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  1. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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