Soil Chemistry 2
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Transcript of 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+