a

quantity intensity ratio: The change in quantity sorbed with change in quantity in solution. It is determined from the slope of the plot of concentration in solution vs. the quantity sorbed.

recessional moraine: An end or lateral moraine, built during a temporary but significant halt in the retreat of a glacier. Also, a moraine built during a minor readvance of the ice front during a period of recession.

redistribution (of soil water): The process of soil-water movement to achieve an equilibrium energy state of water throughout the soil.

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.

relative yield: The harvestable or biomass yield with or without supplementation of the nutrient in question expressed as a percentage of the yield with the nutrient in adequate amounts.

residual fertility: The available nutrient content of a soil carried over to subsequent crops.

residual material: Unconsolidated and partly weathered mineral materials accumulated by disintegration of consolidated rock in place.

residuum: Unconsolidated, weathered, or partly weathered mineral material that accumulates by disintegration of bedrock in place.

reticulate mottling: A network of mottles with no dominant color, most commonly found in deeper horizons of soils containing plinthite.

rhizoplane: Plant root surfaces usually including the adhering soil particles.

rhizosphere: The zone of soil immediately adjacent to plant roots in which the kinds, numbers, or activities of microorganisms differ from that of the bulk soil.

riparian: Land adjacent to a body of water that is at least periodically influenced by flooding.

river wash: In soil survey a map unit that is a miscellaneous area, which is barren alluvial areas of unstablilized sand silt, clay or gravel reworked by frequently by stream activity.

rock land: Areas containing frequent rock outcrops and shallow soils. Rock outcrops usually occupy from 25 to 90% of the area.

rock outcrop: In soil survey a map unit that is a miscellaneous area, which consists of exposures of bedrock other than lava flows and rock-lined pits.

rubble land: Areas with 90% or more of the surface covered with cobbles, stones, and boulders. Commonly occurs as colluvium at the base of mountains but some areas may be left on mountainsides by glaciation or periglacial processes. A miscellaneous area.

runoff: That portion of precipitation or irrigation on an area which does not infiltrate, but instead is discharged from the area. That which is lost without entering the soil is called surface runoff. That which enters the soil before reaching a stream channel is called ground water runoff or seepage flow from ground water. (In soil science runoff usually refers to the water lost by surface flow; in geology and hydraulics runoff usually includes both surface and subsurface flow.)

saline soil: A nonsodic soil containing sufficient soluble salt to adversely affect the growth of most crop plants. The lower limit of saturation extract electrical conductivity of such soils is conventionally set at 4 dS m^-1(at 25 °C). Actually, sensitive plants are affected at half this salinity and highly tolerant ones at about twice this salinity.

salt balance: The quantity of soluble salt removed from an irrigated area in the drainage water minus that delivered in the irrigation water.

salt tolerance: The ability of plants to resist the adverse, nonspecific effects of excessive soluble salts in the rooting medium.

salt-affected soil: Soil that has been adversely modified for the growth of most crop plants by the presence of soluble salts, with or without high amounts of exchangeable sodium.

sapric material: Organic soil material that contains less than 1/6 recognizable fibers (after rubbing) of undecomposed plant remains. Bulk density is usually very low, and water holding capacity very high.

saprolite: Soft, friable, isovolumetrically weathered bedrock that retains the fabric and structure of the parent rock exhibiting extensive inter-crystal and intra-crystal weathering. In pedology, saprolite was formerly applied to any unconsolidated residual material underlying the soil and grading to hard bedrock below.

saturate: (i) To fill all the voids between soil particles with a liquid. (ii) To form the most concentrated solution possible under a given set of physical conditions in the presence of an excess of the solute. (iii) To fill to capacity, as the adsorption complex with a cation species; e.g., H⁺-saturated, etc.

scarp: An escarpment, cliff, or steep slope of some extent along the margin of a plateau, mesa, terrace, or structural bench. A scarp may be of any height.

sediment: Transported and deposited particles or aggregates derived from rocks, soil, or biological material.

segregated ice: Massive ice in a soil pedon, which is relatively free of soil particles.

self-mulching soil: A soil in which the surface layer becomes so well aggregated that it does not crust and seal under the impact of rain but instead serves as a surface mulch upon drying.

