some chemical properties of soils

8/3/2019 Some Chemical Properties of Soils

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Some Chemical

Properties of Soil

Soils Lab. 10.

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Soil Acidity Role of Silicate Clay Minerals

Cation Exchange Capacity

Base Saturation

Exchangeable Acidity

Flocculation - Dispersion

Chemical properties...

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Activity of H ions (H3O+) in the soil

solution

and / or

the acidity associated with the soilsolids

I. Soil Acidity...

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Soil Acidity...

Types of soil acidity:

1. Active Acidity

2. Reserve AcidityExchangeable Acidity

Non-exchangeable Acidity

3. Total Acidity

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1. Active acidity: The activity of H ions in asoil solution

Soil

What happens when youdissolve a small sample of

soil in distilled water

Water

Types of Soil Acidity...

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Soil

H3O+

HAl

Fe

H3O+

Active acidity:

H ion start dissociating from

soil particle in the water and

thereby causing acidity known

as active acidity

Acidic cations still associated

with the soil particle do not

contribute to active acidity


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Soil

H3O+

HAl

Fe

H3O+

These acidic cations that are

still associated with the soil

particle and that do not

contribute towards

active acidity

contribute to the

RESERVE ACIDITY (Exchangeable

and Non-exchangeable Acidity)


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Reserve Acidity

Soil

KCl soln.

Dissolve soil in a KCl

solution to measure the

reserve acidity

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Reserve Acidity

Soil

H3O+

H3O+

KKK

K

K K Al3+

Al3+

K replaces Al ions associated

with the soil. Al in soil solution

reacts with water to produce

H ion, thereby causing acidity

Here Al acts as a exchangeable

acidity

Bound H+ and AL+3 that is not

displaced or slowly displaced is

called non-exchangeable acidity


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Reserve Acidity (Exchangeable and

Non-exchangeable):

Refers to the non-active acidic cations

associated with the soil solids that are

gradually released into soil solution or are

non-exchangeable using a concentrated

neutral salt

e.g. Organically complexed Al

Al-hydroxy cations

Weathering of soilBound Al+3

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The sum total of both types of acidity

i.e. active and the reserve acidity is

known as TOTAL ACIDITY

3. Total Acidity:

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Measuring Soil Acidity

Measured on a pH scale ranging from 0 to 14

expressed in moles L-1 of H ions

in a soil solution.

Most cultivated soils range from 6 - 8 pH

0 7 14Neutral

Acidic Basic


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Sources of Soil Acidity:

Hydrolysis of aluminum Alumino - silicate clay dissociations

Organic matter dissociations

Carbonation

Nitrification

Sulfur oxidation

Soil amendments

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Sources of Soil Acidity...

Hydrolysis of Aluminum

Al3+ (soln.) + H20 Al(OH)2+ + H+

Al(OH)2+ + H20 Al(OH)2+ + H+

Al(OH)2+ + H20 Al(OH)3 + H

+

* Production of H+ ions contributes acidity

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Sources of Soil Acidity...

Hydrolysis of aluminum

Alumino - silicate clay dissociations

Organic matter dissociations

Carbonation

Nitrification

Sulfur oxidation

Soil amendments

All these processes produce H+ ions which

cause soil acidity

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Neutralizing Soil Acidity:

... anything that produces H+ ions in soil

solution causes soil acidity...

Therefore anything that reduces the

activity of H+ ions in the soil solution

would neutralize soil acidity

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Neutralizing Soil Acidity...

Calcium Carbonate (lime)

Magnesium Carbonate

Calcium - Magnesium Carbonate

Calcium oxide (CaO)

Calcium hydroxide (Ca(OH2)

Wood ash

Examples:

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Neutralizing Soil Acidity...

CaCO3 + H+ = Ca2+ + HCO3

HCO3 + H+ = CO2 + H20

Each mole of CaCO3 neutralize 2 moles soil acidity

It is the Carbonate in CaCO3 that neutralizes

soil acidity

Calcium Carbonate: CaCO3


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II. Role of Silicate Clay Minerals:

Clay minerals are minerals that occurin the clay size fraction of soil

Known as ALUMINO-SILICATE clay minerals

Because they contain Si-tetrahedrons and

Al-octohedrons as building blocks

Occur as primary and secondary minerals

in soil.

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Silicate Clay Minerals...

