Glossary of Terms in Soil Science

H layer

See horizon; soil.


The natural environment of an organism.

halomorphic soil

A general term for saline and alkali soils.

halophytic vegetation

Vegetation that grows naturally in soils having a high content of various salts. It usually has fleshy leaves or thorns and resembles desert vegetation.

hardpan layer

See pans.

heat capacity (volume)

The amount of heat required to raise the temperature of a unit volume of soil by one degree. It may also be expressed in terms of weight.

heat flux density

The quantity of heat flowing per unit of time across a unit area.

heavy clay

A textural class. See also texture, soil.

heavy soil

A soil having a high content of the fine separates, particularly clay, or a soil having a high drawbar pull and therefore hard to cultivate. See also fine texture.


Capable of deriving energy for life processes only from the decomposition of organic compounds and incapable of using inorganic compounds as sole sources of energy or for organic synthesis. See also autotrophic.

honeycomb frost

Ice in the soil in insufficient quantity to be continuous, thereby giving the soil an open, porous structure that readily permits water to enter.

horizon, soil

A layer of soil or soil material approximately parallel to the iand surface; it differs from adjacent genetically related layers in properties such as color, structure, texture, consistence, and chemical, bioiogical, and mineralogical composition. A list of the designations and some of the properties of soil horizons and layers foilows. More detailed definitions of some horizons and layers may be found in The Canadian System of Soil Classification".

Organic layers contain 17% or more organic carbon. Two groups of these layers are recognized:

An organic layer developed mainly from mosses, rushes, and woody materials.
least decomposed organic layer, containing large amounts of well-preserved fiber, and called the fibric layer.
An intermediately decomposed organic layer containing less fiber than an Of layer and called the mesic layer.
The most decomposed organic layer, containing only small amounts of raw fiber and calied the humic layer.
Organic layers developed primarily from leaves, twigs, and woody materials, with a minor component of mosses.
The original structures of the organic material are easily recognized.
The accumulated organic material is partly decomposed .
The original structures of the organic material are unrecognizable.
Mineral horizons and layers contain less than 17% organic carbon .
A mineral horizon formed at or near the surface in the zone of removal of materiais in solution and suspension, or maximum in situ accumulation of organic carbon, or both.
A mineral horizon characterized by one or more of the following:
  1. An enrichment in silicate clay, iron, aluminum, or humus.
  2. A prismatic or columnar structure that exhibits pronounced coatings or stainings associated with significant amounts of exchangeable sodium.
  3. An alteration by hydrolysis, reduction, or oxidation to give a change in color or structure from the horizons above or below, or both.
A mineral horizon comparatively unaffected by the pedogenic processes operative in A and B, except gleying, and the accumulation of carbonates and more soluble salts.
Underlying consolidated bedrock that is too hard to break with the hands or to dig when moist.
Roman numerals are prefixed to horizon designations to indicate unconsolidated lithologic discontinuities in the profile. Roman numeral I is understood for the uppermost material and usually is not written. Subsequent contrasting materials are numbered consecutively in the order in which they are encountered downward, that is, II, III, and so on.

Lowercase Suffixes

A buried soil horizon.
A cemented (irreversible) pedogenic horizon. Ortstein, placic, and duric horizons are examples.
A horizon of secondary carbonate enrichment where the concentration of lime exceeds that in the unenriched parent material.
horizon characterized by removal of clay, iron, aluminum, or organic matter alone or in combination and higher in color value by one or more units when dry than an underlying B horizon. It is used with A (Ae).
A horizon enriched with amorphous material, principally Fe and Al combined with organic matter. It usually has a chroma of 3 or more. The criteria for an f horizon except for Bgf are: it contains 0.6% or more pyrophosphate-extractable Fe plus Al in textures finer than sand and 0.4% or more in sands; the ratio of pyrophosphate-extractable Fe plus Al to clay (less than 2 Fm) is greater than 0.5; and organic carbon exceeds 0.5%. These horizons are differentiated on the basis of organic carbon content into: Bf, 0.5% to 5% organic carbon Bhf, more than 5% organic carbon.
A horizon characterized by gray colors, or prominent mottling indicative of permanent or periodic intense reduction, or both; for example, Aeg, Btg, Bg, and Cg.
(used with B)-The dithionite-extractable Fe of this horizon exceeds that of the IC by 1% or more and the dithionite-extractable Al does not exceed that of the IC by more than 0.5%.
A horizon enriched with organic matter.
An A horizon of organic matter accumulation. It contains less than 17% organic carbon. It is one Munsell unit of color value darker than the layer immediately below, or it has at least 0.5% more organic carbon than the IC, or both.
This horizon has been degraded, as evidenced by streaks and splotches of light and dark gray material and often by platy structure.
This horizon contains more than 1% organic carbon and less than 0.3% pyrophosphate-extractable Fe; the ratio of organic carbon to pyrophosphate-extractable Fe is 20 or more.
This is used as a modifier of suffixes e, g, n, and t to denote an expression of, but failure to meet, the specified limits of the suffix it modifies; for example, Aej is an eluvial horizon that is thin, discontinuous, or faintly discernible.
Presence of carbonate.
A horizon slightly altered by hydrolysis, oxidation, or solution, or all three, to give a change in color, or structure, or both.
A horizon in which the ratio of exchangeable Ca to exchangeable Na is 10 or less.
A layer disturbed by man's activities, for example, Ap.
A horizon containing detectable soluble salts.
A horizon of secondary enrichment of salts more soluble than Ca and Mg carbonates, where the concentration of salts exceeds that present in the unenriched parent material .
A horizon enriched with silicate clay, as indicated by a higher clay content (by specified amounts) than the overlying eluvial horizon, a thickness of at least 5 cm, oriented clay in some pores, or on ped surfaces, or both, and usually a higher ratio of fine (less than 0.2 um) to total clay than in the IC horizon.
A horizon of fragipan character.
A horizon affected by cryoturbation.
A perennially frozen layer.


