Soil Glossary

Need a good overview of soil monitoring related words and phrases? Read on for definitions of common words used in the industry.

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    A

    A: Name of a soil horizon. Horizon "A" is a top soil rich in organic matter. Typically found 2 to 10 inches below the surface.

    Aeration: The exchange of air in soil with air from the atmosphere. The air in a well aerated soil is similar to that in the atmosphere; the air in a poorly aerated soil is considerably higher in carbon dioxide and lower in oxygen.

    Alfisols: One of the 12 orders of soil. Alfisols are in semiarid to moist areas. They formed under forest or mixed vegetative cover and are productive for most crops.

    Allowable Depletion: Represents the amount of soil moisture that can be removed by the crop from the soil before the crop begins to stress.

    Aggregate:. Many fine particles held in a single mass or cluster. Natural soil aggregates, such as granules, blocks, or prisms, are called peds. Clods are aggregates produced by tillage or logging.

    Alkali (sodic) soil: A soil having so high a degree of alkalinity (pH 8.5 or higher), or so high a percentage of exchangeable sodium (15 percent or more of the total exchangeable bases), or both, that plant growth is restricted.

    Alluvium: Material, such as sand, silt, or clay, deposited on land by streams.

    Andisols: One of the 12 orders of soil. Andisols tend to be highly productive soils. They are common in cool areas with moderate to high precipitation, especially those areas associated with volcanic materials.

    Aridisols: One of the 12 orders of soil. Aridisols are soils that are too dry for the growth of mesophytic plants. They often accumulate gypsum, salt, calcium carbonate, and other materials that are easily leached from soil in more humid environments. Aridisols are common in the world's deserts.

    Available Water Capacity (AWC): The amount of water in the soil that is available to the plant.

    B

    B: Name of a soil horizon. Horizon "B" is a subsoil, the most diverse horizon and the horizon with the most sub classifications. Typically found 10 to 30 inches below the surface.

    Basal till: Compact glacial till deposited beneath the ice (Lodgement Till is preferred).

    Base saturation: The degree to which material having cation-exchange properties is saturated with exchangeable bases (sum of Ca, Mg, Na, K), expressed as a percentage of the total cation-exchange capacity.

    Bedding planes: Fine stratifications, less than 5 millimeters thick, in unconsolidated alluvial, eolian, lacustrine, or marine sediments.

    Bedding system: A drainage system made by plowing, grading, or otherwise shaping the surface of a flat field. It consists of a series of low ridges separated by shallow, parallel dead furrows.

    Bedrock: The solid rock that underlies the soil and other unconsolidated material or that is exposed at the surface.

    Bisequum: Two sequences of soil horizons, each of which consists of an illuvial horizon and the overlying eluvial horizons.

    Blowout: A shallow depression from which all or most of the soil material has been removed by wind. A blowout has a flat or irregular floor formed by a resistant layer or by an accumulation of pebbles or cobbles. In some blowouts the water table is exposed.

    Bog: Waterlogged, spongy ground, consisting primarily of mosses, containing acidic, decaying vegetation such as sphagnum, sedges, and heaths, that may develop into peat. Compare - fen, marsh, swamp.

    Bottom land: The normal flood plain of a stream, subject to flooding.

    Boulders: Rock fragments larger than 2 feet (60 centimeters) in diameter.

    Bulk Density: A measure of the weight of the soil per unit volume (g/cc), usually given on an oven-dry (110° C) basis

    C

    C: Name of a soil horizon. Horizon "C" is made up of weathered/aged parent material and can usually be found 30 to 48 inches below the surface.

    Calcareous soil: A soil containing enough calcium carbonate (commonly combined with magnesium carbonate) to effervesce visibly when treated with cold, dilute hydrochloric acid.

    Caliche: A more or less cemented deposit of calcium carbonate in soils of warm-temperate, subhumid to arid areas. Caliche occurs as soft, thin layers in the soil or as hard, thick beds just beneath the solum, or it is exposed at the surface by erosion.

    Capillary water: Water held as a film around soil particles and in tiny spaces between particles. Surface tension is the adhesive force that holds capillary water in the soil.

    Carbon dioxide (CO2): Carbon Dioxide is a gas that is found naturally in the atmosphere, in the human blood stream and is used by plants as part of photosynthesis. However, because of its ability to absorb light and stay in the atmosphere for extended periods of time it has been thought that CO2 may be one factor in global warming.

    Carbon sink: A carbon sink is anything that is collecting more CO2 from the atmosphere than it's releasing. Major carbon sinks include the world's oceans and young plants and forests. Carbon sequestering can be used to further enhance the ability of these sinks to capture carbon from the atmosphere.

    Catena: A sequence, or ""chain,'' of soils on a landscape that formed in similar kinds of parent material but have different characteristics as a result of differences in relief and drainage.

