Natural resources
13) Both addition of undesirable substances to as well as removal of desirable substances from water bodies constitutes water pollution.
14) Reducing the dissolved oxygen and nutrients from water bodies is harmful for the aquatic organisms.
15) Aquatic organisms survive best in their optimum temperatures; hence significant sudden changes in water temperatures can disrupt their life processes and / or breeding activities.
16) Weathering of rocks due to physical, chemical and biological processes finally produces the fine particles of soil over long periods of time.
17) The continuous, uneven expansion and contraction of different parts of the rocks during day and night leads to crack formation, followed by breakdown of the large rock pieces into smaller ones.
18) Freezing of water in cracks of rocks causes widening of the cracks.
19) Continuous friction between the various rock pieces carried by flowing water also results in soil formation.
20) Strong wind is another factor resulting in soil formation.
21) Both water and wind deposits soil away from the parent rocks.
22) Soil formation is also carried out by living organisms. Materials released by lichens growing on rock surface powder down the surface, forming soil. Further breakdown of rocks is caused by growth of other small plants in such soil as well as the entry of tree roots into the cracks of rocks.
23) Soil is a mixture of rock particles, humus and microscopic and small organisms.
24) Humus aerates the soil and keeps it porous.
25) The nutrient and humus content of the topsoil influences the biodiversity of a region.
26) An important cause of soil pollution is the recent agricultural methods, which destroys soil structure by killing the nutrient recycling soil microorganisms as well as the earthworms.
27) Trees and other plants prevent soil erosion and helps in deep percolation of water. Hence deforestation accelerates soil erosion greatly.
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NCERT 9 Science