Surface area

Clay ~ the finest texture of soil

has a very high surface area

consequently ... clay minerals provide a

very high reactive surface area

therefore ... they strongly affect the

chemical properties of soil

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Structure:

Silicate clay minerals consist

of layers of Si-tetrahedrons and

Al-octahedrons.

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Structure:

What is a Tetra-hedron?

What is an Octa-hedron?

*For additional info on clay

minerals see the 3D clay mineral

models on the CSES3124 Main

Web Page

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Structures... Tetra-hedron (four surfaces)

1

2

3

4

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Structures... Tetra-hedron (four surfaces)


= Oxygen ion

= Silicon ion

4 Oxygen ions with one Si ion


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Structure ... Tetra-hedral chain

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Diagramatic

representation


Structure ... Tetra-hedral layer (chain)

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Structure ... Octa-hedron (eight surfaces)

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= Oxygen ion

= Aluminum ion

6 Oxygen ions with one Al ion


Structure ... Octa-hedron (eight surfaces)

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Structure ... Octa-hedral layer (chain)

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Structure ... Octa-hedral layer

Digramatic

representation


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Structural types...

1:1 type ~

1 tetrahedral layer and 1 octahedral layer

e.g. Kaolinite

2:1 type ~

2 tetraherdal layers and 1 octahedral layer

e.g. Montmorillonite, Vermiculite, Mica

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KAOLINITE 1:1 Silicate clay mineral

Platelet

Tetrahedral layer

Octahedral layer

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KAOLINITE 1:1 Silicate clay mineral

PlateletO O O O

O O O OH H H H

O O O O

O O O OH H H H

hydroxide

surface

oxygen

surface

Hydrogen bonding

holds platelet

together very

tightly

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O O O O

O O O O

is is is

O O O O

O O O O

is is is

+ H20H20


MONTMORILLONITE 2:1 Silicate clay mineral

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O O O O

O O O O

is is is

O O O O

O O O O

is is is

+ H20H20



Platelet

Cation bridging

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O O O O

O O O O

is is is

O O O O

O O O O

is is is

+ H20H20

Platelet

Cation bridgingOxygen surface

no H bonding

Isomorphic

substitution in

tetrahedral

layer


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VERMICULITE 2:1 Silicate clay mineral

O O O O

O O O O

is

is

is

O O O O

O O O O

is

is

is

+

is is

isis

++ ++

Iso-morphic substitution

is in both tetra-hedral

and octa-hedral layer

Cation bridging

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Montmorillonite is fully expansible upon wetting and

drying while vermiculite is only semi-expansible

This has to do with the magnitude of the net negative

charge and the location of the charge:

Mont has an overall smaller net neagative charge

which is located primarily in the octa layer

(less attraction between platelets)

Verm has an overall greater net negative charge

which is located primarily in the tetra layer

(more attraction between platelets)

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CHLORITE 2:2 or 2:1:1 Silicate clay mineral

O O O O

is

is

is

is is

O O O Ois

is

is

isis

H H H H

H H H H

is

is

is

is

is

is

MgMgAlHydrogen

Bonding and

isomorphic

subst of Al (+3) for

Mg (+2) hold the

additional octa layer

in place and make this mineral non-exspansible

hydroxy interlayer

an octahedral sheet

+

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Iso-morphic Substitution

Iso ~ similar, morphic ~ appearance

(shape & size)

Substitution of Si ion in tetrahedron or

substitution of Al ion in octahedron by a similar

size ion is known as Iso-morphic substitution.

In most cases (IS)gives rise to net negative charge

(Al+3 for Si+4). However, it can also give rise to

positive charge (Al+3

for Mg+2

in chlorite)

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= Silicon ion 4+

= Aluminum ion 3+

Iso-morphic Substitution in Tetrahedron

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Iso-morphic Substitution in Tetrahedron...


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Overall net charge

minus (-) one

= Si 4+

= Al 3+

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Iso-morphic Substitution in Octa-hedron...

= Aluminum ion 3+

= Magnesium ion 2+

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= Aluminum ion 3+

= Magnesium ion 2+

Overall net charge

minus (-) one

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III. Cation Exchange Capacity:

Expressed in cmol+ kg-1 of soil

CEC is the sum total of all the

exchangeable cations present in

the soil solution in equilibrium

with the soil solids.


