Soil Formation, Erosion, Composition, and Its Properties

Igneous rock

Rock formed directly from magma.

Sedimentary rock

Rock that forms when sediments such as muds, sands, or gravels are compressed by overlying sediments.

Metamorphic rock

Rock that forms when sedimentary rock, igneous rock, or other metamorphic rock is subjected to high temperature and pressure.

Rock cycle

The geologic cycle governing the constant formation, alteration, and destruction of rock material that results from tectonics, weathering, and erosion, among other processes.

Erosion

The physical removal of rock fragments from a landscape or ecosystem.

Acid precipitation

Precipitation high in sulfuric acid and nitric acid. Also known as Acid rain.

Chemical weathering

The breakdown of rocks and minerals by chemical reactions, the dissolving of chemical elements from rocks, or both these processes.

Physical weathering

The mechanical breakdown of rocks and minerals.

C horizon

The least-weathered soil horizon, which always occurs beneath the B horizon and is similar to the parent material.

B horizon

Commonly known as subsoil, a soil horizon is composed primarily of mineral material with very little organic matter.

E horizon

A zone of leaching, or eluviation, found in some acidic soils under the O horizon or, less often, the A horizon.

A horizon

Frequently the top layer of soil, a zone of organic material and minerals that have been mixed together. Also known as Topsoil.

Horizon

A horizontal layer in a soil defined by distinctive physical features such as color and texture.

O horizon

The organic horizon at the surface of many soils, composed of organic detritus in various stages of decomposition.

Humus

The most fully decomposed organic matter in the lowest section of the O horizon.

Parent material

The underlying rock material from which the inorganic components of a soil are derived.

Base saturation

The proportion of soil bases to soil acids, expressed as a percentage.

Cation exchange capacity (CEC)

The ability of a particular soil to adsorb and release cations.

Water holding capacity

The amount of water a soil can hold against the draining force of gravity.

Permeability

The ability of water to move through the soil.

Porosity

The size of the air spaces between particles.