The result of processes that entrain and transport earth materials along coastlines, in streams, and on hillslopes. Wind and water are common agents through which forces are applied to resistant rocks, soils, or other unconsolidated materials. Erosion types often are designated on the basis of the agent: wind erosion, fluvial erosion, and glacial erosion. Fluvial erosion usually has been regarded as the most effective type in shaping the land surface during recent geologic time (see illustration). Under certain environmental conditions, however, wind erosion moves considerable quantities of earth materials, as demonstrated during the “dust bowl” years in the United States. Glacial erosion shaped much of the land surface during the Quaternary Period of geologic time. Each type of erosion produces distinctive landforms, contributing to the diversity of terrestrial landscapes. See also: Eolian landforms; Geomorphology; Glaciology; Mass wasting; Quaternary; Stream transport and deposition

The natural medium for growth of tree roots and associated forest vegetation. Proper forest growth and management depend on many factors, including the quality of the forest soil (see illustration). In general, weathering of underlying rock, climatic factors (such as rain, wind, and temperature fluctuations), living organisms, topography, and disturbances (such as glacial action, flooding, erosion, and agriculture) interact over time to produce soil. Soil helps sustain the productivity and diversity of terrestrial life by serving as the source of nutrients and water for plants, by physically anchoring plants, and by serving as the habitat for soil organisms. Far more numbers and species of organisms live belowground than aboveground. Because soil usually develops quite slowly, soil conservation is important for maintaining the productivity of the land. Soil also acts as a water purifier; organic substances and most pollutants are arrested in upper soil horizons, purifying water before it enters streams or aquifers. In addition, soil is the repository of life; as plants and animals die, they become part of the soil. Overall, soil profiles reveal much about soil history and disturbances, including plant and animal remains, flooding, volcanic ash deposits, windthrow events (that is, events in which trees are uprooted by wind), rotting wood, charcoal from fires, and past human activities. See also: Forest; Forest ecosystem; Forestry; Soil; Soil ecology

Soil ingestion by animals. Grazing animals such as sheep and cattle ingest varying amounts of soil when they graze herbage contaminated with it.

A group of substances that are natural products of earth surface environments. Probably the most widely distributed organic carbon–containing materials in terrestrial and aquatic environments, they are dark-colored, predominantly aromatic, acidic, hydrophilic, molecularly flexible polyelectrolytes. Humic substances constitute 70–80% of the organic matter in inorganic soils and are formed from the chemical and biological degradation of plant and animal residues and from synthetic activities of micro-organisms.

Deposits of windblown fine-grained angular particles that are produced by the breakdown of larger particles at desert or glacial margins. Suspension and transport by the wind produces particularly homogenous and well-sorted loess upon deposition. Loess is dominated by silt-sized (2–63 micrometers in diameter) particles of quartz, with smaller amounts of feldspars, calcium carbonate, and clay minerals, and can form successive, blanketing layers of thickness from centimeters to hundreds of meters, as in the Loess Plateau region of north-central China (Fig. 1). Loess occurs globally, encompassing the pampas of Argentina, the central and east European and Russian steppe, and the midcontinent United States, to name some locations. It is porous and unconsolidated, making it a valuable agricultural resource. Loess is highly erodible in wet conditions, although often mechanically strong when dry because calcium carbonate and clay aggregates bind the silt particles together. See also: Calcite; Clay minerals; Depositional systems and environments; Eolian landforms; Feldspar; Quartz

A soil of the past, that is, a fossil soil. Paleosols are most easily recognized when they are buried by sediments. They also include surface profiles that are thought to have formed under very different conditions from those now prevailing, such as the deeply weathered tropical soils of Tertiary geological age that are widely exposed in desert regions of Africa and Australia. Such profiles are generally known as relict paleosols. Those that can be shown to have been buried and then uncovered by erosion are known as exhumed paleosols. The main problem in defining the term paleosol comes not so much from complications such as these arising from its fossil nature, but from defining what is meant by soil, a term that has very different meanings for agronomists, engineers, geologists, and soil scientists. Considering research on soils of Antarctica and Mars and on paleosols in a variety of rocks ranging back to 3.5 × 10⁹ years old, soil can be considered distinct from sediment in that it forms in place, but soil need not necessarily include traces of life. At its most general level, soil is material forming the surface of a planet or similar body and altered in place from its parent material by physical, chemical, or biological processes.

A dark-brown or black residuum produced by the partial decomposition and disintegration of mosses, sedges, trees, and other plants that grow in marshes and other wet places. Forest-type peat, when buried and subjected to geological influences of pressure and heat, is the natural forerunner of most coal.

Perennially frozen ground. Two classes of frozen ground are generally distinguished: seasonally frozen ground, which freezes and thaws on an annual basis; and permafrost, which is formed when subsurface earth materials remain below 0°C (32°F) for more than a year, without regard to composition, phase, or cementation by ice. Permafrost occurs at most locations where the mean annual temperature at the ground surface is below freezing. Because substantial differences often exist between the temperature at the surface and that measured in the air, climate statistics do not provide a reliable guide to the existence or distribution of permafrost. Although permafrost is ultimately a climatically determined phenomenon, its presence or absence is strongly influenced by local factors, including microclimatic variations, circulation of ground water, type of vegetation cover, and thermal properties of subsurface materials.

The biological, physical, and chemical processes involved in the cycling of phosphorus in the environment. The cycle comprises a geological (long-term) cycle and a biological (short-term) cycle, the latter with both terrestrial and aquatic components (Fig. 1). The phosphorus cycle differs markedly from other nutrient cycles (carbon, nitrogen, oxygen, and hydrogen), because phosphorus has no significant gaseous phase and, therefore, no major atmospheric component to its cycle. See also: Biogeochemistry; Phosphorus

The use of plants to clean up environmental contamination of surface soils. Phytoremediation is more cost-effective and environmentally appealing than other currently available methods for soil detoxification. The most common approach for soil cleanup involves the excavation and removal of polluted soil to a chemical treatment facility or a long-term storage landfill facility. This method is very costly for large-scale decontamination and can be destructive to the environment. Phytoremediation, on the other hand, costs significantly less and does not require the same degree of environmental perturbation. See also: Environmental toxicology; Soil chemistry; Soil conservation