Glossary of Terms in Soil Science


Horizons or layers in soils that are strongly compacted, indurated, or very high in clay content. See also caliche, claypan, fragipan, genetic pan, iron pan, and pressure or induced pan.

parent material

The unconsolidated and more or less chemically weathered mineral or organic matter from which the solum of a soil has developed by pedogenic processes.

parent rock

The rock from which the parent materials of soils are formed.

partial sterilization

The elimination of a portion of a population of microorganisms, usually by heat or chemical treatment. The process is selective, and certain organisms or groups of organisms are destroyed to a greater extent than others.

particle density

The mass per unit volume of the soil particles. It is usually expressed in grams per cubic centimetre. Has been called grain density.

particle size

See: bulk density, soil.

particle-size analysis

The determination of the various amounts of the different separates in a soil sample, usually by sedimentation, sieving, micrometry, or combinations of these methods. Has been called grain-size analysis or mechanical analysis.

particle-size distribution

particle-size distribution

parts per million (ppm)

Weight units of any given substance per one million equivalent weight units of ovendry soil; or, for soil or other solutions, the weight units of solute per million weight units of solution.


Unconsolidated soil material consisting iargely of undecomposed or only slightly decomposed organic matter.

peat soil

An organic soil having no, or only slight, decomposition.


A unit of soil structure such as a prism, block, or granule, which is formed by natural processes, in contrast with a clod, which is formed artificially.


See genesis, soil(1.)


The aspects of soil science dealing with the origin, morphology, genesis, distribution, mapping, and taxonomy of soils, and classification in terms of their use. See also science, soil.


A rugged area that was high at one time, but has been reduced by erosion to a low gently rolling surface resembling a plain.


The ease with which a probe can be pushed into the soil. It may be expressed in units of distance speed, force, or work, depending on the type of penetrometer used.

perched water table

A water table due to the ""perching"" of water on a relatively impermeable layer at some depth within the soil. The soil within or below the impermeable layer is not saturated with water.

percolation (of soil water)

The downward movement of water through soil; specifically, the downward flow of water in saturated or nearly saturated soil at hydraulic gradients of 1.0 or less.


Indicative of all cold-climate processes, whether or not they occur in the immediate vicinity of glaciers.


(1) Perennially frozen material underlying the solum. (2) A perennially frozen soil horizon.

permafrost table

The upper boundary of permafrost, usually coincident with the lower limit of seasonal thaw. See also permafrost (1.).

permanent charge

The net negative or positive charge of clay particles inherent in the crystal lattice of the particle. It is not affected by changes in pH or by ion-exchange reactions.

permeability (soil )

(1) The ease with which gases and liquids penetrate or pass through a bulk mass of soil or a layer of soil. Because different soil horizons vary in permeability, the specific horizon should be designated. (2) The property of a porous medium that relates to the ease with which gases or liquids can pass through it. Previously, it was considered to be the ""k"" in Darcy's law. It is the ""K"" in intrinsic permeability. See also intrinsic permeability, Darcy's law and water / soil.


Chemicals that kill organisms that are injurious to man or to the crops and animals upon which he depends for food, fiber and shelter. These organisms include insects, mites, microorganisms, weeds and rodents. Pesticides include insecticides, fungicides, herbicides and others.

pF (Obsolete)

The logarithm of the soil-moisture tension measured by the height in centimetres of a column of water that produces a tension of equal force.

pH (soil)

The negative logarithm of the hydrogen-ion activity of a soil. The degree of acidity or alkalinity of a soil as determined by means of a glass, quinhydrone, or other suitable electrode or indicator at a specified moisture content or soil-water ratio, and expressed in terms of the pH scale.

pH-dependent cation exchange capacity

The difference between the effective cation exchange capacity and the cation exchange capacity of a soil measured at a pH higher than that of its natural value.

phase (soil)

A subdivision of a soil type or other unit of classification having characteristics that affect the use and management of the soil, but that do not vary sufficiently to differentiate it as a separate type. A variation in a property or characteristic such as degree of slope, degree of erosion, or content of stones.


A mosaic map made from aerial photographs showing physical and cultural features as on a planimetric map. See also planisaic and toposaic.


Silicate structures in which the SiO4 tetrahedra are linked together in infinite two-dimensional sheets and are condensed with layers of AlO or MgO octahedra in the ratio 2:1 or 1:1. Isomorphous substitution of certain elements often occurs.

physical properties of soils

The characteristics, processes or reactions of a soil that are caused by physical forces, and are described by, or expressed in physical terms or equations. Sometimes physical properties are confused with and hard to separate from chemical properties; hence the terms "physical-chemical" or "physicochemical". Examples of physical properties are bulk density, water-holding capacity, hydraulic conductivity, porosity and pore-size distribution.

physical weathering

The breakdown of rock and mineral particles into smaller particles by physical forces such as frost action and wind. See also weathering.


The process of attachment of nonionic substances such as polar water molecules or acetic acid molecules or nucleic acids to clays or other solid-phase surfaces. The attachment of large molecules to clay particles by ionic processes is not physiosorption.

phytogenic soils (Obsolete)

Soils developed under the dominant influence of natural vegetation, mainly in temperate regions.


