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igneous rock: Rock formed from the cooling and solidification of magma, and that has not been changed appreciably by weathering since its formation.

illite: (i) As a general term, refers to either a discrete non-expansible mica of detrital or authigenic origin or to the micaceous component of interstratified systems, as in illite-smectite. If used to refer to the species, it should meet the following requirements: a) The micaceous layers ideally are non-expansible; b) the octahedral sheet is dioctahedral and aluminous; c) the interlayer cation is primarily potassium; and (4) the composition deviates from that of muscovite in two main ways: 1) A phengitic component is present in which substitution of R2+ cations for octahedral Al is balanced by addition of tetrahedral Si beyond the ideal Si:Al ratio of 3:1 for muscovite. This substitution gives the octahedral sheet an overall negative charge of about 0.2 to 0.3 per formula unit. 2) Interlayer vacancies or water molecules amounting to about 0.2 to 0.4 atoms per formula unit are compensated by additional tetrahedral Si cations beyond those required by the phengitic component. Where reference is made to the species illite, a clear statement should be made to that effect in order to avoid confusion with the general usage. (ii) In soil taxonomy, the presence of a 1 nm x-ray diffraction peak and greater than or equal to 4% K2O is used to denote the presence of illite.

illuvial horizon: A soil layer or horizon in which material carried from an overlying layer has been precipitated from solution or deposited from suspension. The layer of accumulation.

illuviation: The process of deposition of soil material removed from one horizon to another in the soil; usually from an upper to a lower horizon in the soil profile.

imagery: General term for base map or reference map materials.

Imhoff cone: A graduated volumetric cone used for determining settleable solids in liquid suspensions.

immobilization: The conversion of an element from the inorganic to the organic form in microbial or plant tissues.

immunofluorescence: Fluorescence resulting from a reaction between a substance and a specific antibody that is bound to a fluorescent dye.

imogolite: A poorly crystalline aluminosilicate mineral with an ideal composition SiO2 Al2O3 2.5H2)( + ). It appears as threads consisting of assemblies of a tube unit with inner and outer diameters of 1.0 and 2.0 nm, respectively. Imogolite is commonly found in association with allophane, and is similar to allophane in chemical properties. Imogolite is mostly found in soils derived from volcanic ash, and in weathered pumices and Spodosols.

impeded drainage: A condition which hinders the movement of water through soils under the influence of gravity.

impervious: Resistant to penetration by fluids or by roots.

Inceptisols: Mineral soils that have one or more pedogenic horizons in which mineral materials other than carbonates or amorphous silica have been altered or removed but not accumulated to a significant degree. Under certain conditions, Inceptisols may have an ochric, umbric, histic, plaggen or mollic epipedon. Water is available to plants more than half of the year or more than 90 consecutive days during a warm season.

inclusion: One or more polypedons or parts of polypedons within a delineation of a map unit, not identified by the map unit name; i.e., is not one of the named component soils or named miscellaneous area components. Such soils or areas are either too small to be delineated separately without creating excessive map or legend detail, or occur too erratically to be considered a component, or are not identified by practical mapping methods.

indicator plants: Plants characteristically associated with specific soil or site conditions, such as soil acidity, alkalinity, wetness, or a chemical element.

indurated: A very strongly cemented soil horizon. 

infiltrability: The flux (or rate) of water infiltration into soil when water at atmospheric pressure is maintained on the atmosphere-soil boundary, with the flow direction being one-dimensionally downward.

infiltration: The entry of water into soil.

infiltration flux (or rate): The volume of water entering a specified cross-sectional area of soil per unit time [L t-1].

inoculate: To treat, usually seeds, with microorganisms to create a favorable response. Most often refers to the treatment of legume seeds with Rhizobium or Bradyrhizobium to stimulate dinitrogen fixation but also refers to the introduction of microbial cultures into sterile growth medium.

integrated drainage: A general term for a drainage pattern in which stream systems have developed to the point where all parts of the landscape drain into some part of a stream system, the initial or original surfaces have essentially disappeared and the region drains to a common base level.

interflow: That portion of rainfall that infiltrates into the soil and moves laterally through the upper soil horizons until intercepted by a stream channel or until it returns to the surface at some point downslope from its point of infiltration.

interfluve: A landform composed of the relatively undissected upland or ridge between two adjacent valleys or drainageways.

intergrade: (i) A taxonomic class at the subgroup level of soil taxonomy having properties typical of the great group of which it is a member and that are characteristic of some class in a higher category (any order, suborder or great group) and indicates a transition to that kind of soil. (ii) A soil that is a member of one such subgroup.

intermittent stream: A stream, or reach of a stream, that does not flow year-round and that flows only when a) it receives baseflow solely during wet periods, or b) it receives ground-water discharge or protracted contributions from melting snow or other erratic surface and shallow subsurface sources.

