Glossary of Soil Science Terms - Browse
P2O5 Phosphorus pentoxide; designation on the fertilizer label that denotes the percentage of available phosphorus reported as phosphorus pentoxide.
packing voids (compound) Voids formed by the random packing of peds that do not accommodate each other.
packing voids (simple) Voids formed by the random packing of single skeletal grains.
paha (colloquial: Midwestern USA) Commonly a low, elongated, rounded ridge or hill of drift, rock, or windblown sand, silt, or clay and capped with a thick cover (e.g., up to 10 m) of loess; found especially in northeast Iowa. Height varies between 10 and 30 m ( Jackson, 1997).
pahoehoe lava A type of basaltic lava (material) with a characteristically smooth, billowy or rope-like surface and vesicular interior ( Jackson, 1997; MacDondal and Abbott, 1970). Compare a a lava, block lava, pillow lava.
pahoehoe lava flow A type of basaltic lava flow with a characteristically smooth, billowy or rope-like surface ( Jackson, 1997; MacDonald and Abbott, 1970). Compare aa lava flow, block lava flow, pillow lava flow.
Paleocene The earliest epoch (from 55.8 to 65.5 million years ago) of the Tertiary Period of geologic time, that follows the Cretaceous Period and precedes the Eocene Epoch; also the corresponding (time-stratigraphic) “series” of earthy materials (Schoeneberger and Wysocki, personal communication, 2013).
paleosol A soil that formed on a landscape in the past with distinctive morphological features resulting from a soil-forming environment that no longer exists at the site. The former pedogenic process was either altered because of external environmental change or interrupted by burial. A paleosol (or component horizon) may be classed as relict if it has persisted in a land-surface position without major alteration of morphology by processes of the prevailing pedogenic environment. An exhumed paleosol is one that formerly was buried and has been re-exposed by erosion of the covering mantle. Most paleosols have been affected by some subsequent modification of diagnostic horizon morphologies and profile truncation (Hawley and Parsons, 1980).
paleoterrace An erosional remnant of a terrace which retains the surface form and alluvial deposits of its origin but was not emplaced by, and commonly does not grade to a present-day stream or drainage network. Compare alluvial plain remnant (Schoeneberger and Wysocki, personal communication, 2013).
palsa An elliptical dome-like permafrost mound containing alternating layers of ice lenses and peat or mineral soil, commonly 3 to 10 m high and 2 to 25 m long, occurring in subarctic bogs of the tundra and often surrounded by water; plural palsen (National Research Council of Canada, 1988).
palygorskite Si8Mg2Al2O20(OH)2(OH2)• 4H2O (a) A fibrous clay mineral composed of two silica tetrahedral sheets and one aluminum and magnesium octahedral sheet that make up the 2:1 layer that occurs in strips. The strips that have an average width of two linked tetrahedral chains are linked at the edges forming tunnels where water molecules are held. Palygorskite is most common in soils of arid regions. Synonym attapulgite. (b) A magnesium aluminum silicate clay used in fertilizer production, including conditioning of fertilizer products, and asa suspending agent in suspension fertilizers.
pan (a) [genetic] A natural subsurface soil layer with low or very low hydraulic conductivity and differing in certain physical and chemical properties from the soil immediately above or below the pan. Refer to caliche, claypan, fragipan, and hardpan, all of which are genetic pans. (b) (pressure or induced) A subsurface horizon or soil layer having a higher bulk density and a lower total porosity than the soil directly above or below it, as a result of pressure that has been applied by normal tillage operations or by other artificial means. Frequently referred to as plowpan, plowsole, or traffic pan. Compare tillage, plow pan and tillage, pressure pan.
papule Glaebule composed dominantly of clay minerals with continuous and/or lameliar fabric, and sharp external boundaries.
parabolic dune A sand dune with a long, scoopshaped form, convex in the downwind direction so that its horns point upwind, whose ground plan, when perfectly developed, approximates the form of a parabola ( Jackson, 1997).
paralithic contact Similar to a lithic contact except that it is softer, can be dug with difficulty with a spade, if a single mineral has a hardness <3 (Mohs scale), and gravel-size chunks will partially disperse within 15 h shaking in water or sodium hexametaphosphate solution.
parallel drainage pattern A drainage pattern in which the streams and their tributaries are regularly spaced and flow parallel or subparallel to one another and tributaries characteristically join the mainstream at approximately the same angle, over a considerable area. It is indicative of a region having a pronounced, uniform slope and a homogeneous lithology and rock structure, such as young coastal plains and large basalt flows. (Schoeneberger and Wysocki, personal communication, 2013, Jackson, 1997; Way, 1973).
paraplow Refer to tillage, paraplow.
pararock fragments Discrete, water stable bodies or fragments of soft rock (<Strongly Cemented). Compare rock fragments, artifact.
parasitism (a) Feeding by one organism on the cells of a second organism, which is usually larger than the first. The parasite is, to some extent, dependent on the host at whose expense it is maintained. (b) An association whereby one organism (parasite) lives in or on another organism (host) and benefits at the expense of the host.
paratill Refer to tillage, paratill.
parent material The unconsolidated and more or less chemically weathered mineral or organic matter from which the solum of soils is developed by pedogenic processes.
park (colloquial – Rocky Mountains, USA; not preferred – refer to valley, intermontane basin) (a) An ecological term for a grassy or shrubby, wide, open valley lying at high elevation and confined between forested mountain slopes, as in a high meadow; sometimes marshy. Compare glade. (b) (refer to intermontane basin) A level valley between mountain ranges (Jackson, 1997; Schoeneberger and Wysocki, personal communication, 2013).
parna A term used, especially in southeast Australia and the southwestern USA, for silt and sand-size aggregates of eolian clay occurring in sheets or dunes.
parna dune A dune largely composed of silt and sandsized aggregates of clay; sometimes called a clay dune or lunette. Compare parna (Hawley and Parsons, 1980).
partial ballena (not preferred; use ballena).
particle density The density of the soil particles, the dry mass of the particles being divided by the solid (not bulk) volume of the particles, in contrast with bulk density; units are Mg m-3.
