Do you know that the lump of soil you hold does not result from weather rocks alone? In this article, you will find out that five factors have been identified as being crucial to soil formation.
These factors play different roles to make it possible for us to have soil. As you will see later, to get a mature soil takes time and it is also depends on the type of parent material and the relief.
Furthermore, the organic matter of the soil environment and the climatic conditions are all important. Can you recognize the five factors none? If you can, you have done well, if you cannot, just go on and you will soon master them.
The Make-up of Soil Forming Factors
You probably have the idea that soil derived from the residual inorganic material resulting from the physical disintegration and chemical decomposition of the crust.
However, not all weathered rocks become soil. It is therefore, necessary to know that certain factors act together to make soil formation possible.
The soil forming factors are classified into active and passive factors. Those active factors are climate and organisms while the passive ones are relief, parent material and time.
Do you know that an American soil scientist, Jenny (1941), functionally related the soil forming factors into an equation? Here is it:
S = f (U, O, R, P, T) (1)
Where S is soil property 0 is organisms
R is relief
P is parent material T is time
Climate is an active factor of soil formation. Through its component elements of temperature, precipitation, humidity and wind, climate is generally regarded as the most important factor in soil formation.
This is because climate plays both a direct and an indirect role in soil formation. What are those functions that make climate a major foremost soil forming factor? Some of these are:
Promotion of the production of soil water and soil warmth which constitute the most important agents of soil formation;
Association of world’s soil types in broad terms with climate types
Determination of the rate of organic matter decomposition,
Responsibility for the richness, sparseness or absence of vegetative cover which, in turn, affects the total nature of soil.
Indeed, we can see that climate is almost everything for soil. This is why it is termed an active factor as it supplies the energy that the deed rock material needs to come alive.
Organisms or biological activities are made up of both plants and animals. These also contribute to the formation of soils actively. However, plants play a greater role.
The role of organisms in soil formation is both direct and indirect. When organic materials decompose in the soil it produces organic acids which directly intensify pedogenic processes.
Soil fauna (animals) especially bacteria, worms, termites and man also participate directly in many ways in soil formation.
The indirect role of organisms takes many former. For example, tree roots extend the depth of soil profile while vegetation influences the operation of climatic elements particularly precipitation and temperature by creating a micro-environment.
It is also known that most processes of soil formation such as humification and mineralization are all closely linked with biological activities.
Relief affects soil formation through altitude, slope and aspect.
Altitude usually creates microclimate e.g. where altitude is high temperature is low thereby encouraging physical weathering to predominate.
In this case soil will be coarse and stony and may remain immature and skeletal for a very long time.
Altitude may also cause orographic rainfall whereby the windward side is weather with deeper soil than the leeward side with shallower soil.
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Slope affects depth of soil. The higher the relief, the steeper the slop is likely to be. This affects the rate of materials movement from the slope. Usually depth of soil is thin where slope is steep and deep where slope is gentle.
Aspect is a facet of relief that influences soil formation in higher latitudes. Usually, slope facing the sun receives more isolation and are therefore warmer and drier than the slope not facing the sun.
It should be noted that aspect is especially important in high-latitude regions. It is therefore found in the northern hemisphere that the south-facing slopes receive greater isolation leading to the formation of deeper soils on such slopes than on their north-facing counterparts.
Invariably, this explains why vegetation on the south-facing slopes is luxuriant while that of the north-facing slope is sparse in the northern hemisphere.
4. Parent Material
Parent material may be regarded as rocks from which soils are formed. There are two categories. In situ materials are those on the original spot. That is, they are not moved, such as igneous, sedimentary and metamorphic rocks.
Deposited materials are those that have been eroded transported to where they become soils. Examples include alluvial, colluvial, aeolian, morrainic and littoral materials.
Most of the inorganic fractions of the soil are obtained from the parent material which are always released by weathering.
Parent materials mostly determine the textural classes and also that the type of inorganic fractions found in any soil depends on its character.
In essence, acid rocks often yield a large amount of quartz fragments whereas basic rocks often yield clayey substances.
In the cases of deposited materials soils developed on them are usually sandy with the exception of alluvial materials. However, the influence of parent material may no longer be visible on very mature soils.
Time is a factor soil formation occupies a special position. This is because not only do the processes of soil formation operate in a temporal framework but that the other factors also change through time both in themselves and in their relationships.
Therefore, it is known that the formation of a mature soil requires a lot of time which also depends on the type of rocks on which such a soil is being formed.
For example, it may take over 1000 years for 3cm of granitic rock to become soil whereas a 3cm soil depth has been known to have been formed on a volcanic eruption made up of ash in less than 30 years.
In conclusion, the factors of soil formation are considered in this unit. These factors are climate, organisms, relief, parent material and time. These soil forming factors are usually grouped into active and passive factors. Climate and organisms constitute the active factors while relief, parent material and time are the passive factors.
The major climate element influencing soil formation are temperature and rainfall. Studies have shown a high correspondence between soil and climate maps of the world. Organisms include plants and animals i.e. living things. These are actually responsible for the living entity of the soil.
The location of soil is also important and is depicted by relief in respect of altitude, slope and aspect. Parent material is the rock from which soil mineral matter is formed after it has been weathered.
The formation of soil is a long and complex process so it could take a long time before a weathered material becomes soil. This is why time is also a crucial factor of soil formation.
There are five factors which Jenny (1941) has put into an equation thus:
S = f (U, O, R, P, T)
Where S is soil property
U is climate
O is organisms
R is relief
P is parent material
T is time
These factors are grouped into active and passive. The first two, climate and organisms are regarded as active factors while relief, parent material and time are passive factors.
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We can call the active factors as the life wire of soil. They give it the living entity. While climate especially through its elements of temperature and rainfall, provides warmth and moisture, water organisms i.e. directly influence decomposition of organic matter apart from their indirect influence on soil processes.
The latter may include, for example, the extension of the depth of soil profile by three roots. It is also noted that the influence of climate on soil formation is so high to the extent that world’s soil types are closely associated with world’s climate types e.g. lateritic soils are found mostly in the tropics irrespective of their location.
Relief affects soil formation through altitude which creates microclimate which can affect the soil e.g. where altitude is high soil may be coarse and stony.
Another way relief affects soil formation is through slope whereby soil depth is affected e.g. where slope is steep, there is thin soil whereas where slope is gentle, there is deep soil.
Aspect is the third way relief affects soil formation. It is all about areas facing or backing the sun.
Parent materials are of two types viz insituand deposited materials, and most of the inorganic fractions of the soil are derived from these. The character of the parent material determines whether a soil is acidic or basic.
Time is important in soil formation because the formation of a mature soil requires a long time. Besides, the, type of rock on which soil is being formed also has temporal dimension. While granitic rock may take a very long time to form soil volcanic materials may take a short time.