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"Soils are developed; they are not merely an accumulation of debris resulting from decay of rock and organic materials.... In other words, a soil is an entity--an object in nature which has characteristics that distinguish it from all other objects in nature."

C. E. Millar and L. M. Turk, 1943

Note: A full glossary of soil science terms can be found at the Web site of the Soil Science Society of America (http://www.


A Horizon (Chapter 3)--The natural surface layer of a mineral soil.

Acellular (Chapter 9)--Organisms lacking structures such as cell nucleus, cell membrane, and cell wall associated with cellular organisms.

Acidity (Chapter 8)--The extent to which a soil has a pH < 7.0.

Acidophilic (Chapter 9)--Preferentially growing in acidic conditions.

Active Acidity (Chapter 8)--The amount of acidity in soil solution directly measured by pH strips or a pH meter.

Active and Passive SOM (Chapter 11)--Two fractions of SOM that are readily available and recalcitrant to decomposition, respectively.

Adsorption (Chapter 14)--The process by which atoms, molecules, or ions are taken up and retained by solid surfaces by physical and chemical mechanisms.

Aeration (Chapter 4)--The extent to which an environment has available oxygen.


Anaerobes (Chapter 9)--Aerobes grow in the presence of 02, anaerobes grow only in its absence, and facultative anaerobes can grow in either the presence or absence of [O.sub.2].

Air Dry (Chapter 6)--Soil that has been allowed to dry to the maximum point in ambient conditions through evaporation.

Albedo (Chapter 13)--The ratio of the amount of solar radiation reflected relative to that received expressed as a percent. The albedo of the Earth is approximately 34 percent.

Alfisol (Chapter 3)--One of the soil orders in the U.S. taxonomic system. Alfisols are typically fertile deciduous forest soils that have an accumulation of clay in the B horizon.

Alkali Soil (Chapter 14)--Synonymous with sodic soil, it is a soil with sufficient sodium (Na) to interfere with plant growth.

Alkalinity (Chapter 8)--The extent to which a soil has a pH greater than 7.0.

Alkalophilic (Chapter 9)--Preferentially growing in alkaline environments.

Allelopathic (Chapter 12)--Toxic or inhibitory, specifically, related to the inhibition of one plant by the products of another.

Alluvial (Chapter 2)--Having formed from the deposits of rivers and streams, in general the deposits of any running water.

Alluvial Fan (Chapter 2)--Deposits from a stream as it enters a plain or larger stream.

Ammensalism (Chapter 9)--An interaction in which the activity of one organism is harmful to the growth of another.

Ammonification (Chapter 10)--The mineralization or decomposition of organic N with the release of [NH.sub.4.sup.+].

Anaerobic (Chapter 10)--Lacking oxygen or oxygen-free; contrast this with aerobic, which means an environment containing oxygen.

Andisol (Chapter 3)--One of the soil orders in the U.S. taxonomic system. Andisols are fertile soils that form from volacanic deposits.

Anion Exchange Capacity (AEC) (Chapters 8, 14)--The total capacity of the adsorptive components of a soil to attract and exchange anions.

Anthropogenic (Chapters 2, 3)--Of or pertaining to human influence. With respect to soil it is the transformation of soils by human activities such as tillage and earth moving.

Aquicludes (Chapter 13)--An impervious clay and/or rock layer that prevents water movement.

Aquifer (Chapter 13)--An underground layer of permeable material that can store and supply water.

Aquitard (Chapter 13)--A relatively imperious layer in soil that retards water movement.

Archaea (Chapter 9)--A branch of prokaryotic organisms.

Argillic Horizon (Chapter 3)--The B horizon of a soil that contains more clay than the overlying A horizon due to illuviation.

Aridisol (Chapter 3)--One of the soil orders in the U.S. taxonomic system. Aridisols are hot desert soils that have a cambic or argillic horizon. They can be fertile if irrigated.

Aromatic Compounds (Chapter 11)--Compounds that have as their basis a benzene ring.

Assimilation (Chapter 10)--Incorporation of organic and inorganic compounds and/or elements into living organisms.

Atmosphere (Chapter 1)--The gaseous environment surrounding earth and extending into the soil profile.

Autotroph (Chapter 9)--An organism that obtains its carbon for growth from C[O.sub.2] or bicarbonate.


B Horizon (Chapter 3)--A subsoil horizon underlying either an A or E horizon. B horizons may be exposed if surface erosion is significant.

Bacteriophage (Chapter 9)--Viruses that infect bacteria.

Base Cation Saturation (BCS) (Chapter 8)--The extent to which a soil is saturated with base cations such as [Ca.sup.2+], [Mg.sup.2+], and [K.sup.+].

Base Cations (Chapter 7)--Commonly refers to the [Ca.sup.2+], [Mg.sup.2+], [K.sup.+], and [Na.sup.+] in soil.

Bedrock (Chapter 2)--The solid rock underlying unconsolidated material.

Biogeochemistry (Chapter 10)--The study of the interaction of chemical and biochemical transformations of elements, minerals, and organic matter in Earth's environment.

Biosphere (Chapter 1)--The living organisms in soil.

Bioturbation (Chapter 9)--The mixing or disturbing of soil by the burrowing or excavating activity of soil organisms.

Buffer Capacity (Chapter 8)--The capacity of a soil to resist change, such as change in pH, and the compounds in soil that contribute to that resistance.

