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83-402: Chernozem ( / ˈ tʃ ɜːr n ə z ɛ m / CHUR-nə-zem ), also called black soil , regur soil or black cotton soil , is a black-colored soil containing a high percentage of humus (4% to 16%) and high percentages of phosphorus and ammonia compounds. Chernozem is very fertile soil and can produce high agricultural yields with its high moisture-storage capacity. Chernozems are

166-1063: A soil type in the Canadian system of soil classification and the World Reference Base for Soil Resources (WRB). Chernozemic soil type "equivalents", in the Canadian system, WRB , and U.S. Department of Agriculture soil taxonomy : As seen in the list above, the 19th and 20th-century discussions on the pedogenesis of Chernozem originally stemmed from climatic conditions from the early Holocene to roughly 5500 BC. However, no single paleo-climate reconstruction could accurately explain geochemical variations found in Chernozems throughout central Europe. Evidence of anthropomorphic origins of stable pyrogenic carbon in Chernozem led to improved formation theories. Vegetation burning could explain Chernozem's high magnetic susceptibility ,

249-551: A Reference Soil Group of the World Reference Base for Soil Resources (WRB). The name comes from the Russian terms for black (чёрный čjornyj ) and soil, earth or land (земля zemlja ). The soil, rich in organic matter presenting a black color, was first identified by the Russian geologist Vasily Dokuchaev in 1883 in the tallgrass steppe or prairie of Eastern Ukraine and Western Russia . Chernozem covers about 230 million hectares of land. There are two "chernozem belts" in

332-643: A dry bulk density (density of soil taking into account voids when dry) between 1.1 and 1.6 g/cm , though the soil particle density is much higher, in the range of 2.6 to 2.7 g/cm . Little of the soil of planet Earth is older than the Pleistocene and none is older than the Cenozoic , although fossilized soils are preserved from as far back as the Archean . Collectively the Earth's body of soil

415-557: A goethite pseudomorph if proper conditions are met. It may also be precipitated by groundwater or in other sedimentary conditions, or form as a primary mineral in hydrothermal deposits. Goethite has also been found to be produced by the excretion processes of certain bacteria types. Goethite is found all over the planet, usually in the form of concretions , stalactitic formations, oolites (a form consisting of tiny round grains cemented together), reniform (kidney shapes) or botryoidal (globular, like bunches of grapes) accumulations. It

498-451: A medium for plant growth , making it a critically important provider of ecosystem services . Since soil has a tremendous range of available niches and habitats , it contains a prominent part of the Earth's genetic diversity . A gram of soil can contain billions of organisms, belonging to thousands of species, mostly microbial and largely still unexplored. Soil has a mean prokaryotic density of roughly 10 organisms per gram, whereas

581-419: A net absorption of oxygen and methane and undergo a net release of carbon dioxide and nitrous oxide . Soils offer plants physical support, air, water, temperature moderation, nutrients, and protection from toxins. Soils provide readily available nutrients to plants and animals by converting dead organic matter into various nutrient forms. Components of a silt loam soil by percent volume A typical soil

664-549: A pH of 9, plant growth is reduced. High pH results in low micro-nutrient mobility, but water-soluble chelates of those nutrients can correct the deficit. Sodium can be reduced by the addition of gypsum (calcium sulphate) as calcium adheres to clay more tightly than does sodium causing sodium to be pushed into the soil water solution where it can be washed out by an abundance of water. There are acid-forming cations (e.g. hydronium, aluminium, iron) and there are base-forming cations (e.g. calcium, magnesium, sodium). The fraction of

747-453: A pyrogenic carbon component. Given the symphony of pedogenic processes that contribute to the formation of dark earth, Chernozem summarizes different types of black soils with the same appearance but different formation histories. Soil Soil consists of a solid phase of minerals and organic matter (the soil matrix), as well as a porous phase that holds gases (the soil atmosphere) and water (the soil solution). Accordingly, soil

