores Larger soil pores, generally having a Liebig’s law The growth and reproduction of an diameter greater than 0.06 mm, from which water organism are determined by the nutrient substance drains readily by gravity. (oxygen, carbon dioxide, calcium, etc.) that is available in minimum quantity with respect to organic needs; map unit (mapping unit), soil A conceptual group of one to many component soils, delineated or identified by the the limiting factor. Also attributed to Sprengel . same name in a soil survey, that represent similar land- light soil (Obsolete in scientific use) A coarse-textured scape areas. See also delineation, soil consociation, soil soil; a soil with a low drawbar pull and hence easy to complex, soil association, and undifferentiated group. cultivate. See also coarse texture; soil texture. marl Soft and unconsolidated calcium carbonate, usu- lignin The complex organic constituent of woody fibers ally mixed with varying amounts of clay or other in plant tissue that, along with cellulose, cements the impurities. cells together and provides strength. Lignins resist micro- bial attack and after some modification may become part marsh Periodically wet or continually flooded area with the surface not deeply submerged. Covered dominantly of the soil organic matter. with sedges, cattails, rushes, or other hydrophytic plants. lime (agricultural) In strict chemical terms, calcium Subclasses include freshwater and saltwater marshes. oxide. In practical terms, a material containing the car- bonates, oxides, and/or hydroxides of calcium and/or mass flow Movement of nutrients with the flow of water to plant roots. magnesium used to neutralize soil acidity. lime requirement The mass of agricultural limestone, matric potential That portion of the total soil water potential due to the attractive forces between water and or the equivalent of other specified liming material, soil solids as represented through adsorption and capil- required to raise the pH of the soil to a desired value larity. It will always be negative. under field conditions. limestone A sedimentary rock composed primarily of cal- mature soil A soil with well-developed soil horizons produced by the natural processes of soil formation and cite (CaCO3). If dolomite (CaCO3 • MgCO3) is present in essentially in equilibrium with its present environment. appreciable quantities, it is called a dolomitic limestone. limiting factor See Liebig’s law. maximum retentive capacity The average moisture content of a disturbed sample of soil, 1 cm high, liquid limit (LL) See Atterberg limits. which is at equilibrium with a water table at its lower surface. lithosequence A group of related soils that differ, one from the other, in certain properties primarily as a mechanical analysis (Obsolete) See particle size analysis; result of parent material as a soil-forming factor. particle size distribution. loam The textural-class name for soil having a moder- medium texture Intermediate between fine-textured and ate amount of sand, silt, and clay. Loam soils contain 7 coarse-textured (soils). It includes the following textural to 27% clay, 28 to 50% silt, and 23 to 52% sand. classes: very fine sandy loam, loam, silt loam, and silt. GLOSSARY 939
  • 15. Z03_BRAD9383_14_SE_GLOS.QXD 8/14/07 2:42 AM Page 940 melanic epipedon A diagnostic surface horizon formed matter. Usually contains <20% organic matter, but may in volcanic parent material, that contains more than contain an organic surface layer up to 30 cm thick. 6% organic carbon, is dark in color, and has a very low mineralization The conversion of an element from bulk density and high anion adsorption capacity. an organic form to an inorganic state as a result of mellow soil A very soft, very friable, porous soil with- microbial decomposition. out any tendency toward hardness or harshness. See minimum tillage See tillage, conservation. also consistence. minor element (Obsolete) See micronutrient. mesic A soil temperature class with mean annual temperature 8 to 15°C. moderately coarse texture Consisting predominantly of coarse particles. In soil textural classification, it mesofauna Animals of medium size, between approx- includes all the sandy loams except the very fine sandy imately 2 and 0.2 mm in diameter. loam. See also coarse texture. mesophilic Pertaining to moderate temperatures in moderately fine texture Consisting predominantly of the range of 15 to 35°C, the range in which mesophilic intermediate-sized (soil) particles or with relatively organisms grow best and in which mesophilic com- small amounts of fine or coarse particles. In soil textural posting takes place. classification, it includes clay loam, sandy loam, sandy metamorphic rock A rock that has been greatly altered clay loam, and silty clay loam. See also fine texture. from its previous condition through the combined moisture potential See soil water potential. action of heat and pressure. For example, marble is a metamorphic rock produced from limestone, gneiss is mole drain Unlined drain formed by pulling a bullet- produced from granite, and slate is produced from shale. shaped cylinder through the soil. methane, CH4 An odorless, colorless gas commonly mollic epipedon A diagnostic surface horizon of min- produced under anaerobic conditions. When released eral soil that is dark colored and relatively thick, con- to the upper atmosphere, methane contributes to tains at least 0.6% organic carbon, is not massive and global warming. See also greenhouse effect. hard when dry, has a base saturation of more than 50%, has less than 250 mg/kg P2O5 soluble in 1% citric acid, micas Primary aluminosilicate minerals in which two and is dominantly saturated with bivalent cations. silica tetrahedral sheets alternate with one alumina/ magnesia octahedral sheet with entrapped potassium Mollisols An order in Soil Taxonomy. Soils with nearly atoms fitting between sheets. They separate readily into black, organic-rich surface horizons and high supply of visible sheets or flakes. bases. They have mollic epipedons and base saturation greater than 50% in any cambic or argillic horizon. microfauna That part of the animal population which They lack the characteristics of Vertisols and must not consists of individuals too small to be clearly distin- have oxic or spodic horizons. guished without the use of a microscope. Includes pro- tozoans and nematodes. molybdenosis A nutritional disease of ruminant ani- mals in which high Mo in the forage interferes with microflora That part of the plant population which copper absorption. consists of individuals too small to be clearly distin- guished without the use of a microscope. Includes montmorillonite An aluminosilicate clay mineral in the actinomycetes, algae, bacteria, and fungi. smectite group with a 2:1 expanding crystal lattice, with two silicon tetrahedral sheets enclosing an aluminum micronutrient A chemical element necessary in only octahedral sheet. Isomorphous substitution of magne- extremely small amounts (<50 mg/kg in the plant) for sium for some of the aluminum has occurred in the octa- the growth of plants. Examples are B, Cl, Cu, Fe, Mn, hedral sheet. Considerable expansion may be caused by and Zn. (Micro refers to the amount used rather than to water moving between silica sheets of contiguous layers. its essentiality.) See also macronutrient. mor Raw humus; type of forest humus layer of unin- micropores Relatively small soil pores, generally found corporated organic material, usually matted or com- within structural aggregates and having a diameter less pacted or both; distinct from the mineral soil, unless the than 0.06 mm. Contrast to macropores. latter has been blackened by washing in organic matter. microrelief Small-scale local differences in topogra- moraine An accumulation of drift, with an initial topo- phy, including mounds, swales, or pits that are only graphic expression of its own, built within a glaciated 1 m or so in diameter and with elevation differences of region chiefly by the direct action of glacial ice. Examples up to 2 m. See also gilgai. are ground, lateral, recessional, and terminal moraines. mineral (i) An inorganic compound of defined composi- morphology, soil The constitution of the soil, includ- tion found in rocks. (ii) An adjective meaning inorganic. ing the texture, structure, consistence, color, and other mineral nutrient An element in inorganic form used physical, chemical, and biological properties of the var- by plants or animals. ious soil horizons that make up the soil profile. mineral soil A soil consisting predominantly of, and hav- mottling Spots or blotches of different color or shades ing its properties determined predominantly by, mineral of color interspersed with the dominant color. 940 Glossary
