Soil formation, also known as pedogenesis, is the process of
soil
Soil, also commonly referred to as earth or dirt
Dirt is an unclean matter, especially when in contact with a person's clothes, skin, or possessions. In such cases, they are said to become dirty.
Common types of dirt include:
* Debri ...
genesis as regulated by the effects of place, environment, and history.
Biogeochemical
Biogeochemistry is the scientific discipline that involves the study of the chemical, physical, geological, and biological processes and reactions that govern the composition of the natural environment (including the biosphere, the cryosphere, th ...
processes act to both create and destroy order (
anisotropy
Anisotropy () is the property of a material which allows it to change or assume different properties in different directions, as opposed to isotropy. It can be defined as a difference, when measured along different axes, in a material's physic ...
) within soils. These alterations lead to the development of layers, termed
soil horizon, distinguished by differences in
color
Color (American English) or colour (British English) is the visual perceptual property deriving from the spectrum of light interacting with the photoreceptor cells of the eyes. Color categories and physical specifications of color are assoc ...
,
structure,
texture
Texture may refer to:
Science and technology
* Surface texture, the texture means smoothness, roughness, or bumpiness of the surface of an object
* Texture (roads), road surface characteristics with waves shorter than road roughness
* Texture ...
, and
chemistry. These
features
Feature may refer to:
Computing
* Feature (CAD), could be a hole, pocket, or notch
* Feature (computer vision), could be an edge, corner or blob
* Feature (software design) is an intentional distinguishing characteristic of a software item ...
occur in patterns of
soil type distribution, forming in response to differences in soil forming factors.
Pedogenesis is studied as a branch of
pedology
Pedology (from Greek: πέδον, ''pedon'', "soil"; and λόγος, ''logos'', "study") is a discipline within soil science which focuses on understanding and characterizing soil formation, evolution, and the theoretical frameworks for modeling ...
, the study of soil in its natural environment. Other branches of pedology are the study of
soil morphology
Soil morphology is the study of the formation and description of soil types within various soil horizons. C.F. Marbut championed reliance on soil morphology instead of on theories of pedogenesis for soil classification because theories of soil gen ...
, and
soil classification
Soil classification deals with the systematic categorization of soils based on distinguishing characteristics as well as criteria that dictate choices in use.
Overview
Soil classification is a dynamic subject, from the structure of the system, ...
. The study of pedogenesis is important to understanding soil distribution patterns in current (
soil geography) and past (
paleopedology
Paleopedology (palaeopedology in the United Kingdom) 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 ...
) geologic periods.
Overview
Soil develops through a series of changes. The starting point is
weathering
Weathering is the deterioration of rocks, soils and minerals as well as wood and artificial materials through contact with water, atmospheric gases, and biological organisms. Weathering occurs ''in situ'' (on site, with little or no movement) ...
of freshly accumulated
parent material
Parent material is the underlying geological material (generally bedrock or a superficial or drift deposit) in which soil horizons form. Soils typically inherit a great deal of structure and minerals from their parent material, and, as such, are ...
. A variety of soil microbes (
bacteria
Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a few micrometr ...
,
archaea,
fungi
A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from ...
) feed on simple compounds (
nutrients
A nutrient is a substance used by an organism to survive, grow, and reproduce. The requirement for dietary nutrient intake applies to animals, plants, fungi, and protists. Nutrients can be incorporated into cells for metabolic purposes or excre ...
) released by
weathering
Weathering is the deterioration of rocks, soils and minerals as well as wood and artificial materials through contact with water, atmospheric gases, and biological organisms. Weathering occurs ''in situ'' (on site, with little or no movement) ...
, and produce organic acids and specialized proteins which contribute in turn to mineral weathering. They also leave behind
organic residues which contribute to
humus formation. Plant roots with their symbiotic
mycorrhizal fungi are also able to extract nutrients from rocks.
New soils increase in depth by a combination of weathering, and further
deposition. The
soil production rate due to weathering is approximately 1/10 mm per year. New soils can also deepen from
dust deposition. Gradually soil is able to support higher forms of plants and animals, starting with
pioneer species
Pioneer species are hardy species that are the first to colonize barren environments or previously biodiverse steady-state ecosystems that have been disrupted, such as by wildfire.
Pioneer flora
Some lichens grow on rocks without soil, so ...
, and proceeding along
ecological succession to more complex
plant and animal communities.
Topsoil
Topsoil is the upper layer of soil. It has the highest concentration of organic matter and microorganisms and is where most of the Earth's biological soil activity occurs.
Description
Topsoil is composed of mineral particles and organic matt ...
s deepen with the accumulation of humus originating from dead remains of
higher plants
Vascular plants (), also called tracheophytes () or collectively Tracheophyta (), form a large group of land plants ( accepted known species) that have lignified tissues (the xylem) for conducting water and minerals throughout the plant. They ...
and soil microbes. They also deepen through
mixing of organic matter with weathered minerals. As soils mature, they develop
soil horizons as organic matter accumulates and mineral weathering and leaching take place.
Factors of soil formation
Soil formation is influenced by at least five classic factors that are intertwined in the evolution of a soil. They are: parent material, climate, topography (relief), organisms, and time.
When reordered to climate, organisms, relief, parent material, and time, they form the acronym CLORPT.
Parent material
The mineral material from which a soil forms is called
parent material
Parent material is the underlying geological material (generally bedrock or a superficial or drift deposit) in which soil horizons form. Soils typically inherit a great deal of structure and minerals from their parent material, and, as such, are ...
. Rock, whether its origin is
igneous
Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or ...
,
sedimentary
Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...
, or
metamorphic
Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, causi ...
, is the source of all soil mineral materials and the origin of all
plant nutrients with the exceptions of
nitrogen
Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
,
hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...
and
carbon
Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon mak ...
. As the parent material is chemically and physically weathered, transported, deposited and precipitated, it is transformed into a soil.
Typical soil parent mineral materials are:
*
Quartz
Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical ...
: SiO
2
*
Calcite: CaCO
3
*
Feldspar
Feldspars are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) felds ...
: KAlSi
3O
8
*
Mica (biotite):
Parent materials are classified according to how they came to be deposited. Residual materials are mineral materials that have weathered in place from primary
bedrock. Transported materials are those that have been deposited by water, wind, ice or gravity. Cumulose material is organic matter that has grown and accumulates in place.
Residual soils are soils that develop from their underlying
parent rock In the earth sciences, parent rock, also sometimes ''substratum'', is the original rock from which younger rock or soil is formed. In soil formation the parent rock (or parent material) normally has a large influence on the nature of the resulting ...
s and have the same general chemistry as those rocks. The soils found on
mesa
A mesa is an isolated, flat-topped elevation, ridge or hill, which is bounded from all sides by steep escarpments and stands distinctly above a surrounding plain. Mesas characteristically consist of flat-lying soft sedimentary rocks capped by a ...
s,
plateau
In geology and physical geography, a plateau (; ; ), also called a high plain or a tableland, is an area of a highland consisting of flat terrain that is raised sharply above the surrounding area on at least one side. Often one or more sides ...
x, and
plain
In geography, a plain is a flat expanse of land that generally does not change much in elevation, and is primarily treeless. Plains occur as lowlands along valleys or at the base of mountains, as coastal plains, and as plateaus or uplands ...
s are residual soils. In the United States as little as three percent of the soils are residual.
Most soils derive from transported materials that have been moved many miles by wind, water, ice and gravity.
*
Aeolian processes
Aeolian processes, also spelled eolian, pertain to wind activity in the study of geology and weather and specifically to the wind's ability to shape the surface of the Earth (or other planets). Winds may erode, transport, and deposit mate ...
(movement by wind) are capable of moving
silt
Silt is granular material of a size between sand and clay and composed mostly of broken grains of quartz. Silt may occur as a soil (often mixed with sand or clay) or as sediment mixed in suspension with water. Silt usually has a floury feel ...
and fine
sand
Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural class o ...
many hundreds of miles, forming
loess soils (60–90 percent silt), common in the Midwest of North America, north-western Europe, Argentina and Central Asia. Clay is seldom moved by wind as it forms stable aggregates.
* Water-transported materials are classed as either
alluvial
Alluvium (from Latin ''alluvius'', from ''alluere'' 'to wash against') is loose clay, silt, sand, or gravel that has been deposited by running water in a stream bed, on a floodplain, in an alluvial fan or beach, or in similar settings. All ...
,
lacustrine
A lake is an area filled with water, localized in a basin, surrounded by land, and distinct from any river or other outlet that serves to feed or drain the lake. Lakes lie on land and are not part of the ocean, although, like the much larger ...