Bulk Density (Chapter 5)--By definition, the mass of soil divided by the volume of soil; the greater the bulk density, the less porous a soil.


C Horizon (Chapter 3)--The least developed soil horizon that lies beneath the B horizon.

C Sequestration (Chapter 16)--The storage or retention of carbon in plants, precipitates, and organic matter in soil that makes it temporarily unavailable.

Calcareous (Chapter 14)--High in calcium.

Caliche (Chapter 3)--A type of impermeable layer in soil composed of cemented carbonates.

Cambic Horizon (Chapter 3)--A weakly developed subsoil or B horizon.

Capillarity (Chapter 6)--The attraction of water to itself and to soil particles that allows water to rise through soil pores from above a water table.

Cast (Chapter 9)--The mixed mineral and organic deposits left by earthworms on the surface of soils.

Catena (Chapter 15)--A chain or sequence of soils from the top of a hill to the foot slope. The soils will have formed from the same parent material, and be of approximately the same age, but will differ with respect to such other factors as relief and drainage.

Cation Exchange Capacity (CEC) (Chapters 8, 14)--The capacity of the adsorptive components of a soil to attract and exchange cations.

Chelate (Chapters 10, 11)--From the Greek "claw," this refers to the attraction of reactive constituents of compounds to ions in soil which they bind. Chelating agents bind metals and ions in solutions and keep them in soluble and therefore available form.

Chemodenitrification (Chapter 10)--A chemical process in which N-oxides are lost in gaseous form during N reduction.

Chemotroph (Chapter 9)--An organism that gets its energy from breaking chemical bonds in either organic or inorganic compounds.

Chernozem (Chapter 3)--"Black Earth," a term equivalent to Mollisol used in other soil taxonomic systems.

Chlorosis (Chapter 10)--A lack of chlorophyll in plant tissue that leads to light green or yellowish color and is typically associated with nitrogen or iron deficiency.

Chroma (Chapter 3)--One of the three variables of soil color. Chroma reflects the relative strength or purity of a color.

Clastic (Chapter 2)--Rock formed from mineral or rock fragments or other rocks.

Clay (Chapter 4)--By definition a mineral particle smaller than two micrometers in size. Clay can also refer to specific types of soil minerals such as silicate clays.

Coarse Fragments (Chapter 4)--Mineral soil or material too coarse to pass through a 2-mm sieve.

Coenocytic (Chapter 9)--Fungi that lack internal cell walls in the hyphae.

Colloid (Chapters 7, 14)--Clay or organic particles so small that they remain in suspension in standing water.

Colluvium (Chapter 2)--The soil or rock material deposited at the foot of a slope, primarily through the force of gravity. This includes talus and cliff debris.

Commensalism (Chapter 9)--An interaction in which the activity of one organism is beneficial to the growth of another.

Compaction (Chapter 12)--Reduction in soil volume by mechanical pressure leading to decreased porosity and degraded soil properties.

Competition (Chapter 9)--An interaction in which two organisms have simultaneous demands for the same limiting substrate.

Complex (Chapter 15)--A type of map unit used in soil surveys that reflects the appearance of one or more intermingled soil series.

Conduction (Chapter 6)--Heat transfer brought about by movement of kinetic energy from one body in direct contact with another.

Conservation Tillage (Chapter 12)--One of several methods of tillage that minimizes soil disturbance.

Convection (Chapter 6)--Heat transfer due to the movement of fluid (either liquid or air).

Convective Precipitation (Chapter 13)--Occurs when a moist layer of air near the Earth's surface is heated, rapidly lifted, and cooled. If the uplifting is high enough in the atmosphere ice crystals can result.

CRP (Chapter 16)--Conservation Reserve Program.

Crusting (Chapter 4)--Formation of dense, hard, or brittle layers at the soil surface.

Cryophiles/Psychrophiles (Chapter 9)--Organisms that preferentially grow in cold environments.

Cryptozoans (Chapter 9)--Organisms that preferentially inhabit shaded locations beneath rocks and wood.


Denitrification (Chapter 10)--A respiratory process occurring in anaerobic environments in which N-oxides are used as electron acceptors in place of oxygen.

Desertification (Chapter 12)--A process in which arable land is replaced by progressively desertlike conditions.

Diagnostic Soil Horizons (Chapter 3)--The horizons that are most important for soil classification in assigning soils to representative soil orders.

Diffusion (Chapter 14)--The movement, particularly with respect to nutrients, of compounds in soil in response to concentration gradients.

Discharge (Chapter 13)--The amount of water flow in a stream. Discharge (Q) is the product of velocity (VI and cross-sectional area (A) of the stream (Q = VA).

Disproportionation (Chapter 10)--A chemical process occurring anaerobically in which S compounds are transformed to yield oxidized and reduced molecules.

Dissolution (Chapter 7)--Another term for dissolve.

Divalent Cations or Anions (Chapters 7, 8, 14)--Ions have two charges, either positive (divalent cation, e.g., [Ca.sup.2+]) or negative (divalent anion, e.g., S[O.sub.4.sup.2-]).

Dolomite (Chapter 14)--Calcium-magnesium carbonate, also referred to as limestone.

Drainageways (Chapter 15)--Planned or natural areas through which water drains.