A piant or group of plants used to measure the physical factors of the habitat in terms of physiological activities.

phytomorphic soils

Well-drained soils of an association that have developed under the dominant influence of the natural vegetation characteristic of a region. The zonal soils of an area.


At or near the foot of a mountain.


An instrument for measuring the pressure head of liquids.

piezometric surface

The surface at which water will stand in a series of piezometers.


A slender structural element that is driven or introduced by other means into the soil usually to provide vertical or lateral support.


This layer (commonly <5 mm thick) or series of thin layers is irregular or involuted, hard, impervious, commonly vitreous and dark reddish brown to black; placic horizons may be cemented by Fe, Al-organic complexes (Bhfc or Bfc), hydrated Fe oxides (Bgfc) or a mixture of Fe and Mn oxides.


A photomap in which the planimetric detail is shown by overprints in color. See also photomap and toposaic.

plastic limit

(Atterberg limit) (1) The water content corresponding to an arbitrary limit between the plastic and the semisolid states of consistence of a soil. (2) The water content at which a soil will just begin to crumble when rolled into a thread approximately 3 mm in diameter.

plastic soil

A soil capable of being molded or deformed continuously and permanently into various shapes by moderate pressure. See also consistence.

plasticity constants

plasticity constants See liquid limit, plastic limit and plasticity number.

plasticity number

The numerical difference between the liquid limit and the plastic limit, or, synonymously, between the lower plastic limit and the upper plastic limit. Sometimes called ""plasticity index"".

plasticity range

The range of moisture weight percentage within which a small sample of soil exhibits plastic properties.

plate count

A count of the number of colonies formed on a culture medium that has been inoculated with a small amount of soil in order to estimate the number of certain organisms present in the soil sample.


Consisting of soil aggregates that have developed predominantly along the horizontal axes; laminated; flaky. See also structure types, soil.

plow pan (plow sole)

See pressure or induced pan.


Before 1968 Podzol was a great group in the Podzolic order that included soils having Ae and podzolic B horizons. See also Podzolic.


An order of soils having podzolic B horizons (Bh, Bhf or Bf) in which amorphous combinations of organic matter dominantly fulvic acid, Al and usually Fe are accumulated. The sola are acid and the B horizons have a high pH-dependent charge. See also: Podzolic Soil Classification.

podzolic B

Consists of one or more of Bf, Bhf, and Bh. See also horizon, soil.


A process of soil formation resulting in the genesis of Podzolic soils.

population / soil

All the organisms living in the soil including plants and animals.

pore space

The total space not occupied by soil particles in a bulk volume of soil.

pore-size distribution

The volume of the various sizes of pores in a soil. They are expressed as percentages of the buik voiume (soil plus pore space).

pores (soil)

The part of the bulk volume of soil not occupied by soil particles. Interstices or voids.


The volume percentage of the total bulk not occupied by solid particles.

potassium fixation

The process of converting exchangeable or water-soluble potassium to moderately soluble potassium; that is, to a form not easily exchanged from the adsorption compiex with a cation of a neutral salt solution.

potassium-supplying power of soils

The capacity of soil to supply potassium to growing plants from both the exchangeable and the moderately available forms.

potential acidity

In a soil, the amount of exchangeable hydrogen ion that can be made free or active in the soil solution by cation exchange. It is usually expressed in milliequivalents per unit mass of soil.

potential cation exchange capacity

The total number of cations that a soil can adsorb at a specified pH.

potential evapotranspiration

See evapotranspiration.

precipitation interception

The stopping, interrupting, and temporary holding of precipitation in any form by a vegetative canopy or vegetation residue.

preconsolidation pressure (or prestress)

The greatest effective pressure to which a soil has been subjected.

pressure membrane

A membrane, permeable to water and onliy very slightly permeable to gas when wet through which water can escape from a soil sample in response to a pressure gradient.

pressure or induced pan

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 other artificial means. it is also referred to as plow pan, plow sole, or traffic pan.

primary mineral

A mineral that has not been altered chemically since deposition and crystallization from molten lava. See also secondary mineral.

primary particles

Individual soil particles after a standard dispersion treatment.


A soil structure type having prismlike aggregates that have vertical axes much longer than the horizontal axes. See also structure types, soil.

productive soil

A soil in which the chemical, physical and biological conditions are favorable for the economical production of crops suited to a particular area.

productivity (soil)

The capacity of a soil in its normal environment to produce a specified plant or sequence of plants under a specified system of management. The ""specified"" limitations are needed because no soil can produce all crops with equal success and a single system of management cannot produce the same effect on all soils. Productivity means the capacity of soil to produce crops and is expressed in terms of yields.

profile (soil)

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


Unicellular organisms belonging to the animal kingdom.


Plants that grow best in or tolerate sand particularly fine to medium sand.


A microorganism capable of thriving at temperatures as low as 0°C.

pure culture

A nutrient medium containing the growth of a single strain of an organism free from other living species or strains.