Soil Science Terms Glossary

The following is an alphabetical list of simple definitions of soil science jargon.


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Absorbed energy

In remote sensing and meteorology, radiant energy that is neither reflected nor transmitted by opaque or transparent materials. Absorption converts the energy to another form, usually heating the absorbing object.

Accelerated erosion

Water or wind erosion at more rapid than normal or geological rates, usually associated with human activities. See water erosion and geological erosion.

Accessory cation

A cation paired with an anion to complete a molecule or crystal structure. In aluminosilicate minerals, a cation (usually Ca2+, Mg2+, K+, or Na+) not coordinated with O in tetrahedra or octahedra.


Gather together; in cells or organisms, to absorb an element so that its concentration is greater than in the outside medium.


A substance that releases H+ ions; a condition in which the activity of H+ ions exceeds that of hydroxyl.


The process of making something acid; lowering pH.


A major group of filamentous bacteria, some of which are abundant in soils; once classified as intermediate between fungi and bacteria,

Active remote sensing

A method for acquiring visual images of an object (i.e., pictures) in which the energy recorded is provided by the observer. Examples are radar imagery and flash photography.

Adenosine triphosphate (ATP)

A nucleotide compound formed during biochemical processes to store energy in a form that can be released (with breakdown to adenosine diphosphate and phosphate ion).


The molecular .attraction between surfaces that holds substances together. Water adheres to soil particles.


The process of attachment of a substance to the surface of a solid or liquid.

Adsorption, specific

Adsorption, usually chemical (chemisorption), that is selective for a particular ion or substance.

Adsorptive force

The cause (energy or power) of adsorption.

Aeolian Material

Accumulated through wind action..Loess and sand dunes are examples. Also spelled eolian.


Living or active only in the presence of oxygen.


Individual sand, silt, and clay particles bound together into a larger particle. Aggregates may be spheres, blocks, plates, prisms, or columns.


Plural form is algae. An aquatic, eucaryotic, plantlike, photosynthetic organism, mostly microscopic, often single celled. The term blue-green algae, sometimes still used, refers to blue-green bacteria.


Containing or releasing an excess of hydroxyl over hydrogen ions; pH > 7.0.


A general term describing noncrystalline clay-sized soil minerals. Also, a short-range-order aluminosilicate mineral.


Loose (unconsolidated) sediments deposited by flowing water.


Minerals whose major elements are silicon (Si) , aluminum (Al), and oxygen (O).

Amendment, soil

A substance (e.g., lime, gypsum, or peat) added to soil to improve its pH or physical properties.

Amino acid

An organic acid containing an amino group (-NH2); building unit of peptides and proteins.


Without crystal structure. In amorphous minerals, the atoms are not in a regular order that repeats extensively in three dimensions.


A group of aluminosilicate minerals having a structure containing double chains of linked silica tetrahedra.


Without molecular oxygen. The opposite of aerobic.


Fertilizer trade jargon. The percentage of the desired nutrient element(s) in the fertilizer. Also, methods used to·determine plant or soil composition.


blocky A structure whose units (aggregates) are nearly equidimensional, square, or rectangular and the corners are not rounded.

Anion sorption

The process by which an anion, such as phosphate, replaces OH-groups on mineral surfaces and edges.


Shortage of oxygen.

Aquic conditions

Continuous or periodic saturation and reduction. The presence of aquic conditions is indicated by redoximorphic features and can be verified by measurement of saturation and reduction.


Describes land suitable for the production of cultivated crops.


The direction that a slope faces. A south facing slope has a south aspect.

Assimilation (of nutrients)

Metabolic use of absorbed nutrients to make cell constituents.


limits An engineering classification of soil consistence at different moisture states.


An organism that need not take in organic food to get energy. For example, photoautotrophs use light, and chemoautotrophs obtain energy from oxidation of inorganic substances. See heterotroph.

Availability (of a nutrient)

Adequacy of supply, freedom, ease of release, and mobility. The amount of a nutrient in chemical forms accessible to plant roots.


The rapid downslope movement of soil and rock under the force of gravity.


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Plural of bacterium. A diverse group of microbes containing the procaryotic organisms. Mostly single celled; sometimes occurring in chains or packets. Some, such as actinomycetes, are filamentous. Some, such as blue-green bacteria, are capable of photosynthesis.

Banding, band placement

Application of fertilizer below the ground surface in a narrow strip or band.


A substance that reacts with H+ ions or releases hydroxyl ions; a substance that neutralizes acid and raises pH.

Base, exchangeable

In soil science, an exchangeable cation other than H+ or Al3+.


In mining and fertilizer production, treatment to improve the quality and handling of pulverized ore by processes such as sieving and flotation to remove unwanted material.

Biological nitrogen fixation

Reduction and assimilation of N2, a capability of certain free-living and symbiotic bacteria.

Biological (or biochemical) oxygen demand (BOD)

A measure of the soluble organic content in waste products.


Two or more soils in which the soil forming factor that varies the most is the vegetation (usually kind of vegetation). Other soil forming factors are constant or vary much less than vegetation.

Biotic factor

One of the five soil-forming factors. Biotic refers to the living organisms that influence soil formation.


An important soil-forming aluminosilicate mineral; a black, platy mica found in some igneous rocks such as granite. It is trioctahedral. All possible positions in the octahedral sheet are occupied by atoms.

Bioturbation Mixing

(turbation) of soil by organisms (biota).


A soil structure type whose vertical and horizontal axes are nearly equal. See angular blocky and subangular blocky.

Broadcasting, broadcast application

Scattering, dropping, or spreading fertilizer or other materials on the soil surface.


Processes that constrain the shift in pH when acids or bases are added. More generally, processes that constrain shifts in the dissolved concentration of any ion when it is added to or removed from the system.

Bulk density

In soils, the dry mass (weight) of soil per unit bulk volume.


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Forces between water and soil surfaces in the small (capillary) pores.


An organic substance with the general formula (CH2O)n-for example, sugars and polysaccharides.

Carboxyl group

(-)COOH, the most common active group of organic acids.

Cation exchange capacity (CEC)

The total amount of positive ions (cations) that a soil can adsorb exchangeably.


