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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 Outcome
Briefly describe the soil forming factors Briefly describe the soil forming processes Understand the inter-relationship between these factors and processes
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Soil Forming Processes The different physical and chemical characteristics of each type of soil are the result of several soil forming processes that occur because 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 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.
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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.
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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:
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Representation of a soil forming process Source: www.geologylearn.blogspot.co.za
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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 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).
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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). 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.
Quartzite
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Topography The term topography refers to the form of landscape. Several morphological terrain units are distinguished, although all 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.
Terrain morphological units on a Concave Slope
Crest
Escarpment
Middle slope
Footslopes
Valley botto
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Deep soils are found on the lowest parts of the mid-slope, foot-slopes 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.
Crest
Middle slope
Foot slope
Valley botto
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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 the slope faces (i.e. North, South, East or West). In southern hemisphere countries such as South Africa the sun shines diagonally from the North, since the equator is North of us. Sunshine is thus more concentrated on North facing slopes, making them warmer than South facing slopes.
The influence of aspect
a b
North South
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The importance of the aspect as a topographical soil forming factor. North facing slopes are warmer due to concentrated heating.
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 because 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.
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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.
Different terraces as an example of the influence of time on soil formation
Oldest terrace:
Mature soil Clayey subsoil
Middle terrace:
Alluvial stratification eliminated Clay movement insignificant
