Soil-Source of Minerals

• Soil is the uppermost layer of the Earth's crust which is composed of different minerals/ nutrients that helps in generation of biodiversity. A layer of dead and decayed plant and animal remains called humus is also present in the soil.

Soil

Humus

• Soil also provides the vital nutrients required for the growth of the plants. All living organisms depend directly or indirectly on soil.

Soil Formation:

• Soil formation is a slow, stepwise process and it takes thousands of years to form a layer of soil just a few centimeters thick.
• It is formed by the breaking of rocks at or near the surface of the earth due to various physical, chemical or biological processes.

Processes involved in soil formation:

Weathering:

• Weathering is the disintegration of rocks on the Earth's surface caused by exposure to natural forces such as wind, water, frost, roots of plants, etc.
• Weathering is of two types:

1. Physical weathering

2. Chemical weathering

3. Biological weathering

Physical Weathering:

• This is done by various factors like sun (temperature variation), water, wind and other natural physical forces.
• Sun heats up the sources due to which rocks expand and at night as the temperature falls rocks contract again. Such expansion and contraction during day/night and different seasons creates cracks in them, facilitating their breakdown.
• Water gets into the cracks which later freezes and widens the cracks leading to its breakdown. Also flowing water can wear away the rocks which when rubs each other causes the abrasions.
• Winds can also rub the rocks and wear them into smaller particles. Abrasion is therefore one of the main cause behind soil formation.

Chemical Weathering:

• It involves the reaction of components present in the rocks with the other surrounding like water, acid etc. the reactions involved are oxidation, reduction, hydration, hydrolysis etc. The products produced constitute the soil.

Biological weathering:

• Weathering can also be caused by living agents.

Example:

• Bacteria or lichens growing over the surface of the rocks secrete some chemicals which can decompose the rocks or produce the crevices in them.
• Mosses or big trees growing here can deepen the crevices and form the cracks.
• These cracks gradually widen and cause the fragmentation.

Paedogenesis (soil development):

• Growth of bacteria or other living organisms in soil leads to its enrichment or the development of humus in it.
• Example: Detrivores like nematodes, earthworms feed on organic matter present in the soil and adds excretory nitrogen and other products in it hence enriching the quality of soil.

Soil profile:

• Due to the physical, chemical, and biological activities that take place during soil formation, well-developed layers of soil are formed, one below the other.
• Typically, four distinct soil layers can be seen, distinguishable due to differences in colour and size of the particles. These layers are referred to as horizons.
• A vertical section that shows the distinct layers of soil or horizons when we cut straight down into the soil is known as a soil profile.

Horizon A or Topsoil:

• It is the uppermost layer of soil. It consists of fine particles, is rich in humus and therefore dark in color. It is soft, porous, and can hold water. It is covered by a layer of organic matter.

Horizon B or Subsoil:

• It is rich in minerals and other materials, which seep down along with water. It also contains compactly packed fine particles of soil.

Horizon C:

• It contains partly weathered pieces of rock. It is usually characterized by cracks and crevices. It is very difficult to dig beyond this layer.

Horizon R or Bedrock:

• It is the lowermost layer and mainly consists of the parent rock. It undergoes weathering and starts forming the soil. The properties of this rock greatly determine the composition of the soil.

Composition of soil:

• The main constituents of soil are: Sand, Clay, Silt, Pebbles and Humus. Type of plants that can grow in the soil depends on three factors:
• Nutrient content of the soil
• Amount of humus
• Depth of the top soil

Types of Soil:

• The difference in particle size of the various soil constituents gives rise to a property called texture.
• Based on the proportion of sand, silt, clay, and humus present, soil is of different types.

 Sandy soil:

• More than two-thirds of sandy soil is sand, and about one-tenth is clay.

• It is made up of large particles with large air spaces.
• The porosity of the soil prevents retention of water. So it dries up very easily.
• It is well suited for fruit and vegetable cultivation.

Clayey soil:

• More than two-thirds of clayey soil is clay.
• It is made up of fine particles with small air spaces.

• This soil retains moisture and becomes very sticky when wet.
• Dry clayey soil becomes hard and forms deep cracks.
• To improve this soil, chalk can be added.

Loamy soil:

• Loamy soil has sand and silt in equal amounts, about two-fifths each.

• One-fifth of loamy soil is clay.
• It has high humus content.
• It is porous and also holds moisture.
• It is good for cultivation of crops and gardening.

Uses of soil: