The Physical Soil Properties Environmental Sciences Essay

The Physical landed estate Properties Environmental Sciences Essay consentaneous grounds be composed of five main components mineral particles derived from rocks by persisting organic materials hoummos from dead and decaying fix material grease wet supply in which nutritive elements ar dissolved modify line of descent both carbon dioxide and atomic number 8 and living organisms including bacteria that help form decomposition. flaws differ in their generousness levels, beca riding habit they slang incompatible proportions of these components and because the mineral particles throw been bear on to diametric degrees by weathering. Age of defect minerals, prevailing temperatures, rainfall, l all(prenominal)ing and body politic physico-chemistry ar the main factors which de conditionine how much a particular smear will weather (Sinha and Shrivastava, 2000). filth thus, is burning(prenominal) to everyone either directly or indirectly. It is the natural bodi es on which bucolic yields grow and it has fragile ecosystem (Sinha and Shrivastava, 2000). South Africa ranks among the countries with the exaltedest treasure of income ine part in the world (Aliber, 2009). Compargond to other middle income countries, it has extremely utmost levels of absolute pauperization and sustenance insecurity threat (FAO, 2009). As part of this, a potential drop contributor to food security might be wretchedly-scale agricultural w atomic number 18. Aliber (2009) indicated that input support targeting small switcher farmers could boost production and food security. Utilisation of wild arable lands and subsistence agriculture might be one option to total to incomes and/or savings, as well as to encourage food variegation (Altman et al., 2009). Land with superior agricultural suitability is considered to redeem great spacious security with regards to both agricultural production and development. From a planning perspective, high agricultural f lexibility is so considered an appropriate ginmill of high quality agricultural land that is highly productive and fertile.Only a small proportion of worlds demesnes have a very good level of birthrate, some of which have only good to medium birth rate and some have very low fetidness, and argon lots referred to as marginal vulgarisms (Ashman and Puri, 2002). long-familiar fertile basiss ar deep alluvial body politics formed from river mud, organic function- rich daubs on loess material, nutrient rich Vertisols and vol sub bodily twistic landed estates (Brady and Weil, 2004). Under low exercisement, country rankness can be seriously depleted and undercoats may conk use slight for agriculture.2.2. SOIL PHYSICO-CHEMISTRY obscenity is a natural medium on which agricultural products grow and it is dependent on several factors much(prenominal) as fertility to be considered productive (Shah et al., 2011). The fertility of the dent is depended on preoccupation of foulness nutrients, organic and inorganic materials and urine. These blot physico- chemic properties argon classified as being physical, chemical and biological, which greatly ferment obscenity fertility (Ramaru et al., 2000). To manage land fertility, knowledge and understanding of these properties is required (as discussed below).2.2.1. Physical farming properties(i) dominion textureSoil texture refers to the relative proportions of the various size groups of person particles or grains in a state (Rowell, 1994). It is dependent on the mixture of the different particle sizes present in the dry land. Based on these different sizes, blur particles are classified as sand (0.05- 2mm), choke off (0.002-0,5mm) and frame (Clay particles hold larger quantities of irrigate and nutrients, because of their large come in areas (Brady and Weil, 1999). This dimension causes the swelling and shrinking of the Great Compromiser defacements, but only those with smectitic group of form minerals. The large surface area of trunk particles gives nutrients numerous binding sites in particular when the surface charge density is high, which is part of the reason that fine rough-textured disgraces have such high abilities to retain nutrients (Velde, 1995). The pores amongst clay particles are very small and complex, so bm of both air and water is very slow (Brady and Weil, 1999). Clay particles are negatively chargedly supercharged because of their mineralogical composition. Soils with such particles unremarkably have high CEC and can retain water and engraft nutrients thus such primers are considered to be fertile and good for go under crop (Brady and Weil, 1999).The knowledge of the proportions of different-sized particles in soils is critical to understand soil vogue and their way. Since sand particles are comparatively large, so are the voids surrounded by them, which put up free drainage of water and entry of air into the soil (Brady and W eil, 2002). The intimation of free drainage in sandy soil is that soil nutrients are easily washed surmount into the soil and become