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92-445: Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolinite , Al 2 Si 2 O 5 ( OH ) 4 ). Most pure clay minerals are white or light-coloured, but natural clays show a variety of colours from impurities, such as a reddish or brownish colour from small amounts of iron oxide . Clays develop plasticity when wet but can be hardened through firing . Clay

184-424: A mean prokaryotic density of roughly 10 organisms per gram, whereas 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

276-573: A gel of orthosilicic acid ).) The clay minerals formed depend on the composition of the source rock and the climate. Acid weathering of feldspar -rich rock, such as granite , in warm climates tends to produce kaolin. Weathering of the same kind of rock under alkaline conditions produces illite . Smectite forms by weathering of igneous rock under alkaline conditions, while gibbsite forms by intense weathering of other clay minerals. There are two types of clay deposits: primary and secondary. Primary clays form as residual deposits in soil and remain at

368-834: A great capacity to take up water, and they increase greatly in volume when they do so. When dried, they shrink back to their original volume. This produces distinctive textures, such as mudcracks or "popcorn" texture, in clay deposits. Soils containing swelling clay minerals (such as bentonite ) pose a considerable challenge for civil engineering, because swelling clay can break foundations of buildings and ruin road beds. Clay minerals most commonly form by prolonged chemical weathering of silicate-bearing rocks. They can also form locally from hydrothermal activity. Chemical weathering takes place largely by acid hydrolysis due to low concentrations of carbonic acid , dissolved in rainwater or released by plant roots. The acid breaks bonds between aluminium and oxygen, releasing other metal ions and silica (as

460-594: A habitat for soil organisms , a recycling system for nutrients and organic wastes , a regulator of water quality , a modifier of atmospheric composition , and 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

552-401: A major challenge in civil engineering . The defining mechanical property of clay is its plasticity when wet and its ability to harden when dried or fired. Clays show a broad range of water content within which they are highly plastic, from a minimum water content (called the plastic limit ) where the clay is just moist enough to mould, to a maximum water content (called the liquid limit) where

644-549: 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

736-687: A reduction in the other. The pore space allows for 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

828-731: 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

920-401: 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

1012-408: A silt loam soil by percent volume A typical soil 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

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1104-412: 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

1196-399: 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

1288-472: 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

1380-449: Is Armenian bole , which is used to soothe an upset stomach. Some animals such as parrots and pigs ingest clay for similar reasons. Kaolin clay and attapulgite have been used as anti-diarrheal medicines. Clay as the defining ingredient of loam is one of the oldest building materials on Earth , among other ancient, naturally occurring geologic materials such as stone and organic materials like wood. Between one-half and two-thirds of

1472-553: 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,

1564-603: 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

1656-412: Is a rare silicate mineral formed during contact metamorphism of clay minerals . It can form two stoichiometric forms: 3 Al 2 O 3 2 Si O 2 or 2Al 2 O 3 SiO 2 . Unusually, mullite has no charge-balancing cations present. As a result, there are three different aluminium sites : two distorted tetrahedral and one octahedral . Mullite was first described in 1924 for an occurrence on

1748-509: 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

1840-618: Is a unique type of marine clay indigenous to the glaciated terrains of Norway , North America , Northern Ireland , and Sweden . It is a highly sensitive clay, prone to liquefaction , and has been involved in several deadly landslides . Modelling clay is used in art and handicraft for sculpting . Clays are used for making pottery , both utilitarian and decorative, and construction products, such as bricks, walls, and floor tiles. Different types of clay, when used with different minerals and firing conditions, are used to produce earthenware, stoneware, and porcelain. Prehistoric humans discovered

1932-424: 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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2024-464: 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

2116-401: 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

2208-458: Is clay with visible annual layers that are formed by seasonal deposition of those layers and are marked by differences in erosion and organic content. This type of deposit is common in former glacial lakes . When fine sediments are delivered into the calm waters of these glacial lake basins away from the shoreline, they settle to the lake bed. The resulting seasonal layering is preserved in an even distribution of clay sediment banding. Quick clay

2300-421: 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

2392-672: Is important to soil fertility. Clay is a common component of sedimentary rock . Shale is formed largely from clay and is the most common of sedimentary rocks. However, most clay deposits are impure. Many naturally occurring deposits include both silts and clay. Clays are distinguished from other fine-grained soils by differences in size and mineralogy. Silts , which are fine-grained soils that do not include clay minerals, tend to have larger particle sizes than clays. There is, however, some overlap in particle size and other physical properties. The distinction between silt and clay varies by discipline. Geologists and soil scientists usually consider