particle size The effective diameter of a particle measured by sedimentation, sieving, or micrometric methods.
particle-size analysis Determination of the various amounts of the different soil separates in a soil sample, usually by sedimentation, sieving, micrometry, or combinations of these methods.
particle-size classes [soil taxonomy] The first part of the Family name. They are used to characterize the grain-size composition of the whole soil including both the fine earth and the rock and pararock fragments up to the size of a pedon (USDA, 1999).
particle-size distribution The fractions of the various soil separates in a soil sample, often expressed as mass percentages.
particulate organic matter (POM) The microbially active fraction of soil organic mattter consisting of fine particles of partially decomposed plant tissues.
parts per million (ppm) (no longer used in SSSA publications) (a) The concentration of solutions expressed in weight or mass units of solute (dissolved substance) per million weight or mass units of solution.(b) A concentration in solids expressed in weight or mass units of a substance contained per million weight or mass units of solid, such as soil.
pasteurization Partial sterilization of soil, liquid, or other natural substances by temporary heat treatment.
patina A general term for a colored film or thin outer layer produced on the surface of a rock or other material by weathering after long exposure. Compare rock varnish ( Jackson, 1997).
patterned ground A general term for any ground surface exhibiting a discernibly ordered, more-or-less symmetrical, morphological pattern of ground and, where present, vegetation. Patterned ground is characteristic of, but not confined to, permafrost regions or areas subjected to intense frost action; it also occurs in tropical, subtropical, and temperate areas. Patterned ground is classified by type of pattern and presence or absence of sorting and includes nonsorted and sorted circles, net, polygons, steps and stripes, garlands, and solifluction features. In permafrost regions, the most common macroform is the ice-wedge polygon and a common microform is the nonsorted circle. Stone polygons generally form on slopes of less than 8%, while garlands and stripes occur on slopes of 8 to 15% and more than 15%, respectively. (National Research Council of Canada, 1988; Hawley and Parsons, 1980).
pavement karst Areas of bare limestone, usually sculpted by solution erosion into karren of various types and where soils have been stripped off, commonly by glaciation in alpine areas (e.g., Rocky Mountains – USA) and high latitudes, and by water erosion in arid karst areas (Schoeneberger and Wysocki, personal communication, 2013; White, 1988). Compare fluviokarst, glaciokarst, sinkhole karst, karst.
PCR (polymerase chain reaction) An in vitro method for amplifying defined segments of DNA. PCR involves a repeated cycle of oligonucleotide hybridization and extension on single-stranded DNA templates.
pe The negative logarithm of the apparent electron activity, which can be calculated by including the apparent activity of electrons in equilibrium calculations of redox half-cell reactions. In practice it is used as an alternative to EH and at 25°C can be calculated from EH values expressed in volts by dividing by 0.059.
peak Sharp or rugged upward extension of a ridge chain, usually at the junction of two or more ridges; the prominent highest point of a summit area (Hawley and Parsons, 1980).
peat Largely undecomposed oganic soil material that is the least decomposed. Refer to fibric soil material.
peat plateau A generally flat-topped expanse of peat, elevated above the general surface of a peatland, and containing segregated ice that may or may not extend downward into the underlying mineral soil. Controversy exists as to whether peat plateaus and palsen are morphological variations of the same feature (National Research Council of Canada, 1988).
peat soil An organic soil in which the plant residues are recognizable. The sum of the thicknesses of the organic layers is usually greater than the sum of the thicknesses of the mineral layers. Compare Histosol, muck, muck soil, and peat.
peatland A generic term for any wetland that accumulates partially decayed plant matter. Mire, moor, and muskeg are terms for European and Canadian peatlands. Compare bog and fen.
pebbles (obsolete: use gravel) Rounded or partially rounded rock or mineral fragments between 2 and 75 mm in diameter. Size may be further refined as fine pebbles (2to 5-mm diameter), medium pebbles (5to 20-mm diameter), and coarse pebbles (20to 75-mm diameter). Compare rock fragments.
Peclet number A dimensionless parameter, calculated from the product of the pore water velocity and the linear distance traveled divided by the dispersion coefficient, used to describe the shape of a solute breakthrough curve.
ped A unit of soil structure such as a block, column, granule, plate, or prism, formed by natural processes (in contrast with a clod, which is formed artificially). Compare shrinkage, soil, ped (shrinkage).
pedal Applied to soil materials, most of which consists of peds.
pedalfer [soil classification] (obsolete; not used in soil taxonony) A subdivision of Zonal soils comprising a large group of soils in which sesquioxides (aluminum and iron) increased relative to silica during soil formation. Compare pedocals (Baldwin et al., 1938).
pediment A gently sloping, erosional surface developed at the foot of a receding hill or mountain slope, commonly with a slightly concave-upward profile, that cross-cuts rock or sediment strata that extend beneath adjacent uplands. The erosion surface can be bare bedrock (i.e., rock pediment), or it may be thinly mantled (e.g., 1 to 3 m) with alluvium and colluvium (i.e., pediment) in transport to the adjacent valley (Schoeneberger and Wysocki, personal communication, 2013; Hawley and Parsons, 1980; Ruhe, 1975). Compare pedisediment.
pediplain (not recommended) A geomorphic term for an outwash plain landform.
pedisediment A sediment layer, eroded from the shoulder and back slope of an erosional slope, that lies on and is, or was, being transported across a pediment.
pedocal [soil classification] (obsolete; not used in soil taxonony) A subdivision of a Zonal soils comprising a large group of soils in which calcium accumulated during soil formation (Baldwin et al., 1938). Compare Pedalfers.
pedogenic imprinting Refer to pedogenic overprinting.
pedogenic overprinting 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’ or merging of attributes the two soils; analogous to the geologic concept of ‘overprinting’. Sometimes referred to as soil welding.
pedological features Recognizable units within a soil material that are distinguishable from the enclosing material for any reason such as origin (deposition as an entity), differences in concentration of some fraction of the plasma, or differences in arrangement of the constituents (fabric).
pedology The scientific study of soils and their weathering profiles.
