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Soil

2007 Schools Wikipedia Selection. Related subjects: Geology and geophysics

   Loess field in Germany
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   Loess field in Germany
   Soil horizons are formed by combined biological, chemical and physical
   alterations.
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   Soil horizons are formed by combined biological, chemical and physical
   alterations.
   Project leader and students discuss soil quality in a garden the
   students built themselves.
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   Project leader and students discuss soil quality in a garden the
   students built themselves.
   Motorcycle rider digging into soil
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   Motorcycle rider digging into soil

   Soil is the collection of natural bodies that form in earthy material
   on the land surface. The term is popularly applied to the material on
   the surface of the earth's moon and Mars, a usage acceptable within a
   portion of the scientific community.

   Soil consists of mineral and organic matter, as well as living
   organisms. Soil, comprising the pedosphere, is positioned at the
   interface of the lithosphere with the biosphere, atmosphere, and
   hydrosphere. Soil formation, or pedogenesis, is the combined effect of
   physical, chemical, biological, and anthropogenic processes on soil
   parent material resulting in the formation of soil horizons.

   Soil is among our most important natural resources because of its
   position in the landscape and its dynamic, physical, chemical, and
   biologic functions. Indeed, engineers, agronomists, chemists,
   geologists, geographers, biologists, microbiologists, sylviculturists,
   sanitarians, archaeologists, and specialists in regional planning, all
   depend on, and contribute to, knowledge of soils. While the general
   concept of soil is well established, the definition of soil varies,
   according to the perspective of the discipline or occupation using soil
   as a resource.

   The understanding of soil is incomplete. Despite the duration of
   mankind's dependence on and curiosity about soil, exploring the
   diversity and dynamic of this resource continues to yield fresh
   discoveries and insights. New avenues of soil research are compelled by
   our need to understand soil in the context of climate change,
   greenhouse gases, and carbon sequestration. Our interest in maintaining
   the planet's biodiversity and in exploring past cultures has also
   stimulated renewed interest in achieving a more refined understanding
   of soil.

Soil classification

   As of 2006, the World Reference Base for Soil Resources (WRB) is the
   international standard soil classification system. Development was
   coordinated by the International Soil Reference and Information Centre
   (ISRIC) and sponsored by the International Union of Soil Sciences
   (IUSS) and the Food and Agriculture Organization (FAO) via its Land &
   Water Development division. It replaces the previous FAO soil
   classification.

   The WRB borrows heavily from modern soil classification concepts,
   including USDA soil taxonomy. The classification is based mainly on
   soil morphology as an expression pedogenesis. A major difference with
   USDA soil taxonomy is that soil climate is not part of the system,
   except in so far as climate influences soil profile characteristics.

   Vernacular soil classification systems are developed by the land users.
   Their structure is either nominal, giving unique names to soils or
   landscapes, or descriptive, naming soils by their characteristics such
   as red, hot, fat, or sandy. Soils are distinguished by obvious
   characteristics, such as physical appearance (e.g., colour, texture,
   landscape position), performance (e.g., production capability,
   flooding), and accompanying vegetation. A vernacular distinction
   familiar to many is classifying texture as heavy or light. Light soils
   have lower clay content than heavy soils. They often drain better and
   dry out sooner, giving them a lighter colour. Lighter soils, with their
   lower moisture content and better structure, take less effort to turn
   and cultivate. Contrary to popular belief light soils do not weigh less
   than heavy soils on an air dry basis nor do they have more porosity.

Soil characteristics

   Soils tend to develop an individualistic pattern of horizontal zonation
   under the influence of site specific soil-forming factors. The
   composition of these individual soil horizons, and their relationship
   within the soil profile is key to understanding behavior. Soil colour,
   soil structure, and soil texture are especially important components of
   soil morphology.

   Soil colour is the first impression one has when viewing soil. Striking
   colors and contrasting patterns are especially memorable. The Red River
   (Mississippi watershed) carries sediment eroded from extensive reddish
   soils like Port Silt Loam in Oklahoma. The Yellow River in China
   carries yellow sediment from eroding loessal soils. Mollisols in the
   Great Plains are darkened and enriched by organic matter. Podsols in
   boreal forests have highly contrasting layers due to acidity and
   leaching.