830-731: A result of a trade-off between toxicity and requirement most nutrients are better available to plants at moderate pH, although most minerals are more soluble in acid soils. Soil organisms are hindered by high acidity, and most agricultural crops do best with mineral soils of pH 6.5 and organic soils of pH 5.5. Given that at low pH toxic metals (e.g. cadmium, zinc, lead) are positively charged as cations and organic pollutants are in non-ionic form, thus both made more available to organisms, it has been suggested that plants, animals and microbes commonly living in acid soils are pre-adapted to every kind of pollution, whether of natural or human origin. In high rainfall areas, soils tend to acidify as

913-401: A result, layers (horizons) form in the soil profile. The alteration and movement of materials within a soil causes the formation of distinctive soil horizons . However, more recent definitions of soil embrace soils without any organic matter, such as those regoliths that formed on Mars and analogous conditions in planet Earth deserts. An example of the development of a soil would begin with

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996-412: A soil can be said to be developed, and can be described further in terms of color, porosity, consistency, reaction ( acidity ), etc. Water is a critical agent in soil development due to its involvement in the dissolution, precipitation, erosion, transport, and deposition of the materials of which a soil is composed. The mixture of water and dissolved or suspended materials that occupy the soil pore space

1079-399: A soil determines its ability to supply available plant nutrients and affects its physical properties and the health of its living population. In addition, a soil's chemistry also determines its corrosivity , stability, and ability to absorb pollutants and to filter water. It is the surface chemistry of mineral and organic colloids that determines soil's chemical properties. A colloid

1162-472: A valence of two, converts to (40 ÷ 2) × 1 milliequivalent = 20 milliequivalents of hydrogen ion per 100 grams of dry soil or 20 meq/100 g. The modern measure of CEC is expressed as centimoles of positive charge per kilogram (cmol/kg) of oven-dry soil. Most of the soil's CEC occurs on clay and humus colloids, and the lack of those in hot, humid, wet climates (such as tropical rainforests ), due to leaching and decomposition, respectively, explains

1245-553: Is 0.04%, but in the soil pore space it may range from 10 to 100 times that level, thus potentially contributing to the inhibition of root respiration. Calcareous soils regulate CO 2 concentration by carbonate buffering , contrary to acid soils in which all CO 2 respired accumulates in the soil pore system. At extreme levels, CO 2 is toxic. This suggests a possible negative feedback control of soil CO 2 concentration through its inhibitory effects on root and microbial respiration (also called soil respiration ). In addition,

1328-603: Is a measure of the acidity or alkalinity of the soil. More precisely, it is a measure of hydronium concentration in an aqueous solution and ranges in values from 0 to 14 (acidic to basic) but practically speaking for soils, pH ranges from 3.5 to 9.5, as pH values beyond those extremes are toxic to life forms. At 25 °C an aqueous solution that has a pH of 3.5 has 10 moles H 3 O (hydronium ions) per litre of solution (and also 10 moles per litre OH ). A pH of 7, defined as neutral, has 10 moles of hydronium ions per litre of solution and also 10 moles of OH per litre; since

1411-498: Is a mineral of the diaspore group, consisting of iron(III) oxide-hydroxide , specifically the α- polymorph . It is found in soil and other low-temperature environments such as sediment. Goethite has been well known since ancient times for its use as a pigment (brown ochre ). Evidence has been found of its use in paint pigment samples taken from the caves of Lascaux in France . It was first described in 1806 based on samples found in

1494-509: Is a small, insoluble particle ranging in size from 1 nanometer to 1 micrometer , thus small enough to remain suspended by Brownian motion in a fluid medium without settling. Most soils contain organic colloidal particles called humus as well as the inorganic colloidal particles of clays . The very high specific surface area of colloids and their net electrical charges give soil its ability to hold and release ions . Negatively charged sites on colloids attract and release cations in what

1577-551: Is a three- state system of solids, liquids, and gases. Soil is a product of several factors: the influence of climate , relief (elevation, orientation, and slope of terrain), organisms, and the soil's parent materials (original minerals) interacting over time. It continually undergoes development by way of numerous physical, chemical and biological processes, which include weathering with associated erosion . Given its complexity and strong internal connectedness , soil ecologists regard soil as an ecosystem . Most soils have

