Soil Glossary

A soil horizon is a layer parallel to the soil surface, with physical characteristics that differ from the layers above and below. Each soil type usually has three or four horizons. Horizons are defined in most cases by obvious physical features, mainly colour and texture.

A subsurface soil horizon characterised by the in-situ alteration of soil material or the accumulation of clay, iron, and other materials from the topsoil. The soil material shows substantial change relative to the parent material, and generally contains less organic matter and lower levels of biological activity than the surface topsoil.

The cambic horizon (from Latin cambiare, to change) is a subsurface horizon that shows evidence of alteration relative to the underlying horizons. Cambic horizons are subsurface soil layers that exhibit pedogenic change without significant accumulation of illuviated material such as clay, organic carbon, iron and aluminium oxyhydroxides, carbonate, gypsum, or soluble salts. A cambic horizon always contains less carbonate than an underlying horizon with calcium carbonate accumulation.

A vertical section of soil horizons from the upper layer to the parent material, showing the arrangement of soil horizons typical for individual soil types and used as a basis for soil classification.

The original material (rock, such as limestone, flysch, or tonalite) from which the soil profile has developed through pedogenesis, is usually found at the base of the profile as weathered but otherwise unaltered mineral or organic material.

An unconsolidated residual or transported material that overlies the solid rock on the earth.

Silt is soil or rock formed from granular material of a specific grain size, which retains water for longer periods and has a low exchange capacity. Due to its moisture-retentive properties, silty soil is cold and drains poorly. The mineral fraction of the soil consists of particles ranging from 0.002 to 0.063 mm in diameter.

The arrangement of soil particles in various aggregates differing in shape, size, stability, and degree of adhesion to one another. Soil porosity depends on the distribution of these structural aggregates.

Peds are aggregates of soil particles formed by pedogenic processes; this natural organisation of particles creates discrete units separated by pores or voids. They are aggregates of soil mineral particles bound together with or without organic materials.

Soil acidity is measured in pH units and indicates the concentration of hydrogen (H) ions in the soil solution. The lower the soil pH, the greater the acidity. pH is measured on a logarithmic scale from 1 to 14, with 7 being neutral. A soil with a pH of 4 is ten times more acidic than a soil with a pH of 5 and one hundred times more acidic than a soil with a pH of 6.

Exchangeable cations are those that can be replaced by a cation from an added salt solution.

The total amount of exchangeable anions that a soil can adsorb, expressed in milliequivalents per 100 grams of soil or other adsorbing material, such as clay.

A chemical element or substance (such as nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S)) that is essential in relatively large amounts for the growth and health of a living organism.

Humus is the organic material in soil. It is not a form of soil itself, but rather the decomposed remains of leaves, grass, and other organic matter, broken down by bacteria and fungi and incorporated into the soil. Humus is highly nutritious, rich in minerals, microbes, and fungi, and is vital for healthy plant growth. It can also retain 80–90% of its own weight in moisture.

Groundwater is naturally recharged by rainwater, snowmelt, or water that leaks through the bottom of some lakes and rivers. It is also the water present beneath the Earth’s surface in soil pore spaces and in the fractures of rock formations.

Removal of dissolved or suspended material from one or more soil layers occurs when water movement, due to rainfall exceeding evaporation, transports these materials. Such loss of material in solution is often called leaching. The process of eluviation affects soil composition.

Erosion is the wearing away of land or soil caused by water (including rill, inter-rill, gully, snowmelt, and river and lake bank erosion), wind (desiccation and wind-blown erosion), translocation (tillage, land levelling, harvesting of root crops, trampling, and burrowing animals), and geological processes (internal subterranean erosion by groundwater, coastal erosion, and landslides). Erosion can also be exacerbated by poor land management, such as overgrazing, deforestation, or inappropriate use of mechanisation (for example, ploughing down a hill slope).

Gully erosion occurs when water is channelled across unprotected land (up to 30 cm) and washes away soil along the drainage lines.

A fertile soil developed from deposited alluvium by water flowing over flood plains or in river beds. Alluvial soils are rich agricultural lands.

Having, at a depth between 20 and 100 cm from the soil surface, with a base saturation of 50% or more.

A well-drained, reddish, clayey to silty clayey soil with neutral pH conditions is typical of the Mediterranean region. The reddish colour of terra rossa results from the preferential formation of hematite over goethite. This soil type typically occurs as a discontinuous layer, ranging from a few centimetres to several metres in thickness, covering limestone and dolomite bedrock in karst regions. The high internal drainage and neutral pH conditions of terra rossa result from the karstic nature of the underlying limestone and dolomite.