Drilosphere (Chapter 9)--The zone of soil impacted by the burrowing activity of earthworms.

Duripan (Chapter 3)--A relatively impermeable layer in soil composed of cemented silica.


Ecosystem (Chapter 16)--A given environment and all its components and those external forces working upon it.

Ectomycorrhizae (Chapter 9)--A type of mycorrhizal association in which the symbiotic fungi forms an extensive mantle around the plant roots.

Eluvial (Chapter 2)--Material that has been removed by suspension or solution; usually in reference to a soil horizon from which material has been removed. Leaching is usually used to describe the loss of material in solution.

Eluviation (Chapter 3)--Removal (loss) of soil material in suspension or solution from a soil layer.

Encyst (Chapter 9)--To enter a state of low metabolic activity protected from the environment until growth conditions improve.

Endomycorrhizae (Chapter 9)--A type of mycorrhizal association that is obligatory.

Entisol (Chapter 3)--One of the soil orders in the U.S. taxonomic system. Entisols are "new" or weakly developed soils.

Eolian (Chapter 2)--Material deposited by wind, such as sand dunes, sand sheets, and particularly loess.

Epipedon (Chapter 3)--The diagnostic surface soil horizon.

Equilibrium (Chapter 8)--A state in which change can occur, but no net change in state is observable.

EQUIP (Chapter 16)--Environmental Quality Incentives Program.

Equivalent Surface Depth (Chapter 6)--The amount of water required to fill a uniform layer of soil completely.

Erodibility (Chapter 4)--The extent to which soil can be eroded or lost.

Erosion (Chapter 12)--The process by which soil is moved, suspended, washed, or blown from one location to another.

Essential Nutrients (Chapter 14)--The elements in the periodic table essential for the life of plants and animals.

Eukaryotes (Chapter 9)--Organisms that possess a cell nucleus.

Eutrophication (Chapters 12, 13)--Excessive growth. Having conditions where nutrients are optimal or excessive for growth. Usually with respect to uncontrolled growth in aquatic environments leading to degraded water quality.

Evapotranspiration (ET) (Chapter 13)--The combined water loss from a given area of evaporation from the soil surface and transpiration by plants.

Exchange Capacity (Chapter 8)-The total ionic charge of the adsorption complex that is active in ion adsorption.

Exchangeable Acidity (Chapter 8)--Acidic cations that can be exchanged from soil colloids by salt solutions.

Exchangeable Bases (Chapter 14)--Typically refers to [Ca.sup.2+], [Mg.sup.2+], and [K.sup.+] that are adsorbed and desorbed from the CEC.

Expanding Clays (Chapter 14)--2:1 layer silicate clays that shrink and swell in response to water.


FACTA (Chapter 16)--Food, Agriculture, Conservation, and Trade Act.

FRIRA (Chapter 16)--Federal Agriculture Improvement and Reform Act.

Fallow (Chapter 12)--Leaving uncropped or untilled.

Family (Chapter 3)--The fifth level of classification in the U.S. system of soil taxonomy. Family names use descriptive terms to describe soil features such as texture, mineralogy, and temperature.

Fauna (Chapter 9)--The animals (macro-and microscopic) present at a site.

Feldspar (Chapter 4)--The most common primary mineral in the Earth's crust.

Fenestration (Chapter 9)--The opening of the leaf epidermis and exposure of tissue to microbial attack; the initial step in the decomposition of leaves.

Ferment (Chapter 9)--Growth by internal cycling of electrons and substrate-level phosphorylation.

Fermentation (Chapter 10)--An oxygen-free process in which a compound acts as both source and sink of electrons, resulting in the release of oxidized and reduced products and the production of high-energy metabolic intermediates.

Fertility (Chapter 1)--The nutrient status of a soil; reflects a soil's capacity to promote growth.

Field Capacity (Chapter 6)--A description of soil water content in which all but the largest freely draining pores are water-filled; typically expressed as the percent of water by weight held by a soil by capillary action after larger pores have drained by the force of gravity.

Flocculate (Chapter 5)--The aggregation of soil particles, particularly in suspension, that causes them to grow in size.

Floodplain (Chapter 2)--The land bordering a stream that has been built up by sedimentation when the stream overflows, and which is subject to periodic inundation.

Flora (Chapter 9)--The plants (macro-and microscopic) present at a site.

Fragipan (Chapter 2)--Dense subsoil layers that are relatively impermeable, brittle when moist and dry but not when wet.

FSA (Chapter 16)--Food Security Act.

Fulvic Acid (Chapter 11)--Yellow to brown material soluble in alkali and acid. Fulvic acid consists of compounds of lower molecular weight and greater oxidation than humic acid.


Gelisol (Chapter 3)--One of the soil orders in the U.S. taxonomic system. Gelisols are soils of cold regions typified by permafrost.

Generalized Soil Maps (Chapter 15)--A simplified survey map used at the county level to reflect the major soil associations and their distribution.

Glacial Drift (Chapter 2)--Deposits made by glaciers and their outwash.

Glacial Till (Chapter 2)--Unsorted debris left by a glacier.

Glaciation (Chapter 2)--The process by which large, slowly moving masses of ice transport soil and other debris across a landscape.

Glomalin (Chapter 11)--A glycoprotein produced by mycorrhizal fungi that helps develop soil aggregation.

Granite (Chapter 2)--A light-colored, crystalline igneous rock containing about 25 percent quartz.