inaccessible for use by seed downs (Brady and Weil, 2002). Sandy soils are considered non-cohesive and because of their large size, have low particularized surface areas and thus have low nutrient retention susceptibility (Rowell, 1994). Sand particles can hold little water due to low specific surface area and are prone to drought, therefore have a very low CEC and fertility status (Petersen et al., 1996).The pores between silt particles are much smaller than those in sand, so silt retains to a greater extent water and nutrients (Rowell, 1994). Soils predominate by silt particles therefore have a higher fertility status than sandy soils and provides favorable conditions for be egression when other growth factors are favorable (Miller and Donahue, 1992).(ii) Soil structureThe term soil structure refers to the ar undulatement of soil particles i nto kernels (Six et al., 2000). Soil structure is affected by biological activities, organic subject field, and cultivation practices (Rowell, 1994). It influences soil water movement and retention, erosion, nutrient recycling, sealing and crusting of the soil surface, together with aeproportionn and soils geomorphologic stability, root penetration and crop yield (Lupwayi et al., 2001).Soil structure can be platy, prismatic, granular, crumbly, columniform and blocky (RCEP, 1996). An ideal soil structure for lay down growth is often described as granular or crumb-like, because it provides good movement for air and water through a variety of different pore sizes and it also affects root penetration (RCEP, 1996). An ideal soil structure is also motionless and resistant to erosion (Duiker et al., 2003). entire publication and humification processes reform structural stability, and can rebuild degraded soil structures (Brady and Weil, 1999). and so it is vital to return or add o rganic material to the soil and to maintain its biological performance in order to enhance soil structure for appoint growth. golden soil structure and high aggregate stability are therefore vital to improving soil fertility, increasing agronomic productivity, enhancing porosity and change magnitude erodibility.(iii) Water retention strengthWater holding cognitive subject matter refers to the measuring stick of water that the soil is capable of storing for use by marks (Brady and Weil, 1999). Soil water is held in, and flows through pore spaces in soils. Soil water can be described into the following stages gravitational, capillary, and hygroscopic, based upon the naught with which water is held by the soil solids, which in turn governs their behavior and availability to plants (Rowell, 1994).Water holding qualification is an primary(prenominal) factor in the choice of plants or crops to be liberal and in the design and management of irrigation systems (Brady and Weil, 1999). The total amount of water on tap(predicate) to plants maturement in field soils is a function of the rooting foresight of the plant and sum of the water held between field capacitance and wilt share in each of the horizons explored by the roots (Brady and Weil, 1999). Field capacity is the amount of soil moisture or water content held in soil after superabundance water has drained away and the rate of downward movement has materially mitigated, which usually takes place within 2-3 long time after a rain or irrigation in pervious soils of reproducible structure and texture (Govers, 2002).The ability of the soil to provide water for plants is an of the essence(predicate) fertility characteristic (RCEP, 1996). The capacity for water storage varies, depending on soil properties such as organic matter, soil texture, mountain density, and soil structure (RCEP, 1996). This is explained by the degree of soil calculus, where problems will arise if excessive compaction breathes which would results in developmentd bulk density, a decrease in porosity and aeration and poor water drainage (Gregory et al., 2006), all resulting in poor plant growth.(iv) Electrical Conductivity (EC)Soil galvanising conductivity (EC), is the ability of soil to conduct electrical current (Doerge, 1999). EC is expressed in milliSiemens per thousand (mS/m) or cm (cm/m). Traditionally, soil scientists used EC to estimate soil salinity (Doerge, 1999). EC esteemments also have the potential for estimating variation in some of the soil physical properties such as soil moisture and porosity, in a field where soil salinity is not a problem (Farahani and Buchleiter, 2004). Soil salinity refers to the presence of major dissolved inorganic solutes in the soil aqueous phase, which consist of soluble and readily dissoluble salts including charged species (e.g., Na+, K+, Mg+2, Ca+2, Cl, HCO3, NO3, SO42 and CO32), non-ionic solutes, and ions that combine to form ion pairs (Smith and Doran, 1996).Salt tolerances are usually given in terms of the stage of plant growth over a range of electrical conductivity (EC) levels. EC greater than 4dS/m are considered saline (Munshower, 1994). Salt sensitive plants may be affected by conductivities below 