, or marine. Alluvial materials are those moved and deposited by flowing water.
Sedimentary deposits settled in lakes are called lacustrine.
Lake Bonneville
Lake Bonneville was the largest Late Pleistocene paleolake in the Great Basin of western North America. It was a pluvial lake that formed in response to an increase in precipitation and a decrease in evaporation as a result of cooler temperature ...
and many soils around the
Great Lakes
The Great Lakes, also called the Great Lakes of North America, are a series of large interconnected freshwater lakes in the mid-east region of North America that connect to the Atlantic Ocean via the Saint Lawrence River. There are five lak ...
of the United States are examples. Marine deposits, such as soils along the Atlantic and
Gulf Coast
The Gulf Coast of the United States, also known as the Gulf South, is the coast, coastline along the Southern United States where they meet the Gulf of Mexico. The list of U.S. states and territories by coastline, coastal states that have a shor ...
s and in the
Imperial Valley
, photo = Salton Sea from Space.jpg
, photo_caption = The Imperial Valley below the Salton Sea. The US-Mexican border runs diagonally across the lower left of the image.
, map_image = Newriverwatershed-1-.jpg
, map_caption = Map of Imperial ...
of California of the United States, are the beds of ancient seas that have been revealed as the land uplifted.
* Ice moves parent material and makes deposits in the form of terminal and lateral
moraines in the case of stationary glaciers. Retreating glaciers leave smoother ground moraines and in all cases,
outwash plain
An outwash plain, also called a sandur (plural: ''sandurs''), sandr or sandar, is a plain formed of glaciofluvial deposits due to meltwater outwash at the terminus of a glacier. As it flows, the glacier grinds the underlying rock surface and c ...
s are left as alluvial deposits are moved downstream from the glacier.
* Parent material moved by gravity is obvious at the base of steep slopes as
talus cones and is called
colluvial material.
Cumulose parent material is not moved but originates from deposited organic material. This includes
peat
Peat (), also known as turf (), is an accumulation of partially decayed vegetation or organic matter. It is unique to natural areas called peatlands, bogs, mires, moors, or muskegs. The peatland ecosystem covers and is the most efficient ...
and
muck soils and results from preservation of plant residues by the low oxygen content of a high water table. While peat may form sterile soils, muck soils may be very fertile.
Weathering
The
weathering
Weathering is the deterioration of rocks, soils and minerals as well as wood and artificial materials through contact with water, atmospheric gases, and biological organisms. Weathering occurs ''in situ'' (on site, with little or no movement) ...
of parent material takes the form of physical weathering (disintegration), chemical weathering (decomposition) and chemical transformation. Weathering is usually confined to the top few meters of geologic material, because physical, chemical, and biological stresses and fluctuations generally decrease with depth. Physical disintegration begins as rocks that have solidified deep in the Earth are exposed to lower pressure near the surface and swell and become mechanically unstable. Chemical decomposition is a function of mineral solubility, the rate of which doubles with each 10 °C rise in temperature, but is strongly dependent on water to effect chemical changes. Rocks that will decompose in a few years in tropical climates will remain unaltered for millennia in deserts.
Structural changes are the result of
hydration Hydration may refer to:
* Hydrate, a substance that contains water
* Hydration enthalpy, energy released through hydrating a substance
* Hydration reaction, a chemical addition reaction where a hydroxyl group and proton are added to a compound
* ...
,
oxidation
Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a ...
, and
reduction. Chemical weathering mainly results from the excretion of
organic acids
An organic acid is an organic compound with acidic properties. The most common organic acids are the carboxylic acids, whose acidity is associated with their carboxyl group –COOH. Sulfonic acids, containing the group –SO2OH, are r ...
and
chelating
Chelation is a type of bonding of ions and molecules to metal ions. It involves the formation or presence of two or more separate coordinate bonds between a polydentate (multiple bonded) ligand and a single central metal atom. These ligands are ...
compounds by bacteria and fungi,
thought to increase under present-day
greenhouse effect
The greenhouse effect is a process that occurs when energy from a planet's host star goes through the planet's atmosphere and heats the planet's surface, but greenhouse gases in the atmosphere prevent some of the heat from returning directly ...
.
* ''Physical disintegration'' is the first stage in the transformation of parent material into soil. Temperature fluctuations cause expansion and contraction of the rock, splitting it along lines of weakness. Water may then enter the cracks and freeze and cause the physical splitting of material along a path toward the center of the rock, while temperature gradients within the rock can cause exfoliation of "shells". Cycles of wetting and drying cause soil particles to be abraded to a finer size, as does the physical rubbing of material as it is moved by wind, water, and gravity. Water can deposit within rocks minerals that expand upon drying, thereby stressing the rock. Finally, organisms reduce parent material in size and create crevices and pores through the mechanical action of plant roots and the digging activity of animals. Grinding of parent material by rock-eating animals also contributes to incipient soil formation.
* ''Chemical decomposition'' and ''structural changes'' result when minerals are made soluble by water or are changed in structure. The first three of the following list are solubility changes and the last three are structural changes.
# The ''
solution
Solution may refer to:
* Solution (chemistry), a mixture where one substance is dissolved in another
* Solution (equation), in mathematics
** Numerical solution, in numerical analysis, approximate solutions within specified error bounds
* Soluti ...
'' of salts in water results from the action of bipolar
water molecules
Water () is a polar inorganic compound that is at room temperature a tasteless and odorless liquid, which is nearly colorless apart from an inherent hint of blue. It is by far the most studied chemical compound and is described as the "un ...
on
ionic salt
In chemistry, a salt is a chemical compound consisting of an ionic assembly of positively charged cations and negatively charged anions, which results in a compound with no net electric charge. A common example is table salt, with positively ch ...
compounds producing a solution of ions and water, removing those minerals and reducing the rock's integrity, at a rate depending on
water flow
Environmental flows describe the quantity, timing, and quality of water flows required to sustain freshwater and estuarine ecosystems and the human livelihoods and well being that depend on these ecosystems. In the Indian context river flows requir ...
and pore channels.
# ''
Hydrolysis
Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution, elimination, and solvation reactions in which water is the nucleophile.
Biological hydrolys ...
'' is the transformation of minerals into
polar
Polar may refer to:
Geography
Polar may refer to:
* Geographical pole, either of two fixed points on the surface of a rotating body or planet, at 90 degrees from the equator, based on the axis around which a body rotates
* Polar climate, the c ...
molecules by the splitting of intervening water. This results in soluble
acid-base pairs. For example, the hydrolysis of
orthoclase
Orthoclase, or orthoclase feldspar ( endmember formula K Al Si3 O8), is an important tectosilicate mineral which forms igneous rock. The name is from the Ancient Greek for "straight fracture," because its two cleavage planes are at right angles ...
-
feldspar
Feldspars are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) felds ...
transforms it to acid
silicate clay and basic
potassium hydroxide
Potassium hydroxide is an inorganic compound with the formula K OH, and is commonly called caustic potash.
Along with sodium hydroxide (NaOH), KOH is a prototypical strong base. It has many industrial and niche applications, most of which exp ...
, both of which are more soluble.
# In ''
carbonation
Carbonation is the chemical reaction of carbon dioxide to give carbonates, bicarbonates, and carbonic acid. In chemistry, the term is sometimes used in place of carboxylation, which refers to the formation of carboxylic acids.
In inorganic ch ...
'', the solution of
carbon dioxide
Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...
in water forms
carbonic acid. Carbonic acid will transform
calcite into more soluble
calcium bicarbonate
Calcium bicarbonate, also called calcium hydrogencarbonate, has the chemical formula Ca(HCO3)2. The term does not refer to a known solid compound; it exists only in aqueous solution containing calcium (Ca2+), bicarbonate (), and carbonate () ions ...
.
# ''
Hydration Hydration may refer to:
* Hydrate, a substance that contains water
* Hydration enthalpy, energy released through hydrating a substance
* Hydration reaction, a chemical addition reaction where a hydroxyl group and proton are added to a compound
* ...
'' is the inclusion of water in a mineral structure, causing it to swell and leaving it stressed and easily
decomposed.
# ''
Oxidation
Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a ...
'' of a mineral compound is the inclusion of
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...
in a mineral, causing it to increase its
oxidation number
In chemistry, the oxidation state, or oxidation number, is the hypothetical charge of an atom if all of its bonds to different atoms were fully ionic. It describes the degree of oxidation (loss of electrons) of an atom in a chemical compound. C ...
and swell due to the relatively large size of oxygen, leaving it stressed and more easily attacked by water (hydrolysis) or carbonic acid (carbonation).