pedon A three-dimensional body of soil with lateral dimensions large enough to permit the study of horizon shapes and relations. Its area ranges from 1 to 10 m2. Where horizons are intermittent or cyclic, and recur at linear intervals of 2 to 7 m, the pedon includes one-half of the cycle. Where the cycle is <2 m, or all horizons are continuous and of uniform thickness, the pedon has an area of approximately 1 m2. If the horizons are cyclic, but recur at intervals >7 m, the pedon reverts to the l-m2 size, and more than one soil will usu-ally be represented in each cycle.
pedoturbation Mixing within a soil or sediment profile by various natural processes, such as animal burrowing, tree throw, freeze-thaw cycles, etc. It usually involves disturbance of the skeletal fabric as opposed to redistribution of only the fine particles.
peneplain (obsolete; not recommended) An area which has been reduced by erosion to a low, gently rolling surface resembling a plain.
penetrability The ease with which a probe can be pushed into the soil. (May be expressed in units of distance, speed, force, or work depending on the type of penetrometer used.)
penetration resistance The force per unit area on a standard ASAE cone necessary for penetration by the cone. Compare cone index.
penetrometer Refer to cone penetrometer.
peninsula (a) An elongated body or stretch of land nearly surrounded by water (e.g., on three sides) and connected with a larger tract of land area, usually by a neck or an isthmus. (b) A relatively large tract of land jutting out into the water, with or without a well-defined isthmus; for example, the Italian peninsula ( Jackson, 1997).
peraquic [soil taxonomy] A soil moisture regime in which the ground water is always at or very close to the surface; a subset of aquic soil moisture regime. Examples are soils in tidal marshes or in closed depressions fed by perennial streams. The distinction between the aquic moisture regime and the peraquic moisture regime is not closely defined because neither regime is used as a criterion for taxa above the series level. A mostly reducing soil moisture regime virtually free of dissolved oxygen due to saturation by groundwater or its capillary fringe and occurring at periods when the soil temperature at 50 cm below the surface is >5°C (5°C is considered to be “biological zero” in soil taxonomy; USDA, 2006). Refer to aquic.
percent area wetted Refer to irrigation, percent area wetted.
Percolation theory ”Percolation theory describes emergent properties related to the connectivity of large numbers of objects”, that is, their spatial relationships (Hunt and Ewing, 2009).
percolation test A procedure used to determine the rate at which water moves out of an auger hole.
percolation, soil water The downward movement of water through soil. Especially, the downward flow of water in saturated or nearly saturated soil at hydraulic gradients of the order of 1.0 or less.
perennial stream A stream or reach of a stream that flows continuously throughout the year and whose surface is generally lower than the water table adjacent to the region adjoining the stream. Compare ephemeral stream, intermittent stream.
pergelic [soil taxonomy] (obsolete; not used in current soil taxonomy) Prior to 1987, this was a soil temperature regime that has mean annual soil temperatures of <0°C. Permafrost is present. Compare permafrost (USDA, 1975).
periglacial (adjective) Pertaining to processes, conditions, areas, climates, and topographic features occurring at the immediate margins of glaciers and ice sheets, and influenced by cold temperature of the ice.
permafrost (a) [soil taxonomy] Ground, soil, or rock that remains at or below 0°C for at least two years. It is defined on the basis of temperature and is not necessarily frozen. Compare continuous permafrost, discontinuous permafrost, sporadic permafrost, thaw-stable permafrost, thaw sensitive permafrost. (National Research Council of Canada, 1988) (b) A perennially frozen soil horizon.
permafrost table The upper boundary of the permafrost coincident with the lower limit of seasonal thaw. Compare permafrost (i).
permanent charge The net negative (or positive) charge of clay particles inherent in the crystal structure of the particle; not affected by changes in pH or by ion-exchange reactions.
permanent wilting point The largest water content of a soil at which indicator plants, growing in that soil, wilt and fail to recover when placed in a humid chamber. Often estimated by the water content at –1.5 MPa soil matric potential.
permeability, soil (a) The ease with which gases, liquids, or plant roots penetrate or pass through a bulk mass of soil or a layer of soil. Since different soil horizons vary in permeability, the particular horizon under question should be designated. (b) The property of a porous medium itself that expresses the ease with which gases, liquids, or other substances can flow through it, and is the same as intrinsic permeability k. Compare Darcy’s law, intrinsic permeability, and soil water.
permeameter A device for confining a sample of soil or porous medium and subjecting it to fluid flow, in order to measure the hydraulic conductivity or intrinsic permeability of the soil or porous medium for the fluid.
permittivity Refer to dielectric constant.
Perox [soil taxonomy] A suborder of Oxisols that are well drained and have a perudic soil moisture regime (USDA, 1999).
perudic [soil taxonomy] A subset of the udic soil moisture regime in which water moves through the soil (i.e., precipitation exceeds evapotranspiration) in all months of normal years when the soil is not frozen. The distinction between perudic and udic can be made at the series level (USDA, 1999). Refer to udic soil moisture regime.
petrocalcic horizon [soil taxonomy] A diagnostic, continuous, cemented or indurated horizon that is cemented by calcium carbonate and, in some places, with magnesium carbonate. It cannot be penetrated with a spade or auger when dry, dry fragments do not slake in water, and it is impenetrable to rootsexcept in cracks (USDA, 1999).
petroferric contact [soil taxonomy] A diagnostic boundary between soil and a continuous layer of indurated soil in which iron is an important cement. Contains little or no organic matter.
petrogypsic horizon [soil taxonomy] A diagnostic continuous, strongly cemented, massive, gypsic horizon that is cemented by calcium sulfate. It can be chipped with a spade when dry. Dry fragments do not slake in water, and it is impenetrable to roots , except in cracks (USDA, 1999).
pH, soil The pH of a solution in equilibrium with soil. It is determined by means of a glass, quinhydrone, or other suitable electrode or indicator at a specified soil– solution ratio in a specified solution, usually distilled water, 0.01 M CaCl2, or 1 M KCl.
pH-dependent charge The portion of the cation or anion exchange capacity that varies with pH. Compare acidity, residual and variable charge.
