Soil Colloids: Types and Properties

After reading this article you will learn about the types and properties of soil colloids.

Types of Soil Colloids:

The seat of chemical activity in soils resides in the soil colloidal particles. All soils excluding pure sands contain particles of colloidal size. Soil colloids possess properties very much like those of typical colloidal substance like gelatin, starch etc.

Soil colloids are of two kinds:

(i) Inorganic and

(ii) Organic.

The former is represented by the clay of size smaller than 0.001 mm or 1 micron (µ) while the latter is represented by the humus.

Clays are complex alumino silicates which are constituted of plate like units and therefore possess considerable amount of surface area, both external and internal. Considerable amount of internal surface occurs between the plate like units especially of smectites and vermiculites. The minute (size 1 micron or less) are known as micelle (microcell).

So each clay micelle is surrounded by cations. The concentration of these cations is greatest near the negatively charged surface of the clay micelle and gradually decreases with the increase in the distance in the distance between clay micelle surface and the cation as illustrated in Fig. 7.8. Some species of cation especially Na⁺⁺, H⁺ and Mg⁺⁺ are hydrated (water molecules have not been shown in Fig. 7.8.

The organic colloid is represented by the humus. The humic micelle [M] is constituted mainly of carbon and oxygen and subordinately of hydrogen and nitrogen and possesses reactive (functional) groups which are protruded from the main mass of it as mentioned below:

Properties of Soil Colloids:

All the properties of colloids, as mentioned earlier are also shown by soil colloids whose important properties are given below:

1. Soil colloidal particles appear to oscillate in their suspension in water when the soil colloid-water suspension is observed under the microscope. The soil colloidal particles appear to colloide with one another and also with the solvent molecules (water). This is Brownian movement. Faraday and Tyndall effects are also exhibited by the soil colloids.

2. Soil collidal suspension in water can be flocculated by adding solution of strong electrolyte like sodium chloride or hydrochloric acid etc.

3. Soil colloidal particles commonly bear negative charge. Whenever soil colloidal suspension is placed between two electrodes of unlike sign, the soil colloids i.e. clay and humic particles move towards the positive electrode i.e. anode. The magnitude of the negative charge borne by them is called the zeta potential.

Soil colloids acquire negative charge by the under-mentioned mechanisms:

(i) Isomorphous Ion Substitution means the replacement of one ion by another ion of comparable size. E.g. S₂⁺⁺⁺ ions of size 0.39 Ã… are replaced from the tetrahedral interstices by Al⁺⁺⁺ ions of size 0.57 Ã…. Similarly Al⁺⁺⁺ ions of size 0.57A are replaced from the octahedral interestices of smectites and vermiculites and illites by Mg⁺⁺ ions of size 0.78 Ã… and/or by Fe⁺⁺ ions of size 0.83 Ã….

One positive charge falls short as the consequence of each these isomorphous ion substitutions which means one negative charge become excess. This is permanent negative charge.

(ii) When the pH of the soil increases, then hydrogen H⁺ ions dissociate from the hydroxyl group located on the broken edge of kaolinite and also from alcoholic hydroxyl group, phenolic hydroxyl group and carboxylic group of the humic micelle when negatively charged spot are exposed. This is pH dependent negative charge or pH induced negative charge.

4. Some of the soil colloids also bear a little positive charge. Hydroxyl groups occurring at the broken edge of kaolinite and those of hydrous oxides of iron and aluminum as well as the amine groups of humus get protonated. The positive charge arises in this way, on them.

5. Soil colloidal particles selectively adsorb ions. They adsorb some ions more readily than others e.g. NH₄⁺ K⁺ Mg⁺⁺ and H₂PO₄ ions adsorbed more readily while Ca⁺⁺, Na⁺ and SO₄^— ions are adsorbed less readily and NO₃^–, CI and HCO₃ ions are not adsorbed or very little if at all. This adsorption of ions mostly takes place by exchange of ions.

The adsorption of ions by colloids depends both on the type of colloid and type of ions as well as on the ionic concentration. The strength of adsorption of cations by the soil colloid increases with the increases in the valency of the cations.

So aluminum ions are held most tightly. Calcium and magnesium ions are adsorbed more tightly than sodium and potassium ions. But hydrogen ions, even though they are monovalent are held more tightly than the divalent cations Ca⁺⁺ Mg⁺⁺ etc. If the valency of the cations is same, then their strength of adsorption by soil collidal particles increases with the increase in the atomic weight of the concerned element.

The adsorption of cations also increases with the increase in their concentration in the soil solution. The adsorption of cations also depend on the nature of the soil colloids e.g. smectites adsorb divalent cations more tightly than kaolinites.

The adsorption of anions by the soil colloids depends on the type of soil colloids, the type of anions and the soil pH. Anions are adsorbed more readily by hydrous oxides of iron and aluminum than by kaolinite. They possess considerable capacity to adsorb anions. Phosphates are adsorbed by the maximum possible extent by them. Sulphate are also adsorbed to a great extent. Anion adsorption increases with the decrease in soil pH.

The property of adsorption helps in the maintenance of soil fertility. Soil colloids adsorb nutrient ions and water molecules which remains available to the plant roots. Hence the soil colloids serve as a reservoir of plant nutrients.

Soluble nutrients which are applied to the soil in the form of fertilizers are retained in it on account of this property of soil colloid. Plant roots assimilate these nutrients.

6. If the zeta potential of the soil colloidal particles e.g. clay particles are very high, they continue to repel one another in their suspension enough to stabilize their suspension, when they remain deflocculated or dispersed.

When their zeta potential (magnitude of negative charge) goes down, they unite together to form units called floccules. This phenomenon is known as flocculation. Acids e.g. hydrochloric acid or neutral salts e.g. calcium chloride or sodium chloride whenever added to a deflocculated clay suspension floccu­lates it.

The acid is better in this respect than its salt. Flocculation is the prerequisite to the aggregation of colloidal particles for the development of granular and crumb structure for maintaining proper tilth for crop growth.

7. A colloid, whenever it is brought in contact with water absorbs water and swells up. This phenomenon has been designated as imbibition. It actually causes the disappearance of certain amount of water.

Similarly when soil colloidal particles come in contact with water, they imbibe water to and swell up. Smectite clay imbibes water and swells up more than kaolinitic clay.

8. When water is absorbed by soil colloids, they liberate certain quantity of heat which is known as heat of wetting. Smectite clay evolves more heat than kaolinitic clay.

9. Whenever the soil moisture content decreases, clay particles adhere (stick) together and also with sand particles. These phenomena are called cohesion in the former case and adhesion in the latter case. Cohesion is primarily due to the attraction of the clay particles for the remaining water molecules.

Hydrogen bonding develops between the clay particles and the remaining water molecules. Hydrogen bonding is responsible for cohesion. Smectite clay exhibit greater cohesion and adhesion than kaolinitic clay. These phenomena ultimately results in the development of stable soil structure.

10. Plasticity is the ability of the soil to be moulded to any desired shape and to retain that shape when the external force is no longer being applied to the soil. This property is due to plate like nature of the clay particles. Sodium saturated clay exhibit plasticity more than the calcium saturated clay.