shrinkage, soil: The process of soil material contracting to a lesser volume when subject to loss of water.

skeleton grains: Individual grains that are relatively stable and not readily translocated, concentrated, or reorganized by soil-forming processes; they include mineral grains and resistant siliceous and organic bodies larger than colloidal size.

skew planes: Planar voids that traverse the soil material in an irregular manner, having no specific distribution or orientation pattern between individuals.

slick spots: Areas having a puddled or crusted, very smooth, nearly impervious surface. The underlying material is dense and massive. A also miscellaneous area.

slump: (i) A mass movement process characterized by a landslide involving a shearing and rotary movement of a generally independent mass of rock or earth along a curved slip surface (concave upward) and about an axis parallel to the slope from which it descends, and by backward tilting of the mass with respect to that slope so that the slump surface often exhibits a reversed slope facing uphill. (ii) The landform or mass of material slipped down during, or produced by a slump.

smectite: A group of 2:1 layer silicates with a high cation exchange capacity, about 110 cmol^c kg^-1for soil smectites, and variable interlayer spacing. Formerly called the montmorillonite group. The group includes dioctahedral members montmorillonite, beidellite, and nontronite, and trioctahedral members saponite, hectorite, and sauconite.

sodic soil: A nonsaline soil containing sufficient exchangeable sodium to adversely affect crop production and soil structure under most conditions of soil and plant type. The sodium adsorption ratio of the saturation extract is at least 13.

sodication: The process whereby the exchangeable sodium content of a soil is increased.

soil: (i) The unconsolidated mineral or organic material on the immediate surface of the earth that serves as a natural medium for the growth of land plants. (ii) The unconsolidated mineral or organic matter on the surface of the earth that has been subjected to and shows effects of genetic and environmental factors of: climate (including water and temperature effects), and macro- and microorganisms, conditioned by relief, acting on parent material over a period of time. A product-soil differs from the material from which it is derived in many physical, chemical, biological, and morphological properties and characteristics.

soil aeration: The condition, and sum of all processes affecting, soil pore-space gaseous composition, particularly with respect to the amount and availability of oxygen for use by soil biota and/or soil chemical oxidation reactions.

soil chemistry: The branch of soil science that deals with the chemical constitution, chemical properties, and chemical reactions of soils.

soil compaction: Increasing the soil bulk density, and concomitantly decreasing the soil porosity, by the application of mechanical forces to the soil.

soil fertility: The quality of a soil that enables it to provide nutrients in adequate amounts and in proper balance for the growth of specified plants or crops.

soil formation factors: The variables, usually interrelated natural agencies, that are active in and responsible for the formation of soil. The factors are usually grouped into five major categories as follows: parent material, climate, organisms, topography, and time.

soil horizon: A layer of soil or soil material approximately parallel to the land surface and differing from adjacent genetically related layers in physical, chemical, and biological properties or characteristics such as color, structure, texture, consistency, kinds and number of organisms present, degree of acidity or alkalinity, etc.

soil management: (i) The sum total of all tillage and planting operations, cropping practices, fertilizer, lime, irrigation, herbicide and insecticide application, and other treatments conducted on or applied to a soil for the production of plants.

soil mineral: (i) Any mineral that occurs as a part of or in the soil. (ii) A natural inorganic compound with definite physical, chemical, and crystalline properties (within the limits of isomorphism), that occurs in the soil.

soil morphology: (i) The physical constitution of a soil profile as exhibited by the kinds, thickness, and arrangement of the horizons in the profile, and by the texture, structure, consistence, and porosity of each horizon. (ii) The visible characteristics of the soil or any of its parts.

soil organic matter: The organic fraction of the soil exclusive of undecayed plant and animal residues. (See also humus.)

soil productivity: The capacity of a soil to produce a certain yield of crops or other plants with a specified system of management.

soil qualities Inherent attributes of soils which are inferred from soil characteristics or indirect observations (e.g., compactibility, erodibility, and fertility).

soil quality: The capacity of a soil to function within ecosystem boundaries to sustain biological productivity, maintain environmental quality, and promote plant and animal health.