    Cation: An ion carrying a positive charge of electricity. The common soil cations are calcium, potassium, magnesium, sodium, and hydrogen.

    Cation-exchange capacity: The total amount of exchangeable cations that can be held by the soil, expressed in terms of milliequivalents per 100 grams of soil at neutrality (pH 7.0) or at some other stated pH value. The term, as applied to soils, is synonymous with base-exchange capacity but is more precise in meaning.

    Cement rock: Shaly limestone used in the manufacture of cement.

    Channery soil: A soil that is, by volume, more than 15 percent thin, flat fragments of sandstone, shale, slate, limestone, or schist as much as 6 inches along the longest axis. A single piece is called a channer.

    Chiseling: Tillage with an implement having one or more soil-penetrating points that shatter or loosen hard compacted layers to a depth below normal plow depth.

    Clay: As a soil separate, the mineral soil particles less than 0.002 millimeter in diameter. As a soil textural class, soil material that is 40 percent or more clay, less than 45 percent sand, and less than 40 percent silt.

    Clay film: A thin coating of oriented clay on the surface of a soil aggregate or lining pores or root channels. Synonyms: clay coating, clay skin.

    Claypan: A slowly permeable soil horizon that contains much more clay than the horizons above it. A claypan is commonly hard when dry and plastic or stiff when wet.

    Climax vegetation: The stabilized plant community on a particular site. The plant cover reproduces itself and does not change so long as the environment remains the same.

    Coarse fragments: If round, mineral or rock particles 2 millimeters to 25 centimeters (10 inches) in diameter; if flat, mineral or rock particles (flagstone) 15 to 38 centimeters (6 to 15 inches) long.

    Coarse textured soil: A soil with USDA Soil textures of loamy fine sand or coarser (loamy sand or sand).

    Cobblestone (or cobble): A rounded or partly rounded fragment of rock 3 to 10 inches (7.5 to 25 centimeters) in diameter.

    Colluvium: Soil material, rock fragments, or both moved by creep, slide, or local wash and deposited at the base of steep slopes.

    Complex slope: Irregular or variable slope. Planning or constructing terraces, diversions, and other water-control measures on a complex slope is difficult.

    Complex: A map unit of two or more kinds of soil in such an intricate pattern or so small in area that it is not practical to map them separately at the selected scale of mapping. The pattern and proportion of the soils are somewhat similar in all areas.

    Concretions: Grains, pellets, or nodules of various sizes, shapes, and colors consisting of concentrated compounds or cemented soil grains. The composition of most concretions is unlike that of the surrounding soil. Calcium carbonate and iron oxide are common compounds in concretions.

    Congeliturbate: Soil material disturbed by frost action.

    Conservation tillage: A tillage system that does not invert the soil and that leaves a protective amount of crop residue on the surface throughout the year.

    Consistence:. The feel of the soil and the ease with which a lump can be crushed by the fingers. Terms commonly used to describe consistence are:

    • Loose: Noncoherent when dry or moist; does not hold together in a mass.
    • Friable: When moist, crushes easily under gentle pressure between thumb and forefinger and can be pressed together into a lump.
    • Firm: When moist, crushes under moderate pressure between thumb and forefinger, but resistance is distinctly noticeable.
    • Plastic: When wet, readily deformed by moderate pressure but can be pressed into a lump; will form a ""wire'' when rolled between thumb and forefinger.
    • Sticky: When wet, adheres to other material and tends to stretch somewhat and pull apart rather than to pull free from other material.
    • Hard: When dry, moderately resistant to pressure; can be broken with difficulty between thumb and forefinger.
    • Soft: When dry, breaks into powder or individual grains under very slight pressure.
    • Cemented: Hard; little affected by moistening.

    Contour stripcropping: Growing crops in strips that follow the contour. Strips of grass or close-growing crops are alternated with strips of clean-tilled crops or summer fallow.

    Control section: The part of the soil on which classification is based. The thickness varies among different kinds of soil, but for many it is that part of the soil profile between depths of 10 inches and 40 or 80 inches.

    Coprogenous earth (sedimentary peat): Fecal material deposited in water by aquatic organisms.

    Corrosive: High risk of corrosion to uncoated steel or deterioration of concrete.

    Cover crop: A close-growing crop grown primarily to improve and protect the soil between periods of regular crop production, or a crop grown between trees and vines in orchards and vineyards.

    D

    Dense layer (in tables): A very firm, massive layer that has a bulk density of more than 1.8 grams per cubic centimeter. Such a layer affects the ease of digging and can affect filling and compacting.

    Depth to rock (in tables): Bedrock is too near the surface for the specified use.