The carbohydrate most abundant in plants; a polysaccharide made up of glucose units.


A natural stream or an excavation constructed to carry water.


A compound in which a polyvalent metal cation is firmly combined with a (usually organic) molecule by multiple bonds. The binding molecule is called a chelating agent or chelan.


The process of forming a chelate.


A nonexpanding 2:1 aluminosilicate mineral with a sheet of magnesium and hydroxyl atoms in the interlayer space.


Green pigment, a complex chelate of Mg; traps light for photosynthesis in plants, algae, and some bacteria.


Pale color, yellowing. Caused by lack of chlorophyll in plants.


Two or more soils in which the soil-forming factor that varies the most is time. Other soil-forming factors are constant or vary much less than time.


A mineral particle size less than 0.002 mm diameter. Also, a specific group of aluminosilicate minerals with a high surface area and exchange capacity.

Clay film

A coating of oriented clay particles on larger particle, pore, or aggregate surfaces.


Two or more soils in which the soil forming factor that varies the most is the climate (usually amount of precipitation). Other soil forming factors are constant or vary much less than climate.


A process that blocks pores and reduces water flow in soils. Mechanisms can be physical, chemical, and biological.


In forestry, trees with crowns forming the general level of the canopy and receiving full light from above but little from the sides.

Coefficient of linear extensibility (COLE)

A measure of the potential one-directional change in the volume of a soil as the soil moisture level changes.


Attraction between like molecules. Water molecules cohere to one another. Cohesionless soils In engineering, sands with little clay or silt.

Cohesive soils

In engineering, clay-and silt-rich soils.


A material made of organic or inorganic particles with diameter from 0.001 to 0.1 µm.


Unconsolidated, unsorted earth material that has moved downhill under the force of gravity and has accumulated at the base or lower slopes of the hill.


An aggregate shape that is longer than it is wide, with a rounded top and edges.


Transformation of an organic compound (the substrate) by a microorganism without the organism deriving benefit (energy, carbon, or nutrients) from the substrate.


A process of rearranging soil particles to decrease pore space and increase bulk density. It produces a compacted, dense soil layer.

Compound slide

A combination of a fall and a rotational slide.

Compressive strength

In engineering, a soil's ability to resist compressive (squeezing) forces.

Concentration (of material)

The amount of the material per unit volume of solution, gas, or solid. Also, the process of increasing the concentration at a certain place.

Concentration gradient

The rate of change of concentration per unit distance.

Conductive tissue

In plants, tissue that freely transmits water and dissolved compounds; vascular tissue, comprising xylem and phloem.


A measure of the ease with which a material or object conducts or transmits something such as heat (thermal conductivity), water (hydraulic conductivity), or diffusing substances (diffusion coefficient).


A material that conducts-allows the diffusive transmission of (usually) either electricity or heat.


Protection from loss or waste. In soil science, most often denotes efforts to reduce soil loss by wind and water erosion.

Conservation tillage

Any tillage sequence that leaves at least 30 percent crop cover on the soil surface, with the goal of minimizing soil and water loss. It ranges from omitting one or more tillage operations to eliminating all tillage.


The most detailed soil-mapping unit, composed of delineations that show the size, shape, and location of the unit.

Contact time

In waste disposal, the time that the waste is in contact with soil surfaces as it flows through the soil.


Length of unbroken, continuous, or coherent soil pores.

Control section

A defined portion of the soil profile used in determining the soil's classification.


Bulk mixing of fluid and of heat and materials contained in the fluid. Usually caused by density differences from uneven heating or cooling or by turbulence.


In general, the degree of (numerical) relationship between associated variables. In a soil survey, comparing observed profiles with defined series to ensure proper classification.


Outer layer; in plant roots and stems, the layer between the epidermis and the central conductive tissue.

Coulombic attraction

Electrical attraction between things (ions, surfaces, and the like) with opposite charges.

Covalent (chemical bond)

Caused by sharing of electrons. Cover crop Any crop that is grown to provide soil protection from erosion between periods of normal crop production or between trees in orchards and vines in vineyards.


In soil science, the slow movement of soil down a slope under the force of gravity. Also, the rolling or pushing of particles along the ground surface by wind. See soil creep.

Crown drip

The portion of rainfall, snowmelt, or fog that drips from vegetation. May be an important part of nutrient cycling in some ecosystems.


A layer of increased bulk density of the top few millimeters of a soil caused by raindrop impact! clay dispersion, and clay translocation that blocks pores. See seal.


An arrangement of atoms in minerals and other substances in an ordered, systematic, repeating fashion.


Metal-organic substances used in transfer of electrons to O2 during cell respiration.


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Debris flow

Downslope movement of water lubricated soil and rock. See mass wasting.

Deflocculation (of particles, colloids)

DisperSIon, or separation, of clusters of particles into separate individual particles.


Microbial reduction of nitrate to form gaseous N2 or N20.

Depletion zone

The narrow zone next to the root where immobile nutrient concentrations in soil become markedly lowered.


A chemical reaction involving removal of a proton (H+). Desorption Migration of an ion or molecule from a surface. The opposite of sorption.


Molecular movement along a gradient. Water diffusion occurs from wet areas to dry ones. Gas and solute diffusion occur from zones of high concentration to zones of low concentration.

Diffusion coefficient

A numerical measure of the ease with which something can diffuse through something else (both must be specified, along with the temperature). A proportionality constant relating diffusion rate to concentration gradient.


In soil science, to cause aggregates to separate into individual soil particles. Dissipation Spreading in various directions; scattering, dispersing, wasting. Dissociation Separation of a molecule into component atoms or ions-for example, H2O = H+ + OH-.


The process by which a solid passes into solution. Dominant In forestry, the major tree species in a canopy.


In waste disposal, the process of adding waste at intervals to keep the soil aerobic and to reduce clogging.

Drain, tile

A subsurface tunnel drain consisting of loosely fitting ceramic pipe sections into which water can flow from saturated soil. The term is still used for new perforated plastic drainpipe.


Lowering of the water table by drain or well; a change in the shape of the water table near the point of water withdrawal.