# ''
Reduction'', the opposite of oxidation, means the removal of oxygen, hence the oxidation number of some part of the mineral is reduced, which occurs when oxygen is scarce. The reduction of minerals leaves them electrically unstable, more soluble and internally stressed and easily decomposed. It mainly occurs in
waterlogged conditions.
Of the above, hydrolysis and carbonation are the most effective, in particular in regions of high rainfall, temperature and physical
erosion
Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is dis ...
.
Chemical weathering
Weathering is the deterioration of rocks, soils and minerals as well as wood and artificial materials through contact with water, atmospheric gases, and biological organisms. Weathering occurs ''in situ'' (on site, with little or no movement ...
becomes more effective as the
surface area of the rock increases, thus is favoured by physical disintegration. This stems in latitudinal and altitudinal climate gradients in
regolith
Regolith () is a blanket of unconsolidated, loose, heterogeneous superficial deposits covering solid rock. It includes dust, broken rocks, and other related materials and is present on Earth, the Moon, Mars, some asteroids, and other terrestr ...
formation.
Saprolite is a particular example of a residual soil formed from the transformation of granite, metamorphic and other types of bedrock into clay minerals. Often called
eathered granite saprolite is the result of weathering processes that include:
hydrolysis
Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution, elimination, and solvation reactions in which water is the nucleophile.
Biological hydrolys ...
,
chelation
Chelation is a type of bonding of ions and molecules to metal ions. It involves the formation or presence of two or more separate coordinate bonds between a polydentate (multiple bonded) ligand and a single central metal atom. These ligands are ...
from organic compounds,
hydration Hydration may refer to:
* Hydrate, a substance that contains water
* Hydration enthalpy, energy released through hydrating a substance
* Hydration reaction, a chemical addition reaction where a hydroxyl group and proton are added to a compound
* ...
and physical processes that include
freezing
Freezing is a phase transition where a liquid turns into a solid when its temperature is lowered below its freezing point. In accordance with the internationally established definition, freezing means the solidification phase change of a liquid ...
and
thawing
Melting, or fusion, is a physical process that results in the phase transition of a substance from a solid to a liquid. This occurs when the internal energy of the solid increases, typically by the application of heat or pressure, which ...
. The mineralogical and chemical composition of the primary bedrock material, its physical features, including
grain size and degree of consolidation, and the rate and type of weathering transforms the parent material into a different mineral. The texture, pH and mineral constituents of saprolite are inherited from its parent material. This process is also called ''arenization'', resulting in the formation of sandy soils (granitic arenas), thanks to the much higher resistance of quartz compared to other mineral components of granite (
micas
Micas ( ) are a group of silicate minerals whose outstanding physical characteristic is that individual mica crystals can easily be split into extremely thin elastic plates. This characteristic is described as perfect basal cleavage. Mica is ...
,
amphiboles
Amphibole () is a group of inosilicate minerals, forming prism or needlelike crystals, composed of double chain tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures. Its IMA symbol ...
,
feldspars
Feldspars are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) feldsp ...
).
Climate
The principal climatic variables influencing soil formation are effective
precipitation
In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. ...
(i.e., precipitation minus
evapotranspiration
Evapotranspiration (ET) is the combined processes by which water moves from the earth’s surface into the atmosphere. It covers both water evaporation (movement of water to the air directly from soil, canopies, and water bodies) and transpi ...
) and temperature, both of which affect the rates of chemical, physical, and biological processes. Temperature and moisture both influence the organic matter content of soil through their effects on the balance between
primary production
In ecology, primary production is the synthesis of organic compounds from atmospheric or aqueous carbon dioxide. It principally occurs through the process of photosynthesis, which uses light as its source of energy, but it also occurs through ...
and
decomposition
Decomposition or rot is the process by which dead organic substances are broken down into simpler organic or inorganic matter such as carbon dioxide, water, simple sugars and mineral salts. The process is a part of the nutrient cycle and is e ...
: the colder or drier the climate the lesser atmospheric carbon is fixed as organic matter while the lesser organic matter is decomposed.
Climate is the dominant factor in soil formation, and soils show the distinctive characteristics of the
climate zone
Climate classifications are systems that categorize the world's climates. A climate classification may correlate closely with a biome classification, as climate is a major influence on life in a region. One of the most used is the Köppen climate ...
s in which they form, with a feedback to climate through transfer of carbon stocked in soil horizons back to the atmosphere.
If warm temperatures and abundant water are present in the profile at the same time, the processes of
weathering
Weathering is the deterioration of rocks, soils and minerals as well as wood and artificial materials through contact with water, atmospheric gases, and biological organisms. Weathering occurs ''in situ'' (on site, with little or no movement) ...
,
leaching
Leaching is the loss or extraction of certain materials from a carrier into a liquid (usually, but not always a solvent). and may refer to:
* Leaching (agriculture), the loss of water-soluble plant nutrients from the soil; or applying a small amou ...
, and
plant growth Important structures in plant development are buds, shoots, roots, leaves, and flowers; plants produce these tissues and structures throughout their life from meristems located at the tips of organs, or between mature tissues. Thus, a living plant a ...
will be maximized. According to the climatic determination of
biomes, humid climates favor the growth of trees. In contrast, grasses are the dominant native vegetation in subhumid and
semiarid
A semi-arid climate, semi-desert climate, or steppe climate is a dry climate sub-type. It is located on regions that receive precipitation below potential evapotranspiration, but not as low as a desert climate. There are different kinds of semi- ...
regions, while shrubs and brush of various kinds dominate in arid areas.
Water is essential for all the major chemical weathering reactions. To be effective in soil formation, water must penetrate the
regolith
Regolith () is a blanket of unconsolidated, loose, heterogeneous superficial deposits covering solid rock. It includes dust, broken rocks, and other related materials and is present on Earth, the Moon, Mars, some asteroids, and other terrestr ...
. The seasonal rainfall distribution, evaporative losses, site
topography
Topography is the study of the forms and features of land surfaces. The topography of an area may refer to the land forms and features themselves, or a description or depiction in maps.
Topography is a field of geoscience and planetary sc ...
, and
soil permeability interact to determine how effectively precipitation can influence soil formation. The greater the depth of water penetration, the greater the depth of weathering of the soil and its development. Surplus water percolating through the soil profile transports soluble and suspended materials from the upper layers (
eluviation
In geology, eluvium or eluvial deposits are those geological deposits and soils that are derived by ''in situ'' weathering or weathering plus gravitational movement or accumulation.
The process of removal of materials from geological or soil horiz ...
) to the lower layers (
illuviation Illuvium is material displaced across a soil profile, from one layer to another one, by the action of rainwater. The removal of material from a soil layer is called eluviation. The transport of the material may be either mechanical or chemical. The ...
), including clay particles and
dissolved organic matter
Dissolved organic carbon (DOC) is the fraction of organic carbon operationally defined as that which can pass through a filter with a pore size typically between 0.22 and 0.7 micrometers. The fraction remaining on the filter is called part ...
. It may also carry away soluble materials in the surface
drainage
Drainage is the natural or artificial removal of a surface's water and sub-surface water from an area with excess of water. The internal drainage of most agricultural soils is good enough to prevent severe waterlogging (anaerobic condition ...
waters. Thus, percolating water stimulates weathering reactions and helps differentiate soil horizons. Likewise, a deficiency of water is a major factor in determining the characteristics of soils of dry regions. Soluble salts are not leached from these soils, and in some cases they build up to levels that curtail plant and microbial growth. Soil profiles in arid and semi-arid regions are also apt to accumulate carbonates and certain types of expansive clays (
calcrete or
caliche
Caliche () is a sedimentary rock, a hardened natural cement of calcium carbonate that binds other materials—such as gravel, sand, clay, and silt. It occurs worldwide, in aridisol and mollisol soil orders—generally in arid or semiarid regions ...
horizons). In tropical soils, when the soil has been deprived of vegetation (e.g. by
deforestation
Deforestation or forest clearance is the removal of a forest or stand of trees from land that is then converted to non-forest use. Deforestation can involve conversion of forest land to farms, ranches, or urban use. The most concentrated ...
