Cation Exchange Capacity (CEC)
What is Cation Exchange Capacity (CEC)?
- CEC is the sum total of exchangeable cations a soil can adsorb or exchange. It is a key measure of soil fertility and nutrient-holding capacity.
How is CEC Expressed?
- Units: Centimoles of charge per kilogram of soil (Cmol/kg) or meq/100g soil.
Example:
- If a soil has a CEC of 10 Cmol H⁺/kg, it means:
- It can hold 10 Cmol of H⁺ ions OR
- 10 Cmol of monovalent cations (e.g., K⁺, Na⁺) OR
- 5 Cmol of divalent cations (e.g., Ca²⁺, Mg²⁺) OR
- 33 Cmol of trivalent cations (e.g., Al³⁺).
Cation Exchange is Chemically Equivalent
Since different cations carry different charges, they exchange in proportion to their charge:
1 mol of charge is provided by:
- 1 mol of monovalent cations (H⁺, K⁺, Na⁺).
- ½ mol of divalent cations (Ca²⁺, Mg²⁺).
- ⅓ mol of trivalent cations (Al³⁺).
Example Calculation of Lime Requirement (CaCO₃) for Soil Neutralization
To replace 1 Cmol H⁺/kg soil with Ca²⁺:
- 2 g Ca²⁺/kg soil is required.
For 1 hectare furrow slice (2.2 million kg of soil):
- 2 × 2.2 × 10⁶ = 440 kg Ca²⁺ needed.
To supply this Ca²⁺ using limestone (CaCO₃):
- CaCO₃ contains 40% Ca²⁺, so:
440 × (100/40) = 1100 kg CaCO₃/ha.
Why is CEC Important?
✔Determines soil fertility: Higher CEC = more nutrients retained.
✔Affects nutrient availability: Soils with low CEC leach nutrients quickly.
✔Influences soil pH management: Acidic soils require lime application.
✔Guides fertilizer recommendations: Helps optimize nutrient applications.
Summary of CEC and Its Practical Importance in Agriculture
📌 High CEC soils (e.g., clay, organic soils) retain more nutrients and need fewer fertilizer applications.
📌 Low CEC soils (e.g., sandy soils) lose nutrients easily, requiring more frequent fertilization.
📌 Lime application is crucial for acidic soils to maintain optimal pH and nutrient availability.