Factors Of Soil Formation

Five variables influence the formation and development of soils. These five variables are the factors of soil formation. They include climate of a region, living organisms, relief (topography), parent material and time. These variables are interdependent in that all the
factors are required for the process of soil formation to take place.

The differences that exist in soils are a result of both the differences in the nature of, and the varying degrees to which these factors interrelate. Howbeit, there are situations where the differences between soils are due to the variations of only one soil-forming factor, with all other soil-forming factors remaining constant. This leads to recognizable sequences of soil which includes; chronosequences (variations in time only), lithosequences (variations in parent material only), toposequences (variations in relief/topography only), biosequences (variations in living organisms only) and climosequences (variations in climate only).

The soil-forming factors

 

Parent Material (pm): The soil comes from this material. It is often viewed as the unconsolidated, chemically weathered mineral or organic matter from which the soil developed by pedogenic processes. In other words, without parent material there will be no soil. Parent materials influence soil formation/development through the plant nutrient they contain, the dominant particle size they weather into and their rates of weathering. Therefore, different types of parent material will result in different types of soil. The effect and influence of parent material is visible in any given soil, regardless of its level of development.

Climate (cl): Temperature and precipitation are the major variables by which climate affect the process of soil formation. Other variables include relative humidity, wind velocity, wind direction, sunshine intensity and sunshine duration etc. The impact of climate on soil formation in a particular area depends on four major conditions, which are:

a)   The major climatic variable of that area: This refers to the climatic variable that is most active in a particular area, e.g. precipitation in the humid areas and wind velocity and direction or sunshine intensity and duration in the drier areas. Some areas may have more than one major climatic variable.

b)   The intensity of the major climatic variable of that area: This refers to the severity of the most active climatic variable of a particular area e.g. rainfall volume, wind velocity etc.

c)   The seasonal variation in the major climatic variable of that area: This refers to variations in the intensity and duration of the major climatic variable as affected by changes in seasons. In humid areas for example, rainfall intensity and duration, as well as wind velocity, is greater in the rainy season than at any other season.

d)   The availability, intensity and counterbalancing effects of the other climatic variables: In an area where the major climatic variable is precipitation (rainfall and snow), occasional intense and lengthy sunshine will lead to alternate wetting and drying (rainfall) or freezing and thawing (snow), which will in turn, lead to the physical disintegration of rocks. Have you ever wondered why ice at the Polar Regions does not melt? More so, if you look closely, you will notice that there is hardly any soil in those Regions.

Living Organisms (o): Plants, animals and microorganisms play an active role in soil formation. Their mechanical actions, such as burrowing and root penetration contributes to the physical disintegration of parent materials. Root exudates and excretions from animals as well as chemical excretions from microorganisms, do contribute to the chemical weathering of rocks. Living organisms do also affect soil formation by depositing organic materials in and on the soil. Their presence and activities also affects the rate and intensity of soil erosion along with other factors. The activities of burrowing animals often lead to a constant mixing within the profile; technically known as pedoturbation.

Relief/Topography: Relief refers to the lay of the land surface. Simply, it considers the shape of the land surface, denoting from the area of highest elevation to the area of lowest elevation. The most important factors considered in relief are:

·        Altitude – the height (elevation) of an area of interest (either above or below sea level) and;

·        Slope – this is the inclination from the point of highest elevation to the place of lowest elevation. The angle or steepness of the slope as well as its length is what is most considered.

The impact of topography is, chiefly, through its moderation of the other state factors of soil formation – mostly climate and living organisms. With a change in altitude, there are noticeable variations in species, population and interaction of living organisms. Variations in the intensity and duration of the climatic variables also exist in the same manner. This also causes opposite sides of a mountainous slope to have different climatic regimes.

The angle/steepness of slope modifies the rate and intensity of erosion, as well as providing a medium for the transport of detached parent material. On the other hand, slope length determines the point of deposition of all transported soil and parent materials.

Time: It is very difficult to define time as an independent factor of soil formation. It is rather easier to look at time as a factor that integrates into other factors, or as a factor on which all other factors are dependent. Therefore, time, as a factor of soil formation, is the duration within which the influences of all other soil-forming factors are measured. Time, therefore, influences soil formation by providing a system that enables the measurement of soil development.