Natural Resources

1. Biosphere — the zone of life

Earth is special among the planets of the Solar System because it supports life. Life is possible here due to the right temperature range, the presence of water, and an atmosphere that provides oxygen and shields us from harmful radiation.

The region of Earth where life exists is called the biosphere. It is a narrow zone that extends from the deep ocean floor to a few kilometres into the atmosphere. The biosphere interacts with three non-living (abiotic) domains:

• Lithosphere — the solid rocky crust of the Earth, including the soil. It provides minerals and nutrients.
• Hydrosphere — all the water on Earth: oceans, rivers, lakes, groundwater, ice caps, and water vapour. It covers about 71% of the surface.
• Atmosphere — the gaseous envelope surrounding Earth, extending up to about 500 km. It protects us and makes life possible.

Living organisms (biotic components) interact constantly with these abiotic domains — taking up resources and releasing products — keeping the planet dynamic and life-supporting.

2. Air — composition and its role in climate

Air is a mixture of gases, not a single substance. The approximate composition of dry air is:

• Nitrogen (N₂) — 78% (by volume). Nitrogen is relatively inert and dilutes oxygen to a safe level for breathing.
• Oxygen (O₂) — 21%. Essential for respiration of almost all living organisms and for combustion.
• Carbon dioxide (CO₂) — 0.03 to 0.04%. Raw material for photosynthesis; a greenhouse gas that traps heat.
• Water vapour — variable (0 to 4%). Influences weather, clouds and rain.
• Argon, neon, helium and other noble gases — trace amounts.
• Dust, smoke and aerosols — tiny solid/liquid particles suspended in air; important for cloud formation.

Air is vital because:

• O₂ is needed for cellular respiration in almost every living thing.
• CO₂ is the carbon source for photosynthesis — the base of every food chain.
• N₂ participates in the nitrogen cycle, making amino acids and nucleic acids possible.
• The ozone (O₃) layer in the stratosphere absorbs harmful ultraviolet (UV) radiation from the Sun.

3. Role of the atmosphere — maintaining temperature

Without an atmosphere, the temperature on Earth would swing wildly — scorching hot in daytime and freezing cold at night, like on the Moon. The atmosphere prevents this in two ways:

• Absorbing and retaining heat — Greenhouse gases (CO₂, water vapour, methane) absorb infrared radiation radiated by the Earth's surface and re-radiate it back, keeping the surface warm. This is the natural greenhouse effect.
• Mixing by winds — The atmosphere redistributes heat from the equator to the poles and from day-side to night-side, moderating temperature extremes.

The Moon has no atmosphere. On the Moon, the temperature rises to about +120 degrees C during the day and drops to minus 180 degrees C at night. Earth's average surface temperature is a comfortable +15 degrees C — entirely because of the atmosphere.

The ozone layer (in the stratosphere, 15 to 35 km above Earth) absorbs most of the Sun's harmful UV-B and UV-C radiation. UV radiation damages DNA and can cause skin cancer, cataracts and immune suppression. Depletion of this layer — caused by chlorofluorocarbons (CFCs) — is a serious environmental concern.

4. Wind — uneven heating and local breezes

Air moves from regions of high pressure to regions of low pressure. This movement of air is called wind. The primary driver of wind is uneven heating of the atmosphere by the Sun.

Why uneven heating occurs

• The equator receives more direct sunlight than the poles, so it heats up more.
• Land heats and cools faster than water bodies (specific heat of water is much higher).
• Dark surfaces (forests, asphalt) absorb more radiation than light surfaces (deserts, snow).

Warm air is lighter (less dense) and rises, creating a low-pressure zone. Cool air is denser and sinks, creating a high-pressure zone. Air flows from high to low pressure — that flow is wind.

Sea breeze (daytime)

During the day, land heats up faster than the sea. Air over land rises; cooler air from the sea rushes in to replace it. This is a sea breeze — it blows from sea to land.

Land breeze (night-time)

At night, land cools faster than the sea. Now the sea surface is warmer; air over the sea rises and cooler air from the land blows out to replace it. This is a land breeze — it blows from land to sea.

Large-scale winds and monsoons

On a global scale, the same principle drives trade winds and monsoons. In summer, the Indian subcontinent heats up, creating a large low-pressure zone. Moist air from the Indian Ocean rushes in as the South-West Monsoon, bringing India's crucial seasonal rainfall.