1660-411: Is about 50% solids (45% mineral and 5% organic matter), and 50% voids (or pores) of which half is occupied by water and half by gas. The percent soil mineral and organic content can be treated as a constant (in the short term), while the percent soil water and gas content is considered highly variable whereby a rise in one is simultaneously balanced by a reduction in the other. The pore space allows for

1743-424: Is also a net sink of methane (CH 4 ) but a net producer of methane (a strong heat-absorbing greenhouse gas ) when soils are depleted of oxygen and subject to elevated temperatures. Soil atmosphere is also the seat of emissions of volatiles other than carbon and nitrogen oxides from various soil organisms, e.g. roots, bacteria, fungi, animals. These volatiles are used as chemical cues, making soil atmosphere

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1826-552: Is also a very common pseudomorph. It is frequently encountered in the swampy areas at the head of spring waters (' bog iron '), on cave floors, and on the bottom of lakes and small creeks. The boxworks or gossan resulting from the oxidation of sulfide ore deposits is formed of goethite along with other iron oxides and quartz. Significant deposits of goethite are found in England ; Cuba ; and Minnesota , Missouri , Colorado , Alabama , Georgia , Virginia , and Tennessee , in

1909-556: Is an important characteristic of soil. This ventilation can be accomplished via networks of interconnected soil pores , which also absorb and hold rainwater making it readily available for uptake by plants. Since plants require a nearly continuous supply of water, but most regions receive sporadic rainfall, the water-holding capacity of soils is vital for plant survival. Soils can effectively remove impurities, kill disease agents, and degrade contaminants , this latter property being called natural attenuation . Typically, soils maintain

1992-521: Is as an iron ore , being referred to as brown iron ore . Goethite is an important component of ochre pigments, and has been heat-treated for use as a red pigment since Paleolithic times. Iron-rich lateritic soils that have developed over serpentinite rocks in tropical climates are mined for their iron content, as well as other metals. Fine goethite specimens are rare and therefore are valued collectibles. Banded or iridescent varieties are cut and polished into cabochons for jewelry making. In

2075-464: Is assumed acid-forming cations). Base saturation is almost in direct proportion to pH (it increases with increasing pH). It is of use in calculating the amount of lime needed to neutralise an acid soil (lime requirement). The amount of lime needed to neutralize a soil must take account of the amount of acid forming ions on the colloids (exchangeable acidity), not just those in the soil water solution (free acidity). The addition of enough lime to neutralize

2158-414: Is called the pedosphere . The pedosphere interfaces with the lithosphere , the hydrosphere , the atmosphere , and the biosphere . Soil has four important functions : All of these functions, in their turn, modify the soil and its properties. Soil science has two basic branches of study: edaphology and pedology . Edaphology studies the influence of soils on living things. Pedology focuses on

2241-401: Is called the soil solution. Since soil water is never pure water, but contains hundreds of dissolved organic and mineral substances, it may be more accurately called the soil solution. Water is central to the dissolution , precipitation and leaching of minerals from the soil profile . Finally, water affects the type of vegetation that grows in a soil, which in turn affects the development of

2324-421: Is dependent on the type of parent material , the processes that modify those parent materials, and the soil-forming factors that influence those processes. The biological influences on soil properties are strongest near the surface, though the geochemical influences on soil properties increase with depth. Mature soil profiles typically include three basic master horizons: A, B, and C. The solum normally includes

2407-401: Is induced. Capillary action is responsible for moving groundwater from wet regions of the soil to dry areas. Subirrigation designs (e.g., wicking beds , sub-irrigated planters ) rely on capillarity to supply water to plant roots. Capillary action can result in an evaporative concentration of salts, causing land degradation through salination . Soil moisture measurement —measuring

2490-531: Is likely home to 59 ± 15% of the species on Earth. Enchytraeidae (worms) have the greatest percentage of species in soil (98.6%), followed by fungi (90%), plants (85.5%), and termites ( Isoptera ) (84.2%). Many other groups of animals have substantial fractions of species living in soil, e.g. about 30% of insects , and close to 50% of arachnids . While most vertebrates live above ground (ignoring aquatic species), many species are fossorial , that is, they live in soil, such as most blind snakes . The chemistry of

