The principal raw material used in moulding is the moulding sand because it provides several major characteristics that may not be obtained from other materials. Moulding sand is defined as granular particles resulting from the breakdown of rocks, due to the action of natural forces, such as frost, wind, rain, heat and water currents. Rocks have a complex composition and sand contains most of the elements of the rocks.
Due to this reason, moulding sand differs considerably in different parts of the world. In nature, it is found on the bottom and banks of rivers and lakes. Moulding sand is classified into different categories according to the nature of its origin.
It is also called green sand and is collected from natural resources. It contains water as the only binder. It has the advantage of maintaining moisture content for a long time, having a wide working range of moisture content, permitting easy patching and finishing of moulds.
It is an artificial sand obtained by mixing relatively clay free sand, binder (water and bentonite) and other materials as required. It is a better moulding sand as its properties can be easily controlled by varying the mixture content.
In addition to it, there are certain varieties of special sands such as Zirconite, Olivin etc. These special sands are more expensive than silica and are, therefore, used only where their use is justified.
When sand is in its natural (more or less moist) state, it is referred to as green sand. It is a mixture of silica sand, with 18 to 30% clay and 6 to 8% water. The clay and water give bonding strength to green sand.
Green sand is generally used for casting small or medium sized moulds. Larger output can be obtained from a given floor space as the cost and delay involved in drying the moulds is saved. Coal dust is mixed in green sand to prevent defects in castings.
Dry sand moulding is employed for large castings. The moulds prepared in green sand are dried or baked to remove, almost, all moisture of the moist sand. The structure in the moulding boxes after drying becomes stronger and compact. Venting is therefore necessary but not to that extent, as in the case of green sand mould. For larger heavy moulds, cowdung, horse manure, etc. are mixed with the sand of coarser grains.
Loam sand contains upto 50% clay and dries hard. It also contains fire clay. It must be sufficiently adhesive to hold on to the vertical surfaces of the rough structure of the mould. Chopped stray and manure are commonly used to assist in binding. The moisture content is from 18 to 20%.
It is used directly next to the surface of the pattern and it comes into contact with the molten metal. Since, it is subjected to the most severe conditions, it must possess high strength and refractoriness. It is made of silica sand and clay, without the addition of used sand.
Different forms of carbon known as facing materials, (e.g., plumbago powder, ceylon lead or graphite) are used to prevent the metal from burning into the sand. Sometimes they are mixed with 6 to 15 times fine moulding sand to make mould facings.
The old, repeatedly used moulding sand, black in colour due to addition of coal dust and burning or coming in contact with molten metal is known as backing sand or floor sand or black sand. It is used to fill in the mould at the back of facing layer. It is weak in bonding strength because the sharp edges of sand grain become rounded due to high temperature of molten metal and burning of clay content.
The moulding boxes are separated from adhering to each other by spreading a fine sharp dry sand called parting sand. Parting sand is also used to keep the green sand from sticking to the pattern. It is clean clay- free silica sand. Burnt core sand could also be used for this purpose.
It is used for making cores. It is silica sand mixed with core oil (linseed oil, rosin, light mineral oil and other binders). For the sake of economy pitch or flour and water may be used as core sand for large cores.
In CO2 sand, the silica grains, instead of being coated with natural clay, are coated with sodium silicate. This mixture is first packed around the pattern and then hardened by passing CO2 through the interstices for about a minute. The sand thus sets hard and produces a strong mould.
Shell sands are synthetic sands coated with phenol or urea-formaldehyde resins and cured against a heated pattern to produce very strong, thin shell. No back up sand is required to provide support for the weight of the casting. Since, alloys solidify at high temperatures, the resins are not dissociated. But moulds disintegrate when casting has solidified due to breaking up of chemical bond by heat from solidifying casting.
Usually, facing sand is first applied on the pattern, so that only it comes in contact with the molten metal. This sand is refractory enough so as not to get fused and burnt on coming in contact with the metal.
These are slurries of fine ceramic grains. These are applied over the mould surfaces to minimise fusing of the facing sand grains. These also produce smoother surface on casting due to filling up of the interstices.
Silica, clay (bond) and moisture are three chief constituents of the moulding sand. Silica in the form of granular quartz (itself a sand) is the chief constituent of moulding sand. Silica sand contains from 80 to 90% silicon oxide and is characterised by high softening temperature and thermal stability. It imparts refractoriness, chemical resistivity and permeability to sand.
It is specified according to size and shape. Sand grains could be fine, medium, or coarse as regards size, and could be rounded, semi-angular or compounded, as regards shape. Fine sand is desirable for small and intricate castings. As fine grains lie close, permeability is poor. Medium sized sand is used in bench work and light floor work. Coarse particles are used for large castings to permit gases to escape.
