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Selected Terms


All terms defined using R.W. Miller & D.T. Gardiner, 1998. Soils in our Environment, 8th Edition. Prentice-Hall Publishers, Upper Saddle River, NJ.


Aeration, soil - the process by which air in the soil is replaced by air in the atmosphere.  Poorly aerated soils usually contain a much higher percentage of carbon dioxide and a correspondingly lower percentage of oxygen than the atmosphere above the soil. 

Alb - an albic horizon.

Allophane - amorphous (noncrystalline) clay-sized aluminosilicate.

Anaerobic - 1.  the absence of molecular oxygen.  2.   growing in the absence of molecular oxygen (such as anaerobic bacteria).  3.   occuring in the absence of molecular oxygen (as a biochemical process).

Anion exchange - replacement by an anion in solution for an absorbed anion of positively charged sites of a solid.

Aquic conditions - a soil-water regime, mostly too wet (reducing conditions, waterlogged) for parts of the year.  See Chapter 7.

Aquifer - a geologic formation that transmits water underground, usually sands, gravel, and fractured, porous, cavernous, and vesicular rock.

Argillic horizon - a diagnostic of clay accumulation often designated as Bt.  See Chapter 2 of Soils in Our Environment.

Atterberg limits - water content of manipulated soil at different consistency.  See Liquid limit, Plastic limit.

Available water capacity - the capacity of soils to hold water available for use by most plants, usually defined as water bewteen -33 kPa and -1500 kPal.   In a 2 meter profile, or a more shallow limiting layer, the values are as following:

            Very low              0-3 in                                      0-7.5 cm
            Low                      3-6 in                                      7.5-15 cm
            Moderate            6-9 in                                      15-23 cm
            High                     9-12 in                                    23-30 cm
            Very high            More than 12 in                   More than 30 cm

Banding - applying fertilizer or other amendment into the soil (7-15 cm, or 2.7-6 in, deep) in a thin narrow strip (band), as beside or beneath a planted row of seeds or plants.

Base saturation percentage (base cation saturation) - the degree to which the adsorption complex of a soil is saturated with basic cations (cations other than hydrogen and alluminum), usually expressed in percentage.

Benchmark soil -

Biomass - total amount of living organisms and their residues in a volume or mass of the environment.

Biological oxygen demand (BOD) - the amount of dissolved oxygen in water that will be consumed as the organic matter present is decomposed.  High BOD means low water quality and probably the development of anaerobic waters.  It usually results when waters have received organic wastes.   See also Chemical oxygen demand.

Bor - cool.

Broadcast - fertilizer is uniformly spread on the soil surface.  It may or may not be incorporated into the soil.

Bulk density - the mass (weight) of dry soil per unit bulk volume.

Calcic horizon - a diagnostic mineral horizon of carbonate accumulation.  Indicated by the letter k.

Cambic horizon - a weakly developed diagnostic subsoil horizon.  Indicated by the letter w.

Cation exchange - replacement by a cation in solution for an absorbed cation of negatively charged sites of a solid.

Cation exchange capacity (CEC) - the sum total of exchangeable cations that a soil can absorb, expressed in centimolesc per kg of soil or colloid.

Chelation - the formation of strong bonds between metals and organic compounds.  Some chelates are insoluble, such as in soil humus.

Chlorite - a nonexpanding clay mineral having a silica tetrahedral, an alumina octahedral, a silica tetrahedral, and a magnesium hydroxide (brucite) octahedral layer, has a 2:2 or 2:1:1 crystal structure.

Chroma - the relative purity, strength, or saturation of a color.

Clay - 1.   a mineral soil separate consisting of particles less than 0.002 mm in equivalent diameter.  2.  a soil textural class.  3.  a fine-grained soil that has a high plasticity index in relation to the liquid limits (engineering).  4.   a specific mineral structure.

Compaction -

Conservation tillage - The U.S. Natural Resource Conservation Service defines conservation tillage as any tillage system that leaves at least 30% of the surface covered by plant residues for control of erosion by water; for controlling erosion by wind, it means leaving at least 1120 kg/ha (1000 lb/a) of small-grained-straw-equivalent during the critical wind erosion period.   The amount of residue needed depends on the kind of residue and whether it is standing or flat.

