A natural way of SOIL formation from ROCKS | How Rock Cycle forms Soil | Pedology & Edaphology

A Natural Way of Soil Formation from Rocks.

 Our earth is full of life i.e. its oceans, atmosphere, and land. Water plays a very important role in maintaining life on Earth. Birds fly in the sky and animals that walk on land are very dependent on water but there’s also another very important thing that helps to maintain this life on earth and that thing is SOIL.

Plants like trees are very dependent on soil for growth and development. Soil is the source of minerals for plants and also the source of food and water.

Now, what is soil?

The upper or superficial layer of the earth’s crust is the soil on which most of the life (like plants and many microbes) are present. After the death of living organisms on land, their decay matter is also added to the soil.

The branch of the science in which we studied soil, Its origin, and the description and classification of soil based on its different characteristics is called Pedology. And the effect of soil on life especially plants is studied under another branch of science which we called Edaphology. 

Why and how soil is very important for land plants?

Soil is very important for plant growth as it plays different roles:

• The roots of the plants penetrate the soil which forms a good and healthy base of plants. Because of this strong base (provided by roots with soil), trees can stand straight and can bear the pressure of air.
• Soil is also the source of food and water for plants.
• Soil provides a medium for seed germination of plants.
• Soil is very complex to understand because huge-sized communities of microbes are present in the soil. One of these microbes is nitrogen-fixing bacteria that can fix nitrogen for plant use.
• A plant’s size, height, or erectness is very dependent on the soil’s health.
• Flower and fruit appearance on plants, woodiness, number of root hairs on roots, and number of flowers or fruits per plant can also be affected by the soil’s health and conditions. In short, the plant is very dependent on soil.

 

Now, how does soil form?

Soil is formed by weathering of rocks. This weathering can be physical or chemical. Generally, rocks are divided as:

1. Igneous rocks (formed by lava)
2. Sedimentary rocks
3. Metamorphic rocks

• Igneous rocks are formed by the cooling and hardening of any kind of lava. Granite is the most common example of igneous rock.
• Sedimentary rocks are derived from igneous rocks. It forms when igneous rocks are broken into pieces and these pieces are moved (primarily by water) and settled on any place with some pressure on it which leads to the formation of sedimentary rocks. As the name shows, sediments of igneous rocks form sedimentary rocks. Examples of sedimentary rocks are limestone and sandstone.
• Metamorphic rocks are next to igneous and sedimentary rocks. They are formed under high pressure and temperature. There is a considerable change in the mineral and texture composition of these rocks because of heat and pressure. Marble and slate are examples of metamorphic rocks.

These conversions look like cyclic which forms a rock cycle.

 

Now, what is the rock cycle, and How it forms Soil?

The rock cycle is the formation and conversion of rocks from one form to the other. 

• It usually begins with very hot magma or lava. This magma is a stream of fire that comes from the earth’s inner side. This magma contains many important particles which are formed by natural processes inside the earth. This magma when cooled and hardened in structure, it forms igneous rocks.
• The igneous rocks are then affected by wind and water which then forms sedimentary rock by breaking, moving, and compacting or cementing the sediments of igneous rocks.
• The sedimentary rocks are sometimes buried under the layers of the earth. These layers may increase the pressure and temperature of these rocks and this may result in the formation of metamorphic rocks. 
• The metamorphic rocks may be converted into igneous rocks if they melt with the earth’s magma and becomes its part. In this way, a natural cycle runs which we called a rock cycle.

The soil is formed during this rock cycle by weathering of rocks. This weathering may occur on the upper surface of the earth (Pedochemical weathering) or on the inner surface of the earth (Geochemical weathering). There are many physicals as well as chemical factors that can affect the process of weathering or soil formation. But firstly we have to understand the process of weathering.

 

What is weathering of rocks?

• Weathering of rocks means the breakdown of rock into small size particles. 
• First, a small crack appears on the surface of the rock. Water enters from these cracks. It results in freezing and expands the crack which results in the crack going deeper and deeper inside the stone. 
• Because of the increase in the crack’s size, more water can enter and this process continues. 
• It takes time but in the end, the rock is broken into small pieces. In this way, the process of weathering rocks takes place, and soil is formed in this process.

Now, how water did the weathering of rocks?

Water molecules react with minerals present in rocks. This process is called hydration and the association between water molecules and minerals of rocks formed a hydrate which serves as a bridge for the further attack of hydronium ions.

Al2O3 + 3H2O → Al2O3•3H2O (Hydrate)

 

Here are some factors that can detach the rock particles and can cause rock weathering.

• Water as we discussed earlier that water is an important factor in the weathering of rocks.
• Wind can accelerate the process of weathering and can detach the rock’s particles by exerting pressure on it.
• Lichens and mosses can grow on rock surfaces and cause gradual disintegration.
• The roots of plants can also detach the rock particles.

 Chemical weathering of Rocks.

Oxidation and reduction processes are the best examples of chemical weathering of rocks. 

• Oxidation takes place when a rock surface faces air or atmosphere. It is the condition when the supply of oxygen is high and biological demand is very low.

Oxidation (Fe2+ à Fe3+)  

• While in the reduction process, the biological demand for oxygen is high but its supply is very low.  

Reduction (Fe3+ à Fe2+)

  

As soil is formed as the result of the weathering of rocks and all the minerals in it come from rocks. And the rock cycle or rock formation usually begins within the earth where it is present in the form of magma. And when this magma comes to the earth's surface, it starts to cool and hardened in structure and thus the formation of rocks (i.e. igneous rocks). It's all the natural processes and nature plays it well. We can understand this process by observing the layers of soil. We can understand its (soil) profile.

 

Now what is meant by soil profile?

As we discussed soil formation and so we know soil has different layers which show different colors and other properties. Soil profile can be known by cutting a vertical section of soil through all its horizons. This section is cut from the upper layer of soil downward and goes as deep as you can. This shows a vertical or wall-like appearance of soil with many layers of different colors. 

The soil profile is important to determine the health of the soil and the concentration of minerals in the soil. It's not always important to go deep in the soil, sometimes we take a meter of the vertical sample of soil and sometimes it exceeds depending on the purpose we are interested in knowing the soil profile.  

Soil profile gives information about soil. This information can be observable (physical properties) like the color of soil, texture, particle size, and structure or can be testable (chemical properties) like pH, cation exchange capacity (CEC), etc.

 

• Color of soil.

Soil color is very useful for identification purposes the bright color of soil indicates drainage and aeration while the grayish color of soil indicates about saturated soil.

• Soil texture.

Healthy soil is known for its texture. Plants for any soil can be known by their texture. Soil has three components:

1. Sand
2. Silt
3. Clay

A soil having an ideal profile is more than the cave of gold because life is more dependent on soil than gold.