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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)].
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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.
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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.
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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.
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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.
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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.
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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.
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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
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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.
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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.
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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
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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.
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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