phagotophic Form of feeding where micro-and mesofauna (i.e., protozoa) engulf particulate nutrients, such as bacteria or organic fragments.
phase A utilitarian grouping of soils defined by soil or environmental features that are not class differentia used in U.S. system of soil taxonomy, for example, surface texture, surficial rock fragments, rock outcrops, substratum, special soil water conditions, salinity, physiographic position, erosion, thickness, etc. Phase identifications are introduced into soil names by adding them to a taxon name as modifiers. Compare taxon, component soil and taxon.
phase lag The time difference between the maximum temperature at one depth and the maximum temperature at a second depth.
phosphate In fertilizer trade terminology, phosphate is used to express the sum of the water-soluble and the citrate-soluble phosphoric acid (P2O5); also referred to as the available phosphoric acid (P2O5).
phosphate rock A microcrystalline, calcium fluorophosphate of sedimentary or igneous origin of varying P content. It is usually concentrated and solubilized to be used directly or concentrated in manufacture of commercial phosphate fertilizers.
phosphobacteria Bacteria able to convert organic phosphorus into orthophosphate.
phosphoric acid In commercial fertilizer manufacturing, it is used to designate orthophosphoric acid, H3PO4. In fertilizer labeling, it is the common term used to represent the phosphate concentration in terms of available P, expressed as percentage of P2O5.
phosphorus fixation (no longer used in SSSA publications) The immobilization of phosphorus by strong adsorption or precipitation.
photolithotroph An organism that uses light as a source of energy and CO2 or carbonates as the source of carbon for cell biosynthesis. Compare autotroph.
photomap A mosaic map made from aerial photographs with physical and cultural features, marginal data, and other map information as shown on a planimetric map.
phototropic The response of a biological organism to the presence of light.
phreatic level (surface) Free water surface where soil water is at atmospheric pressure.
phyllosilicate mineral terminology Phyllosilicate minerals have layer structures composed of shared octahedral and tetrahedral sheets. Compare Appendix I, Table A3.interlayer Materials between structural layers of minerals, including cations, hydrated cations, organic molecules, and hydroxide octahedral groups and sheets.layer A combination of sheets in a 1:1 or 2:1 assemblage.plane (of atoms) A flat (planar) array of atoms of one atomic thickness. Example: plane of basal oxygen atoms within a tetrahedral sheet.sheet (of polyhedra) Flat array of more than one atomic thickness and composed of one level of linked coordination polyhedra. A sheet is thicker than a plane and thinner than a layer. Example: tetrahedral sheet, octahedral sheet.unit structure The total assembly of a layer plus interlayer material.
phyllosphere The surface of aboveground living plant parts.
physical nonequilibrium Soil condition in which movement of solute occurs between flow regions, for example, macropore – micorpore, by diffusion as a result of a gradient in solute concentration between the regions.
physical properties (of soils) Those characteristics, processes, or reactions of a soil that are caused by physical forces and that can be described by, or expressed in, physical terms or equations. Examples of physical properties are bulk density, hydraulic conductivity, porosity, pore-size distribution, etc.
physical weathering The breakdown of rock and mineral particles into smaller particles by physical forces such as frost action, salt-crystal growth, absorption of water, etc., involving little or no chemical change. Synonym mechanical weathering. Compare weathering.
Physiographic Division A large portion of a continent of which all parts are similar in geologic structure and climate at a small scale (e.g., 1:5,000,000) and which has consequently had a unified geomorphic history and whose pattern of relief or landforms differ significantly from that of adjacent areas. Examples: the Laurentian Upland, Rocky Mountain System, and Interior Highlands of the USA (Schoeneberger and Wysocki, personal communication, 2013).
Physiographic Province A region of which all parts are similar in geologic structure and climate and which has consequently had a unified geomorphic history; a region whose pattern of features or landforms differ significantly from that of adjacent regions; that is a subset within a Physiographic Division. Examples: the Valley and Ridge, Blue Ridge, and Piedmont provinces in the eastern USA, and the Basin and Range, Rocky Mountains, and Great Plains provinces in the western USA ( Jackson, 1997).
Physiographic Section An area which all parts are similar in geologic structure and climate at a relatively small scale and which has consequently had a unified geomorphic history, and whose pattern of relief or landforms differ significantly from that of adjacent areas [= Fenneman’s (1957) “Section”]; that is, a subset within a Physiographic Province). Examples: the Mohawk, Green Mountain, and Floridian Sections in the eastern USA and the Sacramento Section, Puget Trough, and Klamath Mountains in the western USA (Schoeneberger and Wysocki, personal communication, 2013).
physiosorption (no longer used in SSSA publications) The process of attachment of non-ionic substances such as polar water molecules, acetic acid molecules, or nucleic acids to clays or to other solid-phase surfaces. The attachment of large molecules to clay particles by ionic processes is not physiosorption.
phytoliths Inorganic bodies derived from replacement of plant cells; they are usually opaline.
phytometer A plant or plants used to measure the physical factors of the habitat in terms of physiological activities.
phytomorphic soils (Canada) Well-drained soils of an association which that developed under the dominant influence of the natural vegetation characteristic of a region. The zonal soils of an area.
phytotoxic The property of a substance at a specified concentration that restricts or constrains plant growth.
piedmont (adjective) Lying or formed at the base of a mountain or mountain range; for example, a piedmont terrace or a piedmont pediment ( Jackson, 1997).
piedmont slope (colloquial – western USA) The dominant gentle slope at the foot of a mountain; generally used in terms of intermontane-basin terrain in arid to subhumid regions. Main components include:(a) An erosional surface on bedrock adjacent to the receding mountain front (pediment, rock pediment);(b) A constructional surface comprising individual alluvial fans and interfan valleys, also near the mountain front; and (c) A distal complex of coalescent fans (bajada), and alluvial slopes without fan form. Piedmont slopes grade to basin-floor depressions with alluvial and temporary lake plains or to surfaces associated with through drainage (e.g., axial streams; Hawley and Parsons, 1980). Compare bolson, fan piedmont.