  • 16. Z03_BRAD9383_14_SE_GLOS.QXD 8/14/07 2:42 AM Page 941 mucigel The gelatinous material at the surface of roots microorganisms have removed most of the soluble grown in unsterilized soil. nitrate from the soil solution. muck Highly decomposed organic material in which nitrification The biochemical oxidation of ammonium the original plant parts are not recognizable. Contains to nitrate, predominantly by autotrophic bacteria. more mineral matter and is usually darker in color than nitrogen assimilation The incorporation of nitrogen peat. See also muck soil; peat. into organic cell substances by living organisms. muck soil (1) A soil containing 20 to 50% organic nitrogen cycle The sequence of chemical and biological matter. (2) An organic soil in which the organic matter changes undergone by nitrogen as it moves from the is well decomposed. atmosphere into water, soil, and living organisms, and mulch Any material such as straw, sawdust, leaves, plas- upon death of these organisms (plants and animals) is tic film, and loose soil that is spread upon the surface of recycled through a part or all of the entire process. the soil to protect the soil and plant roots from the effects nitrogen fixation The biological conversion of ele- of raindrops, soil crusting, freezing, evaporation, etc. mental nitrogen (N2) to organic combinations or to mulch tillage See tillage, conservation. forms readily utilized in biological processes. mull A humus-rich layer of forested soils consisting of nodule bacteria See rhizobia. mixed organic and mineral matter. A mull blends into nonacid cations Those cations that do not react with the upper mineral layers without an abrupt change in water by hydrolysis to release H+ ions to the soil solu- soil characteristics. tion. These cations do not remove hydroxyl ions from Munsell color system A color designation system that solution, but form strongly dissociated bases such as specifies the relative degrees of the three simple vari- potassium hydroxide (K+ + OH). Formerly called base ables of color: cations or base-forming cations in soil science literature. chroma The relative purity, strength, or saturation nonacid saturation The proportion or percentage of a of a color. cation-exchange site occupied by nonacid cations. hue The chromatic gradation (rainbow) of light Formerly termed base saturation. that reaches the eye. value The degree of lightness or darkness of the nonhumic substances The portion of soil organic mat- color. ter comprised of relatively low molecular weight organic substances; mostly identifiable biomolecules. mycelium A stringlike mass of individual fungal or actinomycetes hyphae. nonlimiting water range The region bounded by the upper and lower soil water content over which water, myco Prefix designating an association or relationship oxygen, and mechanical resistance are not limiting to with a fungus (e.g., mycotoxins are toxins produced by plant growth. Compare with available water. a fungus). nonpoint source A pollution source that cannot be mycorrhiza The association, usually symbiotic, of traced back to a single origin or source. Examples fungi with the roots of seed plants. See also ectotrophic include water runoff from urban areas and leaching mycorrhiza; endotrophic mycorrhiza; arbuscular my- from croplands. corrhiza. no-tillage See tillage, conservation. natric horizon A diagnostic subsurface horizon that satisfies the requirements of an argillic horizon, but nucleic acids Complex organic acids found in the that also has prismatic, columnar, or blocky structure nuclei of plant and animal cells; may be combined and a subhorizon having more than 15% saturation with proteins as nucleoproteins. with exchangeable sodium. O horizon Organic horizon of mineral soils. necrosis Death associated with discoloration and dehy- dration of all or parts of plant organs, such as leaves. ochric epipedon A diagnostic surface horizon of min- eral soil that is too light in color, too high in chroma, nematodes Very small (most are microscopic) unseg- too low in organic carbon, or too thin to be a plaggen, mented round worms. In soils they are abundant and mollic, umbric, anthropic, or histic epipedon, or that is perform many important functions in the soil food both hard and massive when dry. web. Some are plant parasites and considered pests. octahedral sheet Sheet of horizontally linked, neutral soil A soil in which the surface layer, at least to octahedral-shaped units that serve as the basic structural normal plow depth, is neither acid nor alkaline in reac- components of silicate (clay) minerals. Each unit consists tion. In practice this means the soil is within the pH of a central, six-coordinated metallic atom (e.g., Al, Mg, range of 6.6 to 7.3. See also acid soil; alkaline soil; pH; or Fe) surrounded by six hydroxyl groups that, in turn, reaction, soil. are linked with other nearby metal atoms, thereby serv- nitrate depression period A period of time, beginning ing as interunit linkages that hold the sheet together. shortly after the addition of fresh, highly carbonaceous oligotrophic Environments, such as soils or lakes, organic materials to a soil, during which decomposer which are poor in nutrients. GLOSSARY 941
  • 17. Z03_BRAD9383_14_SE_GLOS.QXD 8/14/07 2:42 AM Page 942 order, soil The category at the highest level of general- outer-sphere complex A relatively weak (easily reversed) ization in Soil Taxonomy. The properties selected to chemical association or general attraction between an distinguish the orders are reflections of the degree of ion and an oppositely charged soil colloid via mutual horizon development and the kinds of horizons present. attraction for intervening water molecules. organic farming A system/philosophy of agriculture outwash plain A deposit of coarse-textured materials that does not allow the use of synthetic chemicals to (e.g., sands and gravels) left by streams of meltwater produce plant and animal products, but instead flowing from receding glaciers. emphasizes the management of soil organic matter and biological processes. In many countries, products are oven-dry soil Soil that has been dried at 105°C until it reaches constant weight. officially certified as being organic if inspections con- firm that they were grown by these methods. oxic horizon A diagnostic subsurface horizon that is at organic fertilizer By-product from the processing of least 30 cm thick and is characterized by the virtual animal or vegetable substances that contain sufficient absence of weatherable primary minerals or 2:1 lattice plant nutrients to be of value as fertilizers. clays and the presence of 1:1 lattice clays and highly insoluble minerals, such as quartz sand, hydrated organic soil A soil in which more than half of the pro- oxides of iron and aluminum, low cation exchange file thickness is comprised of organic soil materials. capacity, and small amounts of exchangeable bases. organic soil materials (As used in Soil Taxonomy): oxidation The loss of electrons by a substance; there- (1) Saturated with water for prolonged periods unless fore, a gain in positive valence charge and, in some artificially drained and having 18% or more organic cases, the chemical combination with oxygen gas. carbon (by weight) if the mineral fraction is more than 60% clay, more than 12% organic carbon if the mineral oxidation ditch An artificial open channel for partial fraction has no clay, or between 12 and 18% carbon if digestion of liquid organic wastes in which the wastes the clay content of the mineral fraction is between 0 are circulated and aerated by a mechanical device. and 60%. (2) Never saturated with water for more than oxidation-reduction potential See Eh and pe. a few days and having more than 20% organic carbon. Histosols