   Soil color is primarily influenced by soil mineralogy. The extensive
   and various iron minerals in soil are responsible for an array of soil
   pigmentation. Color development and distribution of colour within a
   soil profile result from chemical weathering, especially redox
   reactions. As the primary minerals in soil-parent material weather, the
   elements combine into new and colorful compounds. Iron forms secondary
   minerals with a yellow or red colour; organic matter decomposes into
   black and brown compounds; and manganese forms black mineral deposits.
   These pigments give soil its various colors and patterns and are
   further affected by environmental factors. Aerobic conditions produce
   uniform or gradual colour changes while reducing environments result in
   disrupted color flow with complex, mottled patterns and points of
   colour concentration.

   Soil structure is the arrangement of soil particles into aggregates.
   These may have various shapes, sizes and degrees of development or
   expression. Soil structure influences aeration, water movement, erosion
   resistance, and root penetration. Observing structure gives clues to
   texture, chemical and mineralogical conditions, organic content,
   biological activity, and past use, or abuse.

   Surface soil structure is the primary component of tilth. Where soil
   mineral particles are both separated and bridged by
   organic-matter-breakdown products and soil-biota exudates, it makes the
   soil easy to work. Cultivation, earthworms, frost action and rodents
   mix the soil. This activity decreases the size of the peds to form a
   granular (or crumb) structure. This structure allows for good porosity
   and easy movement of air and water. The combination of ease in tillage,
   good moisture and air-handling capabilities, good structure for
   planting and germination are definitive of good tilth.

   Soil texture refers to sand, silt and clay composition in combination
   with gravel and larger-material content. Sand and silt are the product
   of physical weathering while clay is the product of chemical
   weathering. Clay content is particularly influential on soil behaviour
   due to a high retention capacity for nutrients and water. Due to
   superior aggregation, clay soils resist wind and water erosion better
   than silty and sandy soils. In medium-textured soils, clay can tend to
   move downward through the soil profile to accumulate in the subsoil.
   The lighter-textured, surface soils are more responsive to management
   inputs, but also more vulnerable to erosion and contamination.

   Texture influences many physical aspects of soil behaviour. Available
   water capacity increases with silt and, more importantly, clay content.
   Nutrient-retention capacity tends to follow the same relationship.
   Plant growth, and many uses which rely on soil, tends to favour
   medium-textured soils, such as loam and sandy loam. A balance in air
   and water-handling characteristics within medium-textured soils are
   largely responsible for this.

Soil and its environment

Soil in nature

   Soil formation processes never stop which require that soil is always
   changing. The long periods over which change occurs and the multiple
   influences of change mean that simple soils are rare. While soil can
   achieve relative stability in properties for extended periods of time,
   the soil life cycle ultimately ends in soil conditions that leave it
   vulnerable to erosion. Little of the soil continuum of the earth is
   older than Tertiary and most no older than Pleistocene. Despite the
   inevitability of soils retrogression and degradation, most soil cycles
   are long and productive. How the soil "life" cycle proceeds is
   influenced by at least five classic soil forming factors: regional
   climate, biotic potential, topography, parent material and the passage
   of time.

   An example of soil development from bare rock occurs on recent lava
   flows in warm regions under heavy and very frequent rainfall. In such
   climates plants become established very quickly on basaltic lava, even
   though there is very little organic material. The plants are supported
   by the porous rock becoming filled with nutrient bearing water, for
   example carrying dissolved bird droppings or guano. The developing
   plant roots themselves gradually breaks up the porous lava and organic
   matter soon accumulates but, even before it does, the predominantly
   porous broken lava in which the plant roots grow can be considered a
   soil.
   Sample of an aerial photo from a published soil survey
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   Sample of an aerial photo from a published soil survey

   Most of our knowledge of soil in nature comes from soil survey efforts.
   Soil survey, or soil mapping, is the process of determining the soil
   types or other properties of the soil cover over a landscape, and
   mapping them for others to understand and use. It relies heavily on
   distinguishing the individual influences of the five classic soil
   forming factors. This effort draws upon geomorphology, physical
   geography, and analysis of vegetation and land-use patterns. Primary
   data for the soil survey are acquired by field sampling and supported
   by remote sensing.

