The formation of soil starts with the parent material and continues for a very long period of time taking 1000 years or more. As the parent material is weathered and / or transported, deposited and precipitated it is transformed into a soil. The parent material may be in the form of bedrock, glacial deposits, and loose deposits under water or material moving down sloping land.
Factors Affecting Formation of Soil
Soil formation process is influenced by the following factors
- Composition of Parent Material
Bare rocks exposed to warm climate, frequent and heavy annual rainfalls causes faster development of soil.
It is a natural process of breaking down of rocks, and minerals as well as artificial materials due to its prolonged exposure to the environment. These particles when moved away by gravity, water, ice, wind etc is then known as erosion. In short, when a particle is loosened by a chemical or physical process, but does not move, it is called Weathering. When the particle moves, it is called Erosion.
Types of Weathering
Depending upon the mechanism and causes, weathering can be divided into three broad categories
- Physical Weathering
- Chemical Weathering
- Biological Weathering
Physical Weathering also known as Mechanical Weathering may occur due to the following reasons
1. High Speed Winds
High Speed winds causes the erosion of surface material
2. Thermal Stress
Thermal stresses are caused by repeated heating and cooling. In deserts the temperature at day time is more causing expansion of bodies while at night the temperature decreases causing contraction eventually leading to breaking of rocks
3. Freezing and Thawing
The moisture or water penetrated into cracks when freezes, it expands causing stresses and breaking of rock
4. Pressure Releases
When some of the material is removed from the surface, the load is reduced and the underlying body expands up causing cracking.
5. Hydraulic Actions
Water from power full waves when strike against the rock causes erosion.
6. Salt-Crystals Growth
When saline solutions seep into cracks and joints in the rocks and evaporate it leaves salt crystals behind. These salt crystals expand as they are heated up thus exerting pressure on the confining rock. In Physical Weathering the broken down particles have same properties as the parent material
Chemical Weathering may occur due to the following processes and reactions
1) Dissolution and Carbonation
In Chemical Weathering the broken particles have different composition from the parent material and change into different substances
1) Dissolution and Carbonation
- In unpolluted environments, the rainfall pH is around 5.6.
- Acid rain occurs when gases such as sulfur dioxide and nitrogen oxides are present in the atmosphere.
- These oxides react with the rain water to produce stronger acids and lower the pH to 4.5 or even 3.0 causing solution weathering to the rocks on which it falls.
- When rain combines with carbon dioxide or an organic acid it forms a weak carbonic acid which reacts with calcium carbonate (Rock) and forms calcium bicarbonate.
- The process speeds up with a decrease in temperature because colder water holds more dissolved carbon dioxide gas
In this process rock minerals absorb water and expand, creating stress which causes the disintegration of rocks.
Iron Oxide + Water ---> Iron Hydroxide (Increased Volume)
Anhydrite + Water ---> Gypsum (Increased Volume)
Anhydrite is Unhydrated Calcium Sulphate.
Gypsum is Hydrated Calcium Sulphate
In this process a chemical reaction takes place between the minerals (Silicates and Carbonates) in the rock and Hydrogen (H+) in rain water.
In this process oxygen combine with water and minerals in the rock such as iron, calcium and magnesium to form their oxides
When iron reacts with oxygen, reddish-brown iron oxide is formed. The iron-oxide crust crumbles easily and weakens the rock. This process is commonly known as Rusting
Iron + Oxygen ---> Iron Oxide (Crumbles)