Some soils are similar but many are different to various degrees. It is important to understand the relationship between soils and the basic factors that cause them to differ from one area to another.
The processes of additions, removals, transformations, and translocation in producing soils are controlled by five groups of independent variables. These are referred to in many scientific texts as soil formation factors.
In the original formulation, hydrology was separated from relief while nowadays, man because he can substantially alter the physical environment, is considered a separate factor in soil formation.
However, for convenience of our study, the factors of soil formation will be taken as five (as originally formulated by Jenny).
These five soil formation factors are:
- Organisms (vegetation and Fauna)
- Relief (shares of the land surface)
- Parent material
These factors set the conditions that bring about the processes which in turn bring about the properties of the soil.
1. Climate and Vegetation
Affect the parent material and their action are conditioned by relief over some time. The ability to predict soil characteristics and to understand those that are observed is determined by a knowledge of these five soil formation factors.
There have been several attempts to formulate theoretical relations between soil forming factors and soil properties. In considering soil formation, the soil themselves becomes the dependent variable and the other factors independent and causative.
The equation of Jenny being the initial letter of the factor is S = F (C, O, r, p, t).
Where S = the total effect of all the factors F = function of any soil properties. S = F (C), o, r, p, t.
This states that any soil property will be a function of climate if other factors are held constant. Similarly, the equation can be applied to other factors (o, r, p, t.).
The equations are intended to be symbolic rather than utilitarian and should be regarded only as a conceptual model.
Read Also: Approaches to the Study of the Soil
It exerts its influence through this action.
- Rainfall (precipitation)
The most important components of climate that affect the process of soil formation are rainfall and temperature. Both influence the intensity and nature of rock weathering.
Even after the rock has been completely weathered, rainfall and temperature influence the trend of soil development from weathered materials. Climate affects soil formation in at least 4 ways:
- Weathering of parent material
- Transportation of parent material
Rainfall affects the intensity of weathering and leaching within the soil. It is necessary for the important processes of hydrolysis, hydration, carbonation, and other associated weathering processes to take place. It determines whether leaching occurs in the soil. It affects plant growth and vegetation and hence the content of soils.
Rainfall distribution in space and time results in climate periodicity in certain localities that tremendously influence the processes of pedogenesis; for example; the alternation of a wet season with a dry season is important in Nigeria. Wind and sunshine affect the effectiveness of rainfall and the intensity of temperature.
Climate is the most important soil forming factor affecting organic matter content and associated properties (notable nitrogen), reaction, and base saturation. It has a substantial influence on profile depth and texture and is one of several factors influencing the type of clay mineral synthesized.
2. Living Organisms
The factor termed organism covers vegetation and soil Fauna. The major role of living organisms in profile differentiation cannot be over-emphasized. Profile mixing by earthworms, crawfish, nutrient cycling, and structural stability are all made possible by the presence of organisms in the soil. Also, nitrogen is added to the soil system by microorganisms alone or in association with plants.
Vegetation covers the ground of the soil surface and protects it from erosions and other physical traumas. It provides the very essential component of soil, the organic matter. It does the nutrient recycling that is so important for soil maintenance. The micro-climate provided by the vegetation is extremely important.
We can differentiate between grassland, forest, and desert. The micro-climate under these different vegetation communities influences the nature of the soils developed under them.
For example, the organic matter content of Savannah soils is generally much lower owing to the sparse vegetation. Also, structural stability tends to be encouraged by the forest vegetation here in Nigeria.
Climate determines the types of vegetation and the type of soil. The soil determines the types of vegetation which also influences the trends of pedogenesis. There is a two-way interaction between vegetation and soil.
Man influences the trend of soil development in several ways. Man changes the type of vegetation, reclaims lands from swamps and the sea, does a lot of land leveling and soil transfer, cultivates the soil, and plants various types of crops which influence the soil.
The spatial distribution of soil types is related to relief at all levels of scale. Relief influences soil development because of its influences in:
- Water relation
Topography or relief is commonly considered to be a factor that modifies the effects of other factors of soil formation such as established before soil formation begins.
Soils formed on upper slopes are characteristically different from those of middle slopes which also differ from those of lower slopes.
4. Parent Materials
Soil parent materials represent the initial state of the soil system. There are essentially 2 types of the original parent material.
1. Residual Parent Materials
This is weathered material that is derived from the underlying rock below the soil profile. Soil that develops from this type of parent material is known as “Sedentary’, a term indicating that the soil has been formed in place, in situ.
2. Transported Parent Materials
These are classified based on how they came into existence e.g. colluvial marine, Lacustrine, Classical, and alluvial. The characteristics of the parent materials influence the characteristics of the soil which is formed from it.
Parent material affects the type of nutrient, particularly when the soil is still young and has not been severely leached. Soil parent material also affects the texture, soil reaction, and other properties of the soil. The importance of parent material tends to decline with age or degree of soil development.
In this connection time strictly cannot be considered as a soil-forming factor as other factors mainly because it does not exert any effect on the soil as such. However, time or age as one of the factors of soil formation is important because of the length of time during which other factors persist.
The amount of time a soil requires to develop varies widely according to the action of the other soil-forming factors.
Young soils may develop in a few days from the alluvium (sediments left by flood) or from the ash from volcanic eruptions. Other soils may take hundreds of thousand or a million of years to form.