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Soil formation, also known as pedogenesis, is the process of soil genesis as regulated by the effects of place, environment, and history. Biogeochemistry processes act to both create and destroy order (anisotropy) within soils. These alterations lead to the development of layers, termed soil horizons, distinguished by differences in soil color, soil structure, soil texture, and chemistry. These soil morphology occur in patterns of soil type distribution, forming in response to differences in soil forming factors.
Pedogenesis is studied as a branch of pedology, the study of soil in its natural environment. Other branches of pedology are the study of soil morphology and soil classification. The study of pedogenesis is important to understanding soil distribution patterns in current (soil survey) and past (paleopedology) geologic periods.
New soils increase in depth by a combination of weathering and further deposition. In Sicily under Mediterranean climate 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 and proceeding along ecological succession to more complex plant and animal communities. deepen with the accumulation of humus originating from dead remains of vascular plants and soil microbes. They also deepen through mixing of organic matter with weathered minerals. As soils mature, they develop as organic matter accumulates and mineral weathering and leaching take place.
Typical soil parent mineral materials are:
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 rocks and have the same general chemistry as those rocks. The soils found on , , and 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:
Cumulose parent material is not moved but originates from deposited organic material. This includes peat 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.
Of the above, hydrolysis and carbonation are the most effective, in particular in regions of high rainfall, temperature and physical erosion. Chemical weathering 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 formation.
Saprolite is a particular example of a residual soil formed from the transformation of granite, Metamorphic rock and other types of bedrock into Clay mineral. Often called weathered granite, saprolite is the result of weathering processes that include: hydrolysis, chelation from organic compounds, hydration and physical processes that include freezing and thawing. 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, thanks to the much higher resistance of quartz compared to other mineral components of granite (e.g., mica, amphibole, feldspar).
Climate is the dominant factor in soil formation, and soils show the distinctive characteristics of the 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, leaching, and plant growth 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 regions, while shrubs and brush of various kinds dominate in Desert climate areas.
Water is essential for all the major chemical weathering reactions. To be effective in soil formation, water must penetrate the regolith. The seasonal rainfall distribution, evaporative losses, site topography, 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) to the lower layers (illuviation), including clay particles and dissolved organic matter. It may also carry away soluble materials in the surface drainage 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 horizons). In tropical soils, when the soil has been deprived of vegetation (e.g. by deforestation) and thereby is submitted to intense evaporation, the upward Capillary action movement of water, which has dissolved iron and aluminum salts, is responsible for the formation of a superficial hard pan of laterite or bauxite, respectively, which is improper for cultivation, a known case of irreversible soil degradation.
The direct influences of climate include:
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 to (1987). 9780632019052 ISBN 9780632019052 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 in rock minerals, and thus their mechanical disaggregation, a process called thermal fatigue. By the same process freeze-thaw cycles are an effective mechanism which breaks up rocks and other consolidated materials.
Steep slopes encourage rapid soil loss by erosion and allow less rainfall to enter the soil before running off and hence, little mineral deposition in lower profiles (illuviation). 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 sun path will be drier than soils on slopes that do not.
In swales 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 soils may develop. Depressions allow the accumulation of water, minerals and organic matter, and in the extreme, the resulting soils will be or .
Recurring patterns of topography result in toposequences or soil catenas. These patterns emerge from topographic differences in erosion, deposition, soil fertility, soil moisture, plant cover, soil biology, fire history, and exposure to the elements. Gravity transports water downslope, together with mineral and organic solutes and , increasing Particulates and base content at the foot of hills and mountains. However, many other factors like drainage and erosion interact with slope position, blurring its expected influence on crop yield.
Soil is the most speciose (species-rich) ecosystem on Earth, but the vast majority of organisms in soil are microbes, a great many of which have not been described. There may be a microbial 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 number of organisms and species can vary widely according to soil type, location, and depth.
Plants, animals, fungi, bacteria and humans affect soil formation (see Soil Biomantle and stonelayer). Soil animals, including soil macrofauna (e.g. Earthworm, Termite, Darkling beetle, Gopher, moles) and soil mesofauna (e.g. Enchytraeidae, Springtail, Mite), mix soils as they form and Porosity, allowing moisture and gases to move about, a process called bioturbation. In the same way, plant roots penetrate soil horizons and open channels upon decomposition. Plants with deep can penetrate many metres through the different soil layers to bring up nutrients from deeper in the profile. Plants have fine roots that excrete organic compounds (Sugar, Organic acid, mucilage), slough off cells (in particular at their tip), and are easily decomposed, adding organic matter to soil, a process called rhizodeposition.
Microorganism, 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 growth of roots through the soil stimulates microbial populations, stimulating in turn the activity of their predators (notably amoeba), thereby increasing the mineralization rate, and in last turn root growth, a positive feedback called the soil microbial loop. Out of root influence, in the bulk soil most bacteria are in a quiescent stage, forming micro-aggregates, i.e. mucilage colonies to which clay particles are glued, offering them a protection against desiccation and predation by soil microfauna (bacteriophagous protozoa and nematodes). Microaggregates (20–250 μm) are ingested by soil fauna, and bacterial bodies are partly or totally digested in their guts.
Humans impact soil formation by removing vegetation cover through tillage, application of Herbicide, fire and leaving soils bare. This can lead to erosion, waterlogging, lateritization or Podsolisation (according to climate and topography). Tillage 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.
Earthworm, Ant, Termite, moles, , as well as some Millipede and Darkling beetle 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. 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 production, both within the intestine and as a lining in their galleries, exert a Organic matter 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, 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, the same for the deposition of casts at the soil surface by earthworms. Large animals such as gophers, moles, and Prairie dog 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 or krotovinas.
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, 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 Mycorrhiza fungi and rhizosphere bacteria, and improve the soil structure. The type and amount of vegetation depend 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 humans, and by association, fire, are state factors placed within the organisms state factor. Humans can import or extract nutrients and energy in ways that dramatically change soil formation. Accelerated soil erosion from overgrazing, and Pre-Columbian terraforming in the Amazon basin resulting in terra preta are two examples of the effects of human management.
It is believed that Native Americans regularly set fires to maintain several large areas of prairie grasslands in Indiana and Michigan, 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 of the soil for crop production has abruptly modified soil formation. Likewise, irrigating soil in an arid 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 in the genus Euchondrus in the Negev desert are noted for eating growing under the surface limestone rocks and slabs (endolithic lichens). The grazing activity of these ecosystem engineers disrupts the limestone, resulting in the weathering and the subsequent formation of soil. (2025). 9780691044378, Princeton University Press. ISBN 9780691044378 They have a significant effect on the region: the 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 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 capable of producing these distinct deep rich black soils are not easily observed. It is now accepted that Neolithic human-caused Wildfire enriched soils in charcoal (also called black carbon) and played a prominent role in the formation of the fertile and terra preta, actively used for the sake of agriculture.
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 , in which soils of different ages but with minor differences in other soil-forming factors can be compared. are soils formed during previous soil forming conditions.
This is often remembered with the mnemonic 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) to be added as our understanding becomes more refined.
There are two principal methods by which 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 had led to the development of empirical models to describe pedogenesis, such as climofunctions, biofunctions, topofunctions, lithofunctions, and chronofunctions. Since Jenny published his formulation in 1941, it has been used by innumerable all over the world as a qualitative list for understanding the factors that may be important for producing the soil pattern within a region.
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