) and thereby is submitted to intense evaporation, the upward
capillary
A capillary is a small blood vessel from 5 to 10 micrometres (μm) in diameter. Capillaries are composed of only the tunica intima, consisting of a thin wall of simple squamous endothelial cells. They are the smallest blood vessels in the bod ...
movement of water, which has dissolved iron and aluminum salts, is responsible for the formation of a superficial hard pan of
laterite or
bauxite
Bauxite is a sedimentary rock with a relatively high aluminium content. It is the world's main source of aluminium and gallium. Bauxite consists mostly of the aluminium minerals gibbsite (Al(OH)3), boehmite (γ-AlO(OH)) and diaspore (α-AlO ...
, respectively, which is improper for cultivation, a known case of irreversible
soil degradation
Soil retrogression and degradation are two regressive evolution processes associated with the loss of equilibrium of a stable soil. Retrogression is primarily due to soil erosion and corresponds to a phenomenon where succession reverts the land to ...
(
lateritization, bauxitization).
The direct influences of climate include:
* A shallow accumulation of lime in low rainfall areas as
caliche
Caliche () is a sedimentary rock, a hardened natural cement of calcium carbonate that binds other materials—such as gravel, sand, clay, and silt. It occurs worldwide, in aridisol and mollisol soil orders—generally in arid or semiarid regions ...
* Formation of acid soils in humid areas
* Erosion of soils on steep hillsides
* Deposition of eroded materials downstream
* Very intense chemical weathering, leaching, and erosion in warm and humid regions where soil does not freeze
Climate directly affects the rate of weathering and leaching. Wind moves sand and smaller particles (dust), especially in arid regions where there is little plant cover, depositing it close or far from the entrainment source. The type and amount of precipitation influence soil formation by affecting the movement of ions and particles through the soil, and aid in the development of different soil profiles. Soil profiles are more distinct in wet and cool climates, where organic materials may accumulate, than in wet and warm climates, where organic materials are rapidly consumed. The effectiveness of water in weathering parent rock material depends on seasonal and daily temperature fluctuations, which favour
tensile stress
In continuum mechanics, stress is a physical quantity. It is a quantity that describes the magnitude of forces that cause deformation. Stress is defined as ''force per unit area''. When an object is pulled apart by a force it will cause elonga ...
es in rock minerals, and thus their mechanical
disaggregation
In macroeconomics, aggregate demand (AD) or domestic final demand (DFD) is the total demand for final goods and services in an economy at a given time. It is often called effective demand, though at other times this term is distinguished. This i ...
, a process called
thermal fatigue Thermo-mechanical fatigue (short TMF) is the overlay of a cyclical mechanical loading, that leads to fatigue of a material, with a cyclical thermal loading. Thermo-mechanical fatigue is an important point that needs to be considered, when construct ...
. By the same process
freeze-thaw
Frost weathering is a collective term for several mechanical weathering processes induced by stresses created by the freezing of water into ice. The term serves as an umbrella term for a variety of processes such as frost shattering, frost wedg ...
cycles are an effective mechanism which breaks up rocks and other consolidated materials.
Climate also indirectly influences soil formation through the effects of vegetation cover and biological activity, which modify the rates of chemical reactions in the soil.
Topography
The
topography
Topography is the study of the forms and features of land surfaces. The topography of an area may refer to the land forms and features themselves, or a description or depiction in maps.
Topography is a field of geoscience and planetary sc ...
, or
relief
Relief is a sculptural method in which the sculpted pieces are bonded to a solid background of the same material. The term '' relief'' is from the Latin verb ''relevo'', to raise. To create a sculpture in relief is to give the impression that th ...
, is characterized by the inclination (
slope
In mathematics, the slope or gradient of a line is a number that describes both the ''direction'' and the ''steepness'' of the line. Slope is often denoted by the letter ''m''; there is no clear answer to the question why the letter ''m'' is use ...
),
elevation
The elevation of a geographic location is its height above or below a fixed reference point, most commonly a reference geoid, a mathematical model of the Earth's sea level as an equipotential gravitational surface (see Geodetic datum § Ver ...
, and orientation of the terrain (
aspect
Aspect or Aspects may refer to:
Entertainment
* ''Aspect magazine'', a biannual DVD magazine showcasing new media art
* Aspect Co., a Japanese video game company
* Aspects (band), a hip hop group from Bristol, England
* ''Aspects'' (Benny Carter ...
). Topography determines the rate of precipitation or
runoff
Runoff, run-off or RUNOFF may refer to:
* RUNOFF, the first computer text-formatting program
* Runoff or run-off, another name for bleed, printing that lies beyond the edges to which a printed sheet is trimmed
* Runoff or run-off, a stock marke ...
and the rate of formation or erosion of the surface
soil profile
A soil horizon is a layer parallel to the soil surface whose physical, chemical and biological characteristics differ from the layers above and beneath. Horizons are defined in many cases by obvious physical features, mainly colour and texture. ...
. The topographical setting may either hasten or retard the work of climatic forces.
Steep slopes encourage rapid soil loss by
erosion
Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is dis ...
and allow less rainfall to enter the soil before running off and hence, little mineral deposition in lower profiles (
illuviation Illuvium is material displaced across a soil profile, from one layer to another one, by the action of rainwater. The removal of material from a soil layer is called eluviation. The transport of the material may be either mechanical or chemical. The ...
). In semiarid regions, the lower effective rainfall on steeper slopes also results in less complete vegetative cover, so there is less plant contribution to soil formation. For all of these reasons, steep slopes prevent the formation of soil from getting very far ahead of soil destruction. Therefore, soils on steep terrain tend to have rather shallow, poorly developed profiles in comparison to soils on nearby, more level sites.
Topography determines exposure to weather, fire, and other forces of man and nature. Mineral accumulations, plant nutrients, type of vegetation, vegetation growth, erosion, and water drainage are dependent on topographic relief. Soils at the bottom of a hill will get more water than soils on the slopes, and soils on the slopes that
face
The face is the front of an animal's head that features the eyes, nose and mouth, and through which animals express many of their emotions. The face is crucial for human identity, and damage such as scarring or developmental deformities may aff ...
the
sun's path will be drier than soils on slopes that do not.
In
swale
Swale or Swales may refer to:
Topography
* Swale (landform), a low tract of land
** Bioswale, landform designed to remove silt and pollution
** Swales, found in the formation of Hummocky cross-stratification
Geography
* River Swale, in North ...
s and depressions where runoff water tends to concentrate, the regolith is usually more deeply weathered and soil profile development is more advanced. However, in the lowest landscape positions, water may saturate the regolith to such a degree that drainage and aeration are restricted. Here, the weathering of some minerals and the decomposition of organic matter are retarded, while the loss of iron and manganese is accelerated. In such low-lying topography, special profile features characteristic of
wetland
A wetland is a distinct ecosystem that is flooded or saturated by water, either permanently (for years or decades) or seasonally (for weeks or months). Flooding results in oxygen-free (anoxic) processes prevailing, especially in the soils. The p ...
soils may develop. Depressions allow the accumulation of water, minerals and organic matter and in the extreme, the resulting soils will be
saline marsh
A salt marsh or saltmarsh, also known as a coastal salt marsh or a tidal marsh, is a coastal ecosystem in the upper coastal intertidal zone between land and open saltwater or brackish water that is regularly flooded by the tides. It is dominated ...
es or
peat bog
A bog or bogland is a wetland that accumulates peat as a deposit of dead plant materials often mosses, typically sphagnum moss. It is one of the four main types of wetlands. Other names for bogs include mire, mosses, quagmire, and muskeg; a ...
s.
Recurring patterns of topography result in toposequences or
soil catenas. These patterns emerge from topographic differences in
erosion
Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is dis ...
,
deposition,
fertility
Fertility is the capability to produce offspring through reproduction following the onset of sexual maturity. The fertility rate is the average number of children born by a female during her lifetime and is quantified demographically. Fertili ...
,
soil moisture,
plant cover
The abundances of plant species are often measured by plant cover, which is the relative area covered by different plant species in a small plot. Plant cover is not biased by the size and distributions of individuals, and is an important and oft ...
,
soil biology
Soil biology is the study of microbial and faunal activity and ecology in soil.
Soil life, soil biota, soil fauna, or edaphon is a collective term that encompasses all organisms that spend a significant portion of their life cycle within a soil ...
,
fire history Fire history, the ecological science of the study of the history of wildfires, is a subdiscipline of fire ecology. Patterns of forest fires in historical and prehistorical time provide information relevant to the pattern of vegetation in modern land ...
