Topic 4: Questions and Answers

• Answer the questions below.
• Give yourself one hour.
• Check your answers afterwards and do corrections.

QUESTIONS

1. What are the three main components of soil? (3)
2. Soils have a distinct profile consisting of horizontal layers. How are these layers formed? (4)
3. Give five reasons why soil is important. (5)
4. Explain why humus is important in agriculture. (5)
5. How can aeration in the soil be improved? (4)
6. Describe the three characteristics used in mineral identification. (6)
7. Briefly describe how igneous rocks are formed. Give examples of the different types of igneous rocks. (10)
8. Describe the processes of physical weathering. (15)
9. Name the three ways in which water causes chemical weathering. (3)
10. Soil formation can be represented by the equation S = F (P, R, Cl, O, T). What do these letters stand for? (7)
11. How do animals contribute to soil formation? (3)
12. Briefly describe four soil-forming processes. (8)

[Total marks: 70]

Answers

1. Minerals from rocks; organic matter; living organisms that live in the soil.
2. Chemical weathering; eluviations; illuviation; organic decomposition.
3.                    
   • It provides plants with anchorage
   • It provides nutrients (minerals and water for photosynthesis)
   • Soil is the habitat for soil micro- and macro-organisms
   • Some soil particles are used for commercial purposes (e.g. glass)
   • Clay soil can be used to prevent sunburn and for cultural decorations or clay pots
   • It cleans water as it percolates to form spring water
   • Soil prevents floods and drought by absorbing rainwater, storing it and releasing it later. (Any 5)
4.                          
   • It is the ‘life-force’ of soil as it helps the soil retain moisture and encourages the formation of good soil structure.
   • It helps to suppress diseases in the soil.
   • The dark colour helps to absorb heat in the soil for microbial activities, seed germination and chemical reactions.
   • Improves soil structure by binding loose soil and preventing soil compaction.
   • Increases the water-holding capacity of the soil and prevents drainage and erosion.
   • Has great cation holding capacity, which makes it very fertile.
   • Has a lasting effect as a source of plant nutrients in the soil.
   • Releases minerals stored in decomposable materials into the soil for plants. (Any 5)
5.                            
   • Cultivation or tillage manually with farm tools
   • Ploughing
   • Adding bulky organic material
   • Draining waterlogged areas.
6.                      
   • Colour: the colour of a mineral, when powdered or scratched.
   • Lustre: the way in which a mineral reflects light.
   • Hardness, cleavage and fracture: Diamond is the hardest and talc is the softest mineral. Cleavage refers to the way minerals break. Minerals like Mica that break along smooth, flat surfaces have perfect cleavage. Minerals like quartz that break with curved, rough surfaces have fracture.
7. Igneous rocks are formed when molten magma or lava cools and solidifies – usually during volcanic activity. If it is formed by the crystallisation of magma within the Earth, it is called intrusive rocks or plutonic rocks. Intrusive igneous rocks have very large crystal sizes, because the cooling of magma deep in the interior of the Earth is much slower than the cooling process outside the Earths’ crust (e.g. dolerite).
   If the crystallisation of the magma takes place on the surface of the Earth, the igneous rocks are called extrusive or volcanic rocks. Extrusive rocks are fine-grained in texture because they cool down faster on the surface and there is not enough time for large crystals to develop, e.g. basalt
8.                     
   • The primary process in physical weathering is abrasion. It is the disintegration of rocks without chemical change. However, chemical and physical weathering often go hand in hand, e.g. cracks due to physical weathering will increase the surface area exposed to chemical action. The chemical action of minerals in cracks can aid the disintegration process. The agents of physical weathering are wind and water.
   • Strong winds have much energy that can carry soil particles in their way. These soil particles can hit the surfaces of exposed rocks with a strong force that can further remove particles from the rock surfaces. This is common in desert areas.
   • Running water (e.g. streams and rivers) has the energy to carry loose stones downstream. The swiftness of the running water removes soil particles from the sides and the riverbed. Loose stones carried by the running water rub against each other, causing them to break into smaller particles. Strong sea waves carry and roll sea stones towards the beach. This causes abrasion.
   • Temperature changes can cause weathering if there are extreme temperature variations.
9. Dissolution / solution; hydration; hydrolysis.
10.                
    • S = Soil
    • F = Factors
    • P = Parent material
    • R = Relief/topography
    • Cl = Climate
    • O = Organisms
    • T = Time
11.                        
    • Macro- and micro-organisms in the soil affect decomposition of waste materials to form soil.
    • Animals like rats, mice and rabbits that burrow deep in the soil scratch parent materials to form soil.
    • Droppings and beddings of animals decompose to form soil.
12.                        
    1. Mineralisation is the release of organic compounds during decomposition of organic residues by oxidation to form soluble or gaseous chemical compounds. The chemi- cal compounds may then take part in further soil processes or be utilised by plant life.
    2. Humification is the process whereby the carbon of organic residues is transformed and converted into humic substances (humus) through biochemical processes.
    3. Leaching is the removal of soluble nutrients from an upper soil horizon to a lower soil zone beyond the reach of roots of plants.
    4. Luviation is the movement of soluble minerals or colloidal suspension (substances with large molecules) from one place to another within the soil. Soil horizons that:
       • lose materials through luviations are called the eluvial layer
       • receive material are the illuvial layer.
    5. Gley soil is formed in waterlogged areas where there is little oxygen in the soil. The grey colour is the result of the reduction, under anaerobic conditions, of ferric iron to the ferrous state.
    6. A plinthite formation is a highly weathered mixture of clay with quartz and other diluents.
    7. Inversion: Human activities, e.g. ploughing tilling, contribute to soil inversion: the topsoil is fully turned upside down. Inversion buries weeds deep in the soil.
    8. Bioturbation: The churning of soil by organisms and plants roots – organisms (e.g. earthworms) and burrowers (e.g. moles, rats, rabbits) dig deep into the soil and push subsoil to the soil surface. This leads to a change in the composition of the soil.(Any 4)

[Total marks: 70]