Glossary

A HORIZON The surface horizon of a mineral soil having maximum biological activity, or eluviation (removal of materials dissolved or suspended in water), or both.

ABC SOIL A soil with a complete profile, including an A, a B, and a C horizon.

ABSORBING COMPLEX The materials in the soil that hold water and chemical compounds, mainly on their surfaces. They are chiefly the fine mineral matter and organic matter.

AC SOIL A soil with an incomplete profile, including an A and a C horizon, but no B horizon. Commonly such soils are young, like those developing from alluvium or on steep, rocky slopes.

ACID SOIL Generally, a soil that is acid throughout most or all of the parts of it that plant roots occupy. Commonly applied to only the surface-plowed layer or to some other specific layer or horizon of a soil. Practically, this means a soil more acid than pH 6.6; precisely, a soil with a pH value less than 7.0. A soil having a preponderance of hydrogen over hydroxyl ions in the soil solution.

ACTINOMYCETES A group of soil microorganisms which produce an extensive threadlike network. They resemble the soil molds in some respects but are more like the bacteria in size.

ADDITIVE A material added to fertilizer to improve its chemical or physical condition. An additive to liquid fertilizer might prevent crystals from forming in the liquid at temperatures where crystallization would normally take place.

ADSORB Removal of a substance in solution to a solid surface or a separate phase; to accumulate on a surface.

ADSORPTION The attachment of compounds or ionic parts of salts to a surface or another phase. Nutrients in solution (ions) carrying a positive charge become attached to (adsorbed by) negatively charged soil particles.

AERATION, SOIL The exchange of air in soil with air from the atmosphere. The composition of the air in a well-aerated soil is similar to that in the atmosphere; in a poorly aerated soil, the air in the soil is considerably higher in carbon dioxide and lower in oxygen than the atmosphere above the soil.

AEROBIC (1) Conditions with oxygen gas as a part of the environment. (2) Living or acting only in the presence of air or free oxygen. (3) Pertaining to the activity of organisms that grow under aerobic conditions, such as aerobic decomposition.

AGGREGATE (OF SOIL) Many fine soil particles held in a single mass or cluster, such as a clod, crumb, block, or prism. Many properties of the aggregate differ from those of an equal mass of unaggregated soil.

ALKALI SOIL Generally, a highly alkaline soil. Specifically, an alkali soil has so high a degree of alkalinity pH 8.5 or higher or so high a percentage of exchangeable sodium-15 percent or higher or both, that the growth of most crop plants is reduced. (In former years this term was also applied loosely to both alkali and saline soils. The term is also applied by some to those uncommon soils that contain highly alkaline salts, such as sodium carbonate.)

ALKALINE SOIL Generally, a soil that is alkaline throughout most or all of the parts of it occupied by plant roots; although the term is commonly applied to only a specific layer or horizon of a soil. Precisely, any soil horizon having a pH value greater than 7.0; practically, a soil having a pH above 7.3.

ALLUVIAL SOILS Soils developing from transported and relatively recently deposited material (alluvium) with little or no modification of the original materials by soil-forming processes. (Soils with well-developed profiles that have formed from alluvium are grouped with other soils having the same kinds of profiles, not with the alluvial soils.)

ALLUVIUM Sand, mud, and other sediments deposited on land by streams.

ALUMINO-SILICATES Compounds containing aluminum, silicon, and oxygen atoms as main constituents.

AMENDMENT Any material, such as lime, gypsum, sawdust, or synthetic conditioners, that is worked into the soil to make it more productive. Strictly, a fertilizer is also an amendment, but the term "amendment" is used most commonly for added materials other than fertilizer.

AMINO ACIDS Amino acids are nitrogen-containing organic compounds, large numbers of which link together in the formation of a protein molecule. Each amino acid molecule contains one or more amino ( NH₂) groups and at least one carboxyl ( COOH) group. In addition, some amino acids (cystine and methionine) contain sulfur.

AMMONIA A colorless gas composed of one atom of nitrogen and three atoms of hydrogen. Ammonia liquefied under pressure is used as a fertilizer.

AMMONIFICATION The formation by organisms of ammonium compounds from nitrogen-containing organic materials.

AMMONIUM ION The positively charged NH₄+ ion. The form in which nitrogen occurs in many commercial fertilizers.

ANAEROBIC Living or functioning in the absence of air or free oxygen.