Grassed Waterway (Chapter 12)--A grass-covered channel in fields used to carry or divert surface runoff.

Gravimetric Water Content (Chapter 6)--The water content in soil on a mass/mass basis.

Gravitational Potential (Chapter 6)--A mathematical description of the tendency of water to move by the force of gravity.

Great Group (Chapter 3)--The third level of classification in the U.S. system of soil taxonomy. Great group formative elements provide additional information about a soil.

Groundwater (Chapter 13)--Water beneath the Earth's surface in saturated soil or porous rock.

Gully Erosion (Chapter 12)--A type of soil erosion, along with ephemeral gully erosion, in which a permanent channel caused by extreme soil loss occurs.


Hardpan (Chapter 3)--A soil layer that acts as a barrier to root and water movement.

Heterotroph (Chapter 9)--An organism that obtains its energy and C for growth from organic matter.

Heterotrophic Nitrification (Chapter 10)--A process carried out by heterotrophic bacteria and fungi in which reduced organic N is oxidized to yield N[O.sub.2.sup.-] and N[O.sub.3.sup.-].

Highly Erodible Land (Chapter 15)--Land susceptible to erosion. By definition, based on the RUSLE, these lands have an erodibility index greater than 8.

Histosol (Chapter 3)--One of the soil orders in the U.S. taxonomic system. Histosols are typified by accumulation of organic material.

Horizon (Chapters 1, 2, 3)--A natural layer in soil in either the surface or subsurface that forms parallel to the land surface during the process of soil formation. Horizons differ from one another by physical, chemical, and biological properties, texture consistency, acidity, and color, among other properties.

Hue (Chapter 3)--One of the three variables of soil color. Hue reflects the light of certain wavelengths, or perceived color.

Humic Acid (Chapter 11)--brown to black organic material extractable from soil by alkali solution and insoluble in acid.

Humification (Chapter 9)--The formation of soil humus through digestion, fragmentation, and metabolism.

Humin (Chapter 11)--The fraction of SOM that cannot be extracted from soil with an alkali solution and that consists of high-molecular-weight compounds resistant to decomposition.

Humus (Chapter 11)--Brown to black substances of relatively high molecular weight formed during random synthesis reactions in soil. The stable fraction of the organic matter in soil that persists once readily decomposable material is gone.

Hydration (Chapter 7)--The adding of water to form new chemical bonds.

Hydrolysis (Chapter 2)--A chemical reaction in which hydrogen and hydroxyl from a water molecule react with a soil mineral to form an acid and base.

Hydrometer (Chapter 4)--A device used to measure soil texture based on the density of a suspension.

Hydrophytic (Chapter 13)--Water loving, particularly with respect to plant growth.

Hydrosphere (Chapter 1)--The water environment of Earth consisting of seas, lakes, and streams, but also extending to the water films surrounding soil particles.

Hygroscopic Coefficient (Chapter 6)--The water content at which no additional water can be lost through evaporation.

Hysteresis (Chapter 6)--The phenomenon in soil in which water content during wetting and drying cycles does not coincide.


Igneous (Chapter 2)--Rock formed by the cooling and solidification of molten parts (e.g., lava) of the lithosphere. Igneous rock is relatively unchanged since its formation.

Illite (Chapter 7)--A type of silicate clay material composed of hydrous mica.

Illuviation (Chapter 3)--The process of gaining or depositing material removed by one soil horizon or layer from another that typically lies above it.

Immobilization (Chapter 10)--Synonymous with assimilation, usually used in the sense of making elements unavailable for use by other organisms in soil.

Inceptisol (Chapter 3)--One of the soil orders in the U.S. taxonomic system. Inceptisols are moderately developed soils.

Inclusions (Chapter 15)--Locations of different soil series within a larger soil series that do not constitute a large enough area to be mapped separately.

Infiltration (Chapter 4)--Downward entry of water into soil.

Inorganic (Chapter 10)--Elemental or simple molecular forms of compounds.

Interveinal (Chapter 14)--Between veins.

Ion Exchange (Chapter 7)--The exchange between soil solution and soil solids of cations and anions.

Isomorphous Substitution (Chapters 7, 14)--The exchange of atoms of equal size but lesser charge for those of greater charge in silicate clays that leads to a permanent negative charge.


Kaolinite (Chapter 7)--A type of silicate clay. Kaolinite is a 1:1 layer clay with the general formula [A1.sub.2] [Si.sub.2] [O.sub.5] (OH) 4.


Lacustrine (Chapter 2)--Referring to lakes or lake water. Lacustrine deposits were deposited in lake water and then exposed either through drainage of the lake or elevation of the land.

Land Capability Class (Chapter 15)--A parallel system of classification to taxonomic classification that assigns values based on potential productivity and use.

Landform (Chapters 2, 3)--A natural feature of the Earth's surface.

Landscape Position (Chapter 15)--Where a soil exists within a landscape, a critical feature in soil genesis.

Leaching (Chapter 2)--One type of elluvial activity. It is the removal of materials (solutes) in solution typically by downward movement through soil.

Levee (Chapter 12)--A natural or constructed barrier for flood protection.

Lignin (Chapter 11)--A complex polymer of phenyl propanoid subunits that forms a major part of the structural integrity of plants and is a significant soil organic matter precursor.