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Sources of CEC:

1. Clay minerals

2. Organic Matter

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Sources of CEC:

1. Clay minerals:

Isomorphic substitution: e.g. Substitution

of Al3+ for Si4+ give rise to overall one net

minus charge.

This increases the negative charge on the soil and

consequently the CEC of the soil.

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Sources of CEC:

CEC range from 2 to 150 cmol+ kg-1 of soil

Kaolinite ~ 2-5 cmol+ kg-1

Mica ~ 10-20 cmol+ kg-1

Montmorillonite ~ 70 - 100 cmol+ kg-1

Vermiculite ~ 100-150 cmol+ kg-1

CEC of some Clay minerals:

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Sources of CEC:

Highily decomposed organic matter known

as humus has very high surface area (greater

than clay).

It develops negative charge due to release of H+

ions from its funtional groups i.e. carboxyls,

hydroxyls, phenolics, etc.

2. Organic matter: ~ 200 cmol+ kg-1

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CEC:

CEC is expressed in cmol+ of cations kg-1of soil

WHAT is cmol+ !

e.g. Calcium Ca2+ has two positive charges

Therefore 1 mole of Ca = 2 mole+ of positive charge

1 mole of K+ = 1 mole+ of positive charge

1 mole of Al+3 = 3 mole+ of Al

1 mole = 100 cmol or 1mole+ = 100 cmol+

Units:

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IV. Base Saturation:

Is the percentage of soils CEC which is

occupied by Basic cations

Basic cations : K, Ca, Mg, Na

Acidic cations : Fe, Al, H

recall... CEC is total of all exchangeable

cations in soil....


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Base Saturation...

Example:

1 kg

soil

H

H

Al AlCa

K

K2 cmol of K = 2 cmol+ of K2 cmol of H = 2 cmol+ of H

2 cmol of Al = 6 cmol+ of Al

1 cmol of Ca = 2 cmol+ of Ca

TOTAL CEC = 12 cmol+ kg-1

Basic cation (K + Ca) = 4cmol+

Therefore % of basic cations = 33 %

B.S. = 33 %

(represents 1 cmol)

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V. Exchangeable Acidity:

The percentage of soil CEC which isoccupied by acidic cations.

previous example:

Basic cation = 4 cmol+

Acidic cation = 8 cmol+

Total CEC = 12 cmol+ kg-1

E.A. = (8/12) * 100 = 66% or expressed in

cmol+ it would be 8 cmol+

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VI. Flocculation - Dispersion:

Flocculation: Soil particles are attracted

to each other and form aggregates

Dispersion: Soil particles are repelled

away from each other and the particles

act independently

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Flocculation - Dispersion...

A B

Dispersed Flocculated

Soil particles

are aggregated

and settled at

bottom

Soil particles

dispersed

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Factors affecting flocculation dispersion:

1. Affect of Cation Type

2. Affect of electrolyte solution concentration

3. pH effects

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1. Affect of Cation Type

Cations with a relatively small hydrated radius

(Ca++, Mg++) form thin double layer, soil particles

get close enough for short range attractive forces to

cause flocculation.

Cations with a relatively large hydrated radius(Na+)

form thick double layer, soil particles cannot

approach each other causing dispersion

(recall... Calgon ~ PSA)


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SOIL


Soil particles are

negatively charged

What is Double layer !

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++

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+


SOIL

Soil particles are

negatively charged

these charges are

satisfied by positively

charged cations

SINGLE LAYER

What is Double layer !

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++

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

-

+

+

+

+

-

++-


SOIL

Double layer !

Soil particles are

negatively charged

these charges are

satisfied by positively

charged cations

followed by second

layer which becomes

increasingly diffuse

and eventually reaches

the same conc as the

bulk solution.

+

+

+

+-

-

DOUBLE LAYER

-

-

-

-

-

-

+

-

-

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2. Affect of Electrolyte concentration:

High electrolyte concentration forms thin double

layer therfore causes flocculation

Low electrolyte concentration forms thick double

layer therfore causes dispersion

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3. pH effect:

Multifaceted effects:

Surface charge properties

Dissolution reactions Composition of the exchange complex

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Dispersion could be a problem in soils

under irrigated conditions, as sodium

tends to accumulate in the soil solution.

some chemical properties of soils

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