Grain size is determined by passing sand through screens of sieves. Rounded grains have least contact with one another and lack in strength; permeability is high. Sub- angular grains are comparatively less permeable than round grains. Angular grains having defined edges give more strength and less permeability. Compounded grains being hard lumps are not preferred.
The felspar when decomposed, becomes clay which imparts plasticity to the moulding sand in the moist state, i.e. imparts necessary bonding action and strength in the presence of moisture and increases its strength after drying. Normally, the amount of clay found in silica sand varies from 6 to 10%.
Clay actually consists of fine silt (mineral deposit having no bonding property) and fine clay, which imparts necessary bonding strength to mould sand so that mould does not lose its shape after ramming. However, it decreases permeability. Too much of clay causes cracking of the mould after drying.
Oxides of iron magnesia, soda potash, lime and water are the other substances, which are found in the moulding sand. A good moulding sand contains impurities below 2 per cent. Green sand moulding is carried with sand of low moisture content (3 to 5%). In dry sand moulding, more water is present when making mould, as it is beneficial in promoting dry bond strength after storing.
Many times coal dust is also added which makes the sand more open and helps to cool the mould after metal is poured. It absorbs fairly high amount of heat, preventing the sand grains from overheating and fusing. It also releases CO2 whose protective film helps to keep the metal and sand separated from each other.
Fig. 3.17 shows four type of sand grains. The success of a casting process depends to a great extent on compressive strength, permeability, (gas flow rate through sand specimens under specified differential pressure across it), deformation, flow ability, (ability of sand to flow around and over the pattern during ramming) and refractoriness of the moulding sand. Fig. 3.18 shows, how the moisture content in moulding sand (for a given sand-clay ratio) affects all these important properties of the moulding sand.
The general sources of receiving molding sands are the sea shores, rivers, lakes, deserts and granular elements of rocks. Molding sands can be classified mainly into two types namely natural or synthetic. Natural molding sands contains sufficient amount of binder material. Whereas synthetic molding sands are prepared artificially using basic sand molding constituents (silica sand in 85-91%, binder 6-11%, water or moisture content 2-8%) and other additives in proper proportion by weight with perfect mixing and mulling in suitable equipments.
Silica sand in form of granular quarts is the main constituent of molding sand having enough refractoriness which can impart strength, stability and permeability to molding and core sand. But along with silica small amounts of iron oxide, alumina, lime stone (CaCO3), magnesia, soda and potash are present as impurities. The chemical composition of silica sand gives an idea of the impurities like lime, magnesia, alkalis etc. present. The presence of excessive amounts of iron oxide, alkali oxides and lime can lower the fusion point to a considerable extent which is undesirable. The silica sand can be specified according to the sand grain size and the shape (angular, sub-angular and rounded) of the sand.
Binders can be either inorganic or organic substance. Binders included in the inorganic group are clay sodium silicate and port land cement etc. In foundry shop, the clay acts as binder which may be Kaolinite, Ball Clay, Fire Clay, Limonite, Fullers earth and Bentonite. Binders included in the organic group are dextrin, molasses, cereal binders, linseed oil and resins like phenol formaldehyde, urea formaldehyde etc. Binders of organic group are mostly used for core making. Among all the above binders, the bentonite variety of clay is the most commonly used. However, this clay alone cant develop bonds among sand grins without the presence of moisture content in molding sand and core sand.
The amount of moisture content in the molding sand varies from 2 to 8%. This amount is added to the mixture of clay and silica sand for developing bonds. This is the amount of water required to fill the pores between the particles of clay without separating them. This amount of water is held rigidly by the clay and is mainly responsible for developing the strength in the sand. The effect of clay and water decreases permeability with increasing clay and moisture content. The green compressive strength first increases with the increase in clay content, but after a certain value, it starts decreasing. For increasing the molding sand characteristics some other additional materials besides basic constituents are added which are known as additives.
Additives are the materials generally added to the molding and core sand mixture to develop some special property in the sand. Some commonly used additives for enhancing the properties of molding and core sands are coal dust, corn flour, dextrin, sea coal, pitch, wood flour, silica flour.
Coal dust is added mainly for producing a reducing atmosphere during casting process. This reducing atmosphere results in any oxygen in the poles becoming chemically bound so that it cannot oxidize the metal. It is usually added in the molding sands for making molds for production of grey iron and malleable cast iron castings.
Corn flour belongs to the starch family of carbohydrates and is used to increase the collapsibility of the molding and core sand. It is completely volatilized by heat in the sand mould, thereby leaving space between the sand grains. This allows free movement of sand grains, which finally gives rise to mould wall movement and decreases the mold expansion and hence defects in castings. Corn sand if added to molding sand and core sand improves significantly strength of the mold and core.
