– The formation of these horizons is due to the presence
of clay materials transferred by the mechanical
migration method (Lessivage).
– The clay horizon may be developed under different
climatic conditions (wetter) than it is now.
– The clay in the subsurface layers do not move from the
surface but rather are formed in the same place.
• The process of biological or physical mixing of soil
materials (Pedoturbation): In this process, the surface
horizons are mixed with the subsurface horizons, through
the transfer of the soil through thick and deep cracks. As
in the case of Xererts in some regions of Al Jabal Al
Akhdar.
• The process of formation of a new mineral or organic
species (Synthesis): These processes require an abundant
amount of water. Thus, they are almost absent or
non-existent in the Libyan soils, especially in arid regions
(i.e., southern), and among these processes are the following:
– The process of breaking down the mineral and
organic materials (Decomposition): Many soil living
organisms are responsible for this process. This process results in intermediate compounds that are not
completely degraded, known as Humus (Organic
Colloids). Hence, it plays a crucial role in improving
soil properties. However, the production of humus in
Libyan soils is very small due to the high temperature
and good aeration conditions.
– The process of hydrolysis and formation of silicate
clay minerals:
– During this process, the minerals in the rocks are
chemically destroyed. Thus, new minerals (clay minerals) are formed. Likewise, water must be available
for this process to occur. Accordingly, the formation
process of silicate clay minerals in Libyan soils is very
little or no process. Given the fact that the availability
of these minerals in Libyan soils, it can be suggested
that is due to their presence in the parent material
(inherited traits) (Ben Mahmoud 1995).
• Reactions that change the chemical composition of soil
mineral matter: The occurrence of these processes
depends on the availability of water and aeration. Thus, it
can occur under special circumstances in Libyan soils,
and it includes many processes, namely (Ben Mahmoud
1995; Naguib and Khidr 1989)
– Hydration: It is the chemical combination of the polar
water molecules with another substance. The water
molecules do not dissociate, but attach as a shell to the
other substance. For example, the transformation of
Hematite (red) into Limonite (yellow):
2Fe 2 O 3 þ 3H 2 O À À À À À ! 2Fe 2 O 3 :3H 2 O
Hematite red
ð Þ À À À À À ! Limonite (yellow)
The formation of limonite from hematite is a typical
hydration reaction in soils and one that causes soil color to
change.
• Carbonation: It is the dissolution of carbon dioxide in
water. First, carbonic acid H 2 CO 3 is formed by the
reaction of CO 2 with H 2 O and released into the soil
solution by root or microbial respiration. Then, basic
oxides are converted to carbonates through a reaction
with carbonic acid, which has an effective role in dissolving some minerals in rocks, such as calcium and
magnesium carbonates, to more water-soluble forms:
CaO 2 þ H 2 O À À À À À ! H 2 CaO 3
CaCO 3 þ H 2 CO 3 À À À À À ! Ca(HCO 3 Þ 2
Calcium carbonate þ CarbonicAcid À À À À
À ! Calcium bicarbonate
– Oxidation-Reduction: It is a set of coincident reactions
involving electron transfer between atoms. This process
affects the rocks and minerals containing the oxidized or
reduced atom, and in turn, the compound’s susceptibility
to physical or chemical weathering. This process leads to
chemical changes that help break down rocks and minerals. For example, the oxidation of the minerals containing Ferrous iron to Ferric iron, under proper
conditions of water and air:
Fe 2 O 3 ÀO 2 reduction
ð
Þ
! 2FeO þ O 2 reduction
ð
Þ
! Fe 2 O 3
Ferric oxide red
ð Þ!Ferrous oxide black
ð
Þ!Ferric oxide (red)
Ferrous iron is highly soluble and mobile in the soil
profile, but exposure to O 2 easily oxidizes it to ferric iron.
Ferric oxide is the tan, brown or red concretions, and particle
coatings can be visibly observed in the soil.
• The microbiological processes of soil microorganisms
(Decomposition): These processes occur by particular
types of bacteria at the sites where temperature and
moisture provide conditions that are conducive for the
multiplication of microorganisms. These bacteria perform
some transformations and form new chemical compounds, and among these processes are the following:
3 Soil Forming Factors and Processes
45
of clay materials transferred by the mechanical
migration method (Lessivage).
