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Earth

Specialty Definition: Earth

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Specialty Definition: Earth

(From Wikipedia, the free Encyclopedia)

Earth Larger image Orbital characteristics Mean radius 149,597,870 km Perihelion 0.983 AU Aphelion 1.017 AU Eccentricity 0.01671022 Orbital period 365.25636 days Avg. Orbital Speed 29.7859 km/s Inclination 0.00005° Satellitess 1 (the Moon) Satellite of Sun Physical characteristics Equatorial diameter 12,756.3 km Surface area 5.10072×108 km2 Mass 5.9742×1024 kg Mean density 5.515 g/cm3 Surface gravity 9.78 m/s2 Escape velocity 11.18 km/s Rotation period 23.9345 hours Axial tilt 23.45° Albedo 37-39% Surface temperature min mean max 184 K 282 K 333 K Atmospheric characteristics Pressure 101.325 kPa nitrogen 78% oxygen 21% argon 1% carbon dioxide water vapor trace

''For alternate uses see: Earth (disambiguation).

Earth is the planet we live on, the third from the Sun and the largest and most massive of the terrestrial planets in our solar system. It has one natural satellite, the Moon. Earth is the only planet in the universe known by us to be hosting life, as extraterrestrial life has not yet been found.

Alternative names for the planet include "the Earth", "Terra" or "Tellus", the latter sometimes being used in science fiction.

''The image to the right is of Africa, Antarctica, and the Arabian Peninsula as taken en route to the Moon by Apollo 17 on December 7, 1972.

Physical characteristics

Structure

The interior of Earth, like that of the other terrestrial planets, is chemically divided into an outer siliceous solid crust, a highly viscous mantle, an outer core that is less viscous than the mantle, and an inner core. The planet is big enough to have the core differentiated into an liquid outer core, which gives rise to a weak magnetic field due to the convection of its electrically conductive material, and a solid inner core.

New material constantly finds its way to the surface through volcanoes and cracks in the ocean floors (see seafloor spreading). Much of the Earth's surface is less than 100,000,000 years old.

Interior

The interior of the Earth reaches temperatures of 5270 K. The planet's internal heat was originally released during its accretion (see gravitational binding energy), and since then additional heat has continued to be generated by the decay of radioactive elements such as uranium, thorium, and potassium. The heat flow from the interior to the surface is only 1/20,000 as great as the energy received from the Sun.

• 0-60 km -- Lithosphere
  • 0-30/35 km -- Crust
• 30/35-2900 km -- Mantle
  • 100-700 km -- Asthenosphere
• 2900-5100 km -- Outer Core
• 5100-~6375 km -- Inner Core

The Core

The outer core has a radius of ~3500 km. The inner core has a radius of ~1250 km.

The average density of Earth is 5,515 kg/m³, making it the densest planet in the Solar system. Since the average density of surface material is around 3000 kg/m³, this indicates that denser materials exist within the core. (see: planetary differentiation) It is thought that the core is largely composed of iron (80%), along with nickel and silicon; with other dense elements such as lead and uranium either being too rare to be significant or being felsic-seeking in nature (and thus concentrated in the crust rather than the core).

The Earth was entirely molten about 4.6 billion years ago. Gravity would have caused denser substances to sink towards the center in a process called chemical differentiation, while less dense substances would have migrated to the crust.

The inner core is generally believed to be solid and to be composed entirely of iron and some nickel. Some believe it may be entirely composed of a single iron crystal. The inner core is surrounded by the outer core, which is believed to be liquid iron mixed with liquid nickel.

Recent evidence has suggested that the inner core of Earth may rotate slightly faster than the rest of the planet, by ~2° per year (Comins DEU-p.82). It is generally believed that the rotation of the inner core (which is primarily composed of iron) creates the Earth's magnetic field. It is not known, exactly, why this occurs. (See also: dynamo theory)

Mantle

The Earth's mantle extends to a depth of 2,900 km. The pressure, at the bottom of the mantle, is ~1.4 Matm (140 GPa). It is largely composed of substances rich in iron and magnesium. The melting point of a substance depends on the pressure it is under. As there is intense and increasing pressure as one travels deeper into the mantle, the lower part of this region is thought solid while the upper mantle is plastic (semi-molten). The viscosity of the upper mantle ranges between 10²¹ and 10²⁴ Poise, depending on depth [1]. Thus, the upper mantle can only flow very slowly.