    Dioxane (1,4-dioxane): This clear organic, carcinogenic compound is oftentimes used as a solvent in manufacturing processes. It has been found in contaminated groundwater.

    Diversion (or diversion terrace): A ridge of earth, generally a terrace, built to protect downslope areas by diverting runoff from its natural course.

    Drainage class (natural): Refers to the frequency and duration of periods of saturation or partial saturation during soil formation, as opposed to altered drainage, which is commonly the result of artificial drainage or irrigation but may be caused by the sudden deepening of channels or the blocking of drainage outlets.

    Drainage, surface: Runoff, or surface flow of water, from an area.

    E

    E: Name of a soil horizon. Horizon "E" has been leached of organic or mineral content and is light in color.

    Electrical conductivity (EC): Measured in Siemens per meter, soil electrical conductivity is indicative of dissolved salts, dissolved solids, and fertilizers. It may also be indicative of very high pH conditions.

    Eluviation: The movement of material in true solution or colloidal suspension from one place to another within the soil. Soil horizons that have lost material through eluviation are eluvial; those that have received material are illuvial.

    Eolian soil material: Earthy parent material accumulated through wind action; commonly refers to sandy material in dunes or to loess in blankets on the surface.

    Entisols: One of the 12 orders of soil. Entisols occur in areas of recently deposited parent materials or in areas where erosion or deposition rates are faster than the rate of soil development; such as dunes, steep slopes and flood planes.

    Erosion: The wearing away of the land surface by water, wind, ice, or other geologic agents and by such processes as gravitational creep.

    Erosion (geologic): Erosion caused by geologic processes acting over long geologic periods and resulting in the wearing away of mountains and the building up of such landscape features as flood plains and coastal plains. Synonym: natural erosion.

    Erosion (accelerated). Erosion much more rapid than geologic erosion, mainly as a result of the activities of man or other animals or of a catastrophe in nature, for example, fire, that exposes the surface.

    Erosion pavement: A layer of gravel or stones that remains on the surface after fine particles are removed by sheet or rill erosion.

    F

    Fallow: Cropland left idle in order to restore productivity through accumulation of moisture. Summer fallow is common in regions of limited rainfall where cereal grains are grown. The soil is tilled for at least one growing season for weed control and decomposition of plant residue.

    Fen: Waterlogged, spongy ground containing alkaline decaying vegetation, characterized by reeds, that develops into peat. It sometimes occurs in sinkholes of karst regions. Compare - bog, marsh, swamp.

    Fast intake (in tables): The rapid movement of water into the soil.

    Fertility. The quality that enables a soil to provide plant nutrients, in adequate amounts and in proper balance, for the growth of specified plants when light, moisture, temperature, tilth, and other growth factors are favorable.

    Fibric soil material (peat): The least decomposed of all organic soil material. Peat contains a large amount of well preserved fiber that is readily identifiable according to botanical origin. Peat has the lowest bulk density and the highest water content at saturation of all organic soil material.

    Field moisture capacity: The moisture content of a soil, expressed as a percentage of the ovendry weight, after the gravitational, or free, water has drained away; the field moisture content 2 or 3 days after a soaking rain; also called normal field capacity, normal moisture capacity, or capillary capacity.

    Fine textured soil: Sandy clay, silty clay, and clay.

    First bottom: The normal flood plain of a stream, subject to frequent or occasional flooding.

    Flagstone: A thin fragment of sandstone, limestone, slate, shale, or (rarely) schist, 6 to 15 inches (15 to 38 centimeters) long.

    Flood plain: A nearly level alluvial plain that borders a stream and is subject to flooding unless protected artificially.

    Foot slope: The inclined surface at the base of a hill.

    Forb: Any herbaceous plant not a grass or a sedge.

    Fragipan: A loamy, brittle subsurface horizon low in porosity and content of organic matter and low or moderate in clay but high in silt or very fine sand. A fragipan appears cemented and restricts roots. When dry, it is hard or very hard and has a higher bulk density than the horizon or horizons above. When moist, it tends to rupture suddenly under pressure rather than to deform slowly.

    Frost action (in tables): Freezing and thawing of soil moisture. Frost action can damage roads, buildings and other structures, and plant roots.

    G

    Genesis: The mode of origin of the soil. Refers especially to the processes or soil-forming factors responsible for the formation of the solum, or true soil, from the unconsolidated parent material.

    Gelisols: One of the 12 orders of soil. Gelisols are soils that have permafrost near the soil surface, have evidence of frost churning, or ice segregation. These are common in the higher latitudes or high elevations.

    Gilgai: Commonly a succession of microbasins and microknolls in nearly level areas or of microvalleys and microridges parallel with the slope. Typically, the microrelief of Vertisols; clayey soils having a high coefficient of expansion and contraction with changes in moisture content.