2484-649: 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 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

2576-401: 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

2668-531: 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

2760-477: 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

2852-437: 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 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,

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2944-587: Is produced during various melting and firing processes, and is used as a refractory material , because of its high melting point of 1840 °C. In 2006 researchers at University College London and Cardiff University discovered that potters in the Hesse region of Germany since the late Middle Ages had used mullite in the manufacture of a type of crucible (known as Hessian crucibles ), that were renowned for enabling alchemists to heat their crucibles to very high temperatures. The formula finally replicated in

3036-409: 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

3128-427: Is the longest-known ceramic material. Prehistoric humans discovered the useful properties of clay and used it for making pottery . Some of the earliest pottery shards have been dated to around 14,000 BCE, and clay tablets were the first known writing medium. Clay is used in many modern industrial processes, such as paper making, cement production, and chemical filtering . Between one-half and two-thirds of

3220-508: Is used in many industrial processes, such as paper making, cement production, and chemical filtering . Bentonite clay is widely used as a mold binder in the manufacture of sand castings . Clay is a common filler used in polymer nanocomposites . It can reduce the cost of the composite, as well as impart modified behavior: increased stiffness , decreased permeability , decreased electrical conductivity , etc. Traditional uses of clay as medicine go back to prehistoric times. An example

3312-545: 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

3404-599: The Isle of Mull , Scotland . It occurs as argillaceous inclusions in volcanic rocks in the Isle of Mull, inclusions in sillimanite within a tonalite at Val Sissone , Italy and with emerylike rocks in Argyllshire , Scotland. Mullite (porcelainite) can be found as a constituent mineral in a type of thermally-metamorphosed rock called porcellanite . Mullite is present in the form of needles in porcelain . It

3496-490: 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

3588-444: 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 the formation, description (morphology), and classification of soils in their natural environment. In engineering terms, soil

3680-561: 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,

3772-425: 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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3864-613: 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 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

3956-448: 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

4048-583: 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. Mullite Mullite or porcelainite

4140-407: The above studies (using kaolinitic clay and then firing it at temperatures above 1100 °C) was kept a closely guarded secret by those crucible makers since the 15th century. Mullite morphology is also important for its application. In this case, there are two common morphologies for mullite. One is a platelet shape with low aspect ratio and the second is a needle shape with high aspect ratio. If

4232-406: 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

4324-446: 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

4416-452: 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

4508-426: 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 ,

4600-414: The cohesion that makes it plastic. In kaolinite clay, the bonding between plates is provided by a film of water molecules that hydrogen bond the plates together. The bonds are weak enough to allow the plates to slip past each other when the clay is being moulded, but strong enough to hold the plates in place and allow the moulded clay to retain its shape after it is moulded. When the clay is dried, most of

4692-436: 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

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4784-513: The first known writing medium. Clay was chosen due to the local material being easy to work with and widely available. Scribes wrote on the tablets by inscribing them with a script known as cuneiform , using a blunt reed called a stylus , which effectively produced the wedge shaped markings of their writing. After being written on, clay tablets could be reworked into fresh tablets and reused if needed, or fired to make them permanent records. Purpose-made clay balls were used as sling ammunition . Clay

4876-419: 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

4968-403: The material. The clay mineral kaolinite is transformed into a non-clay material, metakaolin , which remains rigid and hard if moistened again. Further firing through the stoneware and porcelain stages further recrystallizes the metakaolin into yet stronger minerals such as mullite . The tiny size and plate form of clay particles gives clay minerals a high surface area. In some clay minerals,

5060-721: The moulded clay is just dry enough to hold its shape. The plastic limit of kaolinite clay ranges from about 36% to 40% and its liquid limit ranges from about 58% to 72%. High-quality clay is also tough, as measured by the amount of mechanical work required to roll a sample of clay flat. Its toughness reflects a high degree of internal cohesion. Clay has a high content of clay minerals that give it its plasticity. Clay minerals are hydrous aluminium phyllosilicate minerals , composed of aluminium and silicon ions bonded into tiny, thin plates by interconnecting oxygen and hydroxide ions. These plates are tough but flexible, and in moist clay, they adhere to each other. The resulting aggregates give clay

5152-431: The needle shape mullite can form in a ceramic body during sintering , it has an effect on both the mechanical and physical properties by increasing the mechanical strength and thermal shock resistance. The most important condition relates to ceramic chemical composition. If the silica and alumina ratio with low basic materials such as sodium and calcium is adjusted, the needle shape mullite forms at about 1400 °C and

