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Phrase:Definition:Other Notes:
A Horizon: The surface horizon of a mineral soil which contains organic matter.  
Abrasion: The physical weathering of a rock surface by running water, glaciers or wind laden with fine particles. See Ventifact.  
Absorption: The physical uptake of water and/or ions by a substance. For example, soils absorb water.  
Accelerated Erosion: An increased rate of erosion caused by humans.  
Accessory Minerals: Minerals occuring in small quantities in a rock whose presence or absence does not affect the true nature of the rock.  
Accumulation: The build-up or increase of one or more constituents in the soil at a given position as a result of translocation. The build-up may be a residue due to the translocation of material out of the horizon or may be due to an addition of material. Usually refers to soluble substances and clay particles.  
Acicular: Needle shaped.  
Acid Rock: An igneous rock that contains more than 60 per cent silica and free quartz.  
Acid Soil: A soil with a pH<6.5.  
Acidity: The hydrogen ion activity in the soil solution expressed as a pH value.  
Actinomycetes: Family of microorganisms belonging to a group intermediary between bacteria and molds (fungi); a form of filamentous, branching bacteria.  
Adsorption Complex: The various substances in the soil that are capable of adsorption, these are mainly clay or humus.  
Adsorption, also adsorb, fixation: The attachment of a particle, ion or molecule to a surface. Calcium is adsorbed onto the surface of clay or humus.  
Adsorption: The attachment of a particle, ion or molecule to a surface. Calcium is adsorbed onto the surface of clay or humus.  
Aeolian Deposits: Fine sediments transported and deposited by wind; they include loess, dunes,desert sand and some volcanic ash.  
Aeolian: Pertaining to wind action.  
Aeration: The process by which atmospheric air enters the soil. The rate and amount of aeration depends on the size and continuity of the pore spaces and the degree of water logging.  
Aerial Photograph: A photograph of the Earth's surface taken from an aeroplane or some other type of airborne equipment.  
Aerobic Organism: Organisms living or becoming active in the presence of molecular oxygen.  
Aerobic: Conditions having a continuous supply of molecular oxygen. compost - composting environment characterized by bacteria active in the presence of oxygen (aerobes); generates more heat and is a faster process than anaerobic composting. Aerobic temperatures may reach over 60 °C high enough to destroy pathogens, weed seeds, and fly ova; creates no excessive unpleasant odors; the most rapid composting process occurs with enclosed aerobic systems.  
AFP: Air filled porosity; the air capacity of a compost.  
Aggregates: Discrete clusters of particles formed naturally of artificially and including such particles as crumbs, peds, clods, faecal pellets, fragments of faecal pellets and concretions.  
Aggregation: The process by which particles coalesce to form aggregates.  
Agricultural Waste: Waste materials produced from the raising of plants and animals, including manures, bedding, plant stalks, hulls, leaves and vegetable matter.  
Agronomy: That part of agriculture devoted to the production of crops and soil management - the scientific utilization of agricultural land.  
Algae: Unicellular of multicellular plants containing chlorophyll. The are aquatic or occur in damp situations and include most seaweeds.  
Alkaline Soil: A soil with pH >7.3.  
Alluvial Plain: A flat area built up of alluvium.  
Alluvial Soil: A general term for those soils developed on a fairly recent alluvium.  
Alluvium: A sediment deposited by streams and varying widely in particle size. The stones and boulders when present are round or sub-rounded. Some of the most fertile soils are derived from alluvium of medium or fine texture.  
Amendment (Soil): A material that is added to soil to improve chemical or physical characteristics or as a means of treating a waste material.  
Ammonia Fixation: Adsorption of ammonium ions by clay minerals, rendering them insoluble and non-exchangeable.  
Ammonification: The production of ammonia by microorganisms through the decomposition of organic matter.  
Anaerobic Organism: One that lives in an environment without molecular oxygen.  
Anaerobic: Conditions that are free of molecular oxygen. In soils this is usually caused by excessive wetness.  
Anion Exchange Capacity: The total amount of anions that a soil can adsorb, usually expressed as meg kg-1 soil.  
Anion: An ion having a negative charge.  
Anisotropic: 1. General: possessing different physical properties in different directions 2. General: having physical properties that depend on direction 3.Minerals or parts of soils: alternately bright and dark between crossed polars when the microscope stage is rotated. The bright position is due to the formation of interference colors. SEE INTERFERENCE COLOURS.  
Annelid: Red blooded worm such as an earthworm.  
Annual Plant: A plant that completes its life cycle within one year.  
Arid: A term applied to a region or climate in which precipitation is too low to support crop production.  
Arthropod: A member of the phylum arthropoda which is the largest in the animal kingdom. It includes insects, spiders,centipedes, crabs, etc.  
Aspect: The compass direction of a slope.  
Autotrophic Organism: Organisms that utilize carbon dioxide as a source of carbon and obtain their energy from the sun or by oxidizing inorganic substances such as sulphur, hydrogen, ammonium, and nitrate salts. The former include the higher plants and algae and the latter various bacteria, cf. HETEROTROPHIC.  
Available Elements: The elements in the soil solution that can readily be taken up by plant roots.  
Available Nutrients:   SEE AVAILABLE ELEMENTS.
Available Water Capacity: The weight percentage of water which a soil can store in a form available to plants. It is equal to the moisture content at field capacity minus that at the wilting point.  
Available Water: That part of the water in the soil that can be taken up by plant roots.  
B horizon: A subsoil layer altered by soil forming processes.  
B.S.: An abbreviation for Base Saturation.  
Bacteria: Unicellular or multicellular microscopic organisms. They occur everywhere and in very large numbers in favorable habitats such as soil and sour milk where they number many millions per gram.  
Bar: 10 to the power of 5 Pascal or 10 to the power of 5(Nm to the power of -2). One bar = 100 kPa, often used in soil water studies.  
Basalt: A fine grained igneous rock forming lava flows or minor intrusions. It is composed of plagioclase, augite and magnetite; olivine may be present.  
Base saturation: The extent to which the exchange sites of a material are occupied by exchangeable basic cations; expressed as % of the cation exchange capacity.  
Basic Rock: An igneous rock that contains less than 55% silica.  
Bedrock: The solid rock at the surface of the earth or at some depth beneath the soil and superficial deposits.  
Biennial: A plant that completes its life cycle in two years.  
Bioassay: A laboratory assay (test) using a biological test organism.  
Bioavailable: Available for biological uptake.  
Biodegradability: The potential of an organic component for conversion into simpler structures by enzymatic activity.  
Biogenic Waste - (Germany): The separated organic fraction of household waste; consists of yard and food waste.  
Biological Oxygen Demand (BOD): The amount of oxygen used in the biochemical oxidation of organic matter; an indication of compost maturity and a tool for studying the composting process.  
