Factors influencing precipitation (Integrated)

Precipitation
Precipitation is any form of the product of the condensation of water vapour that falls due to gravity. Some forms include hail, snow, sleet, rain, drizzle, and graupel. 

For Precipitation to occur, air must achieve saturation, either when it is cooled to its dew point, which is the temperature to which a parcel must be cooled in order to achieve saturation and condense to form clouds, or when sufficient moisture is added to the air to achieve saturation. Condensation nuclei, in the form of dust, ice, or salt particles is usually necessary to facilitate condensation

Monsoon Systems (Integrated)

Monsoon Systems
Monsoons are seasonal changes in atmospheric circulation and precipitation associated with the asymmetric heating of the earth's land and sea It consists of a monsoonal dry phase and a monsoonal wet phase. 

The major monsoon systems of the world consist of the West African and the Asia-Australian monsoons.

Weathering (integrated)

Weathering is the in-situ (on site) breakdown of rocks, soils, and minerals, as well as wood an artificial materials, by natural agents.

Soil Classification and Pedogenesis (integrated)

A soil horizon is a layer generally parallel to the soil crust, whose physical characteristics differ from the layers above and beneath. Each soil type usually has three or four horizons. Horizons are defined in most cases by obvious physical features, chiefly colour and texture. These may be described both in absolute terms (particle size distribution for texture, for instance) and in terms relative to the surrounding material (i.e., "coarser" or "sandier" than the horizons above and below). The differentiation of the soil into distinct horizons is largely the result of influences, such as air, water, solar radiation and plant material, originating at the soil-atmosphere interface. Since the weathering of the soil occurs first at the surface and works its way down, the uppermost layers have been changed the most, while the deepest layers are most similar to the original parent material.

Identification and description of the horizons present at a given site is the first step in soil classification at higher levels, through the use of systems such as the USDA soil taxonomy or the Australian Soil Classification. The World Reference Base for Soil Resources lists 40 diagnostic horizons.^[1] Soil scientists often dig a large hole, called a soil pit (sometimes several meters deep and about a meter wide) to expose soil horizons for study.

The vertical section exposing a set of horizons, from the ground surface to the parent rock, is termed a soil profile. Most soils, especially in temperate climates, conform to a similar general pattern of horizons, often represented as "ideal" soil in diagrams. Each main horizon is denoted by a capital letter, which may then be followed by several alphanumerical modifiers highlighting particular outstanding features of the horizon. While the general O-A-B-C-R sequence seems fairly universal, some variation exists between the classification systems in different parts of the world. In addition, the exact definition of each main horizon may differ slightly – for instance, the US system uses the thickness of a horizon as a distinguishing feature, while the Australian system does not. It should be emphasised that no one system is more correct – as artificial constructs, their utility lies in their ability to accurately describe local conditions in a consistent manner. Also, many subtropical and tropical areas have soils such as oxisols or aridisols that have very different horizons from "ideal" soil, or no horizons at all.

O horizon[edit]

Main article: Organic matter

This layer generally forms above the mineral soil or occurs in an organic soil profile. The "O" stands for organic matter. It is a surface layer dominated by the presence of large amounts of organic material derived from dead plant and/or animal residues which is in varying stages of decomposition. The O horizon is generally absent in grassland regions. The O horizon usually occurs in forested areas and is commonly referred to as the forest floor. The O horizon should be considered distinct from the layer of leaf litter covering many heavily vegetated areas, which contains no weathered mineral particles and is not part of the soil itself. O horizons may be divided into O1 and O2 categories, whereby O1 horizons contain decomposed matter whose origin can be spotted on sight (for instance, fragments of rotting leaves), and O2 horizons contain only well-decomposed organic matter, the origin of which is not readily visible. O horizons may also be divided into three subordinate O horizons denoted as: Oi, Oe, and Oa.

P horizon[edit]

These horizons are also heavily organic, but are distinct from O horizons in that they form under waterlogged conditions. The "P" designation comes from their common name, peats. They may be divided into P1 and P2 in the same way as O Horizons. This layer accumulates iron, clay, aluminium and organic compounds, a process referred to as illuviation.^[4]

A horizon[edit]

Main article: Topsoil

The A horizon is the topmost mineral horizon, often referred to as the 'topsoil'. This layer generally contains enough partially decomposed (humified) organic matter to give the soil a color darker than the lower horizons. The A horizons are often coarser in texture, having lost some of the finer materials by translocation to lower horizons and by erosion. This layer is known as the zone in which the most biological activity occurs. Soil organisms such as earthworms, potworms (enchytraeids), arthropods, nematodes, fungi, and many species of bacteria and archaea are concentrated here, often in close association with plant roots. Thus the A horizon may be referred to as the biomantle.^[5][6] However, since biological activity extends far deeper into the soil, it cannot be used as a chief distinguishing feature of an A horizon.