Consistency - 1.  the resistance of a material to deformation or rupture.  2.  The degree of cohesion or adhesion of the soil mass.  Used for describing consistency of soil materials at various soil moistures and degrees of cementation.

Consolidation -

Creep (soils) - slow mass movement of soil and soil material down relatively steep slopes, primarily under the influence of gravity but facilitated by saturation with water and by alternate freezing and thawing.

Denitrification - the biochemical reduction of nitrate or nitrite to gaseous nitrogen, either as molecular nitrogen or as an oxide of nitrogen.

Diffusion - the slow movement of an ion in water mostly by its own kinetic motion.

Easement -

EC (electrical conductivity) - measured in Siemens/meter.  ECe = value from a saturated soil paste extract.

Edaphology - the science that deals with the influence of soils on living things, particularly plants, including human use of land for plant growth.

Eminent domain -

Evapotranspiration (ET) - water transpired by vegetation plus that evaporated from the soil.   Approximate synonym:  consumptive use.

Fertigation - a term coined for application of fertilizers in irrigation waters, usually through sprinkler systems.  See also Chemigation.

Field (water) capacity - the amount of water remaining in a soil after the soil layer has been saturated and the free (drainable) water has been allowed to drain away (a day or two).  Estimated at -33 kPa water potential.

Foliar feeding -

Furrow -

Geological erosion - See Erosion

Gley - some layer of mineral soil developed under conditions of poor drainage (poor aeration), resulting in reduction of iron and other elements and in gray colors and mottles (blobs of variously colored soils).

Gravitational potential - the amount of work an infinitesimal amount of pure free water can do at the site of the soil solution as a result of the force of gravity.

Gully - large eroded channels.  See also Erosion.

Hapl - minimum horizon development.

Horizon - a layer of soil or soil material approximately parallel to the land surface and differing from adjacent genetically related layers in physical, chemical, and biological properties or characteristics, such as color, structure, texture, consistency, amount of organic matter, and degree of acidity or alkalinity.

Hue - one of the three variables of color, the rainbow color of light reflected from each soil.

Humus - the fraction of the soil organic matter remaining, usually amorphous and dark colored, after the major portion of added residues have decomposed.

Hydraulic conductivity - (K) the proportionality factor in Darcy's law, indicating the soil's ability to transmit flowing water.

Hydric (soil) - a soil with periods of wetness that exhibits evidence of that wetness (mottles, gleying, redox conditions at times).

Igneous rock - formed by solidification, from a molten or partially molten state.  Synonym:  primary rock.  Example:  granite.

Illite - a hydrous alluminosilicate clay mineral with structurally mixed mica and smectite or vermiculite, similar to montmorillonite but containing potassium between the crystal layers.  Also referred to as hydrous mica or mica.

Illuvial horizon - a soil layer or horizon in which material carried from an overlying layer has been precipitated from solution or deposited from suspension.  The layer of accumulation.   Contrast to eluviation.

Immobilization - the transfer of an element from the soluble inorganic into the organic form of microbial or plant tissues.

Infiltration - entry of water downward into the soil surface.

Isomorphous substitution - the replacement of one atom by another of similar size in a crystal lattice during crystal growth without changing the crystal structure.

Kaolinite - hydrous aluminosilicate clay mineral of the 1:1 crystal structure group - - that is, consisting of one silicon tetrahedral sheet and one aluminum oxide-hydroxide octahedral sheet.

Land capability class - one of eight classes of land in the land capability classification of the USDA -- Soil Conservation Service, distinguished according to the risk of land damage or the difficulty of land use.

Landform - a discernible natural landscape, such as a floodplain, stream terrace, plateau, or alluvial fan.

Leaching - the downward removal of materials in solution from the soil.

Leaching fraction - the extra fraction of the amount of water needed to wet the soil that must be added to keep soil salinity below a predetermined tolerance concentration.

Liquid limit - in engineering, the water percentage between a soil's defined liquid and plastic states (consistence).

Loess - material transported and deposited by wind; predominantly silt sized.

