First lec soil chemistry

Download First lec soil chemistry

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  • CHEMICAL PROPERTIES OF SOIL COLLOIDES

  • : , , 2 : ( )

    Cation Exchange :

  • * A Clay Particle ( Clay Micelle) edge surface

  • Clay Particle--------------------------Clay particles carry negative charges ( )

  • 1- 2- OH-

  • *

    Isomorphous Substitution Mg2+ Al3+ Si4+ ( )Clay Particle with Net negative Charge

  • pH : alkaline mediaClay micelle-Si-OH + OH- Si-O- + H2O : acid media Clay micelle-Al-OH + H+ Al-OH2+ pH dependent charge

  • With Magnets--+-Unlikes AttractLikes RepelIn Soil+-++ ( )

  • ( )H+1Na+1 K+1 Li+1 Cs+1 Rb+1 Ca+2 Mg+2 Sr+2 Ba+2Al+3 Fe+3

    ....... .....

  • : , : Rb+ > K+ > Na+ > Li+

  • Al+3 > Ca2+ > Mg2+ > K+ > Na+ Al3+ > Ca2+ > Mg2+ >> NH4+ > K+ > H+ > Na+ > Li+

  • 1234

  • Cation Exchange CapacityCEC : (+) / cmol (+) /kg soil (+) / 100 meq(+) / 100 g soil

  • 1- the number of cation adsorption sites per unit weight of soil 2- : the sum total of exchangeable cations that a soil can adsorb.

    * CEC is expressed in milliequivalents (meq) per 100 g of oven dry soil.

  • eq meq Equivalent weight = 1 = 1000 1 ( ) 6.022 * 1023 ... 1 6.022 * 1020

  • 1 meq wt. of CEC has 6.022 x 10 20 adsorption sitesmeq of Common CationsElement Na+ K+ Ca++ Mg++Valence 1 1 2 2eq. Wt 23/1=23 39/1=39 40/2=20 24/2 = 121 meq wt .023 .039 .02 .012

  • 0.023g 0.023g , 0.023g , 0.023g

  • ,

    :

  • Assume Avg CEC for % OM = 200 meq/100gAssume Avg CEC for % clay = 50 meq/100g

    CEC = (% OM x 200) + (% Clay x 50)From soil data: soil with 2% OM and 10% Clay200 x .02 + 50 x .1 = 4 + 5 = 9 meq/100 g

    : 35% , 1.5%

  • Sand = 0-3 meq/100 g Silt = 10 15 Clay = 30 - 50 (depends on kind of clay)

    : CEC : 30% , 40% : CEC = (30% clay X 30 + 40% silt x 15 + 40% sand x 3 ) = 9 + 6 + 1.2 = 16.2 meq/100g

    CEC = (30% clay x 50 + 40% silt x 10 + 30% sand x 2 ) = 15 + 4 + .6 = 19.6 meq/100g

  • *

    MineralSpecific surface (m2/g)C.E.C (meq/100g)Kaolinite10-203-10Illite80-10020-30Montmorillonite80080-120Chlorite8020-30

  • : CEC : 40% clay, , 35% silt, 1.2%

    CEC = 40% clay x 100 + 35% silt x 12 + 25% sand x 3 + 200 x 1.2% OM =40 + 4.2 + 0.75 + 2.4 = 47.35 meq/100g

    .

  • :

    High humus content High swelling capacity 1:2 High clay content Fine Texture

  • Base Saturation% Base Saturation = meq bases cations CEC x 100% Hydrogen Saturation = meq H CEC x100

    Example: in an Ap Soil HorizonCations-- H+ Ca++ Mg++ K+ Na+ meq/100g 9.4 14 3 0.5 0.1

    CEC = 27 meq/100g (sum of cations)% base saturation = 17.6 27 x 100 = 65%% hydrogen saturation = 9.4 27 x 100 = 35%

  • Soil Solution ,

  • *free waterdouble layer water

  • * free water

  • , 99% micelles (clay particles & organic matter)

    mainly Ca++, Mg++, K+ and Na+)

    ,

    Cation Exchange Capacity or CEC Micelle

  • + + CO2 + , ,

  • Soil ReactionSoil pH pH

  • H2O H+ + OH-

    What is pH?:

  • : : 1- CaCO3 + H2O Ca+2 + HCO3- + OH-2- , ( ) Anorthite + H2O Aluminosilicate + Ca(OH)2

  • : : 1- 2- 3- 4- 5-

  • Carbonic acid formationforms in soil waterCO2 + H2O H2CO3H2CO3 H+ + HCO3-----------------------------CO2 + H2O H+ + HCO3-

    [CO2] is higher in soils than aboveground

  • pH

  • AcidicBasicpH MeasurementsPossible pH Ranges Under Natural Soil Conditionscarrot: 5.5-7.0cucumber: 5.5-7.0spinach: 6.0-7.5tomato: 5.5-7.5white pine: 4.5-6.0black walnut: 6.0-8.0wheat: 5.5-7.5cranberry:4.2-5.0apple: 5.0-6.5Laboratory Analysis

  • BACTERIA

  • pH Measurements

  • The types of soil acidity Active acidity : Free hydrogen ions Soil potential acidity : , Bound hydrogen ions

  • : Exchangeable ( or Active ) acidity [(Al+3 + H+) / CEC] : Residual or bound or non-exchangeable acidity , The types of soil acidity

  • Hydrolysis of Al+3 H2O OH- + H+

    Al+3 + HOH Al(OH)+2 + H+ Al(OH)2+ + HOH Al(OH)2+1 + H+ Al(OH)2+1 + HOH Al(OH)3 + H+ Al(OH)3 + HOH Al(OH)4-1 + H+

  • Soil Buffering Capacity pH

    ( ) ( )

    *****The clay particle derives its net negative charge from the isomorphous substitution and broken bonds at the boundaries. **************The negatively charged faces of clay particles attract cations in the water. The concentration of the cations decreases exponentially with the increasing distance from the clay particle. The negatively charged clay surface and the positively charged cations near the particle form two distinct layers, known as electric double layer or simply double layer.***************