develop on these organic soil materials. There Oxisols An order in Soil Taxonomy. Soils with residual are three kinds of organic materials: accumulations of low-activity clays, free oxides, kaolin, fibric materials The least decomposed of all the and quartz. They are mostly in tropical climates. organic soil materials, containing very high pans Horizons or layers in soils that are strongly com- amounts of fiber that are well preserved and readily pacted, indurated, or very high in clay content. See also identifiable as to botanical origin; with very low caliche; claypan; fragipan; hardpan. bulk density. hemic materials Intermediate in degree of decom- parent material The unconsolidated and more or less position of organic materials between the less chemically weathered mineral or organic matter from decomposed fibric and the more decomposed sapric which the solum of soils is developed by pedogenic materials. processes. sapric materials The most highly decomposed of particle density The mass per unit volume of the soil the organic materials, having the highest bulk den- particles. In technical work, usually expressed as metric sity, least amount of plant fiber, and lowest water tons per cubic meter (Mg/m3) or grams per cubic cen- content at saturation. timeter (g/cm3). orographic Influenced by mountains (Greek oros). particle size The effective diameter of a particle Used in reference to increased precipitation on the measured by sedimentation, sieving, or micrometric windward side of a mountain range induced as clouds methods. rise over the mountain, leaving a rain shadow of reduced precipitation on the leeward side. particle size analysis Determination of the various amounts of the different separates in a soil sample, usu- ortstein An indurated layer in the B horizon of ally by sedimentation, sieving, micrometry, or combi- Spodosols in which the cementing material consists of nations of these methods. illuviated sesquioxides (mostly iron) and organic matter. particle size distribution The amounts of the various osmotic potential That portion of the total soil water soil separates in a soil sample, usually expressed as potential due to the presence of solutes in soil water. It weight percentages. will generally be negative. particulate organic matter A microbially active frac- osmotic pressure Pressure exerted in living bodies as a tion of soil organic matter consisting largely of fine result of unequal concentrations of salts on both sides particles of partially decomposed plant tissue. of a cell wall or membrane. Water moves from the area having the lower salt concentration through the mem- partitioning The distribution of organic chemicals brane into the area having the higher salt concentra- (such as pollutants) into a portion that dissolves in the tion and, therefore, exerts additional pressure on the soil organic matter and a portion that remains undis- side with higher salt concentration. solved in the soil solution. 942 Glossary
  • 18. Z03_BRAD9383_14_SE_GLOS.QXD 8/15/07 4:59 PM Page 943 pascal An SI unit of pressure equal to 1 newton per permafrost (1) Permanently frozen material underly- square meter. ing the solum. (2) A perennially frozen soil horizon. peat Unconsolidated soil material consisting largely permanent charge See constant charge. of undecomposed, or only slightly decomposed, permanent wilting point See wilting point. organic matter accumulated under conditions of exces- sive moisture. See also organic soil materials; peat soil. permeability, soil The ease with which gases, liquids, or plant roots penetrate or pass through a bulk mass of peat soil An organic soil containing more than 50% soil or a layer of soil. organic matter. Used in the United States to refer to the stage of decomposition of the organic matter, peat petrocalcic horizon A diagnostic subsurface horizon referring to the slightly decomposed or undecomposed that is a continuous, indurated calcic horizon cemented deposits and muck to the highly decomposed materials. by calcium carbonate and, in some places, with magne- See also muck; muck soil; peat. sium carbonate. It cannot be penetrated with a spade or auger when dry; dry fragments do not slake in water; ped A unit of soil structure such as an aggregate, crumb, and it is impenetrable by roots. prism, block, or granule, formed by natural processes (in contrast to a clod, which is formed artificially). petrogypsic horizon A diagnostic subsurface horizon that is a continuous, strongly cemented, massive gyp- pedology The science that deals with the formation, sic horizon that is cemented by calcium sulfate. It can morphology, and classification of soil bodies as land- be chipped with a spade when dry. Dry fragments do scape components. not slake in water and it is impenetrable by roots. pedon The smallest volume that can be called a soil. It pH, soil The negative logarithm of the hydrogen ion has three dimensions. It extends downward to the activity (concentration) of a soil. The degree of acidity depth of plant roots or to the lower limit of the genetic (or alkalinity) of a soil as determined by means of a glass soil horizons. Its lateral cross section is roughly hexag- or other suitable electrode or indicator at a specified onal and ranges from 1 to 10 m2 in size, depending on moisture content or soil-to-water ratio, and expressed in the variability in the horizons. terms of the pH scale. pedosphere The conceptual zone within the ecosystem phase, soil A subdivision of a soil series or other unit consisting of soil bodies or directly influenced by them. of classification having characteristics that affect the A zone or sphere of activity in which mineral, water, air, use and management of the soil but do not vary suffi- and biological components come together to form soils. ciently to differentiate it as a separate series. Included Usage is parallel to that for “atmosphere” or “biosphere.” are such characteristics as degree of slope, degree of pedoturbation Physical disturbance and mixing of soil erosion, and content of stones. horizons by such forces as burrowing animals (faunal pH-dependent charge That portion of the total charge pedoturbation) or frost churning (cryoturbation). of the soil particles that is affected by, and varies with, peneplain A once high, rugged area that has been changes in pH. reduced by erosion to a lower, gently rolling surface photomap A mosaic map made from aerial pho- resembling a plain. tographs to which place names, marginal data, and penetrability The ease with which a probe can be other map information have been added. pushed into the soil. May be expressed in units of dis- phyllosphere The leaf surface. tance, speed, force, or work depending on the type of penetrometer used. physical properties (of soils) Those characteristics, pro- cesses, or reactions of a soil that are caused by physical penetrometer An instrument consisting of a rod with forces and that can be described by, or expressed in, a cone-shaped tip and a means of measuring the force physical terms or equations. Examples of physical required to push the rod into a specified increment of properties are bulk density, water-holding capacity, soil. hydraulic conductivity, porosity, pore-size distribution, perc test See percolation test. and so on. percolation, soil water The downward movement of physical weathering The breakdown of rock and min- water through soil. Especially, the downward flow of eral particles into smaller particles by physical forces water in saturated or nearly saturated soil at hydraulic such as frost action. See also weathering. gradients of the order of 1.0 or less. phytotoxic substances Chemicals that are toxic to percolation test A measurement of the rate of percola- plants. tion of water in a soil profile, usually to determine the placic horizon A diagnostic subsurface horizon of a suitability of a soil for use as a septic tank drain field. black to dark reddish mineral soil that is usually thin perforated plastic pipe Pipe, sometimes flexible, with but that may range from 1 to 25 mm in thickness. holes or slits in it that allow the entrance and exit of air The placic horizon is commonly cemented with iron and water. Used for soil drainage and for septic effluent and is slowly permeable or impenetrable to water and spreading into soil. roots. GLOSSARY 943