Why is the inner core thought solid, the outer core thought liquid, and the mantle solid/plastic? The melting point of iron rich substances are higher than pure iron. The core is composed almost entirely of pure iron, while iron rich substances are more common outside the core. So, surface iron-substances are solid, upper mantle iron-substances are semi-melted (as it is hot and they are under relatively little pressure), lower mantle iron-substances are solid (as they are under tremendous pressure), outer core pure iron is liquid as it has a very low melting point (despite enormous pressure), and the inner core is solid due to the overwhelming pressure found at the center of the planet.

Crust

The crust ranges from 5-35 km in depth. It is composed of silicon-based rocks. The crust-mantle boundary occurs as two physically different events. Firstly, there is a discontinuity in the seismic velocity which is known as the Mohorovicic discontinuity or Moho. The cause of the Moho is thought to be a change in rock composition from rocks containing plagioclase feldspar (above) to rocks that contain none (below). The second event is a chemical discontinuity between ultramafic cumulates and tectonized hartzburgites which has been observed from parts of the oceanic crust that have been obducted.

Biosphere

Earth is the only place in the universe where life has been reliably observed. The layer of life is called the biosphere.

The biosphere is divided into a number of biomes, or areas inhabited by a broadly similar flora and fauna. On land, biomes are separated primarily by latitude. Terrestrial biomes lying within the Arctic and Antarctic Circles are relatively barren of plant and animal life, while most of the richest ones lie near the Equator.

Terrestrial organisms in temperate and arctic biomes have relatively small amounts of total biomass, smaller energy budgets, and display prominent adaptations to cold, including world-spanning migrations, social adaptations, homeothermy, estivation and multiple layers of insulation. Some theorists therefore believe that the Earth is poorly suited to life.

However, every part of the planet supports life, from the polar ice caps to the Equator. Recent advances in microbiology have proven that microscopic life lives inside rocks under the Earth's surface, and that the total mass of microbial life in so-called "uninhabitable zones" may, in terms of sheer biomass, outweigh all animal and plant life combined on the surface of the Earth.

Oceans mediate the cold and distribute nutrients. The Antarctic krill, Euphausia superba, for example, is generally considered to be the most successful animal of the planet, with a biomass probably over 500 million tonnes (c.f. human biomass of about 250 million tonnes).

Atmosphere

Water covers 71% of Earth's surface (97% of it being sea water and 3% fresh water [1]) and divides it into five oceans and seven continents. It has a relatively thick atmosphere composed of 78% nitrogen, 21% oxygen, and 1% argon, plus traces of other gases including carbon dioxide and water. The atmosphere acts as a buffer between Earth and the Sun. The layers, troposphere, stratosphere, mesosphere, thermosphere, and the exosphere, vary around the globe and in response to seasonal changes. This is sometimes described as the "third atmosphere" to distinguish it from earlier atmospheric compositions. See also: Earth's atmosphere.

Hydrosphere

Earth is the only planet in our solar system, or even the known universe, whose surface has liquid water. Earth's solar orbit, vulcanism, gravity, greenhouse effect, magnetic field and oxygen-rich atmosphere seem to combine to make Earth a water planet.

Earth is actually beyond the outer edge of the orbits which would be warm enough to form liquid water. Without some form of a greenhouse effect, the Earth's water would freeze. Paleontological evidence indicates that at one point after blue-green bacteria (Archaea) had colonized the oceans, the greenhouse effect failed, and the Earth froze solid for 10 to 100 million years.

On other planets, such as Venus, gaseous water is cracked by solar ultraviolet, and the hydrogen is ionized and blown away by the solar wind. This effect is slow, but inexorable. It is believed that this is the reason why Venus has no water. Without hydrogen, the oxygen interacts with the surface and is bound up in solid minerals.

On Earth, a shield of ozone absorbs most of this energetic ultraviolet high in the atmosphere, reducing the cracking effect. The magnetosphere also shields the ionosphere from direct scouring by the solar wind.

Finally, vulcanism, aided by the moon's tidal effects, continuously emits water vapor from the interior. Earth's plate tectonics recycle carbon and water as limestone fields are subducted into magma and volcanically emitted as gaseous carbon dioxide and steam.

