Soil Soil is one of the most important natural resources. It supports the growth of plants by holding the roots firmly and supplying water and nutrients. It is the home for many organisms. Soil is essential for agriculture. Agriculture provides food, clothing and shelter for all. Soil is thus an inseparable part of our life. The earthy fragrance of soil after the first rain is always refreshing. 9.1 SOIL TEEMING WITH LIFE One day during the rainy season Paheli and Boojho observed an earthworm coming out of the soil. Paheli wondered whether there were other organisms also in the soil. Let us find out. Activity 9.1 Collect some soil samples and observe them carefully. You can use a hand lens. Examine each sample carefully and fill in Table 9.1. • Discuss your observations with your friends. • Are the soil samples collected by your friends similar to the ones collected by you? Boojho and Paheli have used soil in many ways. They enjoy playing with it. It is a great fun indeed. Make a list of the uses of soil.Table 9.1 S.No. Soil source Plants Animals Any other observations 1. Garden soil Grass, ………. Ant, ………. 2. Soil from the roadside ……………… 3. Soil from the area where construction is going on ……………… 4. ……………… ……………… 5. ……………… ……………… 9.2 SOIL PROFILE Soil is composed of distinct layers. Perform the following activity to find out how these layers are arranged. Activity 9.2 Take a little soil. Break the clumps with your hand to powder it. Now take a glass tumbler, three quarters filled with water, SOIL and then add a handful of soil to it. Stir it well with a stick to dissolve the soil. Now let it stand undisturbed for some time (Fig. 9.2). Afterwards, observe it and answer the following questions: humus water clay sand gravel • Do you see layers of particles of different sizes in the glass tumbler? • Draw a diagram showing these layers. • Are there some dead rotting leaves or animal remains floating on water? The rotting dead matter in the soil is called humus. You probably know that 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 any soil depends upon the rocks from which it has been formed and the type of vegetation that grows in it. A vertical section through different layers of the soil is called the soil profile. Each layer differs in feel (texture), 97 colour, depth and chemical composition. soil fertile and provides nutrients to These layers are referred to as horizons growing plants. This layer is generally (Fig. 9.3). soft, porous and can retain more water. It is called the topsoil or the A-horizon. We usually see the top surface of the soil, not the layers below it. If we look at the sides of a recently dug ditch, we can see the inner layers of the soil, too. Such a view enables us to observe the soil profile at that place. Soil profile can also be seen while digging a well or laying the foundation of a building. It can also be seen at the sides of a road on a hill or at a steep river bank. The uppermost horizon is generally dark in colour as it is rich in humus and minerals. The humus makes the 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. 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. The third layer is the C-horizon, which is made up of small lumps of rocks with cracks and crevices. Below 98 SCIENCE this layer is the bedrock, which is hard and difficult to dig with a spade. 9.3 SOIL TYPES As you know, weathering of rocks produces small particles of various materials. These include sand and clay. The relative amount of sand and clay depends upon the rock from which the particles were formed, that is the parent rock. The mixture of rock particles and humus is called the soil. Living organisms, such as bacteria, plant roots and earthworm are also important parts of any soil. The soil is classified on the basis of the proportion of particles of various sizes. If soil contains greater proportion of big particles it is called sandy soil. If the proportion of fine particles is relatively higher, then it is called clayey soil. If the amount of large and fine particles is about the same, then the soil is called loamy. Thus, the soil can be classified as sandy, clayey and loamy. The sizes of the particles in a soil have a very important influence on its properties. Sand particles are quite large. They cannot fit closely together, so there are large spaces between them. These spaces are filled with air. We say that the sand is well aerated. Water can drain quickly through the spaces between the sand particles. So, sandy soils tend to be light, well aerated and rather dry. Clay particles, being much smaller, pack tightly together, leaving little space for air. Unlike sandy soil, SOIL Now let us perform an activity to understand this. Activity 9.4 For this activity divide yourself into three teams. Name the teams A, B and C. You will be finding out how fast the water passes down the soil. You will need a hollow cylinder or a pipe. Ensure that each team uses pipes of the same diameter. Some suggestions for obtaining such a pipe are given below: 1. If possible, get a small tin can and cut off its bottom. 2. If PVC pipe (approx. diameter 5 cm) is available, cut it into 20 cm long pieces and use them. At the place where you collect the soil, place the pipe about 2 cm deep in the ground. Pour 200 mL water in the pipe slowly. For measuring 200 mL water you can use any empty 200 mL bottle. Note SCIENCE the time when you start pouring water. When all the water has percolated leaving the pipe empty, note the time again. Be careful not to let the water spill over or run down on the outside of the pipe while pouring. Calculate the rate of percolation by using the following formula: amount of water (mL) percolation rate (mL/min) = percolation time (min) For example, suppose that for a certain sample, it took 20 minutes for 200 mL to percolate. So, 200 mL rate of percolation = = 10mL/min 20 min Calculate the rate of percolation in your soil sample. Compare your findings with others and arrange the soil samples in the increasing order of the rate of percolation. 