Glossary of Terms in Soil Science

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.

neutral soil

A soil in which the surface layer to plow depth is neither acid nor alklaine in reaction. See also acid soil; alkaline soil; pH, soil; and reaction, soil.

nitrate reduction

The biochemical reduction of nitrate.


The biochemical oxidation of ammonium to nitrate.

nitrogen assimilation

The incorporation of nitrogen into organic cell substances by living organisms.

nitrogen cycle

The sequence of biochemical changes by which nitrogen is used by a living organism, liberated upon the death and decomposition of the organism, and converted to its original state of oxidation.

nitrogen fixation

The conversion of elemental nitrogen (N2) to organic combinations or to forms readily utilizable in biological processes.

nodule (soil)

A rounded unit within the soil matrix that differs from the surrounding material because of the concentration of some constituent or a change in fabric.

noncapillary porosity (Obsolete)

(obsolete) See air porosity.


Nonsoil is the collection of soil material or soil-like material that does not meet the definition of soil. It includes soil displaced by unnatural processes and unconsolidated material unaffected by soil-forming processes, except for the material that occurs within 15 cm (6 inches) below soil as defined. Nonsoil also includes unconsolidated mineral or organic material thinner than 10 cm (4 inches) overlying bedrock; organic material thinner than 40 cm (16 inches) overlying a hydric layer; and soil covered by more than 60 cm (24 inches) of water in the driest part of the year. See soil.

normal erosion

The gradual erosion of land used by man. It does not greatly exceed natural erosion. See natural erosion.