piezometer An open-ended tube that measures the pressure head at the point of opening.
piezometer head The elevation at which water stands in a piezometer with respect to a point in question in the soil. Refer to pressure potential (pressure head).
pillow lava A general term for lava displaying pillow structure (discontinuous, close-fitting, bun-shaped or ellipsoidal masses, generally <1 m in diameter); considered to have formed in a subaqueous environment; such lava is usually basaltic or andesitic (Schoeneberger and Wysocki, personal communication, 2013; Jackson, 1997; Green and Short, 1971). Compare aa lava, block lava, pahoehoe lava.
pillow lava flow A lava flow or body displaying pillow structure and considered to have formed in a subaqueous environment (underwater); usually basaltic or andesitic in composition (Schoeneberger and Wysocki, personal communication, 2013; Green and Short, 1971). Compare aa lava flow, block lava flow, pahoehoe lava flow.
pimple mound (colloquial: Gulf Coast USA) Low, flattened, approximately circular or elliptical features composed of sandy loam that is coarser than, and distinct from, the surrounding soil; the basal diameter ranges from 3 m to more than 30 m, and the height from 30 cm to more than 2 m. Compare mima mound, patterned ground, shrub-coppice dune ( Jackson, 1997).
pingo A large frost mound; especially a relatively large conical mound of soil-covered ice (commonly 30 to 50 m high and up to 400 m in diameter) raised in part by hydrostatic pressure within and below the permafrost of Arctic regions, and of more than 1 year’s duration ( Jackson, 1997).
pinnacle [geomorphology] A tall, slender, tapering tower or spire-shaped pillar of rock, either isolated, as on steep slopes or cliffs formed in karst or other massive rocks, or at the summit of a hill or mountain (Schoeneberger and Wysocki, personal communication, 2013; Jackson, 1997; White, 1988). Compare erosion remnant, hoodoo.
pinnate drainage pattern A variation of the dendritic drainage pattern in which the main stream receives many closely spaced, subparallel tributaries that join it at slightly acute angles upstream, resembling in plan a feather. They typically form on steep slopes with soils that have a high silt content; such as loess landscapes or fine-textured flood plains (Schoeneberger and Wysocki, personal communication, 2013; Jackson, 1997; Way, 1973).
pipe flow (piping) A preferential flow process in which water flows rapidly through a large discrete pore causing tunnel erosion.
pipette analysis A sedimentation procedure that utilizes pipette sampling at controlled depths and times.
piston flow A jump in effluent from background concentration to the input concentration at 1 pore volume of flow.
pit and mound topography (not recommended) Use tree-tip pit and mound topography. Complex microrelief created by numerous cradle knolls and their attendant pits. Usually associated with forested sites or cleared sites that have not been plowed. Compare microrelief.
pits [soil survey] A miscellaneous area (map unit) applied to open excavations from which soil and commonly, underlying material have been removed exposing either rock or other material that supports few or no plants; includes mine pits, gravel pits, and quarry pits. A miscellaneous area (USDA, 1993).
pitted outwash Outwash deposits with surficial pits or kettles, produced by the partial or complete burial of glacial ice by outwash and the subsequent thaw of the ice and collapse of the surficial materials ( Jackson, 1997). Compare pitted outwash plain.
pitted outwash plain An outwash plain marked by many irregular depressions such as kettles, shallow pits, and potholes which formed by melting of incorporated ice masses; much of the gradient and internal structures of the original plain remain intact; many are found in Wisconsin, Minnesota, Michigan, and Indiana. Compare collapsed outwash plain, outwash, pitted outwash ( Jackson, 1997).
pitted outwash terrace A relict glaciofluvial terrace that retains its original attitude, composed of undistorted outwash sediments and depositional structures and whose surface is pock-marked with numerous potholes or kettle depressions. Compare collapsed outwash plain (Schoeneberger and Wysocki, personal communication, 2013).
placic horizon [soil taxonomy] A diagnostic, black to dark reddish mineral soil horizon that is usually thin but that may range from 1 to 25 mm in thickness. The placic horizon is commonly cemented with iron and is slowly permeable or impenetrable to water and roots (USDA, 1999; Appendix 1).
plaggen epipedon [soil taxonomy] A diagnostic, man-made surface horizon more than 50 cm thick that is formed by long-continued manuring and mixing.
Plaggepts [soil taxonomy] (obsolete; not used in current soil taxonomy) Prior to 2000, this was a suborder of Inceptisols that have a plaggen epipedon (USDA, 1999).
plagioclase feldspar Framework silicates with Al substituting for Si with accompanying Na and/or Ca.
plain A general term referring to any broad, flat lowland area, large or small of low relief. Specifically, any extensive region of comparatively smooth and level to gently undulating land. A plain has few or no prominent hills or valleys but sometimes has considerable slope, and usually occurs at low elevation relative to surrounding areas. Where dissected, remnants of a plain can form the local uplands. A plain may be forested, cropped, grasslands or barren and may be formed by deposition or erosion. Compare lowland, plateau (Schoeneberger and Wysocki, personal communication, 2013; Jackson, 1997).
plane (of atoms) Refer to phyllosilicate mineral terminology.
Planosol [soil classification] (obsolete; not used in soil taxonomy) A great soil group of the intrazonal order and hydromorphic suborder consisting of soils with eluviated surface horizons underlain by B horizons more strongly eluviated, cemented, or compacted than associated normal soil (Baldwin et al., 1938).
plant analysis The determination of the nutrient concentration in plants or plant parts with analytical procedures.
plant food The inorganic compounds elaborated within a plant to nourish its cells; a frequent synonym for plant nutrients, particularly in the fertilizer trade.
plant growth-promoting rhizobacteria Diverse group of rhizosphere bacteria that impart beneficial effects on plant growth as root colonizers.
plant nutrient An element that is absorbed by plants and is necessary for completion of the normal life cycle. These include C, H, O, N, P, K, Ca, Mg, S, Cu, Fe, Zn, Mn, B, Cl, Ni, and Mo.