    Glacial drift:. Pulverized and other rock material transported by glacial ice and then deposited. Also, the sorted and unsorted material deposited by streams flowing from glaciers.

    Glacial outwash:. Gravel, sand, and silt, commonly stratified, deposited by glacial meltwater.

    Glacial till: Unsorted, nonstratified glacial deposits consisting of clay, silt, sand, and boulders transported and deposited by glacial ice.

    Glacial fluvial deposits:. Material moved by glaciers and subsequently sorted and deposited by streams flowing from the melting ice. The deposits are stratified and occur as kames, eskers, deltas, and outwash plains.

    Glacial lacustrine deposits: Material ranging from fine clay to sand derived from glaciers and deposited in glacial lakes mainly by glacial meltwater. Many deposits are interbedded or laminated.

    Gleyed soil: Soil that formed under poor drainage, resulting in the reduction of iron and other elements in the profile and in gray colors and mottles.

    Graded stripcropping: Growing crops in strips that grade toward a protected waterway.

    Grassed waterway: A natural or constructed waterway, typically broad and shallow, seeded to grass as protection against erosion. Conducts surface water away from cropland.

    Gravel: Rounded or angular fragments of rock up to 3 inches (2 millimeters to 7.6 centimeters) in diameter. An individual piece is a pebble.

    Gravelly soil material: Material that is 15 to 50 percent, by volume, rounded or angular rock fragments, not prominently flattened, up to 3 inches (7.6 centimeters) in diameter.

    Ground water: Water filling all the unblocked pores of underlying material below the water table.

    H

    Hardpan: A hardened or cemented soil horizon, or layer. The soil material is sandy, loamy, or clayey and is cemented by iron oxide, silica, calcium carbonate, or other substance.

    Hemic soil material (mucky peat): Organic soil material intermediate in degree of decomposition between the less decomposed fibric and the more decomposed sapric material.

    Histosols: One of the 12 orders of soil. Histosols have a high content of organic matter and no permafrost. Most are saturated year round, but a few are freely drained. They are commonly called bogs, moors, pears or mucks.

    Horizon: Soil horizons are distinct layers of soil that form naturally in undisturbed soil over time. The types of horizons are indicative of the soil order. Like other natural processes, the age of the horizon increases with depth.

    Humus: The well decomposed, more or less stable part of the organic matter in mineral soils.

    Hydrologic soil groups: Refers to soils grouped according to their runoff-producing characteristics. The chief consideration is the inherent capacity of soil bare of vegetation to permit infiltration. The slope and the kind of plant cover are not considered but are separate factors in predicting runoff. Soils are assigned to four groups. In group A are soils having a high infiltration rate when thoroughly wet and having a low runoff potential. They are mainly deep, well drained, an-d sandy or gravelly. In group D, at the other extreme, are soils having a very slow infiltration rate and thus a high runoff potential. They have a claypan or clay layer at or near the surface, have a permanent high water table, or are shallow over nearly impervious bedrock or other material. A soil is assigned to two hydrologic groups if part of the acreage is artificially drained and part is undrained.

    I

    Illuviation: The movement of soil material from one horizon to another in the soil profile. Generally, material is removed from an upper horizon and deposited in a lower horizon.

    Impervious soil: A soil through which water, air, or roots penetrate slowly or not at all. No soil is absolutely impervious to air and water all the time.

    Inceptisols: One of the 12 orders of soil. Inceptisols are soils of semiarid to humid environments that generally exhibit only moderate degrees of soil weathering and development. These occur in a wide variety of climates.

    Increasers: Species in the climax vegetation that increase in amount as the more desirable plants are reduced by close grazing. Increasers commonly are the shorter plants and the less palatable to livestock.

    Infiltration: The downward entry of water into the immediate surface of soil or other material, as contrasted with percolation, which is movement of water through soil layers or material.

    Infiltration capacity: The maximum rate at which water can infiltrate into a soil under a given set of conditions.

    Infiltration rate: The rate at which water penetrates the surface of the soil at any given instant, usually expressed in inches per hour. The rate can be limited by the infiltration capacity of the soil or the rate at which water is applied at the surface.

    Intake rate: The average rate of water entering the soil under irrigation. Most soils have a fast initial rate; the rate decreases with application time. Therefore, intake rate for design purposes is not a constant but is a variable depending on the net irrigation application.

    Irrigation
    : Application of water to soils to assist in production of crops.

    L

    Lacustrine deposit: Material deposited in lake water and exposed when the water level is lowered or the elevation of the land is raised.

    Landslide: The rapid downhill movement of a mass of soil and loose rock, generally when wet or saturated. The speed and distance of movement, as well as the amount of soil and rock material, vary greatly.