, and exposure to the elements. As a matter of rule,
gravity
In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stro ...
transport water downslope, together with mineral and organic
solutes
In chemistry, a solution is a special type of homogeneous mixture composed of two or more substances. In such a mixture, a solute is a substance dissolved in another substance, known as a solvent. If the attractive forces between the solvent ...
and
colloids, increasing
particulate
Particulates – also known as atmospheric aerosol particles, atmospheric particulate matter, particulate matter (PM) or suspended particulate matter (SPM) – are microscopic particles of solid or liquid matter suspended in the air. The te ...
and base content at the foot of hills and mountains. However, many other factors like
drainage
Drainage is the natural or artificial removal of a surface's water and sub-surface water from an area with excess of water. The internal drainage of most agricultural soils is good enough to prevent severe waterlogging (anaerobic condition ...
and
erosion
Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is dis ...
interact with slope position, blurring its expected influence on
crop yield.
Organisms
Each soil has a unique combination of microbial, plant, animal and human influences acting upon it. Microorganisms are particularly influential in the mineral transformations critical to the soil forming process. Additionally, some bacteria can fix atmospheric nitrogen and some fungi are efficient at extracting deep soil phosphorus and increasing soil carbon levels in the form of
glomalin
Glomalin is a glycoprotein produced abundantly on hyphae and spores of arbuscular mycorrhizal (AM) fungi in soil and in roots. Glomalin was discovered in 1996 by Sara F. Wright, a scientist at the USDA Agricultural Research Service. The name com ...
. Plants hold soil against erosion, and accumulated plant material build soil
humus levels. Plant
root exudation supports microbial activity. Animals serve to decompose plant materials and mix soil through
bioturbation.
Soil is the most speciose
ecosystem
An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient cycles and energy flows. Energy enters the syste ...
on Earth, but the vast majority of organisms in soil are
microbes
A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in olde ...
, a great many of which have not been described.
There may be a population limit of around one billion cells per gram of soil, but estimates of the number of species vary widely from 50,000 per gram to over a million per gram of soil.
The total number of organisms and species can vary widely according to soil type, location, and depth.
Plants,
animals
Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage in ...
, fungi,
bacteria
Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a few micrometr ...
and humans affect soil formation (see
soil biomantle The soil biomantle can be described and defined in several ways. Most simply, the soil biomantle is the organic-rich bioturbated upper part of the soil, including the topsoil where most biota live, reproduce, die, and become assimilated. The biomant ...
and
stonelayer). Soil animals, including soil
macrofauna
Fauna is all of the animal life present in a particular region or time. The corresponding term for plants is ''flora'', and for fungi, it is '' funga''. Flora, fauna, funga and other forms of life are collectively referred to as '' biota''. Zoo ...
and
soil mesofauna
Soil mesofauna are invertebrates between 0.1mm and 2mm in size, which live in the soil or in a leaf litter layer on the soil surface. Members of this group include nematodes, mites, springtails (collembola), proturans, pauropods, rotifers, ear ...
, mix soils as they form
burrows and
pores, allowing moisture and gases to move about, a process called
bioturbation. In the same way,
plant roots
In vascular plants, the roots are the organs of a plant that are modified to provide anchorage for the plant and take in water and nutrients into the plant body, which allows plants to grow taller and faster. They are most often below the sur ...
penetrate soil horizons and open channels upon decomposition. Plants with deep
taproot
A taproot is a large, central, and dominant root from which other roots sprout laterally. Typically a taproot is somewhat straight and very thick, is tapering in shape, and grows directly downward. In some plants, such as the carrot, the taproo ...
s can penetrate many metres through the different soil layers to bring up
nutrients
A nutrient is a substance used by an organism to survive, grow, and reproduce. The requirement for dietary nutrient intake applies to animals, plants, fungi, and protists. Nutrients can be incorporated into cells for metabolic purposes or excre ...
from deeper in the profile. Plants have fine roots that excrete organic compounds (
sugars
Sugar is the generic name for sweet-tasting, soluble carbohydrates, many of which are used in food. Simple sugars, also called monosaccharides, include glucose, fructose, and galactose. Compound sugars, also called disaccharides or double ...
,
organic acids
An organic acid is an organic compound with acidic properties. The most common organic acids are the carboxylic acids, whose acidity is associated with their carboxyl group –COOH. Sulfonic acids, containing the group –SO2OH, are r ...
,
mucilage
Mucilage is a thick, gluey substance produced by nearly all plants and some microorganisms. These microorganisms include protists which use it for their locomotion. The direction of their movement is always opposite to that of the secretion of m ...
), slough off cells (in particular at their tip) and are easily decomposed, adding organic matter to soil, a process called ''
rhizodeposition''. Micro-organisms, including fungi and bacteria, effect chemical exchanges between roots and soil and act as a reserve of nutrients in a soil biological ''hotspot'' called
rhizosphere
The rhizosphere is the narrow region of soil or substrate that is directly influenced by root secretions and associated soil microorganisms known as the root microbiome. Soil pores in the rhizosphere can contain many bacteria and other microo ...
. The growth of roots through the soil stimulates
microbial populations, stimulating in turn the activity of their
predator
Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill th ...
s (notably
amoeba
An amoeba (; less commonly spelled ameba or amœba; plural ''am(o)ebas'' or ''am(o)ebae'' ), often called an amoeboid, is a type of cell or unicellular organism with the ability to alter its shape, primarily by extending and retracting pseudop ...
), thereby increasing the
mineralization rate, and in last turn root growth, a
positive feedback
Positive feedback (exacerbating feedback, self-reinforcing feedback) is a process that occurs in a feedback loop which exacerbates the effects of a small disturbance. That is, the effects of a perturbation on a system include an increase in th ...
called the soil
microbial loop
The microbial loop describes a trophic pathway where, in aquatic systems, dissolved organic carbon (DOC) is returned to higher trophic levels via its incorporation into bacterial biomass, and then coupled with the classic food chain formed by phy ...
. Out of root influence, in the
bulk soil Bulk soil is soil outside the rhizosphere that is not penetrated by plant roots. The bulk soil is like an ecosystem, it is made up of many things such as: nutrients, ions, soil particles, and root exudates. There are many different interactions tha ...
, most bacteria are in a quiescent stage, forming micro
aggregates, i.e.
mucilaginous
Mucilage is a thick, gluey substance produced by nearly all plants and some microorganisms. These microorganisms include protists which use it for their locomotion. The direction of their movement is always opposite to that of the secretion of ...
colonies to which clay particles are glued, offering them a protection against
desiccation and
predation
Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill th ...
by soil
microfauna
Microfauna (Ancient Greek ''mikros'' "small" + New Latin ''fauna'' "animal") refers to microscopic animals and organisms that exhibit animal-like qualities. Microfauna are represented in the animal kingdom (e.g., nematodes, small arthropods) and t ...
(
bacteriophagous protozoa and
nematodes
The nematodes ( or grc-gre, Νηματώδη; la, Nematoda) or roundworms constitute the phylum Nematoda (also called Nemathelminthes), with plant-parasitic nematodes also known as eelworms. They are a diverse animal phylum inhabiting a broa ...
). Microaggregates (20-250 μm) are ingested by
soil mesofauna
Soil mesofauna are invertebrates between 0.1mm and 2mm in size, which live in the soil or in a leaf litter layer on the soil surface. Members of this group include nematodes, mites, springtails (collembola), proturans, pauropods, rotifers, ear ...
and
macrofauna
Fauna is all of the animal life present in a particular region or time. The corresponding term for plants is ''flora'', and for fungi, it is '' funga''. Flora, fauna, funga and other forms of life are collectively referred to as '' biota''. Zoo ...
, and bacterial bodies are partly or totally digested in their
guts
The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The GI tract contains all the major organs of the digestive system, in humans and ...
.
Humans impact soil formation by removing vegetation cover through tillage, application of
biocides, fire and leaving soils bare. This can lead to
erosion
Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is dis ...
,
waterlogging,
lateritization or
podzolization Podsolisation is an extreme form of leaching which causes the eluviation of iron and aluminium sesquioxides.
The process generally occurs in areas where precipitation is greater than evapotranspiration. The minerals are removed by a process kno ...
(according to climate and topography). Their
tillage
Tillage is the agricultural preparation of soil by mechanical agitation of various types, such as digging, stirring, and overturning. Examples of human-powered tilling methods using hand tools include shoveling, picking, mattock work, hoein ...
also mixes the different soil layers, restarting the soil formation process as less weathered material is mixed with the more developed upper layers, resulting in net increased rate of mineral weathering.
