This glossary defines over 100 soil science terms in 1-3 sentences each. Some key terms include: - A horizon - The surface horizon with the most organic matter and biological activity. - Argillic horizon - A subsurface horizon characterized by illuvial accumulation of clay. - Base saturation percentage - The extent to which a soil's cation exchange capacity is occupied by cations other than hydrogen and aluminum. - B horizon - A subsurface horizon characterized by accumulation of materials like clay, iron, aluminum, and humus.

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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.
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14. Z03_BRAD9383_14_SE_GLOS.QXD 8/14/07 2:42 AM Page 939
layer (Clay mineralogy) A combination in silicate loamy Intermediate in texture and properties between
clays of (tetrahedral and octahedral) sheets in a 1:1, fine-textured and coarse-textured soils. Includes all tex-
2:1, or 2:1:1 combination. tural classes with the words loam or loamy as a part of
the class name, such as clay loam or loamy sand. See
leaching The removal of materials in solution from also loam; soil texture.
the soil by percolating waters. See also eluviation.
lodging Falling over of plants, either by uprooting or
leaching requirement The leaching fraction of irriga- stem breakage.
tion water necessary to keep soil salinity from exceed-
ing a tolerance level of the crop to be grown. loess Material transported and deposited by wind and
consisting of predominantly silt-sized particles.
leaf area index The ratio of the area of the total upper
leaf surface of a plant canopy and the unit area on luxury consumption The intake by a plant of an essen-
which the canopy is grown. tial nutrient in amounts exceeding what it needs. For
example, if potassium is abundant in the soil, alfalfa
legume A pod-bearing member of the Leguminosae may take in more than it requires.
family, one of the most important and widely distributed
plant families. Includes many valuable food and forage lysimeter A device for measuring percolation (leach-
species, such as peas, beans, peanuts, clovers, alfalfas, ing) and evapotranspiration losses from a column of
sweet clovers, lespedezas, vetches, and kudzu. Nearly all soil under controlled conditions.
legumes are associated with nitrogen-fixing organisms.
macronutrient A chemical element necessary in large
lichen A symbiotic relationship between fungi and amounts (usually 50 mg/kg in the plant) for the growth
cyanobacteria (blue-green algae) that enhances colo- of plants. Includes C, H, O, N, P, K, Ca, Mg, and S.
nization of bare minerals and rocks. The fungi supply (Macro refers to quantity and not to the essentiality of
water and nutrients, the cyanobacteria the fixed nitro- the element.) See also micronutrient.
gen and carbohydrates from photosynthesis.
macropores Larger soil pores, generally having a
Liebig’s law The growth and reproduction of an diameter greater than 0.06 mm, from which water
organism are determined by the nutrient substance drains readily by gravity.
(oxygen, carbon dioxide, calcium, etc.) that is available
in minimum quantity with respect to organic needs;
map unit (mapping unit), soil A conceptual group of one
to many component soils, delineated or identified by the
the limiting factor. Also attributed to Sprengel .
same name in a soil survey, that represent similar land-
light soil (Obsolete in scientific use) A coarse-textured scape areas. See also delineation, soil consociation, soil
soil; a soil with a low drawbar pull and hence easy to complex, soil association, and undifferentiated group.
cultivate. See also coarse texture; soil texture.
marl Soft and unconsolidated calcium carbonate, usu-
lignin The complex organic constituent of woody fibers ally mixed with varying amounts of clay or other
in plant tissue that, along with cellulose, cements the impurities.
cells together and provides strength. Lignins resist micro-
bial attack and after some modification may become part
marsh Periodically wet or continually flooded area with
the surface not deeply submerged. Covered dominantly
of the soil organic matter.
with sedges, cattails, rushes, or other hydrophytic plants.
lime (agricultural) In strict chemical terms, calcium Subclasses include freshwater and saltwater marshes.
oxide. In practical terms, a material containing the car-
bonates, oxides, and/or hydroxides of calcium and/or
mass flow Movement of nutrients with the flow of
water to plant roots.
magnesium used to neutralize soil acidity.
lime requirement The mass of agricultural limestone, matric potential That portion of the total soil water
potential due to the attractive forces between water and
or the equivalent of other specified liming material,
soil solids as represented through adsorption and capil-
required to raise the pH of the soil to a desired value
larity. It will always be negative.
under field conditions.
limestone A sedimentary rock composed primarily of cal- mature soil A soil with well-developed soil horizons
produced by the natural processes of soil formation and
cite (CaCO3). If dolomite (CaCO3 • MgCO3) is present in
essentially in equilibrium with its present environment.
appreciable quantities, it is called a dolomitic limestone.
limiting factor See Liebig’s law. maximum retentive capacity The average moisture
content of a disturbed sample of soil, 1 cm high,
liquid limit (LL) See Atterberg limits. which is at equilibrium with a water table at its lower
surface.
lithosequence A group of related soils that differ, one
from the other, in certain properties primarily as a mechanical analysis (Obsolete) See particle size analysis;
result of parent material as a soil-forming factor. particle size distribution.
loam The textural-class name for soil having a moder- medium texture Intermediate between fine-textured and
ate amount of sand, silt, and clay. Loam soils contain 7 coarse-textured (soils). It includes the following textural
to 27% clay, 28 to 50% silt, and 23 to 52% sand. classes: very fine sandy loam, loam, silt loam, and silt.
GLOSSARY 939

15. Z03_BRAD9383_14_SE_GLOS.QXD 8/14/07 2:42 AM Page 940
melanic epipedon A diagnostic surface horizon formed matter. Usually contains <20% organic matter, but may
in volcanic parent material, that contains more than contain an organic surface layer up to 30 cm thick.
6% organic carbon, is dark in color, and has a very low mineralization The conversion of an element from
bulk density and high anion adsorption capacity. an organic form to an inorganic state as a result of
mellow soil A very soft, very friable, porous soil with- microbial decomposition.
out any tendency toward hardness or harshness. See minimum tillage See tillage, conservation.
also consistence.
minor element (Obsolete) See micronutrient.
mesic A soil temperature class with mean annual
temperature 8 to 15°C. moderately coarse texture Consisting predominantly
of coarse particles. In soil textural classification, it
mesofauna Animals of medium size, between approx- includes all the sandy loams except the very fine sandy
imately 2 and 0.2 mm in diameter. loam. See also coarse texture.
mesophilic Pertaining to moderate temperatures in moderately fine texture Consisting predominantly of
the range of 15 to 35°C, the range in which mesophilic intermediate-sized (soil) particles or with relatively
organisms grow best and in which mesophilic com- small amounts of fine or coarse particles. In soil textural
posting takes place. classification, it includes clay loam, sandy loam, sandy
metamorphic rock A rock that has been greatly altered clay loam, and silty clay loam. See also fine texture.
from its previous condition through the combined moisture potential See soil water potential.
action of heat and pressure. For example, marble is a
metamorphic rock produced from limestone, gneiss is mole drain Unlined drain formed by pulling a bullet-
produced from granite, and slate is produced from shale. shaped cylinder through the soil.
methane, CH4 An odorless, colorless gas commonly mollic epipedon A diagnostic surface horizon of min-
produced under anaerobic conditions. When released eral soil that is dark colored and relatively thick, con-
to the upper atmosphere, methane contributes to tains at least 0.6% organic carbon, is not massive and
global warming. See also greenhouse effect. hard when dry, has a base saturation of more than 50%,
has less than 250 mg/kg P2O5 soluble in 1% citric acid,
micas Primary aluminosilicate minerals in which two and is dominantly saturated with bivalent cations.
