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Soil Formation

Soil formation

Pedogenesis or soil evolution (formation) is the process by which soil is created. It is the major topic of the science of pedology, whose other aspects include the soil macro- and micro-morphology (description of the nature of soils), classification (taxonomy) of soils, and their distribution in nature, present and past (soil geography and paleopedology).

Soil

Soil is unconsolidated rock particles mixed with organic matter from plant decay. Soil is vital to all life on Earth because it supports the growth of plants, which supply food and oxygen and absorbs carbon dioxide and nitrogen. Soil serves as a habitat for animal life from microorganisms to small animals.

Soil components

Soils vary widely in composition and structure from place to place. Soils are formed through the weathering of rock and the breakdown of organic matter. Weathering is the action of wind, rain, ice, sunlight and biological processes on rocks, which breaks them down into small particles. The proportions of minerals and organic matter determine the structure and other characteristics of a particular soil. Soils can be divided into two general layers or strata: topsoil, the topmost layer, where most plant roots, microorganisms, and other animal life are located, and subsoil, which is deeper and often more dense and less rich in organic matter. Water and air are also components of most soils. Air, trapped in spaces between soil particles, and water, trapped in spaces and on the surface of particles, comprises about half of the soil by volume. Both are important to plant growth and other life in the soil profile of a particular ecosystem. The rock and mineral content of soil is categorized according to particle size as sand (coarsest), silt or clay (finest); the ratio of these particles to a great degree determines the soil classification and characteristics. Former soils which become buried below the effects of organisms are called paleosols. Soil develops naturally over time through the action of plants, animals, and weathering. Soil is also affected by human habitation. People can alter soil to make it more suitable for plant growth through the addition of organic material and natural or synthetic fertilizer, and by improving its drainage or water-retaining capacity. Human actions also can degrade soil through the depletion of nutrients, pollution, contamination, and compaction, and by increasing the rate of erosion, which is the relocation of soil through the movement of water or wind.

Natural soil development

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 break 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 soil.

Chemical processes in soils

Weathering releases ions such as Potassium (K⁺) and Magnesium (Mg²⁺) into the soil solution. Some of these elements (as 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, e.g. by acid rain, 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 may apply alkaline materials such as slaked lime. Although the elements nitrogen, potassium and phosphorus, which are necessary for plant growth, may be abundant in soil, only a fraction of these elements may be in a chemical form which plants can use. In processes such as nitrification and mineralisation, bacteria and other organisms convert unusable forms (such as NH₄⁺) 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 component of soils originate in plant debris (such as fallen leaves), animal excreta, and other decomposing organic materials. These materials, when broken down, form humus, a dark, nutrient-rich material. Chemically, humus is composed of very large molecules including esters of carboxylic acid, phenolic compounds, and derivatives of benzene. Organic material in soil provides nutrients necessary for plant growth. Organic material also contributes to water retention, drainage ability, and oxygenation of soil. If oxygen enters a wet soil, because of lowered ground water table, organic matter in the soil will be broken down further by oxidation, which can lead to subsidence. An example of this can be seen in soils in the Everglades region of Florida, which have been drained by canals for agriculture, primarily sugar production. Originally very high in organic content, oxygenation and compaction have led to breakdown of the soil structure and nutrient content, and degradation of the soil's ability to support continued high crop yields.

Biological processes in soil

Wetland soil processes

The diffusion of dissolved oxygen in saturated soils is slower than in unsaturated soils. Wetland (also referred to as hydric) soils form due to soil microbial cellular respiration in excess of soil oxygen supply, resulting in oxygen depletion. Anaerobic soil chemistry results, which creates a reducing environment. This eliminates plants and creatures not adapted for life in saturated soil conditions.

Biological soil crusts

Biological soil crusts are formed by living organisms and their by-products, creating a surface crust of soil particles bound together by organic materials.

References

- Soil Survey Staff. (1975) Soil Taxonomy: A basic system of soil classification for making and interpreting soil surveys. USDA-SCS Agric. Handb. 436. U.S. Gov. Print. Office. Washington, DC.
- Soil Survey Division Staff. (1993) Soil survey manual. Soil Conservation Service. U.S. Department of Agriculture Handbook 18.
- Logan, W. B., Dirt: The ecstatic skin of the earth. 1995 ISBN 1573220043
- Faulkner, William. Plowman's Folly. New York, Grosset & Dunlap. 1943. ISBN 0933280513
- Jenny, Hans, Factors of Soil Formation: A System of Quantitative Pedology 1941
- [http://jan.ucc.nau.edu/~doetqp-p/courses/env320/lec1/Lec1.html Why Study Soils?]
- [http://www.hort.purdue.edu/newcrop/tropical/lecture_06/chapter_12l_R.html Soil notes]
- [http://www.home2garden.org/soil.html Soil articles]