The aspect of color that is determined by the wavelengths of light, and changes with the wavelength. Munsell hue notations indicate the visual relationship of a color to red, yellow, green, blue, or purple, or an intermediate of these hues. See also Munsell color system; chroma; and value, color.

humic acids

A mixture of various dark-colored organic substances precipitated by acidifying a dilute alkali extract from the soil. The term is used by some workers to designate only the alcohol-insoluble part of this precipitate.

Humic Gleysol

A great group of soils in the Gleysolic order. A dark-colored A (Ah or Ap) horizon more than 8 cm (3 inches) thick is underlain by mottled gray or brownish gleyed mineral material. It may have up to 40 cm (16 inches) of mixed peat (bulk density 0.1 or more) or up to 60 cm (24 inches) of fibric moss peat (bulk density less than 0.1) on the surface. This group includes soils formerly classified as Dark Gray Gleysolic and Meadow.

humic layer

A layer of highly decomposed organic soil material containing little fiber.

Humic Podzol

A great group of soils in the Podzolic order occurring in cool humid coastal regions, cool humid inland locations at higher altitudes, and some peaty depressions. The soils have a dark brown to black Bh horizon at least 10 cm (4 inches) thick, having rriore than 1% organic carbon, less than 0.3% pyrophosphate-extractable Fe, a ratio of organic carbon to pyrophosphate-extractable Fe of 20 or more, and a very low base saturation (NaCi extraction). A thin iron pan or a series of very thin (totaling less than 2.5 cm or 1 inch) iron pans may be present.


The processes by which organic matter decomposes to form humus. In humus the initial structures or shapes can no longer be recognized. See also humus.


The fraction of the soil organic matter that is not dissolved when the soil is treated with dilute alkali.


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

Humo-Ferric Podzol

A great group of soils in the Podzolic order. The upper 10 cm (4 inches) of the B horizon (Bf) contains between 0.5% and 5% organic carbon and 0.6% or more pyrophosphate-extractable Al and Fe (0.4% for sands). The ratio of organic carbon to pyrophosphate-extractable Fe is less than 20. Most of the typical Podzols are classified as Humo-Ferric Podzols.


(1) The fraction of the soil organic matter that remains after most of the added plant and animal residues have decomposed. It is usualiy dark colored. (2) Humus is also used in a broader sense to designate the humus forms referred to as forest humus. They include principally mor, moder and mull. See also organic matter / soil; mor; moder; mull; and horizon/ soil. (3) All the dead organic material on and in the soil that undergoes continuous breakdown, change and synthesis.


Chemical combination of water with another substance.

hydraulic conductivity

See water / soil.

hydraulic gradient

See water/ soil.

hydraulic head

See water/ soil.

hydric layer

A layer of water in the control section of Organic soils, extending from a depth of not less than 40 cm (16 inches) to a depth of more than 160 cm (64 inches).

hydrogenic soil

Soil developed under the influence of water standing within the profile for prolonged periods; it is formed mainly in cold, humid regions.

hydrologic cycle

The conditions through which water naturally passes from the time of precipitation until it is returned to the atmosphere by evaporation and is again ready to be precipitated.


The process by which a substrate is split to form two end products by the intervention of a molecule of water.

hydromorphic soil

A general term for soils that develop under conditions of poor drainage in marshes, swamps, seepage areas, or flats. See also Gleysolic.

hydrous mica

A term used in two different ways: (1) The groups of clay-sized micas that have a higher lattice water content and lower potassium content than ideal mica (illite). (2) Interstratified montmorillonite- and vermiculite-mica minerals in which mica predominates.

hygroscopic coefficient (Obsolete)

The weight percentage of water held by, or remaining in, the soil (1) After the soil has been air-dried. (2) After the soil has reached equilibrium with an unspecified environment of high relative humidity, usually near saturation, or with a specified relative humidity at a specified temperature.

hygroscopic water

Water adsorbed by a dry soil from an atmosphere of high relative humidity; water lost from an air-dry soil when it is heated to 105°C water held by the soil when it is at equilibrium with an atmosphere of a specified relative humidity at a specified temperature, usually 98% relative humidity at 25°C.

hymatomelanic acid

The fraction of humus that is soluble in alcohol after having been extracted with alkali and precipitated with acid and when the alcohol is distilled, forms a brittle mass that is insoluble in alcohol.