  • 19. Z03_BRAD9383_14_SE_GLOS.QXD 8/14/07 2:42 AM Page 944 plaggen epipedon A diagnostic surface horizon that is or crop characteristics of many very small subunits of human-made and more than 50 cm thick. Formed by land. This technique commonly uses variable rate equip- long-continued manuring and mixing. ment, geo positioning systems and computer controls. plant nutrients See essential element. preferential flow Nonuniform movement of water and its solutes through a soil along certain pathways, plastic limit (PL) See Atterberg limits. which are often macropores. plastic soil A soil capable of being molded or deformed primary consumer An organism that subsists on plant continuously and permanently, by relatively moderate material. pressure, into various shapes. See also consistence. primary mineral A mineral that has not been altered platy Consisting of soil aggregates that are developed chemically since deposition and crystallization from predominantly along the horizontal axes; laminated; molten lava. flaky. primary producer An organism (usually a photosyn- plinthite (brick) A highly weathered mixture of sesquiox- thetic plant) that creates organic, energy-rich material ides of iron and aluminum with quartz and other dilu- from inorganic chemicals, solar energy, and water. ents that occurs as red mottles and that changes irre- versibly to hardpan upon alternate wetting and drying. primary tillage See tillage, primary. plow layer The soil ordinarily moved when land is priming effect The increased decomposition of rela- plowed; equivalent to surface soil. tively stable soil humus under the influence of much enhanced, generally biological, activity resulting from plow pan A subsurface soil layer having a higher bulk the addition of fresh organic materials to a soil. density and lower total porosity than layers above or below it, as a result of pressure applied by normal plow- prismatic soil structure A soil structure type with ing and other tillage operations. prismlike aggregates that have a vertical axis much longer than the horizontal axes. plowing A primary broad-base tillage operation that is performed to shatter soil uniformly with partial to Proctor test A laboratory procedure that indicates the complete inversion. maximum achievable bulk density for a soil and the optimum water content for compacting a soil. point of zero charge The pH value of a solution in equilibrium with a particle whose net charge, from all productivity, soil The capacity of a soil for producing a sources, is zero. specified plant or sequence of plants under a specified system of management. Productivity emphasizes the point source A pollution source that can be traced capacity of soil to produce crops and should be back to its origin, which is usually an effluent discharge expressed in terms of yields. pipe. Examples are a wastewater treatment plant or a factory. Opposite of nonpoint source. profile, soil A vertical section of the soil through all its horizons and extending into the parent material. polypedon (As used in Soil Taxonomy) Two or more contiguous pedons, all of which are within the defined prokaryote An organism whose cells do not have a limits of a single soil series; commonly referred to as a distinct nucleus. soil individual. protein Any of a group of nitrogen-containing pool A portion of a larger store of a substance defined organic compounds formed by the polymerization of a by kinetic or theoretical properties. For example, the large number of amino acid molecules and that, upon passive pool organic matter is defined by its very slow hydrolysis, yield these amino acids. They are essential rate of microbial turnover. Compare to fraction. parts of living matter and are one of the essential food substances of animals. pore size distribution The volume of the various sizes of pores in a soil. Expressed as percentages of the bulk protonation Attachment of protons (H+ ions) to volume (soil plus pore space). exposed OH groups on the surface of soil particles, resulting in an overall positive charge on the particle porosity, soil The volume percentage of the total soil surface. bulk not occupied by solid particles. protozoa One-celled eukaryotic organisms, such as potential acidity The acidity that could potentially be amoeba. formed if reduced sulfur compounds in a potential acid sulfate soil were to become oxidized. puddled soil Dense, massive soil artificially compacted when wet and having no aggregated structure. The potential acidity The extent to which soils containing condition commonly results from the tillage of a iron sulfides or sulfidic material could become more clayey soil when it is wet. acidic through oxidation process if these soils were dis- turbed and exposed to air. rain, acid See acid rain. precision farming The spatially variable management of reaction, soil (No longer used in soil science) The a field or farm based on information specific to the soil degree of acidity or alkalinity of a soil, usually expressed 944 Glossary
  • 20. Z03_BRAD9383_14_SE_GLOS.QXD 8/14/07 2:42 AM Page 945 as a pH value or by terms ranging from extremely rhizosphere That portion of the soil in the immediate acid for pH values Ͻ4.5 to very strongly alkaline for pH vicinity of plant roots in which the abundance and values Ͼ9.0. composition of the microbial population are influ- reactive nitrogen All forms of nitrogen that are read- enced by the presence of roots. ily available to biota (mainly ammonia, ammonium, rill A small, intermittent water course with steep sides; and nitrate with smaller quantities of other com- usually only a few centimeters deep and hence no pounds including nitrogen oxide gases) as opposed to obstacle to tillage operations. unreactive nitrogen that exists mostly as inert N2 gas. rill erosion An erosion process in which numerous small recharge area A geographic area in which an other- channels of only several centimeters in depth are formed; wise confined aquifer is exposed to surficial percolation occurs mainly on recently cultivated soils. See also rill. of water to recharge the groundwater in the aquifer. riparian zone The area, both above and below the redox concentrations Zones of apparent accumula- ground surface, that borders a river. tions of Fe-Mn oxides in soils. riprap Coarse rock fragments, stones, or boulders redox depletions Zones of low chroma (<2) where placed along a waterway or hillside to prevent erosion. Fe-Mn oxides, and in some cases clay, have been stripped from the soil. rock The material that forms the essential part of the earth’s solid crust, including loose incoherent masses redox potential The electrical potential (measured in such as sand and gravel, as well as solid masses of gran- volts or millivolts) of a system due to the tendency of ite and limestone. the substances in it to give up or acquire electrons. root interception Acquisition of nutrients by a root as redoximorphic features Soil properties associated with a result of the root growing into the vicinity of the wetness that result from reduction and oxidation of nutrient source. iron and manganese compounds after saturation and desaturation with water. See also redox concentrations; root nodules Swollen growths on plant roots. Often in reference to those in which symbiotic microorganisms redox depletions. live. reduction The gain of electrons, and therefore the loss of positive valence charge, by a substance. In some rotary tillage See tillage, rotary. cases, a loss of oxygen or a gain of hydrogen is also r-strategist Opportunistic organisms with short repro- involved. ductive times that allow them to respond rapidly to the presence of easily metabolized food sources. Contrast regolith The unconsolidated mantle of weathered with k-strategist. See also zymogenous organisms. rock and soil material on the Earth’s surface; loose earth materials above solid rock. (Approximately runoff The portion of the precipitation on an area that equivalent to the term soil as used by many engineers.) is discharged from the area through stream channels. That which is lost without entering the soil is called relief The relative differences in elevation between surface runoff and that which enters the