Cation Exchange (Base Exchange)
Definition:
Cation exchange is the reversible process where positively charged cations in soil solution are replaced by cations adsorbed on soil colloids.
Mechanism of Cation Exchange:
- Soil colloids are negatively charged, attracting positively charged cations like Ca²⁺, Mg²⁺, K⁺, Na⁺, Al³⁺, and H⁺.
- These cations can be replaced by other cations in the soil solution.
- Example: H⁺ from decomposing organic matter can replace Na⁺ or Ca²⁺ adsorbed on colloids.
Key Properties of Cation Exchange:
✔Reversible Process: If Ca²⁺ is removed from the soil, H⁺ or Na⁺ can replace it.
✔Influenced by Climate:
- High rainfall → Leaching of Ca²⁺, Mg²⁺ → Soil becomes acidic.
- Low rainfall → Accumulation of Ca²⁺ → Soil becomes neutral or alkaline.
✔Cation Exchange Capacity (CEC): Measures the soil’s ability to hold and exchange cations.
Cation Exchange Under Natural Conditions
In humid regions, cation exchange is complex due to leaching and acid formation. Typical cation distribution in soil colloids:
|
Cation |
Percentage in Soil Micelle |
|
Ca²⁺ |
40% |
|
Al³⁺ |
20% |
|
H⁺ |
20% |
|
Mg²⁺ & K⁺ |
20% |
Rainfall effects:
- In humid areas, carbonic acid (H₂CO₃) leaches Ca²⁺ and other cations, making the soil more acidic.
- In dry regions, less leaching occurs, allowing Ca²⁺ to accumulate, keeping the soil neutral to alkaline.
Influence of Lime & Fertilizers on Cation Exchange
a) Lime Application to Acidic Soils:
- Adding calcium compounds (CaCO₃, Ca(OH)₂) increases soil pH by replacing H⁺ ions:
- H⁺ ions are then neutralized by CO₃²⁻ or OH⁻, raising soil pH.
b) Effect of Acidic Fertilizers on Alkaline Soils:
- Adding sulfur (S) or ammonium fertilizers increases H⁺ concentration, lowering soil pH:
Fertilizer Influence on Cation Exchange:
a) KCl fertilizer application:
- K⁺ replaces Ca²⁺, H⁺, and other cations from soil colloids.
- The adsorbed K⁺ is retained, reducing leaching losses and ensuring availability to plants.
Practical Applications of Cation Exchange in Agriculture:
✔Lime application to correct soil acidity.
✔Fertilizer management to improve nutrient availability.
✔Soil structure improvement using gypsum (CaSO₄) to replace Na⁺ in sodic soils.
✔Crop selection based on soil CEC and nutrient availability.