Soil Colloids: Properties, Nature, Types and Significance. sources of charges

Soil Colloids

1. Soil Colloids: Properties, Nature, Types and Significance. Layer silicates: Genesis and sources of charges Soil Colloid The term colloid is coined from the Greek word “ Kolla ” meaning glue. The fraction of the soil particles having size less than 0.002 mm in size. Particles having size less than 0.001 mm possess colloidal properties and are known as soil colloids. These are also called as chemically reactive particles Colloid is a state of matter consisting of very fine particles but never approach molecular size. Examples: Milk, Cheese, Clouds, Fog etc. Dr.AB Jadhav , SSAC,AC, Pune

2. Dr.AB Jadhav , SSAC,AC, Pune

3. Dr.AB Jadhav , SSAC,AC, Pune

4. Dr.AB Jadhav , SSAC,AC, Pune

5. Dr.AB Jadhav , SSAC,AC, Pune

6. Silica Tetrahedron Aluminium or Magnesium octahedron Dr.AB Jadhav , SSAC,AC, Pune

7. Size 2. Surface area 3. Surface charges i ) pH dependent charge ii) Isomorphus substitution iii) Brocken edges of bonds 4. Adsorption of cations 5) Adsorption of water 6) Cohesion 7) Adhesion 8) Swelling and shrinkage 9) Dispersion and flocculation 10) Brownian movement 11) Non permeability General Properties of Soil Colloids Dr.AB Jadhav , SSAC,AC, Pune

8. Sources of charge on colloids 1) pH dependent charge 2) Isomorphus substitution 3) Brocken edges of bonds pH dependent charge The total c harge soil particles frequently varies with pH The positive charge developed at low pH and the excess negetive charge developed at high pH are collectively called as pH dependent charge. Most soils have negative charge because of the negative charges on layer silicates and organic matter. But some highly weathered soils dominated by allophane may have net positive charge. The primary source of pH dependent charge is considered to be gain or loss of H from functional groups on the surfaces of soil colloids Dr.AB Jadhav , SSAC,AC, Pune

9. 2) Isomorphus Substitution This is the major source in 2:1 and 2:1:1 type layer silicates Definition: It is the substitution of one ion for another of similar size within a crystal lattice Si (0.42 A o ) or 0.042 mm in the tetrahedral layer is replaced by ion of similar size Al 3+ (0.51 A o ) or 0.051 mm. In the same manner, part of Al 3+ in the octohedral sheet may be replaced by Mg 2+ (0.66 A o ) without disturbing the crystal structure. The resulting – ve charge is considered as permanent charge, since it will not change with pH. The substituting ion may have a greater, equal or lower charge than the ion for which it substitute. If a cation of lower valence substitute for one of higher valence like Mg2+ for Al3+ or Al3+ for Si4+, negative chrge of O2- and OH- ions in the mineral structure is left unbalanced by yielding a net negative charge on the mineral. Dr.AB Jadhav , SSAC,AC, Pune