4dS/m and salt tolerant species may not be impacted by concentrations of up to twice this maximum agricultural tolerance limitation (Munshower, 1994). Electrical conductivity is the ability of a solution to transmit an electrical current. The conduction of electrical energy in soil takes place through the moisture-filled pores that occur between individual soil particles. Therefore, the EC of soil is nail downd by the following soil properties (Doerge, 1999). Porosity, where the greater soil porosity, the more than easily electricity is conducted. Soil with high clay content has higher porosity than sandier soil. infatuation normally increases soil EC.. Water content, dry soil is much reduce in conductivity than mois t soil.. Salinity level, increasing concentration of electrolytes (salts) in soil water will dramatically increase soil EC.. Cation mass meeting capacity (CEC), mineral soil check intoing high levels of organic matter (humus) and/or 21 clay minerals such as montmorillonite, illite, or vermiculite have a much higher ability to retain positively charged ions (such as Ca, Mg, K, Na, NH4, or H) than soil lacking these constituents. The presence of these ions in the moisture-filled soil pores will enhance soil EC in the same way that salinity does.. Temperature, as temperature decreases toward the freezing point of water, soil EC decreases slightly. to a lour place freezing, soil pores become increasingly insulated from each other and overall soil EC declines rapidly.Plants are detrimentally affected, both physically and chemically, by excess salts in some soils and by high levels of mass meetingable Na in others. Soils with an accumulation of put backable Na are often characterize d by poor tilth and low permeability and therefore low soil fertility status, making them unfavorable for plant growth (Munshower, 1994).(v) Bulk Density (BD)Soil bulk density is defined as the mass of dry soil (g) per unit flashiness (cm3) and is routinely used as a measure of soil compaction (Gregory et al., 2006). The total volume includes particle volume, inter-particle void volume and internal pore volume (Gregory et al., 2006). Bulk density takes into account solid space as well as pore space (Greenland, 1998). consequently soils that are porous or well-aggregated (e.g. clay soil) will have lower bulk densities than soils that are not aggregated (sand) (Greenland, 1998).Plant roots cannot cut through compacted soil as freely as they would in non-compacted soil, which limits their access to water and nutrients present in sub-soil and inhibits their growth (Hagan et al., 2010). Compacted soil requires more frequent applications of irrigation and fertiliser to sustain plant gr owth, which can increase runoff and nutrient levels in runoff (Gregory et al., 2006).The bulk density of soil depends greatly on the soils mineral make up and the degree of compaction. High bulk density usually indicate a poorer environment for root growth, reduced aeration and undesirable changes in hydrologic function, such as reduced infiltration (Brady and Weil, 1999). The presence of soil organic matter, which is considerably lighter than mineral soil, can help decrease bulk density and thereby enhancing soil fertility (Hagan et al., 2010).2.2.2. Soil Chemical propertiesSoil chemical properties which include the concentrations of nutrients, cations, anions, ion exchange reactions and redox properties, but for the purpose of this check focus will be based on properties that have an implication on soil fertility including(i) Soil pHSoil pH is an central soil property that affects several soil reactions and processes and is defined as a measure of the acidity or alkalinity of th e soil (Bohn, 2001). It has considerable ensnare on soil processes including ion exchange reactions and nutrient availability (Rowell, 1994). Soil pH is measured on a scale of 0 to 14, where a pH of 7.0 is considered neutral, readings higher than 7.0 are alkaline, and readings lower than 7.0 are considered acrid (McGuiness, 1993). around plants are tolerant of a pH range of 5.5-6.5 which is near neutral pH range (Bohn, 2001). Soil pH is one of the around important characteristics of soil fertility, because it has a direct impact on nutrient availability and plant growth. Most nutrients are more soluble in acid soils than in neutral or slightly alkaline soils (Bohn, 2001). In strongly acidic soils the availability of macronutrients (Ca, Mg, K, P, N and S) as well as molybdenum and atomic number 5 is reduced. In contrast, availability of micronutrient cations (Fe, Mn, Zn, Cu and Al) is increased by low soil pH, even to the extent of toxicity of higher plants and microorganisms (Bo hn, 2001).The pH of a soil is also reported to affect so m whatever other soil properties (Brady and Weil, 1999), including nutrient availability, effects on soil organisms, fungi thrive in acidic soils, CEC and plant preferences of either acidic or alkaline soils. Most plants prefer alkaline soils, but there are a a few(prenominal) which need acidic soils and will die if placed in an alkaline environment (Brady and Weil, 1999).