plasma That part of the soil material that is capable of being or has been moved, reorganized, and/or concentrated by the processes of soil formation. It includes all the material, mineral or organic, of colloidal size and relatively soluble material that is not contained in the skeleton grains.
plasmic fabric The arrangement of plasma, skeleton grains, and associated simple packing voids.
plasmids Extrachromosomal DNA.
plastic limit The minimum water mass content at which a small sample of soil material can be deformed without rupture. Synonymous with “lower plastic limit.” Compare Atterberg limits, consistency, liquid limit, and plasticity number.
plastic soil A soil capable of being molded or deformed continuously and permanently, by relatively moderate pressure, into various shapes. Compare consistence.
plasticity range The range of water mass content within which a small sample of soil exhibits plastic properties.
plasticity constants Refer to Atterberg limits, consistency, liquid limit, plastic limit, and plasticity number.
plasticity number The numerical difference between the liquid and the plastic limit or, synonymously, between the lower plastic limit and the upper plastic limit. Sometimes called “plasticity index.” Compare Atterberg limits, consistency, liquid limit, and plastic limit.
plate count A count of the number of colonies formed on a solid culture medium when inoculated with a small amount of soil. The technique has been used to estimate the number of certain organisms present in the soil sample.
plateau [geomorphology] A comparatively flat area of great extent and elevation; specifically an extensive land region considerably elevated (more than 100 m) above adjacent lower-lying terrain, and is commonly limited on at least one side by an abrupt descent, has a flat or nearly level surface. A comparatively large part of a plateau surface is near summit level. Compare hill, foothill, mountain, mesa, plain ( Jackson, 1997).
platy Consisting of soil aggregates that are developed predominantly along the horizontal axes; laminated; flaky.
platy soil structure A type of soil structure. Compare soil structure and soil structure types.
playa The usually dry and nearly level lake plain that occupies the lowest parts of closed depressions, such as those occurring on intermontane basin floors. Temporary flooding occurs primarily in response to precipitation-runoff events. Playa deposits are fine grained and may or may not have high water table and saline or sodic conditions. Compare flood-plain playa, playas [Soil Survey] (Hawley and Parsons, 1980).
playa dune (colloquial – Southern High Plains) A linear or curvilinear ridge of windblown, granular material (generally sand or parna) removed from the adjacent basin by wind erosion (deflation), and deposited on the leeward (prevailing downwind) margin of a playa, playa basin, or salina basin. The dune may be barren or vegetated (Schoeneberger and Wysocki, personal communication, 2013). Compare dune.
playa floor (colloquial – Southern High Plains) The lowest extensive, flat to slightly concave surface within a playa basin, consisting of a dry lake bed or lake plain underlain by stratified clay, silt or sand, and commonly by soluble salts (Schoeneberger and Wysocki, personal communication, 2013). Compare playa step.
playa lake A shallow, intermittent lake in a arid or semi-arid region, covering or occupying a playa in the wet season but drying up in summer; an ephemeral lake that upon evaporation leaves or forms a playa ( Jackson, 1997).
playa rim (colloquial – Southern High Plains) The convex, upper margin (shoulder) of a playa basin where the playa slope intersects the surrounding terrain (Schoeneberger and Wysocki, personal communication, 2013). Compare playa slope.
playa slope (colloquial – Southern High Plains) The generally concave to slightly convex area within a playa basin that lies between the relatively level playa floor below (or playa step, if present) and the convex playa rim above. Overland flow is typically parallel down slope (Schoeneberger and Wysocki, personal communication, 2013). Compare playa step, playa rim.
playa step (colloquial – Southern High Plains) The relatively level or gently inclined “terrace-like” bench or toeslope within a large playa basin flanking and topographically higher than the playa floor and below the playa slope; a bench or step-like surface within a playa basin that breaks the continuity of the playa slope and modified by erosion and/or deposition. Temporary ponding may occur in response to precipitation/runoff events (Schoeneberger and Wysocki, personal communication, 2013). Compare playa slope.
playas [soil survey] A miscellaneous area (map unit) applied to dominantly barren, saline or sodic lake beds in semi-arid or arid regions; largely obsolete – now recognized as wet soils (USDA, 1993). Refer to playa.
playette A very small, playa-like, shallow, closed depression typically with a salt-encrusted surface, little or no vegetation in semi-arid to arid climates and infrequently subject to ponding from precipitation events; commonly lacks the component parts of a playa except for a small playa floor (Schoenenberger and Wysocki, personal communication, 2013; Gile et al., 1981). Compare playa.
Pleistocene The epoch of the Quaternary Period of geologic time (from about 10 to 12 thousand to 2.6 million years ago), that follows the Pliocene Epoch and precedes the Holocene; also the corresponding (time-stratigraphic) “series” of earth materials (Schoeneberger and Wysocki, personal communication, 2013; Hawley and Parsons, 1980).
plinthite A weakly cemented iron-rich, humus-poor mixture of clay with other diluents that commonly occurs as dark red redox concentrations that form platy, polygonal, or reticulate patterns. Plinthite changes irreversibly to ironstone hardpans or irregular aggregates on exposure to repeated wetting and drying.
Pliocene The last epoch of the Tertiary Period of geologic time (from 2.6 to 5.3 million years ago), that follows the Miocene Epoch and precedes the Pleistocene Epoch; also, the corresponding (time-stratigraphic) “series” of earth materials (Schoeneberger and Wysocki, personal communication, 2013; Hawley and Parsons, 1980).
plow layer Refer to tillage, plow layer.
plow pan Refer to tillage, plow pan.
plow-planting Refer to tillage, plow-planting.
plowing Refer to tillage, plowing.
plowless farming Refer to tillage, plowless farming.
plug [volcanic] Not preffered – use volcanic plug.
plug dome A volcanic dome characterized by an upheaved, consolidated conduit filling ( Jackson, 1997).
pluton A deep-seated igneous intrusion ( Jackson, 1997).
plutonic Pertaining to igneous rocks formed at great depth, but also including associated metamorphic rocks ( Jackson, 1997).
pluvial lake A lake formed in an extended period of exceptionally heavy rainfall (Schoeneberger and Wysocki, personal communication, 2013; Jackson, 1997). Compare also pluvial lake [relict], glacial lake, proglacial lake.
pneumatic pressure Air pressure above atmospheric applied to soil to impart an equivalent soil water matric potential.
pocosin (colloquial: southeastern USA) A large, raised wet area, commonly a swamp, on broad, nearly level interfluves in the Atlantic coastal plain usually containing organic soil A Native American term for “swamp on a hill”. Not to be confused with Carolina Bays of the southeastern United States (Raymond Daniels, personal communication, 2013). Compare raised bog.