Biomass: a) the weight of a given organism in a volume of soil that is one m squared at the surface and extending down to the lower limit of the organism's penetration. b) The weight of organisms in a given area or volume.  
Biosolids: Primarily organic solid product produced by the wastewater treatment process that can be benefically recycled.  
Blocky: Many sided with angular or rounded corners, used for describing peds.  
Bog Iron Ore: A ferruginous deposit in bogs and swamps formed by oxidizing algae, bacteria or the atmosphere on iron in solution.  
Boulder Clay:   SEE TILL.
Boulder: Rock particles over 200 mm diameter.  
Buffer: A substance that prevents a rapid change in pH when acids or alkalis are added to the soil, these include clay, humus and carbonates.  
Bulk Density: Mass per unit volume of undisturbed soil, dried to constant weight at 105 degrees C. Usually expressed as g/cc.  
C horizon: An unconsolodated subsoil layer slightly altered by soil forming processes.  
Calcareous Soil: A soil that contains enough calcium carbonate so that it effervesces when treated with hydrochloric acid.  
Calcification: Used by some to refer to the processes of calcium carbonate accumulation.  
Calcite: Crystalline calcium carbonate, CaCO3. Crystalizes in the hexagonal system, the main types of crystals in soils being dog-tooth, prismatic, nodular,fibrous granular and compact.  
Caliche: A layer or horizon cemented by the deposition of calcium carbonate. It usually occurs within the soil but may be at the surface due to erosion.  
Capillarity: The process by which moisture moves in any direction through the fine pore spaces and as films around particles.  
Capillary Fringe: The zone just above the water-table that remains practically saturated with water.  
Capillary Moisture: That amount of water that is capable of movement after the soil has drained. It is held by adhesion and surface tension as films around particles and in the finer pore spaces.  
Carbohydrates: Various kinds of sugars, generally easily assimilated by bacteria.  
Carbon to Nitrogen Ratio (C:N Ratio): Ratio representing the quantity of carbon (C) in relation to the quantity of nitrogen (N) in a soil or organic material; determines the composting potential of a material and serves to indicate product quality.  
Catena: A sequence of soils developed from similar parent material under similar climatic conditions but whose characteristics differ because of variations in relief and drainage.  
Cation Exchange Capacity (CEC): The total potential of soils for adsorbing cations, expressed in milligram equivalents per kg of soil. Determined values depend somewhat on the method employed. A routine measure of the binding potential of a soil; measures the soil's ability to remove negative ions from metals and other compounds, allowing the ions to form insoluble compounds and precipitate in the soil; determined by the amount of organic matter and the proportion of clay to sand; the higher the CEC, the greater the soil's ability to bind metals.  
Cation Exchange: The exchange between cations in solution and cations held on the exchange sites of minerals and organic matter.  
Cation: An ion having a positive electrical charge.  
CEC: An abbreviation of Cation Exchange Capacity.  
Cellulose: Carbon component of plants, not easily digested by microorganisms.  
Cemented: Massive and either hard or brittle depending on the content of cementing substances such as calcium carbonate, silica, oxides of iron and aluminum, or humus.  
Chalk: The term refers to either (a) soft white limestone which consists of very pure calcium carbonate and leaves little residue when treated with hydrochloric acid, and sometimes consists largely of the remains of foraminifera, echinoderms, molluscs, and other marine organisms, or(b) The upper or final member of the cretaceous system.  
Channel: A tubular-shaped pore of biological origin < 20 mm in diameter.  
Chemical Oxygen Demand: A measure of the oxygen equivalent of that portion of organic matter in a sample that is susceptible to oxidation by a strong chemical oxidant; an important, rapidly measured parameter for stream and industrial waste studies and for control of waste treatment plants.  
Chlorosis: The formation of pale green or yellow leaves in plants resulting in the failure of chlorophyll to develop. It is often caused by a deficiency in an essential element.  
Chroma: The relative purity of a color directly related to the dominance of the determining wavelength. One of the three variables of color.  
Chronosequence: A sequence of soils that changes gradually from one to the other with time.  
Clay Coating:   SEE COATING.
Clay Mineral: Crystalline or amorphous mineral material,<2mm in diameter.  
Clay Pan: A middle or lower horizon containing significantly more clay than the horizon above. It is usually very dense and has a sharp upper boundary. Clay pans generally impede drainage, are usually plastic and sticky when wet and hard when dry.  
Clay: Either 1. Mineral material <0.002 mm. 2. A class of texture. 3. Silicate clay materials.  
Clod: A mass of soil produced by disturbance.  
Coating: A layer of a substance completely or partly covering a surface. Coatings are composed of a variety of substances separately or in combination. They include clay coatings (clay skins), calcite coatings, whole soil coatings, etc. Coatings may become incorporated into the matrix or be fragmented.  
Coefficient of Linear Extensibility: The ratio of the difference between the moist and dry lengths of a clod to its dry length, (Lm-Ld)/Ld when Lm is the moist length at (1/3 atmospheres) and Ld is the air-dry length. The measure correlates with the volume change of a soil upon wetting and drying.  
COLE: An abbreviation of coefficient of linear extensibility.  
Colloid: The organic and inorganic material with very fine particle size and therefor high surface area which usually exhibits exchange properties.  
Colluvium: Soil materials with or without rock fragments that accumulate at the base of steep slopes by gravitational action.  
Compaction: Increase in bulk density due to mechanical forces such as tractor wheels.  
Composite Structure: Any combination of different types of peds.  
Compost: composting environment characterized by bacteria active in the absence of oxygen (anaerobes). In anaerobic composting, the microflora obtain oxygen from the waste; peak temperatures may reach 38 to 55° C; digestion requires more time, foul odors are created and pathogens may survive.  
Compost: Plant and animal residues that are arranged into piles and allowed to decompose, sometimes soil or mineral fertilizers may be added. The stabilized product of composting which is beneficial to plant growth; it has undergone an initial, rapid stage of decomposition and is in the process of humification.  
Composting, Municipal: Solid waste management method whereby the organic component of the solid waste stream is biologically decomposed under controlled conditions; an aerobic process in which waste organic materials are ground or shredded and then decomposed to humus in windrow piles or in mechanical digesters, drums, or similar enclosures; results in volume and odor reduction, waste stabilization, destruction of pathogens, larvae and weed seeds; the final product is sufficiently stable for storage and land application without adverse environmental effects.  
Composting: The biodegradation, usually aerobic and thermophilic, that: involves a heterogeneous organic substrate in the solid state; evolves by passing through a thermophilic stage with a temporary release of phytotoxins; results in the production of carbon dioxide, water, minerals and stabilized organic matter.  
Compound Structure: Large peds such as prisms and columns that are themselves composed of smaller incomplete peds.  