E horizon[edit]

Main article: Eluvium

"E", being short for eluviated, is most commonly used to label a horizon that has been significantly leached of clay, iron, and aluminium oxides, which leaves a concentration of resistant minerals, such as quartz, in the sand and silt sizes. These are present only in older, well-developed soils, and generally occur between the A and B horizons.The E horizon often has a pale color that is generally lighter in color than either the horizon above or below it. E horizons are commonly found in soils developed under forests, but are rare in soils developed under grasslands. In regions where this designation is not employed, leached layers are classified firstly as an A or B according to other characteristics, and then appended with the designation "e" (see the section below on horizon suffixes). In soils that contain gravels, due to animal bioturbation, a stonelayer commonly forms near or at the base of the E horizon.

The above layers may be referred to collectively as the "solum". The layers below have no collective name but are distinct in that they are noticeably less affected by surface soil-forming processes.

B horizon[edit]

Main article: Subsoil

B horizons form below an O, A, or E horizon and they have undergone sufficient changes during soil genesis, such that the properties of their original parent material are no longer discernible. The B horizon is commonly referred to as the "subsoil". In humid regions, B horizons are the layers of maximum accumulation of materials such as silicate clays, iron (Fe) and aluminium (Al) oxides, and organic material. These materials typically accumulate through a process termed illuviation, wherein the materials gradually wash in from the overlying horizons. Accordingly, this layer is also referred to as the "illuviated" horizon or the "zone of accumulation". In addition, it is defined as having a distinctly different structure or consistency than the horizon(s) above and the horizon(s) below. The B horizon may also have stronger colors (higher chroma) than the A horizon. In arid and semiarid regions, calcium carbonate or calcium sulfate may accumulate in the B horizon.

As with the A horizon, the B horizon may be divided into B1, B2, and B3 types under the Australian system. B1 is a transitional horizon of the opposite nature to an A3 – dominated by the properties of the B horizons below it, but containing some A-horizon characteristics. B2 horizons have a concentration of clay, minerals, or organics and feature the highest soil development within the profile. B3 horizons are transitional between the overlying B layers and the material beneath it, whether C or D horizon.

The A3, B1, and B3 horizons are not tightly defined, and their use is generally at the discretion of the individual worker.

Plant roots penetrate through this layer, but it has little humus. It is usually brownish or reddish due to residual clay and iron oxides.

C horizon[edit]

Main article: Parent rock

The C horizon (parent material) is below the B Horizon. This layer is barely affected by soil-forming processes and they thus have a lack of pedological development. In other words, the C horizon is the unconsolidated material underlying the solum (A and B horizons). It may or may not be the same as the parent material from which the solum formed. The C horizon forms as the R horizon weathers and rocks break up into smaller particles. The C horizon is below the zones of greatest biological activity and it has not been sufficiently altered by soil genesis to qualify as a B horizon. In dry regions, carbonates and gypsum may be concentrated in the C horizon. While loose enough to be dug with a shovel, C horizon material often retains some of the structural features of the parent rock or geologic deposits from which it formed. The A and B layers usually originated from the C horizon. The upper layers of the C horizon may in time become a part of the solum as weathering and erosion continue. The C Horizon may contain lumps or more likely large shelves of unweathered rock, rather than being made up solely of small fragments as in the solum. It contains rocks with cracks and crevices.

D horizon[edit]

D horizons are not universally distinguished, but in the Australian system are "any soil material below the solum that is unlike the solum in general character, is not C horizon, and cannot be given reliable designation… [it] may be recognized by the contrast in pedologic organization between it and the overlying horizons" (MacDonald et al., 1990, p. 106).

R horizon[edit]

Main article: Bedrock

R horizons are the layers of partially weathered bedrock at the base of the soil profile. Unlike the above layers, R horizons are composed largely of consolidated masses of hard rock that cannot be excavated by hand. Soils formed in situ will exhibit strong similarities to this bedrock layer.