silica tetrahedral sheets alternate with one alumina/
magnesia octahedral sheet with entrapped potassium Mollisols An order in Soil Taxonomy. Soils with nearly
atoms fitting between sheets. They separate readily into black, organic-rich surface horizons and high supply of
visible sheets or flakes. bases. They have mollic epipedons and base saturation
greater than 50% in any cambic or argillic horizon.
microfauna That part of the animal population which They lack the characteristics of Vertisols and must not
consists of individuals too small to be clearly distin- have oxic or spodic horizons.
guished without the use of a microscope. Includes pro-
tozoans and nematodes. molybdenosis A nutritional disease of ruminant ani-
mals in which high Mo in the forage interferes with
microflora That part of the plant population which copper absorption.
consists of individuals too small to be clearly distin-
guished without the use of a microscope. Includes montmorillonite An aluminosilicate clay mineral in the
actinomycetes, algae, bacteria, and fungi. smectite group with a 2:1 expanding crystal lattice, with
two silicon tetrahedral sheets enclosing an aluminum
micronutrient A chemical element necessary in only octahedral sheet. Isomorphous substitution of magne-
extremely small amounts (<50 mg/kg in the plant) for sium for some of the aluminum has occurred in the octa-
the growth of plants. Examples are B, Cl, Cu, Fe, Mn, hedral sheet. Considerable expansion may be caused by
and Zn. (Micro refers to the amount used rather than to water moving between silica sheets of contiguous layers.
its essentiality.) See also macronutrient.
mor Raw humus; type of forest humus layer of unin-
micropores Relatively small soil pores, generally found corporated organic material, usually matted or com-
within structural aggregates and having a diameter less pacted or both; distinct from the mineral soil, unless the
than 0.06 mm. Contrast to macropores. latter has been blackened by washing in organic matter.
microrelief Small-scale local differences in topogra- moraine An accumulation of drift, with an initial topo-
phy, including mounds, swales, or pits that are only graphic expression of its own, built within a glaciated
1 m or so in diameter and with elevation differences of region chiefly by the direct action of glacial ice. Examples
up to 2 m. See also gilgai. are ground, lateral, recessional, and terminal moraines.
mineral (i) An inorganic compound of defined composi- morphology, soil The constitution of the soil, includ-
tion found in rocks. (ii) An adjective meaning inorganic. ing the texture, structure, consistence, color, and other
mineral nutrient An element in inorganic form used physical, chemical, and biological properties of the var-
by plants or animals. ious soil horizons that make up the soil profile.
mineral soil A soil consisting predominantly of, and hav- mottling Spots or blotches of different color or shades
ing its properties determined predominantly by, mineral of color interspersed with the dominant color.
940 Glossary

16. Z03_BRAD9383_14_SE_GLOS.QXD 8/14/07 2:42 AM Page 941
mucigel The gelatinous material at the surface of roots microorganisms have removed most of the soluble
grown in unsterilized soil. nitrate from the soil solution.
muck Highly decomposed organic material in which nitrification The biochemical oxidation of ammonium
the original plant parts are not recognizable. Contains to nitrate, predominantly by autotrophic bacteria.
more mineral matter and is usually darker in color than nitrogen assimilation The incorporation of nitrogen
peat. See also muck soil; peat. into organic cell substances by living organisms.
muck soil (1) A soil containing 20 to 50% organic nitrogen cycle The sequence of chemical and biological
matter. (2) An organic soil in which the organic matter
changes undergone by nitrogen as it moves from the
is well decomposed.
atmosphere into water, soil, and living organisms, and
mulch Any material such as straw, sawdust, leaves, plas- upon death of these organisms (plants and animals) is
tic film, and loose soil that is spread upon the surface of recycled through a part or all of the entire process.
the soil to protect the soil and plant roots from the effects
nitrogen fixation The biological conversion of ele-
of raindrops, soil crusting, freezing, evaporation, etc.
mental nitrogen (N2) to organic combinations or to
mulch tillage See tillage, conservation. forms readily utilized in biological processes.
mull A humus-rich layer of forested soils consisting of nodule bacteria See rhizobia.
mixed organic and mineral matter. A mull blends into
nonacid cations Those cations that do not react with
the upper mineral layers without an abrupt change in
water by hydrolysis to release H+ ions to the soil solu-
soil characteristics.
tion. These cations do not remove hydroxyl ions from
Munsell color system A color designation system that solution, but form strongly dissociated bases such as
specifies the relative degrees of the three simple vari- potassium hydroxide (K+ + OH). Formerly called base
ables of color: cations or base-forming cations in soil science literature.
chroma The relative purity, strength, or saturation nonacid saturation The proportion or percentage of a
of a color.
cation-exchange site occupied by nonacid cations.
hue The chromatic gradation (rainbow) of light Formerly termed base saturation.
that reaches the eye.
value The degree of lightness or darkness of the nonhumic substances The portion of soil organic mat-
color. ter comprised of relatively low molecular weight
organic substances; mostly identifiable biomolecules.
mycelium A stringlike mass of individual fungal or
actinomycetes hyphae. nonlimiting water range The region bounded by the
upper and lower soil water content over which water,
myco Prefix designating an association or relationship oxygen, and mechanical resistance are not limiting to
with a fungus (e.g., mycotoxins are toxins produced by
plant growth. Compare with available water.
a fungus).
nonpoint source A pollution source that cannot be
mycorrhiza The association, usually symbiotic, of traced back to a single origin or source. Examples
fungi with the roots of seed plants. See also ectotrophic
include water runoff from urban areas and leaching
mycorrhiza; endotrophic mycorrhiza; arbuscular my-
from croplands.
corrhiza.
no-tillage See tillage, conservation.
natric horizon A diagnostic subsurface horizon that
satisfies the requirements of an argillic horizon, but nucleic acids Complex organic acids found in the
that also has prismatic, columnar, or blocky structure nuclei of plant and animal cells; may be combined
and a subhorizon having more than 15% saturation with proteins as nucleoproteins.
with exchangeable sodium.
O horizon Organic horizon of mineral soils.
necrosis Death associated with discoloration and dehy-
dration of all or parts of plant organs, such as leaves.
ochric epipedon A diagnostic surface horizon of min-
eral soil that is too light in color, too high in chroma,
nematodes Very small (most are microscopic) unseg- too low in organic carbon, or too thin to be a plaggen,
mented round worms. In soils they are abundant and mollic, umbric, anthropic, or histic epipedon, or that is
perform many important functions in the soil food both hard and massive when dry.
web. Some are plant parasites and considered pests.
octahedral sheet Sheet of horizontally linked,
neutral soil A soil in which the surface layer, at least to octahedral-shaped units that serve as the basic structural
normal plow depth, is neither acid nor alkaline in reac- components of silicate (clay) minerals. Each unit consists
tion. In practice this means the soil is within the pH of a central, six-coordinated metallic atom (e.g., Al, Mg,
range of 6.6 to 7.3. See also acid soil; alkaline soil; pH; or Fe) surrounded by six hydroxyl groups that, in turn,
reaction, soil. are linked with other nearby metal atoms, thereby serv-
nitrate depression period A period of time, beginning ing as interunit linkages that hold the sheet together.
shortly after the addition of fresh, highly carbonaceous oligotrophic Environments, such as soils or lakes,
organic materials to a soil, during which decomposer which are poor in nutrients.