2573-590: Is located around Nimmitabel , some of the richest soils on the continent. Previously, there was a black market for the soil in Ukraine. The sale of agricultural land was illegal in Ukraine from 1992 to 2020, but the soil, transported by truck, could be traded legally. According to the Kharkiv -based Green Front NGO, the black market for illegally acquired chernozem in Ukraine was projected to reach approximately US$ 900 million per year in 2011. Chernozemic soils are

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2656-524: Is marked by the oxidation state change of Fe (ferrous) to Fe (ferric), which allows for goethite to exist at surface conditions. Because of this oxidation state change, goethite is commonly seen as a pseudomorph . As iron-bearing minerals are brought to the zone of oxidation within the soil, the iron turns from iron(II) to iron(III), while the original shape of the parent mineral is retained. Common goethite pseudomorphs include pyrite , siderite , and marcasite , though any iron(II)-bearing mineral could become

2739-477: Is more advanced. Most plant nutrients, with the exception of nitrogen , originate from the minerals that make up the soil parent material. Some nitrogen originates from rain as dilute nitric acid and ammonia , but most of the nitrogen is available in soils as a result of nitrogen fixation by bacteria . Once in the soil-plant system, most nutrients are recycled through living organisms, plant and microbial residues (soil organic matter), mineral-bound forms, and

2822-439: Is more typically massive. Feroxyhyte and lepidocrocite are both polymorphs of the iron oxyhydroxide FeO(OH) which are stable at the pressure and temperature conditions of the Earth's surface. Although they have the same chemical formula as goethite, their different crystalline structures make them distinct minerals. Additionally, goethite has several high-pressure and high-temperature polymorphs , which may be relevant to

2905-409: Is referred to as cation exchange . Cation-exchange capacity is the amount of exchangeable cations per unit weight of dry soil and is expressed in terms of milliequivalents of positively charged ions per 100 grams of soil (or centimoles of positive charge per kilogram of soil; cmol c /kg ). Similarly, positively charged sites on colloids can attract and release anions in the soil, giving

2988-545: Is very little organic material. Basaltic minerals commonly weather relatively quickly, according to the Goldich dissolution series . The plants are supported by the porous rock as it is filled with nutrient-bearing water that carries minerals dissolved from the rocks. Crevasses and pockets, local topography of the rocks, would hold fine materials and harbour plant roots. The developing plant roots are associated with mineral-weathering mycorrhizal fungi that assist in breaking up

3071-816: The Great Plains of the United States as far south as Kansas. Chernozem layer thickness may vary widely, from several centimetres up to 1.5 metres (60 inches) in Ukraine, as well as the Red River Valley region in the northern United States and Canada (location of the prehistoric Lake Agassiz ). The terrain can also be found in small quantities elsewhere (for example, in 1% of Poland, Hungary, and Texas). It also exists in Northeast China , near Harbin . The only true chernozem in Australia

3154-666: The United States . Deposits significant in location, if not in abundance, have been found in the Martian crater Gusev by NASA's Spirit rover, providing strong evidence for the presence of liquid water on the planet in an earlier stage of its evolution. Limpets ' teeth are composed of about 80% goethite fibres of only tens of nanometers in diameter, small enough to be flaw-insensitive , which accounts for their extreme tensile strength of 3.5–6.0 GPa and elastic modulus of 120 ± 30 GPa . Its main modern use

3237-490: The aeration of the soil and the ability of water to infiltrate and to be held within the soil. Soil water content can be measured as volume or weight . Soil moisture levels, in order of decreasing water content, are saturation, field capacity , wilting point , air dry, and oven dry. Field capacity describes a drained wet soil at the point water content reaches equilibrium with gravity. Irrigating soil above field capacity risks percolation losses. Wilting point describes

3320-561: The humus form ), the copedon (in intermediary position, where most weathering of minerals takes place) and the lithopedon (in contact with the subsoil). The soil texture is determined by the relative proportions of the individual particles of sand , silt , and clay that make up the soil. The interaction of the individual mineral particles with organic matter, water, gases via biotic and abiotic processes causes those particles to flocculate (stick together) to form aggregates or peds . Where these aggregates can be identified,