Sea coal is the fine powdered bituminous coal which positions its place among the pores of the silica sand grains in molding sand and core sand. When heated, sea coal changes to coke which fills the pores and is unaffected by water. Because to this, the sand grains become restricted and cannot move into a dense packing pattern. Thus, sea coal reduces the mould wall movement and the permeability in mold and core sand and hence makes the mold and core surface clean and smooth.
Pitch is distilled form of soft coal. It can be added from 0.02 % to 2% in mold and core sand. Pitch enhances hot strengths, surface finish on mold surfaces and behaves exactly in a manner similar to that of sea coal.
Wood flour is a fibrous material mixed with a granular material like sand. Wood flour is relatively long thin fibers prevent the sand grains from making contact with one another. wood flour can be added in between 0.05 % to 2% in mold and core sand. Wood flour volatilizes when heated, thus allowing the sand grains room to expand. Wood flour will increase mould wall movement and decrease expansion defects. Wood flour also increases collapsibility of both mold and core.
Silica flour is called as pulverized silica. Pulverized silica can be easily added up to 3% which increases the hot strength and finish on the surfaces of the molds and cores. It also reduces metal penetration in the walls of the molds and cores.
Backing sand or floor sand is used to back up the facing sand and is used to fill the whole volume of the molding flask. Backing sand is sometimes called black sand because of old, repeatedly used molding sand is black in color due to addition of coal dust and burning on coming in contact with the molten metal.
Core sand is used for making cores and it is sometimes also known as oil sand. Core sand is highly rich silica sand mixed with oil binders such as core oil which composed of linseed oil, resin, light mineral oil and other bind materials. Pitch or flours and water may also be used in large cores for the sake of economy.
Green sand that has been dried or baked in suitable oven after the making mold and cores is called dry sand. It possesses more strength, rigidity and thermal stability. Dry sand is mainly used for larger castings. Mold prepared in this sand are known as dry sand molds.
Facing sand forms the face of the mould. It is next to the surface of the pattern and it comes into contact with molten metal when the mould is poured. Initial coating around the pattern and hence for mold surface is given by facing sand. Facing sand have high strength refractoriness. Facing sand is made of silica sand and clay, without the use of already used sand. Different forms of carbon are used in facing sand to prevent the metal burning into the sand. A facing sand mixture for green sand of cast iron may consist of 25% fresh and specially prepared and 5% sea coal. They are sometimes mixed with 6-15 times as much fine molding sand to make facings. The layer of facing sand in a mold usually ranges between 20-30 mm. From 10 to 15% of the whole amount of molding sand is the facing sand.
Green sand is also known as tempered or natural sand which is a just prepared mixture of silica sand with 18 to 30% clay, having moisture content from 6 to 8%. The clay and water furnish the bond for green sand. It is fine, soft, light, and porous. Green sand is damp, when squeezed in the hand and it retains the shape and the impression to give to it under pressure. Molds prepared by this sand are not requiring backing and hence are known as green sand molds. Green sand is easily available and it possesses low cost. Green sand is commonly employed for production of ferrous and non-ferrous castings.
Loam sand is mixture of sand and clay with water to a thin plastic paste. Loam sand possesses high clay as much as 30-50% and 18% of water. Patterns are not used for loam molding and shape is given to mold by sweeps. Loam sand is particularly employed for loam molding used for large grey iron castings.
Parting sand without binder and moisture is used to keep the green sand not to stick to the pattern and also to allow the sand to the parting surface the cope and drag to separate without clinging. Parting sand is clean clay-free silica sand which serves the same purpose as parting dust.
In mechanized foundries where machine molding is employed. System sand is used to fill the whole molding flask. In mechanical sand preparation and handling units, facing sand is not used. The used sand is cleaned and re-activated by the addition of water and special additives. This is known as system sand. Since the whole mold is made of this system sand, the properties such as strength, permeability and refractoriness of the molding sand must be higher than those of backing sand.
Cohesiveness is property of molding sand by virtue which the sand grain particles interact and attract each other within the molding sand. Thus, the binding capability of the molding sand gets enhanced to increase the green, dry and hot strength property of molding and core sand.
After the molten metal in the mould gets solidified, the sand mould must be collapsible so that free contraction of the metal occurs and this would naturally avoid the tearing or cracking of the contracting metal. In absence of collapsibility property the contraction of the metal is hindered by the mold and thus results in tears and cracks in the casting. This property is highly required in cores.