Concretion: Small, hard local concentrations of material such as calcite, gypsum, iron oxide, or aluminum oxide. Usually spherical or subspherical but may be irregular in shape.  
Conductivity: A measure of the soluble salts in the soil; used as an overall indicator of the level of macro- and micronutrients in the soil.  
Conglomerate: A sedimentary rock composed mainly of rounded boulders.  
Consistence: The resistance of the soil to deformation or rupture as determined by the degree of cohesion or adhesion of the soil particles to each other.  
Consolidated: A term that usually refers to compacted or cemented rocks.  
Contaminant: Foreign material lending impurity to a primary material; physical contaminants of compost include glass and plastic, chemical contaminants include heavy metals and toxic organic compounds.  
Continuously Anaerobic(very poorly drained): A horizon that is saturated with water throughout the year, it is blue, olive or grey.  
Creep: Slow movement of masses of soil down slopes that are usually steep. The process takes place in response to gravity facilitated by saturation with water.  
Crotovina: See Krotovina.  
Crust: A surface layer of soils that becomes harder than the underlying horizon.  
Cutans: Coatings or deposits of material on the surface of peds, stones, etc.A common type is the clay cutan caused by translocation and deposition of clay particles on ped surfaces.  
Decomposition: Conversion of organic matter as a result of microbial and/or enzymatic interactions; initial stage in the degradation of an organic substrate, characterized by processes of destabilization of the pre-existing structure.  
Deficiency: The lack of an adequate amount of a plant nutrient.  
Deflation: Preferential removal of fine soil particles from the surface soil by wind. SEE DESERT PAVEMENT.
Deflocculate: To separate of disperse particles of clay dimensions from a flocculated condition.  
Delta: A roughly triangular area of the mouth of a river composed of river transported sediment.  
Denitrification: The biological reduction of nitrogen to ammonia, molecular nitrogen or oxides of nitrogen, resulting in the loss of nitrogen into the atmosphere and therefor undesirable in agriculture.  
Denudation: Sculpturing of the surface of the land by weathering and erosion;levelling mountains and hills to flat or gently undulating plains.  
Deposit: Material placed in a new position by the activity of humans or natural processes such as wind, water ice or gravity.  
Desert Crust: A hard surface layer in desert regions containing calcium carbonate, gypsum, or other cementing materials.  
Desert Pavement: A layer of gravel or stones remaining on the surface of the ground in deserts after the removal of fine material by wind. SEE DEFLATION AND HAMADA.
Desert Varnish: A glossy sheen or coating on gravel or stones in arid regions.  
Dewatered Sewage Sludge: Sewage sludge with a total solids content of 6% or greater that can be transported and handled as a solid material; usually done by belt press, screw press, vacuum filtration or centrifuge.  
Diatoms: Algae that possess a siliceous cell wall which remains preserved after the death of the organisms. They are abundant in both fresh and salt water and in a variety of soils.  
Digester: An enclosed composting system with a device to mix and aerate the waste materials.  
Digestion - composting: The most active stage of the composting process; carried out in open windrows or in enclosures; the objective is to create an environment in which microorganisms will rapidly decompose the organic portion of the refuse.  
Dispersion: The process whereby the structure or aggregation of the soil is destroyed so that each particle is separate and behaves as a unit.  
Doline or Dolina: A closed depression in a karst region often rounded or elliptical in shape, forms by the solution and subsidence of the limestone near the surface. Sometimes at the bottom is a sink hole into which surface water flows and disappears underground.  
Domain: A bundle of clay particles that is only visible in crossed polarized light.  
Drift: A generic term for superficial deposits including till (boulderclay), outwash gravel and sand, alluvium, solifluction deposits and loess.  
Drumlin: A small hill, composed of glacial drift with hog back outline, oval plan, and long atlas oriented in the direction of ice movement. Drumlins usually occur in groups, forming what is known as basket of eggs topography.  
Dry-Farming: A method of farming in arid and semi-arid areas without using irrigation, the land being treated so as to conserve moisture. The technique consists of cultivating a given area in alternate years allowing moisture to be stored in the fallow year. Moisture losses are reduced by producing a mulch and removing weeds. In Siberia, where melting snow provides much of the moisture for spring crops, the soil is ploughed in the autumn providing furrows in which snow can collect, preventing it from being blown away and evaporated by strong winds. Usually alternate narrow strips are cultivated in an attempt to reduce erosion in the fallow year. Dry farming methods are employed in the drier regions of India, USSR, Canada and Austria.  
Dunes, Sand Dunes: Ridges or small hills of sand which have been piled up by wind action on sea coasts, in deserts, and elsewhere. Barkhans are isolated dunes with characteristic crescentic forms.  
Ecology: The study of interrelationships between individual organisms, and between organisms and their environments.  
Ecosystem: A group of organisms interacting among themselves and with their environment.  
Edaphic: (1) Of or pertaining to the soil. (2) Influenced by soil factors.  
Edaphology: The study of the relationships between soil and soil including the use of the land by humans.  
Efflorescence: The accumulation of dissolved substance (usually simple salts) at a surface due to evaporation.  
Eluvial Horizon: A horizon from which material has been removed either in solution or suspension.  
Eluviation: Removal of material from the upper horizon in solution or suspension.  
Erosion Pavement: A layer of gravel or stones left on the surface of the ground after the removal of the fine particles by erosion.  
Erosion: The removal of material from the surface of the land by weathering, running water, moving ice, wind and mass movement.  
Esker: A long narrow ridge, chiefly of gravel and sand formed by a melting glacier or ice sheet.  
Essential Element: The elements C, H, O, P, K, N, S, Ca, Mg, K, B, Mn, Cu, Zn, Mo, Cl, Co, Si and F. These must be taken up and utilized in sufficent quantities for plants to complete their life cycles.  
Eutrophic: Containing an optimum concentration of plant nutrients.  
Evapotranspiration: The combined processes of evaporation and transpiration.  
Excessively Aerobic: A horizon which is usually too dry to support adequate plant growth.  
Excessively Drained: A soil that loses water very rapidly because of rapid percolation.  
Exchangeable Cation: A cation such as calcium that is adsorbed onto a surface, usually clay or humus and is capable of being easily replaced by another cation such as potassium. Exchangeable cations are readily available to plants.  
Exfoliation: A weathering process during which thin layers of rock peel off from the surface. This is caused by the heating of the rock surface during the day and cooling at night leading to alternate expansion and contraction. This process is sometimes termed onion skin weathering.  
Facultative Aerobic Organisms: Organisms capable of growing under both aerobic and anaerobic conditions.  
Faecal Material: The various types of faeces or excrement produced by soil fauna.  
Fallow: Leaving the land uncropped for a period of time. This may be to accumulate moisture, improve structure or induce mineralization of a nutrient.  
