Glossary of Terms in Soil Science

A horizon

See: horizon; soil

Ablation till

A surface deposit of loose permeable somewhat stratified sandy and stony till overlying denser till.

absorbed water

Water held mechanically in a soil mass and having physical properties similar to ordinary water at the same temperature and pressure.

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.

acid soil

A soil material having a pH of less than 7.0. See also soil reaction.

actinomycetes

Unicellular filamentous microorganisms that branch monopodially or more rarely dichotomously and form radiating colonies; mainly found in the soil, and cause of its characteristic odor.

active acidity

The activity of hydrogen ion in the aqueous phase of a soil. It is measured and expressed as a pH value.

adaptation

Change in an organism resulting from the action of natural selection on variation so that the organism is fitted more perfectly for existence in its environment.

adaptive_enzyme

An enzyme produced by an organism in response to the presence of its substrate or a related substance. It is also called an induced enzyme

adsorbed water

Water held mechanically in a soil mass and having physical properties similar to ordinary water at the same temperature and pressure.

adsorption complex

The group of substances in the soil capable of adsorbing other materials. Organic and inorganic colloidal substances form the greater part of the adsorption complex. The noncolloidal materials, such as silt and sand exhibit adsorption to a much lesser extent than the colloidal materials.

aerate

To impregnate with a gas usually air.

aeration soil

The process by which air in the soil is replaced by air from the atmosphere. In a well-aerated soil, the soil air is similar in composition to the atmosphere above the soil. Poorly aerated soils usually contain a much higher percentage of carbon dioxide and a correspondingly lower percentage of oxygen than the atmosphere. The rate of aeration depends largely on the volume and continuity of pores in the soil.

aerobic

(1) Having molecular oxygen as a part of the environment. (2) Growing only in the presence of molecular oxygen such as aerobic organisms. (3) Occurring only in the presence of molecular oxygen as applied to certain chemical or biochemical processes such as aerobic decomposition.

aggradation

Filling in or leveling by deposition.

aggregate

A group of soil particles cohering in such a way that they behave mechanically as a unit.

agronomy

The branch of agriculture that deals with the theory and practice of field-crop production and the scientific management of soil.

air (soil)

The soil atmosphere, the gaseous phase of the soil, is the volume not occupied by solid or liquid.

air porosity

The portion of the bulk volume of soil that is filled with air at any given time or under a given condition such as specified soil water potential. Usually this portion is made up of large pores that is those drained by a tension of less than about 100 cm of water. See also soil moisture tension.

air-dry

(1) The state of dryness of a soil at equilibrium with the moisture content of the surrounding atmosphere. The moisture content depends on the relative humidity and the temperature of the surrounding atmosphere. (2) To allow to reach equilibrium in moisture content with the surrounding atmosphere.

alkali soil

(1) A soil having a high degree of alkalinity (pH of 8.5 or higher) or having a high exchangeable sodium content (15% or more of the exchange capacity) or both. (2) A soil that contains enough alkali (sodium) to interfere with the growth of most crop plants. See also saline-alkali soil and sodic soil.

alkaline soil

Any soil that has a pH greater than 7.0. See also - alkalization - saline-alkali soil - sodic soil - soil alkalinity - soil reaction

alkalinity; soil

The degree or intensity of alkalinity of a soil expressed by a value greater than 7.0 on the pH scale See also - alkaline soil - alkalization - saline-alkali soil - sodic soil - soil reaction

alkalization

The process whereby the exchangeable sodium content of a soil is increased. See also - alkaline_soil - saline-alkali soil - sodic soil - soil alkalinity - soil reaction

allophane

An amorphous hydrated aluminosilicate of variable composition.

alluvial fan

A fan-shaped deposit of alluvium laid down by a stream where it emerges from an upland into less steeply sloping terrain.

alluvium

Material such as clay, silt, sand, and gravel deposited by modern rivers and streams.

alpine soil

Mountain soil occurring above the timberline.

amendment (soil)

(1) An alteration of the properties of a soil and thereby of the soil by the addition of substances such as lime, gypsum and sawdust to make the soil more suitable for the growth of plants. (2) Any substance used for this purpose. Fertilizers constitute a special group of soil amendments.

amino acid

An organic acid containing both amino (NH2) and carboxyl (COOH) groups, and usually having the general formula R-CH-COOH | NH2

ammonification

The biochemical process whereby ammoniacal nitrogen is released from nitrogen-containing organic compounds.

ammonium fixation

The adsorption or absorption of ammonium ions by the mineral or organic fractions of the soil in such a way that the ions are relatively insotuble in water and relatively unexchangeable by the usual methods of cation exchange.

amorphous mineral

(1) A mineral that has no definite crystalline structure. (2) A mineral that has a definite crystalline structure but appears amorphous because of the small crystallite size.

anaerobic

Having no molecular oxygen in the environment. Growing in the absence of molecular oxygen such as anaerobic bacteria. Occurring in the absence of molecular oxygen as in a biochemical process.

angular cobbly

A descriptive term applied to coarse fragments. It is used for irregular and angular rock or mineral particles 7.5 to 25 cm (3 to 10 inches) in diameter. See also coarse fragments.

anion exchange capacity

The total amount of exchangeabie anions that a soil can adsorb. It is expressed as milliequivalents per 100 g of soil or other adsorbing material such as clay.

anisotropic mass

A mass whose properties have different values when they are measured in different directions at any given point.

antagonism

The mutual killing, injury or inhibition of growth of dissimilar organisms occupying the same ecological niche.

antibiotic

A substance that is produced by a species of microorganism and, in dilute solution, has the capacity to inhibit the growth of or kill certain other organisms.

apparent density

See bulk density; soil.

apparent specific gravity

See bulk specific gravity.

arable soil

Soil suitable for plowing and cultivation.

arthropod

A jointed-legged invertebrate such as an insect or a crustacean.

artificial manure

In European usage artificial manure may denote commercial fertilizers. See compost.

associate / soil

A nontaxonomic but cartographic grouping of soils or land segments, which combines related soils into units having similarity in geomorphic position, landform, edaphic and mechanical properties of soils (climate, drainage, particle size, etc.), and to some degree similarity in the geological nature of the soil materials and taxonomic classes.

association (soil)

A natural grouping of soil associates based on similarities in climatic or physiographic factors and soil parent materials. It may include a number of soil associates provided that they are all present in significant proportions.

Atterberg limits

See - liquid limit - plastic limit

auger (soil)

A tool for boring into the soil and withdrawing a small sample for observation in the field or laboratory. The different kinds of augers include those having worm-type bits, unenclosed; those having worm-type bits enclosed in a hollow cylinder; and those having a hollow half-cylinder with cutting edge on the side that rotates around a stabilizing vane.

autotrophic

Capable of utilizing inorganic carbon as the main source of carbon and of obtaining energy for life processes from the oxidation of inorganic elements (chemotrophic) or from radiant energy (phototrophic).

available nutrient

The portion of any element or compound in the soil that can be readily absorbed and assimilated by growing plants. (Available should not be confused with exchangeable.)

available water

The portion of water in a soil that can be readily absorbed by plant roots. Most workers consider it to be the water held in the soil against a pressure of up to approximately 15 bars. See also - field capacity - moisture tension, soil

azonal soil

Soil without distinct genetic horizons. Such soils may have non-Chernozemic Ah horizons or thin Chernozemic Ah horizons but lack B horizons. In Canada they are included with Regosolic soils.