Lime (Chapter 14)--Ground limestone, either calcite or dolomite.

Liming Agent (Chapter 14)--A material that raises the pH of a soil.

Lithosphere (Chapter 1)--The rocks and minerals in the Earth's surface.

Lithotroph (Chapter 9)--An organism that obtains its energy from inorganic compounds.

Loam (Chapter 4)--A classification of soil texture reflecting a mixture of < 52 percent sand, 28-50 percent silt, and 7-27 percent clay-sized particles.

Loess (Chapter 2)--A type of eolian deposit. Loess is fine material (silt-sized particles) transported by wind and deposited elsewhere.

Luxury Consumption (Chapter 14)--Uptake of nutrients in excess of a plant's growth needs.


Macrofauna (Chapter 9)--Large, visible organisms such as insects.

Macronutrients (Chapter 14)--Elements required in the greatest amount by plants and animals.

Macropores (Chapter 5)--Large soil pores > 100 [micro]m in diameter.

Manure--The excrement of animals.

Mapping Unit (Chapters 3, 15)--A conceptual unit in a soil survey of one to many delineations that represent one or more soils either individually or as a mixture.

Mass Action (Chapter 14)--Mass flow, usually with respect to nutrients, in response to water movement.

Mass Flow (Chapter 14)--Flow of water in response to gradients.

Mass Wasting (Chapter 12)--Downslope movement of large volumes of soil or rock due to gravity.

Matric Potential (Chapter 6)--A mathematical description of the force or attraction of water to solid particles in soil.

Mesofauna (Chapter 9)--Multicellular eukaryotic animals such as mites and nematodes.

Mesophiles (Chapter 9)--Organisms that preferentially grow in moderate temperature regimes.

Mesopores (Chapter 5)--Medium-sized pores 30-100 [micro]m in diameter.

Metamorphic (Chapter 2)--Rock that has been formed by recrystallization of primary and secondary materials (igneous or sedimentary rock) under conditions of high temperature and pressure or chemical reactions.

Methemoglobnemia (Chapter 13)--A condition, particularly in infants, in which excess N[O.sub.3.sup.-] consumption causes hemoglobin to lose its ability to effectively transport oxygen.

Micelle (Chapter 14)--A negatively charged colloid particle.

Microfauna (Chapter 9)--Microscopic eukaryotic animals such as protozoa.

Micronutrients (Chapter 14)--Elements required in small, sometimes microgram, amounts by plants and animals.

Microorganisms (Chapter 9)--Small acellular, single-celled, and multicellular organisms typically invisible to the naked eye.

Micropores (Chapter 5)--Small pores < 30 [micro]m in diameter.

Mineralization (Chapter 10)--Decomposition or transformation of organic compounds into inorganic molecules and carbon dioxide.

Mites (Chapter 9)--Macro-to microscopic arthropods in soil.

Mollisol (Chapter 3)--One of the soil orders in the U.S. taxonomic system. Mollisols are deep, fertile soils that typically develop in grassland.

Moraine (Chapter 2)--A blanket or ridge of unconsolidated material left by a glacier.

Mottling (Chapter 3)--Spotted areas of color in soil reflecting periodic saturation.

Mulch (Chapter 12)--A layer of material, either organic or inorganic, covering the soil surface that acts as a barrier to evaporation.

Mutualism (Chapter 9)--An interaction in which the activities of two organisms are mutually beneficial.

Mycorrhiza (pl. mycorrhizae) (Chapter 9)--A generic term for fungi that form symbiotic associations with plant roots.


Neutralism (Chapter 9)--Lack of interaction between two organisms.

Nitrification (Chapter 10)--The oxidation of reduced inorganic and organic N to ultimately release N[O.sub.3.sup.-].

Nitrogen Fixation (Chapters 9, 10)--Biological (bacterial) conversion of molecular dinitrogen gas ([N.sub.2]) to ammonia and subsequently organic nitrogen.

Nitrogenase (Chapter 10)--The enzyme complex responsible for biological nitrogen fixation.

Nonexchangeable Ions (Chapter 14)--Ions so tightly adsorbed to exchange surfaces that they do not participate in cation or anion exchange reactions.

Nonhumic Compounds (Chapter 11)--Organic compounds in soil that retain some recognizable chemical identity.

No-Tillage (Chapter 12)--A conservation tillage practice in which the soil is not disturbed except for a small slit for seed placement.


O Horizon (Chapter 3)--A surface organic layer overlying the mineral layer.

Ochric Epipedon (Chapter 3)--A surface soil that is pale throughout or consists of a thin dark layer over a pale layer of soil.

Octahedral (Chapter 7)--Refers to the overall arrangement or pattern of the aluminum, oxygen, and hydroxyls in silicate clay layers.

Organic (Chapter 10)--Containing C and/or other elements such as N and S and synthesized by living organisms.

Orographic Precipitation (Chapter 13)--Orographic precipitation occurs when warm air is lifted and cooled as air masses go over mountains. The lifting and cooling results in precipitation commonly occurring in the upper elevations of the windward side of the mountain (where the air mass is coming from) and little to no precipitation on the leeward side of the mountain (the opposite side of the mountain). This is commonly called the rain shadow effect.

Orthophosphate (Chapter 10)--The soluble form of inorganic P, at typical soil pH it is usually a mixture of HP[O.sub.4.sup.2-] and [H.sub.2] P[O.sub.4.sup.-].