Earthworms,
ants
Ants are Eusociality, eusocial insects of the Family (biology), family Formicidae and, along with the related wasps and bees, belong to the Taxonomy (biology), order Hymenoptera. Ants evolved from Vespoidea, vespoid wasp ancestors in the Creta ...
,
termites
Termites are small insects that live in colonies and have distinct castes (eusocial) and feed on wood or other dead plant matter. Termites comprise the infraorder Isoptera, or alternatively the epifamily Termitoidae, within the order Blattod ...
,
moles Moles can refer to:
* Moles de Xert, a mountain range in the Baix Maestrat comarca, Valencian Community, Spain
* The Moles (Australian band)
*The Moles, alter ego of Scottish band Simon Dupree and the Big Sound
People
*Abraham Moles, French engin ...
,
gophers, as well as some
millipedes
Millipedes are a group of arthropods that are characterised by having two pairs of jointed legs on most body segments; they are known scientifically as the class Diplopoda, the name derived from this feature. Each double-legged segment is a resu ...
and
tenebrionid
Darkling beetle is the common name for members of the beetle family Tenebrionidae. The number of species in the Tenebrionidae is estimated at more than 20,000 and the family is cosmopolitan in distribution.
Taxonomy
''Tenebrio'' is the Latin ge ...
beetles mix the soil as they burrow, significantly affecting soil formation.
Earthworms ingest soil particles and organic residues, enhancing the availability of plant nutrients in the material that passes through their bodies. They aerate and stir the soil and create stable soil aggregates, after having disrupted links between soil particles during the intestinal transit of ingested soil, thereby assuring ready infiltration of water. In addition, as ants and termites build mounds, earthworms transport soil materials from one horizon to another. Other important functions are fulfilled by earthworms in the soil ecosystem, in particular their intense
mucus
Mucus ( ) is a slippery aqueous secretion produced by, and covering, mucous membranes. It is typically produced from cells found in mucous glands, although it may also originate from mixed glands, which contain both serous and mucous cells. It ...
production, both within the intestine and as a lining in their galleries, exert a
priming effect on soil microflora, giving them the status of
ecosystem engineers, which they share with ants and termites.
In general, the mixing of the soil by the activities of animals, sometimes called
pedoturbation
Perturbation (from Latin: ''perturbare'' "to confuse, disorder, disturb", from ''per''- "through" + ''turbare'' "disturb, confuse," from ''turba'' "turmoil, crowd") is a set of pedology (soil study) and sedimentary geology processes relating to c ...
, tends to undo or counteract the tendency of other soil-forming processes that create distinct horizons. Termites and ants may also retard soil profile development by denuding large areas of soil around their nests, leading to increased loss of soil by erosion. Large animals such as
gopher
Pocket gophers, commonly referred to simply as gophers, are burrowing rodents of the family Geomyidae. The roughly 41 speciesSearch results for "Geomyidae" on thASM Mammal Diversity Database are all endemic to North and Central America. They are ...
s,
moles Moles can refer to:
* Moles de Xert, a mountain range in the Baix Maestrat comarca, Valencian Community, Spain
* The Moles (Australian band)
*The Moles, alter ego of Scottish band Simon Dupree and the Big Sound
People
*Abraham Moles, French engin ...
, and
prairie dogs bore into the lower soil horizons, bringing materials to the surface. Their tunnels are often open to the surface, encouraging the movement of water and air into the subsurface layers. In localized areas, they enhance mixing of the lower and upper horizons by creating, and later refilling the tunnels. Old animal burrows in the lower horizons often become filled with soil material from the overlying A horizon, creating profile features known as ''crotovinas''.
Vegetation impacts soils in numerous ways. It can prevent erosion caused by excessive rain that might result from
surface runoff. Plants shade soils, keeping them cooler and slowing evaporation of
soil moisture. Conversely, by way of
transpiration
Transpiration is the process of water movement through a plant and its evaporation from aerial parts, such as leaves, stems and flowers. Water is necessary for plants but only a small amount of water taken up by the roots is used for growth ...
, plants can cause soils to lose moisture, resulting in complex and highly variable relationships between
leaf area index (measuring light interception) and moisture loss: more generally plants prevent soil from
desiccation during driest months while they dry it during moister months, thereby acting as a buffer against strong moisture variation. Plants can form new chemicals that can break down minerals, both directly and indirectly through mycorrhizal fungi
and rhizosphere bacteria, and improve the soil structure. The type and amount of vegetation depends on climate, topography, soil characteristics and biological factors, mediated or not by human activities. Soil factors such as density, depth, chemistry, pH, temperature and moisture greatly affect the type of plants that can grow in a given location. Dead plants and fallen leaves and stems begin their decomposition on the surface. There, organisms feed on them and mix the organic material with the upper soil layers; these added organic compounds become part of the soil formation process.
The influence of man, and by association, fire, are state factors placed within the organisms state factor. Man can import, or extract, nutrients and energy in ways that dramatically change soil formation. Accelerated soil erosion due to
overgrazing
Overgrazing occurs when plants are exposed to intensive grazing for extended periods of time, or without sufficient recovery periods. It can be caused by either livestock in poorly managed agricultural applications, game reserves, or nature res ...
, and
Pre-Columbian
In the history of the Americas, the pre-Columbian era spans from the original settlement of North and South America in the Upper Paleolithic period through European colonization, which began with Christopher Columbus's voyage of 1492. Usually, ...
terraforming
Terraforming or terraformation ("Earth-shaping") is the hypothetical process of deliberately modifying the atmosphere, temperature, surface topography or ecology of a planet, moon, or other body to be similar to the environment of Earth to make ...
the Amazon basin resulting in
Terra Preta
''Terra preta'' (, locally , literally "black soil" in Portuguese) is a type of very dark, fertile anthropogenic soil ( anthrosol) found in the Amazon Basin. It is also known as "Amazonian dark earth" or "Indian black earth". In Portuguese its f ...
are two examples of the effects of man's management.
Human activities widely influence soil formation. For example, it is believed that
Native Americans regularly set fires to maintain several large areas of
prairie
Prairies are ecosystems considered part of the temperate grasslands, savannas, and shrublands biome by ecologists, based on similar temperate climates, moderate rainfall, and a composition of grasses, herbs, and shrubs, rather than trees, as the ...
grasslands in
Indiana
Indiana () is a U.S. state in the Midwestern United States. It is the 38th-largest by area and the 17th-most populous of the 50 States. Its capital and largest city is Indianapolis. Indiana was admitted to the United States as the 19th s ...
and
Michigan
Michigan () is a U.S. state, state in the Great Lakes region, Great Lakes region of the Upper Midwest, upper Midwestern United States. With a population of nearly 10.12 million and an area of nearly , Michigan is the List of U.S. states and ...
, although climate and mammalian
grazers (e.g.
bisons) are also advocated to explain the maintenance of the
Great Plains of North America. In more recent times, human destruction of natural vegetation and subsequent
tillage
Tillage is the agricultural preparation of soil by mechanical agitation of various types, such as digging, stirring, and overturning. Examples of human-powered tilling methods using hand tools include shoveling, picking, mattock work, hoein ...
of the soil for
crop
A crop is a plant that can be grown and harvested extensively for profit or subsistence. When the plants of the same kind are cultivated at one place on a large scale, it is called a crop. Most crops are cultivated in agriculture or hydropon ...
production has abruptly modified soil formation. Likewise,
irrigating soil in an
arid
A region is arid when it severely lacks available water, to the extent of hindering or preventing the growth and development of plant and animal life. Regions with arid climates tend to lack vegetation and are called xeric or desertic. Most ...
region drastically influences soil-forming factors, as does adding fertilizer and lime to soils of low fertility.
Distinct ecosystems produce distinct soils, sometimes in easily observable ways. For example, three species of
land snails in the genus ''
Euchondrus'' in the
Negev desert are noted for eating
lichens growing under the surface
limestone
Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...
rocks and slabs (
endolithic
An endolith or endolithic is an organism (archaeon, bacterium, fungus, lichen, algae or amoeba) that is able to acquire the necessary resources for growth in the inner part of a rock, mineral, coral, animal shells, or in the Porosity, pores bet ...
lichens). The grazing activity of these
ecosystem engineers disrupts and eats the limestone, resulting in the
weathering
Weathering is the deterioration of rocks, soils and minerals as well as wood and artificial materials through contact with water, atmospheric gases, and biological organisms. Weathering occurs ''in situ'' (on site, with little or no movement) ...
of the stones, and the subsequent formation of soil.
They have a significant effect on the region: the total population of snails is estimated to process between 0.7 and 1.1 metric ton per hectare per year of limestone in the Negev desert.