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17. Z03_BRAD9383_14_SE_GLOS.QXD 8/14/07 2:42 AM Page 942
order, soil The category at the highest level of general- outer-sphere complex A relatively weak (easily reversed)
ization in Soil Taxonomy. The properties selected to chemical association or general attraction between an
distinguish the orders are reflections of the degree of ion and an oppositely charged soil colloid via mutual
horizon development and the kinds of horizons present. attraction for intervening water molecules.
organic farming A system/philosophy of agriculture outwash plain A deposit of coarse-textured materials
that does not allow the use of synthetic chemicals to (e.g., sands and gravels) left by streams of meltwater
produce plant and animal products, but instead flowing from receding glaciers.
emphasizes the management of soil organic matter and
biological processes. In many countries, products are
oven-dry soil Soil that has been dried at 105°C until it
reaches constant weight.
officially certified as being organic if inspections con-
firm that they were grown by these methods. oxic horizon A diagnostic subsurface horizon that is at
organic fertilizer By-product from the processing of least 30 cm thick and is characterized by the virtual
animal or vegetable substances that contain sufficient absence of weatherable primary minerals or 2:1 lattice
plant nutrients to be of value as fertilizers. clays and the presence of 1:1 lattice clays and highly
insoluble minerals, such as quartz sand, hydrated
organic soil A soil in which more than half of the pro- oxides of iron and aluminum, low cation exchange
file thickness is comprised of organic soil materials. capacity, and small amounts of exchangeable bases.
organic soil materials (As used in Soil Taxonomy): oxidation The loss of electrons by a substance; there-
(1) Saturated with water for prolonged periods unless fore, a gain in positive valence charge and, in some
artificially drained and having 18% or more organic cases, the chemical combination with oxygen gas.
carbon (by weight) if the mineral fraction is more than
60% clay, more than 12% organic carbon if the mineral oxidation ditch An artificial open channel for partial
fraction has no clay, or between 12 and 18% carbon if digestion of liquid organic wastes in which the wastes
the clay content of the mineral fraction is between 0 are circulated and aerated by a mechanical device.
and 60%. (2) Never saturated with water for more than oxidation-reduction potential See Eh and pe.
a few days and having more than 20% organic carbon.
Histosols develop on these organic soil materials. There Oxisols An order in Soil Taxonomy. Soils with residual
are three kinds of organic materials: accumulations of low-activity clays, free oxides, kaolin,
fibric materials The least decomposed of all the and quartz. They are mostly in tropical climates.
organic soil materials, containing very high pans Horizons or layers in soils that are strongly com-
amounts of fiber that are well preserved and readily pacted, indurated, or very high in clay content. See also
identifiable as to botanical origin; with very low caliche; claypan; fragipan; hardpan.
bulk density.
hemic materials Intermediate in degree of decom- parent material The unconsolidated and more or less
position of organic materials between the less chemically weathered mineral or organic matter from
decomposed fibric and the more decomposed sapric which the solum of soils is developed by pedogenic
materials. processes.
sapric materials The most highly decomposed of particle density The mass per unit volume of the soil
the organic materials, having the highest bulk den- particles. In technical work, usually expressed as metric
sity, least amount of plant fiber, and lowest water tons per cubic meter (Mg/m3) or grams per cubic cen-
content at saturation. timeter (g/cm3).
orographic Influenced by mountains (Greek oros). particle size The effective diameter of a particle
Used in reference to increased precipitation on the measured by sedimentation, sieving, or micrometric
windward side of a mountain range induced as clouds methods.
rise over the mountain, leaving a rain shadow of
reduced precipitation on the leeward side. particle size analysis Determination of the various
amounts of the different separates in a soil sample, usu-
ortstein An indurated layer in the B horizon of ally by sedimentation, sieving, micrometry, or combi-
Spodosols in which the cementing material consists of nations of these methods.
illuviated sesquioxides (mostly iron) and organic matter.
particle size distribution The amounts of the various
osmotic potential That portion of the total soil water soil separates in a soil sample, usually expressed as
potential due to the presence of solutes in soil water. It weight percentages.
will generally be negative.
particulate organic matter A microbially active frac-
osmotic pressure Pressure exerted in living bodies as a tion of soil organic matter consisting largely of fine
result of unequal concentrations of salts on both sides
particles of partially decomposed plant tissue.
of a cell wall or membrane. Water moves from the area
having the lower salt concentration through the mem- partitioning The distribution of organic chemicals
brane into the area having the higher salt concentra- (such as pollutants) into a portion that dissolves in the
tion and, therefore, exerts additional pressure on the soil organic matter and a portion that remains undis-
side with higher salt concentration. solved in the soil solution.
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18. Z03_BRAD9383_14_SE_GLOS.QXD 8/15/07 4:59 PM Page 943
pascal An SI unit of pressure equal to 1 newton per permafrost (1) Permanently frozen material underly-
square meter. ing the solum. (2) A perennially frozen soil horizon.
peat Unconsolidated soil material consisting largely permanent charge See constant charge.
of undecomposed, or only slightly decomposed,
permanent wilting point See wilting point.
organic matter accumulated under conditions of exces-
sive moisture. See also organic soil materials; peat soil. permeability, soil The ease with which gases, liquids,
or plant roots penetrate or pass through a bulk mass of
peat soil An organic soil containing more than 50% soil or a layer of soil.
organic matter. Used in the United States to refer to the
stage of decomposition of the organic matter, peat petrocalcic horizon A diagnostic subsurface horizon
referring to the slightly decomposed or undecomposed that is a continuous, indurated calcic horizon cemented
deposits and muck to the highly decomposed materials. by calcium carbonate and, in some places, with magne-
See also muck; muck soil; peat. sium carbonate. It cannot be penetrated with a spade or
auger when dry; dry fragments do not slake in water;
ped A unit of soil structure such as an aggregate, crumb, and it is impenetrable by roots.
prism, block, or granule, formed by natural processes (in
contrast to a clod, which is formed artificially). petrogypsic horizon A diagnostic subsurface horizon
that is a continuous, strongly cemented, massive gyp-
pedology The science that deals with the formation, sic horizon that is cemented by calcium sulfate. It can
morphology, and classification of soil bodies as land-
be chipped with a spade when dry. Dry fragments do
scape components.
not slake in water and it is impenetrable by roots.
pedon The smallest volume that can be called a soil. It pH, soil The negative logarithm of the hydrogen ion
has three dimensions. It extends downward to the
activity (concentration) of a soil. The degree of acidity
depth of plant roots or to the lower limit of the genetic
(or alkalinity) of a soil as determined by means of a glass
soil horizons. Its lateral cross section is roughly hexag-
or other suitable electrode or indicator at a specified
onal and ranges from 1 to 10 m2 in size, depending on
moisture content or soil-to-water ratio, and expressed in
the variability in the horizons.
terms of the pH scale.
pedosphere The conceptual zone within the ecosystem phase, soil A subdivision of a soil series or other unit
consisting of soil bodies or directly influenced by them.
of classification having characteristics that affect the
A zone or sphere of activity in which mineral, water, air,
use and management of the soil but do not vary suffi-
and biological components come together to form soils.
ciently to differentiate it as a separate series. Included
Usage is parallel to that for “atmosphere” or “biosphere.”