   Geologists have a particular interest in the patterns of soil on the
   surface of the earth. Soil texture, colour and chemistry often reflect
   the underlying geologic parent material and soil types often change at
   geologic unit boundaries. As of 2002, geologists classify surface soils
   using the 1938 USDA soil taxonomy but use the current version of USDA
   soil taxonomy to classify the buried soils that make up the
   paleopedological record. Buried paleosols mark previous land surfaces
   and record climatic conditions from previous eras. Geologists use this
   paleopedological record to understand the ecological relationships in
   past ecosystems. According to the theory of biorhexistasy, prolonged
   conditions conducive to forming deep, weathered soils result in
   increasing ocean salinity and the formation of limestone.

   Geologists and pedologists use soil profile features to establish the
   duration of surface stability in the context of geologic faults or
   slope stability. An offset subsoil horizon indicates rupture during
   soil formation and the degree of subsequent subsoil formation is relied
   upon to establish time since rupture.

   Soil examined in shovel test pits is used by archaeologists for
   relative dating based on stratigraphy (as opposed to absolute dating).
   Most typical is to use soil profile features to determine the maximum
   reasonable pit depth than needs to be examined for archaeological
   evidence in the interest of cultural resources management.

   Soils altered or formed by man (anthropic and anthropogenic soils) are
   also of interest to archaeologists. An example is Terra preta do Indio.

Soil uses

   A homeowner tests soil to apply only the nutrients needed. Farmers
   practice the same testing procedure.
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   A homeowner tests soil to apply only the nutrients needed. Farmers
   practice the same testing procedure.
   Due to their thermal mass, rammed earth walls fit in with environmental
   sustainability aspirations.
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   Due to their thermal mass, rammed earth walls fit in with environmental
   sustainability aspirations.
   A homeowner sifts soil made from his compost bin in background.
   Composting is an excellent way to recycle household and yard wastes.
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   A homeowner sifts soil made from his compost bin in background.
   Composting is an excellent way to recycle household and yard wastes.
   Light colored soils in northeast Iowa have lost their topsoil. These
   soils are highly erodible and very steep.
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   Light colored soils in northeast Iowa have lost their topsoil. These
   soils are highly erodible and very steep.

   Gardening and landscaping provide common and popular experience with
   soils. Homeowners and farmers alike test soils to determine how they
   can be maintained and improved. Plant nutrients such as nitrogen,
   phosphorus, and potassium are tested for. If specific soil is deficient
   in these substances, fertilizers may provide them. Extensive academic
   reseach is performed in an effort to expand the understanding of
   agricultural soil science.

   Earth sheltering is the architectural practice of using soil for
   external thermal mass against building walls. The principle is that
   earthen material undergoes slow temperature changes and thus presents a
   fairly constant surface temperature at the wall. In higher latitudes
   with low average annual air temperature, the potential for heat
   leaching requires floor and base wall insulation. Earth-based,
   wall-construction materials include adobe, chirpici, cob, mudbrick,
   rammed earth, and sod. An earthen wall facing the mid-day sun can be
   designed as a trombe wall. A trombe wall is glazed on the exterior to
   enhance heat gain. Heat is vented to the interior at night.

   Organic soils, especially peat, serve as a significant fuel resource.
   Peat deposits are found in many places around the world. The majority
   of peatlands are found in high latitudes; approximately 60% of the
   world's wetlands are peat. Peatlands cover around 3% of the global land
   mass or 3,850,000 to 4,100,000 km². Peat is available in considerable
   quantities in Scandinavia: some estimates put the amount of peat in
   Finland alone to be twice the size of North Sea oil reserves. Peat is
   used to produce both heat and electricity, often mixed with wood. Peat
   accounts for 6.2% of Finland's yearly energy production, second only to
   Ireland. Peat is arguably a slowly renewable biofuel but is more
   commonly classified as a fossil fuel.