Drill (agricultural)

A machine for laying seed, fertilizer, and the like below the ground surface. Driving force n See potential gradient.


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Earth flow

The process of saturated soil moving down a slope under the force of gravity. The term is also used to describe the results of the process.

Effective precipitation

The amount or proportion of precipitation that infiltrates into soil. Effluent Liquid waste from either a septic tank or a sewage treatment plant.

Electromagnetic spectrum

Range of energy and wavelengths of electromagnetic radiation. The range from shortwave high-energy radiation to longwave low-energy radiation includes X-ray, ultraviolet, visible light, infrared, and radio waves.


A small, negatively charged atomic particle. Electrostatic (attraction or repulsion) Interaction between electrically charged objects.


Any substance that cannot be further separated except by nuclear disintegration. See the periodic table at the front of the book.

Eluvial horizon

A soil layer (horizon) formed by the removal of constituents such as clay or iron.


Process of removing from a soil layer any soil constituents in suspension. Emissivity Relative measure of an object'S ability to emit radiant energy at a given temperature.


The light-sensitive coating on photographic film, usually containing a silver salt such as silver chloride.


Protein catalysts produced in cells of living organisms that direct and control the cells' chemical reactions.


In plants, the outside layer of cells.


Surface layers of soil with specific characteristics used in classifying soils by soil taxonomy (e.g., the mollic epipedon).


The condition of a chemical reaction or an entire ecosystem in which there are only minor changes in conditions over time.


The unit formerly used to describe cation exchange capacity or quantities of ions. One gram atomic weight of hydrogen or the amount of any other ion that will combine with or displace this amount of hydrogen. The amount that provides 1 mol of charge. Equals 1 molec.


The wearing away of the land surface by water, wind, ice, or gravity.


In biology, referring to the type of cells with a distinct nucleus with a nuclear membrane; characteristic of fungi, protozoa, algae, plants, and animals. See procarotic.


Pollution with unwanted nutrients.


Vapor loss from soil or free water directly into the atmosphere.


Evaporation plus transpiration.

Exchangeable bases

Exchangeable cations other than Al3+ and H+.

Exchangeable base saturation

Exchangeable cations other than Al3+ and H+, expressed as a percentage of cation exchange capacity measured at neutrality.

Exchangeable ions

Ions (charged atoms or molecules) held by electrical attraction at charged surfaces; can be displaced by exchange with other ions.

Exchangeable sodium percentage (ESP) (of soil)

Amount of exchangeable Na expressed as a percentage of total exchangeable cations.

Exchange complex

All the materials (clay, humus) that contribute to a soil's exchange capacity. Extracellular Outside the cell. Extracellular enzymes are excreted by some bacteria and fungi.

Extracellular enzyme

A protein substance that acts as an organic catalyst excreted outside the bacterial or plant cell. Also called an exoenzyme.

Extrusive rock

Any rock that forms from hot molten material by solidifying at the earth's surface.


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Factorial trial

A systematic comparison of the effects of all combinations of two or more variables. Fall occurs when part of a cliff or steep hillside falls vertically downward under gravitational stress.

False-color infrared

Color film carrying an emulsion sensitive to infrared energy. The color is false because the invisible infrared records as a visible color (e.g., green objects appear red on the resulting photograph).

Feldspar (felspar)

A group of primary aluminosilicate minerals with a three-dimensional lattice. Important to soil genesis and fertility as source of the K, Na, and Ca they contain as accessory cations.


A set of metabolic processes by which anaerobic organisms obtain energy by converting sugars to alcohols or acids and CO2,


The addition of 'soluble fertilizer through an irrigation system.

Field capacity

The amount of water retained in soil after it has been saturated and allowed to drain freely for 2 or 3 days.

Film speed

A measure of the emulsion's sensitivity, which determines the exposure required to produce the desired image.


Joining of colloidal particles to form clusters (flocs). Opposite of deflocculation, dispersion.


Pertaining to leaves (e.g., foliar intake of nutrients, directly by leaves).

Frictional resistance

Resistance to the movement of fluids or particles caused by the interaction (rubbing) of surfaces.


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The genetic constitution of an organism. A group of organisms that are genetically alike.

Geographic data

Information that can best be displayed on maps. The distribution of soils on the earth's surface is one example.

Geological erosion

As opposed to accelerated erosion; erosion at natural rates, unaffected by human activity.


Statistics that describe the variability of properties from place to place.

Glacial outwash

Geological material moved by glaciers and subsequently sorted and deposited by streams flowing from melting ice. Also called glaciofluvial deposits.

Glacial till

Unsorted and unstratified geological material deposited directly by glacial ice.


The process of geological erosion by means of glacial ice.


A process that produces reduction of iron and other elements under conditions of prolonged saturation.


A common sugar with six carbon atoms per molecule; present in all cells. A constituent of cellulose, starch, and other polysaccharides.


In furrow irrigation, flood irrigation, and stream flow, the slope of the flowing water surface; the effective slope down which the water runs.


The rate at which a variable changes with distance. See concentration gradient, potential gradient, and temperature gradient.


Spherically shaped; used to describe soil aggregates, fertilizer particles, and the like.

Grass waterway

A natural or constructed waterway covered with erosion-resistant grasses and used to carry water and reduce erosion.


Pertaining to weighing. Gravimetric methods determine amounts of a substance by direct or indirect weighing.

Gravity erosion

The movement of soil or rock by gravity.

Greenhouse effect

The warming of the earth's surface and atmosphere owing to absorption of outgoing radiation by CO2, CH4, and H20 (like absorption by glass).

Green manure

Plants grown to be cultivated into soil to improve soil fertility.

Ground truth

In remote sensing, the actual scene or data from the site that is being remotely sensed.


A channel cut by the concentrated flow of water during rainstorms. A gully is deeper than a rill and cannot be removed by normal tillage. Gypsum A mineral or rock composed of CaSO4·2H2O.


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Any plant that grows in saline environments and readily takes up salts.

Heat capacity

The amount of heat that an object must absorb or lose to produce a 10 change in its temperature.

Heat conductivity

A measure of the ease with which a material or object conducts or transmits heat.


