cie-igcse-envmgmt-0680-theory-v1-znotes
1. ROCKS AND MINERALS AND THEIR EXPLOITATION
The rock cycle represents the changes between the three rock types and the processes causing them.
1.1 Formation of rocks
Types of rocks:
Igneous rocks:
Made when liquid magma cools to form solid rock.
Magma is molten rock below the surface, while lava is magma that reaches the surface.
Extrusive igneous rock: cools quickly, small crystals (e.g., basalt).
Intrusive igneous rock: cools slowly, large crystals (e.g., granite).
Sedimentary rocks:
Formed by the weathering of existing rocks at the Earth’s surface.
Fossils may be present.
Sediments accumulate into layers and get pressurized.
Sediments are transported by water and wind (erosion).
Particles include clays, silts, sands, gravels, and small boulders.
Examples: limestone, sandstone, and shale.
Metamorphic rocks:
Formed from existing rock when heat and/or pressure causes changes in the rock crystals without melting it.
The changes can be physical, chemical, or both.
Examples: marble and slate.
1.2 Extraction of rocks and minerals from the Earth
Exploring for minerals:
Prospecting: Searching for minerals by examining the surface of the rocks.
Remote sensing: Gathering information about the Earth’s surface from above.
Photographs of the area are taken from air.
The images are carefully analyzed for mineral presence.
Aerial photography can cover more ground than a person on the surface.
Radiation detection:
Mineral deposits weather at the Earth’s surface, forming mineral oxides.
Detected by their unique radiation pattern recorded by a satellite and analyzed by a computer.
Satellite signals:
Some satellites send signals to the Earth’s surface and collect the reflected signals, indicating the presence of minerals.
The system works in all weather conditions.
Satellite images:
Computers process data from a region of interest to check for mineral presence.
Geologists confirm the presence of the mineral by visiting the location (recorded by the satellite’s positioning system).
Geologists can further check the availability of the mineral in nearby areas.
Using satellites saves time and costs less.
Geochemical analysis: analyzing the chemical properties of rocks (by taking samples).
Samples can be taken from stream sediments, soil, or rocks (using shallow drilling).
The location of the sample points can be accurately found using the Global Positioning System (GPS).
Geophysics: method to identify mineral ores present in rocks using their physical properties.
A series of vibrations (seismic waves) are sent through the Earth’s surface.
Several sensors are placed at different distances from the source of vibrations on the surface.
Vibrations create shock waves that travel down into the rock layers.
They are reflected back to the sensors on the surface.
Shock waves record different patterns depending on the mineral present in the rock layers.
Methods of extraction:
Surface mining: includes open-cast (open-pit, open- cut) and strip mining.
Open-pit mining is used when a valuable deposit is located near the surface.
The vegetation is cleared and topsoil removed.
The rocks are broken up and loosened with explosives.
The loose rock is removed using diggers.
The rock or mineral is tipped into trucks or railway wagons.
Building materials such as sand, gravel and stone are removed from open pits called quarries.
Strip mining is used to mine a seam of mineral.
The overburden (overlying rock and soil) is removed as a thin strip.
It is mainly used to mine coal.
Sub-surface mining: includes deep and shaft mining.
A vertical shaft is sunk down to the rock layer containing minerals.
A horizontal tunnel is made, following the mineral layer.
The minerals are extracted by digging (by machines and miners).
The loose rock is brought from the mine and piled up on waste heaps on the surface.
The minerals are brought to the surface and transported in trucks or trains.
Factors that affect the decision to extract rocks and minerals:
The costs of exploration and extraction:
Probable cost of extracting one tonne is calculated.
Fewer technical difficulties of mining on a large scale using open-pit mining as there’d be low extraction costs per tonne.
Shaft mining is costlier to set up and maintain as the cost per tonne will be higher. So, only deposits of higher value can be mined in this way.
Geology:
High-grade ores yield more of the required chemical elements than low-grade ores.
Small deposits of high-grade ore are worth mining.
Small deposits of low-grade ore that cannot be mined at a profit are left as reserves.
Accessibility:
Transporting the ore from the mine to processing plants can be difficult and expensive.
The cost of building road or rail links to the processing plant or to the nearest port for export has to be considered.
Carrying out some processing at the mine reduces transport costs.
The mining company must be given a licence before extracting a deposit.
A long-term agreement between the government and mining company must be reached to avoid rapid rises in the tax, which makes the mining unprofitable.
