Agriculture for Engineers 4 (3+1)

Lesson 9. Ion Exchange in Soil

Ion exchange is two types: (1) cation exchange or base exchange and (2) anion exchange or acid exchange. Ion exchange is a reversible process in which cation and anion exchanged between solid and liquid phase.

 9.2  CATION EXCHANGE OR BASE EXCHANGE

In a near neutral soil, Ca is remaining adsorbed on colloidal particle H ion generated as organic and mineral acid formed due to decomposition of organic matter. In colloid, H ion is adsorbed more strongly than is the Ca and H is chemically equivalent.

The reaction is as follow:

This phenomenon of the exchange of cations between soil and salt solution is known as calion exchange or Base Exchange and the cations that take part in this reaction are called exchangeable cations. It is the interchange between a cation in solution and another cation on the surface of clay or organic matter (colloids). Thompson and Way (1892) first recognised cation exchange phenomenon. They passed a solution of (NH₄)₂SO₄ through a soil column and found that effluent contained no NH4^- salt but CaSO₄. This phenomenon is always in equivalent quantities of ions.

 

A. Types of silicate clay mineral

In case of kaolinite, monovalent cations are more strongly held than divalent, while in case of montmorillonite, divalent cations are strongly held than monovalent cations. The normal order of adsorbed cations on clay particles is:

H⁺ > Al⁺³ > Ca ⁺² = Mg⁺² > K⁺ = NH₄⁺> Na⁺.

B. Significance of cation exchange

Cation exchange is an important reation 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.

Cation exchange is very important in soils because of the following relationships:

(l) The exchangeable K is a major source of plant K.

(2) The exchangeable Mg is often, a major source of plant Mg.

(3) The amount of lime required to raise the pH of an acidic soil is greater as the CEC is greater.

(4) Cation exchange sites hold Ca⁺², Mg⁺², K⁺, Na⁺ and NH₄⁺ ions and slow down their losses by leaching.

(5) Cation exchange sites hold- fertilizer K+ and NH₄⁺ and greatly reduces their mobility in soils.

(6) Cation exchange sites adsorb many metals (Cd⁺², Zn⁺², Ni⁺² and Pb⁺²) that might be present in wastewater. Adsorption removes them from the percolating water, thereby cleansing the water that drains into groundwater.

9.3 Anion exchange

The process of anion exchange is similar to that of cation exchange. Under certain conditions hydrous oxides of iron and aluminium shows evidence of having positive charges on their crystal surfaces. The positive charges of colloids are due to addition of hydrogen (H⁺) in hydroxyl group (OH^-) resulted in net positive charge (OH²⁺). This positive charge will attract anions. The capacity for holding anions increases with increase in acidity. The lower the pH the greater is the adsorption. All anions are not adsorbed equally readily. Some anions such as H₂PO₄ are adsorbed very quickly 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 neutral soil, while NO₃^- ions are not adsorbed at all. Hence, at the pH commonly prevailing in cultivated soils- nitrate (NO₃), chloride (Cl) and sulphate (SO₄) ions are easily lost by leaching. In general, the relative order of anion exchange is:

OH^- > H₂PO₄^-> SO₄^- > NO₃^-

This adsorbed negatively changed anion is replaceable as under:

In this way there is fixation of phosphate ion, in which OH ion of silicate clay mineral is substitute by H₂PO₄ ion by isomorphrous substitution. The H₂PO₄ ion becomes a part of the silicate clay minerals and the phosphate fixed by this way is known as colloid bound phosphate. This phosphate is not exchangeable and available to the plant. The anions like NO₃ and OH are not bound at pH above 7. The adsorbing power of anions is in the order of:

Importance of anion exchange in nutrient availability

The phenomenon of anion exchange assumes in relation to phosphate ions and their fixation. The exchange is brought mainly by the replacement of OH ions of the clay minerals.

The adsorption of phosphate ions by clay particles from soil solution reduces its availability to plants. This is also known as colloid bound phosphate fixation. The phosphate ion again becomes available when lime is applied to increase the pH of acidic soil.

The OH ion originated not only from silicate clay minerals but also from hydrous oxides of iron and aluminum present in the soil. The phosphates ions react with the hydrous oxides also get fixed forming insoluble hydroxyl phosphate of ion and aluminum. Which is called saloid bound phosphate.

 

 If the reaction takes place at a low pH under strongly acid conditions, the phosphate ions are irreversibly fixed and are totally unavailable for the use of plants.