SOIL – Class 7

Soil is the upper most part of the surface of land. It contains many particles in it.

FORMATION OF SOIL

Weathering is a process in which huge rocks are broken down by the action of heat, wind, water and other natural and anthropogenic way to form small particles called soil.

Formation of soil is a slow and a regular process.

AGENTS OF WEATHERING OF ROCKS

1. Heating and cooling of rock

• Heating of rocks in hot sun during the day and cooling at night or sudden cooling of hot rock by rain breaks the rocks to smaller pieces.
• Change in season plays an important role in the breaking of rocks forming soil, that result in contraction and expansion of rock.

2. RUNNING WATER

• With the action of running water, the rocks do get cracks and thus facilitate the weathering of rocks. The cracks keeps on widening by the action of running water and changes into soil.

3. BLOWING WIND

• The action of high wind also acts as an agent of weathering. The fast moving wind helps in breaking of the partially weathered rocks.

4. FORMS OF WATER

• Frost condenses to form water which gets into the cracks in the rocks.
• Water from the rain or through the river enters into the cracks of rocks.
• When it is severe cold during winter, water in the cracks on rocks freeze forming ice. Thus, the ice and water in the solid as well as liquid state helps in the process of weathering of rocks.

5. LIVING ORGANISMS

• Lichens are symbionts (relationship between an alga and a fungus).
• They grow and get attached to the rocks.
• The carbon dioxide released by lichens during respiration gets dissolved in the moisture present in between the lichen and rock and forms carbonic acid. Water and Carbon dioxide result in the formation of carbonic acid.
• This acid reacts with the rock and disintegrates the rock into smaller soil particles.
• Small plants like Moss and other plants start growing on this newly formed soil and thus adds humus to it.
• Plant roots:
• The roots of plant grows deep into the soil.
• These roots develop cracks on the rocks and thus helps in weathering of the rock.
• Animals:
• The burrowing animals dig the surface of earth to build burrows, thus in this action the small rocks are also broken down and thus helps in soil formation.
• Human beings are also an important part of soil formation, from breaking of rocks for making buildings to breaking of rocks for other purpose, human beings contribute to the formation of soil.

SOIL PROFILE

• The vertical cut of soil from the ground surface to the bed rock that exposes different layers of soil is called soil profile.
• The different layers of soil is called horizons.
• A-HORIZON
  • A-Horizon is called top soil.
  • The uppermost layer of soil exposed to air.
  • This layer is soft, porous and capable of holding good amount of water.
  • This layer is rich in minerals.
  • This layer is rich in humus, thus is the darkest layer of the soil.
  • Helps most of the herbs in drawing water and minerals from it.
  • This layer is the most fertile layer.

• B-Horizon
• The layer of soil is below the topsoil and is called the subsoil.
• This layer is poor in humus but rich in minerals.
• This layer is lighter in color due to less humus in it.
• This layer is compact than the topsoil.
• This layer is rich in soil water.
• Soil water is the water that is available to plants, the water that is absorbed by the particles of soil is called soil water.
• This layer is home to many burrowing animals.
• This layer helps deep growing roots from shrubs and trees with water and minerals.

• C- HORIZON
• This is the layer below the subsoil, which is known as substratum.
• The substratum includes big pieces of broken-rocks which is formed as a result of partial weathering of rocks.
• The substratum hold water and some minerals.
• Water from this horizon is available to the trees all the year round.

• PARENT ROCK
• The parent rock is hard solid rock layer.
• This layer is impervious to water as the rocks are non- porous.
• As the rocks are non porous the spaces between the rocks have water accumulated known as groundwater.
• The groundwater contains water as well as dissolved minerals.
• Wells or tube wells are dug to reach the bedrock to draw groundwater collected on it.

SOIL COMPOSITION

• On the basis of particle size, the soil can be divided into:
  • CLAY
  • SILT
  • SAND
  • GRAVEL
• CLAY:
  • The size of clay particles are less than 0.002 mm
  • They have highest water retention capacity.
  • The clay particles do not percolate water.
• SILT:
  • The size of silt particles is between 0.2 mm to 0.002 mm
  • The silt soil is slippery when wet.
• SAND:
  • The size of sand particle is between 2.0 mm to 0.2 mm
  • The sand particles do not have high water retention capacity.
  • The sand particles have high percolation rate.
• GRAVEL:
  • The size of gravel is larger than 2mm.
  • They are non porous in nature.