3403-425: The mineralogy of those particles can strongly modify those properties. The mineralogy of the finest soil particles, clay, is especially important. Large numbers of microbes , animals , plants and fungi are living in soil. However, biodiversity in soil is much harder to study as most of this life is invisible, hence estimates about soil biodiversity have been unsatisfactory. A recent study suggested that soil

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3486-448: The A and B horizons. The living component of the soil is largely confined to the solum, and is generally more prominent in the A horizon. It has been suggested that the pedon , a column of soil extending vertically from the surface to the underlying parent material and large enough to show the characteristics of all its horizons, could be subdivided in the humipedon (the living part, where most soil organisms are dwelling, corresponding to

3569-646: The CEC increases. Hence, pure sand has almost no buffering ability, though soils high in colloids (whether mineral or organic) have high buffering capacity . Buffering occurs by cation exchange and neutralisation . However, colloids are not the only regulators of soil pH. The role of carbonates should be underlined, too. More generally, according to pH levels, several buffer systems take precedence over each other, from calcium carbonate buffer range to iron buffer range. Goethite Goethite ( / ˈ ɡ ɜːr t aɪ t / , US also / ˈ ɡ oʊ θ aɪ t / )

3652-690: The Hollertszug Mine in Herdorf , Germany . The mineral was named after the German polymath and poet Johann Wolfgang von Goethe (1749–1832). Goethite is an iron oxyhydroxide containing ferric iron. It is the main component of rust and bog iron ore. Goethite's hardness ranges from 5.0 to 5.5 on the Mohs Scale , and its specific gravity varies from 3.3 to 4.3. The mineral forms prismatic needle-like crystals ("needle ironstone" ) but

3735-406: The acronym CROPT. The physical properties of soils, in order of decreasing importance for ecosystem services such as crop production , are texture , structure , bulk density , porosity , consistency, temperature , colour and resistivity . Soil texture is determined by the relative proportion of the three kinds of soil mineral particles, called soil separates: sand , silt , and clay . At

3818-446: The addition of any more hydronium ions or aluminum hydroxyl cations drives the pH even lower (more acidic) as the soil has been left with no buffering capacity. In areas of extreme rainfall and high temperatures, the clay and humus may be washed out, further reducing the buffering capacity of the soil. In low rainfall areas, unleached calcium pushes pH to 8.5 and with the addition of exchangeable sodium, soils may reach pH 10. Beyond

3901-454: The anions. The sites filled by iron ions form paired chains running the length of the crystal, with the two chains in each pair joined by hydroxide ions. Goethite often forms through the weathering of other iron-rich minerals, and thus is a common component of soils , concentrated in laterite soils. nanoparticulate authigenic goethite is a common diagenetic iron oxyhydroxide in both marine and lake sediments. The formation of goethite

3984-452: The apparent sterility of tropical soils. Live plant roots also have some CEC, linked to their specific surface area. Anion exchange capacity is the soil's ability to remove anions (such as nitrate , phosphate ) from the soil water solution and sequester those for later exchange as the plant roots release carbonate anions to the soil water solution. Those colloids which have low CEC tend to have some AEC. Amorphous and sesquioxide clays have

4067-437: The atmosphere due to increased biological activity at higher temperatures, a positive feedback (amplification). This prediction has, however, been questioned on consideration of more recent knowledge on soil carbon turnover. Soil acts as an engineering medium, a habitat for soil organisms , a recycling system for nutrients and organic wastes , a regulator of water quality , a modifier of atmospheric composition , and

4150-426: The basic cations are forced off the soil colloids by the mass action of hydronium ions from usual or unusual rain acidity against those attached to the colloids. High rainfall rates can then wash the nutrients out, leaving the soil inhabited only by those organisms which are particularly efficient to uptake nutrients in very acid conditions, like in tropical rainforests . Once the colloids are saturated with H 3 O ,

4233-435: The conditions of the Earth's interior. These include ε-FeOOH, which has an orthorhombic crystal structure, a cubic pyrite -type polymorph with or without losing hydrogen and an ultradense hexagonal structure. Goethite has the same crystal structure as diaspore , the analogous aluminium oxide-hydroxide mineral. Oxygen and hydroxide ions form a hexagonal close-packed structure, with iron ions filling octahedral sites between