(ii) Cation Exchange Capacity (CEC)Cation exchange capacity is defined as the sum of the total of the exchangeable cations that a soil can hold or adsorb (Brady and Weil, 1999). A cation is a positively charged ion and most nutrients cations are Ca2+, Mg2+, K +, NH4+, Zn2+, Cu2+, and Mn2+. These cations are in the soil solution and are in dynamic equilibrium with the cations adsorbed on the surface of clay and organic matter (Brady and Weil, 1999).Clay and organic matter are the main descents of CEC (Peinemann et al., 2002). The more clay and organic matter (humus) a soil contains, the higher its CEC and the greater the potential fertility of that soil. CEC varies according to the type of clay. It is highest in montmorillonite clay, lowest in heavily weathered kaolinite clay and slightly higher in the less weathered illite clay (Peinemann et al., 2002). Sand particles have no capacity to exchange cations because it has no electrical charge (Brady and Weil, 1999).CEC is used as a measure of soil nutrient retention capacity, and the capacity to protect groundwater from cation contamination (Brady and Weil, 1999). It buffers fluctuations in nutrient availability and soil pH (Bergaya and Vayer, 1997). Plants obtain many of their nutrients from soil by an electrochemical process called cation exchange. This process is the key to understanding soil fertility (Rowell, 1994). Nutrients that are held by charges on a soil are termed exchangeable as they become readily procurable to plants (Rowell, 1994).The higher the CEC of a soil, the more n utrients it is likely to hold and the higher will be its fertility level (Fullen and Catt, 2004).Factors affecting cation exchange capacityThe factors affecting cation exchange capacity include the following (Brady and Weil 1999), soil texture, soil humus content, nature of clay and soil reaction.Soil texture influences the CEC of soils in a way that it increases when soils percentage of clay increases i.e. the finer the soil texture, the higher the CEC as indicated in remand 2. CEC depends on the nature of clay minerals present, since each mineral has its own capacity to exchange and hold cations e.g. the CEC of a soil dominated by vermiculite is much higher than the CEC of another soil dominated by kaolinite, as vermiculite is high activity clay unlike kaolinte which is low activity clay. When the pH of soil increases, more H+ ions dissociate from the clay minerals especially kaolinite, thus the CEC of soil dominated by kaolinite also increases. CEC varies according to the type o f soil. Humus, the end product of decomposed organic matter, has the highest CEC value because organic matter colloids have large quantities of negative charges. Humus has a CEC devil to five times greater than montmorillonite clay and up to 30 times greater than kaolinite clay, so is very important in improving soil fertility.Table 2.1 CEC values for different soil textures (Brady and Weil, 1999)Soil textureCEC range (meq/100g soil)Sand2-4Sandy loam2-12Loam7-16Silt loam9-26Clay, clay loam4-60(iii) natural MatterThe importance of soil organic matter in relation to soil fertility andphysical condition is widely know in agriculture. However, organic matter kick ins to the fertility or productivity of the soil through its positiveeffects on the physical, chemical and biological properties of the soil (Rowell, 1994), as follows physical stabilizes soil structure, improves water holding characteristics, lowers bulk density, bluish color may alter thermal properties chemical higher CEC, acts as a pH buffer, ties up metals, interacts with biological supplies energy and body-building constituents for soil organisms, increases microbial populations and their activities, source and sink for nutrients, ecosystem resilience, affects soil enzymes. Soil organic matter consists of a wide range of organic substances, including living organisms, carboneous remains of organisms which once work the soil, and organic compounds produced by current and past transfiguration of the soil (Brady and Weil, 1999).Soil organic matter plays a critical role in soil processes and is a key element of integrated soil fertility management (ISFM) (Brady and Weil, 2004). Organic matter is widely considered to be the single most important indicator of soil fertility and productivity (Rowell, 1994). It consists primarily of decayed or decaying plant and animal residues and is a very important soil component. Benefits of Organic matter in soil according to Ashman and Puri, (2002) include i ncreasing the soils cation exchange capacity and acting as food for soil organisms from bacteria to worms and is an important component in the nutrient and carbon cycles.Organic matter, like clay, has a high surface area and is negatively charged with a high CEC, making it an excellent supplier of nutrients to plants. In addition, as organic matter decomposes, it releases nutrients such as N, P and S that are bound in the organic matters structure, essentially imitating a slow release fertilizer (Myers, 1995). Organic matter can also hold large amounts of water, which helps nutrients move from soil to plant roots (Mikkuta, 2004).An important characteristic of organic matter in soil fertility is C N ratio. The C N ratio in organic matter of arable surface horizons commonly ranges from 81 to 151, the median being near 121 (Brady and Weil, 1999). The CN ratio in organic residues apply to soils is important for two reasons intense competition among the micro-organisms for available soi l nitrogen which occurs when residues having a high CN ratio are added to soils and it also helps determine their rate of decay and the rate at which nitrogen is made available to plants (Brady and Weil, 1999).