The effects of ancient ecosystems are not as easily observed, and this challenges the understanding of soil formation. For example, the
chernozem
Chernozem (from rus, чернозём, p=tɕɪrnɐˈzʲɵm, r=chernozyom; "black ground"), also called black soil, is a black-colored soil containing a high percentage of humus (4% to 16%) and high percentages of phosphorus and ammonia compou ...
s of the North American tallgrass prairie have a humus fraction nearly half of which is
charcoal. This outcome was not anticipated because the antecedent prairie
fire ecology
Fire ecology is a scientific discipline concerned with natural processes involving fire in an ecosystem and the ecological effects, the interactions between fire and the abiotic and biotic components of an ecosystem, and the role as an ecosystem ...
capable of producing these distinct deep rich black soils is not easily observed. The role of soil engineers in the formation of charcoal-enriched horizons of
Terra preta
''Terra preta'' (, locally , literally "black soil" in Portuguese) is a type of very dark, fertile anthropogenic soil ( anthrosol) found in the Amazon Basin. It is also known as "Amazonian dark earth" or "Indian black earth". In Portuguese its f ...
(Amazonian Black Earths) is now acknowledged and was verified experimentally on the
pantropical
A pantropical ("all tropics") distribution is one which covers tropical regions of both hemispheres. Examples of species include caecilians, modern sirenians and the plant genera ''Acacia'' and ''Bacopa''.
''Neotropical'' is a zoogeographic term ...
earthworm ''
Pontoscolex
''Pontoscolex'' is a genus of earthworms in the family Glossoscolecidae, or alternatively, in the family Rhinodrilidae.
General characteristics
The type species ''Pontoscolex corethrurus'' has a circumtropical distribution, although it originat ...
corethrurus''.
Time
Time is a factor in the interactions of all the above.
While a mixture of sand, silt and clay constitute the
texture
Texture may refer to:
Science and technology
* Surface texture, the texture means smoothness, roughness, or bumpiness of the surface of an object
* Texture (roads), road surface characteristics with waves shorter than road roughness
* Texture ...
of a soil and the
aggregation of those components produces
ped
Peds are aggregates of soil particles formed as a result of pedogenic processes; this natural organization of particles forms discrete units separated by pores or voids. The term is generally used for macroscopic (visible; i.e. greater than 1 mm i ...
s, the development of a distinct
B horizon
A soil horizon is a layer parallel to the soil surface whose physical, chemical and biological characteristics differ from the layers above and beneath. Horizons are defined in many cases by obvious physical features, mainly colour and texture. ...
marks the development of a soil or pedogenesis. With time, soils will evolve features that depend on the interplay of the prior listed soil-forming factors.
It takes decades to several thousand years for a soil to develop a profile,
although the notion of soil development has been criticized, soil being in a constant state-of-change under the influence of fluctuating soil-forming factors.
That time period depends strongly on climate, parent material, relief, and biotic activity. For example, recently deposited material from a flood exhibits no soil development as there has not been enough time for the material to form a structure that further defines soil. The original soil surface is buried, and the formation process must begin anew for this deposit. Over time the soil will develop a profile that depends on the intensities of biota and climate. While a soil can achieve relative stability of its properties for extended periods,
the soil life cycle ultimately ends in soil conditions that leave it vulnerable to erosion. Despite the inevitability of soil retrogression and degradation, most soil cycles are long.
Soil-forming factors continue to affect soils during their existence, even on ''stable'' landscapes that are long-enduring, some for millions of years.
Materials are deposited on top or are blown or washed from the surface. With additions, removals and alterations, soils are always subject to new conditions. Whether these are slow or rapid changes depends on climate, topography and biological activity.
Time as a soil-forming factor may be investigated by studying soil
chronosequence
A chronosequence describes a set of ecological sites that share similar attributes but represent different ages.
A common assumption in establishing chronosequences is that no other variable besides age (such as various abiotic components and bio ...
s, in which soils of different ages but with minor differences in other soil-forming factors can be compared.
Paleosol
In the geosciences, paleosol (''palaeosol'' in Great Britain and Australia) is an ancient soil that formed in the past. The precise definition of the term in geology and paleontology is slightly different from its use in soil science.
In geolo ...
s are soils formed during previous soil forming conditions.
History of research
Dokuchaev's equation
Russian geologist
Vasily Dokuchaev
Vasily Vasilyevich Dokuchaev (russian: Васи́лий Васи́льевич Докуча́ев; 1 March 1846 – 8 November 1903) was a Russian geologist and geographer who is credited with laying the foundations of soil science. The ...
, commonly regarded as the father of pedology, determined in 1883 that soil formation occurs over time under the influence of
climate
Climate is the long-term weather pattern in an area, typically averaged over 30 years. More rigorously, it is the mean and variability of meteorological variables over a time spanning from months to millions of years. Some of the meteorologi ...
,
vegetation
Vegetation is an assemblage of plant species and the ground cover they provide. It is a general term, without specific reference to particular taxa, life forms, structure, spatial extent, or any other specific botanical or geographic characte ...
,
topography
Topography is the study of the forms and features of land surfaces. The topography of an area may refer to the land forms and features themselves, or a description or depiction in maps.
Topography is a field of geoscience and planetary sc ...
, and parent material. He demonstrated this in 1898 using the soil forming equation:
:
(where
cl or
c climate,
o biological processes,
p parent material)
tr relative time (young, mature, old)
Hans Jenny's state equation
American soil scientist
Hans Jenny published in 1941 a state equation for the factors influencing soil formation:
:
*
S soil formation
*
cl (sometimes
c)
climate
Climate is the long-term weather pattern in an area, typically averaged over 30 years. More rigorously, it is the mean and variability of meteorological variables over a time spanning from months to millions of years. Some of the meteorologi ...
*
o organisms (
soil microbiology
Soil microbiology is the study of microorganisms in soil, their functions, and how they affect soil properties. It is believed that between two and four billion years ago, the first ancient bacteria and microorganisms came about on Earth's oceans. ...
,
soil mesofauna
Soil mesofauna are invertebrates between 0.1mm and 2mm in size, which live in the soil or in a leaf litter layer on the soil surface. Members of this group include nematodes, mites, springtails (collembola), proturans, pauropods, rotifers, ear ...
,
soil biology
Soil biology is the study of microbial and faunal activity and ecology in soil.
Soil life, soil biota, soil fauna, or edaphon is a collective term that encompasses all organisms that spend a significant portion of their life cycle within a soil ...
)
*
r relief
Relief is a sculptural method in which the sculpted pieces are bonded to a solid background of the same material. The term '' relief'' is from the Latin verb ''relevo'', to raise. To create a sculpture in relief is to give the impression that th ...
*
p parent material
Parent material is the underlying geological material (generally bedrock or a superficial or drift deposit) in which soil horizons form. Soils typically inherit a great deal of structure and minerals from their parent material, and, as such, are ...
*
t time
Time is the continued sequence of existence and events that occurs in an apparently irreversible succession from the past, through the present, into the future. It is a component quantity of various measurements used to sequence events, ...
This is often remembered with the
mnemonic
A mnemonic ( ) device, or memory device, is any learning technique that aids information retention or retrieval (remembering) in the human memory for better understanding.
Mnemonics make use of elaborative encoding, retrieval cues, and imag ...
Clorpt.
Jenny's state equation in Factors of Soil Formation differs from the Vasily Dokuchaev equation, treating time (t) as a factor, adding topographic relief (r), and pointedly leaving the ellipsis "open" for more factors (
state variables
A state variable is one of the set of variables that are used to describe the mathematical "state" of a dynamical system. Intuitively, the state of a system describes enough about the system to determine its future behaviour in the absence of a ...
) to be added as our understanding becomes more refined.
There are two principal methods that the state equation may be solved: first in a theoretical or conceptual manner by logical deductions from certain premises, and second empirically by experimentation or field observation.
The empirical method is still mostly employed today, and soil formation can be defined by varying a single factor and keeping the other factors constant. This led to the development of empirical models to describe pedogenesis, such as climofunctions, biofunctions, topofunctions, lithofunctions, and chronofunctions. Since Hans Jenny published his formulation in 1941, it has been used by innumerable
soil survey
Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former term ...
ors all over the world as a qualitative list for understanding the factors that may be important for producing the soil pattern within a region.