– The clay horizon may be developed under different
climatic conditions (wetter) than it is now.
– The clay in the subsurface layers do not move from the
surface but rather are formed in the same place.
• The process of biological or physical mixing of soil
materials (Pedoturbation): In this process, the surface
horizons are mixed with the subsurface horizons, through
the transfer of the soil through thick and deep cracks. As
in the case of Xererts in some regions of Al Jabal Al
Akhdar.
• The process of formation of a new mineral or organic
species (Synthesis): These processes require an abundant
amount of water. Thus, they are almost absent or
non-existent in the Libyan soils, especially in arid regions
(i.e., southern), and among these processes are the following:
– The process of breaking down the mineral and
organic materials (Decomposition): Many soil living
organisms are responsible for this process. This process results in intermediate compounds that are not
completely degraded, known as Humus (Organic
Colloids). Hence, it plays a crucial role in improving
soil properties. However, the production of humus in
Libyan soils is very small due to the high temperature
and good aeration conditions.
– The process of hydrolysis and formation of silicate
clay minerals:
– During this process, the minerals in the rocks are
chemically destroyed. Thus, new minerals (clay minerals) are formed. Likewise, water must be available
for this process to occur. Accordingly, the formation
process of silicate clay minerals in Libyan soils is very
little or no process. Given the fact that the availability
of these minerals in Libyan soils, it can be suggested
that is due to their presence in the parent material
(inherited traits) (Ben Mahmoud 1995).
• Reactions that change the chemical composition of soil
mineral matter: The occurrence of these processes
depends on the availability of water and aeration. Thus, it
can occur under special circumstances in Libyan soils,
and it includes many processes, namely (Ben Mahmoud
1995; Naguib and Khidr 1989)
– Hydration: It is the chemical combination of the polar
water molecules with another substance. The water
molecules do not dissociate, but attach as a shell to the
other substance. For example, the transformation of
Hematite (red) into Limonite (yellow):
2Fe 2 O 3 þ 3H 2 O À À À À À ! 2Fe 2 O 3 :3H 2 O
Hematite red
ð Þ À À À À À ! Limonite (yellow)
The formation of limonite from hematite is a typical
hydration reaction in soils and one that causes soil color to
change.
• Carbonation: It is the dissolution of carbon dioxide in
water. First, carbonic acid H 2 CO 3 is formed by the
reaction of CO 2 with H 2 O and released into the soil
solution by root or microbial respiration. Then, basic
oxides are converted to carbonates through a reaction
with carbonic acid, which has an effective role in dissolving some minerals in rocks, such as calcium and
magnesium carbonates, to more water-soluble forms:
CaO 2 þ H 2 O À À À À À ! H 2 CaO 3
CaCO 3 þ H 2 CO 3 À À À À À ! Ca(HCO 3 Þ 2
Calcium carbonate þ CarbonicAcid À À À À
À ! Calcium bicarbonate
– Oxidation-Reduction: It is a set of coincident reactions
involving electron transfer between atoms. This process
affects the rocks and minerals containing the oxidized or
reduced atom, and in turn, the compound’s susceptibility
to physical or chemical weathering. This process leads to
chemical changes that help break down rocks and minerals. For example, the oxidation of the minerals containing Ferrous iron to Ferric iron, under proper
conditions of water and air:
Fe 2 O 3 ÀO 2 reduction
ð
Þ
! 2FeO þ O 2 reduction
ð
Þ
! Fe 2 O 3
Ferric oxide red
ð Þ!Ferrous oxide black
ð
Þ!Ferric oxide (red)
Ferrous iron is highly soluble and mobile in the soil
profile, but exposure to O 2 easily oxidizes it to ferric iron.
Ferric oxide is the tan, brown or red concretions, and particle
coatings can be visibly observed in the soil.
• The microbiological processes of soil microorganisms
(Decomposition): These processes occur by particular
types of bacteria at the sites where temperature and
moisture provide conditions that are conducive for the
multiplication of microorganisms. These bacteria perform
some transformations and form new chemical compounds, and among these processes are the following:
3 Soil Forming Factors and Processes
45