are such characteristics as degree of slope, degree of
pedoturbation Physical disturbance and mixing of soil erosion, and content of stones.
horizons by such forces as burrowing animals (faunal
pH-dependent charge That portion of the total charge
pedoturbation) or frost churning (cryoturbation).
of the soil particles that is affected by, and varies with,
peneplain A once high, rugged area that has been changes in pH.
reduced by erosion to a lower, gently rolling surface
photomap A mosaic map made from aerial pho-
resembling a plain.
tographs to which place names, marginal data, and
penetrability The ease with which a probe can be other map information have been added.
pushed into the soil. May be expressed in units of dis-
phyllosphere The leaf surface.
tance, speed, force, or work depending on the type of
penetrometer used. physical properties (of soils) Those characteristics, pro-
cesses, or reactions of a soil that are caused by physical
penetrometer An instrument consisting of a rod with forces and that can be described by, or expressed in,
a cone-shaped tip and a means of measuring the force
physical terms or equations. Examples of physical
required to push the rod into a specified increment of
properties are bulk density, water-holding capacity,
soil.
hydraulic conductivity, porosity, pore-size distribution,
perc test See percolation test. and so on.
percolation, soil water The downward movement of physical weathering The breakdown of rock and min-
water through soil. Especially, the downward flow of eral particles into smaller particles by physical forces
water in saturated or nearly saturated soil at hydraulic such as frost action. See also weathering.
gradients of the order of 1.0 or less.
phytotoxic substances Chemicals that are toxic to
percolation test A measurement of the rate of percola- plants.
tion of water in a soil profile, usually to determine the
placic horizon A diagnostic subsurface horizon of a
suitability of a soil for use as a septic tank drain field.
black to dark reddish mineral soil that is usually thin
perforated plastic pipe Pipe, sometimes flexible, with but that may range from 1 to 25 mm in thickness.
holes or slits in it that allow the entrance and exit of air The placic horizon is commonly cemented with iron
and water. Used for soil drainage and for septic effluent and is slowly permeable or impenetrable to water and
spreading into soil. roots.
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plaggen epipedon A diagnostic surface horizon that is or crop characteristics of many very small subunits of
human-made and more than 50 cm thick. Formed by land. This technique commonly uses variable rate equip-
long-continued manuring and mixing. ment, geo positioning systems and computer controls.
plant nutrients See essential element. preferential flow Nonuniform movement of water
and its solutes through a soil along certain pathways,
plastic limit (PL) See Atterberg limits. which are often macropores.
plastic soil A soil capable of being molded or deformed primary consumer An organism that subsists on plant
continuously and permanently, by relatively moderate material.
pressure, into various shapes. See also consistence.
primary mineral A mineral that has not been altered
platy Consisting of soil aggregates that are developed chemically since deposition and crystallization from
predominantly along the horizontal axes; laminated; molten lava.
flaky.
primary producer An organism (usually a photosyn-
plinthite (brick) A highly weathered mixture of sesquiox- thetic plant) that creates organic, energy-rich material
ides of iron and aluminum with quartz and other dilu- from inorganic chemicals, solar energy, and water.
ents that occurs as red mottles and that changes irre-
versibly to hardpan upon alternate wetting and drying. primary tillage See tillage, primary.
plow layer The soil ordinarily moved when land is priming effect The increased decomposition of rela-
plowed; equivalent to surface soil. tively stable soil humus under the influence of much
enhanced, generally biological, activity resulting from
plow pan A subsurface soil layer having a higher bulk the addition of fresh organic materials to a soil.
density and lower total porosity than layers above or
below it, as a result of pressure applied by normal plow- prismatic soil structure A soil structure type with
ing and other tillage operations. prismlike aggregates that have a vertical axis much
longer than the horizontal axes.
plowing A primary broad-base tillage operation that is
performed to shatter soil uniformly with partial to Proctor test A laboratory procedure that indicates the
complete inversion. maximum achievable bulk density for a soil and the
optimum water content for compacting a soil.
point of zero charge The pH value of a solution in
equilibrium with a particle whose net charge, from all productivity, soil The capacity of a soil for producing a
sources, is zero. specified plant or sequence of plants under a specified
system of management. Productivity emphasizes the
point source A pollution source that can be traced capacity of soil to produce crops and should be
back to its origin, which is usually an effluent discharge
expressed in terms of yields.
pipe. Examples are a wastewater treatment plant or a
factory. Opposite of nonpoint source. profile, soil A vertical section of the soil through all its
horizons and extending into the parent material.
polypedon (As used in Soil Taxonomy) Two or more
contiguous pedons, all of which are within the defined prokaryote An organism whose cells do not have a
limits of a single soil series; commonly referred to as a distinct nucleus.
soil individual.
protein Any of a group of nitrogen-containing
pool A portion of a larger store of a substance defined organic compounds formed by the polymerization of a
by kinetic or theoretical properties. For example, the large number of amino acid molecules and that, upon
passive pool organic matter is defined by its very slow hydrolysis, yield these amino acids. They are essential
rate of microbial turnover. Compare to fraction. parts of living matter and are one of the essential food
substances of animals.
pore size distribution The volume of the various sizes
of pores in a soil. Expressed as percentages of the bulk protonation Attachment of protons (H+ ions) to
volume (soil plus pore space). exposed OH groups on the surface of soil particles,
resulting in an overall positive charge on the particle
porosity, soil The volume percentage of the total soil surface.
bulk not occupied by solid particles.
protozoa One-celled eukaryotic organisms, such as
potential acidity The acidity that could potentially be amoeba.
formed if reduced sulfur compounds in a potential acid
sulfate soil were to become oxidized. puddled soil Dense, massive soil artificially compacted
when wet and having no aggregated structure. The
potential acidity The extent to which soils containing condition commonly results from the tillage of a
iron sulfides or sulfidic material could become more
clayey soil when it is wet.
acidic through oxidation process if these soils were dis-
turbed and exposed to air. rain, acid See acid rain.
precision farming The spatially variable management of reaction, soil (No longer used in soil science) The
a field or farm based on information specific to the soil degree of acidity or alkalinity of a soil, usually expressed
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20. Z03_BRAD9383_14_SE_GLOS.QXD 8/14/07 2:42 AM Page 945
as a pH value or by terms ranging from extremely rhizosphere That portion of the soil in the immediate
acid for pH values Ͻ4.5 to very strongly alkaline for pH vicinity of plant roots in which the abundance and
values Ͼ9.0. composition of the microbial population are influ-
reactive nitrogen All forms of nitrogen that are read- enced by the presence of roots.
ily available to biota (mainly ammonia, ammonium, rill A small, intermittent water course with steep sides;
and nitrate with smaller quantities of other com- usually only a few centimeters deep and hence no
pounds including nitrogen oxide gases) as opposed to obstacle to tillage operations.
unreactive nitrogen that exists mostly as inert N2 gas.
rill erosion An erosion process in which numerous small
recharge area A geographic area in which an other- channels of only several centimeters in depth are formed;
wise confined aquifer is exposed to surficial percolation occurs mainly on recently cultivated soils. See also rill.
of water to recharge the groundwater in the aquifer.
riparian zone The area, both above and below the
redox concentrations Zones of apparent accumula- ground surface, that borders a river.
tions of Fe-Mn oxides in soils.
riprap Coarse rock fragments, stones, or boulders
redox depletions Zones of low chroma (<2) where placed along a waterway or hillside to prevent erosion.