(iv) Plant NutrientsPlants require 13 plant nutrients (Table 2.2) (micro and macro nutrients) for their growth. Each is evenly important to the plant, yet each is required in vastly different amounts (Ronen, 2007).Essential elements are chemical elements that plants need in order to apprehend their normal life cycle (Scoones and Toulhim, 1998). The functions of these elements in the plant cannot be fulfil by another, thus making each element essential for plant growth and development (Scoones and Toulhim, 1998).Essential nutrients are divided into macro and micronutrients as illustrated in Table 3. Macronutrients are those that are required in relatively high quantities for plant growth and can be distinguish into two sub groups, primary and secondary ones, (Uchida and Silv a, 2000). The primary macro-elements are most often required for plant growth and also needed in the great total quantity by plants. For most crops, secondary macro nutrients are needed in lesser amounts than the primary nutrients. The second group of plant nutrients which are micronutrients are needed only in trace amounts (Scoones and Toulhim, 1998). These micronutrients are required in very small amounts, but they are unsloped as important to plant development and profitable crop production as the major nutrients (Ronen, 2007).ClassificationElementFunction in plant growthSourceDeficiency symptoms and toxicitiesMacro nutrients PrimaryNitrogen (N) chlorophyl and Protein formationAir/Soil, applied fertilisers long-winded growth, stunted plants, chlorosis, low protein contentPhosphorus (P)Photosynthesis, Stimulates early growth and root formation, hastens maturitySoil and applied fertilisersSlow growth, retard crop maturity, purplish green coloration of leaveschiliad (K)Photosynt hesis and nzyme activity, starch and sugar formation, root growthSoil and applied fertilisersSlow growth, Reduced disease or pest resistance, development of white and white-livered spots on leavesMacronutrients secondaryCalcium (Ca)Cell growth and component of cell wallSoilWeakened stems, death of plants growing points, abnormal dark green appearance on foliage milligram (Mg)Enzyme activation, photosynthesis and influence Nitrogen metabolismSoilInterveinal chlorosis in sometime(a) leaves,curling of leaves, stunted growth,Sulfur (S)Amino acids, proteins and nodule formationSoil and animal manureInterveinal chlorosis on corn leaves, retarded growth, delayed maturity and light green to yellowish color in preadolescent leavesMicronutrients essentialIron (Fe)Photosynthesis, chlorophyll synthesis, constituent of various enzymes and proteinsSoilInterveinal chlorosis, yellowing of leaves between veins, twig dieback, death of entire limp or plantsManganese (Mn)Enzyme activation, metabolis m of nitrogen and organic acids, formation of vitamins and breakdown of carbohydratesSoilInterveinal chlorosis of five-year-old leaves, gradation of pale green coloration with darker color next to veins surface (Zn)Enzymes and auxins component, protein synthesis, used in formation of growth hormonesSoilMottled leaves, dieback twigs, decrease in stem lengthCopper (Cu)Enzyme activation, catalyst for respirationSoilStunted growth, poor pigmentation, wilting of leavesBoron (B)ReproductionSoilThickened, curled, limp and chlorotic leaves reduced floweringMolybdenum (Mo)Nitrogen fixation nitrate reduction and plant growthSoilStunting and lack of vigour (induced by nitrogen deficiency), scorching, cupping or rolling of leavesChlorine (Cl) antecedent growth, photosynthetic reactionsSoilWilting followed by chlorosis, excessive branching of sidelong roots, bronzing of leavesAdditional nutrientsCarbon (C)Constituent of carbohydrates and photosynthesisAir/ Organic matter henry (H)Maintains os motic balance and constituent of carbohydratesWater/Organic mattertype O (O)Constituent of carbohydrates and necessary for respirationAir/Water/ Organic matterTable 2.2 Essential plant elements, their sources and role in plants (Ronen,2007)Deficiency of any of these essential nutrients will retard plant development (Brady and Weil, 2004). Deficiencies and toxicities of nutrients in soil present unfavorable conditions for plant growth, such as poor growth, yellowing of the leaves and by chance the death of the plant as illustrated in