9.5 MOISTURE IN SOIL Have you ever passed through a farmland during a hot summer day? Perhaps you noticed that the air above the land is shimmering. Why is it so? Try out this activity and find the answer. Activity 9.5 Take a boiling tube. Put two spoonfulls of a soil sample in it. Heat it on a flame (Fig. 9.6) and observe it. Let us find out what happens upon heating. Do you see water drops any where? If yes, where did you find them? On heating, water in the soil evaporates, moves up and condenses on the cooler inner walls of the upper part of the boiling tube. SOIL Boiling tube Soil sample Burner On a hot summer day, the vapour coming out of the soil reflect the sunlight and the air above the soil seems to shimmer. After heating the soil, take it out of the tube. Compare it with the soil which has not been heated. Note the difference between the two. 9.6 ABSORPTION OF WATER BY SOIL Do all the soils absorb water to the same extent? Let us find out. Activity 9.6 Take a plastic funnel. Take a filter paper (or a piece of newspaper sheet), fold and place it as shown in the figure. 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. You can use a dropper for this purpose. Do not let all the water fall at one spot. 101 climatic factors, as well as the components of soil, determine the various types of vegetation and crops that might grow in any region. Clayey and loamy soils are both suitable for growing cereals like wheat, and gram. Such soils are good atSOIL • Soil is important for life on the earth. • Soil profile is a section through different layers of the soil, Various layers are called horizons. • Soil is of different types: clayey, loamy and sandy. • Percolation rate of water is different in different types of soil. It is highest in the sandy soil and least in the clayey soil. • Different types of soils are used to cultivate different types of crops. Clay and loam are suitable for growing wheat, gram and paddy. Cotton is grown in sandy loam soil. SCIENCE • Soil holds water in it, which is called soil moisture. The capacity of a soil to hold water is important for various crops. • Clayey soil is used to make pots, toys and statues. Exercises Tick the most suitable answer in questions 1 and 2. 1. In addition to the rock particles, the soil contains (i) air and water (ii) water and plants (iii) minerals, organic matter, air and water (iv) water, air and plants 2. The water holding capacity is the highest in (i) sandy soil (ii) clayey soil (iii) loamy soil (iv) mixture of sand and loam 3. Match the items in Column I with those in Column II: Column I Column II (i) A home for living organisms (a) Large particles (ii) Upper layer of the soil (b) All kinds of soil (iii) Sandy soil (c) Dark in colour (iv) Middle layer of the soil (d) Small particles and packed tight (v) Clayey soil (e) Lesser amount of humus 4. Explain how soil is formed. 5. How is clayey soil useful for crops? 6. List the differences between clayey soil and sandy soil. 7. Sketch the cross section of soil and label the various layers. 8. Razia conducted an experiment in the field related to the rate of percolation. She observed that it took 40 min for 200 mL of water to percolate through the soil sample. Calculate the rate of percolation. 9. Explain how soil pollution and soil erosion could be prevented. 10. Solve the following crossword puzzle with the clues given: SOIL 105 Across 2. Plantation prevents it. 5. Use should be banned to avoid soil pollution. 6. Type of soil used for making pottery. 7. Living organism in the soil. Down 1. In desert soil erosion occurs through. 3. Clay and loam are suitable for cereals like. 4. This type of soil can hold very little water. 5. Collective name for layers of soil. Extended Learning — Activities and Projects 1. Boojho would like to know the difference between raw and baked soil? Investigate how the soil from which matkas are made is different from the soil used to make statues. 2. Paheli is worried. She could see a brick kiln from her house. Bricks were being made there. There was so much smoke coming out of the kiln. She was told that the best quality of clay is required for making pottery, statues and bricks. She has seen truck loads of bricks being taken away for construction of buildings. At this rate, she fears, no soil will be left. Are her fears justified? Discuss this problem with your parents, teachers and other experts of your area and prepare a report. 3. Try to find out the moisture content of a soil sample. One method is given here. 106 SCIENCE Activity: Take 100g soil. (Take help from any shopkeepers to weigh the soil.) Place it on a newspaper in the sun and allow it to dry for two hours. This activity is best done in the afternoon. Take care that the soil does not spill outside the newspaper. After drying it, weigh the soil again. The difference in the weight of the soil before and after drying gives you the amount of moisture contained in 100 g of soil. This is called the percentage moisture content. Suppose your sample of soil loses 10 g on drying. Then wt. of moisture (g) Per cent of moisture in soil = ×100 Original wt. of soil sample (g) In this example 10×100 Per cent of moisture in soil == 10% 100 Did you know? Rivers of north India, which flow from Himalayas, bring a variety of materials including silt, clay, sand and gravel. They deposit their materials called alluvial soil, in the planes of north India. This soil is very fertile and supports nearby half the population of India. SOIL 107

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