A group of complex carbohydrates; polysaccharides that, unlike starch and cellulose, contain other sugars besides glucose. Important to plant cell walls.


Any organism that derives its energy from organic compounds. See autotroph.


A soil layer approximately parallel to the land surface.


The stable, dark-colored organic material that accumulates as a by-product of decomposition of plant or animal residues added to soil. The term is often used synonymously with soil organic matter.


Having water attached or incorporated as part of a chemical substance.


The chemical combination of water with another substance.

Hydraulic conductivity

A measure of the ease (or difficulty) with which a liquid (usually water) flows through a soil (or plant) in response to a given potential gradient. The flux of water per unit gradient of hydraulic potential.

Hydrogen bond

An intermolecular chemical bond between the hydrogen from one molecule and the oxygen from another. Other electronegative atoms, such as N, may also form hydrogen bonds.


A reaction that involves products (H+ or OH-) of the dissociation of water. A mineral weathering reaction that adds H+ to a mineral structure.


Technique of spraying a slurry of fiber, seed, fertilizer, and chemicals onto roadsides for erosion control.

Hydrous oxides, hydroxyoxides

Sesquioxides. Oxides of Fe, Al, or similar metals, with different proportions of water and hydroxyl in the structure.


An (OH)- ion or group.


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Igneous rocks

Rocks formed from the cooling and solidification of hot liquid magma.


The process of deposition in lower soil horizons of material eluviated (transported) from upper horizons.


Unable to transmit water. A term often used with dense soil horizons through which water moves extremely slowly.

Inactivated organisms

In waste disposal, harmful organisms made harmless by reaction with the soil and soil organisms.


One or more polypedons within a map unit, not identified by the map unit name.

Inferred properties

Soil properties that are not seen or measured but are assumed from other properties. Suitability for a use is an inferred property.


Entry of water into the soil.


Refers to the electromagnetic radiation of wavelength longer than light but shorter than radio. Sometimes called heat radiation.


A living culture of an organism in a form suitable for introduction to soil or some other new environment.

In-place parent material

Parent material that has not been transported from its original location; for example, a bedrock is an in-place parent material.


A material (or space) that transmits heat or electricity poorly.


A combined effect of two or more factors or variables such that the variables modify each other's effects.

Interaction, electrostatic

Adsorption caused by the electrical attraction of ions to a charged surface. Intercrop Two or more crops grown together on the same piece of land at the same time.


Between veins.

Interveinal chlorosis

Chlorosis only between leaf veins.

Intrusive rock

Any rock that forms by cooling from a hot molten mass below the earth's surface. Granite is an intrusive rock.

Ion exchange

The interchange between an ion in solution and another ion on the surface of any surface-active material such as clay or humus.

Isomorphous substitution

Substitution of one atom by another of similar size in a mineral without disrupting or changing the mineral structure.


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A family of clay minerals composed of one silicon tetrahedral sheet for each aluminum octahedral sheet.

Kinetic energy

Energy resulting from motion. In equation form, KE = 1/2 mass X (velocity squared).


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A rapid downhill movement of soil and rock under the force of gravity. A term also used to described the resulting landform.

Latent heat

A property of a material. The amount of heat involved per unit mass when the material undergoes a change of phase (e.g., the heat of melting or the heat of vaporization) .

Layer silicates

Another term for aluminosilicate minerals that are platelike in form and composed of layers of atoms. Mica and the aluminosilicate clay minerals are layer silicates.


The removal from soil or plants of soluble materials dissolved in water.


Symbiosis between fungi and algae or bluegreen bacteria, commonly forming a flat, spreading growth on surfaces of rocks and tree trunks.

Ligand exchange

A class of surface reactions on minerals in which anions from solution take the place of some of the anions (ligands) normally bound to cations in the mineral.


A noncarbohydrate organic structural constituent of woody fibers in plant tissues. This slow-to-decompose material is an important part of soil organic matter.


Strictly, calcium oxide and hydroxide. More commonly, ground limestones in which CaCO3 with or without. MgCO3 is the effective constituent.


Organic oils, fats, and waxes. Combinations of organic acids and glycerin.

Liquid limit

In engineering, the water content corresponding to the limit between a soil's liquid and plastic states of consistency.


Two or more soils in which the soil forming factor that varies the most is the parent material. Other soil-forming factors are constant or vary much less than parent material.


A soil textural class name with limits of 7 to 27 percent clay, 28 to 50 percent silt, and 23 to 52 percent sand.


Wind-transported and deposited material of silt and clay size.

London forces

Weak, close-range attraction between atoms, molecules, or colloid particles due to transitory dipole-dipole interactions between their electrons.

Longwave radiation

In meteorology, radiation in the infrared and radio wavelengths, emitted at the earth's surface and partly absorbed in the atmosphere. Distinct from the sun's shortwave radiation.


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A large molecule, typically of colloidal size and made of hundreds or thousands of smaller molecules polymerized (joined together).


Plant nutrient elements needed in the largest amounts. Examples are N, K, and P.


Large pores, often formed by roots and small soil animals and worms.


A hot liquid from within the earth that solidifies to form igneous rock.

Map unit

Defines the contents of an area inside a polygon on a soil map.

Mass flow

Movement of fluid in response to pressure. Movement of heat, gases, or solutes together with the flowing fluid in which they are contained.

Mass wasting

A term describing all of the kinds of movement caused by gravity (such as landslides). Master horizons Major soil horizons identified by capital letters: A, B, C, E, O, and R.

Matric potential

A negative potential resulting from adhesive and capillary forces due to the soil matrix. These forces attract water in soil and lower its potential energy, compared with free water not in contact with soil.


The curved surface of water caused by the affinity of water to surfaces.


The region of tissue in a plant shoot or root where most of the growth by cell division occurs.

Metamorphic rocks

Rocks formed from the alteration of preexisting igneous, sedimentary,' or other metamorphic rocks by heat' and pressure within the earth.


A platy, nonexpanding 2:1 layer aluminosilicate mineral with one octahedral (Al, Mg, Fe), O, OH sheet between two (Si, Al) O sheets. Two common forms are biotite and muscovite.