Morphology

Morphology is a term meaning "study of forms" from the Greek root word μορφή (morphe) meaning "form" or "shape". It takes on the following meanings in particular fields:
- Morphology (linguistics), the study of the structure of word forms
- Morphology (folkloristics), the structure of narratives such as folk tales
- Morphology (biology) is the study of the form or shape of an organism or part thereof
- Morphology (astronomy) is the shape of an astronomical object, like nebulae, galaxy, or other extended objects
- Mathematical morphology, a theoretical model based on Lattice theory

Taxonomy

Taxonomy (from Greek verb tassein = "to classify" and nomos = law, science, cf "economy") may refer to:
- the science of classification (see alpha taxonomy)
- a classification Initially taxonomy was only the science of classifying living organisms, but later the word was applied in a wider sense, and may also refer to either a classification of things, or the principles underlying the classification. Almost anything, animate objects, inanimate objects, places, and events, may be classified according to some taxonomic scheme. Taxonomies are frequently hierarchical in structure. However taxonomy may also refer to relationship schemes other than hierarchies, such as network structures. Other taxonomies may include single children with multi-parents, for example, "Car" might appear with both parents "Vehicle" and "Steel Mechanisms". A taxonomy might also be a simple organization of objects into groups, or even an alphabetical list. In current usage within "Knowledge Management", taxonomies are seen as slightly less broad than ontologies. Mathematically, a hierarchical taxonomy is a tree structure of classifications for a given set of objects. At the top of this structure is a single classification, the root node, that applies to all objects. Nodes below this root are more specific classifications that apply to subsets of the total set of classified objects. So for instance in common schemes of scientific classification of organisms, the root is the Organism (as this applies to all living things, it is implied rather than stated explicitly). Below this are the Domain, Kingdom, Phylum, Class, Order, Family, Genus, and Species, with various other ranks sometimes inserted. Some have argued that the human mind naturally organizes its knowledge of the world into such systems. This view is often based on the epistemology of Immanuel Kant. Anthropologists have observed that taxonomies are generally embedded in local cultural and social systems, and serve various social functions. Perhaps the most well-known and influential study of folk taxonomies is Émile Durkheim's The Elementary Forms of Religious Life. The theories of Kant and Durkheim also influenced Claude Lévi-Strauss, the founder of anthropological structuralism. Lévi-Strauss wrote two important books on taxonomies: Totemism and The Savage Mind. Such taxonomies as those analyzed by Durkheim and Lévi-Strauss are sometimes called folk taxonomies to distinguish them from scientific taxonomies that claim to be disembedded from social relations and thus objective and universal. A recent neologism, folksonomy, should not be confused with Folk Taxonomy (though it is obviously a contraction of the two words). Those who support scientific taxonomies have recently criticized folksonomies by dubbing them fauxonomies. The phrase enterprise taxonomy is used in business to describe a very limited form of taxonomy used only within one organization. The field of solving or best-fitting of numerical equations that characterize all measurable quantities of a set of objects is called cluster analysis; this is a form of taxonomy called numerical taxonomy or taximetrics.

Paleopedology

Paleopedology (palaeopedology in England) is the discipline that studies soils of past geological eras, from quite recent (Quaternary) to the earliest periods of the Earth's history. Paleopedology can be seen either as a branch of soil science or of paleontology, since the mothods it uses are in many ways a well-defined combination of the two disciplines.

History

Paleopedology's earliest developments arose from observations in Scotland circa 1795 whereby it was found that some soils in cliffs appeared to be remains of a former exposed land surface. During the nineteenth century there were many other finds of former soils throughout Europe and North America. However, most of this was only found in the search for animal and/or plant fossils and it was not until soil science first developed that buried soils of past geological ages were considered of any value. It was only when the first relationships between soils and climate were observed in the steppes of Russia and Kazakhstan that there was any interest in applying the finds of former soils to past ecosystems. This occurred because, by the 1920s, some soils in Russia had been found by K.D. Glinka that did not fit with present climates and were seen as relic of warmer climates in the past. Eugene Hilgard, in 1892, had related soil and climate in the United States in the same manner, and by the 1950s analysis of Quaternary stratigraphy to monitor recent environmental changes in the northern hemisphere had become firmly established. These developments have allowed soil fossils to be classified according to USA soil taxonomy quite easily with all recent soils. Interest in earlier soil fossils was much slower to grow, but has steadily developed since the 1960s owing to the development of such techniques as X-ray diffraction which permit their classification. This has allowed many developments in paleoecology and paleogeography to take place because the soils' chemistry can provide a good deal of evidence as to how life moved onto land during the Paleozoic.

