Some Chemical Properties of Soils
Transcript of Some Chemical Properties of Soils
-
8/3/2019 Some Chemical Properties of Soils
1/12
1
1
Some Chemical
Properties of Soil
Soils Lab. 10.
2
Soil Acidity Role of Silicate Clay Minerals
Cation Exchange Capacity
Base Saturation
Exchangeable Acidity
Flocculation - Dispersion
Chemical properties...
3
Activity of H ions (H3O+) in the soil
solution
and / or
the acidity associated with the soilsolids
I. Soil Acidity...
4
Soil Acidity...
Types of soil acidity:
1. Active Acidity
2. Reserve AcidityExchangeable Acidity
Non-exchangeable Acidity
3. Total Acidity
5
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...
6
Types of Soil Acidity...
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
-
8/3/2019 Some Chemical Properties of Soils
2/12
2
7
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)
Types of Soil Acidity...
8
Types of Soil Acidity...
Reserve Acidity
Soil
KCl soln.
Dissolve soil in a KCl
solution to measure the
reserve acidity
9
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
Types of Soil Acidity...
10
Types of Soil Acidity...
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
11
Types of Soil Acidity...
The sum total of both types of acidity
i.e. active and the reserve acidity is
known as TOTAL ACIDITY
3. Total Acidity:
12
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
-
8/3/2019 Some Chemical Properties of Soils
3/12
3
13
Sources of Soil Acidity:
Hydrolysis of aluminum Alumino - silicate clay dissociations
Organic matter dissociations
Carbonation
Nitrification
Sulfur oxidation
Soil amendments
14
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
15
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
16
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
17
Neutralizing Soil Acidity...
Calcium Carbonate (lime)
Magnesium Carbonate
Calcium - Magnesium Carbonate
Calcium oxide (CaO)
Calcium hydroxide (Ca(OH2)
Wood ash
Examples:
18
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
-
8/3/2019 Some Chemical Properties of Soils
4/12
4
19
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.
20
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
21
Silicate Clay Minerals...
Structure:
Silicate clay minerals consist
of layers of Si-tetrahedrons and
Al-octahedrons.
22
Silicate Clay Minerals...
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
23
Silicate Clay Minerals...
Structures... Tetra-hedron (four surfaces)
1
2
3
4
24
Structures... Tetra-hedron (four surfaces)
Silicate Clay Minerals...
= Oxygen ion
= Silicon ion
4 Oxygen ions with one Si ion
-
8/3/2019 Some Chemical Properties of Soils
5/12
5
25
Silicate Clay Minerals...
Structure ... Tetra-hedral chain
26
Diagramatic
representation
Silicate Clay Minerals...
Structure ... Tetra-hedral layer (chain)
27
Silicate Clay Minerals...
Structure ... Octa-hedron (eight surfaces)
28
= Oxygen ion
= Aluminum ion
6 Oxygen ions with one Al ion
Silicate Clay Minerals...
Structure ... Octa-hedron (eight surfaces)
29
Silicate Clay Minerals...
Structure ... Octa-hedral layer (chain)
30
Silicate Clay Minerals...
Structure ... Octa-hedral layer
Digramatic
representation
-
8/3/2019 Some Chemical Properties of Soils
6/12
6
31
Silicate Clay Minerals...
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
32
Silicate Clay Minerals...
KAOLINITE 1:1 Silicate clay mineral
Platelet
Tetrahedral layer
Octahedral layer
33
Silicate Clay Minerals...
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
34
O O O O
O O O O
is is is
O O O O
O O O O
is is is
+ H20H20
Silicate Clay Minerals...
MONTMORILLONITE 2:1 Silicate clay mineral
35
O O O O
O O O O
is is is
O O O O
O O O O
is is is
+ H20H20
Silicate Clay Minerals...
MONTMORILLONITE 2:1 Silicate clay mineral
Platelet
Cation bridging
36
Silicate Clay Minerals...
MONTMORILLONITE 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
+ H20H20
Platelet
Cation bridgingOxygen surface
no H bonding
Isomorphic
substitution in
tetrahedral
layer
-
8/3/2019 Some Chemical Properties of Soils
7/12
7
37
Silicate Clay Minerals...
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
38
Silicate Clay Minerals...
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)
39
Silicate Clay Minerals...
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
+
40
Silicate Clay Minerals...
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)
41
Silicate Clay Minerals...
= Silicon ion 4+
= Aluminum ion 3+
Iso-morphic Substitution in Tetrahedron
42
Silicate Clay Minerals...
Iso-morphic Substitution in Tetrahedron...
-
8/3/2019 Some Chemical Properties of Soils
8/12
8
43
Silicate Clay Minerals...
Iso-morphic Substitution in Tetrahedron...
44
Silicate Clay Minerals...
Iso-morphic Substitution in Tetrahedron...
45
Silicate Clay Minerals...
Iso-morphic Substitution in Tetrahedron...
46
Silicate Clay Minerals...
Iso-morphic Substitution in Tetrahedron...
Overall net charge
minus (-) one
= Si 4+
= Al 3+
47
Silicate Clay Minerals...
Iso-morphic Substitution in Octa-hedron...
= Aluminum ion 3+
= Magnesium ion 2+
48
Silicate Clay Minerals...
Iso-morphic Substitution in Octa-hedron...
-
8/3/2019 Some Chemical Properties of Soils
9/12
9
49
Silicate Clay Minerals...
Iso-morphic Substitution in Octa-hedron...
50
Silicate Clay Minerals...
Iso-morphic Substitution in Octa-hedron...
51
Silicate Clay Minerals...
Iso-morphic Substitution in Octa-hedron...
52
Silicate Clay Minerals...
Iso-morphic Substitution in Octa-hedron...
53
Silicate Clay Minerals...
Iso-morphic Substitution in Octa-hedron...
= Aluminum ion 3+
= Magnesium ion 2+
Overall net charge
minus (-) one
54
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.
-
8/3/2019 Some Chemical Properties of Soils
10/12
10
55
Sources of CEC:
1. Clay minerals
2. Organic Matter
56
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.
57
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:
58
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
59
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:
60
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....
-
8/3/2019 Some Chemical Properties of Soils
11/12
11
61
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)
62
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+
63
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
64
Flocculation - Dispersion...
A B
Dispersed Flocculated
Soil particles
are aggregated
and settled at
bottom
Soil particles
dispersed
65
Flocculation - Dispersion...
Factors affecting flocculation dispersion:
1. Affect of Cation Type
2. Affect of electrolyte solution concentration
3. pH effects
66
Flocculation - Dispersion...
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)
-
8/3/2019 Some Chemical Properties of Soils
12/12
12
67
SOIL
Flocculation - Dispersion...
Soil particles are
negatively charged
What is Double layer !
68
++
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
Flocculation - Dispersion...
SOIL
Soil particles are
negatively charged
these charges are
satisfied by positively
charged cations
SINGLE LAYER
What is Double layer !
69
++
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
-
+
+
+
+
-
++-
Flocculation - Dispersion...
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
-
-
-
-
-
-
+
-
-
70
Flocculation - Dispersion...
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
71
Flocculation - Dispersion...
3. pH effect:
Multifaceted effects:
Surface charge properties
Dissolution reactions Composition of the exchange complex
72
Flocculation - Dispersion...
Dispersion could be a problem in soils
under irrigated conditions, as sodium
tends to accumulate in the soil solution.