Soil Chemistry 2

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    Soil collection of natural bodies of the earths surfacecontaining living matter that is able to support the growth of

    plants

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    The Soil Body

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    y Disintegration of rock by temperature, water, windand other factors

    y Reduces particle size not the chemistry of the material

    y Salts may crystallize in cracks in rock, placing internalpressure on the rock and splitting it apart

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    y Abrasiony particles are moved by physical means such as

    movement of water , action of wind, or action of

    organismsy Wetting and Drying

    y Water penetrates into rock particles and interacts withminerals

    y F

    reez

    ing and Thawingy Water trapped between spaces of rock or mineral freeze,

    causing expansion; thawing of water causes contraction

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    y Changes the chemical makeup of the rock and breaksit down

    y A common process is dissolution where minerals

    simply dissolve slowly in watery In hydrolysis, minerals react with hydrogen in the

    water molecule and splits the water apart

    y In hydration, water molecules join the crystalline

    structure of mineral, creating an easily weatheredmaterial

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    y In Oxidation-Reduction, under oxidizingenvironmental conditions, certain elements of amineral become oxidized causing charge imbalancey

    charge imbalance is neutralized through release ofoxidized ions or dissociation of cations

    y leads to formation of a coating around the mineral,slowing down rate of hydrolysis

    y In complexation, metals are released from mineralsbind with organic compounds to form complexes suchas fulvic acid

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    y Process of creating soil from parent material

    y Residual soils- formed in place of residuum of brokendown bedrock

    y Transported soils- develop from already weatheredmaterial

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    y Microbes influence chemical processes

    y Catalyze oxidation-reduction reactions in soil

    y Exude organic acids w/c play a critical role in acidity

    and trace element cyclingy Hydrogen dissociated attacks and decomposes soil

    minerals and carboxylate ion form soluble complexesby combining with metal cations released by mineral

    weathering

    y Acid concentration is higher that trace elements andhave short lifetimes but are continually produced bymicrobial activity

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    y Exists primarily because of ecological food chains

    y Continues to Carbon cycle because of detrital foodchain

    y Decomposers consume organic matter as food source,returning most of the carbon to atmosphere asCO2 byrespiration leaving behind a residue (humus); plantnutrient tied up in the bodies consumed are released

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    y Immbobilization absorption of available nutrient bysoil organism or plant changing it into unavailableorganic form

    y

    Mineralization conversion of elements in organicforms to inorganic ionic forms by decay

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    y Microbial conversion of gaseous nitrogen to organicnitrogen in the soil

    y They convert N2 into NH3 that plants can use

    y Symbiotic nitrogen fixation bacteria get thenutrients from the plant root and the plant absorbs thefixed nitrogen from the bacteria

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    y Mineralization of immobilized nitrogen occurs whenthe organism harbouring the N2 dies

    y N2 is mineralized into NH4+ and absorbed by plants

    y Some however are oxidized into nitrites and nitratesy NH4+ +2O2 --> NO3- + H2O + 2H+ (Nitrosomonas)

    y NO3- NO2- (Nitrobacter)

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    y Completes N2 cycle by converting nitrate ions into N2gas, w/c filters out of the soil

    y Certain bacteria use NO3- to oxidize organic matter

    during respirationy NO3- NO2- NO N2ON2

    y Intermediate NOx escapes into atmosphere

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    y Ten most abundant elements in the soil:y O, Si, Al, Fe, C, Ca, K, Na, Mg, Ti

    y Solid matter constitutes - 2/3 of soil volume usually

    of inorganic compoundsy Humic substances dark microbially transformed

    materials present in soil matter

    y Humic acids (C187H186O89N9S) and fulvic acids

    (C135H182O95N5S2) from humic substances

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    y Soil Minerals (Precipitation & Dissolution)

    y Organic Matter (Immobilization & Mineralization)

    y Adsorbed Nutrients (Cation Exchange)

    y Dissolved Ions (Soil Solution)

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    Tiny clay and humus particles that carry a slight electricalcharge

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    y Clay formed by association of structural units based onsilica sheets and alumina sheets

    y Si + O silica sheet

    y Alumina sheet aluminum + 6OH-y Most important are kaolinites, montmorillionites,vermiculites, illites, and chlorites

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    y Tiny particles of Fe2O3 and Al(OH)3

    y Common to humid tropical climates since silica andalumina leach out

    y Do not swell, are not sticky and have limited power tostore nutrients

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    y Residues of organic matter decay

    y Not crystalline and form irregular, round shapes

    y More power to adsorb nutrients than clay

    y Unstable and decays to CO2 over time

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    y Soil colloids usually carry a negative charge thatattracts cations from soil solution

    y OH- loses H+ on broken end of micelle

    y Isomorphous substitution- cation replaces anothercation of similar size in clay sheet

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    y Cations can move on & off particles, maintainingequilibrium between adsorbed ions in bulk solution

    y When an ion leaves, it is replaced (Cation exchange)

    y Ability of soil to hold nutrients relates to number ofcations it can attract to soil colloids

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    y Cations cluster most densely near middle of micellesurface, neutralizing negative charge

    y Factors that control selection of cation that leave

    micelle or adsorbed:y Relative bonding strength of each cation

    y Number of each type of cation (mass action)

    y Al3+>H+>Ca2+>Mg2+>K+=NH4+>Na+

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    y Describes acidity or alkalinity of soil

    y Results from interaction of soil minerals, ions insolution and cation exchange

    y High pH is caused by reaction of water and basiccompounds ofCa, Mg and Na

    y Low pH is caused by percolation of mildly acidic waterneutralizing the base and replacing base cations to H+