Glossary of Terms in Soil Science

F layer

See horizon; soil

fabric (soil)

The physical constitution of a soil material expressed by the spatial arrangement of the solid particles and associated voids.

facultative anaerobe

A microorganism that lives under either aerobic or anaerobic conditions.

fallow land

Cultivated land that is not being used for a crop.

family; soil

A category in the Canadian system of soil classification. Differentiae are primarily texture, drainage, thichness of horizons, permeability, mineralogy, consistence and reaction.


Anaerobic oxidation of carbohydrates and carbohydratelike compounds by enzyme action of microorganisms; gaseous oxygen is not involved in this energy-yielding process.

Ferro-Humic Podzol

A great group of soils in the Podzolic order. The upper 10 cm (4 inches) of the Bhf horizon contain 5% or more organic carbon, 0.6% or more pyrophosphate-extractable Al and Fe, and either a ratio of organic carbon to pyrophosphate-extractable Fe of less than 20 or a percentage of pyrophosphate-extractable Fe greater than 0.3 or both. The B horizon is usually overlain by a light-colored, eluviated horizon (Ae) and a mor humus layer.

fertility (soil)

The status of a soil in relation to the amount and availability to plants of elements necessary for plant growth.


Any organic or inorganic material of natural or synthetic origin that is added to a soil to supply certain elements essential to the growth of plants.

fertilizer grade

The guaranteed minimum analysis in percent of the major plant nutrient elements contained in a fertilizer material or in a mixed fertilizer. The analysis usually gives the percentages of N, P2O5 and K2O but proposals have been made to change the designation to the percentages of N, P and K.

fertilizer requirement

The quantity of certain plant nutrient elements needed in addition to the amount supplied by the soil to increase plant growth to a designated optimum.

fibric layer

A layer of organic soil material containing large amounts of weakly decomposed fiber whose botanical origin is readily identifiable.


A great group of soils in the Organic order that are saturated for most of the year. The soils have a dominantly fibric middle tier or middle and surface tiers if a terric, lithic, hydric or cryic contact occurs in the middle tier.

field capacity

The percentage of water remaining in the soil 2 or 3 days after the soil has been saturated and free drainage has practically ceased. The percentage may be expressed in terms of weight or volume. See also moisture tension, soil

fifteen-atmosphere percentage

fifteen-atmosphere percentage See moisture tension; soil.

fifteen-bar percentage

fifteen-bar percentage See moisture tension; soil.

film water

film water A layer of water that surrounds soil particles and varies in thickness from 1 or 2 to 100 or more molecular layers. Usually it is considered to be the water that remains after drainage, because it is not distinguishable in saturated soils.

fine clay

A clay fraction of specified size less than 2 um usually less than 0.2 or 0.08 um.

fine earth

The fraction of mineral soil consisting of particles less than 2 mm in diameter.

fine sand

(1) A soil separate. See also separates, soil. (2) A soil textural class. See also texture, soil.

fine sandy loam

See texture; soil.

fine texture

Consisting of or containing large quantities of the fine fractions, particularly of silt and clay. It includes all the textural classes of clay loams and clays: clay loam, sandy clay loam, silty clay loam, sandy clay, silty clay, and clay. Sometimes it is subdivided into clayey texture and moderately fine texture. See also texture, soil.


A term used in soil mechanics for the portion of a soil finer than a No. 200 (74- µm) U.S. standard sieve.


A term describing the consistence of a moist soil that offers distinctly noticeabie resistance to crushing, but can be crushed with moderate pressure between the thumb and forefinger. See also consistence.

first bottom

The normal floodplain of a stream.


The process or processes in a soil by which certain chemical elements essential for plant growth are converted from a soluble or exchangeable form to a much less soluble or nonexchangeable form, for example, phosphate fixation. See also nitrogen fixation.

fixed phosphorus

(1) Phosphorus that has been changed to a less soluble form as a result of reaction with the soil; moderately unavailable phosphorus. Specifically, it is the quantity of soluble phosphorus compounds that, when added to soil, becomes chemically or biologically attached to the solid phase of soil and cannot be recovered by extracting the soil with a specified extractant under specified conditions. Some of these extractants are water, carbonated water, and dilute solutions of strong mineral acids with or without fluoride or other exchangeable anion. (2) Applied phosphorus that is not absorbed by plants during the first cropping year. (3) Soluble phosphorus that has become attached to the solid phase of the soil in forms highly unavailable to crops; unavailable phosphorus; or phosphorus in other than readily or moderately available forms.


