Glossary of Soil Science Terms - Browse

habitat The place where a given organism lives.

half graben An elongate, structural trough or basin bounded on one side by a normal fault. It may or may not produce a topographic basin ( Jackson, 1997). Compare graben.

Half-Bog soil [soil classification] (obsolete; not used in soil taxonomy) A great soil group of the intrazonal order and hydromorphic suborder consisting of soil with dark-brown or black peaty material over grayish and rust mottled mineral soil; formed under conditions of poor drainage under forest, sedge, or grass vegetation in cool to tropical humid climates (Baldwin, et al., 1938).

halloysite A member of the kaolin subgroup of clay minerals. It is similar to kaolinite in structure and composition except that hydrated varieties occur that have interlayer water molecules. Halloysite usually occurs as tubular or spheroidal particles and is most common in soils formed from volcanic ash. Compare Appendix I, Table A3.

halomorphic soil [soil classification] A suborder of the intrazonal soil order, consisting of saline and sodic soils formed under imperfect drainage in arid regions and including the great soil groups Solonchak or Saline soils, Solonetz soils, and Soloth soils (Not used in soil taxonomy).

hanging valley A tributary valley whose floor at the lower end is notably higher than the floor of the main valley in the area of junction ( Jackson, 1997).

hardpan A soil layer with physical characteristics that limit root penetration and restrict water movement.

hardsetting soil Soils that, following wetting, exhibit transient but only slowly reversible cementation and/ or induration throughout significant fractions of the profile restrictive to seed emergence and root penetration (Australian).

harrowing Refer to tillage, harrowing.

harvest index The quantity of harvestable biomass per unit total biomass produced. If used in relation to nutrients it would be the quantity of biomass produced per unit input of plant nutrient.

head land Refer to tillage, turnrow.

head slope [geomorphology] A geomorphic component of hills consisting of a laterally concave area of a hillside, especially at the head of a drainageway, resulting in converging overland water flow (e.g., sheet wash); head slopes are dominated by colluvium and slope wash sediments (e.g., slope alluvium); contour lines form concave curves. Slope complexity (downslope shape) can range from simple to complex. Headslopes are comparatively moister portions of hillslopes and tend to accumulate sediments (e.g., cummulic profiles) where they are not directly contributing materials to channel flow (Schoeneberger and Wysocki, personal communication, 2013). Compare side slope, nose slope, free face, interfluve, crest, base slope.

head [geomorphology] (a) The source, beginning, or upper part of a stream. (b) The upper part or end of a slope or valley ( Jackson, 1997).

head-of-outwash A sloping and sometimes high relief landform composed predominantly of glaciofluvial sediment that delimits a former ice-margin of a relatively static, rapidly wasting glacier. A steep ice-contact slope forms the ice-proximal face of the landform; a more gently sloping surface dips away on the distal slope, if not slumped. Compare ice-margin complex (Schoeneberger and Wysocki, personal communication, 2013).

headcut Refer to erosion, headcut, nickpoint.

headland [coast] (a) An irregularity of land, especially of considerable height with a steep cliff face, jutting out from the coast into a large body of water (usually the sea or a lake); a bold promontory or a high cape.(b) The high ground flanking a body of water, such as a cove. (c) The steep crag or cliff face of a promontory ( Jackson, 1997).

headwall A steep slope at the head of a valley; e.g., the rock cliff at the back of a cirque. Compare cirque headwall ( Jackson, 1997).

heat A form of energy resident in the random motion of molecules.

heat capacity Amount of heat required to raise the temperature of a given quantity of soil by 1°C.

heat flux Refer to soil heat flux density.

heat of immersion The heat evolved on immersing a soil, at a known initial water content (usually oven dry) in a large volume of water.

heavy metals Those metals which have densities>5.0 Mg m_-3. In soils these include the elements Cd, Co, Cr, Cu, Fe, Hg, Mn, Mo, Ni, Pb, and Zn.

heavy soil (colloquial) An informal name for a soil with a high content of the fine separates, particularly clay, or one with a high drawbar pull and hence difficult to cultivate, especially when wet. Compare fine texture.

hematite Fe O . A red iron oxide mineral that con-

hemic material Organic soil material at an intermediate degree of decomposition that contains 1/6 to 3/4 recognizable fibers (after rubbing) of undecomposed plant remains. Bulk density is usually very low, and water holding capacity very high.

