Fundamentals of Soil Science (1+1)

Introduction

• Ion exchange in soil system refers to exchange of equilavent amounts of ions between two phases in equilibrium in contact in reversible process.
• When cations are involved , the process is termed as cation exchange, while for anions, it is referred to as anion exchange. Cation exchange reaction is considered as the second most important reaction next to photosynthesis.
• The exchanges may take place between soil solid phase and the soil solution phase, or less commonly between the soil solid phases in contact or soil solid phase and growing plant in contact ( contact exchange).

Cation Exchange:

• The Cation Exchange phenomenon was first identified by Harry Stephen Thompson in England during 1850. When soil was leached (washed) with ammonium sulphate, and upon filtration,calcium, and to lesser extent magnesium, potassium ions were detected in the leachate. The total amount of calcium and other cations so released is equivalent to that of ammonium retained.

(NH₄)₂SO₄ + Soil Ca = Soil (NH₄)₂ + CaSO4

• The various cations adsorbed by negatively charged colloids are subject to replacement by other cations through a process called cation exchange.
• The cation exchange reactions takes place reversibly, and the interchange is chemically equilavent.

Cation Exchange Capacity (CEC)

• The CEC is the capacity of soil is defined as the capacity of soil to adsorb and exchange cations.
• The cation exchange capacity is the sum total of the exchangeable cations that a soil can adsorb. The higher the CEC of soil the more cations it can retain. Soils differ in their capacities to hold exchangeable cations.

Unit of expression

• CEC is expressed as milliequivalents of cations per 100 grams of soil (meq /100g soil). After 1982, in the metric system the term equivalent is not used but moles are the accepted chemical unit. The recent unit of expression of CEC is centi moles of protons per kilo gram soil [cmol (p⁺) kg^-1 soil]. One meq/100 g is equal to one cmol (p⁺) kg^-1 soil.