Soil Colloidal

Chemistry

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Compiled and Edited by

Dr. Syed Ismail,

Marthwada Agril. University

Parbhani,MS, India

The Colloidal Fraction

Introduction

What is a colloid?

Why this is important in understanding

soils?

How can we understand this fraction?

What are the fundamental basics of this

fraction?

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Soil Colloids

The most chemically active fraction of soils.

They are very small, less than 2 µm in

diameter

Shape

Colloids can be either

– mineral (clays) or organic (humus)

– crystalline (definite structure) or amorphous.

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Colloids

Properties imparted to soils

– Static Vs. dynamic properties

– Sand and Silt (no colloids)

Static and occupy space

– Clay and Humus (colloids)

Dynamic and very active (charges)

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Colloids

Properties imparted to soils by colloids

– Chemical Vs Physical properties

Chemical

– Sources of ions for plant nutrition

– Source of electro-negativity (CEC)

– Buffering capacity

– Chemical cement agents

Physical

– Large surface area per unit of mass (cm2/g)

– Plasticity

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Types of Soil Colloids

Crystalline silicate clays

Non-crystalline silicate clays

Iron and aluminum oxide

Organic material (Humus)

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Soil Colloids: Silicate Clays

Kaolinite Montmorillonite

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Crystalline Silicate Clays

What is it?

Shape: x, y and z.

Surface area: 2 sources ***

– External Vs Internal

Composition of crystalline structures:

– Silicon-Oxygen and Aluminum- Hydroxide … others

– Silicon Tetrahedral and Aluminum Octahedral sheets

– Si-O Tetrahedral sheet (tetra=four void spaces)

– Al-OH Octahedral sheet (0cta=eight void spaces)

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Charges

Isomorphous Substitutions

Process in which one element substitutes

another of comparable size in the crystalline

structure

Al is slightly larger than Si, consequently Si

may replace Al!

IF Al+3 and Si+2 then what?

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Charges

Permanents

– Isomorphous substitutions

pH dependant (non-permanents)

– Broken Edges

– Al-OH + OH ==H- == Al- O- + H2O

(no charge) (- charge)

– C-OH + OH ==H- == -C- O- + H20

(no charge) (- charge)

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Structures (2D)

Si Tetrahedral Sheet

Al Octahedral Sheet

Al-Si combined

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Example 1:1 type clay

Sheets Layer

Interlayer

Sheets Layer

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Clay silicate crystals

1:1 type KAOLINITE

2:1 type SMECTITE

VERMICULITE

MONTMORILLONITE

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Clay silicate crystal

2:1:1 type CHLORITE

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Clay silicate crystals

1:1 type KAOLINITE

4 O and 1 Si

6 OH and 1 Al

Hydrogen ion

STRONG BOND!

NO WATER and

NO OTHER ION!

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1:1 type clays

Stable and non expanding clay

Low total charges

Relative low specific surface area

pH dependant charges

Good physical properties

Limiting holding capacity for nutrients

Good for roads, buildings, ceramic and bricks.

Hexagonal shape

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Clay silicate crystals

2:1 type Expanding

4 O and 1 Si

Montmorillonite 6 OH and 1 Al

4 O and 1 Si

O bonding (WEAK)

Hydrated

exchangeable

cations

Non Hydrated ions

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Smectite (includes montmorillonite)

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kaolinite illite

mont-

morillonite

humus

(fulvic acid)

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Sheets and Layers

Al sheet

Al sheet

Al sheet

Al sheet

Si sheet

kaolinite

Si sheet

Si sheet

Si sheet

illite smectite and

vermiculite

1:1 non-expanding 2:1 non-expanding 2:1 expanding

+ +

+

+

+ +

+

+

+

+

+ +

+ +

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2:1 Clay expanding type

Expanding clays

Shrinking and swelling constantly

Poor physical characteristics

Abundant charges and surface

Rich in nutrients

Good soils for crops if managed properly

Not affected much by pH

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Clay silicate crystals

2:1 type (Fine Mica) Non expanding

Charges: 20% Al octa-

by Si

tetrahedral

Strong bonding forces

Large Net Charge

Attract ions K+ and NH4+

Fits perfectly in hexahedral

holes 24

Hexahedral holes

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2:1 Clay non expanding type

Limited expanding

Good physical properties

Medium total charges

Lower specific surface area than expanding 2:1 clays

Good soils for crops

Challenging management for K+ and NH4+

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Clay silicate crystal

2:1:1 type

Fe/Mg instead

of Al octahedral

Mg dominated sheet

CHLORITE Hydrogen

STRONG

BOND 28

2:1:1 Clay type

Non Expanding clays

Very limited shrinking and swelling

Good physical characteristics

Limited charges and surface

Good soils for crops if managed properly

Not affected much by pH

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Distance between UNITS of crystalline

structures

1.41 nm Vermiculite

1.00 nm Micas

0.71 Kaolinite

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Non silicate Clays

Alone or mixed with silicate clays

Organic colloids – Humus

– Large molecules (+ and – charges)

Iron and Aluminum oxides

– Modified octahedral sheets with substitutions

– No tetrahedral sheets

Gibbsite (Al(OH)3) Oxisol and Ultisol

Goethite (FeOOH) yellow brown color

Hematite Fe2O3 red color

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Clay Minerals Comparison

Kaolinite Illite Vermiculite Smectite

Tetrahedral 0 20% Al3+ 10% Al3+ 2.5% Al3+

Octahedral 0 0 15% Mg2+ 15% Mg2+

Tetrahedral --- 20% Al3+ 10% Al3+ 2.5% Al3+

CEC me/100g 3-15 (edges) 30 150 80 - 150

Shrink-swell Low None Mod to High High

Interlayer H-bonds Fixed K+ Exch. cations Exch. cations

Origin Recrystallization

under intense

acid weathering

Early

alteration of

micas

Intermediate

alteration of

micas

Recrystallization

under moderate

neutral to

alkaline weath.

Thanks

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