Finding soil fossils and their structure

Remains of former soils can either be found under deposited sediment in unglaciated areas or in extremely steep cliffs where the old soil can be seem below the young present-day soil. In cases where volcanoes have been active, some soil fossils occur under the volcanic ash. If there is continued deposition of sediment, a sequence of soil fossils will form, especially after the retreat of glaciers during the Holocene. Paleosols can also exist where a younger soil has been eroded (for instance by wind), as in the Badlands of South Dakota. (One must exclude areas where present-day soils are relics of former climates, as with Australia and Southern Africa) Paleosols, whether bried or exposed, suffer from alteration. This occurs largely because so many paleosols have lost their former vegetative covering and the organic matter they once supported has been used up by plants since the soil was buried. However, if remains of plants can be found, the nature of the soil fossil can be made a great deal clearer than if no flora can be found because roots can nowadays be identified with respect to the plant group from which they come. Patterns of root traces including their shape and size, is good evidence for the vegetation type the former soil supported. Bluish colours in the soil tend to indicate the plants have mobilised nutrients within the soil. The horizons of fossil soils typically are sharply defined only in the top layers, unless some of the parent material has not been obliterated by soil formation. the kinds of horizons in fossil soils are, though, generally the same as those found in present-day soils.

Analysis

Chemical analysis of soil fossils generally focuses on their lime content, which determines both their pH and how reactive they will be to dilute acids. Chemical analysis is also useful, usually through solvent extraction to determine key minerals. this analysis can be of some use in determining the structure of a soil fossil, but today X-ray diffraction is preferred because it permits the exact crystal structure of the former soil to be determined. With the aid of X-ray diffraction, paleosols can now be classified into one of the 12 orders of Soil Taxonomy (Oxisols, Ultisols, Alfisols, Mollisols, Spodosols, Aridisols, Entisols, Inceptisols, Gelisols, Histosols, Vertisols and Andisols). Many Precambrian soils, however, when examined do not fit the characteristics for any of these soil orders and have been placed in a new order called green clays. The green colour is due to the presence of certain unoxidised minerals found in the primitive earth because O₂ was not present. There are also some forest soils of more recent times that cannot clearly be classified as Alfisols or as Spodosols because, despite their sandy horizons, there are not nearly acidic enough to have the typical features of a Spodosol.

Uses

Paleopedology is a very important discipline today for the understanding of the ecology of ancient ecosystems because it gives a clue as to what soils conditions past animals and plants were required to live under and how the plants obtained essential nutrients. In geochemistry, a knowledge of the structure of former soils is also valuable to understand the composition of certain continental rocks laid down many years ago.

External references

- Retallack, Gregory John; Soils of the past: An introduction to paleopedology (2nd edition). Published 2001 by Blackwell Science; Malden, Massachusetts. Paleopedology Paleopedology

Weathering

Weathering is the process of decomposition and/or disintegration of rocks, soils and their minerals through natural, chemical, and biological processes that is, in place. It is not to be confused with erosion, which is the movement of rocks and/or weathering products by water, wind, ice or gravity. The breakdown products, after chemical weathering of rock and sediment minerals and the leaching out of the more soluble parts, when combined with decaying organic material, is called soil. The mineral content of the soil is determined by the parent material, thus a soil derived from a single rock type can often be deficient in one or more minerals for good fertility, while a soil weather from a mix of rock types (as in glacial, eolian or alluvial sediments) often makes a richer soil.

Mechanical (Physical) Weathering

Mechanical weathering is the cause of the disintegration of rocks or wood. Most of the times it produces smaller angular fragments (like scree) as compared to chemical weathering. However, chemical and physical weathering often go hand in hand. For example, cracks exploited by mechanical weathering will increase the surface area exposed to chemical action. Furthermore, the chemical action at minerals in cracks can aid the disintegration process.

Exfoliation

Exfoliation, also known as onion-skin weathering, often occurs in hot areas, like deserts, where there is a large diurnal temperature range. The temperatures soar high in the day, while dipping to a few degrees at night. As the rock heats up and expands by day, and cools and contracts by night, stress is often exerted on the outer layers. The stress causes the peeling off of the outer layers of rocks in thin sheets. Though this is caused mainly by temperature changes, exfoliation cannot take place without the presence of moisture.