The Earth also suffers from the Chandler wobble.

The Moon

Earth's Satellite

Name	Diameter(km)	Mass (kg)	Mean Orbital Radius (km)	Orbital Period
Moon	3,474.8	7.349 × 1022	384,400	27Days,7hours, 43.7minutes

Earth is unique in its solar system in having a moon, called "the Moon" (or, occasionally, "Luna"), which is a relatively large terrestrial planet-like satellite, about one quarter of Earth's diameter. The natural satellites orbiting other planets are called "moons", after Earth's moon.

The moon may enable life by moderating the weather. Paleontological evidence shows that Earth's axial tilt is stabilised by tidal interactions with its moon. Without this stabilization, the rotational axis might be chaotically unstable, as it is with a sphere. If Earth's axis of rotation were to approach the plane of the ecliptic, extremely severe weather could result as one pole was continually heated and the other cooled. Planetologists who have studied the effect claim that this might kill all large animal and higher plant life. This remains a controversial subject, however, and further studies of Mars - which shares Earth's rotation period and axial tilt, but not its large moon or liquid core - may provide additional information.

The Moon is just far enough away to have, when seen from the Earth, the same apparent angular size as the Sun. This allows a total eclipse to occur on Earth.

Also, the Moon is tidally locked: its rotation period is the same as the time it takes to revolve around the Earth, meaning it always presents the same face to the planet, seeming to disappear and reappear as the solar terminator line moves around the moon.

The origin of the Moon is presently unknown, but one popular theory has it that it was formed from the collision of a Mars-sized protoplanet into the early Earth. This theory explains (among other things) the Moon's relative lack of iron and volatile elements. See Giant impact theory.

Earth also has at least one known co-orbital asteroid, 3753 Cruithne.

Geography

Physical map of Earth.

Map references:

Time Zones, Coordinates.

Biggest geograhic subdivision

Continents.

Area:

• total: 510.072 million km²
• land: 148.94 million km²
• water: 361.132 million km²
• note: 70.8% of the world's surface is covered by water, 29.2% is exposed land

Land boundaries: the land boundaries in the world total 251,480.24 km (not counting shared boundaries twice)

Coastline: 356,000 km

Maritime claims: see United Nations Convention on the Law of the Sea

• contiguous zone: 24 nautical miles (NM) claimed by most, but can vary
• continental shelf: 200 m depth claimed by most or to depth of exploitation; others claim 200 NM or to the edge of the continental margin
• exclusive fishing zone: 200 NM claimed by most, but can vary
• exclusive economic zone: 200 NM claimed by most, but can vary
• territorial sea: 12 NM claimed by most, but can vary
• Note: boundary situations with neighboring states prevent many countries from extending their fishing or economic zones to a full 200 NM; 43 nations and other areas that are landlocked include Afghanistan, Andorra, Armenia, Austria, Azerbaijan, Belarus, Bhutan, Bolivia, Botswana, Burkina Faso, Burundi, Central African Republic, Chad, Czech Republic, Ethiopia, Holy See (Vatican City), Hungary, Kazakhstan, Kyrgyzstan, Laos, Lesotho, Liechtenstein, Luxembourg, Malawi, Mali, Moldova, Mongolia, Nepal, Niger, Paraguay, Rwanda, San Marino, Slovakia, Swaziland, Switzerland, Tajikistan, The Republic of Macedonia, Turkmenistan, Uganda, Uzbekistan, West Bank, Zambia, Zimbabwe

Climate

Two large areas of polar climates separated by two rather narrow temperate zones from a wide equatorial band of tropical to subtropical climates. Precipitation patterns vary widely, ranging from several meters of water per year to less than a millimeter.

Terrain

Elevation extremes: (measured relative to sea level)

• Lowest point on land: Dead Sea -408 m
• Lowest point overall: Marianas Trench in the Pacific Ocean -10,924 m
• Highest point: Mount Everest 8,850 m (1999 est.)