As soon as the molten metal is poured into the mould, the moisture in the sand layer adjacent to the hot metal gets evaporated and this dry sand layer must have sufficient strength to its shape in order to avoid erosion of mould wall during the flow of molten metal. The dry strength also prevents the enlargement of mould cavity cause by the metallostatic pressure of the liquid metal.
Flowability or plasticity is the ability of the sand to get compacted and behave like a fluid. It will flow uniformly to all portions of pattern when rammed and distribute the ramming pressure evenly all around in all directions. Generally sand particles resist moving around corners or projections. In general, flowability increases with decrease in green strength and vice versa. Flowability increases with decrease in grain size of sand. The flowability also varies with moisture and clay content in sand.
The green sand after water has been mixed into it, must have sufficient strength and toughness to permit the making and handling of the mould. For this, the sand grains must be adhesive, i.e. they must be capable of attaching themselves to another body and. therefore, and sand grains having high adhesiveness will cling to the sides of the molding box. Also, the sand grains must have the property known as cohesiveness i.e. ability of the sand grains to stick to one another. By virtue of this property, the pattern can be taken out from the mould without breaking the mould and also erosion of mould wall surfaces does not occur during the flow of molten metal. The green strength also depends upon the grain shape and size, amount and type of clay and the moisture content.
Permeability is also termed as porosity of the molding sand in order to allow the escape of any air, gases or moisture present or generated in the mould when the molten metal is poured into it. All these gaseous generated during pouring and solidification process must escape otherwise the casting becomes defective. Permeability is a function of grain size, grain shape, and moisture and clay contents in the molding sand. The extent of ramming of the sand directly affects the permeability of the mould. Permeability of mold can be further increased by venting using vent rods.
Refractoriness is defined as the ability of molding sand to withstand high temperatures without breaking down or fusing thus facilitating to get sound casting. It is a highly important characteristic of molding sands. Refractoriness can only be increased to a limited extent. Molding sand with poor refractoriness may burn on to the casting surface and no smooth casting surface can be obtained. The degree of refractoriness depends on the SiO2 i.e. quartz content, and the shape and grain size of the particle. The higher the SiO2 content and the rougher the grain volumetric composition the higher is the refractoriness of the molding sand and core sand. Refractoriness is measured by the sinter point of the sand rather than its melting point.
In addition to above requirements, the molding sand should not stick to the casting and should not chemically react with the metal. Molding sand need be economically cheap and easily available in nature. It need be reusable for economic reasons. Its coefficients of thermal expansion need be sufficiently low.
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The dominant minerals in both the sand and silt fractions are residues of the minerals in the original parent material; hence, they are known as primary minerals. Foremost is the weathering-resistant mineral quartz (SiO2). Other minerals often present, though in smaller amounts, are mica, feldspars, zircon, haematite, and limonite. If the soil is not strongly leached, the sand and silt fractions may also contain fragments of calcite and dolomite.
The clay fraction differs fundamentally from the sand and silt fractions, not only in grain size but generally in mineralogy as well. It typically contains a class of minerals that are products of chemical weathering and re-precipitation, known as secondary minerals or clay minerals. They consist largely of aluminosilicates and hydrated oxides. The most prevalent of the clay minerals are crystalline, though some clay minerals are amorphous; e.g., allophane and imogolite. The crystal-structured aluminosilicates are of three main types: (a) minerals with 1:1 alternating sheets of alumina and silica, such as in kaolinite and halloysite; (b) minerals with 2:1 alternating layers of silica and alumina, such as illite, vermiculite, and smectite; and (c) minerals with 2:2 layers, such as chlorite.
The charges on the faces of the layered clay minerals result from the partial isomorphous substitution of cations of lesser valence for those of higher valence. Such charges are permanent in the sense that they are independent of the pH of the enveloping soil solution. In addition, dissociated protons from the edges (rather than the faces) of clay crystals result in negative charges that are pH dependent.
The most loosely structured of the layered clay minerals is smectite, in which the crystal layers are so weakly bonded that water molecules can enter the interlayers and cause the mineral to swell, somewhat like an accordion. The tendency of this mineral to expand upon wetting and to shrink when drying may cause the soil to destabilize building foundations and to warp roadways.
Some of the most important and prevalent primary minerals in soils are the feldspars (Table 2.2). They are common in the sand and silt fractions of soils and can also be found in the clay fraction. They comprise 59.5, 30.0, and 11.5% by weight of igneous rock, shale, and sandstone, respectively. Metamorphic rocks also contain feldspars (Huang, 1989). The K feldspars are important sources of K in soils and often compose a major component of the mineral form of soil K (Sparks, 1987). Feldspars are anhydrous three-dimensional aluminosilicates of linked SiO4 and AlO4 tetrahedra that contain cavities that can hold Ca2+, Na+, K+, or Ba2+ to maintain electroneutrality (Huang, 1989). Feldspars can be divided into two main groups, alkali feldspars, ranging in composition from KAlSi3O8 to NaAlSi3O8, and the plagioclases, ranging from NaAlSi3O8 to CaAl2Si2O8.
