Glossary of Terms in Soil Science

Darcy's law

(1) A law describing the rate of flow of water through porous media. (Named for Henry Darcy of Dijon, who formulated it in 1856 from extensive work on the flow of water through sand filter beds.) As formulated by Darcy the law is: Q = k*S*(H+e)/e where Q is the volume of water passed in unit time, S is the area of the bed, e is the thickness of the bed, H is the height of the water on top of the bed, and "k is a coefficient depending on the nature of the sand" and for cases where the pressure "under the filter is equal to the weight of the atmosphere." (2) Generalization for three dimensions: The rate of viscous flow of water in isotropic porous media is proportionai to, and in the direction of, the hydraulic gradient. (3) Generalization for other fluids: The rate of viscous flow of homogeneous fluids through isotropic porous media is proportional to, and in the direction of, the driving force

Dark Brown

A great group of soils in the Chernozemic order. The soils occur in the cool to cold, semiarid grassland regions and have a dark brown surface (Ah or Ap) horizon on a lighter colored brownish B (Bm, Btj, or Bt) horizon, which may be absent, over a highly base saturated, usually calcareous C horizon.

Dark Gray

A great group of soils in the Chernozemic order. The soils occur in the cool to cold subhumid grassland-forest transitional regions, and have a dark gray partially eluviated surface (Ahe or Ap) horizon and a brownish B (Bm, Btj, or Bt) horizon which may be absent over a highly base saturated usually calcareous C horizon.

Dark Gray Gleysolic

See Humic Gleysol.

deflation

The removal of fine soil particles from the soil by wind.

deflocculate

(1) To separate the individual components of compound particles by chemical or physical means or both. (2) To cause the particles of the disperse phase of a colloidal system to become suspended in the dispersion medium.

degradation

The changing of a soil to a more highly leached and weathered state, usually accompanied by morphological changes such as the development of an eluviated, light-colored A (Ae) horizon.

dehydrogenase

An enzyme that accelerates oxidation of a substrate by removing hydrogen from it.

dehydrogenation

Removal of hydrogen as from a molecule.

delta

A fan-shaped area at the mouth of a river formed by deposition of successive layers of sediments brought down from the land and spread out on the bottom of a basin. Where the stream current reaches quiet water, the bulk of the coarser load is dropped and the finer material is carried farther out. Deltas are recognized by nearly horizontal beds, termed bottomset beds, overlain by more steeply inclined and coarser-textured beds called foreset beds.

denitrification

The gaseous loss of nitrogen by either biological or chemical mechanisms but exclusive of ammonia volatilization.

depleted soil

Soil that has lost most of its available nutrients.

deposit

Material left in a new position by a natural transporting agent such as water, wind, ice or gravity or by human activity.

diatomaceous earth

An earthy deposit of fine, grayish, siliceous material composed chiefly or wholly of the remains of diatoms. It may occur as a powder or as a porous, rigid material.

diatoms

Algae having siliceous ceil walls that persist as a skeleton after death. These microscopic unicellular or colonial algae belong to the class Bacillariophyceae. They are abundant in both fresh and salt waters and their remains are widely distributed in soils.

differential water capacity

See water; soil.

diffusivity

See water; soil.

direct count

In soil microbiology any one of several methods for estimating by direct microscopic examination the total number of microorganisms in a given mass of soil.

discharge velocity

The rate of discharge of water through a porous medium per unit of total area perpendicular to the direction of flow.

disintegration

The breakdown of rock and mineral particles into smaller particles by physical forces such as frost action. See also weathering.

disperse

(1) To break up compound particles such as aggregates into the individual component particles. (2) To distribute or suspend fine particles such as clay in or throughout a dispersion medium such as water.

dispersion medium

The portion of a colloidal system in which the disperse phase is distributed.

diversion dam

A structure or barrier that diverts part of or all the water of a stream to a different course.

double layer

In colloid chemistry, the electric charges on the surface of the disperse phase, usually negative and the adjacent diffuse layer, usually positive, of ions in solution.

drag

The force retarding the flow of water or wind over the surface of the ground.

drain

(1) To provide channels, such as open ditches or drain tile, so that excess water can be removed by surface or by internal flow. (2) To lose water from the soil by percolation.

drain tile

Pipe used to conduct drainage water from the soil.

drumlin

An elongate or oval hill of glacial drift, commonly glacial till, deposited by glacier ice and having its long axis parallel to the direction of ice movement.

dry aggregate

A compound or secondary soil particle that is not broken down by dry sieving.

dry weight percentage

The ratio of the weight of any constituent of a soil to the ovendry weight of the soil. See also ovendry soil.

dryland farming

The practice of growing crops in areas of low rainfall without irrigation.

dunes

Wind-built ridges and hills of sand formed in the same manner as snowdrifts. They are started by some obstruction, such as a bush, boulder or fence, that causes an eddy or otherwise thwarts the sand-laden wind. Once begun, the dunes themselves offer further resistance and they grow to form various shapes.

duric

A Bc horizon that is strongly cemented and usually has an abrupt upper boundary and a diffuse lower boundary. Cementation is usually strongest near the upper boundary. Ordinarily the color is similar to that of the parent material and the structure is amorphous or coarse platy. Air-dried clods do not slake when immersed in water. The duric horizon does not meet the requirements of a podzolic B horizon but may meet those of a Bt horizon.

dust mulch

A loose, finely granular, or powdery layer on the surface of the soil, usually produced by shallow cultivation, and also by deposition.

dy

Finely divided, partly decomposed organic material accumulated in peat soils in the transition zone between the peat and the underlying mineral material. Dy-peat also refers to amorphous material formed from humus soils that have settled in lake waters. Dy is poorer in nutrients than gyttja and is characterized by a high C:N ratio.

dynamometer

An instrument for measuring draft of tillage implements, and for measuring resistance of soil to penetration by tillage implements.

Dystric Brunisol

A great group of soils in the Brunisolic order. The soils may have mull Ah horizons less than 5 cm (2 inches) thick. They have Bm horizons in which the base saturation (NaCI) is usually 65% to 100% and the pH (CaCI2) is usually 5.5 or lower