L horizon[edit]

L (limnic) horizons or layers indicate mineral or organic material that has been deposited in water by precipitation or through the actions of aquatic organisms. Included are coprogenous earth (sedimentary peat), diatomaceous earth, and marl; and is usually found as a remnant of past bodies of standing water.

Pedogenesis (from the Greek pedo-, or pedon, meaning 'soil, earth,' and genesis, meaning 'origin, birth') (also termed soil development, soil evolution, soil formation, and soil genesis) is the process of soil formation as regulated by the effects of place, environment, and history. Biogeochemical 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 color, structure, texture, and chemistry. These features occur in patterns of soil type distribution, forming in response to differences in soil forming factors.^[1]

⚐ Overview

Soil develops through a series of changes. The starting point is weathering of freshly accumulated parent material. Primitive microbes feed on simple compounds (nutrients) released by weathering, and produce acids which contribute to weathering. They also leave behind organic residues. New soils increase in depth by a combination of weathering, and further deposition. Gradually soil is able to support higher forms of plants and animals, starting with pioneer species, and proceeding to more complex plant and animal communities.

Soils deepen with accumulation of organic matter primarily due to the activities of higher plants. Topsoil deepen through soil mixing. Soils develop layers as organic matter accumulates and leaching takes place. This development of layers is the beginning of the soil profile.

A rate of pedogenesis of 1/10 mm per year is in order of magnitude of the global soil production estimates

⚐ Factors of soil formation

Russian geologist Vasily Dokuchaev determined that soil formation occurs over time under the influence of climate, vegetation, topography, and parent material. He demonstrated this in 1898^[5] using the soil forming equation:

soil = f(cl, o, p) tr

(where cl = climate, o = organisms, p = biological processes) tr = relative time (young, mature, old)

❧ Clorpt

Main article: Clorpt

In 1941, the American soil scientist Hans Jenny published Factors of Soil Formation. His equation of state 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:

soil = f(Cl, o, r, p, t, ...)

❧ Climate

Climate regulates soil formation. Soils are more developed in areas with higher rainfall and more warmth. The rate of chemical weathering increases by 2-3 times when the temperature increases by 10 degrees Celsius. Climate also affects which organisms are present, affecting the soil chemically and physically (movement of roots).

❧ Organisms

The organisms living in and on the soil form distinct soil types. Coniferous forests have acidic leaf litter that form soils classed as inceptisols. Mixed or deciduous forests leave a larger layer of humus, changing the elements that are either leeched or accumulated in the soil, and thereby forming soils classed as alfisols. Prairies have very high humus accumulation, creating a dark, thick A horizon characteristic of mollisols.

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 rocks and slabs (endolithic lichens).^[6] They disrupt and eat the limestone.^[6] Their grazing results in the weathering of the stones, and the subsequent formation of soil.^[6] 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.^[6]

❧ Parent material

The rock from which soil is formed is called parent material. The main types are: aeolian sediments, glacial till, glacial outwash, alluvium, lacustrine sediments and residual parent material (coral or bedrock).

Pedologists see soil formation or soil properties as a function of regional climate, biota, topography, parent material, time and other variables.^[7]

Soils develop from parent material by various weathering processes. Organic matter accumulation, decomposition, and humification are as critically important to soil formation as weathering. The zone of humification and weathering is termed the solum.

Soil acidification resulting from soil respiration supports chemical weathering. Plants contribute to chemical weathering through root exudates.

Soils can be enriched by deposition of sediments on floodplains and alluvial fans, and by wind-borne deposits.

Soil mixing (pedoturbation) is often an important factor in soil formation. Pedoturbation includes churning clays, cryoturbation, and bioturbation. Types of bioturbation include faunal pedoturbation (animal burrowing), floral pedoturbation (root growth, tree-uprootings), and fungal pedoturbation (mycelia growth). Pedoturbation transforms soils through destratification, mixing, and sorting, as well as creating preferential flow paths for soil gas and infiltrating water. The zone of active bioturbation is termed the soil biomantle.

Soil moisture content and water flow through the soil profile support leaching of soluble constituents, and eluviation. 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. The interplay of removal and deposition results in contrasting soil horizons.