Fen Peat: Peat that is neutral to alkaline due to the presence of calcium carbonate.  
Ferralitisation: Used by some to refer to the process of formation of ferralitic soils. This term is not specific and should not be used.  
Fertilizer: A material that is added to the soil to supply one or more plant nutrients in a readily available form.  
Fertilizer: A material that is added to the soil to supply one or more plant nutrients in a readily available form.  
Field Capacity or Field Moisture Capacity: The total amount of water remaining in a freely drained soil after the excess has flowed into the underlying unsaturated soil. It is expressed as a percentage of the oven-dry soil. Taken as the water content of a soil sample at -10 kPa.  
Fine Material: Soil material in thin sections composed of particles less than 2mm which are difficult or impossible to resolve with the petrological microscope.  
Flood Plain: The land adjacent to a stream, built of alluvium and subject to repeated flooding.  
Fragipan: Brittle subsurface restricting soil horizon, usually loamy textured and weakly cemented.  
Fragment: A small mass of soil produced by a disturbance.  
Freely Drained: A soil that allows water to percolate freely.  
Friable: A term applied to soils that when either wet or dry crumble easily between the fingers.  
Fulvic Acid: The mixture of organic substances remaining in solution upon acidification of a dilute alkali extract of soil.  
Fungi: Simple plants that lack chlorophyll and are composed of cellular filamentous growth known as hyphae. Many fungi, but their fruiting bodies, viz. mushrooms and puffballs are quite large. in composting:- saprophytic or parasitic multinucleate organisms with branching filaments called hyphae, forming a mass called a mycelium; fungi bring about cellulolysis and humification of the substrate during stabilization.  
Gastropod: A member of the Gastropoda class of molluscs which includes snails and slugs.  
Geomorphology: The study of the origin of physical features of the Earth, as they are related to geological structure and denudation.  
Gilgai: A distinctive microrelief of knolls and basins that develop on clay soils that exhibit a considerable amount of expansion and contraction in response to wetting and drying.  
Glacial Drift: Materials transported by glaciers and deposited directly from the ice of from the meltwater.  
Glacier: A large mass of ice that moves slowly over the surface of the ground or down a valley. They originate in snowfields and terminate at lower elevations in a warmer environment where they melt.  
Glacio-Fluvial Deposits: Material deposited by meltwaters coming from a glacier. These deposits are variously stratified and may form outwash plains, deltas, kames, eskers, and kame terraces. SEE GLACIAL DRIFT AND TILL.
Gleisation: A process in saturated or nearly saturated soils which involves the reduction of iron, its segregation into mottles and concretions, or its removal by leaching from the gleyed horizon. SEE GLEYING.
Gleyed: A soil condition resulting from gleizaion which is manifested by the presence of neutral grey, bluish or greenish colors through the soil matrix or in mottles (spots or streaks) among other colors.  
Gleying: The reduction of iron in an anaerobic environment leading to the formation of grey or blue colors.  
Granite: An igneous rock that contains quartz, feldspar and varying amounts of biotite and muscovite.  
Gravel: Rock material 2 to 200 mm in diameter.  
Gravitational Water: The water that flows freely through soils in response to gravity.  
Great Soil Group: One of the Categories in soil classification.  
Green Waste: Portion of the municipal waste stream consisting of grass clippings, tree trimmings and other vegetative matter.  
Groundwater-Table: The upper limit of the groundwater.  
Growing season: The portion of the year when soil temperatures are above biologic zero 41°F (4°C) as defined by Soil Taxonomy.  
Gully Erosion: A form of catastrophic erosion that forms gullies.  
Gully: A shallow steep-sided valley that may occur naturally or be formed by accelerated erosion.  
Gyttja: Peat consisting of faecal material, strongly decomposed plant remains, shells of diatoms, phytoliths, and fine material particles. Usually forms in standing water.  
Halophyte: A plant capable of growing in salty soil; i.e. a salt tolerant plant.  
Halophytic Vegetation: Vegetation that tolerates or requires saline conditions.  
Hamada: An accumulation of stones at the surface of deserts, formed by the washing or blowing away of the finer material.  
Hardpan: A horizon cemented with organic matter, silica, sesquioxides, or calcium carbonate. Hardness or rigidity is maintained when wet or dry and samples do not slake in water.  
Heavy Metals: Trace elements regulated because of their potential for human, plant, or animal toxicity, including cadmium (Cd), copper (Cu), chromium (Cr), mercury (Hg), nickel (Ni), lead (Pb) and Zinc (Zn).  
Heavy Soil (Obsolete): A soil that has a high content of clay and is difficult to cultivate.  
Heterogeneous: Complex and not easily described.  
Heterotrophic Organisms: Those that derive their energy by decomposing organic compounds, cf. AUTOTROPHIC.  
High water mark: A distinct mark made on vegetation, buildings or rocks that shows the extent of water rise.  
Histosol: A soil order in the United States soil taxonomic system that is composed of mucks and peats that have a high concentration of organic materials in the surface soil or overly rock.  
Holocene Period: The period extending from 10,000-0 years BP.  
Horizon notation: A system of symbols used for labelling soil horizons and aids in comparing and classifying soil profiles e.g. A, B and C horizons.  
Horizon: Relatively uniform materials that extend laterally, continuously or discontinuously throughout the pedounit; runs approximately parallel to the surface of the ground and differs from the related horizons in many chemical, physical and biological properties.  
Hue: The dominant spectral color and one of the three color variables.  
Humic Acid: Usually refers to the mixture of ill-defined dark organic substances precipitated upon acidification of a dilute alkali extract of soil. Some workers use it to include only the alcohol-insoluable portion of the precipitate. The main constituent of humus, composed of proteins and lignins, dark brown to black in color.  
Humification: The decomposition of organic matter leading to the formation of humus. The microbial synthesis of three-dimensional polymers of saccharides and phenols resembling gums and lignin; a process of storing organic energy in compounds of high molecular weight which are slowly degradable (10-100+ years).  
Humus: The well-decomposed, relatively stable part of the organic matter found in aerobic soils. A complex aggregate of amorphous substances, formed during the microbial decomposition or alteration of plant and animal residues and products synthesized by soil organisms; principal constituents are derivatives of lignins, proteins and cellulose; humus has a high capacity for base exchange (CEC), combining with inorganic soil constituents, and for water absorption; finished compost may be designated by the general term humus.  
Hydration: The process whereby a substance takes up water.  
Hydraulic Conductivity: The rate at which water will move through soil.  
Hydrologic Cycle: Disposal of precipitation from the time it reaches the soil surface until it re-enters the atmosphere by evapotranspiration to serve again as a source of precipitation.  
Hydrolysis: In soils it is the process whereby hydrogen ions are exchanged for cations such as sodium, potassium, calcium and magnesium.  
Hydromorphic Soil: Soils developed in the presence of excess water.  