Fe-Mn oxides, and in some cases clay, have been
stripped from the soil.
rock The material that forms the essential part of the
earth’s solid crust, including loose incoherent masses
redox potential The electrical potential (measured in such as sand and gravel, as well as solid masses of gran-
volts or millivolts) of a system due to the tendency of ite and limestone.
the substances in it to give up or acquire electrons.
root interception Acquisition of nutrients by a root as
redoximorphic features Soil properties associated with a result of the root growing into the vicinity of the
wetness that result from reduction and oxidation of nutrient source.
iron and manganese compounds after saturation and
desaturation with water. See also redox concentrations;
root nodules Swollen growths on plant roots. Often in
reference to those in which symbiotic microorganisms
redox depletions.
live.
reduction The gain of electrons, and therefore the loss
of positive valence charge, by a substance. In some
rotary tillage See tillage, rotary.
cases, a loss of oxygen or a gain of hydrogen is also r-strategist Opportunistic organisms with short repro-
involved. ductive times that allow them to respond rapidly to the
presence of easily metabolized food sources. Contrast
regolith The unconsolidated mantle of weathered
with k-strategist. See also zymogenous organisms.
rock and soil material on the Earth’s surface; loose
earth materials above solid rock. (Approximately runoff The portion of the precipitation on an area that
equivalent to the term soil as used by many engineers.) is discharged from the area through stream channels.
That which is lost without entering the soil is called
relief The relative differences in elevation between
surface runoff and that which enters the soil before reach-
the upland summits and the lowlands or valleys of a
ing the stream is called groundwater runoff or seepage flow
given region.
from groundwater. (In soil science runoff usually refers to
residual material Unconsolidated and partly weath- the water lost by surface flow; in geology and hydraulics
ered mineral materials accumulated by disintegration runoff usually includes both surface and subsurface flow.)
of consolidated rock in place.
salic horizon A diagnostic subsurface horizon of
resilience The capacity of a soil (or other ecosystem) enrichment with secondary salts more soluble in cold
to return to its original state after a disturbance. water than gypsum. A salic horizon is 15 cm or more in
rhizobacteria Bacteria specially adapted to colonizing thickness.
the surface of plant roots and the soil immediately saline seep An area of land in which saline water
around plant roots. Some have effects that promote seeps to the surface, leaving a high salt concentration
plant growth, while others have effects that are delete- behind as the water evaporates.
rious to plants.
saline soil A nonsodic soil containing sufficient soluble
rhizobia Bacteria capable of living symbiotically with salts to impair its productivity. The conductivity of a sat-
higher plants, usually in nodules on the roots of urated extract is >4 dS/m, the exchangeable sodium
legumes, from which they receive their energy, and adsorption ratio is less than about 13, and the pH is <8.5.
capable of converting atmospheric nitrogen to com-
bined organic forms; hence the term symbiotic nitrogen-
saline-sodic soil A soil containing sufficient exchange-
able sodium to interfere with the growth of most crop
fixing bacteria. (Derived from the generic name
plants and containing appreciable quantities of soluble
Rhizobium.)
salts. The exchangeable sodium adsorption ratio is
rhizoplane The root surface–soil interface. Used to >13, the conductivity of the saturation extract is >4
describe the habitat of root-surface-dwelling microor- dS/m (at 25°C), and the pH is usually 8.5 or less in the
ganisms. saturated soil.
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salinization The process of accumulation of salts in soil. series, soil The soil series is a subdivision of a family
in Soil Taxonomy and consists of soils that are similar in
saltation Particle movement in water or wind where all major profile characteristics.
particles skip or bounce along the stream bed or soil
surface. sewage effluent The liquid part of sewage or waste-
water; it is usually treated to remove some portion
sand A soil particle between 0.05 and 2.0 mm in of the dissolved organic compounds and nutrients
diameter; a soil textural class. present from the original sewage.
sapric materials See organic soil materials. sewage sludge Settled sewage solids combined with
saprolite Soft, friable, weathered bedrock that retains varying amounts of water and dissolved materials,
the fabric and structure of the parent rock but is porous removed from sewage by screening, sedimentation,
and can be dug with a spade. chemical precipitation, or bacterial digestion. Also
called biosolids if certain quality standards are met.
saprophyte An organism that lives on dead organic
material. shear Force, as of a tillage implement, acting at right
angles to the direction of movement.
saturated paste extract The extract from a saturated
soil paste, the electrical conductivity Ec of which gives sheet (Mineralogy) A flat array of more than one
an indirect measure of salt content in a soil. atomic thickness and composed of one or more levels
of linked coordination polyhedra. A sheet is thicker
saturation extract The solution extracted from a satu- than a plane and thinner than a layer. Examples: tetra-
rated soil paste. hedral sheet, octahedral sheet.
saturation percentage The water content of a satu- sheet erosion The removal of a fairly uniform layer of
rated soil paste, expressed as a dry weight percentage. soil from the land surface by runoff water.
savanna (savannah) A grassland with scattered trees, shelterbelt A wind barrier of living trees and shrubs
either as individuals or clumps. Often a transitional established and maintained for protection of farm
type between true grassland and forest. fields. Syn. windbreak.
second bottom The first terrace above the normal shifting cultivation A farming system in which land is
floodplain of a stream. cleared, the debris burned, and crops grown for 2 to 3
secondary mineral A mineral resulting from the years. When the farmer moves on to another plot, the
decomposition of a primary mineral or from the repre- land is then left idle for 5 to 15 years; then the burning
cipitation of the products of decomposition of a pri- and planting process is repeated.
mary mineral. See also primary mineral. short-range order minerals Minerals, such as allophane,
sediment Transported and deposited particles or aggre- whose structural framework consists of short distances
gates derived from soils, rocks, or biological materials. of well-ordered crystalline structure interspersed with
distances of noncrystalline amorphous materials.
sedimentary rock A rock formed from materials
deposited from suspension or precipitated from solu- shrinkage limit (SL) The water content above which a
tion and usually being more or less consolidated. The mass of soil material will swell in volume, but below
principal sedimentary rocks are sandstones, shales, which it will shrink no further.
limestones, and conglomerates. side-dressing The application of fertilizer alongside
row-crop plants, usually on the soil surface. Nitrogen
seedbed The soil prepared to promote the germina-
materials are most commonly side-dressed.
tion of seed and the growth of seedlings.
self-mulching soil A soil in which the surface layer siderophore A nonporphyrin metabolite secreted by
certain microorganisms that forms a highly stable coor-
becomes so well aggregated that it does not crust and
dination compound with iron.
seal under the impact of rain but instead serves as a sur-
face mulch upon drying. silica/alumina ratio The molecules of silicon dioxide
(SiO2) per molecule of aluminum oxide (Al2O3) in clay
semiarid Term applied to regions or climates where minerals or in soils.
moisture is more plentiful than in arid regions but still
definitely limits the growth of most crop plants. silica/sesquioxide ratio The molecules of silicon diox-
Natural vegetation in uncultivated areas is short ide (SiO2) per molecule of aluminum oxide (Al2O3) plus
grasses. ferric oxide (Fe2O3) in clay minerals or in soils.
separate, soil One of the individual-sized groups of silt (1) A soil separate consisting of particles between
mineral soil particles—sand, silt, or clay. 0.05 and 0.002 mm in equivalent diameter. (2) A soil
textural class.
septic tank An underground tank used in the deposi-
tion of domestic wastes. Organic matter decomposes silting The deposition of waterborne sediments in
in the tank, and the effluent is drained into the sur- stream channels, lakes, reservoirs, or on floodplains, usu-
rounding soil. ally resulting from a decrease in the velocity of the water.
