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Base Saturation

Ion Exchange

Charge
Source

Electroneutrality

Exchange
Process

Electric
Double
Layer

Exchangeable
Cations

Lyotropic
Series

Soil pH
and Al
Cation
Exchange
Capacity
pH dependent
CEC
Base Saturation

Anion Exchange

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Test
Soils
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             Base Saturation              

Fraction of CEC occupied by base cations (Na+, K+, Mg2+, Ca2+

 

         %Base Saturation            
(sum exchangeable bases)/CEC) x 100

          Remainder of CEC occupied by acidic cations (H+, Al3+)

 

  %Base Saturation directly related to pH
  adsorbed cations are in equilibrium with solution cations
 
high pH     -       Al3+ precipitated as Al(OH)3
 
low pH      -       Al3+ replaces Bases, which are leached out

 

                            pH 7   -       BS  =  100%

                            pH 4   -      BS  =      0%

 

Plant Growth
     - reduced as Base Saturation declines fewer nutrients, more toxic Al3+

Soil Classification -     > 35%  =  Alfisol
                                             < 35%  =  Ultisol

Lime Requirement -
        -  amout of lime necessary to lower pH to acceptable level
          -  increases with low BS

Acid Rain resistance -
        
only soils with low base saturation produce acidified lakes

High B.S.      Al(OH)3 + [-]..Ca2+ +  H2SO4  =  Ca2+ + SO42- + [-]..Al3+

Low B.S.       Al(OH)3 + [-]..Al3+   +  H2SO4  =  H+ + Al3+ + SO42- + [-]..Al3+

releases toxic low pH, toxic Al3+ leachate to lakes