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4316-436: The dry limit for growing plants. During growing season, soil moisture is unaffected by functional groups or specie richness. Available water capacity is the amount of water held in a soil profile available to plants. As water content drops, plants have to work against increasing forces of adhesion and sorptivity to withdraw water. Irrigation scheduling avoids moisture stress by replenishing depleted water before stress

4399-527: The formation, description (morphology), and classification of soils in their natural environment. In engineering terms, soil is included in the broader concept of regolith , which also includes other loose material that lies above the bedrock, as can be found on the Moon and other celestial objects . Soil is a major component of the Earth 's ecosystem . The world's ecosystems are impacted in far-reaching ways by

4482-419: The highest AEC, followed by the iron oxides. Levels of AEC are much lower than for CEC, because of the generally higher rate of positively (versus negatively) charged surfaces on soil colloids, to the exception of variable-charge soils. Phosphates tend to be held at anion exchange sites. Iron and aluminum hydroxide clays are able to exchange their hydroxide anions (OH ) for other anions. The order reflecting

4565-463: The highest of the major soil types. Soil magnetism increases when soil minerals goethite and ferrihydrite convert to maghemite on exposure to heat. Temperatures sufficient to elevate maghemite on a landscape scale indicate the influence of fire. Given the rarity of such natural phenomena in the modern day, magnetic susceptibility in Chernozem likely relates to control of fire by early humans . Humification can darken soils ( melanization ) absent

4648-632: The infiltration and movement of air and water, both of which are critical for life existing in soil. Compaction , a common problem with soils, reduces this space, preventing air and water from reaching plant roots and soil organisms. Given sufficient time, an undifferentiated soil will evolve a soil profile that consists of two or more layers, referred to as soil horizons. These differ in one or more properties such as in their texture , structure , density , porosity, consistency, temperature, color, and reactivity . The horizons differ greatly in thickness and generally lack sharp boundaries; their development

4731-429: The negatively charged colloids resist being washed downward by water and are out of reach of plant roots, thereby preserving the soil fertility in areas of moderate rainfall and low temperatures. There is a hierarchy in the process of cation exchange on colloids, as cations differ in the strength of adsorption by the colloid and hence their ability to replace one another ( ion exchange ). If present in equal amounts in

4814-412: The negatively-charged soil colloid exchange sites (CEC) that are occupied by base-forming cations is called base saturation . If a soil has a CEC of 20 meq and 5 meq are aluminium and hydronium cations (acid-forming), the remainder of positions on the colloids ( 20 − 5 = 15 meq ) are assumed occupied by base-forming cations, so that the base saturation is 15 ÷ 20 × 100% = 75% (the compliment 25%

4897-419: The next larger scale, soil structures called peds or more commonly soil aggregates are created from the soil separates when iron oxides , carbonates , clay, silica and humus , coat particles and cause them to adhere into larger, relatively stable secondary structures. Soil bulk density , when determined at standardized moisture conditions, is an estimate of soil compaction . Soil porosity consists of

4980-472: The ocean has no more than 10 prokaryotic organisms per milliliter (gram) of seawater. Organic carbon held in soil is eventually returned to the atmosphere through the process of respiration carried out by heterotrophic organisms, but a substantial part is retained in the soil in the form of soil organic matter; tillage usually increases the rate of soil respiration , leading to the depletion of soil organic matter. Since plant roots need oxygen, aeration

5063-635: The other cations more weakly bound to colloids are pushed into solution as hydrogen ions occupy exchange sites ( protonation ). A low pH may cause the hydrogen of hydroxyl groups to be pulled into solution, leaving charged sites on the colloid available to be occupied by other cations. This ionisation of hydroxy groups on the surface of soil colloids creates what is described as pH-dependent surface charges. Unlike permanent charges developed by isomorphous substitution , pH-dependent charges are variable and increase with increasing pH. Freed cations can be made available to plants but are also prone to be leached from