internal friction: The portion of the shearing strength of a soil indicated by the term σ tan θ in Coulomb's equation τ = c + σ tan θ, where τ is shear stress, σ is normal stress, c is cohesion, and θ is friction angle. It is usually considered to be due to the interlocking of soil grains and the resistance to sliding between the grains.

interstratification: Mixing of different kinds of silicate layers along the c-direction in a given stack. Interstratification may be regular or random. In regular interstratification, the stacking of the component layers follows a periodic succession. In random interstratification, the distribution of the different layers lacks periodicity and is controlled only by the proportions of the various layers.

intrinsic permeability: The property of a porous material that expresses the ease with which gases or liquids flow through it. Often symbolized by k = Kn/pg, where K is the Darcy hydraulic conductivity, n is the fluid viscosity, p is the fluid density, and g is the acceleration of gravity. Dimensionally, k is an area [L2].

iron oxides: Group name for the oxides and hydroxides of iron. Includes the minerals goethite, hematite, lepidocrocite, ferrihydrite, maghemite, and magnetite. Sometimes referred to as "sesquioxides," or "iron hydrous oxides."

ironstone: An in-place concentration of iron oxides that is at least weakly cemented.

irrigation: The intentional application of water to the soil, usually for the purpose of crop production.

irrigation: The intentional application of water to the soil, usually for the purpose of crop production. Related terms include.

joint planes: Planar voids that traverse the soil material in some fairly regular pattern, such as parallel or subparallel sets.

kame: A low mound, knob, hummock, or short irregular ridge, composed of stratified sand and gravel deposited by a subglacial stream as a fan or delta at the margin of a melting glacier; by a supraglacial stream in a low place or hole on the surface of the glacier; or as a ponded deposit on the surface or at the margin of stagnant ice.

kandic horizon: Subsoil diagnostic horizon having a clay increase relative to overlying horizons and has low activity clays i.e., <160 cmolc kg-1 clay.

karst: Topography with sinkholes, caves, and underground drainage that is formed in limestone, gypsum, or other rocks by dissolution.

lacustrine deposit: Clastic sediments and chemical precipitates deposited in lakes.

lacustrine soil: Soil formed on or from lacustrine deposits.

land farming: A process of bioremediation or biodegradation in which wastes are incorporated into soil and allowed to decompose via naturally occurring microbial activity.

landform: Any physical, recognizable form or feature on the earth's surface, having a characteristic shape, and produced by natural causes; it includes a wide range in size such as a shrub-coppice dune than can be several meters across vs. a seif dune which can be up to 100 km long. Landforms provide an empirical description of similar portions of the earth's surface.

landscape: A collection of related landforms; usually the land surface which the eye can comprehend in a single view.

landslide: A general term for a mass movement landform and a process characterized by moderately rapid to rapid (greater than 30 cm per year) downslope transport, by means of gravitational stresses, of a mass of rock and regolith that may or may not be water saturated.

Latosol: A suborder of zonal soils including soils formed under forested, tropical, humid conditions and characterized by low silica-sesquioxide ratios of the clay fractions, low base-exchange capacity, low activity of the clay, low content of most primary minerals, low content of soluble constituents, a high degree of aggregate stability, and usually having a red color.

lava flow: A solidified body of rock formed from the lateral, surficial outpouring of molten lava from a vent or fissure, often lobate in form.

layer charge: Magnitude of charge per formula unit of a clay which is balanced by ions of opposite charge external to the unit layer.

leaching: The removal of soluble materials from one zone in soil to another via water movement in the profile.

light soil (colloquial): A coarse-textured soil; a soil with a low drawbar pull and hence easy to cultivate.

lime-pan: A hardened layer cemented by calcium carbonate. Better term may be caliche.

lithic contact: A boundary between soil and continuous, coherent, underlying material. The underlying material must be sufficiently coherent to make hand-digging with a spade impractical. If a single mineral, its hardness is 3 (Mohs scale), and gravel size chunks that do not disperse with 15 hours shaking in water or sodium hexametaphosphate solution.

Lithosols: A great soil group of azonal soils characterized by an incomplete solum or no clearly expressed soil morphology and consisting of freshly and imperfectly weathered rock or rock fragments.

litter: The surface layer of the forest floor which is not in an advanced stage of decomposition, usually consisting of freshly fallen leaves, needles, twigs, stems, bark, and fruits.

loamy: (i) Texture group consisting of coarse sandy loam, sandy loam, fine sandy loam, very fine sandy loam, loam, silt loam, silt, clay loam, sandy clay loam, and silty clay loam soil textures.

loess: Material transported and deposited by wind and consisting of predominantly silt-sized particles.

loose: A soil consistence term.

luxury uptake: The absorption of nutrients by plants in excess of that quantity needed for optimum growth. Luxury concentrations during early growth may be utilized in later growth.


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