Glossary of Terms in Soil Science

talud

A short steep slope formed gradually at the downslope margin of a field by deposition against a hedge, a stone wall or other similar barrier.

talus

A sloping heap of loose rock fragments Iying at the foot of a cliff or steep slope.

taxon (plural taxa)

A class at any categorical level of a taxonomic system of classification.

tensiometer

A device for measuring the negative pressure or tension, of water in soil in situ; a porous permeable ceramic cup connected through a tube to a manometer or vacuum gauge.

terrace

A nearly level usually narrow plain bordering a river, lake or sea. Rivers sometimes are bordered by a number of terraces at different levels. There are also manmade terraces.

terric layer

An unconsolidated mineral substratum underlying organic soil material.

textural classification

See texture (soil).

texture (soil)

The relative proportions of the various soil separates in a soil as described by the classes of soil texture shown in Figure1. The names of textural soil classes may be modified by adding suitable adjectives when coarse fragments are present in substantial amounts; for example, "Gibraltar stony sandy loam," or "Saint-Jovite silt loam, stony phase." For other modifications see coarse fragments. The sand, loamy sand, and sandy loam are further subdivided on the basis of the proportions of the various sand separates present. The limits of the various classes and subclasses are:

sand (sable)
Soil material that contains 85% or more sand; the percentage of silt plus 1.5 times the percentage of clay does not exceed 15. coarse sand (sable grossier) 25% or more very coarse and coarse sand, and less than 50% any other one grade of sand.
sand (sable)
25% or more very coarse, coarse, and medium sand, and less than 50% fine or very fine sand.
fine sand (sable fin)
50% or more fine sand or less than 25% very coarse, coarse, and medium sand and less than 50% very fine sand.
very fine sand (sable très fin)
50% or more very fine sand.
loamy sand (sable loameux)
Soil material that contains at the upper limit 85 to 90% sand, and the percentage of silt plus 1.5 times the percentage of clay is not less than 15, at the lower limit it contains not less than 70 to 85% sand, and the percentage of silt plus twice the percentage of clay does not exceed 30.
loamy coarse sand (sable grossier loameux)
25% or more very coarse and coarse sand and less than 50% any other one grade of sand.
loamy sand (sable loameux)
25% or more very coarse, coarse, and medium sand and less than 50% fine or very fine sand.
loamy fine sand (sable fin loameux)
50% or more fine sand or less than 25% very coarse, coarse, and medium sand and less than 50% very fine sand.
loamy very fine sand (sable très fin loameux)
0% or more very fine sand.
sandy loam (loam sableux)
Soil material that contains either 20% or less clay, with a percentage of silt plus twice the percentage of clay that exceeds 30, and 52% or more sand; or less than 7% clay, less than 50% silt, and between 43% and 52% sand.
coarse sandy loam (loam sableux grossier)
25% or more very coarse and coarse sand and less than 50% any other one grade of sand.
sandy loam (loam sableux)
30% or more very coarse, coarse, and medium sand, but less than 25% very coarse sand, and less than 30% very fine or fine sand.
fine sandy loam (loam sableux fin)
30% or more fine sand and less than 30% very fine sand or between 15 and 30% very coarse, coarse, and medium sand.
very fine sandy loam (loam sableux très fin)
30% or more very fine sand or more than 40% fine and very fine sand, at least half of which is very fine sand, and less than 15% very coarse, coarse, and medium sand.
loam (loam)
Soil material that contains 7 to 27% clay, 28 to 50% silt, and less than 52% sand.
silt loam (loam limoneux)
Soil material that contains 50% or more silt and 12 to 27% clay, or 50 to 80% silt and less than 12% clay.
silt (limon)
Soil material that contains 80% or more silt and less than 12% clay.
sandy clay loam (loam sablo-argileux)
Soil material that contains 20 to 35% clay, less than 28% silt, and 45% or more sand.
clay loam (loam argileux)
Soil material that contains 27 to 40% clay and 20 to 45% sand.
silty clay loam (loam limono-argileux)
Soil material that contains 27 to 40% clay and less than 20%sand.
sandy clay (argile sableuse)
Soil material that contains 35% or more clay and 45% or more sand.
silty clay (argile limoneuse)
Soil material that contains 40% or more clay and 40% or more silt.
clay (argile)
Soil material that contains 40% or more clay, less than 45% sand, and less than 40% silt.
heavy clay (argile lourde)
Soil material that contains more than 60% clay.