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5146-492: The plant roots release hydrogen ions to the solution. CEC is the amount of exchangeable hydrogen cation (H ) that will combine with 100 grams dry weight of soil and whose measure is one milliequivalents per 100 grams of soil (1 meq/100 g). Hydrogen ions have a single charge and one-thousandth of a gram of hydrogen ions per 100 grams dry soil gives a measure of one milliequivalent of hydrogen ion. Calcium, with an atomic weight 40 times that of hydrogen and with

5229-486: The porous lava, and by these means organic matter and a finer mineral soil accumulate with time. Such initial stages of soil development have been described on volcanoes, inselbergs, and glacial moraines. How soil formation proceeds is influenced by at least five classic factors that are intertwined in the evolution of a soil: parent material, climate, topography (relief), organisms, and time. When reordered to climate, relief, organisms, parent material, and time, they form

5312-414: The processes carried out in the soil, with effects ranging from ozone depletion and global warming to rainforest destruction and water pollution . With respect to Earth's carbon cycle , soil acts as an important carbon reservoir , and it is potentially one of the most reactive to human disturbance and climate change . As the planet warms, it has been predicted that soils will add carbon dioxide to

5395-414: The rate of diffusion of gases into and out of soil. Platy soil structure and soil compaction (low porosity) impede gas flow, and a deficiency of oxygen may encourage anaerobic bacteria to reduce (strip oxygen) from nitrate NO 3 to the gases N 2 , N 2 O, and NO, which are then lost to the atmosphere, thereby depleting the soil of nitrogen, a detrimental process called denitrification . Aerated soil

5478-442: The seat of interaction networks playing a decisive role in the stability, dynamics and evolution of soil ecosystems. Biogenic soil volatile organic compounds are exchanged with the aboveground atmosphere, in which they are just 1–2 orders of magnitude lower than those from aboveground vegetation. Humans can get some idea of the soil atmosphere through the well-known 'after-the-rain' scent, when infiltering rainwater flushes out

5561-441: The soil anion exchange capacity. The cation exchange, that takes place between colloids and soil water, buffers (moderates) soil pH, alters soil structure, and purifies percolating water by adsorbing cations of all types, both useful and harmful. The negative or positive charges on colloid particles make them able to hold cations or anions, respectively, to their surfaces. The charges result from four sources. Cations held to

5644-538: The soil by volatilisation (loss to the atmosphere as gases) or leaching. Soil is said to be formed when organic matter has accumulated and colloids are washed downward, leaving deposits of clay, humus , iron oxide , carbonate , and gypsum , producing a distinct layer called the B horizon. This is a somewhat arbitrary definition as mixtures of sand, silt, clay and humus will support biological and agricultural activity before that time. These constituents are moved from one level to another by water and animal activity. As

5727-420: The soil solution composition (attenuate changes in the soil solution) as soils wet up or dry out, as plants take up nutrients, as salts are leached, or as acids or alkalis are added. Plant nutrient availability is affected by soil pH , which is a measure of the hydrogen ion activity in the soil solution. Soil pH is a function of many soil forming factors, and is generally lower (more acidic) where weathering

5810-402: The soil solution. Both living soil organisms (microbes, animals and plant roots) and soil organic matter are of critical importance to this recycling, and thereby to soil formation and soil fertility . Microbial soil enzymes may release nutrients from minerals or organic matter for use by plants and other microorganisms, sequester (incorporate) them into living cells, or cause their loss from

5893-473: The soil voids are saturated with water vapour, at least until the point of maximal hygroscopicity , beyond which a vapour-pressure deficit occurs in the soil pore space. Adequate porosity is necessary, not just to allow the penetration of water, but also to allow gases to diffuse in and out. Movement of gases is by diffusion from high concentrations to lower, the diffusion coefficient decreasing with soil compaction . Oxygen from above atmosphere diffuses in

5976-430: The soil water solution will be insufficient to change the pH, as the acid forming cations stored on the soil colloids will tend to restore the original pH condition as they are pushed off those colloids by the calcium of the added lime. The resistance of soil to change in pH, as a result of the addition of acid or basic material, is a measure of the buffering capacity of a soil and (for a particular soil type) increases as