Loam - soil material that is 7% to 27% clay, 28%-50% silt, and less than 52% sand.

Lysimeter - container of soil to measure the water movement, gains, or losses through that block of soil, usually undisturbed or in situ.

Mass flow - movement of nutrients with the overall flow of water to plant roots.

Matric potential - the amount of work an infinitesimal quantity of water in the soil can do as it moves from the soil to a pool of free water of the same composition and at the same location.  This work is less than zero, or negative work, thus reported in negative values.  Matric potential nearly equals water potential in nonsalty soils.

Metamorphic rock - igneous or sedimentary rock that has changed because of high temperature, high pressure, and the chemical environment while deep in the crust of the earth.  Examples:  marble, slate, gneiss.

Microirrigation -

Micronutrient - a chemical element necessary in only small amouts (usually less than several parts per million in the plant) for the growth of plants.   Examples:  boron, copper, iron, and zinc.

Mineralization - the conversion of an element from an organic combination to an inorganic form as a result of microbial decomposition.

Mollic horizon - a diagnostic epipedon of dark color, of moderate pH, and quite deep.

Montmorillonite - a hydrous alluminosilicate clay mineral with 2:1 expanding crystal structure - - that is, with two silicon tetrahedral sheets enclosing an aluminum octahedral sheet.  Considerable expansion may be caused by water.

Mulch - a natural or artificial layer of crop residues, leaves, sand, plastic, or paper on the soil surface.

Natric horizon - an argillic horizon with >15% exchangable sodium.

Nitrification - the biological oxidation of ammonium salts to nitrites and the further oxidation of nitrites to nitrates.

Ochric horizon - a diagnostic epipedon of light color, low humus, or shallow depth.

Octahedra -

Ogallala -

Osmosis -

Pale - excessive development, usually very old.

Particle density - the mass per unit volume of the soil particles.   See also Bulk density.

Ped - a unit of soil structure; an aggregate, such as prism, block, or granule, formed by natural processes.

Pedology - the study of soil as a geologic entity.

Pedon - the smallest volume that can be called a soil.  It has three dimensions.  It extends downward to the depth of plant roots or to the lower limit of the genetic soil horizons.   Its lateral cross-section is roughly hexagonal and ranges from 1 to 10 m2 in size, depending on the variability in the horizons. 

Pell - low chroma, dull color.

Percolation, soil water - the downward movement of water through soil, especially the downward flow of water in saturated or nearly saturated soil.

Permanent wilting point - the largest water content in soil at which plants will wilt and not recover when placed in a humid chamber.  It is estimated at about -1.5 MPa matric potential.

pH, soil - a numerical measure of the acidity of hydrogen ion activity of a soil.   Exactly, the negative logarithm of the hydrogen-ion activity of a soil.

Phase change -

Plant available water -

Plastic limit - (engineering) the water content at which the soil rolled to a 1/3-cm wire begins to crumble.

Plasticity index - (engineering) the water content percentage between the liquid limit and the plastic limit.

Porosity - in engineering and soils, the ratio of the volume of voids to the total volume of the soil.  In soils, usually it is given in percentage.

Potash -

Pressure potential - the amount of work an infinitesimal amount of soil water can do in moving from a pool of pure water under pressure common to that soil position to a pool of pure water at the same location and at normal atmospheric pressure.

Psychrometer -

Quick test, soil - simple, routine analysis on soils, usually to measure pH, soluble salts, and nutritional status.

Redox - 1.  a term for the overall reactions in which one substance is oxidized while another is reduced by the electron transfers.  2.  the electron density of the media.  Redox is measured in units of millivolts.

Residual material - unconsolidated and partly weathered mineral materials derived from rock in place.

Resistance blocks -

Respiration -

Rill - a small, eroded ditch, usually only a few inches deep and hence no great obstacle to tillage operations.

Saltation - Synonym:  vaultationSee Erosion.

Salinity -

Sand -

Saturated flow - movement of water through soil by gravity flow, as in irrigation or during a rainstorm.

Secondary nutrient - the nutrient Ca, Mg, and S used in large amounts by plants but less often deficient than the primary nutrients N, P, and K.