Osmosis (Chapter 6)--Usually used in terms of diffusion of water through differentially permeable membranes.

Osmotic Potential (Chapter 6)--A mathematical description of the force by which water is attracted to soil solutes.

Oxidation (Chapter 10)--Loss of electrons from an element or compound.

Oxisol (Chapter 3)--One of the soil orders in the U.S. taxonomic system. Oxisols are weathered soils that have oxic horizons.


Pan (Chapter 3)--Horizons or layers in soil that are compacted.

Parasitism (Chapter 9)--An interaction in which one organism obtains its sustenance from another organism without necessarily killing it except in the long term.

Parent Material (Chapter 2)--The consolidated or unconsolidated material from which a soil forms.

Pedon (Chapter 3)--The smallest volume of soil that can be called a soil. It is a three-dimensional body of soil with dimensions large enough to permit the study of horizon shapes and relations.

Pedogenesis (Chapter 2)--Ped generation. Synonymous with the process of soil formation.

Pedosphere (Chapter 1)--The environment of the soil profile.

Peds (Chapter 1)--Units of soil structure in a soil horizon. They may be blocky, platey, or angular. Soil aggregate and soil ped are synonymous.

Percent Base Saturation (Chapter 14)--The extent to which the total CEC of a soil is saturated with respect to base cations such as [Ca.sup.2+], [Mg.sup.2+], and [K.sup.+].

Percolation (Chapter 4)--The downward movement of water into soil.

Permeability (Chapter 15)--The ease with which gases, liquids, and plant roots pass through the bulk mass of soil or a soil layer.

Pesticide (Chapter 9)--Any chemical used to kill unwanted microorganisms, plants, or animals.

pH (Chapter 8)--A measure of acidity, the negative log of the hydrogen ion (or hydronium) concentration in soil.

Phosphatases (Chapter 10)--Enzymes that cleave orthophosphate from organic P compounds.

Photosynthesis (Chapter 10)--The use of light energy to convert inorganic C into organic C.

Phototroph (Chapter 9)--An organism that generates its energy by photophosphorylation.

Piezometers (Chapter 13)--Hollow tubes with incisions inserted into soil and used to sample fluctuating groundwater tables.

Plow Layer (Chapter 2)--That layer of soil, typically at the soil surface, that is subject to frequent tillage.

Podzol (Chapter 3)--A term used in some taxonomic systems that is equivalent to Spodosol.

Polypedon (Chapter 3)--A group of contiguous, similar pedons.

Pore Size Distribution (Chapters 4, 5)--The percentage of pores of different size classification in a given soil.

Porosity (Chapters 4, 5)--The volume percentage of the total bulk of soil not occupied by solids.

Precipitation (Chapters 2, 13)--Rainfall or snowfall, or the deposition of dissolved solutes.

Predation (Chapter 9)--An interaction in which one organism consumes another.

Primary Minerals (Chapters 4, 7)--Minerals formed naturally by the crystallization of molten rock.

Proctor Density Test (Chapter 12)--An engineering test used to determine the maximum compaction of soils at different water contents.

Productivity (Chapter 1)--The ability to promote or sustain growth.

Profile (Chapter 2)--A two-dimensional, vertical cross-section of a soil through all horizons.

Prokaryotes (Chapter 9)--Organisms lacking a cell nucleus, usually equated with bacteria.

Proteobacteria (Chapter 9)--A taxonomic division of bacteria in soil; the dominant group of bacteria in soil.


Quartz (Chapter 4)--A resistant crystalline mineral composed of silicon and oxygen.


Redox Potential (Chapter 9)--A measure of the extent to which an environment is oxidized or reduced; often equated with the aeration state of an environment.

Redox Reactions (Chapter 10)--Oxidation reduction reactions in which electrons are lost and gained by various compounds in soil.

Reduction (Chapter 10)--Gain of electrons by an element or compound.

Regolith (Chapter 2)--Loose earth material over solid bedrock.

Relief (Chapter 2)--The elevation or differences in elevation in a landscape.

Residual Acidity (Chapter 8)--Soil acidity that is neutralized by lime or other alkaline materials, but that cannot be replaced by an unbuffered salt solution.

Residual Soil (Chapter 2)--A soil that develops in place from the weathering of rock in a location.

Respiration (Chapter 10)--An energy-yielding process in which a compound is oxidized and its electrons used to generate ATP during transport through the cell membrane.

Respire (Chapter 9)--The process of generating energy by electron transport across membranes.

Rhizosphere (Chapter 9)--The zone of soil around the plant root that is influenced by the plant root and its metabolism.

Rill Erosion (Chapter 12)--A type of soil erosion caused by water removing soil from preferential flow channels. The channels can be easily obscured by tillage, which conceals the problem.

Riparian Areas (Chapter 13)--The land surrounding streams or other freshwater bodies.

Runoff (Chapter 12)--That part of precipitation that does not infiltrate the soil but flows over land.


Salinization (Chapter 12)--The process by which salt content in soils increases to levels detrimental to plant growth and soil function.

Saltation (Chapter 12)--An erosion process in which windborne sands strike the ground and disperse silt and clay particles into suspension. The soil particles leap and jump along the soil surface during high winds.

Sand (Chapter 4)--By definition, soil particles smaller than 2 mm and larger than 0.05 mm.