Olivines, pyroxenes, and amphiboles are known as accessory minerals in soils and are found in the heavy specific gravity fractions. Pyroxenes and amphiboles are ferromagnesian minerals with single- and double-chain structures, respectively, of linked silica tetrahedra. They make up 16.8% by weight of igneous rocks (Huang, 1989). Olivines are green neosilicates in which Mg2+ and Fe2+ are octahedrally coordinated by O atoms. They are prevalent in igneous rocks, are sources of soil micronutrients, and are generally present in quantities smaller than those of pyroxenes and amphiboles (Huang, 1989).
Primary minerals are formed at elevated temperatures and inherited from igneous and metamorphic rocks, sometimes through a sedimentary cycle. In most soils, the sand and silt fractions consist largely of primary minerals. In the clay fraction of weakly weathered soils, primary minerals are present but are minor constituents of the clay fraction of most agricultural soils.
The most abundant primary minerals in soils are quartz and feldspars, just as they are the dominant rock-forming minerals in the Earth's crust. Quartz consists of a continuous framework of silica tetrahedra and is the main form of free silica occurring in soils. Feldspars are anhydrous, three-dimensional aluminosilicates containing varying amounts of Na, K, and Ca, and occasionally of other large cations such as Ba. Feldspars are found in virtually all sediments and soils in quantities that vary with the nature of the parent material and the stage of weathering. Weathering of feldspars is an important Earth-surface geochemical process. Feldspars are stores of the macronutrients K and Ca; they play a substantial role in overall K dynamics of soils.
Micas are 2:1 phyllosilicates with tightly held, nonhydrated, interlayer cations balancing a high layer charge. They are largely of primary origin, occur extensively, and serve as precursors for expansible 2:1 phyllosilicates, vermiculites, and smectites in soil environments. Micas are often present in soils as components of particles that have been only partially transformed to expansible 2:1 minerals through interstratification with the other minerals or the formation of mica cores surrounded by expanded zones. Micas are important reserves of soil K for growing plants. The K-supplying power of a soil depends to a considerable extent on the nature and amount of micas present and their dynamics of K release.
Olivines, pyroxenes, and amphiboles are important accessory primary minerals that occur in small but significant amounts. Olivines are olive-green nesosilicates in which Mg and Fe2+ are octahedrally coordinated by O atoms. Pyroxenes and amphiboles, which are inosilicates, are ferromagnesian minerals with single- and double-chain structures, respectively, of linked silica tetrahedra. The variety of isomorphous substitution possible in olivines, pyroxenes, and amphiboles and their relative ease of weathering make them excellent source minerals for Ca, Mg, and trace elements in soils. Furthermore, the ability of nesosilicates and inosilicates to catalyze the abiotic formation of humic substances in soils merits attention.
Soil texture is one of the most common features used by scientists and laymen to describe soils. Texture refers to the relative abundance, by weight, of the size fractions sand, silt, and clay. In all textural classification systems, the sum of all particles (by weight) less than or equal to 2mm is equal to 100% (Figure 1), and is referred to as the fine-earth fraction. There are several textural classification systems used, with varying ranges of particles sizes (Figure 2).
Textural descriptions are commonly used, because it is cumbersome to give the percentage of sand, percentage of silt, and percentage of clay within a description. Commonly, field estimates of sand, silt, and clay are used, and the soils are placed into textural classes. These textures are verified by laboratory analysis. The mineralogy of the clay may affect estimates of clay in field textures. The kaolinitc clays expand less and tend to be less expressed than smectitic clays which may result in an underestimation of the percentage of clay.
Soil texture relates to many of the ways a soil performs. If a soil is coarse (sandy), water tends to move through it quite well, but it may not retain enough water for plant growth. If a soil is clayey, water will probably move slowly but it should retain water for plant growth. The shrinkswell potential of clayey soils varies quite widely depending on the type of clay minerals present. If the soil is high in smectitic clay minerals, it will shrink and swell as the moisture content of the soil changes. If the shrinking and swelling are not considered in the design of infrastructure, it will cause problems for roads and foundations. However, if the clay mineral is primarily kaolinite and other low-activity minerals, construction on such soils should not be a problem. Also, if the particles larger than 2mm are of sufficient quantity to be important, the textural classes are prefixed with modifiers. These separates are classed as gravel, cobbles, or stones (Figure 2). (See TEXTURE.)