Environmental impact assessment:
For a licence application to be approved, the company must have a plan to keep the loss of habitat minimal, followed by the restoration of land proceeding the completion of mining.
The choice of site for mine waste should also be considered.
Supply and demand: the relation between how much of a commodity is available and how much is needed or wanted by the consumers.
Increase in world demand for any mineral ore will elevate the prices.
The profit from a working mine depends on changes in supply and demand.
If the demand is too high, mines that were not profitable before become worth mining.
If the demand falls, working mines may get into a loss due to the transport and extraction expenses.
1.3 Impact of rock and mineral extraction
Environmental impacts:
Ecological impacts:
Loss of habitat as the vegetation is cleared ؞ plants do not have a place to grow, so the animals depending on them for food and shelter are affected.
After deep mining has been working for several years, more habitats will be destroyed due to the increased overburden aboveground.
Pollution:
Noise pollution: due to machinery and explosives ؞ disturbs the behaviour of animal species and causes hearing problems for people.
Water pollution: water supplies may also be polluted, making it unsafe for people to drink.
The water may become acidic and dissolve toxic metal ions-this combination kills many aquatic organisms.
Bioaccumulation: organisms absorb the ions and retain them in their body, reaching concentration higher than that in water.
Biomagnification: the concentrations increase higher up in the food chain and cause the death of top consumers.
Land pollution: toxic nature of the waste doesn’t allow plant growth even years after the mining is stopped.
Air pollution: dust particles settle on the vegetation, not allowing sunlight to reach the leaves and thus, reducing the rate of photosynthesis.
Breathing in dust that remains in the lungs can cause serious lung diseases.
Visual pollution: landscape is damaged.
Waste management: (refer to section 1.4 Managing the impact of rock and mineral extraction)
Economic impacts:
Provides employment for people and taxes for the government.
Jobs are created directly to extract the mineral;
Further jobs are created to supply transport and mining equipment;
More jobs are created when the mineral is refined to make products;
If all these activities occur in the same country, it will generate the most income;
Earn foreign exchange.
The income earned can be used for buying goods and services and investing in infrastructure projects.
Improvements to transport;
Improvements to services, like healthcare and education;
These services can be helpful for miners and their families too;
Investing in infrastructure projects can help the country in building more well-designed communities.
1.4 Managing the impact of rock and mineral extraction
Safe disposal of mining waste:
Mine waste must be stored to prevent collapse.
Site of the mine must prevent the chances of water pollution.
The waste must be monitored to detect any movement or further pollution.
Land restoration:
Soil improvement:
After (sanitary) landfilling, mine waste can be covered by a layer of soil, that can be enriched with fertilisers.
Tree planting:
After improving the soil fertility, plants and trees can be grown in that area, helping an ecosystem to be reborn.
Bioremediation: a process of removing pollutants from waste using living organisms.
In situ treatment: treatment of contaminated waste where it’s left.
Ex situ treatment: removal of contaminated waste from a site to a treatment plant.
Often happens slowly (can be sped up by providing oxygen and nitrogen).
Microorganisms, like bacteria, can absorb pollutants and metabolise them into less harmful substances.
Some plants have the ability to bioaccumulate toxic metals.
After these plants grows for a while, the parts of the plants aboveground are removed so the waste in the ground becomes less toxic.
Making lakes and natural reserves:
Several tree and herb species are introduced.
As their populations grow, they create habitats for many species.
These nature reserves become valuable green spaces for human recreation and help in maintaining biodiversity.
If the rock lining the hole (created by the extraction) is non-toxic and impervious to water, it can be filled with water to form a reservoir or lake.
It is used for irrigating farmland or processed to provide clean, safe drinking water for humans.
Using as landfill sites:
Landfilling: the waste is tipped into a hole; from time to time it is levelled off and compacted.
Sanitary landfilling: As in landfilling, the waste is used to fill the hole, but alternating layers of waste and sand are used.
1.5 Sustainable use of rocks and minerals
Sustainable resource: a resource that can be continuously replenished e.g. agriculture, forestry, etc.
Sustainable development: development that meets the needs of the present, without compromising the ability of future generations to meet their own needs.
Strategies for the sustainable use of rocks and minerals:
Increasing the efficiency of the extraction of rocks and minerals:
Mine wastes must be processed for the second time.
This allows the valuable minerals to be recovered and reduces the risk of pollution due to mine waste.
Chemical treatment of the waste and biological treatment (using microorganisms) extracts much of the valuable mineral still within it.