soil before reach- the upland summits and the lowlands or valleys of a ing the stream is called groundwater runoff or seepage flow given region. from groundwater. (In soil science runoff usually refers to residual material Unconsolidated and partly weath- the water lost by surface flow; in geology and hydraulics ered mineral materials accumulated by disintegration runoff usually includes both surface and subsurface flow.) of consolidated rock in place. salic horizon A diagnostic subsurface horizon of resilience The capacity of a soil (or other ecosystem) enrichment with secondary salts more soluble in cold to return to its original state after a disturbance. water than gypsum. A salic horizon is 15 cm or more in rhizobacteria Bacteria specially adapted to colonizing thickness. the surface of plant roots and the soil immediately saline seep An area of land in which saline water around plant roots. Some have effects that promote seeps to the surface, leaving a high salt concentration plant growth, while others have effects that are delete- behind as the water evaporates. rious to plants. saline soil A nonsodic soil containing sufficient soluble rhizobia Bacteria capable of living symbiotically with salts to impair its productivity. The conductivity of a sat- higher plants, usually in nodules on the roots of urated extract is >4 dS/m, the exchangeable sodium legumes, from which they receive their energy, and adsorption ratio is less than about 13, and the pH is <8.5. capable of converting atmospheric nitrogen to com- bined organic forms; hence the term symbiotic nitrogen- saline-sodic soil A soil containing sufficient exchange- able sodium to interfere with the growth of most crop fixing bacteria. (Derived from the generic name plants and containing appreciable quantities of soluble Rhizobium.) salts. The exchangeable sodium adsorption ratio is rhizoplane The root surface–soil interface. Used to >13, the conductivity of the saturation extract is >4 describe the habitat of root-surface-dwelling microor- dS/m (at 25°C), and the pH is usually 8.5 or less in the ganisms. saturated soil. GLOSSARY 945
  • 21. Z03_BRAD9383_14_SE_GLOS.QXD 8/14/07 2:42 AM Page 946 salinization The process of accumulation of salts in soil. series, soil The soil series is a subdivision of a family in Soil Taxonomy and consists of soils that are similar in saltation Particle movement in water or wind where all major profile characteristics. particles skip or bounce along the stream bed or soil surface. sewage effluent The liquid part of sewage or waste- water; it is usually treated to remove some portion sand A soil particle between 0.05 and 2.0 mm in of the dissolved organic compounds and nutrients diameter; a soil textural class. present from the original sewage. sapric materials See organic soil materials. sewage sludge Settled sewage solids combined with saprolite Soft, friable, weathered bedrock that retains varying amounts of water and dissolved materials, the fabric and structure of the parent rock but is porous removed from sewage by screening, sedimentation, and can be dug with a spade. chemical precipitation, or bacterial digestion. Also called biosolids if certain quality standards are met. saprophyte An organism that lives on dead organic material. shear Force, as of a tillage implement, acting at right angles to the direction of movement. saturated paste extract The extract from a saturated soil paste, the electrical conductivity Ec of which gives sheet (Mineralogy) A flat array of more than one an indirect measure of salt content in a soil. atomic thickness and composed of one or more levels of linked coordination polyhedra. A sheet is thicker saturation extract The solution extracted from a satu- than a plane and thinner than a layer. Examples: tetra- rated soil paste. hedral sheet, octahedral sheet. saturation percentage The water content of a satu- sheet erosion The removal of a fairly uniform layer of rated soil paste, expressed as a dry weight percentage. soil from the land surface by runoff water. savanna (savannah) A grassland with scattered trees, shelterbelt A wind barrier of living trees and shrubs either as individuals or clumps. Often a transitional established and maintained for protection of farm type between true grassland and forest. fields. Syn. windbreak. second bottom The first terrace above the normal shifting cultivation A farming system in which land is floodplain of a stream. cleared, the debris burned, and crops grown for 2 to 3 secondary mineral A mineral resulting from the years. When the farmer moves on to another plot, the decomposition of a primary mineral or from the repre- land is then left idle for 5 to 15 years; then the burning cipitation of the products of decomposition of a pri- and planting process is repeated. mary mineral. See also primary mineral. short-range order minerals Minerals, such as allophane, sediment Transported and deposited particles or aggre- whose structural framework consists of short distances gates derived from soils, rocks, or biological materials. of well-ordered crystalline structure interspersed with distances of noncrystalline amorphous materials. sedimentary rock A rock formed from materials deposited from suspension or precipitated from solu- shrinkage limit (SL) The water content above which a tion and usually being more or less consolidated. The mass of soil material will swell in volume, but below principal sedimentary rocks are sandstones, shales, which it will shrink no further. limestones, and conglomerates. side-dressing The application of fertilizer alongside row-crop plants, usually on the soil surface. Nitrogen seedbed The soil prepared to promote the germina- materials are most commonly side-dressed. tion of seed and the growth of seedlings. self-mulching soil A soil in which the surface layer siderophore A nonporphyrin metabolite secreted by certain microorganisms that forms a highly stable coor- becomes so well aggregated that it does not crust and dination compound with iron. seal under the impact of rain but instead serves as a sur- face mulch upon drying. silica/alumina ratio The molecules of silicon dioxide (SiO2) per molecule of aluminum oxide (Al2O3) in clay semiarid Term applied to regions or climates where minerals or in soils. moisture is more plentiful than in arid regions but still definitely limits the growth of most crop plants. silica/sesquioxide ratio The molecules of silicon diox- Natural vegetation in uncultivated areas is short ide (SiO2) per molecule of aluminum oxide (Al2O3) plus grasses. ferric oxide (Fe2O3) in clay minerals or in soils. separate, soil One of the individual-sized groups of silt (1) A soil separate consisting of particles between mineral soil particles—sand, silt, or clay. 0.05 and 0.002 mm in equivalent diameter. (2) A soil textural class. septic tank An underground tank used in the deposi- tion of domestic wastes. Organic matter decomposes silting The deposition of waterborne sediments in in the tank, and the effluent is drained into the sur- stream channels, lakes, reservoirs, or on floodplains, usu- rounding soil. ally resulting from a decrease in the velocity of the water. 946 Glossary
