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Soil Resources Management


Soil: The unconsolidated portion of the earth's crust modified through physical, chemical, and biotic processes into a medium capable of supporting the growth of land plants. Soil extends from the earth's surface to the lower limit of biological activity. The soil volume includes a mineral fraction derived from geologic materials from the earth's crust; an organic fraction consisting of living, dead, and decomposing organisms and organic residues; and pore space containing air and water in varying percentages. Soil is three-dimensional, with layers (horizons) that vary in arrangement and thickness on different parts of the landscape. Soils are not static, but are in a dynamic equilibrium with the surrounding environment.

Soil horizon: A soil layer, approximately parallel with the earth's surface, having unique characteristics resulting from the position in the profile. Two examples are the A horizon, which usually lies at the soil surface and contains more organic matter than other soil horizons in the same profile, and the B horizon, a subsurface horizon expressing the greatest degree of alteration in the profile from the parent material.

Pedon: A sample unit just large enough to describe the pattern of soil horizon shapes and relations at one position on the landscape, typically 1 m2.

Soil family: The categorical level immediately above the soil series in the hierarchy of the Soil Taxonomy. Soil families provide groupings of soils with ranges in texture, mineralogy, temperature, and thickness, and are used typically in reconnaissance soil surveys.

Soil series: The lowest category in the hierarchy of the Soil Taxonomy, used in soil surveys as a practical grouping of pedons, alike enough in physical and chemical characteristics and in the arrangement and thickness of soil horizons that they behave similarly. Soil series are used typically in detailed soil surveys.

Soil phase: A subdivision of a soil series based on features that affect its use and management, such as slope, stoniness, and flooding. Soil phases are identified in a soil survey by appending the grouping modifier to the taxon name; e.g., Alpha loam, 15-30% slopes, frequently flooded.

Soil mapping unit: The name given to groups of polygons in a soil survey in which the landforms and soils are alike. Most soil mapping units are named after the soil phases present within them. A few, such as “rock outcrop,” are named after features where soil is absent. For each unit, the landscape setting, proportions of soils, and presence of any contrasting inclusions are documented in a map unit description.

Natural soils: Soils that occur naturally on the landscape, and have physical and chemical properties resulting from natural processes. Their characteristics can be reliably predicted from their landscape setting by considering local patterns and processes as they relate to the factors of soil formation.

Anthropic soils: Soils in which modern people have been the dominant soil forming agent. New soil materials often are placed over existing soils, in effect creating a new parent material for soil formation and a new "time zero" from which soil formation begins. The chemistry and physical conditions of anthropic soils is often unique, a result of inclusion of artifacts of human activity, such as brick, concrete, ash, wood, oil, metal, and other similar materials.

Soil genesis: The process of soil development from a parent material as a result of the five factors of soil formation: climate, organisms, topography, parent material, and time. Soils develop gradually over periods ranging from hundreds to thousands of years.

Parent material: The mineral and organic material from which a soil has formed. It may exist in its landscape position naturally or through human manipulation.

Soil management: Actions that affect soil characteristics. Soil is managed to achieve specific goals, such as to perpetuate native ecosystems, maintain crop yields, or reduce soil erosion. Soil management may or may not include soil modification.

Soil modification: Alteration of specific soil properties, usually in response to identification of a specific soil problem. Both soil compaction and soil aeration (to alleviate the adverse impacts of soil compaction) are examples of soil modification for particular objectives.

Soil compaction: Increased bulk density due to reduction in pore space within soil under applied pressure. Some consequences of soil compaction are reduced permeability to water and air and increased penetration resistance and surface runoff. Compaction can be used to strengthen the base of support for roads and structures. However, unintended compaction can slow plant growth, lowering vegetative productivity in compacted areas.

Soil Resources Management Table of Contents | RM#77 Table of Contents
update on 02/05/2004  I   http://www.nature.nps.gov/Rm77/soils/define.cfm   I  Email: Contact Us
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