Hydromulching: An application method using a water jet to spread a mulch emulsion on a land surface.  
Hydrophytic Vegetation: Plants that can exist in water that at least periodically is subject to anaerobic conditions.  
Hygroscopic Water: Water that is adsorbed onto a surface from the atmosphere.  
Igneous Rock: A rock formed by cooling of molten magma including basalt and granite.  
Illuvial Horizon: A horizon that receives material in solution or suspension from some other part of the soil.  
Illuviation: The precess of movement of material from one horizon and its deposition in another horizon of the same soil; usually from an upper horizon to a middle or lower horizon in the pedounit. Movement can also take place laterally.  
Immobilization: Conversion of an element from its inorganic form to its organic form within microbial or plant tissues, rendering it unavailable to other organisms or plants.  
Impeded Drainage: Restriction of the downward movement of water by gravity.  
Imperfectly drained: A soil that shows a small amount of reduction of iron due to short periods of water-logging.  
Impervious: Not easily penetrated by roots or water.  
Induration: The degree to which soil particles are held together by a combination of cementation and close packing.  
Infiltration: The process whereby water enters the soil through the surface.  
Inocula: Preconditioned microorganisms or compost product added to raw material to provide the appropriate microorganisms for decomposition.  
Inorganic: Substance in which carbon-to-carbon bonds are absent; mineral matter.  
Inselberg: (pl. inselberge) A steep sided hill composed predominantly of hard rock and rising abruptly above a plain; found mainly in tropical and subtropical areas.  
Insoluble: Not capable of being dissolved. For instance, insoluble phosphorus is present in the solid phase in soils.  
Interglacial Period: A relatively mild period occuring between two glacial periods.  
Intergrade: A soil which contains the properties of two distinctive and genetically different soils.  
Intrazonal Soils: One of the three orders of the old NZ Genetic system of soil classification. They have well developed characteristics resulting from the dominant influence of a local factor such as topography and parent material.  
Isomorphous Replacement: The replacement of one ion by another in the crystal lattice without changing the structure of the mineral.  
Isotropic: Not visible in cross polarized light. SEE ANISOTROPIC.
Karst Topography: An irregular land surface in a limestone region. The principal features are depression (e.g. dolines which sometimes contain thick soils which have been washed off the rest of the surfaces leaving them bare and rocky.) Drainage is usually by underground streams.  
Krotovina: An animal burrow or cavity > 20 mm diameter which may or may not have been filled with material from another horizon.  
Labile: Term applied to denote element that can be solubilized in a relatively short period of time. For instance, a labile nutrient is not directly available, but will be release relatively quickly.  
Lacustrine Deposit: Materials deposited by lake waters.  
Lacustrine: Pertaining to lakes.  
Land Reclamation: The restoration of productivity to lands made barren through processes such as erosion, mining or land clearing.  
Landslide or Landslip: The movement down the slope of a large mass of soil or rocks from a mountain or cliff. Often occurs after a torrential rain which soaks into the soil making it heavier and more mobile. Earthquakes and the undermining action of the sea are also causative agents.  
Laterisation: Used by some to refer to the processes of formation of laterite or red and yellow tropical soils. This term is not specific and should not be used.  
Lattice Structure: The orderly arrangement of atoms in crystalline material.  
Leachate: Liquid which has percolated through solid wastes and extracted dissolved and suspended materials; liquid that drains from the compost mix.  
Leaching: The washing out of material from the soil, both in solution and suspension. The process by which nutrient chemicals or contaminants are dissolved and carried away by water, or are moved into a lower layer of soil.  
Light Soil: (obsolete) A soil which has a course texture and is easily cultivated.  
Lignin: The component of wood responsible for its rigidity.  
Lime: Compounds of calcium used to correct the acidity of soils.  
Litter: The freshly fallen plant material occuring on the surface of the ground.  
Loading Rate: Measure of application amount, based on nutrients, trace metals or total mass of material.  
Lodging: The collapse of top heavy plants, particularly grain crops because of excess growth or beating by rain.  
Loess: An aeolian deposit composed mainly of silt which originated in arid regions, from glacial outwash or from alluvium. It is usually of yellowish brown color and has a widely varying calcium carbonate content. In the USSR, loess is regarded as having been deposited by water.  
Lysimeter: Apparatus installed in the soil for measuring percolation and leaching.  
Macroelement: Elements such as nitrogen that are needed in large amounts for plant growth. Nutritive elements needed in large quantities to ensure normal plant development (N,P,K, S, Mg, Ca, Fe).  
Macronutrient:   SEE MACROELEMENT.
Macropores: Pores > 0.030 mm in diameter (some definitions use pores > 0.050 mm).  
Mangrove Swamp: A dense jungle of mangrove trees which have the special adaptation of extending from their branches long arching roots which act as anchors and form an almost impenetrable tangle. They occur in tropical and subtropical areas, particularly near the mouths of rivers.  
Manure: Animal excreta with or without a mixture of bedding or litter.  
Matrix: The fine Material ( generally <2mm) forming a continuous phase and enclosing coarser material and/or pores.  
Mesofauna: Small organisms such as worms and insects.  
Metabolism: Sum of the chemical reactions within a cell or whole organism, including the energy-releasing breakdown of molecules (catabolism), and the synthesis of complex molecules and new protoplasm (anabolism).  
Metamorphic Rock: A rock that has been derived from other rocks by heat and pressure. The original rock may have been igneous, sedimentary, or another metamorphic rock.  
Microbe, Soil: A soil microorganism.  
Microbial: Pertaining to microbes.  
Microclimate: The climate of a very small region.  
Microelement: Those elements that are essential for plant growth but are required only in very small amounts. Nutritive elements needed in small quantities for healthy plant development; trace elements (Mn, B, Cl, Zn, Cu, Mo).  
Microfauna: The small animals that can only be seen with a microscope; they include protozoa, nematodes, etc.  
Microflora: The small plants that can only be seen with a microscope; they include algae, fungi, bacteria, etc.  
Micronutrient:   SEE MICROELEMENT.
Microorganisms: The members of the microflora and microfauna that can only be seen with a microscope.  
Micropores: Pores 5-30 µm in diameter.  
Microrelief: Small differences in relief that have differences in elevation up to about 2 m.  
Milliequivalent: A thousandth of an equivalent weight.  
Mineral Soil: A soil that is composed predominantly of mineral material cf. ORGANIC SOIL.  
Mineralisation: The change of an element in an organic form to an inorganic form by microorganisms.  
Mineral-N: Nitrogen in its inorganic form, usually as nitrates or ammonium.  
Mites: Very small members of the arachnid which includes spiders; they occur in large numbers in many organic surface soils.  
Moisture Content: The mass of water lost per unit dry mass when the material is dried at 103°C (217°F) for eight hours or more. The minimum moisture content required for biological activity is 12-15%; it generally becomes a limiting factor below 45 or 50%; expressed as a percentage, moisture content is water weight/wet weight.  
