Subsoil is the layer of soil under the topsoil on the surface of the ground. Like topsoil, it is composed of a variable mixture of small particles such as sand , silt and clay , but with a much lower percentage of organic matter and humus . The subsoil is labeled the B Horizon in most soil mapping systems. Because it has less organic matter than topsoil , subsoil soil colour is mainly derived from iron oxides. Iron oxides and clay minerals form due to weathering . Rainfall moves these weathering products downward as solutes and colloids by rainfall. The subsoil is the depth where these weathering products accumulate. The accumulation of clay minerals, iron, aluminum, and organic compounds is called illuviation .
31-416: Whereas the topsoil tends to be the depth of greatest physical, chemical, and biological activity, the subsoil is the depth of most deposition. Due to physical, chemical, and biological activity, the subsoil generally has a soil structure . The presence of structure distinguishes the subsoil from the underlying substratum. Due to human activity, the topsoil and subsoil in many environments have been mixed. Below
62-419: A bone repair cement. Small amounts of calcined gypsum are added to earth to create strong structures directly from cast earth , an alternative to adobe (which loses its strength when wet). The conditions of dehydration can be changed to adjust the porosity of the hemihydrate, resulting in the so-called α- and β-hemihydrates (which are more or less chemically identical). On heating to 180 °C (356 °F),
93-521: A moldable paste upon hydration and hardens as crystalline calcium sulfate dihydrate. It is also convenient that calcium sulfate is poorly soluble in water and does not readily dissolve in contact with water after its solidification. With judicious heating, gypsum converts to the partially dehydrated mineral called bassanite or plaster of Paris . This material has the formula CaSO 4 ·( n H 2 O), where 0.5 ≤ n ≤ 0.8. Temperatures between 100 and 150 °C (212–302 °F) are required to drive off
124-435: A result of rapid wetting. This is particularly so if soils are sodic ; that is, having a high exchangeable sodium percentage (ESP) of the cations attached to the clays. High sodium levels (compared to high calcium levels) cause particles to repel one another when wet, and the associated aggregates to disaggregate and disperse. The ESP will increase if irrigation causes salty water (even of low concentration) to gain access to
155-505: A significant host response and creates a calcium-rich milieu in the area of implantation. When sold at the anhydrous state as a desiccant with a color-indicating agent under the name Drierite , it appears blue (anhydrous) or pink (hydrated) due to impregnation with cobalt(II) chloride , which functions as a moisture indicator. Up to the 1970s, commercial quantities of sulfuric acid were produced from anhydrous calcium sulfate. Upon being mixed with shale or marl , and roasted at 1400°C,
186-710: A single intense rainfall event." Soil structure is inherently a dynamic and complex system that is affected by different factors such as tillage , wheel traffic, roots , biological activities in soil, rainfall events, wind erosion , shrinking, swelling, freezing and thawing. In turn, reciprocally soil structure interacts and affects the root growth and function, soil fauna and biota, water and solute transport processes, gas exchange , thermal conductivity and electrical conductivity , traffic bearing capacity , and many other aspects in relation with soil. Ignoring soil structure or viewing it as "static" can lead to poor predictions of soil properties and might significantly affect
217-470: A structureless mass that is very difficult to cultivate. They can only be tilled when their moisture content is within a limited range. When they are tilled the result is often a very cloddy surface (poor tilth ). As they dry out the high soil strength often restricts seedling and root growth. Infiltration rates are low and runoff of rain and irrigation limits the productivity of many hardsetting soils. Hardsetting has been defined this way: "A hardsetting soil
248-551: Is a stub . You can help Misplaced Pages by expanding it . Soil structure In geotechnical engineering , soil structure describes the arrangement of the solid parts of the soil and of the pore space located between them. It is determined by how individual soil granules clump, bind together, and aggregate , resulting in the arrangement of soil pores between them. Soil has a major influence on water and air movement, biological activity , root growth and seedling emergence. There are several different types of soil structure. It
279-466: Is also a common component of fouling deposits in industrial heat exchangers, because its solubility decreases with increasing temperature (see the specific section on the retrograde solubility). The solubility of calcium sulfate decreases as temperature increases. This behaviour ("retrograde solubility") is uncommon: dissolution of most of the salts is endothermic and their solubility increases with temperature.The retrograde solubility of calcium sulfate