Podzol [soil classification] (obsolete; not used in soil taxonomy) A great soil group of the zonal order consisting of soils formed in cool-temperate to temperate, humid climates, under coniferous or mixed coniferous and deciduous forest, and characterized particularly by a highly leached, whitish-gray (Podzol) A2 (E) horizon (Baldwin et al., 1938).
podzolization A process of soil formation resulting in the genesis of Podzols and Podzolic soils.
point bar One of a series of low, arcuate ridges of sand and gravel developed on the inside of a growing meander by the slow addition of individual accretions accompanying migration of the channel toward the outer bank ( Jackson, 1997). Compare meander scroll.
point of zero net charge (pznc) The pH value of a solution in equilibrium with a variable charge material or mixture of materials whose net charge from all sources is zero (i.e., anion exchange capacity = effective cation exchange capacity). It is often determined for soils that are low in permanent charge minerals and high in oxides and hydrous oxides of Fe and Al.
Poiseuille's law The law governing flow in an individual tube or pipe in which the flow rate is proportional to the product of the pressure drop per unit distance and the tube radius to the fourth power.
Poiseuille’s law The law governing flow in an individual tube or pipe in which the flow rate is proportional to the product of the pressure drop per unit distance and the tube radius to the fourth power.
polder [coast] A generally fertile tract of flat, low-lying coastal area that is at or below sea level but has been reclaimed and is constantly protected from the sea, or other body of water by an organized system of embankments, dikes, dams, or levees; e.g., a brackish marsh that has been drained and brought under cultivation (Schoeneberger and Wysocki, personal communication, 2013; J. Galbraith, ICOMANTH, personal communication, 2011).
polje (not preferred) refer to synonym: interior valley.
pollution The presence or introduction of a pollutant into the environment.
polygon A type of patterned ground consisting of a closed, roughly equidimensional figure bounded by more or less straight sides; some sides may be irregular. Refer to patterned ground (National Research Council of Canada, 1988). Compare High center polygon, low center polygon, ice wedge polygon, nonsorted polygon.
polymerize To combine two or more molecules of a compound to form a more complex compound with a higher molecular weight.
polymorphism Crystallization into two or more chemically identical but crystallographically distinct forms.
polypedon A group of contiguous similar pedons. The limits of a polypedon are reached at a place where there is no soil or where the pedons have characteristics that differ significantly.
pond (a) A natural body of standing fresh water occupying a small surface depression, usually smaller than a lake and larger than a pool. (b) A small artificial body of water, used as a source of water ( Jackson, 1997). Compare salt pond.
ponding Process through which water stands on the soil surface.
pool A small, natural body of standing water, usually fresh; for example, a stagnant body of water in a marsh, or a transient puddle in a depression following a rain ( Jackson, 1997). Compare pond.
poorly drained A drainage class referring to soils that have evidence (e.g., mottles) of seasonal water tables at depths between 0 and 20 cm (0 and 8 in).
poorly graded Soil material, usually sand or gravel, with a narrow range of particle sizes.
porcellanite A dense, siliceous rock formed as a indurated or baked clay or shale with a dull, light-colored, cherty appearance, often found in the roof or floor of a burned-out coal seam ( Jackson, 1997).
pore water velocity The velocity at which water travels in pores relative to a given axis. It is equal to the flux density divided by the soil water content.
pore ice Frozen water in the interstitial pores of a porous medium.
pore space The portion of soil bulk volume occupied by soil pores.
pore volume Refer to pore space.
pore-size distribution The volume fractions of the various size ranges of pores in a soil, expressed as percentages of the soil bulk volume (soil particles plus pores). Compare Table 3.
porosity The volume of pores in a soil sample (nonsolid volume) divided by the bulk volume of the sample.
porous trickle tubing Refer to irrigation, trickle.
postglacial (not preferred) Use Holocene.
potash Term used to refer to potassium or potassium fertilizers and usually designated as K2O.
potassium fixation The process of converting exchangeable or water-soluble potassium to that occupying the position of K+ in the micas. They are counter-ions entrapped in the ditrigonal voids in the plane of basal oxygen atoms of some phyllosilicates as a result of contraction of the interlayer space. The fixation may occur spontaneously with some minerals in aqueous suspensions or as a result of heating to remove interlayer water in others. Fixed K+ ions are exchangeable only after expansion of the interlayer space. Compare ammonium fixation.
potential energy Energy due to position in space.
pothole [geomorphology] (not preferred) A generic, imprecise term for any pot-shaped pit or hole ( Jackson, 1997).
Prairie soils [soil classification] (obsolete; not used in soil taxonomy). A zonal great soil group consisting of soils formed under temperate to cool-temperate, humid regions under tall grass vegetation (Baldwin et al., 1938). Synonym:-Brunizems.
precipitation interception The stopping, interrupting, or temporary holding of descending precipitation in any form by mulch, a vegetative canopy, vegetation residue or any other physical barrier.
predation A relationship between two organisms whereby one organism (predator) engulfs and digests the second organism (prey).
preferential flow The process whereby free water and its constituents move by preferred pathways through a porous medium. Also called bypass flow.
preplant irrigation Refer to irrigation, preplant irrigation.
pressure membrane A membrane, permeable to water and only very slightly permeable to gas when wet, through which water can escape from a soil sample in response to a pressure gradient.
pressure head Refer to pressure potential.
pressure potential (pressure head) Potential energy of soil water due to the weight of water (hydrostatic pressure) on the point of interest. Pressure head is expressed as energy per weight and is equal to the height of water between the point of interest and the free water surface (water table), hp, also termed the piezometric head. Pressure potential is expressed as energy per unit volume and equals the piezometric head, hp, the water density, r, and the gravitation constant, g (rghp).
