Anion Exchange in Soils

• Anion exchange refers to the interchange of anions between the soil solution and positively charged soil colloids, such as hydrous oxides of iron (Fe) and aluminum (Al). Unlike cation exchange capacity (CEC), which is generally higher in soils, anion exchange capacity (AEC) is limited but plays a key role in soil fertility and nutrient retention.

Anion Exchange Mechanism

1️. Anion Adsorption Mechanism

• Positively charged surfaces (Fe & Al oxides, kaolinite, amorphous minerals) attract anions.
• Anions like SO₄²⁻ and NO₃⁻ can be replaced by other anions, just like cation exchange.
• Anion exchange capacity (AEC) decreases with increasing soil pH (opposite to CEC).

Example of an Anion Exchange Reaction:

Soil – Cl^– + NO₃⁺< —– > Soil – NO­­­^­­₃^– + Cl^–

This reaction is reversible, similar to cation exchange.

2. Inner-Sphere Complexes

• Some anions, like phosphates (H₂PO₄⁻), molybdates, and sulfates, form strong chemical bonds with soil particles.
• Instead of being loosely exchanged, these anions react with hydroxyl (-OH) groups on soil minerals, forming stable inner-sphere complexes.

Example: Phosphate Fixation

Al-OH₂⁺ + H2PO₄^– –> AlH₂PO₄ + H₂O

This reaction makes phosphate strongly bound to the soil, reducing its availability to plants.

 Relative Strength of Anion Exchange

Not all anions are equally exchanged. The relative order of anion adsorption strength is:

OH^– > H2PO₄^– > SO₄^— > NO₃^– = Cl^–

• H₂PO₄⁻ (phosphate) is strongly adsorbed at all pH levels, leading to phosphorus fixation.
• Cl⁻ and NO₃⁻ are weakly held and easily lost by leaching.