A flexible, whiplike, appendage on cells used as an organ of locomotion.


See coarse fragments.


A thin fragment of sandstone, limestone, slate, shale, or rarely of schist, 15 to 37 cm (6 to 15 inches) long. See also coarse fragments.


See irrigation methods.


The land bordering a stream, built up of sediments from overflow of the stream and subject to inundation when the stream is at flood stage. See also first bottom.

flow velocity

The volume of water transferred per unit of time and per unit of area in the direction of the net flow of water in soil.

fluvial deposits

All sediments, past and present, deposited by flowing water, including glaciofluvial deposits. Wave-worked deposits and deposits resulting from sheet erosionand mass wasting are not included.


See glaciofluvial deposits.

foliar diagnosis

An estimation of the extent to which plants are getting certain necessary chemical elements from the soil based on an examination of the color and the growth habits of the foliage of the plants.


A great group of soils in the Organic order. The soils are not usually saturated for more than a few days a year and consist of 10 cm (4 inches) or more of L-H horizons derived from leaf litter, twigs, branches and mosses. A lithic contact or fragmented material occurs at a depth of less than 160 cm (64 inches). Mineral layers less than 10 cm (4 inches) thich may lie above the lithic contact.

forest floor

All dead vegetable and organic matter including litter and unincorporated humus on the mineral soil surface under forest vegetation.

forest soils

(1) Soils developed under forest vegetation. (2) (European usage) Soils formed in temperate climates under forest vegetation.


A Bx or BCx horizon of high bulk density and consistence that is firm and brittle when moist and hard to extremely hard when dry. Commonly it has bleached fracture planes separating very coarse prismatic units and frequently the secondary structure is platy. Usually the fragic horizon is similar in color to the parent material but differs from it in structure, consistence and bulk density. Air-dried clods slake when immersed in water. The upper boundary is usually abrupt and clear but the lower boundary is diffuse. The fragic horizon does not meet the criteria of a podzolic B horizon but may meet those of a Bt (Btx) horizon. See fragipan.


A natural subsurface horizon having a higher bulk density than the solum above, seemingly cemented when dry but showing moderate to weak brittleness when moist. The layer is low in organic matter, mottled and slowly or very slowly permeable to water. It usually has some polygon-shaped bleached cracks. It is found in profiles of either cultivated or virgin soils but not in calcareous material.


Rock and mineral particles larger than 2 mm occupy 90% or more of the soil (by volume).

free acidity

The titratable acidity in the aqueous phase of a soil. It may be expressed in milliequivalents per unit mass of soil or in other suitable units.

free oxides

Oxides and hydroxides of iron, aluminum, silicon, manganese, and titanium, usually of fine particle size, that occur uncombined with other elements and often as coatings on primary and secondary minerals.

freezing index; F degree-days

The number of degree-days between the highest and lowest points on the cumulative degree-days - time curve for one freezing season. It is used as a measure of the combined duration and magnitude of below-freezing temperature occurring during any given freezing season. The index determined for air temperatures 137.3 cm (4.5 ft) above the ground is commonly designated as the air freezing index, whereas that determined for temperatures immediately below a surface is called the surface freezing index.


A consistence term pertaining to the ease of crumbling of soils. See also consistence.

frost action

Freezing and thawing of moisture in materials and the resultant effects on these materials and on the structures of which they are a part or with which they are in contact.

frost heave

The raising of a surface caused by ice in the underlying soil.

fulvic acids

A term with various meanings that usually refers to the mixture of organic substances that remains in solution when a dilute alkali extract from the soil has been acidified.


The allophytic plants that lack chlorophyll and are filamentous in structure; molds.

furrow irrigation

See irrigation methods.