Humans in distant prehistory discovered earth collected from certain soils and sediments could be molded when moist and heated over hot coals to produce durable, heat-resistant vessels. The key component was a fine mineral fraction know as clg (Oxford English Dictionary, 1989).
Drawing on the review by Blott and Pye (2012), the soil physicist Whitney working at the Maryland Agricultural Experiment Station chose 5 m as the grain size separating silt fraction from the clay fraction (Whitney, 1891). The Bureau of Soils adopted the Whitney particle-size classification when Whitney became its head. Swedish soil scientist Atterberg (1905), famous for his soil plasticity classification scheme (Atterberg, 1911), developed a particle-size classification independent of Whitney. Atterberg (1905) chose 2 m as the grain size separating silt from clay. In 1938 the Bureau of Soils officially adopted 2 m as the grain size separating silt from clay (Knight, 1938), making no mention of the Atterberg classification.1 The following quote from Knight (1938) gives the rationale for changing the grain size separating silt from clay.
It is hoped that these changes will make the data from mechanical analysis more useful. The change to 2 microns for the upper limit for clay has the effect of bringing about a better correlation between field texture classification and classification from the data of mechanical analysis. Knight (1938)
Mechanical analysis refers to a method that quantifies grain size by measuring sedimentation rate.2 Field texture classification, on the other hand, relies on the ribbon test to distinguish silt from clay based on plasticity (cf. Box 3.1). The American Society for Testing and Materials ASTM method for measuring the plastic limit (ASTM, 2014) employs a similar technique.
Soil mineral particles are typically separated into three particle-size fractions: sand (0.052.0mm), silt (250 m), and clay (<2 m). Soil texture is usually a complex size distribution represented by the relative proportions of the three particle-size fractions (NCSS, 2014).
A highly plastic soil contains sufficient clay to be molded into a ribbon greater than 5cm in length without cracking. A medium plastic soil forms a ribbon 2.55cm in length without cracking. A slightly plastic soil forms a ribbon less than 2.5cm without cracking (Fig. 3.1). Soil textural classes that are typically nonplastic include sand, loamy sand, and silt. These textural classes contain less than 715% clay, depending on silt content.Fig. 3.1. Molding moist clay between forefinger and thumb produces a clay ribbon.
The 1938 revised standard for the upper limit in clay particle-size (Knight, 1938) is notable for two reasons. First, the modest shift from 5 to 2 m recognized the importance of plasticity in defining clay particle size. Second, the 2-m limit is identical to the limit chosen by Atterberg (1905) who clearly understood the importance of clay plasticity. A third major particle-size classification (Wentworth, 1922) explicitly recognized plasticity when naming the finest particle-size class.
After consideration of a number of alternative terms, the term clay has been selected as most likely to be acceptable to geologists for the finest clastic sediments. A few geologists objected to the term on the ground that it implied plasticity or that it referred to a definite chemical composition. It is the view of the writer and of many other geologists that nearly all clastic materials of this grade consist largely of the hydrous aluminum silicates which make up the clay of the chemist and also that the material is always more or less plastic. There is, therefore, in his opinion a common ground for the geologist and chemist without an insistence on the use of the term clay for the pure chemical compounds kaolin or other minerals of this group. Wentworth (1922)
Besides emphasizing the importance of clay plasticity, Wentworth (1922) also refers to the emerging understanding of clay-fraction mineralogy. Petrographic microscopy could not resolve clay-size particles. Chemical analysis suggested the clay-size fraction had a chemical composition distinct from silt and sand mineral grains.3 Early X-ray diffraction studies revealed the presence of crystalline minerals, resulting to the adoption of the term clay minerals to acknowledge the presence of yet to be identified minerals.
Today the clay minerals connotation encompasses several mineral groups within the phyllosilicate subclass. Clay minerals are phyllosilicates whose natural occurrence is confined to the clay particle-size class. Clay minerals differ from the phyllosilicates found in igneous rocks because they are chemical alteration products formed at Earths surface, where the conditions favor the formation of the hydrous, fine-grain minerals. Clay minerals, by virtue of their extremely small particle size and high surface-to-volume ratio, are one of the most chemically active components of soils and sediments.
Our first task is to identify where clay minerals appear in the geochemical weathering sequence. Understanding mineral structures, while a worthy and interesting subject in and of itself, is generally not essential for environmental chemistry at the level of this book. Clay minerals, however, display physical and chemical behavior that simply cannot be appreciated without a grasp of their crystal structure. This is our second task. With these basics in hand, we are prepared for our final task: a description of clay mineral physical behavior and chemistry.
