Cation Exchange & Nutrient Availability

1️. Cation Saturation & Nutrient Release

  • Higher % saturation of a cation = More available to plants.

Example:

  • If 6 Cmol/kg Ca²⁺ is present in a soil with 8 Cmol/kg CEC (75% Ca saturation) → Ca is readily available.
  • If the same 6 Cmol/kg Ca²⁺ is in a soil with 30 Cmol/kg CEC (20% Ca saturation) → Ca is less available.

 

2️. Influence of Complementary Adsorbed Cations

Adsorption Strength Order:

H⁺ > Al³⁺ > Ca²⁺ > Mg²⁺ > K⁺ = NH₄⁺ > Na⁺

Example:

  • K⁺ is loosely held in acid soils (where H⁺ and Al³⁺ dominate), making it more available for plants but also more prone to leaching.
  • In neutral to alkaline soils (where Ca²⁺ and Mg²⁺ dominate), K⁺ is held more tightly.
  • High Ca levels can inhibit K⁺ uptake (nutrient antagonism).

 

3️. Effect of Colloidal Type

  • Smectites (2:1 clays) hold Ca²⁺ tightly and require 70% base saturation before releasing it readily.
  • Kaolinite (1:1 clay) releases Ca²⁺ more easily due to low charge density.
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