What are the Factors that Contribute to Soil Formation

What are the Factors that Contribute to Soil Formation

The top surface layer of this exposed, solid part of crust containing weathered minerals and humus and capable of supporting plant growth is called soil.

Soil Formation
The process of soil formation is so slow that the soil is regarded as a non-renewable resource.

Pedogenesis
It is the process of formation of soil from rocky earth’s crust. It involves following two processes:

1. Weathering
2. Decomposition of organic matter and subsequent humification and mineralization.

1. Weathering :
It is the process of breaking down of rocks into small, fine mineral particles. It may occur due to physical, chemical or biological means.

1. Physical weathering : It involves pulverisation of rocky matter caused by physical (climatic changes such as heating, cooling, wetting-drying, frost action) and mechanical forces (abrasion by rain and hail, rolling stones, wave action, wind action)
   1. Sun : Under the influence of solar radiations, the rocks heat up and expand. At night, these cool down and contract. Since all parts of the rocks do not expand and contract at the same rate, cracks appear in the rocks and ultimately the large rocks break up into smaller pieces.
   2. Water : Water infuences the formation of soil in two ways : –
      It gets into the cracks in the rocks formed due to uneven heating of different parts of the rocks by the sun. On freezing, the water expands in rock crevices and breaks the rocks.
      Flowing water wears away even hard rocks over long periods of time. Fast flowing water generally carries various-sized particles of rocks downstream. On the way, these moving rock particles rub against other rocks. The resultant abrasion forms still smaller particles. The water takes these particles of rocks and deposits them down its path. In this way, soil is found in places far away from its parent rock.
   3. Wind : Strong winds influence the formation of soil by continuosly rubbing against rocks and eroding them. These also carry sand from one place to other.
   4. Living organisms (biological weathering) : Lichens, mosses (bryophytes) and other plants also influence the formation of soil. The lichens live on the rocks and produce acids. The latter corrode the surface of rocks to form thin layer of soil. Other small plants, e.g., mosses later grow on such surfaces and cause the rocks to break up further. Roots of trees sometimes enter the cracks and provide anchorage. As the trees grow, roots also grow bigger and force of cracks to widen. The whole process of weatherning of rocks involving living organisms is called biological weathering.
2. Chemical weathering of rocks involves a number of chemical processes such as hydrolysis, hydration, oxidation and reduction. For instance, complex compounds present in the rocks are broken down by the action of carbonic acid present in water or by acidic substances derived from the decomposition of organic matter in soil. The end products of chemical weathering are silica, hydrated oxides, inorganic salts etc.
   The process of weathering can be divided into three stages:
   Stage I Weathering, be it physical or chemical, causes large pieces of parent rock near the surface to disintegrate into smaller ones.
   Stage II Smaller rocks continue to undergo weathering, thus allowing a very fine layer of rock particles to form at the surface of the Earth. Bacteria and other microorganisms break down plant and animal remains to add humus to it. This makes the soil fertile and rich in organic materials and minerals, which are vital to plant growth.
   Stage III Minerals and salts seep deeper into the ground along with water to complete the formation of soil and make it favourable for plant growth.
   

2. Decomposition of Organic Matter and Subsequent Humification and Mineralization :
It involves stepwise degradation of organic materials by bacteria and fungi of decay and subsequent humification and mineralization. Dead remains of plants and animals are called detritus. The decomposition involves stepwise degradation of detritus. The process of decomposition of detritus is vital in any ecosystem because it involves release of the nutrients from the dead organic matter. Without this process, all the nutrients would remain locked in the dead remains of plants and animals and will not be available for recreation of living matter.

1. Humification : It is the process by which simplified detritus (partially decomposed organic mater) gets converted into dark coloured amorphous substance called humus in the soil. Humus is the reservoir of nutrients.
2. Mineralization : It is the process which results in the release of inorganic substances such as CO₂, H₂O and nutrients (e.g., NH₄⁺, Ca⁺⁺, Mg⁺⁺, K⁺ etc.) in the soil.

Humus binds the weathered rock particles into aggregate called crumbs. The latter absorb water and hold air in spaces.
Detritivores such as nematodes, earthworms, centipedes, millipedes, mites and ants consume organic matter and add excretory nitrogen to it. It is, thus, the final step in the formation of soil.

Main factors that influence the formation of soil from the rocks are 
There are a number of factors which affect soil formation, namely, climate, characteristics of the parent rock, and slope of land.

1. Climate: Climatic factors or weathering forces such as temperature, rain, wind, etc., play
   an important role in loosening and breaking up rocks (physical weathering). Water in rock revices expands when it freezes, hence, increasing the pressure and causing the rocks to ti crack. Rocks are also worn down by wind.
   Soil formation is fastest in hot, moist climates and slowest in cold, dry climates. In the warm and wet tropics, bacterial activity is high. As a result, soil in these areas have a rich content of humus. On the other hand, desert regions have very little vegetation and animal
   life, and so limited decaying material is available for the soil. The lack of rainfall leads to the formation of coarse-textured soil in dry and arid regions. In areas that experience severe winter, bacterial activity is very slow causing dead plant and animal matter to build up. Therefore, soil in such places forms very slowly. Thus, the rate of soil formation and the type of soil formed depends on the climate of a place.
2. Characteristics of the parent rock: The composition of the parent rock also determines the kind of soil formed. For example, if the parent rock is rich in calcium, the soil formed by its weathering would be rich in calcium, too.
3. Slope of land: The physical features of a place play an important role in the formation of soil. Hilly areas generally have a thin layer of soil because running water and wind tend to carry the soil down the slope. On flat plains, soil remains relatively undisturbed leading to the formation of a thick layer.