Soil CLASS. VII Soil Soil is an inseparable part of our life. Explain Humus Weathering The nature of soil: Soil Profile Horizons.

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1 Soil CLASS. VII Soil : The mixture of soil particles with humus is called soil. Soil is an inseparable part of our life. Explain 1. Soil is one of the most important natural resources. 2. It supports the growth of plants by holding the roots firmly and supplying water and nutrients. 3. It is the home for many organisms. 4. Soil is essential for agriculture. 5. Agriculture provides food, clothing and shelter for all. Humus: The rotting dead matter in the soil is called humus. Weathering: The soil is formed by the breaking down of rocks by the action of wind, water and climate. This process is called weathering. The nature of soil: depends upon 1. the rocks from which it has been formed and 2. The type of vegetation that grows in it. Soil Profile: A vertical section through different layers of the soil is called the oil profile. Each layer differs in feel (texture), colour, depth and chemical composition. These layers are referred to as Horizons. A Horizon: The uppermost horizon is generally dark in colour and it is rich in humus and minerals. The humus makes the soil fertile and provides nutrients to growing plants. This layer is generally soft, porous and can retain more water. It is called the topsoil or the A-horizon. A- Horizon This provides shelter for many living organisms such as worms, rodents, moles and beetles. The roots of small plants are embedded entirely in the topsoil. B- Horizon_ The next layer has a lesser amount of humus but more of minerals. This layer is generally harder and more compact and is called the B-horizon or the middle layer.

2 C- Horizon: The third layer is the C-horizon, which is made up of small lumps of rocks with cracks and crevices. Below this layer is the bedrock, which is hard and difficult to dig with a spade. ( diagram) Types of soil: The soil is classified on the basis of the proportion of particles of various sizes. Thus, the soil can be classified as sandy, clayey and loamy. Sandy soil: if soil contains greater proportion of big particles it is called l sandy soil. Clayey soil: If the proportion of fine particles is relatively higher, then it is called clayey soil. Loamy soil: If the amount of large and fine particles is about the same, then the soil is called loamy. Properties of Sandy soil: 1. Sand particles are quite large. The particles cannot fit closely together, so there are large spaces between them. 2. The spaces are filled with air. 3. The sand is well aerated. 4. Water can drain quickly through the spaces between the sand particles. 5. Sandy soils are light, well aerated and dry. Properties of Clayey Soil: 1. Clay particles, are smaller, packed tightly together, leaving no space for air. 2. In clayey soil water can be held in the tiny gaps between the particles of clay. 3. Clayey soils are heavy as they hold more water. Properties of loamy soil 1. The best topsoil for growing plants is loamy soil. 2. Loamy soil is a mixture of sand, clay and silt. 3. Silt occurs as a deposit in river beds. 4. The size of the silt particles is between those of sand and clay. 5. The loamy soil has humus in it. 6. It has the right water holding capacity for the growth of plants

3 Percolation rate of soil: Percolation rate (ml/min) = amount of water (ml) / percolation time (min) Ex. It took 20 minutes for 200 ml to percolate. Calculate the rate of percolation. Rate of percolation = 200 ml / 20 min = 10 ml/min On a hot summer day soil seems shimmering. Give reason. On a hot summer day, the vapour coming out of the soil reflect the sunlight and the air above the soil seems to shimmer Activity to show that soil contains moisture: Method: Take a boiling tube. Put two spoonful of a soil sample in it. Heat it on a flame and observe it. Observation: We see water drops on the inner walls of the test tube Result: On heating, water in the soil evaporates, moves up and condenses on the cooler inner walls of the upper part of the boiling tube. Activity to determine the amount of water absorbed by the soil: Take a plastic funnel. Take a filter paper (or a piece of newspaper sheet), fold and place it. Weigh 50g of dry, powdered soil and pour it into the funnel. Measure a certain amount of water in a measuring cylinder and pour it drop by drop on the soil. Do not let all the water fall at one spot. Pour water all over the soil. Keep pouring water till it starts dripping. Subtract the amount of water left in the measuring cylinder from the amount you started with. This is the amount of water retained by the soil. Observations: Weight of soil = 50g Initial volume of water in the measuring cylinder = U ml Final volume of water in the measuring cylinder = V ml Volume of water absorbed by the soil = (U V) ml

4 Weight of water absorbed by the soil = (U V) g (1 ml of water has weight equal to 1 g) Percentage of water = U-V/50 X 100 The important climatic factors which affect the soil profile. Soil is affected by wind, rainfall, temperature, light and humidity. Soil types and crops: Clayey and loamy soils are suitable for growing cereals like wheat, and gram. Such soils are good at retaining water. For paddy, soils rich in clay and organic matter and having a good capacity to retain water are ideal. For lentils (masoor) and other pulses, loamy soils, which drain water easily, are required. For cotton, sandy loam or loam, which drain water easily and can hold plenty of air, are more suitable. Crops such as wheat are grown in the fine clayey soils, because they are rich in humus and are very fertile Soil erosion: The removal of land surface by water, wind or ice is known as erosion. No Vegetation, More soil erosion: plant roots firmly bind the soil. In the absence of plants, soil becomes loose. So it can be moved by wind and flowing water. Erosion of soil is more severe in areas of little or no surface vegetation, such as desert or bare lands. Methods to prevent Soil erosion: 1. Prevent Cutting of trees and deforestation 2. Increase the green areas. SOIL FACTS; Which soil would have the highest percolation rate? SANDY SOIL Which soil would have the lowest percolation rate? CLAYEY SOIL

5 Which type of soil will allow water to reach a well faster and in greater amount? SANDY SOIL Which type of soil retains the highest amount of water (CLAYEY SOIL) and which retains the least? (SANDY SOIL) Can you suggest any method to let more rain water percolate and reach the water underground? 1. Make rain pits to collect rain water and allow to recharge. 2. Dig / loosen the top soil regularly. 3. Prevent the free flow of water. 4. Make small earthen bunds.

DELHI PUBLIC SCHOOL TAPI

DELHI PUBLIC SCHOOL TAPI CLASS VII SUBJECT: SCIENCE Ch 9- SOIL Subjective Type Exercises A. Very Short Answer Questions 1. What is weathering? Ans: The process of breaking down of rocks by the action of