chapter_4b_-_cec_v2_HANDOUT__1_

Cation Exchange Capacity (CEC)

• CEC refers to the ability of soil to absorb and exchange cations with soil solution and plant roots.

• It is crucial for soil fertility and nutrient availability.

Definition and Measurement of CEC

• CEC is defined as the sum of negative charges of the soil.

• Soil colloids possess both positive and negative charges.

• The net negative charges, unneutralized by positive charges, determine the CEC of soil.

• The measure of CEC is expressed in centimoles of positive charge per kilogram of soil (cmol/kg) or milli-equivalents per 100 grams of soil.

Cation Exchange Reactions

• Cation exchange reactions in soils are:

  • Instantaneous

  • Reversible

  • Stoichiometric

• A representative reaction:+2H+ + Co++

Units of CEC

• Two common units for expressing CEC:

  • cmol/kg of soil

  • meg/100 g of soil

• Both expressions are numerically the same:

  • Example: 10 cmol/kg = 10 meg/100 g and 25 cmol/kg = 25 meg/100 g

Factors Affecting CEC

1. Soil Texture

   • Clay soils have higher CEC values than sandy soils.

2. Type of Dominant Clay Mineral

   • Montmorillonitic clays have a higher CEC than Kaolinitic clays.

3. Organic Matter (OM)

   • The presence of OM increases CEC value significantly.

Influence of Colloids on CEC

• Different colloids affect CEC values:

  • Montmorillonite: 10 - 40 cmol/kg

  • Illite: 3 - 15 cmol/kg

  • Kaolinite: Lower CEC than Montmorillonite

  • Organic Matter (Humus): Significantly enhances CEC.

• Improving low CEC in kaolinitic soils can be done by increasing organic matter content.

Cation Exchange Process

• Cations in soil colloids exchange in equivalent amounts with those from soil solution or plant roots:

  • Plant roots exchange H+ ions for K+, Ca++, Mg++, etc.

• Base saturation is defined by the occupation of exchange sites in the soil colloids by basic cations.

Calculating CEC and % Base Saturation

• To compute CEC, sum the centimole positive charges in soil:

  • CEC = Sum of positive cation charges (Ca++, Mg++, K+, Na+, H+, Al+++)

  • Example calculation:

    • CEC = 10 + 5 + 0.5 + 1.5 + 5.0 + 4.0

Base Saturation Calculation

• % Base Saturation = [ \frac{\Sigma \text{cmol positive charge of basic cations}}{\text{CEC}} \times 100 ]

• Example: [ % BS = \frac{4 + 2 + 1 + 2 + 3}{12} \times 100 ]

Laboratory Measurement of CEC

• Measurement involves leaching a known soil sample with salt solution containing specific cations (usually NH4+), replacing soil cations with solution cations.

• The resulting leachate is analyzed for cations (Ca, Mg, K, Al, H) to determine CEC.

Summary of CEC from Sample Data

• Ca++: 4 cmol/kg

• Mg++: 2 cmol/kg

• K+: 1 cmol/kg

• H+: 2 cmol/kg

• Al+++: 3 cmol/kg

• Total CEC = 4 + 2 + 1 + 2 + 3

• % Base Saturation calculation based on total cmol values.