Importance of Anion Exchange Capacity (AEC) in Soil
1️. Soil Development (Leaching & Adsorption)
- AEC influences soil formation by controlling how anions move through the soil profile.
- Retains nutrient anions and reduces groundwater contamination.
2️. Nutrient Availability
- Many essential nutrients exist as anions:
- Nitrate (NO₃⁻) – Easily leached in most soils.
- Phosphate (PO₄³⁻) – Strongly adsorbed and often fixed, reducing availability.
- Boron (BO₃³⁻), Chloride (Cl⁻), Molybdate (MoO₄²⁻) – Availability depends on AEC.
- AEC helps prevent excessive leaching of anionic nutrients, improving plant uptake.
3️. Nutrient Fixation & Soil Fertility
- Fe³⁺ and Al³⁺ hydroxyl groups (-OH) exchange with SO₄²⁻, HSO₄⁻, and other anions.
- Volcanic ash soils (amorphous clays) have the highest AEC, while kaolinite has low AEC.
- Phosphorus fixation is a major concern in high-AEC soils, requiring careful fertilizer management.
4️. Environmental Protection
- AEC helps trap pollutants like organic waste anions, reducing their movement into groundwater.
- Restricts sulfate leaching in subsoils, preserving soil fertility in humid regions.
Key Differences Between CEC & AEC
|
Property |
Cation Exchange Capacity (CEC) |
Anion Exchange Capacity (AEC) |
|
Exchangeable ions |
K⁺, Ca²⁺, Mg²⁺, NH₄⁺, Na⁺ |
NO₃⁻, SO₄²⁻, Cl⁻, PO₄³⁻ |
|
Charge of soil colloid |
Negative (-) |
Positive (+) |
|
Effect of pH |
Increases with pH |
Decreases with pH |
|
Major soil components |
Clay minerals, organic matter |
Fe & Al oxides, kaolinite, amorphous minerals |
|
Nutrient loss |
Cations (e.g., K⁺, Ca²⁺) can leach in sandy soils |
Anions (e.g., NO₃⁻, Cl⁻) are highly leachable |
|
Agricultural impact |
Holds plant nutrients, buffers soil pH |
Retains anionic nutrients, reduces leaching |