5244-429: 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

5336-412: 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%

5428-419: 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

5520-635: 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

5612-492: 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

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5704-466: The plates carry a negative electrical charge that is balanced by a surrounding layer of positive ions ( cations ), such as sodium, potassium, or calcium. If the clay is mixed with a solution containing other cations, these can swap places with the cations in the layer around the clay particles, which gives clays a high capacity for ion exchange . The chemistry of clay minerals, including their capacity to retain nutrient cations such as potassium and ammonium,

5796-486: 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

5888-434: The rate of soil respiration , leading to the depletion of soil organic matter. Since plant roots need oxygen, aeration 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,

5980-414: 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

6072-484: The removal of heavy metals from waste water and air purification. 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 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

6164-442: 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

6256-695: The separation to occur at a particle size of 2 μm (clays being finer than silts), sedimentologists often use 4–5 μm, and colloid chemists use 1 μm. Clay-size particles and clay minerals are not the same, despite a degree of overlap in their respective definitions. Geotechnical engineers distinguish between silts and clays based on the plasticity properties of the soil, as measured by the soils' Atterberg limits . ISO 14688 grades clay particles as being smaller than 2 μm and silt particles as being larger. Mixtures of sand , silt and less than 40% clay are called loam . Some clay minerals (such as smectite ) are described as swelling clay minerals, because they have

6348-835: The site of formation. Secondary clays are clays that have been transported from their original location by water erosion and deposited in a new sedimentary deposit. Secondary clay deposits are typically associated with very low energy depositional environments such as large lakes and marine basins. The main groups of clays include kaolinite , montmorillonite - smectite , and illite . Chlorite , vermiculite , talc , and pyrophyllite are sometimes also classified as clay minerals. There are approximately 30 different types of "pure" clays in these categories, but most "natural" clay deposits are mixtures of these different types, along with other weathered minerals. Clay minerals in clays are most easily identified using X-ray diffraction rather than chemical or physical tests. Varve (or varved clay )

6440-575: 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 is called the pedosphere . The pedosphere interfaces with the lithosphere , the hydrosphere ,

6532-441: 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

6624-538: 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

6716-420: 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

6808-402: 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

6900-473: 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

6992-430: 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

7084-433: 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 ),

7176-462: 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

7268-468: 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 a dry bulk density (density of soil taking into account voids when dry) between 1.1 and 1.6 g/cm , though

7360-502: 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

7452-414: 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

7544-409: 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

7636-491: 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

7728-547: The useful properties of clay. Some of the earliest pottery shards recovered are from central Honshu , Japan . They are associated with the Jōmon culture, and recovered deposits have been dated to around 14,000 BCE. Cooking pots, art objects, dishware, smoking pipes , and even musical instruments such as the ocarina can all be shaped from clay before being fired. Ancient peoples in Mesopotamia adopted clay tablets as

7820-473: 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

7912-553: 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

8004-426: The water molecules are removed, and the plates hydrogen bond directly to each other, so that the dried clay is rigid but still fragile. If the clay is moistened again, it will once more become plastic. When the clay is fired to the earthenware stage, a dehydration reaction removes additional water from the clay, causing clay plates to irreversibly adhere to each other via stronger covalent bonding , which strengthens

8096-461: 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

8188-476: 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

8280-734: The world's population live or work in buildings made with clay, often baked into brick, as an essential part of its load-bearing structure. Clay is a very common substance. Shale , formed largely from clay, is the most common sedimentary rock. Although many naturally occurring deposits include both silts and clay, clays are distinguished from other fine-grained soils by differences in size and mineralogy. Silts , which are fine-grained soils that do not include clay minerals, tend to have larger particle sizes than clays. Mixtures of sand , silt and less than 40% clay are called loam . Soils high in swelling clays ( expansive clay ), which are clay minerals that readily expand in volume when they absorb water, are

8372-492: The world's population, in both traditional societies as well as developed countries, still live or work in buildings made with clay, often baked into brick, as an essential part of their load-bearing structure. Also a primary ingredient in many natural building techniques, clay is used to create adobe , cob , cordwood , and structures and building elements such as wattle and daub , clay plaster, clay render case, clay floors and clay paints and ceramic building material . Clay

8464-460: Was used as a mortar in brick chimneys and stone walls where protected from water. Clay, relatively impermeable to water, is also used where natural seals are needed, such as in pond linings, the cores of dams , or as a barrier in landfills against toxic seepage (lining the landfill, preferably in combination with geotextiles ). Studies in the early 21st century have investigated clay's absorption capacities in various applications, such as

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