⚐ Key Soil Formation Processes

Key soil forming processes especially important to macro-scale patterns of soil formation are:^[8]

• Laterization
• Podsolization
• Calcification
• Salinization
• Gleization

Note: These are soil forming processes, which are the mechanisms; soil type is oxisol, mollisol, aridisol, podzol etc. They occur due to leaching.

❧ Background Knowledge: Leaching

In pedology, leaching is the loss of mineral and organic solutes due to very heavy rainfall, high temperature and percolation. It is a mechanism of soil formation distinct from the soil forming process of eluviation, which is the loss of mineral and organic colloids. Leached and elluviated materials tend to be lost from topsoil and deposited in subsoil. A soil horizon accumulating leached and eluviated materials is referred to as a zone of illuviation.

Laterite soil, which develops in regions with high temperature and heavy rainfall, is an example of this process in action.

Mechanical illuviation[edit]

When percolating rain water reaches a drier soil horizon, water from the suspension is removed by capillary action of microchannels, leaving fine deposits (cutans) oriented along percolation macrochannels.

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 process of deposition of illuvium is termed illuviation.^[1] It is a water-assisted transport in a basically vertical direction, as compared to alluviation, the horizontal running water transfer. The resulting deposits are called illuvial deposits.

Illuvial deposits of clays, oxides, and organics accumulate in subsoil as distinctive soil horizons classified as "B horizons" or "zones of illuviation".

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 horizons is called eluviation or leaching. There is a difference in the usage of this term in geology and soil science. In soil science, eluviation is the transport of soil material from upper layers of soil to lower levels by downward precipitation of water across soil horizons, and accumulation of this material (illuvial deposit) in lower levels is called illuviation.^[1][2] In geology, the removed material is irrelevant, and the deposit (eluvial deposit) is the remaining material. Eluviation occurs when precipitation exceeds evaporation.

A soil horizon formed due to eluviation is an eluvial zone or eluvial horizon. In a typical soil profile, the eluvial horizon refers to a light-colored zone located (depending on context and literature) either at the lower part of the A horizon (symbol: Ae) or within a distinct horizon (E horizon) below the A, where the process is most intense and rapid. Yet some sources consider the eluvial zone to be the A horizon plus the (distinct) E horizon, as eluviation technically occurs in both.

In sedimentology, winnowing is the natural removal of fine material from a coarser sediment by wind or flowing water. Once a sediment has been deposited, subsequent changes in the speed or direction of wind or water flowing over it can agitate the grains in the sediment and allow the preferential removal of the finer grains. This action can improve the sorting and increase the mean grain size of a sediment after it has been deposited.^[1]

❧ Laterization*

Laterization is a prolonged process of chemical weathering which produces a wide variety in the thickness, grade, chemistry and ore mineralogy of the resulting soils. The majority of the land area containing laterites is between the tropics of Cancer and Capricorn.

Laterite is a soil and rock type rich in iron and aluminium, and is commonly considered to have formed in hot and wet tropical areas. Nearly all laterites are of rusty-red coloration, because of high iron oxide content. They develop by intensive and long-lasting weathering of the underlying parent rock.

Laterites are formed from the leaching of parent sedimentary rocks (sandstones, clays, limestones); metamorphic rocks (schists, gneisses, migmatites); igneous rocks (granites, basalts, gabbros, peridotites); and mineralised proto-ores;^[3]:5 which leaves the more insoluble ions, predominantly iron and aluminium. The mechanism of leaching involves acid dissolving the host mineral lattice, followed by hydrolysis and precipitation of insoluble oxides and sulfates of iron, aluminium and silica under the high temperature conditions^[8] of a humid sub-tropical monsoon climate.^[9]

An essential feature for the formation of laterite is the repetition of wet and dry seasons.^[10] Rocks are leached by percolating rain water during the wet season; the resulting solution containing the leached ions is brought to the surface by capillary action during the dry season.^[10] These ions form soluble salt compounds which dry on the surface; these salts are washed away during the next wet season.^[10]Laterite formation is favoured in low topographical reliefs of gentle crests and plateaus which prevents erosion of the surface cover.^[5]:4The reaction zone where rocks are in contact with water—from the lowest to highest water table levels—is progressively depleted of the easily leached ions of sodium, potassium, calcium and magnesium.^[10] A solution of these ions can have the correct pH to preferentially dissolve silicon oxide rather than the aluminium oxides and iron oxides.^[10]