    Large stones (in tables): Rock fragments 3 inches (7.6 centimeters) or more across. Large stones adversely affect the specified use of the soil.

    Leaching: The removal of soluble material from soil or other material by percolating water.

    Liquid limit: The moisture content at which the soil passes from a plastic to a liquid state.

    Loess: Fine grained material, dominantly of silt-sized particles, deposited by wind.

    Lowmoor bog: A bog that is at or only slightly above the water table, on which it depends for accumulation and preservation of peat (chiefly the remains of sedges, reeds, shrubs, and various mosses).

    Low strength: The soil is not strong enough to support loads.

    Lower soil moisture limit: The soil moisture value below which the crop will become stressed because it will have insufficient water. When the lower limit is reached, it is time to irrigate.

    M

    Maximum Allowable Depletion (MAD): The fraction of the available water that is 100% available to the crop.

    Medium textured soil: Very fine sandy loam, loam, silt loam, or silt.

    Metamorphic rock: Rock of any origin altered in mineralogical composition, chemical composition, or structure by heat, pressure, and movement. Nearly all such rocks are crystalline.

    Mineral soil: Soil that is mainly mineral material and low in organic material. Its bulk density is more than that of organic soil.

    Minimum tillage: Only the tillage essential to crop production and prevention of soil damage.

    Miscellaneous area: An area that has little or no natural soil and supports little or no vegetation.

    Moderately coarse textured soil: Coarse sandy loam, sandy loam, and fine sandy loam.

    Moderately fine textured soil: Clay loam, sandy clay loam, and silty clay loam.

    Mollisols: One of the 12 orders of soil. Mollisols are soils that have a dark colored surface horizon relatively high in content of organic matter. The soils are base rich throughout and therefore are quite fertile.

    Morphology, soil: The physical makeup of the soil, including the texture, structure, porosity, consistence, color, and other physical, mineral, and biological properties of the various horizons, and the thickness and arrangement of those horizons in the soil profile.

    Moss peat: An accumulation of organic material that is predominantly the remains of mosses (e.g. sphagnum moss). Compare - Herbaceous peat, sedimentary peat, woody peat, peat, muck, and mucky peat.

    Mottling, soil: Irregular spots of different colors that vary in number and size. Mottling generally indicates poor aeration and impeded drainage.

    Muck: Unconsolidated soil material consisting primarily of highly decomposed organic material in which the original plant parts are not recognizable (i.e. "sapric" in Soil Taxonomy). It generally contains more mineral matter and is usually darker in color, than peat.

    Mucky peat: Unconsolidated soil material consisting primarily of organic matter that is in an intermediate stage of decomposition such that a significant part of the original material can be recognized and a significant part of the material can not be recognized (i.e. "hemic" in Soil Taxonomy).

    N

    Neutral soil: A soil having a pH value between 6.6 and 7.3.

    Nitrogen: A chemical compound that can be found in all living organisms, the atmosphere, and animal wastes. It is commonly found in nutrients used in fertilization, and can cause problems to local ecosystems if too much is washed into waterways.

    Nutrient, plant: Any element taken in by a plant essential to its growth. Plant nutrients are mainly nitrogen, phosphorus, potassium, calcium, magnesium, sulfur, iron, manganese, copper, boron, and zinc obtained from the soil and carbon, hydrogen, and oxygen obtained from the air and water.

    O

    O: Name of a soil horizon. Horizon "O" is made up of decaying plants on or near surface and is typically up to 2 inches thick.

    Organic matter: Plant and animal residue in the soil in various stages of decomposition.

    Organic materials: Unconsolidated sediments or deposits in which carbon is an essential, substantial component. Several types of organic materials (deposits) can be identified based on the composition of the dominant fibers (grassy organic materials, herbaceous organic materials, mossy organic materials, woody organic materials). Compare - herbaceous peat, moss peat, sedimentary peat, woody peat.

    Outwash, glacial: Stratified sand and gravel produced by glaciers and carried, sorted, and deposited by glacial meltwater.

    Oxisols: One of the 12 orders of soil. Oxisols are highly weathered soils of tropical and subtropical regions. They characteristically occur on land surfaces that have been stable for a long time. They have low natural fertility as well as a low capacity to retain additions of lime and fertilizer.

    P

    Pan: A compact, dense layer in a soil that impedes the movement of water and the growth of roots. For example, hardpan, fragipan, claypan, plowpan, and traffic pan.

    Parent material: The unconsolidated organic and mineral material in which soil forms.

    Peat: Unconsolidated soil material consisting largely of undecomposed, or slightly decomposed, organic matter (i.e. "fibric" in Soil Taxonomy) accumulated under conditions of excessive moisture. Compare - muck, mucky peat, herbaceous peat.

    Ped: An individual natural soil aggregate, such as a granule, a prism, or a block.