soil science: That science dealing with soils as a natural resource on the surface of the earth including soil formation, classification and mapping; physical, chemical, biological, and fertility properties of soils per se; and these properties in relation to the use and management of soils.

soil solution: The aqueous liquid phase of the soil and its solutes.

soil structure: The combination or arrangement of primary soil particles into secondary units or peds. The secondary units are characterized on the basis of size, shape, and grade (degree of distinctness).

soil survey: (i)The systematic examination, description, classification, and mapping of soils in an area. Soil surveys are classified according to the kind and intensity of field examination. (ii) The program of the National Cooperative Soil Survey that includes developing and implementing standards for describing, classifying, mapping, writing, and publishing information about soils of a specific area.

soil texture: The relative proportions of the various soil separates in a soil as described by the classes of soil texture shown in Fig. 1. The textural classes may be modified by the addition of suitable adjectives when rock fragments are present in substantial amounts; for example, "stony silt loam." (For other modifiers see also rock fragments.) The sand, loamy sand, and sandy loam are further subdivided on the basis of the proportions of the various sand separates present. The limits of the various classes and subclasses are as follows:

soil welding: A process in which pedogenesis in a surface layer of parent rock or sediment feeds physicochemical and other imprints downward into a subjacent buried soil formed in a separate parent rock or sediment, leading to pedogenic 'fusion' of the two soils. Also referred to as pedogenic overprinting or pedogenic imprinting. (added 6/1/2000)

solum: (plural: sola) A set of horizons that are related through the same cycle of pedogenic processes; the A, E, and B horizons.

sombric horizon: A subsurface mineral horizon that is darker in color than the overlying horizon but that lacks the properties of a spodic horizon. Common in cool, moist soils of high altitude in tropical regions.

Spodosols: Mineral soils that have a spodic horizon or a placic horizon that overlies a fragipan.

sorption: The removal of an ion or molecule from solution by adsorption and absorption . It is often used when the exact nature of the mechanism of removal is not known.

spatial variability: The variation in soil properties (i) laterally across the landscape, or (ii) vertically downward through the soil.

specific surface: The solid-particle surface area (of a soil or porous medium) divided by the solid-particle mass or volume, expressed in m² kg^-1or m² m^-3 = m^-1, respectively.

spodic horizon: A mineral soil horizon that is characterized by the illuvial accumulation of amorphous materials composed of aluminum and organic carbon with or without iron. The spodic horizon has a certain minimum thickness, and a minimum quantity of extractable carbon plus iron plus aluminum in relation to its content of clay.

spoil bank: Rock waste, banks, and dump depositions resulting from the excavation of ditches and strip mines.

stones: Rock or mineral fragments between 250 and 600 mm in diameter if rounded, and 380 to 600 mm if flat.

stratified: Arranged in or composed of strata or layers.

straw mulching: The use of straw to create a surface mulch on all or part of the soil surface for soil or water conservation, for soil temperature management or for weed suppression.

stream terrace: One of a series of platforms in a stream valley, flanking and more or less parallel to the stream channel, originally formed near the level of the stream, and representing the dissected remnants of an abandoned flood plain, stream bed, or valley floor produced during a former state of erosion or deposition. Erosional surfaces cut into bedrock and thinly mantled with stream deposits (alluvium) are designated "strath terraces." Remnants of constructional valley floors thickly mantled with alluvium are termed alluvial terraces.

structural charge: The charge (usually negative) on a mineral resulting from isomorphous substitution within the mineral layer. (Expressed as moles (mol) or centimoles (cmol) of charge per kilogram of clay.

substrate: (i) That which is laid or spread under an underlying layer, such as the subsoil. (ii) The substance, base, or nutrient on which an organism grows. (iii) Compounds or substances that are acted upon by enzymes or catalysts and changed to other compounds in the chemical reaction.

sulfidic material: Waterlogged material or organic material that contains 7.5 g kg^-1or more of sulfide-sulfur.

sulfur cycle: The sequence of transformations undergone by sulfur wherein it is used by living organisms, transformed upon death and decomposition of the organism, and ultimately converted to its original oxidation state.

