NCERT 7th Class (CBSE) Science: Soil

Question: Mention two ways by which farmer’s field could get eroded.

Answer: Two ways by which farmer’s field gets eroded are:

1. Inappropriate farming techniques such as deep ploughing the land two or three times a year to produce annual crops can lead to soil erosion.
2. Over cultivation of crops and overgrazing by animals can cause the soil to lose its structure and binding capacity, which makes it easier to erode.

Question: What is weathering? List different types of weathering.

Answer: Weathering is the process of breaking down of rocks into smaller particles by the action of wind and water, change in temperature and penetrating roots of plants.

Weathering is of three types:

1. Physical weathering: The breaking down of big rocks into smaller-sized rock particles with the help of agents like wind, water or change in temperature is called physical weathering. It is also called mechanical weathering.
2. Chemical weathering: Chemical weathering changes the chemical nature of the soil. When rainwater mixes with gases like carbon dioxide, sulphur dioxide and nitrogen oxides in the atmosphere, it forms acid rain. When acid rain falls on the surface of the earth, it makes the soil slightly acidic.
3. Biological weathering: The roots of trees and burrowing of animals inside the soil bring about this type of weathering. Roots of trees and plants grow through the rocks, contributing to weathering. As the roots grow bigger, they penetrate deeper into the soil, forming cracks in rocks and helping soil formation.

Question: How is soil classified based on its absorption capacity? Give one property of each type.

Answer: Soil is classified into three main types based on its absorption capacity:

1. Sandy Soil: It contains large coarse particles, which is called sand. It is gritty, with very little humus and lot of air spaces between the particles. This type of soil is well aerated and cannot hold water for long. This type of soil is found in deserts and marshy areas.
2. Clayey Soil: It contains fine particles made of clay. The soil particles have very little air spaces between them. Hence, this type of soil retains water making soil sticky. Water logging reduces the availability of air between the soil particles, making the soil infertile.
3. Loamy Soil: It contains large and fine particles of sand, silt and clay. This is the most fertile type of soil and proves to be the ideal plant growth. This soil is well aerated and also contains a lot of humus. Silt is a type of soil formed due to depositions in river beds.

Question: With the help of diagrams, explain the stages of soil formation.

Answer: Soil formation takes place in three main stages:

1. First Stage: Parent rocks or bed rocks get broken down into smaller-sized rocks by the action of wind, water or change in temperature. The rocks are broken down with the help of decaying remains of plants and animals present in the parent rock during this stage.
2. Second Stage: Further weathering continues to form smaller rock particles. In this stage, the dead plants and animals present in the soil get decayed by the action of bacteria and other microorganisms to form humus.
3. Third Stage: In this stage, minerals and salts reach deeper into the soil with water, and increase the fertility of the soil to make it suitable for growth of parents. This stage completes the process of soil formation.

Question: Explain the different properties of soil.

Answer: The different properties of soil are:

1. Absorption: The ability of a particular type of soil to absorb and retain is called its water-holding capacity. The water-absorption capacity of clay is more than that of sand. The higher the absorption of water by soil, the higher is the availability of water to plants.
2. Moisture: Moisture is the water present in the soil. Since different soils have different water-holding capacities, the amount of moisture present in each type of soil will be different. Water can easily pass through sandy soil, which is also well aerated. So, it allows water to drain out quickly and does not retain moisture for long. Clayey soil takes up water slowly but can hold a lot of water. Since they cause water stagnate and collect above its surface, especially after a heavy rain, they cause waterlogging, choking the plant roots and finally leading to the death of plants. Loamy soil holds the right amount of moisture for plant growth. Humus and silt present in these soils help in holding the right amount of water required for plant growth.
3. Percolation rate: Percolation means the movement of water through the soil. Percolation rate is the rate (speed) at which water flows through the pores in the soil. The soil having a high water-holding capacity will have a low percolation rate and vice-versa.

Question: List the types of soil suitable for the growth of crops. Give examples of crops that can be grown in each type.

Answer: The kind of minerals present in the soil, the acidity and alkalinity of the soil and humus are the factors that play an important role in the growth of crops. The Indian Council of Agriculture Research (ICAR) has identified the following soil types:

1. Alluvial soil: Alluvial soil is rich in nutrients and may contain metallic ores. This soil is formed in regions having streams and rivers. This soil is light to dark in colour and is rich in potash and humus. Crops grown in alluvial soil are rice, sugarcane, cotton, jute and wheat.
2. Black soil: Black soil is also called black cotton soil. it is dark grey to black in colour and rich in calcium (Ca), magnesium (Mg), Carbonate (Co3) and aluminium (Al). Crops grown in black soil are cotton, sugarcane, goundnut, rice, wheat and oilseeds.
3. Red soil: It is sandier and less clayey. It is slightly acidic and does not retain moisture. Red soil is rich in iron and has less amount of humus. Crops grown in red soil are ragi, groundnut, millet, tobacco, potato, rice, wheat and sugarcane.
4. Laterite soil: The word “laterite” comes from a Latin word meaning “brick”. It is brown to yellowish in colour. This soil is rich in iron (Fe) and aluminium (AI) and is used as a building material. Crops grown in laterite soil are coffee, cashew nut, rubber and coconut.
5. Desert soil: It is porous and coarse. It has 95% sand and 5% clay. This soil is rich in nitrates and phosphates. Drought resist crops such as millets, barley, cactus and dates are grown in desert soil.
6. Mountain soil: It is mainly found on hill slopes. It is formed by the deposition of organic matter. It is rich in humus. Crops grown in mountain soil are tea, coffee, spices and tropical fruits. Under good management practices, this soil can be profitably used for growing a varity of agricultural, horticultural and plantation crops, which can benefit the country’s economy.

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