Freeze-thaw

Freeze-thaw action, sometimes known as ice crystal growth or frost shattering, occurs when water in cracks and joints of rocks freeze and expand. In the expansion, it can exert pressures up to 21 megapascals (MPa) (2100 kgf/cm²) at −22 °C. This pressure is often higher than the resistance of most rocks and causes the rock to shatter. Freeze-thaw action occurs mainly in environments where there is a lot of moisture, and temperatures frequently fluctuate above and below freezing point—that is, mainly alpine and periglacial areas. When water that has entered the joints freezes, the ice formed strains the walls of the joints and causes the joints to deepen and widen. This is because the volume of water expands by 9% when it freezes. When the ice thaws, water can flow further into the rock. When the temperature drops below freezing point and the water freezes again, the ice enlarges the joints further. Repeated freeze-thaw action weakens the rocks which, over time, break up along the joints into angular pieces. The angular rock fragments gather at the foot of the slope to form a talus slope (or scree slope). The spitting of rocks along the joints into blocks is called block disintegration. The blocks of rocks that are detached are of various shapes depending on their rock structure. Ice crystals can also form in the pore spaces of rocks. They grow larger as they attract water that has not frozen from the surrounding pores. The ice crystal growth weakens the rocks which, in time, break up. An example of rocks susceptible to frost action is chalk, which has many pore spaces for the growth of ice crystals. Laboratory tests show that frequent daily freeze-thaw cycles are more condusive than seasonal freeze-thaw cycles to frost shattering.

Pressure release

In pressure release, overlying materials (not necessarily rocks) are removed (by erosion, or other processes), which causes underlying rocks to expand and fracture parallel to the surface. Often the overlying material is heavy, and the underlying rocks experience high pressure under them, for example, a moving glacier. Pressure release may also cause exfoliation to occur. Intrusive igneous rocks (e.g. granite) are formed deep beneath the earth's surface. They are under tremendous pressure because of the overlying rock material. When erosion removes the overlying rock material, these intrusive rocks are exposed and the pressure on them is released. The outer parts of the rocks then tend to expand. The expansion sets up stresses which cause fractures parallel to the rock surface to form. Over time, sheets of rock break away from the exposed rocks along the fractures.

Salt-crystal growth

glacier Salt crystallisation causes disintegration of rocks when saline (see salinity) solutions seep into cracks and joints in the rocks and evaporate, leaving salt crystals behind. These salt crystals expand as they are heated up exerting pressure on the confining rock. Salt crystallisation may also take place when solutions decompose rocks (for example, limestone and chalk) to form salt solutions of sodium sulfate or sodium carbonate, of which the moisture evaporates to form their respective salt crystals. The salts which have proved most effective in disintegrating rocks are sodium sulfate, magnesium sulfate, and calcium chloride. Some of these salts can expand up to three times or even more.

Chemical Weathering

Carbonation-solution

Carbonation occurs on rocks which contain calcium carbonate such as limestone and chalk. This takes place when rain combines with carbon dioxide or an organic acid to form a weak carbonic acid which reacts with calcium carbonate and forms calcium bicarbonate. The reactions as follows: ::CO₂ + H₂O ⇌ H₂CO₃ :carbon dioxide + water ⇌ carbonic acid ::H₂CO₃ + CaCO₃ ⇌ Ca(HCO₃)₂ :carbonic acid + calcium carbonate ⇌ calcium bicarbonate

Hydration

Hydration (not to be confused with hydrolysis) is the process whereby minerals in the rock absorb water and expand, and sometimes change. One example is how anhydrite changes to gypsum by absorbing water. :CaSO₄ + 2H₂O → CaSO₄·2H₂O :anhydrite + water → gypsum Though chemical, this process may contribute to mechanical weathering as well, as some materials expand upon absorption of water. These materials may expand up to sixteen times their size, especially mudstones containing montmorillonite clays or bentonite. bentonite near Angelica, New York]]

Hydrolysis

Hydrolysis involves the action of acidic water on rock forming minerals like pyroxenes, amphiboles, and feldspars. For example feldspar reacts with acidic water to form kaolin clay and quartz. The soluble ions are removed in solution.

Oxidation

Within the weathering environment chemical oxidation of a variety of metals occurs. The most commonly observed is the oxidation of Fe²⁺ and combination with oxygen and water to form Fe³⁺ hydroxides and oxides such as goethite, limonite, and hematite. This gives the affected rocks a reddish-brown colouration on the surface.

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Soil formation

See also

Soil

Soil components

Natural soil development

Chemical processes in soils

Biological processes in soil

Wetland soil processes

Biological soil crusts

References

See also:

Morphology

See also

Taxonomy

See also

Paleopedology

History

Finding soil fossils and their structure

Analysis

Uses

External references

Weathering

Mechanical (Physical) Weathering

Exfoliation

Freeze-thaw

Pressure release

Salt-crystal growth

Chemical Weathering

Carbonation-solution

Hydration

Hydrolysis

Oxidation

See also

Category:Soil science

Category:Islands in English Channel