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Module # 11 – Component # 2
Soil Forming Processes
and Factors
Objectives
To achieve a working knowledge of the different soil forming processes, as well
as understanding the impact that various soil forming factors have on these
processes.
Expected Outcomes
Briefly describe the soil forming factors
Briefly describe the soil forming processes
Understand the inter-relationship between these factors and processes
Soil Forming Processes
The different physical and chemical characteristics of each type of soil are
the result of a number of soil forming processes that occur as a result of a
specific combination of soil forming factors.
Soil forming factors  soil forming processes  different soils
The four most important processes of soil formation processes are:
Additions
Transformations
Translocations
Losses
Processes where additions to a profile take place do not only take place at
the soil surface, but lateral, and even additions from below play an
Soil Forming Processes and Factors
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important role in profile development. Important additions are water by rain
or runoff from higher lying areas, organic material such as decaying plant
matter, mineral material including soil and partly weathered geological
material as well as salts, carbonates and bases.
When microbes decompose raw organic material, an amorphous black
substance called humus is synthesized. This process is called humification,
and is an example of a transformation process. Changes of primary
minerals to secondary layer alumino-silicate clay minerals, as well as
changes of these secondary alumino-silicate clay minerals to sesquixides (oxides
and hydroxides of iron and aluminium) are further examples of transformations
processes.
Simple chemical reactions such as the reduction of ferric (Fe +3 ) to ferrous
(Fe +2 ) iron under conditions of poor aeration such as waterlogging, are
also included in this group.
The moving of material from one position in the soil profile to another in the
same profile is termed translocation. The movement of material takes place
principally in solution or in suspension in water and is mainly vertically
downwards. Movement can however take place upwards as well, for example
through capillary action from a water table.
The movement of material from a certain profile is called eluviation,
whereas the movement of material into a certain part of a profile is termed
illuviation.
Important translocation processes in soil are the translocation of clay, salts,
carbonates and bases, sesquioxides and pedoturbation.
Pedoturbation is a type of translocation where the mixing of soil takes
place and thus prevents horizon differentiation. This is seen for instance in the
cycling of soil by soil fauna such as earthworms.
Losses, means the removal of something from a profile, be it:
water through evapotranspiration
soil through erosion
losses of organic material, bases, clay and iron, and silica.
Losses of these substances occur in all directions – vertically upwards at the
surface, vertically downwards at the bottom of the profile, and laterally.
Soil Forming Processes and Factors
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Soil Forming Factors
The following five soil forming factors are differentiated:
Climate
Parent material
Topography
Living organisms
Time
The specific combination of soil forming factors that prevail at a certain location
will determine the dominant soil forming processes that occur in that location.
This will determine the type of soil profile that develops.
Soil Forming process include:
Vertical additions
Rain water
Organic material
Run-on water
Soil particles (blown)
Salts
Lateral additions
Water
Salts
Iron
Clay
Bases
Transformations
Humification
Clay transformation
Fe (iron) redox
Vertical losses
Water (evaporation)
Humus (decomposition)
Soil particles (erosion)
Soil Forming Processes and Factors
Capillary additions
Water
Salts
Translocations
Iron
Clay
Bases
Carbonates
Silica
Lateral Losses
Water
Salts
Iron
Clay
Bases
Leaching losses
Water
Salts
Bases
Carbonated silica
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Figure # 4 -
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Graphic representation of the various soil forming processes.
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Climate
Climate is by far the most dominant soil forming factor on a global,
national and regional scale. The combination of rainfall and temperature
play a fundamental role in soil formation.
Hot and humid conditions promote the weathering of parent material, since
chemical reactions are accelerated by high temperatures and water
provides the medium in which these reactions can take place, and therefore
soils in the tropics and sub-tropics are in a very advanced stage of
weathering (deep and rich in minerals). With high moisture levels the
reaction products are removed through leaching, and this further accelerates
the reactions in that it prevents the development of an equilibrium.
When conditions are hot and dry, the lack of water retards the weathering
and leaching processes and therefore also soil formation. Under these conditions
heat
actually
impedes
soil
formation,
in
that
it
increases
evapotranspiration and thus reduces the effectiveness of the rainfall.
Weathering is restricted to the depth where water infiltration takes place.
Soils in these regions are consequently shallow and stony and alkaline due to
accumulation of salts, carbonates and bases.
In the areas between the two above examples (semi-arid/semi-humid
boundary) significant soil formation takes place. Equilibrium conditions are
reached soon as reaction products are not leached out, bases accumulate and
soils in such regions are characterised by swelling type clay materials called
smectites. These can be vertic soils (swelling clays throughout the profile)
or duplex soils (soils with a sandy topsoil that abruptly overlies an unstable
swelling clay).
Parent material
This is simply defined as the underlying bedrock on which a soil profile is
based
Different soil profiles can be found in a region with homogenous climatic
conditions. This is the result of a combination of parent material and
topography. In the semi-arid/semi-humid boundary where significant
pedogenesis (formation of soil) takes place, the role of parent material is very
important. Not only the type of profile that arises, but also the inherent
fertility of the soil and the stability thereof against different forms of
degradation (erosion, crust formation and compaction) is influenced by the type
of parent material.
A soil profile does not necessarily originate from the rocks present below
it. It can originate from other overlying material or from more than one
parent material. Different layers in a soil profile can also originate from
different parent material (lithological discontinuity).
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When soil is formed from weathered rocks it is called in-situ. Alternatively it
can form in previously weathered transported material. This is known as
drift.
