Fundamentals of Soil Science (1+1)

Factors affecting Cation Exchange Capacity

Factors affecting Cation Exchange Capacity

• Soil texture: The negatively charged clay colloids attracts positively charged cations and holds them. Therefore, the cation exchange capacity of soils increases with increase in per centage of clay content .
• Clay soils with high CEC can retain large amounts of cations and reduce the loss of cations by leaching. Sandy soils, with low CEC, retain smaller quantities of cations and therefore cations are removed from soil by leaching.
• Soil organic matter: High organic matter content increases CEC. The CEC of clay minerals range from 10 to 150 [cmol (p⁺) kg^-1] and that of organic matter ranges from 200 to 400 [cmol (p⁺) kg^-1].
• Nature of clay minerals: The CEC and specific area of the clay minerals are in the order : smectite>fine mica>kaolinite. Hence the CEC of a soil dominated by smectite type of clay minerals is much higher than kaolinite type dominated soils
• Soil Reaction: As the pH is raised, the hydrogen held by the organic colloids and silicate clays (Kaolinite) becomes ionized and replaceable. The net result is an increase in the negative charge on the colloids and in turn an increase in CEC.

Importance of Cation Exchange

• Cation exchange is an important reaction in soil fertility, in causing and correcting soil acidity and basicity, in changes altering soil physical properties, and as a mechanism in purifying or altering percolating waters.
• The plant nutrients like calcium, magnesium, and potassium are supplied to plants in large measure from exchangeable forms.
• The exchangeable K is a major source of plant K.
• The exchangeable Mg is often a major source of plant Mg.
• The amount of lime required to raise the pH of an acidic soil is greater as the CEC is greater.
• Cation exchange sites hold Ca⁺, Mg⁺, K⁺, Na⁺, and NH₄⁺ ions and slow down their losses by leaching.
• Cation exchange sites hold fertilizer K⁺ and NH₄⁺ and greatly reduce their mobility in soils.
• Cation exchange sites adsorb many metals (Cd²⁺ , Zn²⁺, Ni²⁺, and Pb²⁺) that might be present in wastewater adsorption removes them from the percolating water, thereby cleansing the water that drains into groundwater.

Anion Exchange

• Adsorption of negative ion (anions) e.g. Cl^-, NO₃^-, SO₄^2-, and H₂PO₄^- on positively charged sites of clay and organic matter is known as anion adsorption. These anions are subject to replacement by other anions through a process known as anion excange.

Clay NO₃^- + solution Cl^- = Clay Cl ^-+ Solution NO₃^-

Source of positive charge:

1. Isomorphous substitution: Low valence cations replaced by high valence cations.

2. Surface and exposed broken bonds of clay lattice: OH group in certain acid soils.

3. Complex aluminium and iron hydroxy ions in acid soils.

4. pH dependent charges are important for anion exchange of organic matter

• The basic principles of cation exchange apply also to anion exchange, except that the charges on the colloids are positive and the exchange is among negatively charged anions.
• Anion exchange is an important mechanism for interactions in the soil and between the soil and plant. Together with cation exchange it largely determines the ability of soil to provide nutrients to plants.

Anion exchange capacity

• “The sum total of exchangeable anions held exchangebly by a unit mass of soil , termed as its anion exchange capacity( AEC.)”. It is expressed as cmol / kg or m.eq./ 100 g soil.
• The AEC is much less than CEC of the soil.
• Kaolinitic minerals have a greater anion adsorbing and exchange capacity than montmorillonitic and illitic clays because the exchange is located at only a few broken bonds.
• The capacity for holding anions increases with the increase in acidity.
• Some anions such as H₂PO₄ are adsorbed very readily at all pH values in the acid as well as alkaline range.
• Cl and SO₄ ions are adsorbed slightly at low pH but none at neutrality, while NO₃ ions are not adsorbed at all.
• The affinity for adsorption of some of the anions commonly present in soil is of the order: NO₃ < Cl < SO₄ < PO₄.
• Hence at the pH commonly prevailing in cultivated soils, nitrate, chloride and sulphate ions are easily lost by leaching.

Importance of anion exchange

• The phenomenon of anion exchange assumes importance in relation to phosphate ions and their fixation.
• The adsorption of phosphate ions by clay particles from soil solution reduces its availability to plants. This is known as phosphate fixation. As the reaction is reversible, the phosphate ions again become available when they are replaced by OH ions released by substances like lime applied to soil to correct soil acidity.
• Percent Base Saturation
• The extent to which the adsorption complex of a soil is saturated with exchangeable basic cations is termed as base saturation. It is expressed as a percentage of the total cation exchange capacity.

% base saturation = Excaneable bases(cmol/kg) / CEC(cmol/kg) x 100

• Percent base saturation tells what percent of the exchange sites are occupied by the basic cations. If the percetage base saturation is 50, half of the exchane capacity is satisfied by bases, the other by hydrogen and aluminium.