5. Rain — how precipitation forms

Rain is part of the continuous water cycle (hydrological cycle). The Sun is the energy source that drives it.

How rain forms — step by step

1. Evaporation — Solar energy heats water in oceans, lakes and rivers, converting liquid water to water vapour that rises into the atmosphere.
2. Transpiration — Plants release water vapour through their leaves (stomata). Evaporation plus transpiration together = evapotranspiration.
3. Rising air and cooling — Warm, moist air rises (through convection or forced uplift over mountains). As it rises, it expands and cools.
4. Condensation — When the air cools to its dew point, water vapour condenses around tiny dust or smoke particles (called condensation nuclei), forming tiny water droplets. Billions of droplets together form clouds.
5. Precipitation — As droplets grow larger by colliding and combining, they become too heavy to stay suspended and fall as rain. In colder regions or at high altitudes, precipitation falls as snow or hail.
6. Collection and runoff — Rain water collects in rivers, lakes and groundwater aquifers, and eventually returns to the ocean, completing the cycle.

Why do mountains affect rainfall? When moisture-laden winds hit a mountain range, they are forced to rise. The air cools, condensation occurs, and rain falls on the windward side (facing the wind). On the leeward side (the rain-shadow), the now-dry air descends and warms — little or no rain falls there. This is why the Western Ghats receive heavy rainfall while the Deccan Plateau is relatively dry.

6. Air pollution — causes, effects and control

The addition of harmful substances to the air in amounts that are injurious to living organisms or damage the environment is called air pollution. The added substances are called air pollutants.

Major air pollutants and their sources

• Carbon monoxide (CO) — incomplete combustion of fuels in vehicles and industrial boilers; highly toxic, binds to haemoglobin and blocks oxygen transport.
• Oxides of nitrogen (NO_x) — produced in vehicle engines and power plants at high temperatures; cause smog and acid rain.
• Sulphur dioxide (SO₂) — burning coal and petroleum with sulphur impurities; major cause of acid rain.
• Particulate matter (PM_2.5, PM₁₀) — fine dust, smoke, and soot; damages lungs and reduces visibility.
• Hydrocarbons (unburnt fuel) — from vehicle exhausts; contribute to smog and cancer risk.
• Chlorofluorocarbons (CFCs) — from old refrigerants and aerosol sprays; destroy the ozone layer.
• Carbon dioxide and methane — from burning fossil fuels, cattle, paddy fields; intensify the greenhouse effect and global warming.

Acid rain

SO₂ and NO_x react with water vapour and oxygen in the atmosphere to form sulphuric acid and nitric acid. These dissolve in raindrops and fall as acid rain (pH below 5.6). Effects: damages leaves and soil, corrodes marble monuments (the Taj Mahal is a famous example), and kills aquatic life by acidifying lakes and rivers.

Control measures

• Use of catalytic converters in vehicles to convert CO and NO_x to CO₂ and N₂.
• Switching to cleaner fuels (CNG, LPG, hydrogen) or renewable energy (solar, wind).
• Fitting electrostatic precipitators or scrubbers in factory chimneys.
• Planting trees — they absorb CO₂, release O₂, and trap dust particles.
• Phasing out CFCs under the Montreal Protocol.

7. Water — importance and the water cycle

Water is arguably the most critical natural resource. It is the universal solvent and the medium in which virtually all biochemical reactions in living cells occur.

Importance of water

• Biological solvent — nutrients, gases and waste products are transported in water inside organisms (blood, sap).
• Temperature regulation — the high specific heat of water buffers temperature changes in organisms and in large water bodies, moderating local climates.
• Photosynthesis — water is split (photolysis) during the light reactions to release O₂ and provide electrons and hydrogen ions.
• Agriculture — plants absorb mineral nutrients dissolved in soil water through their roots.
• Industry — used as a coolant, solvent, and raw material in manufacturing.
• Habitat — aquatic ecosystems (freshwater and marine) support millions of species.

Distribution of water on Earth

About 97.5% of all water is saline (in oceans). Of the 2.5% freshwater, most (about 69%) is locked in glaciers and ice caps, and about 30% is groundwater. Only about 0.3% of freshwater is in rivers and lakes — readily accessible to humans and most land life.