For agronomic and other soil-science applications, the textural designations are quite straightforward. It is assumed that the organic fraction is minimal and that only three size separates, sand, silt, and clay fractions, are being considered. In the USDA system, the fractions of the three size separates, sand (20.05mm), silt (0.050.002mm), and clay (0.002mm) are plotted and 12 distinct textures are assigned according to the mix of separates (Figure 8). For the textural diagram, the assumption is:
A review of tracers of past ocean circulation would not be complete without mentioning some of the sedimentological and paleontological approaches to reconstructing past ocean flow. Sediment grain size has been used to reconstruct current intensity since the 1970s (Ledbetter and Johnson, 1976). More recent grain-size studies emphasize the mean size of the sortable silt fraction (>10 m), which is least likely to display a cohesive behavior (McCave et al., 1995). Stronger currents show larger sortable silt size due to changes in both deposition and winnowing with current speed.
Benthic foraminifera assemblages have been related to distinct deep-water masses and have been used for tracking these water masses during the past (Streeter, 1973; Schnitker, 1974). In subsequent years, it has been determined that the deep-sea benthic assemblage responds strongly to the flux of organic material from overlying surface productivity (e.g., Miller and Lohmann, 1982; Loubere, 1991), limiting its use as a water mass tracer to low-productivity regions (Schnitker, 1994).
We investigated the relationships between soil texture and organic C in different organic matter fractions. For the fraction >150 m, we present the sum only of the light, intermediate and heavy fractions, because the density fractions did not give any additional information with respect to the capacity of soils to protect C physically.
There was a highly significant correlation (r=0.91) between the clay and silt content of grassland soils and the amount of C associated with this fraction (Table T, Fig.3; C associated with the fraction <20 m=6.9+0.29 % particles <20 m). The amounts of C in the fractions 20150 m and >150 m did not correlate significantly with the clay and silt fraction and varied considerably between soils with similar clay and silt content (Table1).
Fig.3. Relationship between C in the particle size fraction <20 m (clay and silt in g/kg soil) and the percentage of soil particles <20 m in the top 10 cm of grassland soils, the top 10 cm of an arable field in Tynaarlo, the top 20 cm of a maize field in Cranendonck and the top 25 cm of the FYM treatments of the long term experiment in the sandy and clay soil.
In spite of the fact that the arable field from Tynaarlo, the maize field from Cranendonck and the deeper layers of the grassland and maize field from Cranendonck had much lower total amounts of soil C than the corresponding top layers of the grassland fields, the amounts of C associated with the clay and silt fraction were not less (Table3). The amounts of C in the fractions 20150 m and >150 m were less in the arable field from Tynaarlo and the maize field from Cranendonck than in the corresponding grassland sites, and decreased with increasing depth at Cranendonck (Table3). The sum of C in the different fractions was generally close to the total amount of soil C (Table3).
Table3. Amounts of C in different size fractions and total soil C in the top 10 cm of the grassland and arable soil in Tynaarlo and the top 20 cm and the soil layers at 3040 and 6080 cm depth in the grassland and maize field in Cranendonck (g/kg)
The application of chaff, lucerne and FYM to the sand and clay soil for 25 years (long term experiment of objective 1) resulted in a considerable increase in total soil C in comparison with the treatment where no organic residues were applied (Table4). In the sandy soil, the increase was concentrated in the 20150 m fraction, while in the clay soil most of the increase took place in the <20 m fraction (Table4). In the clay soil, the sum of C in the fractions was close to the total amount of soil C; in the sandy soil, the sum of C in the fractions was 40% higher than total soil C for the no input treatment, while in the other treatments the sum of C in the fractions was again close to total soil C (Table4). No explanation can be offered for the high recovery in the sand soil without input.
Table4. Amounts of C in different size fractions after 25 years of no input or annual applications of chaff, lucerne and FYM (g/kg) and the percentage of increase in soil C (in comparison with the no input treatment) that is associated with the fraction<20 m for the sand and clay soil in Haren
Soil conditions are highly variable across a broad range of spatial and temporal scales, including down to the scale of individual soil microaggregates and pores at which single-celled microorganisms typically interact with the soil environment. This variation in conditions has important consequences for microbial community dynamics, ecosystem processes, and interactions with plants. A landscape, in ecology, is the particular spatial arrangement of components of the environment that are important in some way to population dynamics of a given species. Landscapes usually include patches of multiple habitats, as well as variability in conditions that affect habitat quality. Unlike some definitions of the term landscape, this definition does not link landscapes to a particular spatial scale. Instead, it recognizes that landscapes are different for different organisms, depending on the spatial scales over which the organisms interact with the environment (Wiens, 1997).