Natural resources

• The Earth's crust contains large deposits of fossil fuels: (coal, oil, natural gas, methane clathrate). These deposits are used by humans both for energy production and as feedstock for chemical production.
• Mineral ore bodies have been formed in the Earth's crust by the action of erosion and plate tectonics. These ore bodies form concentrated sources for many metals and other useful elementss.
• The Earth's biosphere produces many useful biological products, including (but far from limited to) food, wood, pharmaceuticals, oxygen, and the recycling of many organic wastes. The land-based ecosystem depends upon topsoil and fresh water, and the oceanic ecosystem depends upon dissolved nutrients washed down from the land.

Some of these resources, such as fossil fuels, are difficult to replenish on a short time scale, called non-renewable resources. The exploitation of non-renewable resources by human civilization has become a subject of significant controversy in modern environmentalism movements.

Land use

• arable land: 10%
• permanent crops: 1%
• permanent pastures: 26%
• forests and woodland: 32%
• other: 31% (1993 est.)

Irrigated land: 2,481,250 km² (1993 est.)

Natural hazards

Large areas are subject to extreme weather such as (tropical cyclones), hurricanes,or typhoons that dominate life in those areas. Many places are subject to earthquakes, landslides, tsunamis, volcanic eruptions, tornadoes, sinkholes, floods, droughts, and other calamities and disasters.

Environment - current issues

Large areas are subject to overpopulation, industrial disasters such as pollution of the air and water, acid rain and toxic substances, loss of vegetation (overgrazing, deforestation, desertification), loss of wildlife, soil degradation, soil depletion, erosion, and introduction of invasive species.

Human population

Earth at night, showing population centres (Larger version)

Nearly all humans live on the Earth: 6,327,152,352 inhabitants (November 1 2003 est.)

In orbit about the Earth: 2 astronauts (November 28 2003), on board the International Space Station.

The northernmost settlement in the world is Alert, Ellesmere Island, Canada.

See also space colonization.

Age structure:

• 0-14 years: 1,818,803,078 (29.92%)
  • male: 932,832,913 (15.35%)
  • female: 885,970,165 (14.57%)
• 15-64 years: 3,840,881,326 (63.19%)
  • male: 1,942,402,264 (31.95%)
  • female: 1,898,479,062 (31.23%)
• 65 years and over: 419,090,130 (6.89%)
  • male: 184,072,470 (3.03%)
  • female: 235,017,660 (3.87%) (2000 est.)

Population growth rate: 1.3% (2000 est.)

Birth rate: 22 births/1,000 population (2000 est.)

Death rate: 9 deaths/1,000 population (2000 est.)

Sex ratio:

• at birth: 1.05 male(s)/female
• under 15 years: 1.05 male(s)/female
• 15-64 years: 1.02 male(s)/female
• 65 years and over: 0.78 male(s)/female
• total population: 1.01 male(s)/female (2000 est.)

Infant mortality rate: 54 deaths/1,000 live births (2000 est.)

Life expectancy at birth:

• total population: 64 years

• male: 62 years
• female: 65 years (2000 est.)

Total fertility rate: 2.8 children born/woman (2000 est.)

Government

The worldwide general international organization is United Nations.

Administrative divisions: 267 nations, dependent areas, other, and miscellaneous entries

The Earth has often been personified as a deity, often a goddess. See Gaea and Mother Earth. In Norse mythology, Earth was the son of Nott and Annar.

The Earth has also been described as a massive spaceship, with a life support system that requires maintenance. See Spaceship Earth.

See also

• Legal system: international law
• Economy: world economy
• earthquake
• Earth's magnetic field
• Equatorial bulge
• Earth in fiction

Reference

• Discovering the Essential Universe (Second Edition) by Comins (2001)

The Solar System

Sun - Mercury - Venus - Earth - Mars - Asteroids - Jupiter - Saturn - Uranus - Neptune - Pluto - Comets - Kuiper belt - Oort cloud

simple:Earth

Source: adapted by the editor from Wikipedia, the free encyclopedia under a copyleft GNU Free Documentation License (GFDL) from the article "Earth."

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Earth (classical element)

(From Wikipedia, the free Encyclopedia)

Earth is one of the four classical elements in ancient Greek philosophy and science. Earth is considered to be both cold and dry, and according to Plato is associated with the cube.

Source: adapted by the editor from Wikipedia, the free encyclopedia under a copyleft GNU Free Documentation License (GFDL) from the article "Earth (classical element)."