    Pedon: The smallest volume that can be called ""a soil.'' A pedon is three dimensional and large enough to permit study of all horizons. Its area ranges from about 10 to 100 square feet (1 square meter to 10 square meters), depending on the variability of the soil.

    Perchloroethylene (PCE): See Tetrachloroethylene (TCE).

    Percolation: The downward movement of water through the soil.

    Percs slowly (in tables): The slow movement of water through the soil, adversely affecting the specified use.

    Permafrost: Layers of soil, or even bedrock, occurring in arctic or subarctic regions, in which a temperature below freezing has existed continuously for a long time.

    Permanent wilting point: Refers to the amount of water in soil that is unavailable to the plant.

    Permeability: The quality of the soil that enables water to move downward through the profile. Permeability is measured as the number of inches per hour that water moves downward through the saturated soil.

    Phase, soil: A subdivision of a soil series based on features that affect its use and management. For example, slope, stoniness, and thickness.

    pH value: A numerical designation of acidity and alkalinity in soil. (See Reaction, soil.)

    Phosphorus: A highly reactive chemical element used in fertilizers to aid in plant growth. Phosphorus can cause oxygen problems and unwanted algae blooms if too much is washed into bodies of water, creating hardship for the local ecosystem.

    Phytoremediation
    : A remediation method by which trees are used to pull contamination out of groundwater. When the trees absorb the water and then off-gas it during photosynthesis, the toxic chemicals are rendered into relatively harmless by-products such as methane and carbon dioxide.

    Piping (in tables): Formation of subsurface tunnels or pipelike cavities by water moving through the soil.

    Pitting (in tables): Pits caused by melting ground ice. They form on the soil after plant cover is removed.

    Plasticity index: The numerical difference between the liquid limit and the plastic limit; the range of moisture content within which the soil remains plastic.

    Plastic limit: The moisture content at which a soil changes from semisolid to plastic.

    Plinthite: The sesquioxide-rich, humus-poor, highly weathered mixture of clay with quartz and other diluents. It commonly appears as red mottles, usually in platy, polygonal, or reticulate patterns. Plinthite changes irreversibly to an ironstone hardpan or to irregular aggregates on repeated wetting and drying, especially if it is exposed to heat from the sun. In a moist soil, plinthite can be cut with a spade. It is a form of laterite.

    Plowpan: A compacted layer formed in the soil directly below the plowed layer.

    Ponding: Standing water on soils in closed depressions. Unless the soils are artificially drained, the water can be removed only by percolation or evapotranspiration.

    Poor filter (in tables): Because of rapid permeability, the soil may not adequately filter effluent from a waste disposal system.

    Poorly graded: Refers to a coarse grained soil or soil material consisting mainly of particles of nearly the same size. Because there is little difference in size of the particles, density can be increased only slightly by compaction.

    Poor outlets (in tables): Refers to areas where surface or subsurface drainage outlets are difficult or expensive to install.

    Probe, soil: The end of a soil probe that is buried or otherwise sunk into the ground in order to take measurement readings of the surrounding soil. The name "soil probe" is also used as a general name for any number of devices designed to measure soil, and is also interchangeably referred to as a "soil sensor".

    Productivity, soil: The capability of a soil for producing a specified plant or sequence of plants under specific management.

    Profile, soil: A vertical section of the soil extending through all its horizons and into the parent material.

    R

    Reaction, soil: A measure of acidity or alkalinity of a soil, expressed in pH values. A soil that tests to pH 7.0 is described as precisely neutral in reaction because it is neither acid nor alkaline.

    Regolith:. The unconsolidated mantle of weathered rock and soil material on the earth's surface; the loose earth material above the solid rock.

    Relief: The elevations or inequalities of a land surface, considered collectively.

    Residuum (residual soil material): Unconsolidated, weathered or partly weathered mineral material that accumulated as consolidated rock disintegrated in place.

    Rill: A steep-sided channel resulting from accelerated erosion. A rill is generally a few inches deep and not wide enough to be an obstacle to farm machinery.

    Rippable: Bedrock or hardpan can be excavated using a single-tooth ripping attachment mounted on a tractor with a 200-300 draw bar horsepower rating.

    Rock fragments: Rock or mineral fragments having a diameter of 2 millimeters or more; for example, pebbles, cobbles, stones, and boulders.

    Rooting depth (in tables): Shallow root zone. The soil is shallow over a layer that greatly restricts roots.

    Root zone: The part of the soil that can be penetrated by plant roots.

    Runoff: The precipitation discharged into stream channels from an area. The water that flows off the surface of the land without sinking into the soil is called surface runoff. Water that enters the soil before reaching surface streams is called ground-water runoff or seepage flow from ground water.

