Textbook Solution: Soil | Class 7 Oxford Science: Chapter Notes, Worksheets & Tests PDF Download

A. Choose the correct option.

1.
Ans: (a)

2.
Ans: (c)

3.
Ans: (d)

4.
Ans: (b)

5.
Ans: (c)

6.
Ans: (a)

7.
Ans: (d)

B. Fill in the blanks with the correct words.

1.
Ans: Breakdown of rocks over a period of time forms soil.

2.
Ans: If the parent rock is dark brown, the soil will be dark brown in color.

3.
Ans: Properties of the soil depend on the properties of the parent rock.

4.
Ans: The difference in particle size of the soil determines its texture.

5.
Ans: Clayey soil becomes sticky and heavy when wet.

6.
Ans: Plants grow well in neutral soils.

7.
Ans: Terrace farming is a method to prevent soil erosion.

8.
Ans: Prevention of soil erosion is called soil conservation.

9.
Ans: Deforestation increases soil erosion.

10.
Ans: We should try to use organic pesticides and fertilizers to decrease soil pollution.

C. Answer the following questions.

1.
Ans: Soil is the brownish or blackish substance that forms the uppermost layer of the Earth's crust on which plants grow. It is necessary for the survival of plants and animals. Soil is formed by a process called weathering, which involves the breakdown of rocks on the Earth's surface due to natural forces such as wind, water, frost, or the action of plant roots. This is a slow and stepwise process taking thousands of years.

2.
Ans: Factors affecting soil formation include climate, properties of the parent rock, slope of land, and biological factors. The climate determines the rate of soil formation and the type of soil formed. The composition of the parent rock influences the kind of soil it forms. The physical features of a region play a role in soil formation, with mountain regions having a thinner layer of soil and plains having a thicker layer. Biological factors like plants, animals, and microorganisms also contribute to soil formation.

3.
Ans: Humus is formed from the dead and decomposed remains of plants and animals. It is produced by microbes such as bacteria and fungi, and it improves the fertility of soil.

4.
Ans: Humus helps plants by improving soil fertility. It enhances the soil's capacity to hold and supply nutrients to plants, increases the soil's water-holding capacity, and improves soil structure for better root growth.

5.
Ans: Soil can be divided into three main types: sandy, clayey, and loamy soil. Sandy soil, made up of a large proportion of sand, drains water easily but retains very little. Clayey soil, mostly made up of clay, retains a lot of water. Loamy soil, a mixture of sand and clay with high humus content, is ideal for growing crops as it retains just enough water for use by plants.

6.
Ans: A soil profile refers to the layers of soil that are formed due to soil formation processes. These typically include the topsoil or A horizon, rich in humus and where plants grow; the subsoil or B horizon, rich in minerals and partly weathered pieces of rock; the C horizon, consisting of partially broken or unbroken rocks; and the bedrock, the parent rock from which soil begins forming.

7.
Ans: You can differentiate soils by feeling them. Sandy soil feels rough and gritty, clayey soil feels smooth and slippery, and loamy soil is crumbly and dark but not gritty, smooth, or slippery.

8.
Ans: The percolation rate of soil refers to the rate at which water can pass through soil. Soils with larger particles, like sandy soil, have a higher percolation rate as they have more space for water to pass through quickly. Soils with smaller particles, like clayey soil, have a lower percolation rate as they hold water for long and do not allow it to pass easily. 

9.
Ans: Moisture in soil is important as it is the water source for plants. The amount of moisture in a soil determines whether plants can grow in it.

10.
Ans: Soil fertility refers to the ability of a soil to provide the required nutrients and water in the right amounts needed for the growth of plants. Soil fertility can be improved by adjusting the soil pH, improving soil aeration, managing soil moisture content, and increasing the nutrient content of the soil through the addition of fertilizers or manure.

11.
Ans: Soil erosion is the loss of topsoil caused by wind and water, which depletes the soil of nutrients. Major causes of soil erosion include deforestation, overgrazing, floods and heavy rainfall, and improper farming practices. It can be reduced through measures like afforestation, controlling overgrazing, flood control, and adopting proper farming practices like terrace farming.

12.
Ans: Soil pollution is caused by the dumping of chemicals from factories, mines, and industries, accumulation and improper dumping of garbage and sewage wastes on land, excessive use of pesticides and fertilizers, and accumulation of non-decomposable wastes like plastics and metals. It can be controlled by properly treating and disposing garbage, composting organic wastes, using organic fertilizers and pesticides, and treating chemical wastes from mines and factories before disposal.