Glossary of Terms in Soil Science

igneous rock

Rock formed by the cooling and solidification of magma. It has not been changed appreciably since its formation.


A hydrous mica. See also hydrous mica.

illuvial horizon

A soil horizon in which material carried from an overlying layer has been precipitated from solution or deposited from suspension as a layer of accumulation. See also eluvial horizon.


The process of depositing soil material removed from one horizon in the soil to another, usually from an upper to a lower horizon in the soil profile. Illuviated substances include silicate clay, hydrous oxides of iron and aluminum, and organic matter.

immature soil

A soil having indistinct or only slightly developed horizons. Also called juvenile soil.


The conversion of an element from the inorganic to the organic form in microbial tissues so that the element is not readily available to other organisms or plants.

impeded drainage

A condition that hinders the movement of water by gravity through soils.


Resistant to penetration by fluids or roots.

impoverishment ( soil)

The process or the result of making the soil less productive.

improvement / soil

The processes for or the results of making the soil more productive for growing plants by drainage, irrigation, addition of fertilizers and soil amendments and so on.

indicator plants

Plants that are characteristic of specific soil or site conditions.

induced pan

See pressure or induced pan.

indurated layer

A soil layer that has become hardened, generally by cementation of soil particles.


The downward entry of water into the soil.

infiltration capacity (Obsolete)

See infiltration rate.

infiltration rate

A soil characteristic determining or describing the maximum rate at which water can enter the soil under specified conditions including the presence of excess water. It has the dimensions of velocity (i.e. cm³ cm-² sec-¹ = cm sec-¹). It was formerly called infiltration capacity. See also infiltration velocity.

infiltration velocity

The actual rate at which water is entering the soil at a given time. It may be less than the maximum (the infiltration rate) because of a limited supply of water (rainfall or irrigation). It is expressed in the same units that are used for the infiltration rate. See also infiltration rate.


A device for measuring the rate of entry of fluid into a porous body, for example, water into soil.


The prevention of growth or multiplication of organisms.


The artificial introduction of microorganisms into a habitat or their introduction into a culture medium.

inorganic soil

A soil made up mainly of mineral particles. See mineral soil.

intergrade (soil)

A soil that possesses moderately well-developed distinguishing characteristics of two or more genetically related taxa.

interstratified clay mineral

An aggregation composed of random or regular intergrowths of two or more clay minerals.

intrazonal soil

A soil having a morphology that shows the influence of some local factor of relief parent material or age rather than of climate and vegetation.

intrinsic permeability

The property of a porous material that is related to the ease with which gases or liquids can pass through it. The Darcy ""k"" multiplied by n/pg where: n is the viscosity of the fluid in poises p is the density of the fluid in g cm-³ and g is the acceleration of gravity in cm sec-². See also permeability of soil; and water / soil.


Atom or group of atoms or compound that is electrically charged as a result of the loss of electrons (cation) or the gain of electrons (anion).

ion activity

The effective concentration of a particular ion in a solution or soil-water system. It is expressed analogously to pH as ""pCa"" or ""pNa"" and so forth.

iron pan

A thin indurated soil horizon in which iron is a major constituent of the cementing material. Several kinds of cementing materials occur: I - iron - organic matter complexes II - hydrous oxides of manganese and iron III - hydrous iron oxides


The artificial application of water to the soil for the benefit of growing crops.

irrigation efficiency

The ratio of the water actually consumed by crops on an irrigated area to the amount of water diverted from the source onto the area.

irrigation methods

The manner in which water is artificially applied to an area. The methods and the manner of applying the water follow:

Water is applied at the upper end of a strip having earthen dikes to confine the water to the strip.
Water is applied rapidly to fairly level plots surrounded by levees. The basin is a small check.
Water is applied to small closely spaced furrows in grain and forage crops to confine the flow of irrigation water to one direction.
Water is released from field ditches and allowed to flood over the land.
Water is applied to row crops in ditches between the rows made by tiilage implements.
Water is sprayed over the soil surface through nozzles from a pressure system.
Water is applied to open ditches or tile lines until the water table is high enough to wet the soil.
wild flooding
Water is released at high points in the field and distribution is uncontrolled.


Points of a cultivating implement having equal dynamometer pull; a line on a map of a cultivated field connecting points of equal dynamometer pull.

isomorphous substitution

The replacement of one atom by another of similar size in a crystal lattice without disrupting or changing the crystal structure of the mineral.