Sandstone (Chapter 2)--A type of sedimentary rock, usually dominated by quartz, bound together by cementing agents such as silica or iron oxide.

Saprophytic (Chapter 10)--Growing on dead and decayed organic compounds.

Saturated (Chapter 6)--A soil condition in which all pore space is occupied by water.

Saturated Flow (Chapter 6)--Water flow in soil in which all pores are water-filled.

Saturation (Chapter 6)--The soil condition when all pores are filled with water or all exchange sites are filled by one or more ions.

Secondary Minerals (Chapters 4, 7)--Minerals formed by the weathering of primary minerals. Examples are kaolinite and all carbonates.

Sedimentary (Chapter 2)--Referring to material that is deposited by water, wind, ice, or gravity.

Sesquioxide (Chapter 14)--A general term for oxides and hydroxides of iron and aluminum.

Shale (Chapter 2)--A type of sedimentary rock made of clay, silt, and very fine sand.

Sheet Erosion (Chapter 12)--A type of erosion in which soil is more or less removed uniformly from an exposed surface.

Side-Dress (Chapter 14)--Application of an amendment to the side of a growing plant.

Siderophores (Chapter 10)--Low-molecular-weight organic compounds with a high affinity for iron.

Silicate Clay (Chapter 14)--A secondary mineral composed of crystalline layers of silica and alumina.

Silt (Chapter 4)--By definition, soil particle smaller than 0.05 mm and larger than 0.002 mm.

Slickenside (Chapters 2, 14)--Polished surfaces on blocks of soil formed by the blocks sliding past one another as occurs in Vertisols.

Smectite (Chapter 7)--A type of 2:1 silicate clay with high cation exchange capacity; montmorillonite is the best known.

Soil Association (Chapter 15)--A kind of map unit used in soil surveys comprised of delineations of landscape units in which two or more soil series appear in a fairly repetitive and describable pattern.

Soil Fertility (Chapter 14)--The status of a soil with respect to its ability to supply the nutrients essential to plant growth.

Soil Management (Chapters 12, 14)--The total of all operations, agricultural or otherwise, that are used to manipulate a soil.

Soil Moisture (Chapter 6)--Water contained in soil.

Soil Order (Chapter 3)--The first level of classification in the U.S. system of soil taxonomy. Soil orders represent the most basic grouping of soils in terms of the soil-forming factors.

Soil Organic Matter (SOM) (Chapter 11)--Usually understood to be the nonliving, organic fraction of soil consisting of decayed and partially decayed plant and animal remains, and unrecognizable humic substances.

Soil Separates (Chapter 4)--Mineral particles < 2 mm in diameter that range between specified size limits.

Soil Series (Chapter 3)--The sixth and final level of soil classification used in the U.S. system of soil taxonomy. Soil series are named after adjacent cities and towns and represent soil types with very similar properties.

Soil Solution (Chapter 14)--The water, including all dissolved solutes, in soil.

Soil Structure (Chapter 5)--The three-dimensional array of solids and pore space.

Soil Survey (Chapter 15)--The identification, classification, mapping, examination, and evaluation of the soils in a landscape.

Soil Taxonomy (Chapter 3)--The book specifying the soil classification system used in the United States.

Soil Textural Classes (Chapter 4)--The terms used to describe the relative proportions of various separates in a soil, such as clay, loamy clay, etc.

Solum (Chapter 2)--The A and B horizons together or either one separately above the C horizon; the true soil.

Specific Surface Area (Chapter 7)--The soil particle surface area divided by the solid particle mass, expressed as m2 kg 1.

Splash Erosion (Chapter 12)--The translocation of soil particles by the impact of falling rain.

Spodosol (Chapter 3)--One of the soil orders in the U.S. taxonomic system. Spodosols have a pronounced spodic horizon where iron, aluminum, and organic matter have accumulated.

Stemflow (Chapter 13)--Precipitation interacting with vegetation that flows down stems and trunks thereby picking up and transporting loose deposits from those locations.

Stream Gauging (Chapter 13)--The-process whereby the height and flow of water past a site is monitored.

Subgroup (Chapter 3)--After "great group," the fourth level of classification in the U.S. system of soil taxonomy. Subgroup names provide information about special features of a soil.

Suborder (Chapter 3)--After "order," the second level of classification in the U.S. system of soil taxonomy. Suborder formative elements provide information about climate and parent material.

Subsidence (Chapter 11)--Generally refers to a specific phenomenon in which the decomposition of high-organic matter soils leads to the rapid lowering of the soil surface.

Subsoil (Chapter 2)--In a fully formed soil it would be that formed below an A horizon. Subsoils can be exposed through erosion and become surface soils, however.

Substrate--Level Phosphorylation (Chapters 9, 10)A metabolic process in which high-energy phosphate bonds are created on intermediate compounds from which the phosphate is later cleaved to yield energy for cellular metabolism.

Surface Creep (Chapter 12)--The process of wind-driven erosion and movement of soil across a landscape.

Symbiosis/Symbiotic (Chapters 9, 10)--An interaction, sometimes obligatory, in which two different organisms grow together for their mutual benefit; contrast with asymbiotic in which no cooperation between two organisms occurs.

Synergism (Chapter 9)--An interaction of two organisms in which the activity of each is enhanced in combination compared to when they are separate.