    S

    Saline soil: A soil containing soluble salts in an amount that impairs growth of plants. A saline soil does not contain excess exchangeable sodium.

    Salty water (in tables): Water that is too salty for consumption by livestock.

    Sand: As a soil separate, individual rock or mineral fragments from 0.05 millimeter to 2.0 millimeters in diameter. Most sand grains consist of quartz. As a soil textural class, a soil that is 85 percent or more sand and not more than 10 percent clay.

    Sandstone: Sedimentary rock containing dominantly sand-size particles.

    Sapric soil material (muck): The most highly decomposed of all organic soil material. Muck has the least amount of plant fiber, the highest bulk density, and the lowest water content at saturation of all organic soil material.

    Saprolite: Unconsolidated residual material underlying the soil and grading to hard bedrock below.

    Sedimentary rock: Rock made up of particles deposited from suspension in water. The chief kinds of sedimentary rock are conglomerate, formed from gravel; sandstone, formed from sand; shale, formed from clay; and limestone, formed from soft masses of calcium carbonate. There are many intermediate types. Some wind-deposited sand is consolidated into sandstone.

    Seepage (in tables): The movement of water through the soil. Seepage adversely affects the specified use.

    Sensor, soil: A device that measures soil parameters such as temperature and soil moisture content. Also know as a "soil probe", these names can be used interchangeably.

    Sequum: A sequence consisting of an illuvial horizon and the overlying eluvial horizon. (See Eluviation.)

    Series, soil: A group of soils that have profiles that are almost alike, except for differences in texture of the surface layer or of the underlying material. All the soils of a series have horizons that are similar in composition, thickness, and arrangement.

    Shale: Sedimentary rock formed by the hardening of a clay deposit.

    Sheet erosion: The removal of a fairly uniform layer of soil material from the land surface by the action of rainfall and surface runoff.

    Shrink-swell: The shrinking of soil when dry and the swelling when wet. Shrinking and swelling can damage roads, dams, building foundations, and other structures. It can also damage plant roots.

    Silica: A combination of silicon and oxygen. The mineral form is called quartz.

    Silica-sesquioxide ratio: The ratio of the number of molecules of silica to the number of molecules of alumina and iron oxide. The more highly weathered soils or their clay fractions in warm-temperate, humid regions, and especially those in the tropics, generally have a low ratio.

    Silt: As a soil separate, individual mineral particles that range in diameter from the upper limit of clay (0.002 millimeter) to the lower limit of very fine sand (0.05 millimeter). As a soil textural class, soil that is 80 percent or more silt and less than 12 percent clay.

    Siltstone: Sedimentary rock made up of dominantly silt-sized particles.

    Similar soils: Soils that share limits of diagnostic criteria, behave and perform in a similar manner, and have similar conservation needs or management requirements for the major land uses in the survey area.

    Sinkhole: A depression in the landscape where limestone has been dissolved.

    Site index: A designation of the quality of a forest site based on the height of the dominant stand at an arbitrarily chosen age. For example, if the average height attained by dominant and codominant trees in a fully stocked stand at the age of 50 years is 75 feet, the site index is 75 feet.

    Slickensides: Polished and grooved surfaces produced by one mass sliding past another. In soils, slickensides may occur at the bases of slip surfaces on the steeper slopes; on faces of blocks, prisms, and columns; and in swelling clayey soils, where there is marked change in moisture content.

    Slick spot: A small area of soil having a puddled, crusted, or smooth surface and an excess of exchangeable sodium. The soil is generally silty or clayey, is slippery when wet, and is low in productivity.

    Slippage (in tables): Soil mass susceptible to movement downslope when loaded, excavated, or wet.

    Slope: The inclination of the land surface from the horizontal. Percentage of slope is the vertical distance divided by horizontal distance, then multiplied by 100. Thus, a slope of 20 percent is a drop of 20 feet in 100 feet of horizontal distance.

    Slope (in tables): Slope is great enough that special practices are required to ensure satisfactory performance of the soil for a specific use.

    Sloughed till: Water-saturated till that has flowed slowly downhill from its original place of deposit by glacial ice. It may rest on other till, on glacial outwash, or on a glaciolacustrine deposit.

    Slow intake (in tables): The slow movement of water into the soil.

    Slow refill (in tables): The slow filling of ponds, resulting from restricted permeability in the soil.

    Small stones (in tables): Rock fragments less than 3 inches (7.6 centimeters) in diameter. Small stones adversely affect the specified use of the soil.

    Soil: A natural, three-dimensional body at the earth's surface. It is capable of supporting plants and has properties resulting from the integrated effect of climate and living matter acting on earthy parent material, as conditioned by relief over periods of time.

    Soil carbon flux: Also known as soil respiration, this is the result of bacteria and other microorganisms in the soil consuming organic material or decaying plant matter, which in turn produces CO2 that is off-gassed into the atmosphere and groundwater.