  • 22. Z03_BRAD9383_14_SE_GLOS.QXD 8/14/07 2:42 AM Page 947 site index A quantitative evaluation of the productiv- soil amendment Any material, such as lime, gypsum, ity of a soil for forest growth under the existing or spec- sawdust, or synthetic conditioner, that is worked into ified environment. the soil to make it more amenable to plant growth. slag A product of smelting, containing mostly sili- soil association A group of defined and named taxo- cates; the substances not sought to be produced as nomic soil units occurring together in an individual matte or metal and having a lower specific gravity. and characteristic pattern over a geographic region, comparable to plant associations in many ways. slash-and-burn See shifting cultivation. slick spots Small areas in a field that are slick when soil auger A tool used to bore small holes up to several meters deep in soils in order to bring up samples of wet because of a high content of alkali or exchangeable material from various soil layers. It consists of a long sodium. T-handle attached to either a cylinder with twisted slickensides Stress surfaces that are polished and stri- teeth or a screwlike bit. ated and are produced by one mass sliding past another. soil classification (Soil Taxonomy) The systematic slope The degree of deviation of a surface from horizon- arrangement of soils into groups or categories on the tal, measured in a numerical ratio, percent, or degrees. basis of their characteristics. See order; suborder; great group; subgroup; family; and series. slow fraction (of soil organic matter) That portion of soil organic matter that can be metabolized with soil complex A mapping unit used in detailed soil sur- great difficulty by the microorganisms in the soil and veys where two or more defined taxonomic units are so therefore has a slow turnover rate with a half-life in intimately intermixed geographically that it is undesir- the soil ranging from a few years to a few decades. able or impractical, because of the scale being used, to Often this fraction is the product of some previous separate them. A more intimate mixing of smaller areas decomposition. of individual taxonomic units than that described smectite A group of silicate clays having a 2:1-type under soil association. lattice structure with sufficient isomorphous substitu- soil compressibility The property of a soil pertaining tion in either or both the tetrahedral and octahedral to its capacity to decrease in bulk volume when sub- sheets to give a high interlayer negative charge and jected to a load. high cation exchange capacity and to permit signifi- cant interlayer expansion and consequent shrinking soil conditioner Any material added to a soil for the and swelling of the clay. Montmorillonite, beidellite, purpose of improving its physical condition. and saponite are in the smectite group. soil conservation A combination of all management and sodic soil A soil that contains sufficient sodium to land-use methods that safeguard the soil against deple- interfere with the growth of most crop plants, and in tion or deterioration caused by nature and/or humans. 21/2([Ca2+] + [Mg2+]) which the sodium adsorption ratio is 13 or greater. soil consociation A kind of soil map unit that is sodium adsorption ratio (SAR) named for the dominant soil taxon in the delineation, [Na+] and in which at least half of the pedons are of the SAR = named soil taxon, and most of the remaining pedons are so similar as to not affect most interpretations. where the cation concentrations are in millimoles of soil correlation The process of defining, mapping, charge per liter (mmolc/L). naming, and classifying the kinds of soils in a specific soil survey area, the purpose being to ensure that soils soft armor The bioengineering use of organic and/or are adequately defined, accurately mapped, and uni- inorganic materials combined with plants to create a formly named. living vegetation barrier of protection against erosion. soil erosion See erosion. soil (1) A dynamic natural body composed of mineral and organic solids, gases, liquids and living organisms soil fertility See fertility, soil. which can serve as a medium for plant growth. (2) The soil genesis See genesis, soil. collection of natural bodies occupying parts of the Earth’s surface that is capable of supporting plant soil geography A subspecialization of physical geogra- growth and that has properties resulting from the inte- phy concerned with the areal distributions of soil types. grated effects of climate and living organisms acting upon parent material, as conditioned by topography, soil horizon See horizon, soil. over periods of time. soil loss tolerance (T value) (i) The maximum average annual soil loss that will allow continuous cropping soil air The soil atmosphere; the gaseous phase of and maintain soil productivity without requiring addi- the soil, being that volume not occupied by soil or tional management inputs. (ii) The maximum soil ero- liquid. sion loss that is offset by the theoretical maximum rate soil alkalinity The degree or intensity of alkalinity of a of soil development, which will maintain an equilib- soil, expressed by a value >7.0 on the pH scale. rium between soil losses and gains. GLOSSARY 947
  • 23. Z03_BRAD9383_14_SE_GLOS.QXD 8/14/07 2:42 AM Page 948 soil loss tolerance (T) The maximum rate of annual peds. These secondary units may be, but usually are soil loss that will permit plant productivity to be main- not, arranged in the profile in such a manner as to give tained economically and indefinitely. a distinctive characteristic pattern. The secondary units are characterized and classified on the basis of size, soil management The sum total of all tillage opera- shape, and degree of distinctness into classes, types, tions, cropping practices, fertilizer, lime, and other and grades, respectively. treatments conducted on or applied to a soil for the production of plants. soil structure classes A grouping of soil structural units or peds on the basis of size from the very fine to soil map A map showing the distribution of soil types very coarse. or other soil mapping units in relation to the prominent physical and cultural features of the Earth’s surface. soil structure grades A grouping or classification of soil structure on the basis of inter- and intraaggregate soil mechanics and engineering A subspecialization of adhesion, cohesion, or stability within the profile. Four soil science concerned with the effect of forces on the grades of structure, designated from 0 to 3, are recog- soil and the application of engineering principles to nized: structureless, weak, moderate, and strong. problems involving the soil. soil structure types A classification of soil structure based soil moisture potential See soil water potential. on the shape of the aggregates or peds and their arrange- soil monolith A vertical section of a soil profile removed ment in the profile, including platy, prismatic, columnar, from the soil and mounted for display or study. blocky, subangular blocky, granulated, and crumb. soil morphology The physical constitution, particu- soil survey The systematic examination, description, larly the structural properties, of a soil profile as exhib- classification, and mapping of soils in an area. Soil sur- ited by the kinds, thicknesses, and arrangement of the veys are classified according to the kind and intensity horizons in the profile, and by the texture, structure, of field examination. consistence, and porosity of each horizon. soil temperature classes A criterion used to differenti- soil order See order, soil. ate soil in Soil Taxonomy, mainly at the family level. Classes are based on mean annual soil temperature and soil organic matter The organic fraction of the soil on differences between summer and winter tempera- that includes plant and animal residues at various tures at a depth of 50 cm. stages of decomposition, cells and tissues of soil organ- isms, and substances synthesized by the soil popula- soil textural class A grouping of soil textural units tion. Commonly determined as the amount of organic based on the relative proportions of the various soil material contained in a soil sample passed through a separates (sand, silt, and clay). These textural classes, 2-mm sieve. listed from the coarsest to the finest in texture, are sand, loamy sand, sandy loam, loam, silt loam, silt, soil porosity See porosity, soil. sandy clay loam, clay loam, silty clay loam, sandy clay, soil productivity See productivity, soil. silty clay, and clay. There are several subclasses of the sand, loamy sand, and sandy loam classes based on the soil profile See profile, soil. dominant particle size of the sand fraction (e.g., loamy soil quality The capacity of a specific kind of soil to fine sand, coarse sandy loam). function, within natural or managed ecosystem bound- soil texture The relative proportions of the various aries, to sustain plant and animal productivity, maintain soil separates in a soil. or enhance water and air quality, and support human health and habitation. Sometimes considered in relation soil water deficit The difference between PET and ET, to this capacity in the undisturbed, natural state. representing the gap between the amount of evapo- transpiration water atmospheric conditions “demand” soil reaction See