Soil forming processes
Soils develop from
parent material
Parent material is the underlying geological material (generally bedrock or a superficial or drift deposit) in which soil horizons form. Soils typically inherit a great deal of structure and minerals from their parent material, and, as such, are ...
by various
weathering
Weathering is the deterioration of rocks, soils and minerals as well as wood and artificial materials through contact with water, atmospheric gases, and biological organisms. Weathering occurs ''in situ'' (on site, with little or no movement) ...
processes.
Organic matter accumulation,
decomposition
Decomposition or rot is the process by which dead organic substances are broken down into simpler organic or inorganic matter such as carbon dioxide, water, simple sugars and mineral salts. The process is a part of the nutrient cycle and is e ...
, and
humification
In classical soil science, humus is the dark organic matter in soil that is formed by the decomposition of plant and animal matter. It is a kind of soil organic matter. It is rich in nutrients and retains moisture in the soil. Humus is the Lati ...
are as critically important to soil formation as weathering. The zone of humification and weathering where pedogenic processes are dominant and where biota play an important role is termed the
solum
The solum (plural, sola) in soil science consists of the surface and subsoil layers that have undergone the same soil forming conditions. The base of the solum is the relatively unweathered parent material.
Solum and soils are not synonymous. Som ...
.
Soil acidification
Soil acidification is the buildup of hydrogen cations, which reduces the soil pH. Chemically, this happens when a proton donor gets added to the soil. The donor can be an acid, such as nitric acid, sulfuric acid, or carbonic acid. It can also be a ...
resulting from
soil respiration
Soil respiration refers to the production of carbon dioxide when soil organisms respire. This includes respiration of plant roots, the rhizosphere, microbes and fauna.
Soil respiration is a key ecosystem process that releases carbon from the ...
supports
chemical weathering
Weathering is the deterioration of rocks, soils and minerals as well as wood and artificial materials through contact with water, atmospheric gases, and biological organisms. Weathering occurs ''in situ'' (on site, with little or no movement ...
. Plants contribute to chemical weathering through root exudates.
Soils can be enriched by deposition of
sediment
Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sa ...
s on
floodplain
A floodplain or flood plain or bottomlands is an area of land adjacent to a river which stretches from the banks of its channel to the base of the enclosing valley walls, and which experiences flooding during periods of high discharge.Goudi ...
s and
alluvial fan
An alluvial fan is an accumulation of sediments that fans outwards from a concentrated source of sediments, such as a narrow canyon emerging from an escarpment. They are characteristic of mountainous terrain in arid to semiarid climates, but a ...
s, and by
wind-borne deposits.
Soil mixing (
pedoturbation
Perturbation (from Latin: ''perturbare'' "to confuse, disorder, disturb", from ''per''- "through" + ''turbare'' "disturb, confuse," from ''turba'' "turmoil, crowd") is a set of pedology (soil study) and sedimentary geology processes relating to c ...
) is often an important factor in soil formation. Pedoturbation includes
churning clays,
cryoturbation
In gelisols ( permafrost soils), cryoturbation (frost churning) refers to the mixing of materials from various horizons of the soil down to the bedrock due to freezing and thawing.
Cryoturbation occurs to varying degrees in most gelisols. The c ...
, and
bioturbation. Types of bioturbation include faunal pedoturbation (animal
burrowing), plant pedoturbation (root growth,
tree uprooting
Uprooting is a form of treefall in which the root plate of a tree is torn from the soil, disrupting and mixing it and leaving a pit-mound.
Transplanting
Small trees can be replanted if their root system is well attached to the trunk. Trees can ...
), and fungal pedoturbation (
mycelial
Mycelium (plural mycelia) is a root-like structure of a fungus consisting of a mass of branching, thread-like hyphae. Fungal colonies composed of mycelium are found in and on soil and many other substrates. A typical single spore germinates ...
growth). Pedoturbation transforms soils through destratification, mixing, and
sorting, as well as creating preferential flow paths for
soil gas Soil gases (soil atmosphere) are the gases found in the air space between soil components. The spaces between the solid soil particles, if they do not contain water, are filled with air. The primary soil gases are nitrogen, carbon dioxide and oxygen ...
and
infiltrating water. The zone of active bioturbation is termed the
soil biomantle The soil biomantle can be described and defined in several ways. Most simply, the soil biomantle is the organic-rich bioturbated upper part of the soil, including the topsoil where most biota live, reproduce, die, and become assimilated. The biomant ...
.
Soil moisture content and water flow through the
soil profile
A soil horizon is a layer parallel to the soil surface whose physical, chemical and biological characteristics differ from the layers above and beneath. Horizons are defined in many cases by obvious physical features, mainly colour and texture. ...
support
leaching
Leaching is the loss or extraction of certain materials from a carrier into a liquid (usually, but not always a solvent). and may refer to:
* Leaching (agriculture), the loss of water-soluble plant nutrients from the soil; or applying a small amou ...
of
solutes
In chemistry, a solution is a special type of homogeneous mixture composed of two or more substances. In such a mixture, a solute is a substance dissolved in another substance, known as a solvent. If the attractive forces between the solvent ...
, and
eluviation
In geology, eluvium or eluvial deposits are those geological deposits and soils that are derived by ''in situ'' weathering or weathering plus gravitational movement or accumulation.
The process of removal of materials from geological or soil horiz ...
. Eluviation is the translocation of
colloid material, such as organic matter, clay and other mineral compounds. Transported constituents are deposited due to differences in soil moisture and soil chemistry, especially
soil pH and
redox potential
Redox potential (also known as oxidation / reduction potential, ''ORP'', ''pe'', ''E_'', or E_) is a measure of the tendency of a chemical species to acquire electrons from or lose electrons to an electrode and thereby be reduced or oxidised respe ...
. The interplay of removal (eluviation) and deposition (
illuviation Illuvium is material displaced across a soil profile, from one layer to another one, by the action of rainwater. The removal of material from a soil layer is called eluviation. The transport of the material may be either mechanical or chemical. The ...
), also called ''pedotranslocation'', results in contrasting soil horizons.
Key soil-forming processes especially important to macro-scale patterns of soil formation are:
*
Laterization
*
Podsolization
*
Calcification
Calcification is the accumulation of calcium salts in a body tissue. It normally occurs in the formation of bone, but calcium can be deposited abnormally in soft tissue,Miller, J. D. Cardiovascular calcification: Orbicular origins. ''Nature Ma ...
*
Salinization
*
Gleization
Examples
A variety of mechanisms contribute to soil formation, including
siltation
Siltation, is water pollution caused by particulate Terrestrial ecoregion, terrestrial Clastic rock, clastic material, with a particle size dominated by silt or clay. It refers both to the increased concentration of suspended sediments and to the ...
,
erosion
Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is dis ...
,
overpressure and
lake
A lake is an area filled with water, localized in a basin, surrounded by land, and distinct from any river or other outlet that serves to feed or drain the lake. Lakes lie on land and are not part of the ocean, although, like the much large ...
bed succession. A specific example of the evolution of soils in prehistoric
lake beds is in the
Makgadikgadi Pan
The Makgadikgadi Pan ( Tswana pronunciation ), a salt pan situated in the middle of the dry savanna of north-eastern Botswana, is one of the largest salt flats in the world. The pan is all that remains of the formerly enormous Lake Makgadi ...
s of the
Kalahari Desert
The Kalahari Desert is a large semi-arid sandy savanna in Southern Africa extending for , covering much of Botswana, and parts of Namibia and South Africa.
It is not to be confused with the Angolan, Namibian, and South African Namib coastal d ...
, where change in an ancient river course led to millennia of salinity buildup and formation of
calcretes and
silcrete
Silcrete is an indurated (resists crumbling or powdering) soil duricrust formed when surface soil, sand, and gravel are cemented by dissolved silica. The formation of silcrete is similar to that of calcrete, formed by calcium carbonate, and ferr ...
s.
Notes
References
* Stanley W. Buol, F.D. Hole and R.W. McCracken. 1997. Soil Genesis and Classification, 4th ed. Iowa State Univ. Press, Ames
* C. Michael Hogan. 2008. ''Makgadikgadi'', The Megalithic Portal, ed. A. Burnha
* Francis D. Hole and J.B. Campbell. 1985. Soil landscape analysis. Totowa Rowman & Allanheld, 214 p.
* Hans Jenny. 1994
Factors of Soil Formation.A System of Quantitative Pedology. New York: Dover Press. (Reprint, with foreword by R. Amundson, of the 1941 McGraw-Hill publication). pdf file format.
* Ben van der Pluijm et al. 2005
from the Global Change 1 Lectures. University of Michigan. Url last accessed on 2007-03-31
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Pedology
Ecological succession