Takings (Chapter 16)--Confiscation of private property by government, usually through the process of eminent domain, in which the property owner is compensated for the loss, or loss of certain uses of their property.

Tensiometers (Chapter 13)--Measure the force or tension that results from a water-filled column in contact with the soil through a porous ceramic cup. They measure the soil water matric potential.

Terrace (Chapter 2)--A bench-like landform occurring on the border of rivers, lakes, and oceans that results from the deposition of material by flooding or surf activity from subsequently receded water. It can also refer to an artificial structure made by earth movement for the purpose of erosion control.

Tetrahedral (Chapter 7)--Refers to the overall arrangement or pattern of the silicon, oxygen, and hydroxyls in silicate clay layers.

Textural Triangle (Chapter 4)--A graphing device used to determine the textural class of a soil based on the percent of sand-, silt-, and clay-sized particles.

Thermophiles (Chapter 9)--Organisms that grow preferentially at elevated temperatures.

Throughfall (Chapter 13)--Precipitation that passes through a forest canopy, but interacts with leaves.

Tile Drain (Chapters 2, 6, 12, 14)--Ceramic or plastic pipe placed at suitable depths and intervals to artificially lower water tables.

Tillage (Chapter 12)--The process of moving, turning, or stirring soil. Mechanical manipulation of the soil for any purpose.

Tilth (Chapter 15)--The physical condition of a soil in relation to the ease of its tillage and fitness as a seedbed for crop growth.

Top Dress (Chapter 14)--Application of an amendment to the soil surface.

Topography (Chapter 2)--The lay of the land, its levelness or hilliness.

Toposequence (Chapter 15)--A sequence of related soils that differ primarily because of topography as the soil-forming factor.

Topsoil (Chapter 2)--Most often refers to the soil immediately involved in cultivation. Often used synonymously with the term A horizon.

Tortuosity (Chapters 5, 6)--Refers to the twisted nature of soil pores.

Trace Element (Chapter 14)--Synonymous with micronutrient, an essential element required in very small amounts.

Transpiration (Chapter 13)--Loss of water to the atmosphere through the leaves of plants.

Truncated (Chapter 3)--Having lost all or part of a soil horizon, or all or part of a metabolic pathway.

Tundra (Chapters 3, 11)--Treeless plains characteristic of arctic environments.


Ultisol (Chapter 3)--One of the soil orders in the U.S. taxonomic system. Ultisols develop in warm, humid forests. They are typically more weathered and less fertile than Alfisols.

Undifferentiated Soils (Chapter 15)--A kind of map unit used in soil surveys to reflect locations in which different soil series are so intermixed as to make individual distinctions irrelevant.

Universal Soil Loss Equation (USLE) (Chapter 12)--A predictive model for soil erosion.

Unsaturated--The condition in which only some pore space is filled with water.

Unsaturated Flow (Chapter 6)--Water movement in soil by capillarity through soil in which the largest pores are air-filled.


Vadose Zone (Chapter 6)--The unsaturated zone of soil extending from the top of the water table to the soil surface.

Value (Chapter 3)--One of the three variables of soil color. Value reflects the relative lightness of intensity of color.

Vermiculite (Chapter 7)--A 2:1 type silicate clay mineral formed from mica. It displays an adsorption preference for potassium and cesium.

Vertisol (Chapter 3)--One of the soil orders in the U.S. taxonomic system. Vertisols are characterized by high content of shrinking and swelling clay that causes soil heaving.

Volumetric Water Content (Chapter 6)--Soil water content expressed as volume of water relative to volume of bulk soil (water content on a vol/vol basis).


Water Erosion Prediction Project (WEPP) (Chapter 12)--A process-driven computer model for soil erosion prediction.

Water Holding Capacity (Chapter 4)--The total amount of water that a soil can hold once all freely draining pores have emptied by the force of gravity.

Water Potential (Chapters 6, 9)--A mathematical description of water availability or the potential of water to move and do work in response to free energy gradients; a measure of the availability of water.

Watershed (Chapter 13)--An area draining ultimately to a particular body of water such as a lake or river.

Water-Stable Aggregate--A ped or aggregate that retains its structure despite rain impact or agitation in water.

Water Table (Chapters 6, 13)--The surface of the groundwater.

Weathering (Chapters 1, 2, 7)--All physical, biological, and chemical changes produced at or near the Earth's surface that result in the disintegration and decomposition of rocks and minerals.

Wetland (Chapters 3, 15, 16)--An area of land characterized by hydric soils and hydrophytic vegetation.

Wilting Point (Chapter 6)--The water content or potential at which a plant is unable to extract water from soil; water availability in soil for plant growth becomes insufficient to supply adequate plant turgor or rigidity.

Wind Erosion Equation (WEQ) (Chapter 12)--The computer-based equivalent of WEPS.

Wind Erosion Prediction System (WEPS) (Chapter 12)--The equivalent of USLE for predicting wind erosion.

Windbreak (Chapter 12)--An artificial or natural plant barrier designed to protect crops and soil from strong winds.


Xeric (Chapter 3)--A soil moisture regime common to Mediterranean climates that is typified by moist, cool winters and warm, dry summers. Soil moisture is usually not present at times for optimal plant growth.
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Publication:Fundamental Soil Science
Article Type:Glossary
Date:Jan 1, 2006
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