Figure 1 Soil textural classes. Percentages of clay and sand in the main textural classes of soils; the remainder of each class is silt.

thermal analysis (or differential thermal analysis)

A method ot analyzing a soil sample for constituents, based on the differential rate of heating of the unknown and standard samples when a uniform source of heat is applied.

thermal conductivity

The rate of transfer of heat to or from a point in the soil.

thermogenic soils

Soils with properties that have been influenced primarily by high soil temperature as the dominant soil-formation tactor developed in subtropical and equatorial regions.

thermophile

An organism that grows best at temperatures of 50°C or higher.

thermosequence

A sequence of related soils that differ, one from the other, primariiy as a result of temperature as a soil-formation factor.

threshold moisture content

The minimum moisture condition measured in terms of either moisture content or moisture stress, at which biological activity just becomes measurable.

tidal flats

Areas of nearly flat, barren mud periodically covered by tidal waters. Normally these materials have an excess of soluble salt. A miscellaneous land type.

tier

A depth subdivision used in the classification of Organic soils. (1) surface tier: The upper 40 cm (16 inches) of peat. (2) middle tier: The tier just below the surface tier. It is 80 cm (32 inches) thick or extends to a lithic or hydric contact. The great group classification of Organic soils is usually based on this tier. (3) bottom tier: The tier below the middle tier. It is 40 cm (16 inches) thick or extends to a lithic or hydric contact. The subgroup classification of Organic soils is based partly on this tier.

tight soil

A compact, relatively impervious and tenacious soil or subsoil which may or may not be plastic.

tile drain

Pipe piaced at suitable depths and spacings in the soil or subsoil to provide water outlets from the soil. The pipe may be concrete, ceramic, fiber, plastic or any other suitable material.

till

Unstratified glacial drift deposited directly by the ice and consisting of clay, sand, gravel, and boulders intermingled in any proportion.

till (to)

To plow and prepare for seeding; to seed or cultivate the soil.

tillage

The operation, practice, or art of tilling land to improve it for agricultural purposes.

tilth

The physical condition of soil as related to its ease of tillage, fitness as a seedbed, and impedance to seedling emergence and root penetration.

topography

The physical features of a district or region, such as those represented on a map, taken collectively; especially the relief and contours of the land.

toposaic

A photomap on which topographic or terrain-form lines are shown as on topographic quadrangles. See also photomap and planisaic.

toposequence

toposequence toposequence A sequence of related soils that differ one from the other primarily because of topography as a soil-formation factor. See also clinosequence.

topsoil

(1) The layer of soil moved in cultivation. See also surface soil. (2) The A horizon. (3) The Ah horizon. (4) Presumably fertile soil material used to topdress road-banks, gardens, and lawns.

total potential (of soil water)

See water / soil.

total pressure

See water, soil.

trace element (Obsolete)

See micronutrient.

traffic pan

See pressure or induced pan.

transitional soil

A soil with properties intermediate between those of two different soils and genetically related to them. See also intergrade / soil.

truncated

Having lost all or part of the upper soil horizon or horizons.

tuff

Volcanic ash usually more or less stratified and in various states of consolidation.

tundra

A level or undulating treeless plain characteristic of arctic regions.

tundra soil

Any soil in the tundra region.

type; soil

(1) A unit in the natural system of soil classification; a subdivision of a soil series consisting of or describing soils that are alike in all characteristics including the texture of the A horizon. (2) In Europe the term is roughly equivalent to a great soil group.