Plant nutrient elements needed in lesser amounts than the macronutrients. Micro does not imply lesser importance.


Pores that are small compared with macropores. These have sometimes been referred to as capillary pores.

Mineralization (of C, N, S, P, and so on)

Release of an element in inorganic form-usually ions during the decay of organic matter containing the element.


Any of a large number of solid nonorganic soil constituents in which Al, Si, O, and cations are the major components.

Mine soil

Soil formed by reclamation of land used for mining.

Minimum tillage

A method of cultivation that reduces the number of machinery operations to the fewest required to create the proper soil condition for seed germination.

Miscellaneous area

A soil map unit mainly composed of disturbed soil or highly variable soil. Urban' areas, beaches, dumps, minespoil, and artificial land are examples.


The central or fmost typical soil series.


In soil classification, a system that uses only one level of classification, in contrast with a multicategorical system.


A single molecule capable of joining with identical or similar monomer molecules to form a large {macro) molecule-that is, a polymer. Glucose polymerizes to form starch and cellulose, and amino acids polymerize to form protein.


Spots or blotches of different colors or shades of color interspersed with the main (or matrix,) soil color.


Any material spread on the soil surface to protect soil from raindrops, sunshine, freezing, or evaporation.


A form of mica. A platy aluminosilicate mineral with an aluminum octahedral· sheet between two silicon tetrahedral sheets. It is a dioctahedral mineral because two out of three possible octahedral positions' in the structure are filled with aluminum atoms.


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Natural classification

A type of classification in which observed properties or objects are classified.


Death of a piece or pieces of tissue·.


An uncharged atomic particle with a mass ofL Neutron probe An instrument that uses radiation to measure the soil water content. .

Nitrogen fixation

Conversion of N2 from air to forms usable by plants-for example, nitrate or, more commonly, ammonia.


A farming system in which a crop is planted in the residue from a previolis crop without soil tillage (such as plowing).

Nucleic acids

Polymers made of nucleotide units that carry and transcribe genetic information in cells.


Compounds consisting of a ring structured organic base, a five-carbon sugar, and one or more phosphate groups. May polymerize to form nucleic acids. See adenosine triphosphate (ATP).


In chemistry, the central part of an atom, consisting of positive protons and neutral neutrons in all atoms except hydrogen. In biology, the central organ within a eucaryotic cell, with a membrane enclosing nucleic acid structures that contain the cell's genetic information.

Nutrient cycling

The movement of nutrients through different forms and places in an ecosystem.

Nutrient elements

Elements or substances that contribute to an organism's growth and health. Some nutrients are essential to completion of the life cycle.


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Oblique photo

In remote sensing, a photograph taken at an angle to the horizon, not perpendicular. See vertical photo.

Observed property

Any property that can be seen or measured.


An eight-sided molecular unit. In mineralogy, a metal cation such as Al or Fe surrounded by six oxygen atoms or hydroxyl groups.

Off-site disposal

Disposal of waste in a location away from where the waste is produced. Onsite disposal Disposal of waste at the site where the waste is produced.

Organic fertilizer

Organic material that releases or supplies useful amounts of a plant nutrient when added to soil.

Organic soil

Soil that contains a high percentage of organic matter throughout the solum. Osmotic potential The negative potential caused by the presence of solutes in soil water; solute potential.

Osmotic potential

The negative potential caused by the presence of solutes in the soil water; solute potential.

Overland flow

Water flow on the soil surface.


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The addition of oxygen. More generally, the removal of electrons from an atom, ion, or molecule during a reaction. Oxidation may increase the positive charge of an element or compound. See reduction.


Soft or weathered rock. Literally, "like rock." In soil classification, the solum may overlie lithic or paralithic material.


In remote sensing, the apparent displacement, or difference, in the apparent direction of an object seen from two different points.

Parent material

The unconsolidated and more or less chemically weathered minerals or organic matter from which the soil solum develops.

Particle detachment

The first step in the erosion process. Wind or water detaches particles from the soil mass.

Particle transport

The second step in the erosion process. Wind or water carries particles away from where they were detached.

Passive remote sensing

Remote sensing in which the energy being sensed is not supplied by the sensor. For example, any sensing that uses the sun's energy is passive remote sensing.


Unconsolidated soil material consisting largely of undecomposed, recognizable plant material.


A natural unit of soil structure. Another term for aggregate.


A three-dimensional body of soil with lateral dimensions large enough to permit the study of horizon shapes and relations. The soil individual.


A small ball of material stuck together. Seed may be pelleted with fertilizer, inoculants, and the like. Powdered fertilizers are pelleted for efficiency and ease of handling.


Amino acid polymers smaller in molecular size than proteins. Protein breakdown products.


The downward movement of fluid (water or waste effluent) in soil.

Perc test

A test to evaluate the suitability of soils for on-site waste disposal, by measuring the rate that water enters (percs) a soil layer or horizon.

Permanent wilting point

The water content of soils at which indicator plants will not regain turgor even if the soil water content is raised. Usually estimated as -1.5 MPa.


The property of a material such as a pesticide or organic matter to resist microbial or chemical decomposition. The more persistent a chemical, the longer time required to decompose it.


A measure of acid intensity. pH = -log[H+].

pH buffering

See buffering.

Phenol, phenolic compounds

Organic compounds with an unsaturated carbon-ring structure with one or more OH groups attached. Components of lignin and humus.


One of two main kinds of conductive (vascular) tissue in plants. Phloem carries dissolved organic materials from leaves to other parts.


The use of plants to remediate (clean) soil by plant uptake and accumulation of soil contaminants.

Plane table

A flat table used in mapping soils before the use of aerial photographs became common.

Plant-available moisture

Soil water held loosely enough that plants can extract it for use.

Plastic index

In engineering, the difference between the liquid limit and the plastic limit.

Plastic limit

In engineering, the minimum soil water content at which the soil consistence is soft and pliable rather than hard and brittle.


A soil structure type whose horizontal axis is much longer than its vertical axis.

Point data

Data for a specific location that are most easily presented in a table or graph. Soil color and texture of a horizon are examples of point data.


In chemistry, an uncharged molecule (such as water) that has an uneven distribution of charge so that it has some of the properties of a charged molecule.