6059-433: The soil water solution: Al replaces H replaces Ca replaces Mg replaces K same as NH 4 replaces Na If one cation is added in large amounts, it may replace the others by the sheer force of its numbers. This is called law of mass action . This is largely what occurs with the addition of cationic fertilisers ( potash , lime ). As the soil solution becomes more acidic (low pH , meaning an abundance of H ),

6142-462: The soil where it is consumed and levels of carbon dioxide in excess of above atmosphere diffuse out with other gases (including greenhouse gases ) as well as water. Soil texture and structure strongly affect soil porosity and gas diffusion. It is the total pore space ( porosity ) of soil, not the pore size, and the degree of pore interconnection (or conversely pore sealing), together with water content, air turbulence and temperature, that determine

6225-502: The soil, a complex feedback which is exemplified in the dynamics of banded vegetation patterns in semi-arid regions. Soils supply plants with nutrients , most of which are held in place by particles of clay and organic matter ( colloids ) The nutrients may be adsorbed on clay mineral surfaces, bound within clay minerals ( absorbed ), or bound within organic compounds as part of the living organisms or dead soil organic matter. These bound nutrients interact with soil water to buffer

6308-414: The soil, possibly making the soil less fertile. Plants are able to excrete H into the soil through the synthesis of organic acids and by that means, change the pH of the soil near the root and push cations off the colloids, thus making those available to the plant. Cation exchange capacity is the soil's ability to remove cations from the soil water solution and sequester those to be exchanged later as

6391-409: The strength of anion adhesion is as follows: The amount of exchangeable anions is of a magnitude of tenths to a few milliequivalents per 100 g dry soil. As pH rises, there are relatively more hydroxyls, which will displace anions from the colloids and force them into solution and out of storage; hence AEC decreases with increasing pH (alkalinity). Soil reactivity is expressed in terms of pH and

6474-491: The two concentrations are equal, they are said to neutralise each other. A pH of 9.5 has 10 moles hydronium ions per litre of solution (and also 10 moles per litre OH ). A pH of 3.5 has one million times more hydronium ions per litre than a solution with pH of 9.5 ( 9.5 − 3.5 = 6 or 10 ) and is more acidic. The effect of pH on a soil is to remove from the soil or to make available certain ions. Soils with high acidity tend to have toxic amounts of aluminium and manganese . As

6557-473: The void part of the soil volume and is occupied by gases or water. Soil consistency is the ability of soil materials to stick together. Soil temperature and colour are self-defining. Resistivity refers to the resistance to conduction of electric currents and affects the rate of corrosion of metal and concrete structures which are buried in soil. These properties vary through the depth of a soil profile, i.e. through soil horizons . Most of these properties determine

6640-553: The water content of the soil, as can be expressed in terms of volume or weight—can be based on in situ probes (e.g., capacitance probes , neutron probes ), or remote sensing methods. Soil moisture measurement is an important factor in determining changes in soil activity. The atmosphere of soil, or soil gas , is very different from the atmosphere above. The consumption of oxygen by microbes and plant roots, and their release of carbon dioxide, decreases oxygen and increases carbon dioxide concentration. Atmospheric CO 2 concentration

6723-461: The weathering of lava flow bedrock, which would produce the purely mineral-based parent material from which the soil texture forms. Soil development would proceed most rapidly from bare rock of recent flows in a warm climate, under heavy and frequent rainfall. Under such conditions, plants (in a first stage nitrogen-fixing lichens and cyanobacteria then epilithic higher plants ) become established very quickly on basaltic lava, even though there

6806-476: The whole soil atmosphere after a drought period, or when soil is excavated, a bulk property attributed in a reductionist manner to particular biochemical compounds such as petrichor or geosmin . Soil particles can be classified by their chemical composition ( mineralogy ) as well as their size. The particle size distribution of a soil, its texture, determines many of the properties of that soil, in particular hydraulic conductivity and water potential , but

6889-744: The world. One is the Eurasian steppe that extends from eastern Croatia ( Slavonia ), along the Danube (northern Serbia , northern Bulgaria ( Danubian Plain ), southern and eastern Romania ( Wallachian Plain and Moldavian Plain ), and Moldova , to northeast Ukraine across the Central-Chernozem Strip of Central and Southern Russia into Siberia . The other stretches from the Canadian Prairies in Manitoba through

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