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Earth (dragonball)

(From Wikipedia, the free Encyclopedia)

This page describes the planet Earth as seen in the fictional Animes Dragon Ball, Dragon Ball Z and Dragon Ball GT.

The Earth is primarily the same as the real Earth, however it does not share the same real information such as countries and places. The technology seen in the series seems to be a mix of current and future technology, such as the "Capsules" invented by Capsule Corp, which look like small pressurised tubes which fit in your palm, but can contain entire houses, for example. The animal population is also slightly different, mainly because dinosaurs are not extinct and freely roam the countryside. The marine life also contains prehistoric elements. Certain animals are highly intelligent, and are often seen mingling with humans, as if they are just another person in the greater population. Some of the main characters are such animals, Oolong is a pig, and Puar is a cat. There is also a talking turtle on Kame Island.

All planets have a Guardian in Dragon Ball, who watches over the planet. Earth thus has one. Throughout most of Dragon Ball Z, the Earth's guardian is Kami, and then later Dende after Kami had fused with Piccolo. Because Kami and Dende are Namekianss they have the ability to create Dragonballs, which have the ability to grant wishes if they are collected. These wishes are used regularly throughout Dragon Ball for many various things, such as bringing people back to life, or wishing for the worlds most comfortable pair of underpants.

The locations depicted in Dragon Ball are obviously heavily influenced by Asian cities, and a lot of their customs are seen in the series. An example of this can be seen in the scenes with any food involved, as they include oriental influences such as the style of food, and even chop sticks are used for eating with. The cars seen in Dragon Ball often float rather than having wheels, suggesting a technologically advanced society.

At one point the Earth is blown up by Kid Buu, but it is wished back by the new Namekian Dragonballs.

Source: adapted by the editor from Wikipedia, the free encyclopedia under a copyleft GNU Free Documentation License (GFDL) from the article "Earth (dragonball)."

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Earth, Texas

(From Wikipedia, the free Encyclopedia)

Earth is a city located in Lamb County, Texas. As of the 2000 census, the city had a total population of 1,109.

Geography

Earth is located at 34°14'4" North, 102°24'19" West (34.234566, -102.405183)¹. According to the United States Census Bureau, the city has a total area of 3.1 km² (1.2 mi²). 3.1 km² (1.2 mi²) of it is land and none of the area is covered with water.

Demographics

As of the census of 2000, there are 1,109 people, 395 households, and 293 families residing in the city. The population density is 356.8/km² (926.2/mi²). There are 458 housing units at an average density of 147.4/km² (382.5/mi²). The racial makeup of the city is 73.13% White, 2.89% African American, 0.81% Native American, 0.00% Asian, 0.00% Pacific Islander, 22.09% from other races, and 1.08% from two or more races. 52.93% of the population are Hispanic or Latino of any race. There are 395 households out of which 38.5% have children under the age of 18 living with them, 59.7% are married couples living together, 10.9% have a female householder with no husband present, and 25.8% are non-families. 22.8% of all households are made up of individuals and 13.7% have someone living alone who is 65 years of age or older. The average household size is 2.81 and the average family size is 3.33. In the city the population is spread out with 31.0% under the age of 18, 9.3% from 18 to 24, 23.5% from 25 to 44, 21.6% from 45 to 64, and 14.5% who are 65 years of age or older. The median age is 34 years. For every 100 females there are 89.9 males. For every 100 females age 18 and over, there are 92.2 males. The median income for a household in the city is $25,595, and the median income for a family is $29,688. Males have a median income of $27,396 versus $15,938 for females. The per capita income for the city is $12,191. 27.3% of the population and 20.0% of families are below the poverty line. Out of the total people living in poverty, 36.5% are under the age of 18 and 20.8% are 65 or older.

Source: adapted by the editor from Wikipedia, the free encyclopedia under a copyleft GNU Free Documentation License (GFDL) from the article "Earth, Texas."

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Earth's magnetic field

(From Wikipedia, the free Encyclopedia)

The cause of Earth's magnetic field (the surface magnetic field) is not known for certain, but is possibly explained by dynamo theory. The magnetic field extends several tens of thousands of kilometers into space.

The field is approximately a magnetic dipole, with one pole near the geographic north pole and the other near the geographic south pole. An imaginary line joining the magnetic poles would be inclined by approximately 11.3° from the planet's axis of rotation.