Land once flooded that has been reclaimed by means of drainage. Pollute To make unclean; to add unwanted substances.


A large molecule made up of many similar small molecules (monomers) joined together. Examples are cellulose, proteins, and nucleic acids.


The process of joining two or more similar molecules to form a larger molecule.


A group of contiguous pedons.


Carbohydrate macromolecules-polymers-made from sugars.


Polymers of sugar acids (sugars with attached carboxyl groups) and related monomer units. Include pectic compounds important to plant cell walls.


Deliberate or accidental buildup of water on the ground surface. Pores The space not occupied by solid particles in the bulk volume of soil. Sometimes referred to as pore space.

Pore size distribution

The volume fractions of the various sizes of pores in a soil.


The volume percentage of the total bulk of soil not occupied by solid particles. The volume of pores in a sample divided by the sample volume.

Positive pore water pressure

Pressure exerted on soil particles by water in a saturated soil.

Potential gradient

The rate of change of potential with distance. The driving force of water through a medium.


In chemistry, the formation of solid material from constituents of a solution. Also, the deposition of water on the ground as rain, snow, dew, and frost or the amount of deposition.

Primary minerals

minerals that have not been altered chemically since they solidified from the molten magma.

Primary particles

Individual mineral particles; sand, silt, and clay are primary soil particles.


A soil structure type whose vertical axis is much longer than its horizontal axis and that has angular edges, compared with a columnar structure, which has a similar shape but curved edges.


In biology, referring to morphologically simple cells, without a membrane-enclosed nucleus; characteristic of bacteria. See eucaryotic.


Vertical gradation or sequence of a soil property-for example, temperature profile or pH profile. The appearance of a pedon's vertical face or section.


Polymers of amino acids that are essential to life. They function as enzymes and structural molecules within cells and cell membranes.


A positively charged primary particle found in the nucleus" of atoms and released by dissociation of water and other acids. H+ ions.


The addition of a proton to a molecule, ion, or surface.


Plural form is protozoa. A microscopic, often single-celled, eucaryotic, usually nonphotosynthetic animal-like organism.


A device to measure soil or leaf moisture or water potential as a function of relative humidity.


Accumulation of anthocyanin pigments in plants, causing a purple coloration of leaves or stem.


A family of single-chain ferromagnesian silicates.


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SiO2. An important rock-forming mineral, resistant to weathering and abundant in many soils.


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Processes in which energy is sent out as waves and particles through space from atoms and molecules as they rotate, vibrate, and undergo internal change.


Land used for free-grazing livestock.


The property of a material to stubbornly resist decomposition. A chemical such as a pesticide that is not decomposed by microorganisms is a recalcitrant pesticide.


In regard to soil, the treatment of soil to correct water logging or severe excesses such as salinity or sodicity.

Redoximorphic features

Soil properties associated with wetness that result from the reduction and oxidation of iron and manganese compounds in the soil after saturation with water and desaturation, respectively.

Redox concentrations

Zones of apparent accumulation of Fe-Mn oxides in soils.

Redox depletions

Zones of low chroma (2 or less) where Fe-Mn oxides alone or both Fe-Mn oxides and clay have been stripped out of the soil.


Addition of electron(s) to an atom or molecule. Opposite of oxidation.

Reemitted energy

Radiation of previously absorbed energy.

Reflected energy

Radiant energy that is thrown back from an object, with no change except direction.

Relative humidity

The concentration of water in the air or soil atmosphere relative to the maximum concentration it can hold at the given temperature.

Remote sensing

Identifying and observing objects at a distance.


Radiant emission of energy previously absorbed.

Residual parent material

Unconsolidated and partly weathered mineral materials accumulated by disintegration of consolidated rock in place. See transported parent material.

Residue conservation

Leaving straw, stubble, trash-crop residues-to rot on or in the ground instead of removing or burning them.


A measure of the difficulty with which a material conducts heat, ions, electricity, or water. Reciprocal of conductance.


The set of metabolic processes in which an organism obtains energy from the oxidation of sugars to CO2 and water, usually with O2 as the oxidizing agent.

Response curve

A graph relating a plant's quantitative response-changes in growth, yield, or any desired attribute-to different levels of a treatment such as fertilizer application.


The region in the soil immediately next to the root surface (within perhaps a millimeter) where the plant most strongly affects soil properties.


A small, intermittent water course with steep sides usually only several centimeters deep and wide.


The process by which reclaimed land develops characteristics suitable for farming. It includes drainage, cultivation, and neutralizing acidity.

Root cap

Cells at the tip of the root protecting the growing cells as the root pushes through the soil.

Root density

Number mass or length of roots within a given soil volume.

Root distribution

Arrangement of roots in space within the soil, especially the vertical root density profile.

Root exudate

A mixture of organic acids, sugars, and other soluble plant components that escape from roots.

Root hair

A hairlike structure produced from a root epidermal cell. Probably helps obtain water and immobile nutrients.

Rotational slide

Mass wasting in which the mass rotates about a point rather than sliding horizontally.


Water that runs off the soil surface instead of infiltrating; the process of running off. Sometimes loosely includes seepage'-that is, the escape of water that has flowed through the soil.


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Containing large concentrations of soluble salts. Operationally defined as the electrical conductivity of a saturation extract of ~4 deciSiemens per meter (dS m-1).


The process of making a soil saline; increasing the concentration of soluble salts.


Movement of particles by bouncing or skipping, under the influence of wind or moving water.

Salting out

Jargon. Precipitation of salts, as when concentrated fertilizer solutions are overcooled.


A mineral particle size ranging between 0.05 and 2.00 mm in diameter.

Sanitary landfill

An area of land used for the disposal of (household) solid waste.


Generally, occupying all of a capacity. With respect to water, the condition of a soil when all pores are filled with water. With respect to a particular cation or group of cations, 'the condition in which cations of the specified kind occupy all the exchange capacity.

Saturation extract

The soil solution removed for analysis from a saturated soil by means of vacuum filtration.

Scattered energy

Radiation reflected from an object in many directions. In remote sensing, this energy is useless for identifying the object.