The habitat that is present in the largest proportion in a landscape, and also has the greatest connectivity, is considered the habitat matrix, within which other habitat patches are distributed. In most soils, the habitat matrix is dominated by minerals and nonparticulate, humified organic matter and is not directly exposed to root exudates. We refer to this as mineral bulk soil. Mineral bulk soil typically harbors a large diversity of microbial species, the majority of microbial biomass, and dominates soil biotic community composition. It is thought that many of the microorganisms in mineral bulk soil are either dormant or nearly dormant, due to a lack of labile organic matter or other resources. However, these dormant microbes rapidly become active if conditions change. Mineral bulk soil can be further divided into pore size classes (micropores, mesopores, and macropores), mineral size classes (clay, silt, and sand fractions), or aggregate size classes (microaggregates and macroaggregates), which may reflect further variability in habitat conditions (Young et al., 2008).
Alternative nonmatrix habitat patches that exist within mineral bulk soil can be created by disturbances or heterogeneity of environmental resources or modulators. After a disturbance, the nonmatrix habitat that is created is defined by an absence of competitively dominant species and is characterized by an abundance of resources due to a lack of competition. Environmental heterogeneity embedded within mineral bulk soil can be caused by many factors, including biological activity, mineralogy, or hydrology. Many soil habitat patches are created through an increased supply of nutrients or labile organic matter and are therefore areas of increased biogeochemical activity. The rhizosphere, fecal matter, and decomposing plant tissue are important examples of this type of habitat. These habitats harbor increased microbial biomass with a differing taxonomic composition (Blackwood and Paul, 2003). Some microorganisms with hyphal growth forms (e.g., many fungi) are able to interact with the environment on a larger spatial scale than are individual rhizospheres or decomposing organic matter particles and therefore integrate over multiple patches of this type. This difference in life-form has important consequences for community and population dynamics and ecosystem processes in the context of spatial variation in soil (Collins et al., 2008; Watkinson et al., 2006).
At larger spatial scales, many types of environmental variation are known to affect the community composition, biomass, and activity of microbes in the soil matrix and other embedded habitats described earlier. Growth of different plant species, and in some cases plant genotypes and developmental stages, cause divergence in the composition of soil microbial communities (e.g., Houlden et al., 2008; Osanai et al., 2013), which can have important consequences for plant health (Berendsen et al., 2012; Berg and Smalla, 2009). Plant species affect microbial communities by releasing different suites of compounds into the rhizosphere and during tissue decomposition, from simple organic acids to complex secondary metabolites. Plants also interact directly with microbial symbionts that may be beneficial or harmful through surface compounds. Microorganisms that proliferate in response to a plant species are typically recruited from the surrounding soil. The microorganisms that proliferate in response to a particular plant species are normally constrained by soil, ecosystem, and land use type (Berg and Smalla, 2009; Lundberg et al., 2012), which can also greatly influence soil microbial community composition. Soil pH is often found to have the best correlation with microbial community composition (Lauber et al., 2009; Tripathi et al., 2012). However, there are many confounded differences among soil types, ecosystem types, and land uses, and it is likely that a complex combination of disturbance and soil factors is involved in differentiating microbial communities at the regional scale.
A summary of the textural characteristics of the Cerrado soils is shown in Fig. 21. The median for sand, silt, and clay percentage were 48.6, 15.3, and 33.5, respectively. The range for the sand fraction (20.05mm) was 4.393.9%, with the vast majority of the soils (90.0%) containing 2060% sand. For the silt fraction (0.050.002mm) the observed range varied from 1.6 to 57.4% and most samples (93.4%) contained less than 30% silt. The data for the clay fraction (<0.002mm) in these soils showed the range from 4.5 to 72.4%. There was a broad distribution for this variable with 89.8% of the samples being in the range of 1560% clay.
These data show that soils under Cerrado vegetation in Brazil vary quite widely in terms of textural classes. They range from clayey to sandy and include most other intermediate classes. This extreme variation in texture seems to be one of the most important parameters that should be considered in the characterization and management of these soils.
Early studies had shown that a variation in texture was reported to have a direct effect on the water-holding capacity of these soils (Medina and Grohman, 1966; Ranzani, 1971; Wolf, 1975b). Also the variation in phosphorus fixation had been related to variation in texture (Weaver, 1974; Bahia Filho, 1974). Textural changes may also be involved in the different behavior and responses to P fertilizer application.
Since the cost of fertilizers for improvement of these soils is relatively high and since good yields normally can be obtained only with rather large rates of fertilizers, efforts must be made to determine the correct economical rates of fertilizer to be used on each soil. It seems that texture will be one of the main parameters to be considered in achieving an adequate quantification of these rates.