Hemists [soil taxonomy] A suborder of Histosols that are wet and have an intermediate degree of plant fiber decomposition (e.g., the botanic origin of much of the organic material cannot be readily determined), and a bulk density between 0.1 and 0.2 g cm_-3. The fiber content of much of the organic material is between one-sixth and two-thirds after rubbing between the thumb and fingers. Hemists are saturated with groundwater at or very near the surface for periods long enough to limit their use for most crops unless they are artificially drained; the ground water fluctuates but seldom drops much below the bottom of the surface tier (USDA, 1999; Appendix I).

herbaceous organic materials Refer to organic materials.

herbaceous peat [soil taxonomy] An accumulation of organic material, decomposed to some degree, that is predominantly the remains of sedges, reeds, cattails and other herbaceous plants. Compare moss peat, sedimentary peat, woody peat, peat, muck, and mucky peat (Soil Survey Staff, 1993).

heterogeneous Media with properties whose variability are not uniform with space.

heterotroph An organism able to derive carbon and energy for growth and cell synthesis by utilizing organic compounds.

heterotrophic nitrification Biochemical oxidation of ammonium and/or organic nitrogen to nitrate and nitrite by heterotrophic microorganisms. Compare nitrification.

high hill A generic name for an elevated, generally rounded land surface with high local relief, rising between 90 meters (approx. 300 ft.) to as much as 300 m (approx. 1000 ft.) above surrounding lowlands (Schoeneberger and Wysocki, personal communication, 2013). Compare low hill, hill, hillock.

high-center polygon A polygon whose center is raised relative to its boundary (National Research Center of Canada, 1988). Compare low center polygon.

highmoor bog A bog, often on the uplands, whose surface is covered by sphagnum mosses which, because of their high degree of water retention, make the bog more dependent upon precipitation than on the water table. The bog often occurs as a raised peat bog or blanket bog ( Jackson, 1997). Compare lowmoor bog, raised bog.

hill [tillage] Refer to tillage, hill.

hill top (not recommended) use summit.

hillock A generic name for a small, low hill, generally between 3 – 30 m in height and slopes between 5 and 50% (e.g., bigger than a mound but smaller than a hill); commonly considered a microfeature (Schoeneberger and Wysocki, personal communication, 2013). Compare mound, hill.

hills A landscape dominated by hills and associated valleys (Schoeneberger and Wysocki, personal communication, 2013).

hillside (not recommended) use hillslope.

hillslope A generic term for the steeper part of a hill between its summit and the drainage line, valley flat, or depression floor at the base of the hill (Hawley and Parsons, 1980). Compare mountain slope.

hillslope terrace [anthropogenic] A raised, generally horizontal strip of earth and/or rock bounded by a down-slope berm or retaining wall, constructed along a contour on a hillslope to make land suitable for tillage and to prevent accelerated erosion; common in steep terrain, both archaic (e.g., Peru) and modern (e.g., Nepal; Schoeneberger and Wysocki, personal communication, 2013). Compare conservation terrace.

hillslope-profile position Discrete slope segments found along a transect line that runs perpendicular to the contour, beginning at a divide and descending to a lower, bounding stream channel or valley floor; a discrete piece of a two-dimensional cross profile of a hill. Positions are commonly separated from one another by inflection points along the line. In descending elevational order, the hillslope-profile positions of a simple hillslope include summit, shoulder, backslope, footslope, and toeslope. Not all of these segments (positions) are necessarily present along a particular hillslope. Complex hillslopes include multiple sequences or partial sequences, or partial sequences (Schoeneberger and Wysocki, personal communication, 2013; Hawley and Parsons, 1980; Ruhe, 1975). Compare geomorphic components hills.

Histels [soil taxonomy] A suborder of Gelisols that are organic-rich and either: (i) overlie cindery, fragmental, or pumiceous materials and/or fill their interstices and directly overly a densic, lithic, or paralithic contact; (ii) when added with the underlying cindery, fragmental, or pumiceous materials, total 40 cm or more between the soil surface and a depth of 50 cm; (iii) or are saturated with water for 30 or more cumulative days during normal years, unless artificially drained, (iv) and have 80% or more, by volume, organic soil materials from the soil surface to a depth of 50 cm (or to a glacic layer or a densic, lithic, or paralithic contact, whichever is shallowest)(USDA, 1999; Appendix 1).

histic epipedon [soil taxonomy] A thin organic soil horizon that is saturated with water at some period of the year unless artificially drained and that is at or near the surface of a mineral soil. The histic epipedon has a maximum thickness depending on the kind of materials in the horizon and the lower limit of its organic carbon content is the upper limit for the mollic epipedon.