The mineralogical and chemical compositions of laterites are dependent on their parent rocks.^[3]:6 Laterites consist mainly of quartz, zircon, and oxides of titanium, iron, tin, aluminium and manganese, which remain during the course of weathering.^[3]:7 Quartz is the most abundant relic mineral from the parent rock.^[3]:7

Laterites vary significantly according to their location, climate and depth.^[8] The main host minerals for nickel and cobalt can be either iron oxides, clay minerals or manganese oxides.^[8] Iron oxides are derived from mafic igneous rocks and other iron-rich rocks; bauxites are derived from granitic igneous rock and other iron-poor rocks.^[10] Nickel laterites occur in zones of the earth which experienced prolonged tropical weathering of ultramafic rocks containing the ferro-magnesian minerals olivine, pyroxene, and amphibole.^[5]

^{Oxisols are an order in USDA soil taxonomy, best known for their occurrence in tropical rain forest, 15-25 degrees north and south of the Equator.}

^{The main processes of soil formation of oxisols are weathering, humification and pedoturbation due to animals. These processes produce the characteristic soil profile. They are defined as soils containing at all depths no more than 10 percent weatherable minerals, and low cation exchange capacity. Oxisols are always a red or yellowish color, due to the high concentration of iron(III) and aluminium oxides and hydroxides. In addition they also contain quartz and kaolin, plus small amounts of other clay minerals and organic matter.}

<sup>Scientists originally thought that the heavy vegetation of tropical rain forests would provide rich nutrients, but as rainfall passes through the litter on the forest floor the rain is acidified and leaches minerals from the above soil layers. This forces plants to get their nutrition from decaying litter as oxisols are quite infertile due to the lack of organic matter and the almost complete absence of soluble minerals leached by the wet and humid climate.

Laterization is a pedogenic process common to soils found in tropical and subtropical environments.  Movements of large amounts of water through the soil cause eluviation and leaching to occur. Almost all of the by products of weathering, very simple small compounds or nutrient ions, are translocated out of the soil profile by leaching if not taken up by plants for nutrition. The two exceptions to this process are iron and aluminum compounds. Iron oxides give tropical soils their unique reddish coloring. Heavy leaching also causes these soils to have an acidic pH because of the net loss of base cations.</sup>

❧ Calcification*

Calcification occurs when evapotranspiration exceeds precipitation causing the upward movement of dissolved alkaline salts from the groundwater. At the same time, the movement of rain water causes a downward movement of the salts. The net result is the deposition of the translocated cations in the B horizon. In some cases, these deposits can form a hard layer called caliche. The most common substance involved in this process is calcium carbonate. Calcification is common in the prairie grasslands.

❧ Salinization*

Salinization is a process that functions in the similar way to calcification. It differs from calcification in that the salt deposits occur at or very near the soil surface. Salinization also takes place in much drier climates.

❧ Podsolization

Podzolization is associated with humid cold mid-latitude climates and coniferous vegetation. Decomposition of coniferous litter and heavy summer precipitation create a soil solution that is strongly acidic. This acidic soil solution enhances the processes of eluviation and leaching causing the removal of soluble base cations and aluminum and iron compounds from the A horizon. This process creates a sub-layer in the A horizon that is white to gray in color and composed of silica sand.

❧ Gleization

Gleization is a pedogenic process associated with poor drainage. This process involves the accumulations of organic matter in the upper layers of the soil. In lower horizons, mineral layers are stained blue-gray because of the chemical reduction of iron.

⚐ Examples

A variety of mechanisms contribute to soil formation, including siltation, erosion, overpressure and lake bed succession. A specific example of the evolution of soils in prehistoric lake beds is in the Makgadikgadi Pans of the Kalahari Desert, where change in an ancient river course led to millennia of salinity buildup and formation of calcretes and silcretes.^[9]

⚐ Soil Production Function

Soil production function refers to the rate of bedrock weathering into soil as a function of soil thickness.

A general model suggested that the rate of physical weathering of bedrock (de/dt) can be represented as an exponential decline with soil thickness:

${\displaystyle de/dt=P_{0}\exp {[-kh]}\ }$

where h is soil thickness [m], P₀ [mm/year] is the potential (or maximum) weathering rate of bedrock and k [m⁻¹] is an empirical constant.

The reduction of weathering rate with thickening of soil is related to the exponential decrease of temperature amplitude with increasing depth below the soil surface, and also the exponential decrease in average water penetration (for freely-drained soils).