ANHYDROUS Dry, or without water. Anhydrous ammonia is water free; in contrast to the water solution of ammonia commonly known as household ammonia.

ANION An ion carrying a negative charge of electricity.

ANTHROPIC SOIL A soil produced from a natural soil or other earthy deposit by the work of man that has new characteristics that make it different from the natural soil. Examples include deep, black surface soils resulting from centuries of manuring, and naturally acid soils that have lost their distinguishing features because of many centuries of liming and use for grass.

ANTIBIOSIS Opposed to living. Antibiotics suppress some micro-organisms.

APATITE A native phosphate of lime. The name is given to the chief mineral of phosphate rock and the inorganic compound of bone.

AQUA AMMONIA A water solution of ammonia.

AQUIFER A water-bearing formation through which water moves more readily than in adjacent formations of lower permeability.

ARID CLIMATE A very dry climate like that of desert or semidesert regions where there is only enough water for widely spaced desert plants. The limits of precipitation vary widely according to temperature, with an upper limit for cool regions of less than 10 inches and for tropical regions of as much as 20 inches. (The precipitation-effectiveness index ranges from 0 to about 16.)

ARID REGION Areas where the potential water losses by evaporation and transpiration are greater than the amount of water supplied by precipitation. In the United States this area is broadly considered to be the dry parts of the 17 Western States.

ASH The nonvolatile residue resulting from the complete burning of organic matter. It is commonly composed of oxides of such elements as silicon, aluminum, iron, calcium, magnesium, and potassium.

ASSIMILATION Conversion of substances taken in from the outside into living tissue of plants or animals.

AUTOTROPHIC Capable of using (oxidizing) simple chemical elements or compounds, such as iron, sulfur, or nitrates, to obtain energy for growth.

AUXINS Organic substances which cause lengthening of the stem when applied in low concentrations to shoots of growing plants.

AVAILABLE NUTRIENT IN SOILS The part of the supply of a plant nutrient in the soil that can be taken up by plants at rates and in amounts significant to plant growth.

AVAILABLE WATER IN SOILS The part of the water in the soil that can be taken up by plants at rates significant to their growth; usable; obtainable.

AZONAL SOILS A general group of soils having little or no soil profile development. Most of them are young. In the United States, Alluvial soils, Lithosols, and Regosols are included in the azonal group.

B HORIZON A soil horizon, usually beneath an A horizon, or surface soil, in which (1) clay, iron, or aluminum, with accessory organic matter, have accumulated by receiving suspended material from the A horizon above it or by clay development in place; (2) the soil has a blocky or prismatic structure; or (3) the soil has some combination of these features. In soils with distinct profiles, the B horizon is roughly equivalent to the general term "subsoil."

BANDING (OF FERTILIZERS) The placement of fertilizers in the soil in continuous narrow ribbons, usually at specific distances from the seeds or plants. The fertilizer bands are covered by the soil but are not mixed with it.

BASE SATURATION The relative degree to which soils have metallic cations absorbed. The proportion of the cation-exchange capacity that is saturated with metallic cations.

BASIN IRRIGATION (OR LEVEL BORDERS) The application of irrigation water to level areas that are surrounded by border ridges or levees. Usually irrigation water is applied at rates greater than the water intake rate of the soil. The water may stand on uncropped soils for several days until the soil is well soaked; then any excess may be used on other fields. The water may stand a few hours on fields having a growing crop.

BASIN LISTING A method of tillage that creates small basins by damming lister furrows at regular intervals of about 4 to 20 feet. This method is a modification of ordinary listing and is carried out approximately on the contour on nearly level or gently sloping soils as a means of encouraging water to enter the soil rather than to run off the surface.

BC SOIL A soil with a B and a C horizon but with little or no A horizon. Most BC soils have lost their A horizons by erosion.

BEDDING SOIL Arranging the surface of fields by plowing and grading into a series of elevated beds separated by shallow ditches for drainage.

BEDROCK The solid rock underlying soils and other earthy surface formations.

BENCH TERRACES An embankment constructed across sloping soils with a steep drop on the downslope side.

BLOWOUT An area from which soil material has been removed by wind. Such an area appears as a nearly barren, shallow depression with a flat or irregular floor consisting of a resistant layer, an accumulation of pebbles, or wet soil lying just above a water table.