Sedimentary rock - rock made up of particles deposited from suspension in water.  Some wind-deposited sand is consolidated into sandstone.

Semi-permeable membrane -

Series, soil - the soil series is a group of soils having horizons similar in differentiating characteristics and arrangement in the soil profile, except for texture of the surface, slope, gravel, stones, and erosion.

Sesquioxides (metal oxides) - a term for minerals containing 1.5 atoms of oxygen per atom of the metal, particularly Al2O3 and Fe2O3.   Often TiO2 is included, although it does not strictly fit the meaning of sesqui ( = 1.5 times).

Silt - 1.   a soil separate consisting of particles between 0.05 and 0.002 mm in equivalent diameter.  2.  A soil textural class.

Sodicity -

Sodium adsorption ratio (SAR) - a value representing the relative hazard of irrigation water because of a high sodium content relative to its calcium plus magnesium content.

Soil -  1.  the unconsolidated mineral and organic material on the immediate surface of the Earth that serves as a natural medium for the growth of land plants.  2.  the unconsolidated mineral matter on the surface of the Earth that has been subjected to and influenced by genetic and environmental factors of parent material, climate, macro- and microorganisms, and topography, all acting over a period of time and producing a product - soil - that differs from the material from which it is derived in many physical, chemical, biological, and morphological properties and characteristics.

Soil colloids -

Soil genesis - formation of the soil with special reference to the processes or soil-forming factors responsible for the development of the solum or true soil from the unconsolidated parent material.  Synonyms:  pedogenesis, soil formation.

Soil horizon - a layer of soil or soil material approximately parallel to the land surface and differing from adjacent genetically realted layers in physical, chemical, and biological properties or characteristics, such as color, structure, texture, consistency, amount of organic matter, and degree of acidity or alkalinity. 

Soil separates - mineral particles, less than 2.0 mm in equivalent diameter, ranging between specified size limits.

Soil survey - the systematic examination, description, classification, and mapping of soils of an area.

Soil test - a chemical, physical, or microbiological operations that estimates a property of a soil.

Soil texture - the relative proportions of the various soil size separates.

Solute potential - the amount of work an infinitesimal quantity of water will do in moving from a pool of free water the same composition as the soil water to a pool of pure water at the same location.  The effect of dissolved substances.   Usually very small.

Solvent -

Specific heat -

Strain -

Stress -

Suspension -

Symbiosis - the living together in intimate association of two dissimilar organisms, the cohabitation being mutually beneficial, such as Rhizobia legume bacteria with the host leguminous plant.

Tensiometer - instrument used for measuring the water potential (suction or negative pressure) of soil water.

Tetrahedra -

Tile drain - lines of concrete or ceramic (clay) pipe placed in the subsoil to collect and drain water from the soil to an outlet.

Turbidity -

Ud - udic moisture regime.

Umbric horizon - a dark-colored, high-organic matter, diagnostic epipedon similar to mollic but more acidic.

Unsaturated flow - the movement of water in a soil that is not filled to capacity with water.  Water moves because of water-potential differences toward areas of lower water potentials (drier soil).

Ust - ustic moisture regime.

Value, color - the relative lightness or darkness of color.

Vermiculite - 1. a clay similar to hydrous mica and having 2:1 layers of 2 tetrahedral sheets to 1 octahedral sheet.  Vermiculite has the layers held together by hydrated cations and has less swelling than montmorillonite.  2. a mica-like silicate mineral that expands into an accordion-like structure when heated to high temperature; it is a high water-holding capacity within the expanding particles and yet good aeration between the large particles.

Volatilization -

Water retention curve - a graph showing the soil-water content versus applied tension, suction, or water potential.  Also called water release characteristic curve.

Weathering - the group of processes (such as chemical action of air and rainwater and the biological action of plants and animals)  whereby rocks and minerals change in character, disintegrate, decompose, and synthesize new compounds and clay minerals.

Wetlands - soil areas that have evidence of saturated conditions part of the year (ponded water, hydric soil, wet-area plants, such as cattails).

Xer - a xeric moisture regime.