Mor: An accumulation of acid organic matter at the soil surface beneath forest.  
Moraine: Any type of constructional topographic form consisting of till and resulting from glacial deposition.  
Mottles: Patches or spots of different colors usually used for the color pattern developed due to partial anaerobism.  
Mottling: Patches or spots of different colors usually used for the color pattern developed due to partial anaerobism.  
Muck: Highly decomposed organic wet soil.  
Mulch: A loose surface horizon that forms naturally or may be produced by cultivation and consists of either inorganic or organic materials. Any suitable protective layer of organic or inorganic material applied or left on or near the soil surface as a temporary aid in stabilizing the surface and improving soil microclimatic conditions for establishing vegetation; mulch reduces erosion and water loss from the soil and can be used to control weeds.  
Mulching: The application of a layer of compost to the surface of the soil, creating an interface that accepts water readily yet resists moisture loss through evapotranspiration.  
Mull: A crumbly intimate mixture of organic and mineral material formed mainly by worms, particularly by earthworms.  
Munsell colour: Soil colour recorded by comparing it with colours on Munsell soil colour charts.  
Mycorrhiza: Soil-borne fungi that invade the roots of vascular plants and establish a symbiotic relationship; mycorrhiza hyphae, filaments that extend from plant roots, increase the surface area for nutrient and water absorption.  
Necessary Nutrient: The elements C, H, O, P, K, N, S, Ca, Mg, K, B, Mn, Cu, Zn, Mo, Cl, Co, Si and F. These must be taken up and utilized in sufficent quantities for plants to complete their life cycles. See essential element.  
Necrosis: The appearance of dead parts of plants due to a lack of plant growth factors or the presence of toxics or disease. Necrosis can also be confused with the normal senescense of plant parts.  
Nematodes: Elongated, cylindrical, unsegmented worms; includes a number of plant parasites (a cause of root damage) and human parasites.  
Neutral Soil: A soil with pH values 6.5-7.3.  
NIMBY: Not in My Back Yard. A NIMBY attitude.  
Nitrification: The oxidation of ammonia to nitrite and nitrite to nitrate by microorganisms.  
Nitrification: The oxidation of ammonia to nitrite and nitrite to nitrate by microorganisms.  
Nitrogen Fixation: The transformation of elemental nitrogen to an organic form by microorganisms.  
Non-Silicate: Rock forming minerals that do not contain silicon.  
Nutrient deficiency: The lack of an adequate amount of a plant nutrient. Nutrient deficiency may result in a number of symptoms, including poor plant growth, chlorosis or necrosis. Nutrient deficiency symptoms can easily be confused with toxicity symptoms.  
Nutrient: he elements C, H, O, P, K, N, S, Ca, Mg, K, B, Mn, Cu, Zn, Mo, Cl, Co, Si and F which are required for plant growth.  
NZ Genetic Soil Classification: A previous system of classifying NZ soils now replaced by the NZ Soil Classification. Soils could be divided into one of three classes based on climate and vegetation effects. Zonal (climate and vegetation effects strongly expressed), intrazonal (climate and vegetation effects partly expressed), or azonal (climate and vegetation effects not expressed). Soil groups included examples such as yellow-brown earths, yellow-brown loams, yellow-grey earths and podzols.  
NZ Soil Classification: The system used to classify NZ soils and replaces the old Genetic classification system. In the NZ system the are 15 soil orders 1) Allophanic Soils, 2) Anthropic Soils, 3) Brown Soils, 4) Gley soils, 5) Granular Soils, 6) Melanic Soils, 7) Organic soils, 8) Oxidic Soils, 9) Pallic Soils, 10) Podzols, 11) Pumice Soils, 12) Raw Soils, 13) Recent Soils, 14) Semiarid Soils, 15) Ultic Soils.  
Obligate Aerobic Organisms: Can only grow in the presence of oxygen.  
Obligate Anaerobic Organisms: Can only grow in the absence of oxygen.  
Onion Skin Weathering:   SEE EXFOLIATION.
Order: The highest level of the NZ soil classification. In the NZ system the are 15 soil orders:  
  1) Allophanic Soils,  
  2) Anthropic Soils,  
  3) Brown Soils,  
  4) Gley soils,  
  5) Granular Soils,  
  6) Melanic Soils,  
  7) Organic soils,  
  8) Oxidic Soils,  
  9) Pallic Soils,  
  10) Podzols,  
  11) Pumice Soils,  
  12) Raw Soils,  
  13) Recent Soils,  
  14) Semiarid Soils,  
  15) Ultic Soils." see SOIL ORDER.
Organic Contaminants: Synthetic trace organics include pesticides and polychlorinated biphenyls (PCB's).  
Organic Matter: Portion of the soil that includes microflora and microfauna (living and dead) and residual decomposition products of plant and animal tissue; any carbon assembly (exclusive of carbonates), large or small, dead or alive, inside soil space; generally consists primarily of humus.  
Organic Soil: A soil that is composed predominantly of organic matter, usually refers to peat. See HISTOSOL.
Organic: Substance which includes carbon-to-carbon bonds.  
Organic-N: Nitrogen in organic material.  
Outwash: Glacially deposited soil parent material worked and graded by water action from the melting glacial ice.  
Oxidation: Energy-releasing process involving removal of electrons from a substance; in biological systems, generally by the removal of hydrogen (or sometimes by the addition of oxygen); chemical and/or biochemical process combining carbon and oxygen and forming carbon dioxide (CO2). see REDUCTION.
Oxygen Demand: See:- BOD and COD.  
Pans: Soil horizons that are strongly compacted, cemented or have a high content of clay.  
Parent Material: The original state of the soil. The relatively unaltered lower material in soils is often similar to the material in which the horizons above have formed.  
Particle Density: The weight per unit volume of soil solids only.  
Pathogen: An organism, chiefly a microorganism, including viruses, bacteria, fungi, and all forms of animal parasites and protozoa, capable of producing an infection or disease in a susceptible host.  
Peat: An accumulation of dead plant material often forming a layer many meters deep. It is only slightly decomposed due to being completely waterlogged.  
Ped: A single individual naturally occuring soil aggregate such as a granule or prism cf. CLOD OR FRAGMENT.  
Pedogenesis: The natural process of soil formation.  
Pedology: The study of soils as naturally occuring phenomena taking into account their composition, distribution, and method of formation.  
Pedoturbation: All mixing of soil components that is not caused by illuviation.  
Pedounit: A selected column of soil containing sufficient material in each horizon for adequate laboratory characterization.  
Peneplain: A large flat or gently undulation area. Its formation is attributed to progressive erosion by rivers and rain, which continues until almost all the elevated portions of the land surface are worn down. When a peneplain is elevated, it may become a plateau which then forms the initial stages in the development of a second peneplain.  
Penetration resistance: The capacity of the soil in its natural confined state to resist penetration by rigid objects. Measured with a penetrometer.  
Peraquic: A condition that results from a high soil water table.  
Perched Water-Table: A water table formed on top of a slowly permeable or impermeable subsurface layer. The upper limit of perched water.  
Percolation: (soil water) The downward or lateral movement of water through soil.  
Permafrost: Permanently frozen subsoil.  
Permanent Wilting Point:   SEE WILTING POINT.