Improvements in the performance of the machines used in mining and processing.
Greater use of data analysis by computers (to predict geological conditions).
Increasing the efficiency of the use of rocks and minerals:
Engineering solutions e.g. design steel beams with same strength but using less steel.
The need to recycle rocks and minerals:
Recycling uses less energy than processing the ores.
Recycling also produces less waste and thus, reduces the risk of pollution.
Legislation:
The governments pass laws that require manufacturers to become responsible for recycling and reuse.
2. ENERGY AND THE ENVIRONMENT
2.1 Fossil fuel formation
Fossil fuels: carbon-based fuels, formed over many millions of years ago from the decay of living matter.
Coal: formed from plants.
Oil and natural gas: formed from sea creatures.
Formation of coal:
Huge forests grew millions of years ago covering most of the Earth.
The vegetation died and formed peat.
The peat was compressed between layers of sediments to form lignite (low-grade coal).
Further compression formed coal.
Formation of oil and natural gas:
Small animals and plants die and fall to the bottom of the sea.
Their remains are covered by sediments.
As the sediments start forming layers, they start to change into sandstone as the temperature and pressure increase.
The heat and pressure turn the remains into crude oil and natural gas.
They separate and rise through the sandstone, filling in the pores.
The rock above the oil and gas is impervious (non- porous).
So, they get trapped underneath it.
2.2 Energy resources and the generation of electricity
The demand for energy is increasing worldwide due to:
Increasing population size.
Increasing industrialisation and urbanisation.
Improvements in standards of living and expectations.
Types of energy sources:
NON-RENEWABLE:
Limited.
Take millions of years to get replenished.
Fossil fuels (coal, oil and natural gas);
Nuclear power (using uranium).
RENEWABLE:
Can be used over and over again.
Can be replenished in a short period of time.
Geothermal power;
Hydro-electric power;
Tidal power;
Wave power;
Wind power;
Solar power;
Biofuels e.g. bioethanol, biogas and wood.
Nuclear fuels last for centuries and are a good replacement for fossil fuels, but the source material (uranium) is limited.
Biofuels may become limited, but it can be renewed by replacing the cut-down trees with new ones to obtain bioethanol and wood.
Biogas can be obtained by recycling waste products.
How energy sources are used to generate electricity:
Turbine: a machine, often containing fins, that is made to revolve by gas, steam or air (it is connected to a generator).
Generator: a machine that converts mechanical energy into electrical energy.
Fossil fuels and biofuels:
These produce a massive amount of energy during combustion that is used to heat water and convert it into steam, which thereby drives the turbines.
Nuclear power:
Uranium, a radioactive element, releases huge amounts of energy when nuclear fission (splitting of the atom) occurs.
This energy is used to heat the water, produce steam, and rotate the turbines.
Geothermal power:
Cold water is pumped under pressure into a layer of hot rocks.
The rocks heat the water.
The hot water returns to the surface under pressure and heats the second supply of water using a heat exchanger.
The steam produced in the second supply moves the turbine, generating electricity.
Wind power:
Wind turbines have shafts (blades) that rotate due to wind.
Gearbox maximises the rotation of the shaft.
Brakes slow down or stop the rotor in very windy conditions, preventing damage to the blade.
As the turbine rotates, the generator produces electricity.
Solar power:
Uses photovoltaic cells that produce a small electric charge when exposed to light.
A bank of cells organized into solar panels produce a significant amount of electricity.
Tidal power:
Uses the natural rise and fall in the level of water in an area.
When the levels drop, water is held back by a tidal barrage (a small dam that releases water back through a turbine).
Wave power:
Also uses turbine and generator.
Uses the smaller differences in water levels that are caused by wind.
Hydro-electric power:
Uses a dam on a river to store water in a reservoir.
Water is released from the reservoir that flows through the turbine, rotating it.
The turbine then activates a generator that generates electricity.
Advantages and disadvantages of:
Fossil fuels:
ADVANTAGES:
Plentiful supply;
Provides job opportunities (mining and processing);
The technology used is well-known and the methods of extraction are well-practised.
DISADVANTAGES:
Carbon dioxide and toxic gases are released when burnt ؞ contributes to global warming;
Damages local area;
Limited supply (non- renewable).
Biofuels:
ADVANTAGES:
Renewable source;
Growing more plants uses carbon dioxide;
Plentiful supply.