The field is similar to that of a bar magnet, but this similarity is superficial. The magnetic field of a bar magnet, or any other type of permanent magnet, is created by the coordinated motions of electrons (negatively charged particles) within iron atoms. The Earth's core, however, is hotter than 1043 K, the temperature at which the orientations of electron orbits within iron become randomized. Such randomization tends to cause the substance to lose its magnetic field. Therefore the Earth's magnetic field is caused not by magnetised iron deposits, but mostly by electric currents (known as telluric currents).

The Earth's magnetic field reverses at intervals, ranging from tens of thousands to hundreds of thousands of years. It is believed that this last occurred some 600,000 years ago (Comins - DEU p.84). The overall geomagnetic field is becoming weaker at a rate which will cause the field to disappear by about 4000 AD.¹ Other sources have put the date of field collapse as early as 3000 AD.

Another feature that distinguishes the Earth magnetically from a bar magnet is its magnetosphere. At large distances from the planet, this dominates the surface magnetic field. In addition, the magnetized elements within the planetary core are undergoing rotation and are not static.

See also: ionosphere, Sherwood machine, Edward Sabine, magnetohydrodynamics, Dynamo theory, South Atlantic Magnetic Anomaly.

References

• Discovering the Essential Universe by Neil F. Comins (2001)
• Introduction to Geomagnetically Trapped Radiation by Martin Walt (1994)

  Source: adapted by the editor from Wikipedia, the free encyclopedia under a copyleft GNU Free Documentation License (GFDL) from the article "Earth's magnetic field."

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Ground

(From Wikipedia, the free Encyclopedia)

In electrical engineering, the term ground has the following meanings:

1. An electrical connection to earth through an earth-electrode subsystem.
2. In an electrical circuit, a common return path that usually (a) is connected to an earth-electrode subsystem and (b) is extended throughout a facility via a facility ground system consisting of the signal reference subsystem, the fault protection subsystem, and the lightning protection subsystem.
3. In an electrical circuit, a common return path that (a) may not necessarily be connected to earth and (b) is the zero voltage reference level for the equipment or system.

See also: Ground constants

Source: from Federal Standard 1037C in support of MIL-STD-188

Source: adapted by the editor from Wikipedia, the free encyclopedia under a copyleft GNU Free Documentation License (GFDL) from the article "Ground."

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Soil

(From Wikipedia, the free Encyclopedia)

Soil is the layer of minerals and organic matter, in thickness from from centimeters to a meter or more, on the land surface. Its main components are mineral matter, organic matter, moisture, and air. Soils differ in the ratio of these components.

Minerals in soil are obtained from a variety of sources, but the process which delivers the bulk is weathering of rocks. Weathering is the actions of wind, rain, ice, sunlight, and biological processes on rocks that break them down into smaller particles.

Weathering also releases ions such as K⁺ and Mg²⁺ in to the soil solution. Some of these ions are taken up by plants, but the majority not left in solution are absorbed through ion exchange by clays such as montmorillonite. When the level of ions is low in the soil an equilibrium process forces ions back into solution, where they can be used by plants.

However if acid is introduced into soil, hydrogen ions bind in preference to clays, forcing ions out where they can be washed away during rain. Acidity also encourages the weathering of clays, releasing toxic aluminium ions (of which clays are composed) into the solution. To stop this occurring farmers apply alkalic materials such as slaked-lime.

Although there exists plenty of elements such as nitrogen, potassium and phosphorus necessary for plant growth in soil, very little of this is in a form which plants can use. In processes such as nitrification and mineralisation, bacteria and other organisms convert unusable forms (such as NH₄⁺) in to usable forms (such as NO₃^-). The raw products are initially present as gases in the atmosphere. Processes such as the nitrogen cycle and carbon cycle continually exchange nutrients between the soil and atmosphere.

The organic store in soil is made up of plant debris, animal excreta and other decomposing materials. A lot of the carbon compounds react to form humus, which is composed of very large molecules including esters of carboxylic acid, phenolic compounds and derivatives of benzene. Organic materia in soil provides nutrients necessary for plant growth.

