Glossary of Terms in Soil Science

ecology

The study of the relationship between organisms and their environment.

ectotrophic mycorrhiza

A mycorrhizal association in which the fungal hyphae form a compact mantle on the surface of the roots. Mycelial strands extend inward between cortical cells and outward from the mantle to the surrounding soil.

edaphic

(1) Of or pertaining to the soil. (2) Resulting from or influenced by factors inherent in the soil or other substrate rather than by climatic factors.

edaphology

The science that deals with the influence of soils on living things, particularly plants including man's use of land for plant growth.

effective cation exchange capacity

The sum of cations that a soil can adsorb in its natural state.

effective precipitation

The portion of the total precipitation that becomes available for plant growth.

electrokinetic potential

(1) In a colloidal system, the difference in potential between the immovable layer attached to the surface of the dispersed phase and the dispersion medium. (2) The work done in bringing a unit charge from infinity to a reference point in the liquid layer attached to the solid phase in a colloidal system.

eluvial horizon

A soil horizon that has been formed by the process of eluviation. See also illuvial horizon.

eluviation

The transportation of soil material in suspension or in solution within the soil by the downward or lateral movement of water.

end moraine or terminal moraine

A ridgelike accumulation of drift built chiefly along the terminal margin of a valley glacier or the margin of an ice sheet. It is mainly the result of deposition by ice or deformation by ice thrust, or both.

endoenzyme or intracellular enzyme

An enzyme formed within the cell and not excreted into the medium.

endotrophic

Nourished or receiving nourishment from within; for example, fungi or their hyphae receiving nourishment from plant roots in a mycorrhizal association.

endotrophic mycorrhiza

A mycorrhizal association in which the fungal hyphae are present on root surfaces as individual threads that may penetrate directly into root hairs, other epidermal cells, and occasionally into cortical cells. Individual threads extend from the root surface outward into the surrounding soil.

engineering (soil)

See mechanics and engineering, soil.

enrichment culture

A technique in which environmental, including nutritional, conditions are controlled to favor the development of a specific organism or group of organisms.

enzyme

A proteinaceous organic substance, produced within an organism, that acts like a catalyst. See also adaptive enzyme, constitutive enzyme, endoenzyme, and exoenzyme.

eolian deposit

Sand, or silt, or both, deposited by the wind. See also loess and dunes.

equivalent diameter

In sedimentation analysis, the diameter assigned to a nonspherical particle. It is numerically equal to the diameter of a spherical particle having the same density and velocity of fall.

equivalent weight of a soil colloid

The weight of clay or organic colloid that has a combining power equal to 1 gram-atomic weight of hydrogen.

erode

To wear away or remove the land surface by wind, water or other agents.

erodible

Susceptible to erosion. It is expressed by terms such as highly erodible and slightly erodible.

erosion

  1. The wearing away of the land surface by running water, wind, ice or other geological agents including such processes as gravitational creep.
  2. Detachment and movement of soil or rock by water, wind, ice or gravity.

The following terms are used to describe different types of water erosion:

  • accelerated erosion Erosion much more rapid than normal, natural, geological erosion, primarily as a result of the influence of the activities of man or animals.
  • geological erosion The normal or natural erosion caused by geological processes acting over long geologic periods and resulting in the wearing away of mountains, the dissection of plains, and the building up of floodplains and coastal plains. Synonymous with natural erosion.
  • gully erosion The erosion process whereby water accumulates in narrow channels and, over short periods, removes the soil from this narrow area to various depths, from about 0.3 m (1 ft) to as much as 30 m (100 ft).
  • natural erosion Wearing away of the earth's surface by water, ice, or other natural agents under natural environmental conditions such as climate and vegetation, undisturbed by man. Synonymous with geological erosion.
  • normal erosion The gradual erosion of land used by man. It does not greatly exceed natural erosion. See natural erosion.
  • rill erosion An erosion process in which many small channels a few centimetres deep are formed; it occurs mainly on recently cultivated soils. See also rill.
  • sheet erosion The removal of a fairly uniform layer of soil from the land surface by runoff water.
  • splash erosion The spattering of small soil particles caused by the impact of raindrops on very wet soils. The loosened and spattered particles may, or may not, be subsequently removed by surface runoff.

erosion classes

A grouping of erosion conditions based on the degree of erosion or on the characteristic patterns. The classes apply to accelerated erosion but not to normal, natural or geological erosion. Four erosion classes are recognized for water erosion anq three for wind erosion. (For details see The Canadian System of Soil Classification.)

erosion pavement

A layer of coarse fragments such as sand, gravel, cobbles, or stones that remains on the surface of the ground after the removal of fine particles by eroslon.

erratic

A transported rock fragment different from the bedrock where it lies. The term is generally applied to fragments transported by glacier ice or by floating ice.

esker

A winding ridge of irregularly stratified sand, gravel, and cobbles deposited under the ice by a rapidly flowing glacial stream.

eubacteriales

An order of the class Schizomycetes. True bacteria.

Eutric Brunisol

A great group of soils in the Brunisolic order. The soils may have mull Ah horizons less than 5 cm (2 inches) thick, and they have Bm horizons in which the base saturation (NaCI) is l00%.

eutrophic

Having concentrations of nutrients optimal or nearly so for piant or animal growth. It is used to describe nutrient or soil solutions.

evapotranspiration

The loss of water from a given area during a specified time by evaporation from the soil surface and by transpiration from the plants. Potential evapotranspiration is the maximum transpiration that can occur in a given weather situation with a low-growing crop that is not short of water and does not completely shade the ground.

excessive drainage

Too great or too rapid a loss of water from soils, either by percolation or by surface flow. The loss is greater than that necessary to prevent the development of an anaerobic condition for an appreciable length of time.

exchange acidity

The titratable hydrogen and aluminum that can be replaced from the adsorption complex by a neutral sait solution. It is usually expressed as milliequivalents per 100 g of soil (meq/ 100 g soil).

exchange capacity

The total ionic charge of the adsorption complex that is active in the adsorption of ions. See also anion exchange capacity and cation exchange capacity.

exchangeable cation percentage

The extent to which the adsorption complex of a soil is occupied by a particular cation. It is expressed as: ECP = [exchangeable cation (meq/100 g soil) / cation exchange capacity (meq/100 g soil)] x 100.

exchangeable phosphate

The phosphate anion reversibly attached to the surface of the solid phase of the soil in such form that it may go into solution by anionic equilibrium reactions with isotopes of phosphorus or with other anions of the liquid phase without solution of the colloid phase to which it was attached.

exchangeable potassium

The potassium that is held by the adsorption complex of the soil and is easily exchanged with the cation of neutral nonpotassium salt solutions.

exchangeable sodium percentage

The extent to which the adsorption complex of a soil is occupied by sodium. It is expressed as: ESP= [exchangeable sodium (meq/ 100 g soil) / cation exchange capacity (meq/ 100 g soil)] x 100.

exoenzyme or extracellular enzyme

An enzyme excreted by a microorganism into the environment. An enzyme that acts outside the cell.

external drainage

The natural elimination or accumulation of precipitation water on the soil surface.

extract (soil)

The solution separated from a soil suspension or from a soil by filtration, centrifugation, suction or pressure.