DISADVANTAGES:
Carbon dioxide and toxic gases are released when burnt;
Lot of land needed.
Shortage of land for agriculture ؞ increase in food prices;
Removal of natural ecosystems ؞ reduction in biodiversity.
Nuclear power:
ADVANTAGES:
Does not produce carbon dioxide;
Large amount of energy is produced with a small amount of fuel;
Provides job opportunities.
DISADVANTAGES:
Risk of radiation leakage;
Radioactive waste cannot be recycled since it is active for centuries;
Limited supply (non- renewable).
Geothermal power:
ADVANTAGES:
Does not produce carbon dioxide ؞ doesn’t contribute to global warming;
Unlimited supply (renewable).
DISADVANTAGES:
Expensive to install;
Only certain areas have suitable conditions.
Wind power:
ADVANTAGES:
Does not produce carbon dioxide ؞ doesn’t contribute to global warming;
Renewable.
DISADVANTAGES:
Only certain locations are suitable;
Generation of electricity is weather-dependent;
Visual impact;
Uses a large area.
Solar power:
ADVANTAGES:
Does not produce carbon dioxide ؞ doesn’t contribute to global warming.
DISADVANTAGES:
Weather- dependent.
Tidal power:
ADVANTAGES:
Does not produce carbon dioxide ؞ doesn’t contribute to global warming.
Tidal movements are not weather-dependent.
DISADVANTAGES:
Limited to specific coastal areas.
Impact on tourism and local fishermen.
Wave power:
ADVANTAGES:
Does not produce carbon dioxide ؞ doesn’t contribute to global warming.
Renewable.
DISADVANTAGES:
Limited to specific areas.
Not very efficient at present.
Hydro-electric power:
ADVANTAGES:
Does not produce carbon dioxide ؞ doesn’t contribute to global warming.
Water can be reused.
DISADVANTAGES:
Dams impact the natural flow of water.
Villages and ecosystems may be destroyed.
2.3 Energy demand
Domestic demand:
Created by affordability, availability and social status.
Most of the purchases that are considered as necessities now increase the demand for energy supplies, notably electricity.
Example:
Fruits and vegetables, that aren’t naturally available in the season locally, are produced in glasshouse or in areas with a favourable climate and are then transported.
In both the scenarios (glasshouse operation and transport), the energy cost is significant.
Industrial demand:
Manufacturing requires the use of large amounts of energy throughout the production e.g. iron and steel production.
Advanced manufacturing techniques made the products, that were once luxury items, cheaper.
So, more people want to buy them.
The demand for the product increases.
The demand for energy (needed for production) also increases.
Transport:
Manufacturers supply customers across the globe.
This decreases production costs in countries that import, but increases the transport costs as they require large amounts of fossil fuels to operate.
Personal and national wealth:
If economic conditions are good:
Higher employment;
More money to spend on luxury items;
Increase in demand for the product;
Increase in demand for energy (for production).
If economic conditions are poor:
Families have less money to spend on luxury items;
Need to make savings;
Reduce the use of fuel;
Reduce the purchase and use of electrical items;
Decrease in the demand for energy.
Decline in the economy of one country can have a global impact.
Reduction in the economy of China meant a worldwide:
Reduction in production of steel.
Decrease in the amount of manufactured goods (transported by ships).
Decrease in the price of oil (energy source).
Climate:
The demand for energy with regard to climate depends on the country.
People living in a temperate climate are likely to experience colder winters, so the energy demand for heating would be far higher.
They also experience fewer hours of daylight.
This increases the usage of electrical lighting.
Climate change (excessive heat or cold) increased the energy consumption (particularly in urban areas).
Need for additional heating.
Installation and operation of air-conditioning units.
2.4 Conservation and management of energy sources
Strategies for the efficient management of energy resources:
Reducing consumption:
Reducing the amount of energy used to heat a building.
Insulation: constructing using material with good insulation properties prevents loss of heat.
Loft insulation: adding an insulation layer into the roof space.
Underfloor insulation: adding an insulation layer on the floor e.g. carpet.
Cavity wall insulation: a gap between inside and outside walls is filled with an insulating material, causing the heat to pass through more slowly.
Double glazing: two panes of glass with a gap in the middle to act as an insulator.
This sealed gap is usually filled with air or an inert gas e.g. argon.
Triple glazing can also be used, but it is too expensive.
Electrical devices must be turned off when not in use.
Devices can be left in ‘standby’ mode and can be accessed rapidly.