sulfuric horizon: A horizon composed either of mineral or organic soil material that has both pH <3.5 and jarosite mottles.

summit: The highest point of any landform remanant, hill, or mountain.

superphosphate: A product obtained when phosphate rock is treated with H₂SO₄, H₃PO₄, or a mixture of those acids.

surface area: The area of the solid particles in a given quantity of soil or porous medium. (i) BET surface area is that area on which gas molecules, such as N₂ or O₂, can adsorb which normally does not include the planar surface of expanding clays such as smectites. (ii) EGME surface area is that area on which ethylene glycol monoethyl ether can adsorb which normally includes the planar surface of expanding clays such as smectites.

surface sealing: The deposition by water, orientation and/or packing of a thin layer of fine soil particles on the immediate surface of the soil, greatly reducing its water permeability.

surface soil: The uppermost part of the soil, ordinarily moved in tillage, or its equivalent in uncultivated soils and ranging in depth from 7 to 25 cm. Frequently designated as the plow layer, the surface layer, the Ap layer, or the Ap horizon.

surfactant: A substance that lowers the surface tension of a liquid.

swamp: An area saturated with water throughout much of the year but with the surface of the soil usually not deeply submerged. Usually characterized by tree or shrub vegetation.

talud: A short, steep slope formed gradually at the downslope margin of a field by deposition against a hedge, a stone wail, or other similar barrier.

talus: Rock fragments of any size or shape (usually coarse and angular) derived from and lying at the base of a cliff or very steep rock slope. The accumulated mass of such loose, broken rock formed chiefly by falling, rolling, or sliding.

taxadjunct: A soil that is correlated as a recognized, existing soil series for the purpose of expediency. They are so like the soils of the defined series in morphology, composition, and behavior that little or nothing is gained by adding a new series.

terminal moraine: An end moraine that marks the farthest advance of a glacier and usually has the form of a massive arcuate or concentric ridge, or complex of ridges, underlain by till and other drift types.

terrace: (i) A step-like surface, bordering a stream or shoreline, that represents the former position of a flood plain, lake, or sea shore. (ii) A raised, generally horizontal strip of earth and/or rock constructed along a hill on or nearly on a contour to make land suitable for tillage and to prevent accelerated erosion. (iii) An earth embankment constructed across a slope for conducting water from above at a regulated flow to prevent accelerated erosion and to conserve water.

thermic: A soil temperature regime that has mean annual soil temperatures of 15°C or more but <22°C, and >5°C difference between mean summer and mean winter soil temperatures at 50 cm below the surface. Isothermic is the same except the summer and winter temperatures differ by <5°C.

thermogenic soils: Soils with properties that have been influenced primarily by high temperature as the dominant soil-formation factor; developed in subtropical and equatorial regions.

till: (i) Unsorted and unstratified earth material, deposited by glacial ice, which consists of a mixture of clay, silt, sand, gravel, stones, and boulders in any proportion. (ii) To prepare the soil for seeding; to seed or cultivate the soil.

till plain: An extensive flat to undulating surface underlain by till.

tillage: The mechanical manipulation of the soil profile for any purpose; but in agriculture it is usually restricted to modifying soil conditions and/or managing crop residues and/or weeds and/or incorporating chemicals for crop production.

toeslope: The hillslope position that forms a gently inclined surface at the base of a slope. Toeslopes in profile are commonly gentle and linear, and are constructional surfaces forming the lower part of a slope continuum that grades to a valley or closed depression.

topsoil: (i) The layer of soil moved in cultivation. Frequently designated as the Ap layer or Ap horizon. (See also surface soil.) (ii) Presumably fertile soil material used to topdress roadbanks, gardens, and lawns.

tortuosity: The nonstraight nature of soil pores.

toxicity: Quality, state, or degree of the harmful effect from alteration of an environmental factor.

transitional soil (intergrades): A soil that possesses properties and distinguishing characteristics of two or more separate soils.

truncated: Having lost all or part of the upper soil horizon or horizons by soil removal (erosion, excavation, etc.).

tuff: A compacted deposit that is 50 percent or more volcanic ash and dust.

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