The water cycle — brief recap

Evaporation from oceans and transpiration from plants produces water vapour that rises and cools, condensation forms clouds, precipitation (rain or snow) falls, runoff enters rivers and lakes or infiltrates as groundwater, and eventually water returns to the ocean. The Sun provides the energy; gravity drives the return flow.

8. Water pollution — causes, effects and control

Any undesirable change in the physical, chemical or biological properties of water that makes it harmful or unusable is called water pollution.

Major sources and pollutants

• Sewage and domestic waste — untreated human waste introduces pathogens (bacteria, viruses, protozoa) causing waterborne diseases like cholera, typhoid and dysentery.
• Industrial effluents — factories discharge heavy metals (mercury, lead, cadmium, arsenic), acids, alkalis and organic solvents into rivers.
• Agricultural runoff — excess fertilisers (nitrates, phosphates) and pesticides wash into water bodies. Excess nutrients cause eutrophication — explosive algal growth (algal bloom) depletes dissolved oxygen, killing fish.
• Oil spills — accidental release of crude oil in oceans destroys marine ecosystems and seabird populations.
• Thermal pollution — hot water discharged from power plant cooling systems raises the temperature of rivers, reducing dissolved oxygen and harming aquatic life.
• Plastic waste — non-biodegradable; accumulates in oceans as microplastics, entering the food chain.

The Ganga Action Plan

The Ganga is one of India's most important rivers. Untreated sewage, industrial waste and religious offerings have severely polluted it. The Ganga Action Plan (launched 1985, continued as Namami Gange Mission) aimed to build sewage treatment plants, set industrial effluent standards, and restore river flow.

Control of water pollution

• Treating sewage in sewage treatment plants (STPs) before discharge.
• Mandatory effluent treatment plants (ETPs) for industries.
• Restricting use of chemical fertilisers and pesticides; promoting organic farming.
• Rainwater harvesting to recharge groundwater and reduce surface runoff.
• Strict laws and enforcement (Water Prevention and Control of Pollution Act, 1974).

9. Soil — formation, profile and erosion

Soil is the thin, loose, upper layer of the Earth's crust that supports plant life. It is formed over thousands to millions of years by the process of weathering.

Formation of soil — weathering

Weathering is the breaking down of rocks into smaller and smaller particles by physical, chemical and biological agents:

• Physical (mechanical) weathering
  • Temperature changes — repeated heating and cooling cause rocks to expand and contract, eventually cracking. Water seeps into cracks, freezes, expands and widens them (frost action).
  • Wind and water abrasion — particles carried by wind and rivers scratch and grind rock surfaces.
  • Glacial erosion — slow-moving glaciers scrape and pulverise bedrock.
• Chemical weathering
  • Rainwater dissolves CO₂ and forms weak carbonic acid that dissolves limestone and marble (calcite).
  • Oxygen and water cause oxidation (rusting) of iron-containing minerals, weakening rocks.
  • Water molecules break chemical bonds in minerals (hydrolysis).
• Biological weathering
  • Lichens (symbiosis of algae and fungi) grow on bare rock surfaces and secrete acids that dissolve the rock — they are often the very first coloniser of bare rock.
  • Plant roots penetrate cracks in rocks and widen them as they grow.
  • Earthworms, bacteria and other soil organisms break down organic material into humus and release minerals, enriching the soil.

Over time, accumulating weathered mineral particles mix with dead organic matter (humus), water, air and living organisms — forming soil. The nature of soil depends on the parent rock, climate, topography and the organisms present.

Soil profile — the four horizons

A vertical cross-section through soil (called a soil profile) reveals distinct layers called horizons:

• O horizon (Humus layer) — topmost layer of freshly fallen and partially decomposed organic matter (leaves, twigs). Present mainly in forests.
• A horizon (Topsoil) — rich in humus (fully decomposed organic matter), minerals and micro-organisms; dark coloured; the most fertile layer where most plant roots grow and seeds germinate.
• B horizon (Subsoil) — less humus; contains clay and minerals leached (washed) down from the A horizon; lighter in colour; some deeper plant roots reach here.
• C horizon (Weathered parent rock or Regolith) — partially broken rock fragments; very little organic matter; transitional between soil and bedrock.
• Bedrock (R horizon) — solid, unweathered parent rock at the bottom.