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22. Z03_BRAD9383_14_SE_GLOS.QXD 8/14/07 2:42 AM Page 947
site index A quantitative evaluation of the productiv- soil amendment Any material, such as lime, gypsum,
ity of a soil for forest growth under the existing or spec- sawdust, or synthetic conditioner, that is worked into
ified environment. the soil to make it more amenable to plant growth.
slag A product of smelting, containing mostly sili- soil association A group of defined and named taxo-
cates; the substances not sought to be produced as nomic soil units occurring together in an individual
matte or metal and having a lower specific gravity. and characteristic pattern over a geographic region,
comparable to plant associations in many ways.
slash-and-burn See shifting cultivation.
slick spots Small areas in a field that are slick when soil auger A tool used to bore small holes up to several
meters deep in soils in order to bring up samples of
wet because of a high content of alkali or exchangeable
material from various soil layers. It consists of a long
sodium.
T-handle attached to either a cylinder with twisted
slickensides Stress surfaces that are polished and stri- teeth or a screwlike bit.
ated and are produced by one mass sliding past another.
soil classification (Soil Taxonomy) The systematic
slope The degree of deviation of a surface from horizon- arrangement of soils into groups or categories on the
tal, measured in a numerical ratio, percent, or degrees. basis of their characteristics. See order; suborder; great
group; subgroup; family; and series.
slow fraction (of soil organic matter) That portion of
soil organic matter that can be metabolized with soil complex A mapping unit used in detailed soil sur-
great difficulty by the microorganisms in the soil and veys where two or more defined taxonomic units are so
therefore has a slow turnover rate with a half-life in intimately intermixed geographically that it is undesir-
the soil ranging from a few years to a few decades. able or impractical, because of the scale being used, to
Often this fraction is the product of some previous separate them. A more intimate mixing of smaller areas
decomposition. of individual taxonomic units than that described
smectite A group of silicate clays having a 2:1-type under soil association.
lattice structure with sufficient isomorphous substitu- soil compressibility The property of a soil pertaining
tion in either or both the tetrahedral and octahedral to its capacity to decrease in bulk volume when sub-
sheets to give a high interlayer negative charge and jected to a load.
high cation exchange capacity and to permit signifi-
cant interlayer expansion and consequent shrinking soil conditioner Any material added to a soil for the
and swelling of the clay. Montmorillonite, beidellite, purpose of improving its physical condition.
and saponite are in the smectite group. soil conservation A combination of all management and
sodic soil A soil that contains sufficient sodium to land-use methods that safeguard the soil against deple-
interfere with the growth of most crop plants, and in tion or deterioration caused by nature and/or humans.
21/2([Ca2+] + [Mg2+])
which the sodium adsorption ratio is 13 or greater. soil consociation A kind of soil map unit that is
sodium adsorption ratio (SAR) named for the dominant soil taxon in the delineation,
[Na+] and in which at least half of the pedons are of the
SAR = named soil taxon, and most of the remaining pedons
are so similar as to not affect most interpretations.
where the cation concentrations are in millimoles of soil correlation The process of defining, mapping,
charge per liter (mmolc/L). naming, and classifying the kinds of soils in a specific
soil survey area, the purpose being to ensure that soils
soft armor The bioengineering use of organic and/or are adequately defined, accurately mapped, and uni-
inorganic materials combined with plants to create a
formly named.
living vegetation barrier of protection against erosion.
soil erosion See erosion.
soil (1) A dynamic natural body composed of mineral
and organic solids, gases, liquids and living organisms soil fertility See fertility, soil.
which can serve as a medium for plant growth. (2) The
soil genesis See genesis, soil.
collection of natural bodies occupying parts of the
Earth’s surface that is capable of supporting plant soil geography A subspecialization of physical geogra-
growth and that has properties resulting from the inte- phy concerned with the areal distributions of soil types.
grated effects of climate and living organisms acting
upon parent material, as conditioned by topography,
soil horizon See horizon, soil.
over periods of time. soil loss tolerance (T value) (i) The maximum average
annual soil loss that will allow continuous cropping
soil air The soil atmosphere; the gaseous phase of
and maintain soil productivity without requiring addi-
the soil, being that volume not occupied by soil or
tional management inputs. (ii) The maximum soil ero-
liquid.
sion loss that is offset by the theoretical maximum rate
soil alkalinity The degree or intensity of alkalinity of a of soil development, which will maintain an equilib-
soil, expressed by a value >7.0 on the pH scale. rium between soil losses and gains.
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23. Z03_BRAD9383_14_SE_GLOS.QXD 8/14/07 2:42 AM Page 948
soil loss tolerance (T) The maximum rate of annual peds. These secondary units may be, but usually are
soil loss that will permit plant productivity to be main- not, arranged in the profile in such a manner as to give
tained economically and indefinitely. a distinctive characteristic pattern. The secondary units
are characterized and classified on the basis of size,
soil management The sum total of all tillage opera- shape, and degree of distinctness into classes, types,
tions, cropping practices, fertilizer, lime, and other and grades, respectively.
treatments conducted on or applied to a soil for the
production of plants. soil structure classes A grouping of soil structural
units or peds on the basis of size from the very fine to
soil map A map showing the distribution of soil types very coarse.
or other soil mapping units in relation to the prominent
physical and cultural features of the Earth’s surface. soil structure grades A grouping or classification of
soil structure on the basis of inter- and intraaggregate
soil mechanics and engineering A subspecialization of adhesion, cohesion, or stability within the profile. Four
soil science concerned with the effect of forces on the grades of structure, designated from 0 to 3, are recog-
soil and the application of engineering principles to nized: structureless, weak, moderate, and strong.
problems involving the soil.
soil structure types A classification of soil structure based
soil moisture potential See soil water potential. on the shape of the aggregates or peds and their arrange-
soil monolith A vertical section of a soil profile removed ment in the profile, including platy, prismatic, columnar,
from the soil and mounted for display or study. blocky, subangular blocky, granulated, and crumb.
soil morphology The physical constitution, particu- soil survey The systematic examination, description,
larly the structural properties, of a soil profile as exhib- classification, and mapping of soils in an area. Soil sur-
ited by the kinds, thicknesses, and arrangement of the veys are classified according to the kind and intensity
horizons in the profile, and by the texture, structure, of field examination.
consistence, and porosity of each horizon. soil temperature classes A criterion used to differenti-
soil order See order, soil. ate soil in Soil Taxonomy, mainly at the family level.