Table 3 (Ahmed et al., 1997). Therefore proper nutrient management is required to achieve maximum plant growth, maximum economic and growth response by the crop, and also for token(prenominal) environmental impact.In addition to the nutrients listed above, plants require carbon, hydrogen, and oxygen, which are extracted from air and water to make up the bulk of plant weight (Brady and Weil, 1999). Achieving balance between the nutrient requirements of plants and the nutrient reserves in soils is essential for maintaining soil fertility and high yields, preventing environmental contamination and degradation, and sustaining agricultural production over the long term.2.2.3. Soil Biological properties(i) Soil organismsSoil organisms include mostly microscopical living organisms such as bacteria and fungi which are the invention of a healthy soil because they are the primary decomposer of organic matter (Brady and Weil, 1999). Soil organisms are grouped into two namely soil microorganisms and soil macro organisms (Table 2.3).Table 2.3 Soil Macro and microorganisms and their role in plant and soil (Brady and Weil, 1999)ClassificationOrganismsFunction in plant and/or soilSourceMicroorganismsBacteriaDecomposition of organic matterSoil surface and humus particlesActinomycetesSource of protein and enhance soil fertilitySurface layers of hatful landsFungiFix atmospheric nitrogen and enhance soil fertilitySoil (without organic matter)Algae Add organic matter to soil, improve aeration of swamp soils, and fix atmospheric nitrogenMoist soilsMacro-organismsNematodesThey can be applied to crops in large quantities as a biological insect powderSoil and plant rootsEarthwormsEnhance soil fertility and structural stabilityAerated soilsAnts and termitesSoil developmentDominant in tropical soilsSoil can contain millions of organisms that feed off decaying material such as old plant material, mulch unprocessed compost (Ashman and Puri, 2002), Microorganisms cost Soil organic matter is the main food and energy source of soil microorganisms (Ashman and Puri, 2002). Through decomposition of organic matter, microorganisms take up their food elements. Organic matter also serves as a source of energy for both macro and micro organisms and helps in performing various honorable functions in soil, resulting in highly productive soil (Mikutta et al., 2004).Macro-organisms such as insects, other arthropods, earthworms and nematodes live in the soil and have an important influence on soil fertility (Amezketa, 1999). They ingest soil material and relocate plant material and form burrows. The effects of these activities are variable. Macro-organisms improve aeration, porosity, infiltration, aggregate stability, litter mixing, improved N and C stabilization, C turnover and change reduction and N mineralization, nutrient availability and metal mobility (Amezketa, 1999 Winsome and McColl, 1998 and embrown et al., 2000).The various groups of soil organisms do not live independently of each other, but form an interlocked system more or less in equilibrium with the environment (Brady and Weil, 1999). Their activity in soil depend on moisture content, temperature, soil enzymes, dissolution of soil minerals and breakdown of toxic chemicals. all told have a tremendous role in the development of soil fertility (Alam, 2001). Their actions involve the formation of structural systems of the soils which help in the increase of a gricultural productivity (Alam, 2001).2.3. SOIL CLAY MINERALOGYThe clay fraction of soil is dominated by clay minerals which control important soil chemical properties including sorption characteristics of soils (Dixon and Schulze, 2002). Minerals are naturally occurring inorganic compounds, with defined chemical and physical properties (Velde, 1995). Minerals that are formed in the depths of a volcano are called primary minerals (Pal et al., 2000). Feldspar, biotite, crystal and hornblende are examples of primary minerals. These minerals and the rocks made from them are often not abiding when exposed to the weathering agents at the surface of the earth (Dixon and Schulze, 2002). These rocks are broken down (weathered) continuously into small pieces by exposure to physical and chemical weathering processes (Dixon and Schulze, 2002). whatsoever of the elements that are released during weathering, reform and crystallise in a different structure forming secondary minerals (Melo et al ., 2002). Secondary minerals tend to be much smaller in particle size than primary minerals, and are most commonly set up in the clay fraction of soils (Guggenheim and Martin, 1995). Soil clay fractions often contain a wide range of secondary minerals such as kaolinite, montmorillonite and aluminum hydrous oxides, whereas the sand or silt particles of soils are dominated by relatively inert primary minerals. The clay fraction is usually dominated by secondary minerals which are more chemically active and contribute the most to soil fertility (Melo et al., 2002). Two major secondary mineral groups, clay minerals and hydrous