Scattering (of light or other radiation)

Reflection in several directions. Scattering from fog, cloud, and haze reduces energy input to the ground.


A thin, dense layer that develops when the soil is wet. See crust.

Secondary minerals

Recrystallized or modified products from the chemical breakdown and/or alteration of primary minerals (clay minerals are an example of secondary minerals).

Section-range system

A legal survey system used to accurately locate a parcel of land.

Sedimentary rocks

Rocks formed by consolidation of materials deposited or precipitated from water or air.


A technique for determining particle size distribution. Particles fall through water at a rate proportional to their size.

Sensible heat

Ordinary everyday heat, sensed by feel or by thermometer.


The general term for the aluminum and iron oxides in soil. See hydrous oxides.

Seventh Approximation

The first widely distributed version of the present U.S. soil classification system.

Shear strength

The resistance of a soil to forces acting at right angles to the soil body. A plow produces shearing forces.

Sheet erosion

Water erosion that removes a uniform layer of soil from the land surface.

Sheet flow

A thin, relatively uniform water runoff (a few millimeters deep), in which the water is not concentrated in channels.

Shortwave radiation

In meteorology, high-energy radiation of the ultraviolet, visible, and near infrared wavelengths; sunshine. Unlike longwave radiation, it passes freely through air.

Shrinkage limit

The maximum water content at which a reduction in water content will not cause a decrease in the soil's volume.


Those minerals in which silicon and oxygen are the major elemental constituents.


A mineral particle size ranging between 0.002 and 0.05 mm in diameter.


Somewhere or something for the disposal of wastewater.

Site index

A quantitative evaluation of a soil's relative site productivity for forest growth under the existing or specified environment.

Size separate

Individual mineral particles less than 2.0 mm in diameter, ranging between specified size limits. Sand, silt, and clay are common soil size separates.


A process of aggregate breakdown caused by internal pressures generated as water enters aggregates and air escapes.


In waste disposal, a semifluid mixture of fine solid particles with a liquid. Also referred to as biosolids.


Collapse of soil and rock due to gravity.


A family of expanding 2:1 (Si tetrahedral sheets to Al octahedral sheets) clay minerals with a high surface area and exchange capacity. Sodic High in sodium; having a high exchangeable Na percentage or high Na adsorption ratio.

Sodium adsorption ratio (SAR)

The concentration of Na divided by the square root of the sum of Ca and Mg concentrations, both expressed as molarities and measured in the saturation extract.

Soil acidity factors

A set of related factors often inhibiting plant growth in acidic soils (deficiency of Ca2+ and excess of H+, Al3+, and Mn2+).

Soil air

The gas within the soil; the soil atmosphere.

Soil association

A group of defined and named soil units occurring together in an individual and characteristic pattern over a geographic area. The individual bodies are large enough to be delineated at a scale of 1:24,000.

Soil complex

A map unit consisting of two or more kinds of soil that occur in a regular, repeating pattern so intricate that its components cannot be separated at the 1:24,000 map scale.

Soil creep

Slow mass movement of soil and soil material down relatively steep slopes under the influence of gravity. See creep.

Soil density

A measure of the relative amount of pores and solid particles. The mass per unit volume.

Soil drainage class

An interpretive grouping of soils based on the level of the water table during the growing season and the rate of water flow through soil.

Soil fabric

The pattern resulting from the arrangement of solids and pores.

Soil-forming factors

The five interrelated natural factors that are active in the formation of soil: parent material, climate, organisms, topography, and time.

Soil-forming processes

The biological, chemical, and physical processes that act under the influence of the soil-forming factors to create soils.

Soil genesis

Soil formation; the process of soil formation.

Soil individual

The pedon; a three-dimensional body large enough to contain all the properties necessary to describe a soil completely.

Soil matrix

Like soil fabric, the combination of solids and pores in a soil.

Soil order

In soil classification, the most general level of classification. All soils fit into 12 orders.

Soil potential

The usefulness of a site for a specific purpose using available technology at a cost expressed in economic, social, or environmental units of value.

Soil series

The most detailed level in soil classification. A soil series consists of soils essentially alike in all major profile characteristics except the texture of the A horizon.

Soil solution

The liquid phase of the soil and its solutes.

Soil survey

The systematic examination, description, classification, and mapping of soils in an area.

Soil taxonomy

The U.S. Department ofAgriculture system of soil classification.

Soil test

A quick, routine extraction and analysis of soil for a diagnostic purpose.

Solid waste

Waste other than sewage, mostly solid. Household garbage is a familiar form of solid waste.


The upper and most weathered part of the soil profile; the A, E, and B horizons.


A material dissolved in a solvent to form a solution.

Specific adsorption

See adsorption, specific and ligand exchange.

Specific gravity

The ratio of the weight or mass of a given volume of a substance to that of an equal volume of a reference substance. Water is the reference for liquids and solids.

Spectral signature

In remote sensing, an object's spectral reflectance pattern. The wavelength and intensity of reflectance.

Splash erosion

The detachment and airborne movement of small soil particles by raindrop impact.

Spoil (overburden)

Material overlying sought after resources that is removed and piled to gain access to the minerals, coal, or other underground materials of value. Also, the material remaining after rock has been crushed to remove valuable minerals.


The central conductive tissue of a root or some kinds of stem.

Stereo pair

Two photographs of the same object taken from slightly different positions that when viewed together give the viewer a three dimensional image.

Stomates (or stomata)

The controllable openings in the epidermis of leaves and other parts of a plant shoot.


Arranged in layers.


A force exerted on a body, especially one that strains or deforms its shape. An adverse condition imposed on an organism.


A technique to reduce soil erosion in which bare fallow or a row crop such as corn or soybeans is grown in conjunction with grass or a legume hay crop.

Strip mining

A process in which rock and top soil strata overlying ore or fuel deposits are scraped away to expose the desired deposits.


In soil, the arrangement of primary particles into secondary units or peds. The units are characterized and classified on the basis of size, shape, and degree of distinctness. In crystals, the arrangement of atoms or molecules. In molecules, the arrangement and bonding of atoms.

Structure grade

A grouping of soil structure based on the cohesion or stability of the structure. Grades are structureless, weak, moderate, and strong.