   Waste management often has a soil component. Using compost and
   vermicompost are popular methods for diverting household waste to build
   soil fertility and tilth. The technique for creating Terra prêta do
   índio in the Amazon basin increasingly appears to have started from
   knowledge of soil first gained at a household level of waste
   management. Industrial waste management similarly relies on soil
   improvement to utilise waste treatment products. Compost and anaerobic
   digestate (also termed biosolids) are used to benefit the soils of land
   remediation projects, forestry, agriculture, and for landfill cover.
   These products increase soil organic content, provide nutrients,
   enhance microbial activity, improve soil ability to retain moisture,
   and have the potential to perform a role in carbon sequestration.

   Compost and digestate are the finished products of treatment. Soil
   performs a more direct treatment role when it comes to septage effluent
   and in land application of industrial waste water.

   Septic drain fields treat septic tank effluent using aerobic soil
   processes to degrade putrescible components. Pathogenic organisms
   vulnerable to predation in an aerobic soil environment are eliminated.
   Clay particles act like electrostatic filters to detain virus in the
   soil adding a further layer of protection. Soil is also relied on for
   chemically binding and retaining phosphorus. Where soil limitations
   preclude the use of a septic drain field, the soil treatment component
   is replaced by some combination of mechanical aeration, chemical
   oxidation, ultraviolet light disinfection, replaceable phosphorus
   retention media and/or filtration.

   For industrial wastewater treatment, land application is a preferred
   treatment approach when oxygen demanding (putrescible) constituents and
   nutrients are the treatment targets. Aerobic soil processes degrade
   oxygen demanding components. Plant uptake and removal through grazing
   or harvest perform nutrient removal. Soil processes have limited
   treatment capacity for treating metal and salt components of waste.

Soil and land degradation

   Land degradation is a human induced or natural process which impairs
   the capacity of land to function. Soils are the critical component in
   land degradation when it involves acidification, contamination,
   desertification, erosion, or salination.

   While soil acidification of alkaline soils is beneficial, it degrades
   land when soil acidity lowers crop productivity and increases soil
   vulnerability to contamination and erosion. Soils are often initially
   acid because their parent materials were acid and initially low in the
   basic cations (calcium, magnesium, potassium, and sodium).
   Acidification occurs when these elements are removed from the soil
   profile by normal rainfall or the harvesting of crops. Soil
   acidification is accelerated by the use of acid-forming nitrogenous
   fertilizers and by the effects of acid precipitation.

   Soil contamination at low levels are often within soil capacity to
   treat and assimilate. Many waste treatment processes rely on this
   treatment capacity. Exceeding treatment capacity can damage soil biota
   and limit soil function. Derelict soils occur where industrial
   contamination or other development activity damages the soil to such a
   degree that the land cannot be used safely or productively. Remediation
   of derelict soil uses principles of geology, physics, chemistry, and
   biology to degrade, attenuate, isolate, or remove soil contaminants and
   to restore soil functions and values. Techniques include leaching, air
   sparging, chemical amendments, phytoremediation, bioremediation, and
   natural attenuation.

   Desertification is an environmental process of ecosystem degradation in
   arid and semi-arid regions, or as a result of human activity. It is a
   common misconception that droughts cause desertification. Droughts are
   common in arid and semiarid lands. Well-managed lands can recover from
   drought when the rains return. Soil management tools include
   maintaining soil nutrient and organic matter levels, reduced tillage
   and increased cover. These help to control erosion and maintain
   productivity during periods when moisture is available. Continued land
   abuse during droughts, however, increases land degradation. Increased
   population and livestock pressure on marginal lands accelerates
   desertification.

   Soil erosional loss is caused by wind, water, ice, movement in response
   to gravity. Although the processes may be simultaneous, erosion is
   distinguished from weathering. Erosion is an intrinsic natural process,
   but in many places it is increased by human land use. Poor land use
   practices include deforestation, overgrazing, and improper construction
   activity. Improved management can limit erosion using techniques like
   limiting disturbance during construction, avoiding construction during
   erosion prone periods, intercepting runoff, terrace-building, use of
   erosion suppressing cover materials and planting trees or other soil
   binding plants.