reaction, soil; pH, soil. and the amount the soil can actually supply. A mea- soil salinity The amount of soluble salts in a soil, sure of the limitation that water supply places on expressed in terms of percentage, milligrams per kilo- plant productivity. gram, parts per million (ppm), or other convenient ratios. soil water potential (total) A measure of the difference soil separates See separate, soil. between the free energy state of soil water and that of pure water. Technically it is defined as “that amount of soil series See series, soil. work that must be done per unit quantity of pure water soil solution The aqueous liquid phase of the soil and in order to transport reversibly and isothermically an its solutes, consisting of ions dissociated from the sur- infinitesimal quantity of water from a pool of pure faces of the soil particles and of other soluble materials. water, at a specified elevation and at atmospheric pres- sure to the soil water (at the point under considera- soil strength A transient soil property related to the tion).” This total potential consists of gravitational, soil’s solid phase cohesion and adhesion. matric, and osmotic potentials. soil structure The combination or arrangement of pri- solarization The process of heating a soil in the field mary soil particles into secondary particles, units, or by covering it with clear plastic sheeting during sunny 948 Glossary
  • 24. Z03_BRAD9383_14_SE_GLOS.QXD 8/14/07 2:42 AM Page 949 conditions. The heat is meant to partially sterilize the subgroup, soil The great groups in Soil Taxonomy are upper 5 to 15 cm of soil to reduce pest and pathogen subdivided into central concept subgroups that show populations. the central properties of the great group, intergrade sub- solum (pl. sola) The upper and most weathered part of groups that show properties of more than one great the soil profile; the A, E, and B horizons. group, and other subgroups for soils with atypical prop- erties that are not characteristic of any great group. sombric horizon A diagnostic subsurface horizon that contains illuvial humus but has a low cation exchange submergence potential The positive hydrostatic pres- capacity and low percentage base saturation. Mostly sure that occurs below the water table. restricted to cool, moist soils of high plateaus and suborder, soil A category in Soil Taxonomy that nar- mountainous areas of tropical and subtropical regions. rows the ranges in soil moisture and temperature sorption The removal from the soil solution of an ion regimes, kinds of horizons, and composition, accord- or molecule by adsorption and absorption. This term is ing to which of these is most important. often used when the exact mechanism of removal is subsoil That part of the soil below the plow layer. not known. subsoiling Breaking of compact subsoils, without species diversity The variety of different biological inverting them, with a special knifelike instrument species present in an ecosystem. Generally, high diversity (chisel), which is pulled through the soil at depths usu- is marked by many species with few individuals in each. ally of 30 to 60 cm and at spacings usually of 1 to 2 m. species richness The number of different species sulfidic Adjective used to describe sulfide-containing present in an ecosystem, without regard to the distri- soil materials that initially have a pH > 4.0 and exhibit bution of individuals among those species. a drop of at least 0.5 pH unit within 8 weeks of aerated, specific gravity The ratio of the density of a mineral moist incubation. Found in potential acid sulfate soils. to the density of water at standard temperature and sulfuric horizon A diagnostic subsurface horizon in pressure. either mineral or organic soils that has a pH < 3.5 and specific heat capacity The amount of kinetic (heat) fresh straw-colored mottles (called jarosite mottles). energy required to raise the temperature of 1 g of a sub- Forms by oxidation of sulfide-rich materials and is stance (usually in reference to soil or soil components). highly toxic to plants. specific surface The solid particle surface area per unit summer fallow See fallow. mass or volume of the solid particles. surface runoff See runoff. splash erosion The spattering of small soil particles surface seal A thin layer of fine particles deposited on caused by the impact of raindrops on very wet soils. the surface of a soil that greatly reduces the permeabil- The loosened and separated particles may or may not ity of the soil surface to water. be subsequently removed by surface runoff. surface soil The uppermost part of the soil, ordinarily spodic horizon A diagnostic subsurface horizon char- moved in tillage, or its equivalent in uncultivated soils. acterized by the illuvial accumulation of amorphous Ranges in depth from 7 to 25 cm. Frequently desig- materials composed of aluminum and organic carbon nated as the plow layer, the Ap layer, or the Ap horizon. with or without iron. surface tension The elasticlike phenomenon resulting Spodosols An order in Soil Taxonomy. Soils with sub- from the unbalanced attractions among liquid mole- surface illuvial accumulations of organic matter and cules (usually water) and between liquid and gaseous compounds of aluminum and usually iron. These soils molecules (usually air) at the liquid–gas interface. are formed in acid, mainly coarse-textured materials in humid and mostly cool or temperate climates. swamp An area of land that is usually wet or sub- merged under shallow fresh water and typically sup- stem flow The process by which rain or irrigation water ports hydrophilic trees and shrubs. is directed by a plant canopy toward the plant stem so as to wet the soil unevenly under the plant canopy. symbiosis The living together in intimate association of two dissimilar organisms, the cohabitation being stratified Arranged in or composed of strata or layers. mutually beneficial. strip-cropping The practice of growing crops that synergism (i) The nonobligatory association between require different types of tillage, such as row and sod, organisms that is mutually beneficial. Both popula- in alternate strips along contours or across the prevail- tions can survive in their natural environment on their ing direction of wind. own, although, when formed, the association offers structure, soil See soil structure. mutual advantages. (ii) The simultaneous actions of two or more factors that have a greater total effect stubble mulch The stubble of crops or crop residues together than the sum of their individual effects. left essentially in place on the land as a surface cover before and during the preparation of the seedbed and talus Fragments of rock and other soil material accu- at least partly during the growing of a succeeding crop. mulated by gravity at the foot of cliffs or steep slopes. GLOSSARY 949
  • 25. Z03_BRAD9383_14_SE_GLOS.QXD 8/14/07 2:42 AM Page 950 taxonomy, soil The science of classification of soils; tillage, which generally leaves at least 30% of the soil laws and principles governing the classifying of soil. surface covered by residues, including the following Also a specific soil classification system developed by systems: the U.S. Department of Agriculture. minimum tillage The minimum soil manipulation necessary for crop production or meeting tillage tensiometer A device for measuring the negative pres- requirements under the existing soil and climatic sure (or tension) of water in soil in situ; a porous, per- conditions. meable ceramic cup connected through a tube to a mulch tillage Tillage or preparation of the soil in manometer or vacuum gauge. such a way that plant residues or other materials are tension, soil-moisture See soil water potential. left to cover the surface; also called mulch farming, trash farming, stubble mulch tillage, and plowless terrace (1) A level, usually narrow, plain bordering a farming. river, lake, or the sea. Rivers sometimes are bordered by no-tillage system A procedure whereby a crop is terraces at different levels. (2) A raised, more or less planted directly into a seedbed not tilled since har- level or horizontal strip of earth usually constructed on vest of the previous crop; also called zero tillage. or nearly on a contour and designed to make the land ridge till