Classes are based on mean annual soil temperature and
soil organic matter The organic fraction of the soil on differences between summer and winter tempera-
that includes plant and animal residues at various tures at a depth of 50 cm.
stages of decomposition, cells and tissues of soil organ-
isms, and substances synthesized by the soil popula- soil textural class A grouping of soil textural units
tion. Commonly determined as the amount of organic based on the relative proportions of the various soil
material contained in a soil sample passed through a separates (sand, silt, and clay). These textural classes,
2-mm sieve. listed from the coarsest to the finest in texture, are
sand, loamy sand, sandy loam, loam, silt loam, silt,
soil porosity See porosity, soil. sandy clay loam, clay loam, silty clay loam, sandy clay,
soil productivity See productivity, soil. silty clay, and clay. There are several subclasses of the
sand, loamy sand, and sandy loam classes based on the
soil profile See profile, soil. dominant particle size of the sand fraction (e.g., loamy
soil quality The capacity of a specific kind of soil to fine sand, coarse sandy loam).
function, within natural or managed ecosystem bound- soil texture The relative proportions of the various
aries, to sustain plant and animal productivity, maintain soil separates in a soil.
or enhance water and air quality, and support human
health and habitation. Sometimes considered in relation soil water deficit The difference between PET and ET,
to this capacity in the undisturbed, natural state. representing the gap between the amount of evapo-
transpiration water atmospheric conditions “demand”
soil reaction See reaction, soil; pH, soil. and the amount the soil can actually supply. A mea-
soil salinity The amount of soluble salts in a soil, sure of the limitation that water supply places on
expressed in terms of percentage, milligrams per kilo- plant productivity.
gram, parts per million (ppm), or other convenient ratios. soil water potential (total) A measure of the difference
soil separates See separate, soil. between the free energy state of soil water and that of
pure water. Technically it is defined as “that amount of
soil series See series, soil. work that must be done per unit quantity of pure water
soil solution The aqueous liquid phase of the soil and in order to transport reversibly and isothermically an
its solutes, consisting of ions dissociated from the sur- infinitesimal quantity of water from a pool of pure
faces of the soil particles and of other soluble materials. water, at a specified elevation and at atmospheric pres-
sure to the soil water (at the point under considera-
soil strength A transient soil property related to the tion).” This total potential consists of gravitational,
soil’s solid phase cohesion and adhesion. matric, and osmotic potentials.
soil structure The combination or arrangement of pri- solarization The process of heating a soil in the field
mary soil particles into secondary particles, units, or by covering it with clear plastic sheeting during sunny
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24. Z03_BRAD9383_14_SE_GLOS.QXD 8/14/07 2:42 AM Page 949
conditions. The heat is meant to partially sterilize the subgroup, soil The great groups in Soil Taxonomy are
upper 5 to 15 cm of soil to reduce pest and pathogen subdivided into central concept subgroups that show
populations. the central properties of the great group, intergrade sub-
solum (pl. sola) The upper and most weathered part of groups that show properties of more than one great
the soil profile; the A, E, and B horizons. group, and other subgroups for soils with atypical prop-
erties that are not characteristic of any great group.
sombric horizon A diagnostic subsurface horizon that
contains illuvial humus but has a low cation exchange submergence potential The positive hydrostatic pres-
capacity and low percentage base saturation. Mostly sure that occurs below the water table.
restricted to cool, moist soils of high plateaus and suborder, soil A category in Soil Taxonomy that nar-
mountainous areas of tropical and subtropical regions. rows the ranges in soil moisture and temperature
sorption The removal from the soil solution of an ion regimes, kinds of horizons, and composition, accord-
or molecule by adsorption and absorption. This term is ing to which of these is most important.
often used when the exact mechanism of removal is subsoil That part of the soil below the plow layer.
not known.
subsoiling Breaking of compact subsoils, without
species diversity The variety of different biological inverting them, with a special knifelike instrument
species present in an ecosystem. Generally, high diversity (chisel), which is pulled through the soil at depths usu-
is marked by many species with few individuals in each. ally of 30 to 60 cm and at spacings usually of 1 to 2 m.
species richness The number of different species sulfidic Adjective used to describe sulfide-containing
present in an ecosystem, without regard to the distri- soil materials that initially have a pH > 4.0 and exhibit
bution of individuals among those species. a drop of at least 0.5 pH unit within 8 weeks of aerated,
specific gravity The ratio of the density of a mineral moist incubation. Found in potential acid sulfate soils.
to the density of water at standard temperature and sulfuric horizon A diagnostic subsurface horizon in
pressure. either mineral or organic soils that has a pH < 3.5 and
specific heat capacity The amount of kinetic (heat) fresh straw-colored mottles (called jarosite mottles).
energy required to raise the temperature of 1 g of a sub- Forms by oxidation of sulfide-rich materials and is
stance (usually in reference to soil or soil components). highly toxic to plants.
specific surface The solid particle surface area per unit summer fallow See fallow.
mass or volume of the solid particles. surface runoff See runoff.
splash erosion The spattering of small soil particles surface seal A thin layer of fine particles deposited on
caused by the impact of raindrops on very wet soils. the surface of a soil that greatly reduces the permeabil-
The loosened and separated particles may or may not ity of the soil surface to water.
be subsequently removed by surface runoff.
surface soil The uppermost part of the soil, ordinarily
spodic horizon A diagnostic subsurface horizon char- moved in tillage, or its equivalent in uncultivated soils.
acterized by the illuvial accumulation of amorphous Ranges in depth from 7 to 25 cm. Frequently desig-
materials composed of aluminum and organic carbon nated as the plow layer, the Ap layer, or the Ap horizon.
with or without iron.
surface tension The elasticlike phenomenon resulting
Spodosols An order in Soil Taxonomy. Soils with sub- from the unbalanced attractions among liquid mole-
surface illuvial accumulations of organic matter and cules (usually water) and between liquid and gaseous
compounds of aluminum and usually iron. These soils molecules (usually air) at the liquid–gas interface.
are formed in acid, mainly coarse-textured materials in
humid and mostly cool or temperate climates. swamp An area of land that is usually wet or sub-
merged under shallow fresh water and typically sup-
stem flow The process by which rain or irrigation water ports hydrophilic trees and shrubs.
is directed by a plant canopy toward the plant stem so as
to wet the soil unevenly under the plant canopy. symbiosis The living together in intimate association
of two dissimilar organisms, the cohabitation being
stratified Arranged in or composed of strata or layers. mutually beneficial.
strip-cropping The practice of growing crops that synergism (i) The nonobligatory association between
require different types of tillage, such as row and sod, organisms that is mutually beneficial. Both popula-
in alternate strips along contours or across the prevail- tions can survive in their natural environment on their
ing direction of wind. own, although, when formed, the association offers
structure, soil See soil structure. mutual advantages. (ii) The simultaneous actions of
two or more factors that have a greater total effect
stubble mulch The stubble of crops or crop residues together than the sum of their individual effects.
left essentially in place on the land as a surface cover
before and during the preparation of the seedbed and talus Fragments of rock and other soil material accu-
at least partly during the growing of a succeeding crop. mulated by gravity at the foot of cliffs or steep slopes.