Histosols [soil taxonomy] An order of organic soils that have organic soil materials in more than half of the upper 80 cm, or that are of any thickness if overlying rock or fragmental materials that have interstices filled with organic soil materials, and lacks permafrost. (An order in the U.S. system of soil taxonomy.) Compare peat (USDA, 1999; Appendix 1).

hoe Refer to tillage, hoe.

hogback A sharp-crested, symmetric ridge formed by highly tilted resistant rock layers; a type of homocline produced by differential erosion of interlayered resistant and weak rocks with dips greater than about 25° (or approximately >45% slopes; Schoenenberger and Wysocki, personal communication, 2013; Hawley and Parson, 1980). Compare homoclinal ridge, cuesta.

Holocene An epoch of the Quaternary Period of geologic time that follows the Pleistocene Epoch (from the present to about 10 to 12 thousand years ago); also corresponding (time-stratigraphic) “series” of earth materials (Schoeneberger and Wysocki, personal communication, 2013).

homoclinal [structural geomorphology] (adjective) Pertaining to strata that dip in one direction with a uniform angle (Hawley and Parsons, 1980). Compare cuesta, hogback, homoclinal ridge.

homoclinal ridge An intermediate form of homocline that forms an asymmetric ridge with a dip slope commonly between 10–25° (15–45% slopes); A homoclinal ridge has steeper dip than a cuesta but less dip than a hogback (Schoeneberger and Wysocki, personal communication, 2013; Fairbridge, 1968). Compare cuesta, hogback.

homocline A general term for a series of rock strata that dip in one direction with a uniform angle; e.g., one limb of a fold, a tilted fault block, or an isocline ( Jackson, 1997). Compare cuesta, homoclinal ridge, hogback.

homogeneous Media with uniform variability with space.

hoodoo A bizarrely shaped column, pinnacle, or pillar of rock produced by differential weathering or erosion in a region of sporadically heavy rainfall. Formation is facilitated by joints and layers of varying hardness ( Jackson, 1997). Compare Compareearth pillar.

horizon Refer to soil horizon.

horizonation The development of horizons in soil as a result of a soil forming process or a combination of soil forming processes. Refer to soil horizon.

horn [geology] A high, rocky, sharp pointed, steep-sided, mountain peak with prominent faces and ridges, bounded by the intersecting walls of three or more cirques that have been cut back into the mountain by headward erosion of glaciers ( Jackson, 1997).

hornblende An amphibole type ferromagnesian silicate.

horst An elongate block that is bounded on both sides by normal faults that dip away from the interior of the horst. It is a structural form and may or may not be expressed geomorphically (at the ground surface; Jackson, 1997).

Hortonian flow Surface runoff as a result of rainfall in excess of the soils infiltration capacity; also called Hortoninan overland flow.

hot spring A natural, geothermally heated spring whose temperature is above that of the human body ( Jackson, 1997). Compare geyser, mud pot.

hue A measure of the chromatic composition of light that reaches the eye; one of the three variables of color. Compare chroma, Munsell color system, and value, color.

human transported material Organic or mineral material (or any other material that can function as a soil material) that has been moved horizontally onto a pedon from a source area outside of that pedon by directed human activity, usually with the aid of machinery. There has been little or no reworking by wind, gravity, water, or ice. Human transported materials are most commonly associated with building sites, mining or dredging operations, land fills, or other similar activities that result in the formation of a constructional anthropogenic landform ( J. Galbraith, ICOMANTH, personal communication, 2011).

humic acid The dark-colored organic material that can be extracted from soil with dilute alkali and other reagents and that is precipitated by acidification to pH 1 to 2.