Planting on ridges formed by cultivation suitable for tillage and to prevent accelerated erosion during the previous growing period. by diverting water from undesirable channels of con- strip till Planting is done in a narrow strip that has centration; sometimes called diversion terrace. been tilled and mixed, leaving the remainder of the tetrahedral sheet Sheet of horizontally linked, soil surface undisturbed. tetrahedron-shaped units that serve as one of the tillage, conventional The combined primary and sec- basic structural components of silicate (clay) miner- ondary tillage operations normally performed in als. Each unit consists of a central four-coordinated preparing a seedbed for a given crop grown in a given atom (e.g., Si, Al, Fe) surrounded by four oxygen geographic area. Usually said of non-conservation atoms that, in turn, are linked with other nearby atoms tillage. (e.g., Si, Al, Fe), thereby serving as interunit linkages to hold the sheet together. tillage, primary Tillage that contributes to the major soil manipulation, commonly with a plow. texture See soil texture. thermal analysis (differential thermal analysis) A method tillage, rotary An operation using a power-driven rotary of analyzing a soil sample for constituents, based on a tillage tool to loosen and mix soil. differential rate of heating of the unknown and standard tillage, secondary Any tillage operations following pri- samples when a uniform source of heat is applied. mary tillage designed to prepare a satisfactory seedbed thermic A soil temperature class with mean annual for planting. temperature 15 to 22 °C. tilth The physical condition of soil as related to its thermophilic Pertaining to temperatures in the range ease of tillage, fitness as a seedbed, and its impedance of 45 to 90 °C, the range in which thermophilic organ- to seedling emergence and root penetration. isms grow best and in which thermophilic composting topdressing An application of fertilizer to a soil after takes place. the crop stand has been established. thermophilic organisms Organisms that grow readily toposequence A sequence of related soils that differ, at temperatures above 45 °C. one from the other, primarily because of topography as a thixotrophy The property of certain clay soils of soil-formation factor, with other factors constant. becoming fluid when jarred or agitated and then set- topsoil (1) The layer of soil moved in cultivation. See ting again when at rest. Similar to quick, as in quick also surface soil. (2) Presumably fertile soil material clays or quicksand. used to top-dress roadbanks, gardens, and lawns. tile, drain Pipe made of burned clay, concrete, or trace elements Elements present in the Earth’s crust in ceramic material, in short lengths, usually laid with concentrations less than 1000 mg/kg. When referring open joints to collect and carry excess water from the to plant nutrients, the term micronutrients is preferred. soil. trioctahedral An octahedral sheet of silicate clays in till (1) Unstratified glacial drift deposited directly by which the sites for the six-coordinated metallic atoms the ice and consisting of clay, sand, gravel, and boul- are mostly filled with divalent cations, such as Mg2+. ders intermingled in any proportion. (2) To plow and prepare for seeding; to seed or cultivate the soil. trophic level Levels in a food chain that pass nutrients and energy from one group of organisms to another. tillage The mechanical manipulation of soil for any purpose; but in agriculture it is usually restricted to the truncated Having lost all or part of the upper soil modifying of soil conditions for crop production. horizon or horizons. tillage, conservation Any tillage sequence that tuff Volcanic ash usually more or less stratified and in reduces loss of soil or water relative to conventional various states of consolidation. 950 Glossary
  • 26. Z03_BRAD9383_14_SE_GLOS.QXD 8/15/07 8:24 PM Page 951 tundra A level or undulating treeless plain characteris- water potential, soil See soil water potential. tic of arctic regions. water table The upper surface of groundwater or that Ultisols An order in Soil Taxonomy. Soils that are low in level below which the soil is saturated with water. bases and have subsurface horizons of illuvial clay accu- mulations. They are usually moist, but during the warm water table, perched The surface of a local zone of saturation held above the main body of groundwater season of the year some are dry part of the time. by an impermeable layer of stratum, usually clay, and umbric epipedon A diagnostic surface horizon of min- separated from the main body of groundwater by an eral soil that has the same requirements as the mollic unsaturated zone. epipedon with respect to color, thickness, organic car- bon content, consistence, structure, and P2O5 content, water use efficiency Dry matter or harvested portion but that has a base saturation of less than 50%. of crop produced per unit of water consumed. universal soil loss equation (USLE) An equation for waterlogged Saturated with water. predicting the average annual soil loss per unit area per watershed All the land and water within the geo- year; A = RKLSPC, where R is the climatic erosivity fac- graphical confines of a drainage divide or surrounding tor (rainfall plus runoff), K is the soil erodibility factor, ridges that separate the area from neighboring water- L is the length of slope, S is the percent slope, P is the sheds. soil erosion practice factor, and C is the cropping and management factor. water-stable aggregate A soil aggregate stable to the action of water, such as falling drops or agitation, as in unsaturated flow The movement of water in a soil wet-sieving analysis. that is not filled to capacity with water. weathering All physical and chemical changes pro- vadose zone The aerated region of soil above the per- duced in rocks, at or near the Earth’s surface, by atmo- manent water table. spheric agents. value (color) See Munsell color system. wetland An area of land that has hydric soil and variable charge See pH-dependent charge. hydrophytic vegetation, typically flooded for part of the year, and forming a transition zone between varnish, desert A glossy sheen or coating on stones aquatic and terrestrial systems. and gravel in arid regions. wetting front The boundary between the wetted soil vermicompost Compost made by earthworms eating and dry soil during infiltration of water. raw organic materials in moist aerated piles, which are kept shallow to avoid heat buildup that could kill the wilting point (permanent wilting point) The moisture worms. content of soil, on an oven-dry basis, at which plants wilt and fail to recover their turgidity when placed in vermiculite A 2:1-type silicate clay, usually formed a dark, humid atmosphere. from mica, that has a high net negative charge stem- ming mostly from extensive isomorphous substitution windbreak Planting of trees, shrubs, or other vegeta- of aluminum for silicon in the tetrahedral sheet. tion perpendicular, or nearly so, to the principal wind direction to protect soils, crops, homesteads, etc., from Vertisols An order in Soil Taxonomy. Clayey soils with wind and snow. high shrink–swell potential that have wide, deep cracks when dry. Most of these soils have distinct wet and dry xenobiotic Compounds foreign to biological sys- periods throughout the year. tems. Often refers to compounds resistant to decom- position. vesicles (1) Unconnected voids with smooth walls. (2) Spherical structures formed inside root cortical cells xerophytes Plants that grow in or on extremely dry by vesicular arbuscular mycorrhizal fungi. soils or soil materials. virgin soil A soil that has not been significantly dis- zero tillage See tillage, conservation. turbed from its natural condition. zymogenous organisms So-called opportunist organ- water deficit (soil) The amount of available water isms found in soils in large numbers immediately fol- removed from the soil within the vegetation’s active lowing addition of readily decomposable organic mate- rooting depth, or the amount of water required to rials. Contrast with autochthonous organisms. See also bring the soil to field capacity. r-strategist. GLOSSARY 951
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