6 Cation Exchange in Soil Science

Cation Exchange Mechanism
- Cation exchange involves the replacement of adsorbed cations in soil with other cations present in the soil solution.
- When leaching soil with water:
- Removes adsorbed cations (e.g., Ca2+) from the soil.
- When leaching soil with KCl:
- K+ from KCl replaces adsorbed cations like Ca2+ on negatively charged soil particles.
- The reaction can be illustrated as follows:
  - Initial State: Soil Particles - - - - Ca2+
  - Addition of KCl: Soil Particles - - - - Ca2+ + 4K+
  - Final State: Soil Particles - - - - K+ K+ K+ K+ + 2Ca2+
  - Resulting Soil Solution now contains higher K+ and less Ca2+.
Cloudiness Observation in Leachates
- Higher cloudiness was observed in the KCl leachate.
- This is due to greater removal of cations from soil particles resulting from the abundance of K+, which facilitates the replacement of other cations.

Soil Texture and CEC Relation
- Soil texture plays a significant role in CEC; different textures (sand, silt, clay) impact the soil's capacity to retain cations.
Organic Matter (OM) and CEC
- The intensity of color in the supernatant after cation exchange indicates cation presence influenced by OM content.
- High intensity of violet in low OM soil (e.g., sand) implies fewer negative charges, meaning Gentian violet (a dye) had minimal color reaction.
- In contrast, high OM soil (e.g., forest soil) displays a significant color change since the increased negative sites react strongly, indicating higher CEC capacity.

Cations in Soil
- Basic cations include K+, Na+, Ca2+, Mg2+.
- Acidic cations include NH4+, H+, AI3+.
Calculating CEC
- Example formula to determine CEC:
- To calculate the contribution of NH4+ in 25 g of soil:
  - (0.072 g NH4+/25 g soil) * (1 mol NH4+ / 18 g NH4+) * (100 cmol NH4+ / 1 mol NH4+) * (1 cmolc / 1 cmol NH4+) * (1,000 g / 1 kg)
- Resulting CEC is 16 cmolc/kg of soil, demonstrating the soil's cation capacity.