These DAV Class 7 Science Notes and DAV Class 7 Science Chapter 10 Notes – Soil act as excellent revision resources, particularly in preparation for board exams.
Soil Class 7 DAV Notes
→ Soil: It is the naturally occurring, loose covering of broken rock particles and decaying organic matter (humus) on the surface of the Earth which can support life.
→ Weathering: A combination of physical, chemical and biological process on rocks is called weathering. It leads to soil formation. Weathering can be of two types, viz. physical and chemical weathering.
→ Agents of Physical Weathering:
- Temperature: Rocks expand and contract due to changes in temperature and become weak. Gradually, the rocks break apart.
- Water: Flowing water washes away the broken particles of rocks and deposits them somewhere else as sediment. Further, the flowing water also exposes the underlying rocks to different weathering agents.
- Wind: Wind has abrasive effect on rocks, due to this rock particles are swept away by wind and deposited elsewhere.
- Plants and animals: Lichens and mosses grow on bare rocks and cause gradual disintegration of rocks. Grasses, shrubs and trees also break rocks because their roots enter the crevices in the rocks.
→ Chemical Weathering: Organic acids released during the decomposition of organic matter lead to weathering of rocks. Salts dissolve in water and the solution causes weathering.
→ Soil Profile: Arrangement of different layers of soil is called soil profile. Different layers of soil; at a particular place; are called soil horizons. Various soil horizons are as follows:
- O Horizon: This is the topmost layer of soil. It is mostly composed of leaf litter and humus.
- A Horizon: This is below the O horizon and is also known as topsoil. It is composed of humus and minerals.
- B Horizon: This is also called sub-soil. It is composed of clay and mineral deposits.
- C Horizon: This is composed of slightly broken up bedrock. Roots of plants do not penetrate this layer.
- R Horizon: This layer is at the bottom and is composed of unweathered rock.
Properties of Soil:
Soil Texture: The bigger and coarse particles are called sand. The smaller and smooth particles are termed clay. The intermediated particles which are neither very rough, nor too smooth are called silt. On the basis of texture, soil can be classified into three types:
(a) Sandy Soil: This soil is composed of bigger particles. This soil is well aerated. This type of soil cannot retain water because water drains out quickly. Sandy soil is not fit for farming.
(b) Clayey Soil: This soil has a higher proportion of fine and smooth particles. This type of soil is not well aerated but can retain lot of water. This type of soil leads to problem of water logging in the fields.
(c) Loamy Soil: This soil is a mixture of sand, silt and clay particles. It can hold reasonable amount of water and air. This soil is best for farming.
→ Soil Colour: Soil colour differs due to mineral content of parent rock and amount of organic matter. Soil can be red, yellow, brown or black. Dark coloured soil often contains a good amount of humus. Red colour of soil is because of high proportion of iron in it.
→ Soil Structure: Soil particles have a tendency to stick together, if the soil contains a high proportion of clay. On the other hand, sandy soil does not stick together.
→ Plasticity and Cohesion: Because of plasticity, a particular type of soil can be given any shape. Sandy soil is non-plastic, while clayey soil is plastic in nature. Plastic soil is cohesive in nature which means the particles of soil stick together.
→ Soil Density: Soil with larger particles is denser than soil with smaller particles.
→ Infiltration and Percolation Rate: Absorption of water by the soil depends on infiltration and percolation rate. The rate at which water enters a soil is called infiltration rate. The rate at which water moves through soil is called percolation rate. A soil with a higher infiltration rate but moderate percolation rate is good for farming.
→ Composition of Soil: Following components are generally found in soil:
→ Inorganic substances: Most of the inorganic components of soil are derived from the parent rock.
→ Organic Material: The organic matter mainly comes from dead and decaying parts of plants and animals. The high humus content means the soil would be highly fertile.
→ Soil water: Soil water is usually present in the spaces between soil particles.
→ Soil Air: Air is present in spaces between soil particles. Soil air is important for respiration by roots.
→ Living Organisms: Many organisms are present in soil. For example; bacteria, fungi, algae, etc. Some insects and small creatures also live in soil.
→ Soil as a Resource: Soil is an important resource for us. We get food from plants and plants need soil to grow. Soil is an important component in mining and construction industries. Soil is the habitat for many organisms.
→ Soil Pollution: Accumulation of toxic compounds, chemicals, salts, radioactive materials, or disease causing agents; which can have adverse effect on plant and animal health; is called soil pollution. Chemical pesticides, excessive use of fertilisers and contamination of surface water are major causes of soil pollution.
→ Soil and Crops: Different types of soil are suitable for different types of crops. Cereal crops; like wheat and legume grow well in clayey soil and loamy soil. Cotton grows in sandy loam or loamy soil.
→ Soil Erosion: The removal of topsoil by agents of weathering is called soil erosion. Human activities hasten the rate of soil erosion and aggravate the situation. Plants prevent soil erosion because they hold the topsoil in place with their roots.
→ Prevention of Soil Erosion: Soil erosion can be prevented by afforestation practices. Afforestation involves planting trees on bare lands and nearby fields.
→ Weathering: The process of gradual breaking of rocks is called weathering.
→ Soil profile: Arrangement of various layers of soil in a place is called soil profile.
→ Humus: The soil content which consists of dead and decaying organic material.
→ Deforestation: Mindless cutting of trees to remove forest cover.
→ Afforestation: Planting new trees to recover forest cover.
→ Soil pollution: Contamination of soil which makes it unsuitable for living beings.
→ Infiltration rate: The rate at which water enters the soil.
→ Percolation rate: The rate at which water flows through porous soil.
→ Soil horizons: Different layers in soil.
→ Soil profile: Arrangement of soil horizons at a place.
→ Weathering: the process of breaking down of rocks by various agents of weathering.