Structure strength

A term sometimes used in place of structure grade.

Subangular blocky

A structure type whose aggregates are nearly equidimensional, square, or block like with rounded edges.


Layers of material underlying the soil horizons-for example, sediment or rock.


A substratum. More often, the reactant or starting compound for a biochemical reaction or a material that provides food for microbes.

Subsurface diagnostic horizons

One or more soil horizons below the surface horizon with specific properties as described in the soil taxonomy (e.g., the argillic horizon).

Subtractive trial

A fertility trial in which deficiencies are detected by observing plant response to the elimination of individual elements from a complete mixture.


A simple carbohydrate, consisting of one or two monomer units, each usually containing five or six C atoms. Rapidly used food sources for microbes in soil. Examples are glucose, fructose, and sucrose.


A phosphate fertilizer made by treating phosphate rock with acid to increase solubility.

Surface chelation

A chelation.reaction that holds a cation at the surface.

Surface tension

The force required per unit length to separate or pull apart a liquid surface.


A fluid, usually liquid, bearing small particles held suspended but not dissolved.


An association between two organisms or populations that, in the absence of environmental change, is stable.


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Technical classification

A soil classification of inferred rather than observed properties.

Temperature gradient

The rate at which temperature changes with distance; in soil, usually vertical distance.

Tensile strength

The resistance of a soil to forces pulling from opposite directions.


A device for measuring the soil-water matric potential in place.


In soil conservation, a more or less level or horizontal strip of earth usually constructed on a contour designed to reduce erosion.


A four-sided arrangement of atoms with a cation, such as Si, in the center surrounded by four oxygen atoms or hydroxyls.


The relative proportion of the various soil separates-sand, silt, and clay-that make up the soil texture classes as described by the textural triangle.

Textural Class

Classifications of texture into groupings. They are typically named for the primary constituent particle size or a combination of the most abundant particles sizes, e.g. "sandy clay" or "silty clay"

Thermal properties

See conductivity and heat capacity.

Tile drain

See Drain, tile.


The mechanical stirring or turning of the soil profile. In agriculture, horticulture, and landscaping, tillage is used to prepare soil for seeds, to incorporate organic matter or chemicals, or to control weeds.

Tissue test

A routine, rapid chemical analysis of leaves or other plant parts for diagnostic purposes.


Two or more soils in which the soilforming factor that varies the most is the landscape (usually elevation or position on a slope). Other soil-forming factors are constant or vary much less than topography.


A term describing the winding, twisting, and crooked nature of soil pores. A factor contributing to the rate of water movement in soil.

Trace elements

Elements contained in plants or soil in small quantities. Sometimes used for micronutrients.

Transitional horizon

A horizon with properties intermediate between the horizons above and below.

Transitional slide

A break causing a mass to slide along the face of the supporting mass under the force of gravity.

Transmitted energy

In remote sensing, energy that passes through an object. Transpiration Evaporation from leaves; the flow of water through plants from soil to atmosphere.


Evaporation from leaves: the flow of water through plants from soil to atmosphere.

Transported parent material

The opposite of residual parent material; unconsolidated material carried by wind or water that, once stabilized, acts as the starting material for soil.


A term often used to describe a field or container experiment designed to determine nutrient deficiencies. See factorial trial and subtractive trial.

Turbulent transfer

Diffusion-like movement of heat, gases, and solutes greatly accelerated by irregular mixing motion-turbulence-in the fluid medium.


The normal state of turgidity in living cells produced by pressure of the cell contents on the cell walls.

Turgor pressure

Pressure exerted on a cell wall by the cell's constituents (and vice versa).


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Undifferentiated group

A soil map unit consisting of two or more similar soil units not in a regular geographic association. The soil units have the same or very similar use and management.

Unit cell

The smallest definable repeating structural unit in a crystalline material.

Universal Soil Loss Equation (USLE)

An equation developed from experimental results that relates the average annual soil loss to rainfall, soil, cover, erosion control, and topographic factors.


A sugar with a COOH group. See polyuronides.


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van derWaal's forces

Combined polar and London attractions between molecules in close proximity.

Variable charge

An electrical charge on clay or organic matter that changes with changes in soil pH.


Having to do with veins. See conductive tissue.


An expanding 2:1 layer aluminosilicate clay mineral, in which one sheet of Al, O, and OH in octahedral coordination lies between two sheets of Si and Al, with O in a tetrahedral coordination.

Vertical photo

In remote sensing, a photo taken from directly above an object rather than at an angle to the object. See oblique photo.

Vesicular pore

A soil pore not connected to other pores.

Void ratio

A measure of porosity. The ratio of void volume to soil bulk volume.



Volcanic ash

Fine particles of rock blown into the air by a volcano. Settles, often in layers, to become soil parent material. Often consists of short-range order minerals.


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Washed-in zone

Formed by clay separating from the top few tenths of a millimeter of soil and being translocated into the next few tenths to clog the pores.

Water erosion

The natural wearing away of the earth's surface by rainfall and surface runoff. See accelerated erosion.


Saturated with water.

Water potential

The tendency of soil water to move; the sum of gravity, pressure, matric, and solute components.

Water profile

The pattern of vertical variation in water content or potential with depth in the soil.

Water retention curve or soil-water characteristic curve

The graphical relationship between soil water content (by mass or volume) and the soil-water matric potential (the energy required to remove the water).

Water table

The level, elevation, or imaginary plane corresponding to the top of a water-saturated zone of soil, sediment, or rock.


The chemical or physical processes at or near the earth's surface that change rocks and minerals exposed to air and water.

Wetting front

The advancing boundary between dry soil and wetted soil during infiltration.


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One of two main kinds of conductive tissue in plants. Xylem has tubelike cells that conduct water and ions rapidly from roots to leaves. Wood is xylem tissue.


Glossary of Soil Science Terms. 1975. Madison, Wis.: Soil Science Society ofAmerica.

Glossary of Soil Science Terms. 1997. Madison, Wis.: Soil Science Society of America.

Resource Conservation Glossary. 1976. Ankeny, Iowa: Soil Conservation Society ofAmerica.