More energy-efficient devices must be bought.
Developing alternative fuels for vehicles and further development in engine technology.
‘Scrappage’ schemes: remove inefficient machines from use (electrical appliances or vehicles).
Energy from waste:
Reusing existing materials to extract energy from them before they are disposed.
Anaerobic digestion: breaking down of organic matter (waste food and vegetation) using bacteria.
This process takes place in a sealed container and releases methane (a flammable gas) that can be used for heating purposes.
The composted waste can be used as organic matter to improve soil structure.
Household rubbish can be incinerated (burnt) to produce heat, that can be used to generate electricity;
ADVANTAGES:
Waste from burning (ash) is small in volume. Thus, it doesn’t take up much space.
DISADVANTAGES:
Produces poisonous gases during combustion.
Vegetable oils, once used, should be disposed;
These oils can be collected and recycled into biofuels suitable for running vehicles;
It can be used exclusively or as an additive.
Education:
Benefits of the technology must be communicated to others;
Promote new ways of thinking;
The message must be that significant savings in energy bills can be made over the longer term, reducing energy use;
Energy-efficiency ratings must be provided for new products to compare with the old ones.
Laws passed by the government to make changes rapidly:
Stricter building regulations: new constructions must be more energy efficient.
Preventing the sales of inefficient types of electrical devices.
Incentives to encourage the purchase of more efficient technologies:
Insulating older houses that are energy efficient;
Replacing older, inefficient electrical devices;
Scrapping older, inefficient cars, that emit more pollutants.
Exploiting existing energy resources:
The type of energy source used depends on social, environmental and economic factors.
The current solution is to use a renewable resource as a primary energy source when possible and have a fossil-fuel (or biofuel) powered station available as a backup when weather conditions are not suitable.
This is a reliable source for industry and households and reduces the amount of fossil fuels used.
Transport policies:
Regulations regarding the quality of exhaust gases from vehicles;
Check on the fuel efficiency;
Restrictions on where vehicles may go;
Taxation on fuels;
Surcharges for travelling to certain places at peak times;
Improving public transport so it is easier and cheaper than using cars;
Improving routes for cyclists and pedestrians;
Encouraging car-sharing;
Restricting when cars can be used e.g. odd even rule in Delhi;
Providing incentives to buy more fuel-efficient vehicles and for vehicles using cleaner technology.
Development of new resources:
Fracking: obtaining oil or gas from shale rock by splitting them open using water, sand and chemicals.
A vertical hole (2-3 km deep) is drilled to reach the fuel-rich rocks (shale rocks).
Water, sand and chemicals are pumped down into the shale rock layer.
This causes the rock to fracture, releasing oil and natural gas, which are forced back to the surface and collected.
Purpose of the three components:
Water: easy to handle (in high pressure).
Chemicals: stop the blockage of pipes.
Sand: keeps the cracks in the rock open.
ADVANTAGES:
Access to more oil and gas;
Less pollution than burning coal;
The need to import reduces;
Provide many jobs locally.
DISADVANTAGES:
Risk of toxins entering the water table;
Chemicals are toxic and may affect local residents;
Uses a lot of water; may cause water scarcity;
Noise pollution;
Natural areas damaged;
May cause additional Earth tremors.
2.5 Impact of oil pollution
Main causes of marine oil spills:
Offshore oil extraction: leakage from the rigs.
Oil pipelines: leaks in the oil pipework.
Shipping: risk of collision or damage to oil tankers.
Effects of an oil spill:
PHYTOPLANKTON:
Oil floats on the surface of the water and blocks the sunlight from entering.
The phytoplankton can’t photosynthesise, so they die.
FISH:
Shortage of food; reduction in phytoplankton.
Oil floating on the surface prevents gas exchange.
Fish become short of oxygen and die;
Direct contact of the fish with oil affects their gills.
BIRDS:
Shortage of food as fish and other creatures die;
May consume oil when eating fish (toxic);
When hunting for food, feathers get covered with oil, affecting their ability to fly.
MAMMALS:
Food sources are depleted;
Mammals may also swallow oil while feeding (toxic);
Coating of oil will affect their skin.
REEFS:
Complete devastation of the reef due to lack of oxygen (species die);
Areas may be covered in oil.
BEACHES:
Oil (washed by tides) coats rocks;
Organisms in shallow water and rock pools may die due to toxic effects of the oil;
Animal food sources and tourism are affected.