310-417: Is around 127 million tonnes per annum. In addition to natural sources, calcium sulfate is produced as a by-product in a number of processes: Related sulfur-trapping methods use lime and some produces an impure calcium sulfite , which oxidizes on storage to calcium sulfate. These precipitation processes tend to concentrate radioactive elements in the calcium sulfate product. This issue is particular with
341-427: Is associated with high organic matter. Improving soil structure in such settings can increase yields significantly. The NSW Department of Land and Water Conservation suggests that in cropping systems, wheat yields can be increased by 10 kg/ha for every extra millimetre of rain that is able to infiltrate due to soil structure. Hardsetting soils lose their structure when wet and then set hard as they dry out to form
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#1732780871081372-435: Is inherently a dynamic and complex system that is affected by different factors. Soil structure describes the arrangement of the solid parts of the soil and of the pore spaces located between them (Marshall & Holmes, 1979). Aggregation is the result of the interaction of soil particles through rearrangement, flocculation and cementation. It is enhanced by: the precipitation of oxides, hydroxides, carbonates and silicates;
403-506: Is known in the E number series as E516 , and the UN's FAO knows it as a firming agent, a flour treatment agent, a sequestrant, and a leavening agent. Calcium sulfate has a long history of use in dentistry. It has been used in bone regeneration as a graft material and graft binder (or extender) and as a barrier in guided bone tissue regeneration. It is a biocompatible material and is completely resorbed following implantation. It does not evoke
434-405: Is one that sets to an almost homogeneous mass on drying. It may have occasional cracks, typically at a spacing of >0.1 m. Air dry hardset soil is hard and brittle, and it is not possible to push a forefinger into the profile face. Typically, it has a tensile strength of 90 kN . Soils that crust are not necessarily hardsetting since a hardsetting horizon is thicker than a crust. (In cultivated soils
465-524: The framing (through combustion of wood members or loss of strength of steel at high temperatures) and consequent structural collapse. But at higher temperatures, calcium sulfate will release oxygen and act as an oxidizing agent . This property is used in aluminothermy . In contrast to most minerals, which when rehydrated simply form liquid or semi-liquid pastes, or remain powdery, calcined gypsum has an unusual property: when mixed with water at normal (ambient) temperatures, it quickly reverts chemically to
496-460: The soil management . [REDACTED] This article incorporates public domain material from the United States Government Calcium sulfate Calcium sulfate (or calcium sulphate ) is the inorganic compound with the formula CaSO 4 and related hydrates . In the form of γ- anhydrite (the anhydrous form), it is used as a desiccant . One particular hydrate is better known as plaster of Paris , and another occurs naturally as
527-605: The hemihydrate and γ-anhydrite, and their easy inter-conversion, is due to their nearly identical crystal structures containing "channels" that can accommodate variable amounts of water, or other small molecules such as methanol . The calcium sulfate hydrates are used as a coagulant in products such as tofu . For the FDA , it is permitted in cheese and related cheese products; cereal flours; bakery products; frozen desserts; artificial sweeteners for jelly & preserves; condiment vegetables; and condiment tomatoes and some candies. It
558-472: The mineral gypsum . It has many uses in industry. All forms are white solids that are poorly soluble in water. Calcium sulfate causes permanent hardness in water. The compound exists in three levels of hydration corresponding to different crystallographic structures and to minerals: The main use of calcium sulfate is to produce plaster of Paris and stucco . These applications exploit the fact that calcium sulfate which has been powdered and calcined forms
589-454: The mineral, which rises slowly until the water is gone, then increases more rapidly. The equation for the partial dehydration is: The endothermic property of this reaction is relevant to the performance of drywall , conferring fire resistance to residential and other structures. In a fire, the structure behind a sheet of drywall will remain relatively cool as water is lost from the gypsum, thus preventing (or substantially retarding) damage to
620-479: The nearly water-free form, called γ-anhydrite (CaSO 4 · n H 2 O where n = 0 to 0.05) is produced. γ-Anhydrite reacts slowly with water to return to the dihydrate state, a property exploited in some commercial desiccants . On heating above 250 °C, the completely anhydrous form called β-anhydrite or "natural" anhydrite is formed. Natural anhydrite does not react with water, even over geological timescales, unless very finely ground. The variable composition of