Humic Gley soil [soil classification] (obsolete; not used in soil taxonomy) Soil of the intrazonal order and hydromorphic suborder that includes Wisenboden and related soils, such as Half-Bog soils, which have a thin muck or peat O₂ (Oi) horizon and an A1 (A) horizon. Developed in wet meadow and in forested swamps.(Thorp and Smith, 1949).

humic substances A series of relatively high-molecular-weight, yellow to black colored organic substances formed by secondary synthesis reactions in soils. The term is used in a generic sense to describe the colored material or its fractions obtained on the basis of solubility characteristics. These materials are distinctive to soil environments in that they are dissimilar to the biopolymers of microorganisms and higher plants (including lignin). Compare humic acid, humin, and fulvic acid.

humification The process whereby the carbon of organic residues is transformed and converted to humic substances through biochemical and abiotic processes.

humin The fraction of the soil organic matter that cannot be extracted from soil with dilute alkali.

hummock [geomorphology] (a) (not preferred–refer to hillock). An imprecise, general term for a rounded or conical mound or other small elevation. (b) (not preferred) A slight rise of ground above a level surface ( Jackson, 1997).

Humods [soil taxonomy] A suborder of Spodosols that are more or less freely-drained, have a large accumulationed of organic carbon and aluminum, but not iron, in the upper part of the spodic horizon. Undisturbed Humods may have either a thin, intermittent or a distinct, continuous albic horizon over a spodic horizon (which in its upper part is nearly black and has reddish hue). Humods are rarely saturated with water or do not have characteristics associated with wetness (USDA, 1999: Appendix 1).

Humox [soil taxonomy] (obsolete; no longer used in soil taxonomy) Prior to 1999, this was a suborder of Oxisols that are moist all or most of the time and that have a high organic carbon content within the upper 1 m. Humox have a mean annual soil temperature of<22°C and a base saturation within the oxic horizon of<35%, measured at pH 7 (USDA, 1999: Appendix 1).

Humults [soil taxonomy] A suborder of Ultisols that are more or less freely-drained and have a high content of organic carbon. Humults are not saturated with water for periods long enough to limit their use for most crops (USDA, 1999: Appendix 1).

humus The well decomposed, more or less stable part of the organic matter in mineral soils. Humus is an organic soil material that is also one of the USDA textures of muck (sapric soil material), mucky peat (hemic soil material), or peat (fibric soil material). Most likely it is muck.

humus form A group of soil horizons located at or near the surface of a pedon, which have formed from organic residues, either separate from or intermixed with mineral material.

hybridization The binding or annealing of two, complementary, single strands of nucleic acid.

hydrated lime A liming material composed mainly of calcium and magnesium hydroxides that reacts quickly to neutralize acid soils.

hydraulic nonequilibrium Condition in which movement of water occurs between flow regions, e.g., macropore to micropore, as a result of gradient in hydrualic potential between the regions.

hydraulic gradient (soil water) A vector (macroscopic) point function that is equal to the decrease in the hydraulic head per unit distance through the soil in the direction of the greatest rate of decrease. In isotropic soils, this will be in the direction of the water flux.

hydraulic conductivity The proportionality factor in Darcy’s law, as applied to viscous flow of water in soil, that represents the ability of soil to conduct water and is equivalent to the flux of water per unit gradient of hydraulic potential.

hydraulic head Refer to soil water and total head.

hydric soils A soil that formed under conditions of saturation, flooding, or ponding long enough during the growing season to develop anaerobic conditions in the upper part (Federal Register, 1994).

hydrodynamic dispersion The process wherein the solute concentration in flowing solution changes in response to the interaction of solution movement with the pore geometry of the soil, a behavior with similarity to diffusion but only taking place when solution movement occurs.

hydrodynamic dispersion coefficient The coefficient in the solute convection equations that accounts for hydrodynamic dispersion, it is usually determined by solving an inverse problem.

hydrogen bond An intramolecular chemical bond between a hydrogen atom of one molecule and a highly electronegative atom (e.g., O, N) of another molecule.

hydrogenic soil Soil developed under the influence of water standing within the profile for considerable periods; formed mainly in cold, humid regions.

hydrogeomorphic models (HGM) A group of conceptual models used in hydric soil determinations for identifying the operative hydrology of the soil zone and associated vegetation communities.

hydrograph A graph of the flow rate, either surface or subsurface flow, with time.

hydrologic cycle The fate of water from the time of precipitation until the water has been returned to the atmosphere by evaporation and is again ready to be precipitated. Refer to hydrology–soil hydrologic cycle.

hydrology The science dealing with the distribution and movement of water:agrohydrology The science dealing with the distribution and movement of rainfall and/or irrigation water and soil solution to and from the root zone in agricultural land, and with the distribution and movement of irrigation and surface water in con-