Soil erosion

The removal of the topsoil (A horizon) by the agents of water, wind and human activity is called soil erosion. Soil erosion is serious because the topsoil is the most fertile layer, and once lost it takes hundreds of years to reform.

Causes of soil erosion:

• Deforestation — tree roots bind soil; removing trees leaves soil bare and vulnerable. Raindrops directly hit bare soil, detaching particles (splash erosion), and runoff carries them away.
• Overgrazing — animals strip vegetation, leaving soil bare.
• Construction and mining — disturb and expose soil.
• Wind erosion — in dry, bare areas, wind lifts and carries away fine soil particles.

Prevention of soil erosion: afforestation (planting trees), check dams on slopes, contour ploughing, terracing on hill slopes, leaving crop residues on fields (mulching), and controlling overgrazing.

10. Soil pollution — causes, effects and remedies

The addition of unwanted substances to the soil that adversely affect soil fertility, plant growth or the health of organisms living in or on the soil is called soil pollution.

Causes

• Excessive chemical fertilisers — long-term use of synthetic fertilisers (urea, superphosphate) changes soil pH, reduces microbial diversity and destroys soil structure.
• Pesticides and herbicides — many are persistent organic pollutants (POPs) that do not break down and accumulate in the soil food web (biomagnification). DDT is a classic example.
• Industrial waste — heavy metals (lead, cadmium, mercury, chromium) from factory effluents and mining tailings contaminate soil, enter plant tissues and eventually human food.
• Solid waste — improper disposal of plastic, glass, electronic waste (e-waste) and municipal solid waste pollutes soil; plastics persist for hundreds of years.
• Acid rain — lowers soil pH, leaches essential minerals and kills beneficial soil bacteria and earthworms.
• Radioactive waste — from nuclear power plants or weapons testing; highly persistent.

Effects

• Loss of soil fertility and reduced crop yields.
• Groundwater contamination (pollutants leach into aquifers).
• Bioaccumulation and biomagnification of toxins in food chains.
• Harm to earthworms, bacteria and other soil organisms that maintain soil health.

Remedies

• Use organic farming techniques — compost, vermicompost, biofertilisers (Rhizobium inoculants, blue-green algae).
• Integrated Pest Management (IPM) — biological control using predators and parasitoids instead of chemical pesticides.
• Proper landfill management and waste segregation; phasing out single-use plastics.
• Bioremediation — using bacteria or plants (phytoremediation) to detoxify polluted soil.

11. Biogeochemical cycles — how elements circulate

The elements and compounds that form the bodies of living organisms are continuously recycled between living organisms and the non-living environment. These recycling pathways are called biogeochemical cycles (bio = living; geo = Earth; chemical = elements). The Sun provides the energy; decomposers close the loop.

A. The Water Cycle (Hydrological Cycle)

Diagram described in words: Solar energy evaporates water from oceans, lakes and rivers; plants simultaneously transpire water vapour. Water vapour rises, cools at altitude, and condenses around dust particles to form clouds. Clouds produce precipitation (rain, snow, hail). On land, water flows as surface runoff into rivers and lakes, or seeps underground as groundwater. Rivers carry water back to the ocean. Glaciers store water as ice for centuries. The cycle then repeats indefinitely.

Key points: The energy source is the Sun; water changes state (liquid to vapour to liquid); the cycle distributes freshwater over land; no water is gained or lost overall from Earth.

B. The Carbon Cycle

Diagram described in words: CO₂ in the atmosphere is fixed (converted to organic molecules) by photosynthesis in green plants and algae: 6CO₂ + 6H₂O gives C₆H₁₂O₆ + 6O₂. Carbon moves from plants to animals through feeding (herbivores eat plants, carnivores eat herbivores). All organisms return CO₂ to the atmosphere through cellular respiration. When organisms die, decomposers (bacteria, fungi) break down their bodies, releasing CO₂ back to the air. Over geological time, buried remains became fossil fuels (coal, petroleum, natural gas) — burning them releases carbon stored over millions of years within decades, upsetting the balance. CO₂ also dissolves in the ocean, where marine organisms use it to build shells (CaCO₃).

Human impact: Burning fossil fuels combined with deforestation is rapidly increasing atmospheric CO₂, driving global warming and climate change.