2.6 Management of oil pollution
Reducing oil spills in marine environments:
MARPOL (Marine Pollution): International Convention for the Prevention of Pollution from Ships.
Regulations of the MARPOL:
Supervise the transport of oil at sea;
All tankers must be certificated to show they have appropriate systems in use;
Else, it can result in a heavy fine or the ship may not be permitted to leave port.
Tanker design:
Oil spill can be caused by damage to the hull (a hole in the hull of the boat causes its contents to leak).
Increase in the number of compartments within the hull of the ship: if one of the compartment’s damaged, the contents of the whole ship aren’t lost.
Double-hulled tankers: if the outer layer’s damaged, the contents are still secure by the inner plate.
Though double-hulled tankers cost more than single- hulled, the risks of oil spill are far less.
Minimising the impact of oil spills:
Floating booms: a floating barrier is used to surround the oil slick, preventing it from spreading.
This process works well when the spill covers a relatively small area and the sea is calm.
Detergent sprays: detergents help break down the oil slick into smaller droplets, that eventually degrade, and disperse it.
They are effective on smaller spills, but cause damage to the coral reefs themselves as they’re not tolerant to detergents.
Skimmers: clean the water using a material that oil easily attaches to.
The skimmer drags oil off the seawater surface, that is then scrapped off into a container.
This system is used when oil slick is contained within a boom and the sea is calm.
When the oil reaches beaches, it can only be removed by hand (difficult and time-consuming).
3. AGRICULTURE AND THE ENVIRONMENT
3.1 Soil composition
Mineral particles: combination of rock fragments and other inorganic substances.
They are formed due to physical, chemical and biological weathering of the parent rock.
Organic content: mixture of living plants, animals, microorganisms and their dead remains.
Air: held within the pore spaces (between the mineral particles and organic content).
Air enters the soil by diffusion.
Water: held within the pore spaces (water that is available for plant growth).
Water enters the soil when there’s precipitation or when the soil is irrigated.
The proportion of these components depends on:
Type of soil;
Way it has been managed;
Local climatic conditions;
Size of the mineral particles.
Soil can be classified into three groups:
SAND:
Size: mm
Texture: Gritty
SILT:
Size: mm
Texture: Silky or soapy
CLAY:
Size: <0.002 mm
Texture:
Sticky when wet
Hard when dried
3.2 Soils for plant growth
Soil is the cheapest and most abundant medium in which water, mineral nutrients, anchorage and oxygen can be supplied to a plant.
Plants require a supply of nitrogen, phosphorus, potassium and a range of other elements to construct proteins and carry out life processes.
NITROGEN: Supplied as Nitrate ions ( -
PHOSPHORUS: Supplied as Phosphate ions ( 3-)
POTASSIUM: Supplied as Potassium ions (+)
Organic content: decomposers that produce humus (rich in nutrients):
Earthworms: break down vegetation; mix the soil; aerate the soil; spread organic matter through the soil.
Fungi: feed directly on dead matter; digest hard woody items; aid plants to take up nutrients through their roots.
Bacteria: work on organic matter; convert waste products to simple chemicals; some convert nitrogen to nitrates ؞ important in nitrogen cycle.
High levels of organic matter:
Increase the water-holding capacity (like a sponge);
Increase air spaces in the soil;
Increase no. of decomposers, tunnels and burrows in the soil, providing additional drainage and less compaction;
Prevent the loss of mineral nutrients (humus holds on to mineral nutrients).
Soil pH:
Depends on the type of parent rock and pH of water that flows into the area;
Affects the uptake of nutrients by plant roots;
Affects the availability of nutrients;
Farmers can try changing the pH of the soil either to acidify it (using fertilisers that have an acidic effect) or make it alkaline (adding ground limestone).
SAND:
Larger air spaces;
Drains well;
Poor retention of humus;
Easier to cultivate.
CLAY:
Poor air spaces;
Poor drainage;
Retains humus;
Hard to cultivate.
Drainage: capacity of the soil to drain water must be medium (no water loss; no surplus amount of water).
Ease of cultivation: how easily the soil can be ploughed.
3.3 Agriculture types
SUBSISTENCE:
Cultivation of food to meet the needs of the farmers and their families;
Surplus is bartered for other goods (or cash).
Examples: wheat and rice.
COMMERCIAL:
Cultivation of food with the main aim of selling them for cash;
Some food may be used by the farmers.
Examples: tea, coffee, cocoa, sugarcane, cotton