    Soil saturation: Refers to the situation where all the soil pores are filled with water. This occurs below the water table and in the unsaturated zone above the water table after a heavy rain or irrigation event. After the rain event, the soil moisture (above the water table) will decrease from saturation to field capacity.

    Soil separates: Mineral particles less than 2 millimeters in equivalent diameter and ranging between specified size limits.

    Solum: The upper part of a soil profile, above the C horizon, in which the processes of soil formation are active. The solum in soil consists of the A, E, and B horizons. Generally, the characteristics of the material in these horizons are unlike those of the underlying material. The living roots and plant and animal activities are largely confined to the solum.

    Spodosols: One of the 12 orders of soil. Spodosols formed from weathering processes that strip organic matter combined with aluminum from the surface layer and deposit them in the subsoil. These tend to be acid and infertile.

    Stone line:
    A concentration of coarse fragments in a soil. Generally, it is indicative of an old weathered surface. In a cross section, the line may be one fragment or more thick. It generally overlies material that weathered in place and is overlain by recent sediment of variable thickness.

    Stones: Rock fragments 10 to 24 inches (25 to 60 centimeters) in diameter.

    Stony: Refers to a soil containing stones in numbers that interfere with or prevent tillage.

    Stripcropping: Growing crops in a systematic arrangement of strips or bands which provide vegetative barriers to wind and water erosion.

    Structure, soil: The arrangement of primary soil particles into compound particles or aggregates.

    Subsoil: Technically, the B horizon; roughly, the part of the solum below plow depth.

    Subsoiling: Breaking up a compact subsoil by pulling a special chisel through the soil.

    Substratum: The part of the soil below the solum.

    Subsurface layer: Any surface soil horizon (A, E, AB, or EB) below the surface layer.

    Summer fallow: The tillage of uncropped land during the summer to control weeds and allow storage of moisture in the soil for the growth of a later crop. A practice common in semiarid regions, where annual precipitation is not enough to produce a crop every year. Summer fallow is frequently practiced before planting winter grain.

    Surface layer: The soil ordinarily moved in tillage, or its equivalent in uncultivated soil, ranging in depth from about 4 to 10 inches (10 to 25 centimeters). Frequently designated as the "plow layer'' or the "AP horizon".

    Surface soil: The A, E, AB, and EB horizons. It includes all subdivisions of these horizons.

    Swamp: An area of low, saturated ground, intermittently or permanently covered with water, and predominantly vegetated by shrubs and trees, with or without the accumulation of peat.

    U

    Ultisols: One of the 12 orders of soil. Ultisols are soils in humid areas. They are typically acid soils in which most nutrients are concentrated in the upper few inches. They have a moderately low capacity to retain additions of lime and fertilizer.

    V

    Valley fill: In glaciated regions, material deposited in stream valleys by glacial meltwater. In nonglaciated regions, alluvium deposited by heavily loaded streams.

    Variant, soil: A soil having properties sufficiently different from those of other known soils to justify a new series name, but occurring in such a limited geographic area that creation of a new series is not justified.

    Variegation: Refers to patterns of contrasting colors assumed to be inherited from the parent material rather than to be the result of poor drainage.

    Varve: A sedimentary layer of a lamina or sequence of laminae deposited in a body of still water within a year. Specifically, a thin pair of graded glaciolacustrine layers seasonally deposited, usually by meltwater streams, in a glacial lake or other body of still water in front of a glacier.

    Vertisols
    : One of the 12 orders of soil. Vertisols have a high content of expanding clay minerals. They undergo pronounced changes in volume with changes in moisture. Because they swell when wet, vertisols transmit water very slowly and have undergone little leeching. They tend to be fairly high in natural fertility.

    W

    Water Fraction by Volume (WFV): This term describes the percentage of water found in the soil displayed in decimal form. For example, a water content of 0.20 wfv means that a one liter soil sample contains 200 ml of water. Full saturation (all the soil pore spaces filled with water) occurs typically between 0.3-0.45 wfv and is quite soil dependent. WFV is a desirable way to measure water content of soils because you can compare the content directly between different types of soil with no conversion between units.

    Weathering: All physical and chemical changes produced in rocks or other deposits at or near the earth's surface by atmospheric agents. These changes result in disintegration and decomposition of the material.

    Well graded: Refers to soil material consisting of coarse grained particles that are well distributed over a wide range in size or diameter. Such soil normally can be easily increased in density and bearing properties by compaction. Contrasts with poorly graded soil.

    Wilting point (or permanent wilting point): The moisture content of soil, on an ovendry basis, at which a plant (specifically a sunflower) wilts so much that it does not recover when placed in a humid, dark chamber.