651-428: The phosphate by-product, since phosphate ores naturally contain uranium and its decay products such as radium-226 , lead-210 and polonium-210 . Extraction of uranium from phosphorus ores can be economical on its own depending on prices on the uranium market or the separation of uranium can be mandated by environmental legislation and its sale is used to recover part of the cost of the process. Calcium sulfate
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#1732780871081682-400: The preferred dihydrate form, while physically "setting" to form a rigid and relatively strong gypsum crystal lattice: This reaction is exothermic and is responsible for the ease with which gypsum can be cast into various shapes including sheets (for drywall ), sticks (for blackboard chalk), and molds (to immobilize broken bones, or for metal casting). Mixed with polymers, it has been used as
713-419: The products of biological activity (such as biofilms , fungal hyphae and glycoproteins ); ionic bridging between negatively charged particles (both clay minerals and organic compounds) by multivalent cations; and interactions between organic compounds ( hydrogen bonding and hydrophobic bonding). The quality of soil structure will decline under most forms of cultivation —the associated mechanical mixing of
744-419: The soil compacts and shears aggregates and fills pore spaces; it also exposes organic matter to a greater rate of decay and oxidation . A further consequence of continued cultivation and traffic is the development of compacted , impermeable layers or 'pans' within the profile. The decline of soil structure under irrigation is usually related to the breakdown of aggregates and dispersion of clay material as
775-593: The soil from raindrop impact. In irrigated agriculture, it may be recommended to: apply gypsum ( calcium sulfate ) to displace sodium cations with calcium and so reduce ESP or sodicity, avoid rapid wetting, and avoid disturbing soils when too wet or dry. The benefits of improving soil structure for the growth of plants, particularly in an agricultural setting, include: reduced erosion due to greater soil aggregate strength and decreased overland flow; improved root penetration and access to soil moisture and nutrients; improved emergence of seedlings due to reduced crusting of
806-568: The soil. A wide range of practices are undertaken to preserve and improve soil structure. For example, the NSW Department of Land and Water Conservation advocates: increasing organic content by incorporating pasture phases into cropping rotations ; reducing or eliminating tillage and cultivation in cropping and pasture activities; avoiding soil disturbance during periods of excessive dry or wet when soils may accordingly tend to shatter or smear; and ensuring sufficient ground cover to protect
837-463: The subsoil is the soil substratum (or C horizon). Clay-based subsoil has been the primary source of material for adobe , cob , rammed earth , wattle and daub , and other earthen construction methods for millennia. Coarse sand , the other ingredient in most of these materials, is also found in subsoil. Although not sterile, subsoil is relatively barren in terms of soil organisms compared to humus-rich topsoil. This horticulture article
868-466: The sulfate liberates sulfur dioxide gas, a precursor to sulfuric acid . The reaction also produces calcium silicate , used in cement clinker production. Some component reactions pertaining to calcium sulfate: The main sources of calcium sulfate are naturally occurring gypsum and anhydrite , which occur at many locations worldwide as evaporites . These may be extracted by open-cast quarrying or by deep mining. World production of natural gypsum
899-403: The surface; and greater water infiltration, retention and availability due to improved porosity. Productivity from irrigated no-tillage or minimum tillage soil management in horticulture usually decreases over time due to degradation of the soil structure, inhibiting root growth and water retention. There are a few exceptions, why such exceptional fields retain structure is unknown, but it
930-421: The thickness of the hardsetting horizon is frequently equal to or greater than that of the cultivated layer.) Hardsetting soil is not permanently cemented and is soft when wet. The clods in a hardsetting horizon that has been cultivated will partially or totally disintegrate upon wetting. If the soil has been sufficiently wetted, it will revert to its hardset state on drying. This can happen after flood irrigation or
961-411: The water within its structure. The details of the temperature and time depend on ambient humidity. Temperatures as high as 170 °C (338 °F) are used in industrial calcination, but at these temperatures γ-anhydrite begins to form. The heat energy delivered to the gypsum at this time (the heat of hydration) tends to go into driving off water (as water vapor) rather than increasing the temperature of