C. The Nitrogen Cycle

Why nitrogen matters: Nitrogen (N) is essential for amino acids, proteins, and nucleic acids (DNA, RNA). Although N₂ makes up 78% of the atmosphere, most organisms cannot use N₂ directly. It must first be "fixed" — converted to a usable form.

Diagram described in words — the five steps:

1. Nitrogen fixation — conversion of atmospheric N₂ to ammonia (NH₃) or nitrates (NO₃^-):
   • Biological fixation — Nitrogen-fixing bacteria: Rhizobium (lives in root nodules of legumes such as peas, beans and groundnut), Azotobacter (free-living in soil), and cyanobacteria such as Anabaena in water and paddy fields.
   • Lightning — the enormous energy of lightning splits N₂ and O₂ molecules; they recombine as NO then NO₂, which dissolves in rain as dilute nitric acid and enters soil as nitrates.
   • Industrial fixation (Haber process) — N₂ + 3H₂ gives 2NH₃ (at high temperature and pressure); used to manufacture fertilisers.
2. Nitrification — soil bacteria (Nitrosomonas, Nitrobacter) convert ammonia to nitrites then to nitrates. Plants absorb nitrates through roots via active transport.
3. Assimilation — plants use nitrates to synthesise amino acids and proteins. Animals obtain nitrogen by eating plants or other animals.
4. Ammonification — when organisms die, decomposer bacteria and fungi break down their proteins, releasing nitrogen back as ammonia (NH₃) into the soil.
5. Denitrification — certain bacteria (Pseudomonas) in waterlogged, anaerobic soils convert nitrates back to N₂ gas, returning it to the atmosphere and completing the cycle.

Exam focus: Questions often ask to name nitrogen-fixing bacteria (Rhizobium, Azotobacter, cyanobacteria) and the role of lightning. Know all five steps and their associated micro-organisms.

D. The Oxygen Cycle

Diagram described in words: O₂ is released into the atmosphere primarily by photosynthesis in green plants, algae and cyanobacteria — it comes from the splitting of water molecules (photolysis). O₂ is consumed by: (1) aerobic respiration in almost all living organisms — O₂ combines with glucose to release energy, CO₂ and water; (2) combustion — burning fossil fuels and wood consumes O₂; (3) weathering — O₂ reacts with minerals in rocks (oxidation). Oxygen is also present in CO₂, water and carbonate rocks. Ozone (O₃) in the stratosphere is continually created from O₂ by UV radiation and destroyed by UV and CFCs. In the atmosphere, O₂ released by photosynthesis and consumed by respiration are roughly in balance — maintaining the 21% level we depend on.

Key link between cycles: Photosynthesis drives both the carbon cycle (fixing CO₂ into glucose) and the oxygen cycle (releasing O₂) simultaneously. Respiration does the reverse. The cycles are deeply interconnected and interdependent.

12. Key terms at a glance

• Biosphere — zone of life on Earth; encompasses parts of lithosphere, hydrosphere and atmosphere.
• Lithosphere — solid rocky outer layer of Earth (crust and upper mantle).
• Hydrosphere — all water on Earth in all forms (liquid, solid, vapour).
• Atmosphere — gaseous envelope surrounding Earth; contains air and ozone layer.
• Greenhouse effect — warming of Earth's surface because greenhouse gases trap outgoing infrared radiation.
• Acid rain — rain with pH below 5.6 due to dissolved sulphuric and nitric acids formed from SO₂ and NO_x.
• Ozone layer — layer of O₃ in the stratosphere (15 to 35 km) that absorbs harmful UV radiation.
• Weathering — physical, chemical and biological breaking down of rocks to form soil particles.
• Humus — fully decomposed organic matter in soil; dark, spongy; increases water retention and fertility.
• Soil profile — vertical cross-section of soil showing O, A, B, C and R horizons.
• Nitrogen fixation — conversion of atmospheric N₂ to ammonia or nitrates, by bacteria or lightning.
• Nitrification — conversion of ammonia to nitrites then nitrates by soil bacteria.
• Denitrification — conversion of nitrates back to N₂ by anaerobic bacteria such as Pseudomonas.
• Eutrophication — excessive nutrient enrichment of a water body causing algal bloom and oxygen depletion.
• Biomagnification — increasing concentration of a non-biodegradable pollutant at each successive trophic level in a food chain.
• Biogeochemical cycle — cyclic movement of elements and compounds between biotic and abiotic components of an ecosystem.