NRES 201 Lecture 25 Outline of Last Lecture I What is a colloid II Particle size and surface area III Importance of the clay fraction IV Clay minerals V Organic matter Outline of Current Lecture VI Cation Exchange VII Cation exchange capacity CEC VIII Determination of CEC Current Lecture Cation Exchange A property of all soil colloids including o Clay minerals o Hydrous oxides o Organic matter Significance of Cation Exchange The process holds nutrients in a form that is o Potentially usable by plants o Protected against leaching Regulates soil pH and provides buffer capacity Maintains a stable ionic environment Determines plant composition Distinguishes soil from solution cultures Mechanisms Nature of cation retention o Cations are not fixed to colloidal surfaces o Thermal energies create a hemisphere of motion o The greater this motion the more exchangeable an ion becomes The exchange process o A cation from the bulk solution must penetrate an exchangeable cation s hemisphere of motion These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute o Exchange occurs when the former is closer than the latter to the colloidal surface Principles of Cation Exchange Reversibility o Adsorbed cations are displaceable Charge equivalence o Exchange occurs on a charge for charge basis Speed o Cation exchange is rapid o Limited by ion movement to and from exchange sites Mass Action o Adsorption is favored by a higher ion concentration Anion Effects o Cation exchange is shifted by an anion that causes a difference in Dissociation Solubility Volatility Cation Selectivity o Cations differ in their affinity for exchange sites depending mainly on Ionic charge A higher valence increases retention Usually the most important factor Ionic radius Smaller ions are held more tightly Extent of hydration Greater hydration increases ionic radius and reduces the tightness of binding Cation Exchange Capacity Defined as o The sum total of cations that can be adsorbed per unit weight of material at a specific pH Equivalent Weight Defined as o The weight of a substance that will contribute or reach with 1 gram of H
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