Permanently flooded: A condition where standing water covers the soil surface throughout normally wet years.  
Permeability: The ease with which air, or plant roots penetrate into or pass through a specific horizon.  
pH: The negative logarithm of the hydrogen ion concentration of a solution. It is the quantitative expression of the acidity and alkalinity of a solution and has a scale that ranges from about 0 to 14. pH 7 is neutral, <7 is acid and >7 is alkaline.  
Physical Weathering: The communition of rocks into smaller fragments by physical forces such as frost action or exfoliation.  
Phytolith: Opaline formation in plant tissue that remains in the soil after the softer plant tissue has decomposed.  
Plastic: A moist or wet soil that can be moulded without rupture.  
Platy: Soil aggregates that are horizontally elongated.  
Podzolisation: Used by some to refer to the process of the formation of a podzol. This term is not specific and should not be used.  
Polder: A term used in Holland for an area reclaimed from the sea or lake. A dyke is constructed around the area which is then drained by pumping the water out. Polders form valuable agricultural land or pasture land for cattle.  
Polygenic Soil: A soil that has been formed by two or more different and contrasting processes so that all of the horizons are not genetically related.  
Pore Space: The continuous and interconnecting spaces in soils.  
Pore: A discrete volume of soil atmosphere completely surrounded by soil (cf. PORE SPACE).  
Porosity: The volume of the soil mass occupied by pores and pore spaces.  
Primary Mineral: 1.A mineral such as feldspar or mica which occurs or occurred originally in an igneous rock. 2. Any mineral which occurs in the parent material of the soil.  
Primary nutrient: The elements P, K and N. These must be taken up and utilized in sufficent quantities for plants to complete their life cycles. Normally present in quantities > 1%.  
Profile: A vertical section through a soil from the surface into the relatively unaltered material.  
Pseudomorph: A mineral having the characteristic outward form of another mineral or object it replaces.  
Puddle: To destroy the structure of the surface soil by physical methods such as the impact of raindrops, poor cultivation with implements, and trampling by animals.  
Pugg: To destroy the structure of the surface soil through the trampling of animals.  
Quaternary Era: The period of geological time following the Tertiary Era, it includes the Pleistocene and Holocene periods and extends from 2,000,000 - 0 years BP.  
Rain Splash Erosion:   SEE RAIN SPLASH.
Rain Splash: The redistribution of soil particles on the surface by the impact of rain drops. On slopes this can cause a large amount of erosion.  
Rainfall Interception: The interception and accumulation of rainfall by the foliage and branches of vegetation.  
Raised Beach: A beach raised by earth movement thus forming a narrow coastal plain. There may be raised beaches at different levels resulting from repeated earth movement.  
Raw Humus: A humus form consisting predominantly of well preserved, though often fragmented plant remains with few faecal pellets.  
Recalcitrant: Term generally applied to organic matter that is quite stable or nutrients that are stable and not subject to release into soluble form.  
Reduction: The process of an element or compound accepting an electron during a chemical reaction. see OXIDATION.
Regolith: The unconsolidated mantle of weathered rock, soil and superficial deposits overlying solid rock.  
Respiration: The metabolic function of consuming oxygen.  
Restricting horizon: The soil horizon that most restricts movement of water or air movement vertically through the soil, or restricts root growth down into soil. Restricting horizons are often termed PANS.  
Retention mechanism: The process by which a substance (i.e. nutrient) is retained within the soil profile. Examples include precipitation, adsorption, nutrient cycling, and binding into organic matter.  
Rhizosphere: The soil close to plant roots where there is usually an abundant and specific microbiological population.  
Rill Erosion: The formation of rills as a consequence of poor cultivation.  
Rill: A small intermittent water course with steep sides.  
Rubification: The development of red color in soil - reddening.  
Runoff: Precipitation that reaches the composting pad directly without going through the composting materials.  
Saline Soil: A soil containing enough soluble salts to reduce its fertility.  
Salinisation: The process of accumulation of salts in soil.  
Sand: Mineral rock fragments that range in diameter from 2-0.06 mm.  
Saprist: Organic soils in which most of the plant matter has decomposed (less than 1/3 of fibers remain visible after rubbing) and the original tissue cannot be recognized.  
Saturated Flow: The movement of water in a soil that is completely filled with water.  
Saturated Soil: A soil for which the entire profile is saturated with water.  
Secondary Mineral: Those minerals that form from the material released by weathering. The main secondary minerals are the clays and oxides.  
Secondary nutrinet: The elements S, Ca, Mg. These must be taken up and utilized in sufficent quantities for plants to complete their life cycles. Normally present in quantities of 0.01-0.5 %. See essential element.  
Sedimentary Rock: A rock composed of sediments with varying degrees of consolidation. The main sedimentary rocks include sandstones, shales, conglomerates and some limestones.  
Self-Mulching Soils: A soil with a naturally formed well aggregated surface which does not crust and seal under the impact of raindrops.  
Sesquioxides: Usually refers to the combined amorphous oxides of iron and aluminium.  
Sheet Erosion: The gradual and uniform removal of the surface soil by water without forming any rills or gullies.  
Silicates: Rock forming minerals that contain silicon.  
Silt: Mineral particles that range in diameter from 0.02-0.002 mm in the international system or 0.06-0.002 mm in the NZ soil description system.  
Slickenside: The polished surface that forms when two peds rub against each other when some soils expand in response to wetting.  
Slickspot: Small areas of surface soil that are slick when wet because of alkalinity or high exchangeable sodium.  
Sludge: Solid residue of the wastewater purification process, a product of screening, sedimentation, filtering, pressing, bacterial digestion, chemical precipitation and oxidation; primary sludge is produced by sedimentation process and secondary sludge is the product of microbial digestion.  
Slurry: A thin watery mixture of a fine insoluble material.  
Soil Amendment/Soil Conditioner: Soil additive which stabilizes the soil, improves resistance to erosion, increases permeability to air and water, improves texture and resistance of the surface to crusting, eases cultivation or otherwise improves soil quality.  
Soil Anisotropy: The occurrence of a vertical horizon sequence in soils causes vertical anisotropy to be an essential characteristic. Frequently this vertical anisotropy can also be observed in thin sections. SEE ANISOTROPIC.
Soil Auger: A tool used for boring into the soil and withdrawing small samples for field or laboratory examination.  
Soil colour: Colour of the matrix, molltes or other features in the soil. Determined by matching soil colour with colours on standard colour charts (e.g. Munsell soil colour charts).  
Soil description: The systematic recording of soil horizon features for a soil profile using standard techniques and terminology.  
Soil Fabric: The arrangement, size, shape and frequency of the individual soil constituents, including pores.  
Soil Horizon:   SEE HORIZON.
Soil Monolith: A vertical section through the soil preserved with resin and mounted for display.  
Soil Order:

