Soil Question Answers: NCERT Class 7 Science

(iii) Minerals, organic matter, air and water

In addition to the rock particles, soil contains: 50% minerals, 25% of water, 20% air and 5% of soil organic matter. And, the soil development contains two stages: Weathering, Humification and Mineralization.

Across
2. Erosion
5. Polythene
6. Clay
7. Earthworm

Down
1. Wind
3. Wheat
4. Sandy
5. Profile

(ii) Clayey soil

The water holding capacity of a soil is a very important for the agriculture. Soils that hold generous amounts of water are less leads to the losses of nutrients or soil-applied pesticides; it contains diameter range below 0.002 mm. The water holding capacity is highest in clayey soil because it is made up of very small tightly packed particles with > 50% clay that do not allow water to percolate that determines the soil texture. 

     Column I                                              Column II

(i) A home for living organisms               (b) All kinds of soil
(ii) Upper layer of the soil                       (c) Dark in colour
(iii) Sandy soil                                         (a) Large particles
(iv) Middle layer of the soil                     (e) Lesser amount of humus
(v) Clayey soil                                        (d) Small particles and packed tight

1. Soil is the topmost layer of the earth’s surface. It consists of rocks and mineral particles, water and air mixed with the soil organic matter. Soil formation begins with the breakdown of rocks called weathering and other materials into smaller pieces on the surface of the earth; and ends with humification and mineralization process.

2. The layers of the soil are called horizons. The uppermost horizon is known as topsoil layer. The topsoil layer is a mixture of sand, slit, clay and broken down organic matter, is called humus. Humus is rich, highly decomposed organic matter mostly made from dead plants, dead insects and twigs; incomplete decomposed black coloured organic matter.

3. Sandy soils contains mainly sand about 70 - 80% of sand, large particles with large spaces. It contains very few amount of humus. It is found in desert  or dried areas. Sandy soil has large particles with large spaces between them which dries out quickly that is not good for plants. Its water holding capacity is very low. 

4. The middle layer is called subsoil. Subsoil is the layer of soil under the topsoil on the surface of the ground. It contains lower amount of humus.

5. Clayey soil contains mainly clay about > 50% clay having very small particles with very small spaces. Clayey soil also contains very few amount of humus. Clayey soil is heavy because it has ability to hold more water than the sandy soil.

Soil is the thin layer of materials covering the earth’s surface and is formed from the weathering of rocks or differentiation from the parent rock. It is made up mainly of minerals particles 50%, 5% soil organic materials, 20% air and 25% water.

Formation of Soil:
Soil is formed from parent's rocks by the process of weathering. In weathering, rocks are broken down very slowly by the physical (temperature, exfoliation, water, glaciers etc.) and the chemical methods (hydrolysis, oxidation, reduction etc.) to form tiny rock particles. These tiny rock particles then mix up with humus to form fertile soil which is known as humification process. And, then release of organically bound nutrients occurs that changes into the inorganic form that are available to plants and microbes; this process is known as mineralization.

Clayey soil is useful for crops because:

1. Clayey soil is rich in organic matter.
2. It provides essential compounds and minerals to the crops so that they can grow better.
3. Clayey soil is more fertile than sandy soil.
4. Clayey soil has very good water-holding capacity due to its very small and tightly packed particles and contains greater than 50% of clay.
5. Clayey soil is rich in humus and very fertile, so it is suitable for the growth of the plants. Such soil is good at retaining water level.

Several layers or also called sequence of soil horizons of the soil develops one over th eother in the progression state of maturity. Usually there are three main horizons and the parent bedrock is present.

The 'A' horizon: It is very loose, fertile horizon or layer of soil. It contains plant roots, small insects, animals etc. This layer is composed of fresh and partially decomposed organic matter; and is dominant reservoir for the plant nutrients.

The 'B' horizon: It is the subsoil which contains few plant roots, some micro flora and fauna with less organic matter. It is composed of mineral soil where mineralization can occurs. it is also known as zone of Illuviation.

The 'C' horizon: It contains or represents unmodified freshy weathered parent material.

Bedrock is the last layer in the soil profile with or without small amount of organic matter and without it no life occurs.  

Given, Time = 40 minutes
Volume of water = 200 ml
Percolation rate = amount of water in ml                                                                                                                                                                                                                            Time taken for percolation
= 200 ml / 40 min
= 5 ml per minute

Soil Pollution: The contamination of soil with waste materials like polythene bags and plastics, pesticides, fertilizers, acid rain and industrial chemical wastes etc. that reduces the productivity level is called Soil pollution.

Prevent by soil pollution: In case of organic farming instead of using chemical fertilizers, insecticides and herbicides, bacterial fertilizer nutrients; use compost, green manure, bacterial culture, organic manure, bio-pesticides, and bio-agents that leads to less soil pollutiion. This reduces the degradation of the soil.
ï‚Ÿ By reducing the use of plastics, we can use jute and paper bags that is how we can reduce soil pollution.

Soil Erosion: The removal and trasportation of fertile top layer of soil from its original position by strong winds or fast running rainwater is called soil erosion.

Prevent by soil Erosion: Crop rotation -Rotating in high residue crops such as corn and small grain etc. that can reduce erosion as the layer of residue protects topsoil from being carried away by wind and water.

ï‚Ÿ By preventing deforestation and avoiding overgrazing of animals.
ï‚Ÿ Planting more and more trees (afforestation) will result in the reduction of soil erosion.