See also; soil life, Soil pH, Soil types, Soil profile, Soil structure

The following text is taken from the Household Cyclopedia of 1881, and should be updated and integrated with the above article:

Soils consist mostly of sand, lime, and clay, with certain saline and organic substances in smaller and varying proportions; but the examination of the ashes of plants shows that a fertile soil must of necessity contain an appreciable quantity of at least eleven different substances, which in most cases exist in greater or less relative abundance in the ash of cultivated plants; and of these the proportions are not by any means immaterial. In general, the soils which are made up of the most various materials are called alluvial; having been formed from the depositions of floods and rivers. Many of them are extremely fertile. Soils consist of two parts; of an organic part, which can readily be burned away when the surface-soil is heated to redness; and of an inorganic part, which remains fixed in the fire, consisting of earthy and saline substances from which, if carbonic acid or any elastic gas be present, it may, however, be driven by the heat. The organic part of soils is derived chiefly from the remains of vegetables and animals which have lived and died in and upon the soil, which have been spread over it by rivers and rains, or which have been added by the industry of man for the purposes of increased fertility.

This organic part varies much in quantity, as well as quality, in different soils. In peaty soils it is very abundant, as well as in some rich, long cultivated lands. In general, it rarely amounts to one-fourth, or 25 per cent. even in our best arable lands. Good wheat soils contain often as little as eight parts in the hundred of organic animal or vegetable matter; oats and rye will grow in a soil containing only 1 1/2 per cent.; and barley when only two or three parts per cent. are present.

The inorganic portion of any given soil, again, is divisible into two portions; that part which is soluble in water, and thus easily taken up by plants, and a much more bulky portion which is insoluble.

Sir Humphry Davy found the following to be the composition of a good productive soil.

• In every 9 parts, 8 consisted of siliceous sand;
• the remaining (one-ninth) part was composed, in 100 parts, as follows:
  • Carbonate of lime (chalk) 63 grains.
  • Pure silex 15 grains.
  • Pure alumina, or the earth of clay 11 grains.
  • Oxide (rust) of iron 3 grains.
  • Vegetable and other saline matter 5 grains.
  • Moisture and loss 3 grains.

Thus the whole amount of organic matter in this instance is only 1 part in 200, or one-half of one per cent.; a fact which, in itself, would demonstrate the fallacy of supposing that decomposed animal and vegetable matter in the soil form the exclusive supply to growing plants.

Clayey soil

A clayey soil, though distinguished by the color which it bears, namely black, white, yellow and red, differs from all other soils, being tough, wet, and cold, and consequently requiring a good deal of labor from the husbandman before it can be sufficiently pulverized, or placed in a state for bearing artificial crops of corn or grass. Clay land is known by the following qualities, or properties.

It holds water like a cup, and once wetted does not soon dry. In like manner, when thoroughly dry, it is not soon wetted; if we except the varieties which have a thin surface, and are the worst of all to manage. In a dry summer, clay cracks and shows a surface full of small chinks, or openings. If ploughed in a wet state, it sticks to the plough like mortar, and in a dry summer, the plough turns it up in great clods, scarcely to be broken or separated by the heaviest roller.

Clayey soils, when sufficiently enriched with manures, are naturally well qualified for carrying crops of wheat, oats, beans, and clover; but are not fitted for barley, turnips, potatoes, etc., or even for being kept under for grass longer than one year. Such soils ought to be regularly summer-fallowed once in six, or at least once in eight years, even when they are comparatively in a clean state, as they contract a sourness and adhesion from wet ploughing, only to be removed by exposure to the sun and wind during the dry months of summer. Soils of this kind receive little benefit from winter ploughing, unless so far as their surface is thereby presented to the frost, which mellows and reduces them in a manner infinitely superior to what could be accomplished by all the operations of man. Still they are not cleaned or made free of weeds by winter ploughing; and therefore this operation can only be considered as a good means for producing a seed-bed, in which the seeds of the future crop may be safely deposited. Hence the necessity of cleansing clay soils during the summer months, and of having always a large part of every clay farm under summer fallow. All clayey soils require great industry and care, as well as a considerable portion of knowledge in dressing or management to keep them in good condition; yet when their natural toughness is got the better of, they always yield the heaviest and most abundant crops. One thing requisite for a clayey soil, is to keep it rich and full of manure; a poor clay being the most ungrateful of all soils, and hardly capable of repaying the expense of labor, after being worn out and exhausted. A clayey soil also receives, comparatively, smell benefit from grass; and when once allowed to get into a sterile condition, the most active endeavors will with difficulty restore fertility to it after the lapse of many years.