The weathering rate from different types of parent material can differ greatly.
The differences are the result of the following:
Type of mineral
Coarseness of granules
Density of rocks
Quartzite is an example of a dense hard rock that weathers slowly, while
material such as mudstone weathers relatively rapidly due to its soft porous
structure.
Topography
The term topography refers to the form of landscape. A number of
morphological terrain units are distinguished, although all of these units do not
necessarily have to occur in all landscapes. On a rolling hill type for instance
there is no escarpment (cliff face) identifiable.
Soils on crests and the upper parts of middle slopes are generally shallow.
The reason for this is that much of the water runs off from these terrain
positions and a good deal of material is also naturally eroded from here.
Pedogenesis is restricted due to the increased run off as well as the fact
that crests consist of more weatherable resistant rock. Soils on these crests
are also generally stony.
Crest
Escarpment
Middle
slope
Footslopes
Figure # 5 - Terrain morphological units on a Concave Slope
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Valley
botto
m
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Crest
Middle
slope
Footslope
Valley
botto
m
Figure # 6 - Terrain morphological units on a Convex Slope
Deep soils are found on the lowest parts of the midslope, footslopes and
valley floors. This is because pedogenesis is promoted by the increased run off
accumulating here, as well as the debris that is transported here from the higher
lying areas.
In rolling landscapes, a repetitive pattern of soils is found associated with
the different terrain morphological units. This is called a toposequence. Where
the parent material from the crest to the valley floor is the same, the soil
pattern coupled with the slope is called a catena.
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Aspect
Topography is also important in terms of terrain form, the aspect of the slope
as well as the gradient thereof.
An important aspect of terrain form is whether the slope is convex, plane or
concave.
Convex slopes are well drained, particularly with respect to surface drainage.
Concave slopes however often show a degree of hydromorphism (water
influences the slope), since water accumulates here.
Straight slopes in high lying positions are usually well drained, whereas flat
straight slopes such as on valley floors may have incomplete drainage with
resultant marsh formation in depressions on such terrain.
Aspect implies the direction in which
East or West). In southern hemisphere
sun shines diagonally from the North,
Sunshine is thus more concentrated on
warmer than South facing slopes.
the slope faces (i.e. North, South,
countries such as South Africa the
since the equator is North of us.
North facing slopes, making them
North
South
a
b
Figure # 7 – The influence of aspect
The importance of the aspect as a topographical soil forming factor.
North facing slopes are warmer due to concentrated heating.
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Soils on North facing slopes are considerably drier than those on South
facing slopes with the same rainfall. This is because North facing slopes are
warmer and evapotranspiration is thus higher on these slopes. Vegetation
is therefore also sparser on the North facing slopes and less organic material
consequently accumulated. The decay of humus is also accelerated in warm
conditions. The soils on North facing slopes are thus poorer in organic
material and are lighter and brighter in colour. They are also leached to a
lesser degree.
A further important factor is shadow casting. This is seen in areas with steep
mountains and hills and in deep ravines. These conditions lead to cool, damp
soils which are rich in humus.
Mountains also play another important role in soil formation due to the effect
that they have on rainfall patterns. Rainfall decreases from the coast to
the inland over the coastal plateau and then increases drastically once the
escarpment is reached. This causes dramatic differences in soil patterns
between the rainy side and the rain shadow side of the mountains.
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Living Organisms
Microbes, plants, soil fauna, and man through cultivation, all play an
important role in soil formation. Microbes, algae and mosses play the most
primitive role in soil forming.
Microbes are responsible for the conversion of organic material to humus,
as well as the decaying of humus. Important weathering agents such as
carbonic acid (from CO2 dissolved in water) and other organic acids are
released from these processes. Certain microbes can also attack inorganic
compositions.
Soil fauna such as termites and earthworms promote weathering in that soil
is being circulated through their digestive systems. Vast quantities of soil are
shifted every year through termites that build mounds. Only soil with a
certain particle size is used by termites to build these mounds. This changes
the texture of the soil and promotes horizon differentiation to a certain
degree. Certain termites accumulate free lime in their mounds.
Soil fauna such as the giant earthworms (up to 7 metres long! [± 21 feet])
that can be seen in the Eastern Cape, can also delay horizon differentiation by
mixing the material from different horizons. This is termed pedoturbation.
Time
Time is an important soil forming factor, as the processes involved in soil
formation are extremely slow.
Sand dunes of various ages are a good example of the influence of time on soil
formation. The oldest dunes are the furthest away from the coastline, red in
colour and sometimes difficult to recognise as dunes. The youngest dunes are
normally grey, whereas those that are a bit older are yellow.
River terraces in semi-arid areas are another good example. Two or more
terraces of differing ages are normally found along rivers, where the oldest
terrace is the furthest and most often also the highest away from the river.
The youngest terrace is right on the banks of the river.
The oldest terraces show clear horizon differentiation, especially in the form
of large differences in clay content between the topsoil and the subsoil. Very
little pedogenesis has occurred in the youngest terrace, and the topsoil
shows only the accumulation of organic material under which the original alluvial
deposits are clearly visible.
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Oldest terrace:
Middle terrace:
 Mature soil
 Clayey
subsoil
 Alluvial
stratification
eliminated
 Clay movement
insignificant
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Youngest terrace:
 Alluvial
stratification still
visible
River
Figure # 8 - Different terraces as an example of the influence of time on soil
formation
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