The highest level of the NZ soil classification. In the NZ system the are 15 soil orders:

1) Allophanic Soils,

2) Anthropic Soils,

3) Brown Soils,

4) Gley soils,

5) Granular Soils,

6) Melanic Soils,

7) Organic soils,

8) Oxidic Soils,

9) Pallic Soils,

10) Podzols,

11) Pumice Soils,

12) Raw Soils,

13) Recent Soils,

14) Semiarid Soils,

15) Ultic Soils.

Soil Profile: A section of two dimensions extending vertically from the earthís surface so as to expose all the soil horizons and a part of the relatively unaltered underlying material.  
Soil Structure:   See STRUCTURE.
Soil Suborder: The 2nd highest taximonic order of the U.S. soil classification system.  
Soil Survey: The systematic examination and mapping of soil.  
Soil Taxonomy: The systematic arrangement of soils into groups or categories on the basis of their characteristics.  
Soil Textural Triangle: A 3-phase scale used to define soil into a soil textural group based on % sand, silt and clay.  
Soil Texture: The size distribution of individual particles of a soil. Often assigned to a textural class e.g. silt loam.  
Soil: (1) A dynamic natural body composed of mineral and organic materials and living forms in which plants grow. (2) The collection of natural bodies occupying parts of the earth's surface that support plants and that have properties due to the integrated effect of climate and living matter acting upon parent material, as conditioned by relief, over periods of time.  
Solid Waste: Garbage, refuse and other discarded solid materials, including such materials resulting from industrial, commercial, and agricultural operation and community activities.  
Solifluction: Slow flow of material on sloping ground, characteristic of , though not confined to regions subjected to alternate periods of freezing and thawing.  
Solum: The part of the soil above the relatively unaltered material.  
Sphericity: Relates to the overall shape of a feature irrespective of the sharpness of its edges and is a measure of the degree of its conformity to a sphere.  
Spodic horizon: A subsurface soil horizon characterized by an accumulation of aluminum, (also potentially iron) and organic matter. This is the diagnostic horizon for the soil order Spodosol.  
Springtails: Very small insects that live in the surface soil and feed on organic matter.  
Strip Cropping: The practice of growing crops in strips along the contour in an attempt to reduce runoff, thereby preventing runoff or conserving moisture.  
Strongly Anaerobic: (poorly drained) Soil that remains very wet or waterlogged for long periods of the year and as a result develops a mottled pattern of greys and browns.  
Structure: The spatial distribution and total organization of the soil system as expressed by the degree and type of aggregation and the nature and distribution of pores and pore spaces.  
Suborder:   See SOIL SUBORDER.
Symbiosis: Two organisms that live together for their mutual benefit. Fungus and alga that forms a lichen or nitrogen fixing bacteria living in roots are examples of symbiosis. The individual organisms are called symbionts.  
Talus: Angular rock fragments that accumulate by gravity at the foot of steep slopes of cliffs.  
Tectonic: Rock structures produced by movements in the earthís crust.  
during eruption. Includes volcanic ash, cinders, lapilli, scoria and pumice.
A collective term for all volcanic materials ejected from a vent  
Terrace: A broad surface running along the contour. It can be a natural phenomenon or specially constructed to intercept runoff, thereby preventing erosion and conserving moisture. Sometimes they are built to provide adequate rooting depths for plants.  
Tertiary Period: The period of time extending from 75,000,000-2,000,000 years BP.  
Texture Triangle: A 3-phase scale used to define soil into a soil textural group based on % sand, silt and clay. See SOIL TEXTURAL TRIANGLE.
Texture: The size distribution of individual particles of a soil. Often assigned to a textural class e.g. silt loam. See SOIL TEXTURE.
Thermophyllic Bacteria: Bacteria which have optimum activity between about 45 degrees and 55 degrees C.  
Tile Drain: Short lengths of concrete or pottery pipes placed end to end at a suitable depth and spacing in the soil to collect water from the soil and lead it to an outlet.  
Till Plain: A level or undulation land surface covered by glacial till.  
Till: An unstratified or crudely stratified glacial deposit consisting of a stiff matrix of fine rock fragments and and old soil containing sub-angular stones of various sizes and composition, many of which may be striated (scratched). It forms a mantle from less than 1 m to over 100 m in thickness covering areas which carried an ice sheet or glaciers during the Pleistocene and Holocene periods.  
Tilth: The physical state of the soil that determines its suitability for plant growth taking into account texture, structure, consistence and pore space. It is a subjective estimation and is judged by experience.  
Toposequence: A sequence of soils whose properties are determined by their particular topographic situation.  
Topsoil: Soil consisting of various mixtures of sand, silt, clay and organic matter; considered to be the nutrient-rich top layer of soil that supports plant growth.  
Toxicity: Adverse biological effect due to toxins and other compounds.  
Trace Metals: Trace elements regulated because of their potential for human, plant, or animal toxicity, including cadmium (Cd), copper (Cu), chromium (Cr), mercury (Hg), nickel (Ni), lead (Pb) and Zinc (Zn).  
Traffic pan: Compacted soil horizon created by the action of machinery, such as trucks, tractors or logging skidders, over the soil.  
Translocation: Migration of material in solution or suspension from one horizon to another.  
Ultramicropores: Pores < 0.005mm in diameter.  
Unavailable Nutrients: Plant nutrients that are present in the soil but cannot be taken up by the roots because they have not been released from the rock or minerals by weathering or from organic matter by decomposition.  
Unavailable Water: Water that is present in the soil but can not be taken up by plant roots because it is strongly adsorbed onto the surface of particles. Water held at tensions of 1500 kPa (wilting point).  
Unconsolidated: Sediments that are loose and not hardened.  
Unsaturated Flow: The movement of water in the soil that is not completely filled with water.  
Upland: An area where soils are generally relatively well drained such that the water table is significantly below the soil surface most of the year.  
USDA Soil Taxonomy: The United Stated Department of Agriculture system of soil classification consisting of 11 soil orders, including 1) Entisols, 2) Inceptisols, 3) Spodosols, 4) Ultisols, 5) Alfisols, 6) Vertisols, 7) Oxisols, 8) Histosols, 9) Andisols, 10) Aridosols, and 11) Mollisols.  
Value: The relative lightness or intensity of color, one of the three color variables.  
Varnish (Desert): A dark shiny coating on stones in deserts, probably composed of compounds of iron and manganese (cf. DESERT VARNISH).  
Varve: A layer representing the annual deposit of sediment, it usually consists of a lighter and darker portion due to the change in rate of decomposition during the year. The material may be of any origin but the term is most often used in connection with glacial lake sediments.  
Ventifact: A pebble faceted or moulded by wind action, usually formed in desert or polar areas. The flat facets meet at sharp angles.  
Vermicomposting: The biological degradation of organic matter contained in agricultural, urban and industrial wastes, occurring when earthworms feed on these materials.  
Vermiculture: Composting by the activity of earthworms; material is eaten by the worms, leaving air passages which maintain aerobic conditions; the process is completed with a curing stage.  
Very Poorly Drained: A soil that remains wet and waterlogged for most of the year so that most of the horizons are blue, olive or gray due to the reducing conditions.  
Volatilization: Gaseous loss of a substance to the atmosphere.  
Volcanic Ash (Volcanic Dust): Fine particles of lava ejected during a volcanic eruption. Sometimes the particles are shot high into the atmosphere and carried long distances by the wind.  
Volcanic Tephra   See Tephra
Waterlogged: Saturated with water.  
Water-Table (Ground): The upper limit in the soil or underlying material permanently saturated with water.  
Water-Table (Perched):   SEE PERCHED WATER-TABLE.
Weakly Anaerobic: A horizon that is anaerobic for short periods and moist for long periods. The colors are less bright than aerobic horizons and they are usually marbled or weakly mottled.  
Weathering: All the physical, chemical and biological processes that cause the disintegration of rocks at or near the surface.  
Well Drained:   SEE AEROBIC.
Wetland: General definition:- Areas that under normal circumstances have hydric soils and hydrophytic vegetation. Legal and political definition:- It should be noted that the specific legal definition of wetland if often hotly debated and wetland experts (such as Dan Quail) are working hard to come up with a good legal definition.  
Wilting Point: The percentage by weight of water remaining in the soil when the plant wilts permanently (-1500 kPa).  
Xerophytes: Plants that grow in extremely dry areas.  
Yard Waste: Grass clippings, leaves and weeds, and prunings from residences or businesses six inches or less in diameter.