Upon light soils the case is very different. These flourish under the grass husbandry; and bare summer fallow is rarely required, because they may be cleaned and cropped in the same year with that valuable esculent, turnip. Upon light soils, however, wheat can seldom be extensively cultivated; nor can a crop be obtained of equal value, either in respect to quantity or quality, as on clay sand loams. The best method of procuring wheat on light lands, is to sow upon a clover stubble, when the soil has got an artificial solidity of body and is thereby rendered capable of sustaining the grain till it arrives at maturity. The same observation applies to soils of a gravelly nature; and upon both barley is generally found of as great benefit as wheat.

Thin clays and peat earths are more friendly to the growth of oats than of other grains, though in favorable seasons a heavy crop of wheat may be obtained from a thin clayey soil, when it has been completely summer-fallowed and enriched with dung. A first application of calcareous manure is generally accompanied with great advantage upon these soils; but when once the effect of this application is over, it can hardly be repeated a second time, unless the land has been very cautiously managed after the first dressing. Neither of these soils is friendly to grass, yet there is a necessity of exercising this husbandry with them, because they are incapable of standing the plough more than a year or two in the course of a rotation.

Wheat ought to be the predominant crop upon all the rich clays and strong loams, and light soils of every kind are well qualified for turnips, barley, etc. Upon the thin and moorish soils, oats must necessarily preserve a prominent rank, and grass seeds may be cultivated upon every one of them, though with different degrees of advantage, according to the natural and artificial richness of each soil, or to the qualities which it possesses for encouraging the growth of clover, in the first instance, and preserving the roots of the plant afterwards.

See also : pedology -- pedogenesis -- soil degradation --soil remediation -- FAO - Soil Unit Classification Scheme -- derelict soil

Source: adapted by the editor from Wikipedia, the free encyclopedia under a copyleft GNU Free Documentation License (GFDL) from the article "Soil."

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Abbreviations & Acronyms: Earth

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Synonyms within Context: Earth

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Modern Usage: Earth

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Commercial Usage: Earth

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Image Slideshow: Earth

Photos: Earth More pictures...

Illustrations: Earth More pictures...

Computer Images: Earth More pictures...

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Photo Album: Earth

Thumbnail	Description & Credit	Thumbnail	Description & Credit
	"Orrery" (movie) by Marijke van Gans. Watch Mercury, Venus, Earth, and Mars orbit the Sun, while the Moon orbits Earth. From inside DPGraph, click on Edit for more information.		Artist Concept of Terra Instruments Scanning the Earth. Credit: NASA.
	Landsat 7 Spacecraft to Join NASA's Earth Science Team. Credit: NASA.		White Earth Limb. Credit: NASA.
	The Sounds of Earth. Credit: NASA.		Taking advantage of Mars's closest approach to Earth in eight years, astronomers using the ... Credit: NASA.
	If springtime on Earth were anything like it will be on Uranus, we would be experiencing waves ... Credit: NASA.		Mars during 1999 opposition with Earth. Credit: NASA.
	Individual frames for each of the six planets imaged, Venus, Earth, Jupiter, Saturn, Uranus, and Neptune. Credit: NASA.		Full view of the Earth, taken by GOES-7(Geostationary Operational Environmental Satellite) on 25 August 1992.The image shows Hurricane Andrew approaching Louisiana. Credit: NASA.
Source: pictures compiled by the editor from various references; see picture credits.

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Digital Photo Gallery: Earth
 

"Earth pencils" by Federico Rocco Commentary: "Pencils."	"Mother Earth" by Lisa Christine Tam Commentary: "From the Montreal Mosaiculture garden sculpture competition."
Source: photographs selected by the editor, with permission from the photographers.

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Sounds Captioned with "Earth".

Play	Caption
	Dirt; construction; worker; digging; dig; excavation; excavating; excavate; hole; earth; .
Source: compiled by the editor from various references; see credits.

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Familiar Quotations: Earth

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Historic Usage: Earth

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Use in Literature: Earth

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Spoken Usage: Earth