pressure ridge [ice] A rugged, irregular wall of broken floating ice buckled upward by the lateral pressure of wind or current forcing or squeezing one floe against another, or against a shore; it may extend for kilometers in length and up to 30 m in height. Along shores they are lower (<10 m tall) and contribute to the temporary or permanent formation of a beach berm or a rim of boulders and stones (Schoeneberger and Wysocki, personal communication, 2013; Jackson, 1997).
primary mineral A mineral that has not been altered chemically since deposition and crystallization from molten lava. Compare secondary mineral.
primary nutrients Refers to N, P, and K in fertilizers. Compare macronutrient.
priming effect Stimulation of microbial activity in soil, usually organic matter decomposition, by the addition of labile organic matter.
prismatic soil structure A type of soil structure. Compare soil structure and soil structure types.
procaryotes Refer to prokaryotes.
productive capacity Refer to soil productivity.
productivity, soil The output of a specified plant or group of plants under a defined set of management practices.
profile, soil A vertical section of the soil through all its horizons and extending into the C horizon.
proglacial lake A type of glacial lake which formed just beyond the margin of an advancing or retreating glacier; generally in direct contact with the ice ( Jackson, 1997). Compare glacial lake, pluvial lake.
prokaryotes (procaryotes) A cell or organism lacking a true nucleus.
propagule Any cell unit capable of developing into a complete organism. For fungi, the unit may be a single spore, a cluster of spores, hyphae, or a hyphal fragment.
protocooperation An association of mutual benefit to two or more species but without the cooperation being obligatory for their existence or the performance of some function.
proximal [sedimentology] (adjective) Said of a sedimentary deposit consisting of coarse clastics and deposited nearest the source area. Compare distal.
Psamments [soil taxonomy] A suborder of Entisols that are sandy in all layers within the particle-size control section. Psamments occur under any climate, but cannot have permafrost within 100 cm of the soil surface. Ground water typically is deeper than 50 cm and commonly is much deeper. Psamment are not saturated with water for periods long enough to limit their use for most crops. They have a relatively low water-holding capacity. Those that are bare and become dry are subject to soil blowing and drifting and cannot easily support wheeled vehicles (USDA, 1999).
pseudomonads Members of the genus Pseudomonas, a large group of gram-negative, obligately respiratory (never fermentative) bacteria.
psychrophile Refer to psychrophilic organism.
psychrophilic organism An organism whose optimum temperature for growth falls in the approximate range of 5 to 15°C. Synonymous with cryophile.
puddling Any process involving both shearing and compactive forces that destroys natural structure and results in a condition of greatly reduced pore space.
puff [gilgai] A surface drape or exposure of up-welled substratum material (e.g., carbonates) forced to the surface and crops out on a low mound or rim (e.g., microhigh); the surface exposure of a chimney [gilgai]; a type of diapir composed of earthy material (Schoeneberger and Wysocki, personal communication, 2013). Compare chimney, microslope, gilgai.
pumice (a) [soils] Rock or pararock fragments > 2 mm in diameter (i.e., retained upon a 2 mm sieve), or coherent rock layers (pumice flow), made of light-colored, vesicular, glassy rock commonly having the composition of rhyolite. The material commonly has a specific gravity of<1.0 and is thereby sufficiently buoyant to float on water (Schoeneberger and Wysocki, personal communication, 2013); pumice-like fragments < 2 mm in size are called pumiceous ash. (Soil Survey Staff, 1975); Compare scoria, tephra; (b) [geology] same as (a) but does not include any size restrictions (Schoeneberger and Wysocki, personal communication, 2013).
pump testing A test made on a well by pumping it down for a period of time and observing the change in hydraulic head to determine the hydraulic properties of the surrounding media. Also called a draw down test or aquifer test.
pure culture A population of microorganisms composed of a single strain. Such cultures are obtained through selective laboratory procedures and are rarely found in a natural environment.
pyroclastic (adjective) Pertaining to clastic rock particles produced by explosive, aerial ejection from a volcanic vent. Such materials may accumulate on land or under water (Hawley and Parsons, 1980). Compare epiclastic, volcaniclastic, clastic.
pyroclastic flow A fast density current of pyroclastic material, usually very hot, composed of a mixture of gasses and a high concentration of pyroclastic particles in a variety of sizes and composition (ash, pumice, scoria, lava fragments, etc.); produced by the explosive disintegration of viscous lava in a volcanic crater, collapse of an eruption column, or by the explosive emission of gas-charged ash from a fissure and which tends to follow topographic lows (e.g., valleys) as it moves; used in a more general sense than ash flow (Schoeneberger and Wysocki, personal communication, 2013; Sigurdsson, 2000; Jackson, 1997). Compare pyroclastic surge, ash flow, nueé ardente, lahar.
pyroclastic surge A low density, dilute, turbulent pyroclastic flow, usually very hot, composed of a generally unsorted mixture of gases and comparatively low concentrations of pyroclastic particles (ash, pumice, and dense rock fragments) that travels across the ground at high speed and less constrained by topography than a pyroclastic flow; several types of pyroclastic surges can be specified (e.g., base surge, ash-cloud-surge; Schoeneberger and Wysocki, personal communication, 2013; Sigurdsson, 2000; Jackson, 1997). Compare pyroclastic flow.
pyroclastics A general term applied to detrital volcanic materials that have been explosively or aerially ejected from a volcanic vent.
pyrophosphate A class of phosphorus compounds produced by the reaction of either anhydrous ammonia or potassium hydroxide with pyrophosphoric acid (H4P2O7). Pyrophosphoric acid is a condensation product of two molecules of orthophosphoric acid (H3PO4). The main polyphosphate species in poly-phosphate fertilizers.
pyrophyllite Si4Al2O10(OH)2. An aluminosilicate mineral with a 2:1 layer structure but without isomorphous substitution. It is dioctahedral. Compare Appendix I, Table A3.
pyroxene A single chain ferromagnesian silicate.