Soils 205-90

Lecture 7 Soil Colloids

Videos                                                            Pages in text.

A. Colloidal Properties

1. Size - extremely small

2. Surface area - very large

3. Surface charge

(a) most soils = electronegative charge dominates

(b) results in ion (cation) adsorption

 

B. Types of Colloids

1. Layer silicate clays

(covered next)

2. Hydrous oxides of Fe and Al

(a) highly weathered soils & coatings

(b) some have structure, others poorly structured

(c) examples: gibbsite, Al(OH)₃; goethite, FeOOH

3. Allophane and other amorphous minerals

ü short-range-order minerals

4. Organic colloids

(a) highly charged (pH dependant)

(b) phenolic and carboxyl OH groups

 

5. Adsorbed cations

(a) arid region soils = "basic" cations

Ca⁺², Mg⁺², K⁺, Na⁺

(b) humid region soils = "acidic" cations as well

Ca⁺², Mg⁺², H⁺ and Al⁺³

(c) strength of adsorption

Al⁺³> Ca⁺² = Mg⁺² > K⁺ = NH₄⁺ > Na⁺

C. Layer Silicate Clay Structure- basic building blocks

1. Tetrahedron - SiO₄

 

2. Octahedron - Al(OH)₆

 

3. Sharing of O or OH groups = sheets and unit layers

(a) tetrahedral sheet

 

(b) octahedral sheet

 

Tetrahedral and octahedral sheets are often drawn as shown below.

 

(c) unit layer and interlayer area

 

(d) crystal or micelle = single particle

(e) Isomorphous substitution

= atom substitution during crystal formation

Zn⁺² for Al⁺³

or Al⁺³ for Si⁺⁴

or Cu⁺² for Mg⁺²

Leads to permanents charge:

Return to contents