   A serious and long-running water erosion problem is in China, on the
   middle reaches of the Yellow River and the upper reaches of the Yangtze
   River. From the Yellow River, over 1.6 billion tons of sediment flow
   each year into the ocean. The sediment originates primarily from water
   erosion in the Loess Plateau region of northwest China.

   One of the main causes of soil erosion in 2006 is slash and burn
   treatment of tropical forests.

   Soil piping is a particular form of soil erosion that occurs below the
   soil surface. It is associated with levee and dam failure as well as
   sink hole formation. Turbulent flow removes soil starting from the
   mouth of the seep flow and subsoil erosion advances upgradient. The
   term sand boil is used to describe the appearance of the discharging
   end of an active soil pipe.

   Soil salination is the accumulation of free salts to such an extent
   that it leads to degradation of soils and vegetation. Consequences
   include corrosion damage, reduced plant growth, erosion due to loss of
   plant cover and soil structure, and water quality problems due to
   sedimentation. Salination occurs due to a combination of natural and
   human caused processes. Aridic conditions favour salt accumulation.
   This is especially apparent when soil parent material is saline.
   Irrigation of arid lands is especially problematic. All irrigation
   water has some level of salinity. Irrigation, especially when it
   involves leakage from canals, often raise the underlying water table.
   Rapid salination occurs when the land surface is within the capillary
   fringe of saline groundwater.

   An example of soil salination occurred in Egypt in the 1970s after the
   Aswan High Dam was built. The source water was saline. The seasonal
   change in the level of ground water before the construction had enabled
   salt flushing, but lack of drainage resulted in the accumulation of
   salts in the groundwater. The dam supported irrigation which raised the
   water table. A stable, shallow water table allowed capillary transport
   and evaporative enrichment of salts at the soil surface, depressing
   crop productivity below pre-project levels.

   Preventing soil salination involves flushing with higher levels of
   applied water in combination with tile drainage.

Fields of study

   Soil occupies the pedosphere, one of Earth's spheres that the
   geosciences use to conceptually organise the Earth. This is the
   conceptual perspective of pedology and edaphology, the two main
   branches of soil science. Pedology is the study of soil in its natural
   setting. Edaphology is the study of soil in relation to soil-dependent
   uses. Both branches apply a combination of soil physics, soil
   chemistry, and soil biology. Due to the numerous interactions between
   the biosphere, atmosphere and hydrosphere that are hosted within the
   pedosphere, more integrated, less soil-centric concepts are also
   valuable. Many concepts essential to understanding soil come from
   individuals not identifiable strictly as soil scientists. This
   highlights the interdisciplinary nature of soil concepts.

History

   Vasily V. Dokuchaev, a Russian geologist, geographer and early soil
   scientist, is credited with identifying soil as a resource whose
   distinctness and complexity deserved to be separated conceptually from
   geology and crop production and treated as a whole.

     Previously, soil had been considered a product of physicochemical
     transformations of rocks, a dead substrate from which plants derive
     nutritious mineral elements. Soil and bedrock were in fact equated.
     Dokuchaev considers the soil as a natural body having its own
     genesis and its own history of development, a body with complex and
     multiform processes taking place within it. The soil is considered
     as different from bedrock. The latter becomes soil under the
     influence of a series of soil-formation factors (climate,
     vegetation, country, relief and age). According to him, soil should
     be called the "daily" or outward horizons of rocks regardless of the
     type; they are changed naturally by the common effect of water, air
     and various kinds of living and dead organisms.

   A 1914 encyclopedic definition: "the different forms of earth on the
   surface of the rocks, formed by the breaking down or weathering of
   rocks." serves to illustrate the historic view of soil which persisted
   from the 19th century. Dokuchaev's late 19th century soil concept
   developed in the 20th century to one of soil as earthy material that
   has been altered by living processes. A corollary concept is that soil
   without a living component is simply dirt.

   Further refinement of the soil concept is occurring in view of an
   appreciation of energy transport and transformation within soil.
   Accurate to this modern understanding of soil is Nikiforoff's 1959
   definition of soil as the "excited skin of the subaerial part of the
   earth's crust".

   Retrieved from " http://en.wikipedia.org/wiki/Soil"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
   of authors and sources) and is available under the GNU Free
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