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taxonomy, soil The science of classification of soils; tillage, which generally leaves at least 30% of the soil
laws and principles governing the classifying of soil. surface covered by residues, including the following
Also a specific soil classification system developed by systems:
the U.S. Department of Agriculture. minimum tillage The minimum soil manipulation
necessary for crop production or meeting tillage
tensiometer A device for measuring the negative pres- requirements under the existing soil and climatic
sure (or tension) of water in soil in situ; a porous, per-
conditions.
meable ceramic cup connected through a tube to a
mulch tillage Tillage or preparation of the soil in
manometer or vacuum gauge.
such a way that plant residues or other materials are
tension, soil-moisture See soil water potential. left to cover the surface; also called mulch farming,
trash farming, stubble mulch tillage, and plowless
terrace (1) A level, usually narrow, plain bordering a farming.
river, lake, or the sea. Rivers sometimes are bordered by
no-tillage system A procedure whereby a crop is
terraces at different levels. (2) A raised, more or less
planted directly into a seedbed not tilled since har-
level or horizontal strip of earth usually constructed on
vest of the previous crop; also called zero tillage.
or nearly on a contour and designed to make the land
ridge till Planting on ridges formed by cultivation
suitable for tillage and to prevent accelerated erosion
during the previous growing period.
by diverting water from undesirable channels of con-
strip till Planting is done in a narrow strip that has
centration; sometimes called diversion terrace.
been tilled and mixed, leaving the remainder of the
tetrahedral sheet Sheet of horizontally linked, soil surface undisturbed.
tetrahedron-shaped units that serve as one of the
tillage, conventional The combined primary and sec-
basic structural components of silicate (clay) miner-
ondary tillage operations normally performed in
als. Each unit consists of a central four-coordinated
preparing a seedbed for a given crop grown in a given
atom (e.g., Si, Al, Fe) surrounded by four oxygen
geographic area. Usually said of non-conservation
atoms that, in turn, are linked with other nearby atoms
tillage.
(e.g., Si, Al, Fe), thereby serving as interunit linkages to
hold the sheet together. tillage, primary Tillage that contributes to the major soil
manipulation, commonly with a plow.
texture See soil texture.
thermal analysis (differential thermal analysis) A method tillage, rotary An operation using a power-driven rotary
of analyzing a soil sample for constituents, based on a tillage tool to loosen and mix soil.
differential rate of heating of the unknown and standard tillage, secondary Any tillage operations following pri-
samples when a uniform source of heat is applied. mary tillage designed to prepare a satisfactory seedbed
thermic A soil temperature class with mean annual for planting.
temperature 15 to 22 °C. tilth The physical condition of soil as related to its
thermophilic Pertaining to temperatures in the range ease of tillage, fitness as a seedbed, and its impedance
of 45 to 90 °C, the range in which thermophilic organ- to seedling emergence and root penetration.
isms grow best and in which thermophilic composting topdressing An application of fertilizer to a soil after
takes place. the crop stand has been established.
thermophilic organisms Organisms that grow readily toposequence A sequence of related soils that differ,
at temperatures above 45 °C. one from the other, primarily because of topography as a
thixotrophy The property of certain clay soils of soil-formation factor, with other factors constant.
becoming fluid when jarred or agitated and then set- topsoil (1) The layer of soil moved in cultivation. See
ting again when at rest. Similar to quick, as in quick also surface soil. (2) Presumably fertile soil material
clays or quicksand. used to top-dress roadbanks, gardens, and lawns.
tile, drain Pipe made of burned clay, concrete, or trace elements Elements present in the Earth’s crust in
ceramic material, in short lengths, usually laid with concentrations less than 1000 mg/kg. When referring
open joints to collect and carry excess water from the to plant nutrients, the term micronutrients is preferred.
soil.
trioctahedral An octahedral sheet of silicate clays in
till (1) Unstratified glacial drift deposited directly by which the sites for the six-coordinated metallic atoms
the ice and consisting of clay, sand, gravel, and boul- are mostly filled with divalent cations, such as Mg2+.
ders intermingled in any proportion. (2) To plow and
prepare for seeding; to seed or cultivate the soil. trophic level Levels in a food chain that pass nutrients
and energy from one group of organisms to another.
tillage The mechanical manipulation of soil for any
purpose; but in agriculture it is usually restricted to the truncated Having lost all or part of the upper soil
modifying of soil conditions for crop production. horizon or horizons.
tillage, conservation Any tillage sequence that tuff Volcanic ash usually more or less stratified and in
reduces loss of soil or water relative to conventional various states of consolidation.
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tundra A level or undulating treeless plain characteris- water potential, soil See soil water potential.
tic of arctic regions.
water table The upper surface of groundwater or that
Ultisols An order in Soil Taxonomy. Soils that are low in level below which the soil is saturated with water.
bases and have subsurface horizons of illuvial clay accu-
mulations. They are usually moist, but during the warm
water table, perched The surface of a local zone of
saturation held above the main body of groundwater
season of the year some are dry part of the time.
by an impermeable layer of stratum, usually clay, and
umbric epipedon A diagnostic surface horizon of min- separated from the main body of groundwater by an
eral soil that has the same requirements as the mollic unsaturated zone.
epipedon with respect to color, thickness, organic car-
bon content, consistence, structure, and P2O5 content, water use efficiency Dry matter or harvested portion
but that has a base saturation of less than 50%. of crop produced per unit of water consumed.
universal soil loss equation (USLE) An equation for waterlogged Saturated with water.
predicting the average annual soil loss per unit area per watershed All the land and water within the geo-
year; A = RKLSPC, where R is the climatic erosivity fac- graphical confines of a drainage divide or surrounding
tor (rainfall plus runoff), K is the soil erodibility factor, ridges that separate the area from neighboring water-
L is the length of slope, S is the percent slope, P is the sheds.
soil erosion practice factor, and C is the cropping and
management factor. water-stable aggregate A soil aggregate stable to the
action of water, such as falling drops or agitation, as in
unsaturated flow The movement of water in a soil wet-sieving analysis.
that is not filled to capacity with water.
weathering All physical and chemical changes pro-
vadose zone The aerated region of soil above the per- duced in rocks, at or near the Earth’s surface, by atmo-
manent water table. spheric agents.
value (color) See Munsell color system. wetland An area of land that has hydric soil and
variable charge See pH-dependent charge. hydrophytic vegetation, typically flooded for part of
the year, and forming a transition zone between
varnish, desert A glossy sheen or coating on stones aquatic and terrestrial systems.
and gravel in arid regions.
wetting front The boundary between the wetted soil
vermicompost Compost made by earthworms eating and dry soil during infiltration of water.
raw organic materials in moist aerated piles, which are
kept shallow to avoid heat buildup that could kill the wilting point (permanent wilting point) The moisture
worms. content of soil, on an oven-dry basis, at which plants
wilt and fail to recover their turgidity when placed in
vermiculite A 2:1-type silicate clay, usually formed a dark, humid atmosphere.
from mica, that has a high net negative charge stem-
ming mostly from extensive isomorphous substitution windbreak Planting of trees, shrubs, or other vegeta-
of aluminum for silicon in the tetrahedral sheet. tion perpendicular, or nearly so, to the principal wind
direction to protect soils, crops, homesteads, etc., from
Vertisols An order in Soil Taxonomy. Clayey soils with wind and snow.
high shrink–swell potential that have wide, deep cracks
when dry. Most of these soils have distinct wet and dry xenobiotic Compounds foreign to biological sys-
periods throughout the year. tems. Often refers to compounds resistant to decom-
position.
vesicles (1) Unconnected voids with smooth walls.
(2) Spherical structures formed inside root cortical cells xerophytes Plants that grow in or on extremely dry
by vesicular arbuscular mycorrhizal fungi. soils or soil materials.
virgin soil A soil that has not been significantly dis- zero tillage See tillage, conservation.
turbed from its natural condition.
zymogenous organisms So-called opportunist organ-
water deficit (soil) The amount of available water isms found in soils in large numbers immediately fol-
removed from the soil within the vegetation’s active lowing addition of readily decomposable organic mate-
rooting depth, or the amount of water required to rials. Contrast with autochthonous organisms. See also
bring the soil to field capacity. r-strategist.
GLOSSARY 951

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