UNITS:

• OVERVIEW
• Unit 1 Basic soil components)
• Unit 2 Minerals (primary and secondary)
• Unit 3 Rocks and their formation 
• Unit 4 Weathering of rocks 
• Unit 5 Soil forming factors 
• Unit 6 Soil-forming processes 
• Topic questions 

Topic 4: Soil Science

Overview

What you will cover in Topic 4

Unit 1 : Basic Soil Components

1.1 What is soil?

• Soil, air and water are the three major natural resources.
• Soil is the upper layer of the Earth → supports all plant and animal life.
• Soil consists of three main components:
  • minerals from rocks (below or nearby the soil)
  • organic matter (most important part of soil)
  • the living organisms that live in the soil.
• Type of soil depends on the proportion of each of the above, as well as on:
  • climate
  • vegetation
  • the surrounding terrain
  • human activities.
• Soils have different textures depending on the size and amount of mineral particles, e.g. sandy, silty and clayey soils.
• Organic matter (the most important part of soil) is partially decomposed organic material, rich in nutrients. The darker the soil, the greater the concentration of organic matter.
• Erosion can take place in soils that are not well looked after.

1.2 The main functions and importance of soil in an ecosystem

• Soil is the basis of life.
• Soil is an ecosystem of plants and animals.
• plants and animals change the composition and structure of soil
• soil is important because it provides plants with food in the form of nutrients (e.g. nitrogen, potassium, phosphorus, boron and zinc).

1.2.1 Plants in a soil ecosystem

• Plant roots get energy to grow from sugars through photosynthesis.
• When roots have developed, they absorb soil nutrients and water to make the plant grow.
• When plants die, the remaining nutrients stay in the roots and enrich the soil.

1.2.2 Animals in a soil ecosystem

• Visible: earthworms, ants and burrowing animals.
• Microscopic: bacteria, fungi and nematodes.

1.2.3 Other reasons why soil is important

• 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)
• It cleans water as it percolates to form spring water
• Soil prevents floods and drought by absorbing rainwater, storing it and releasing it later.

1.3 The major components of soil
1.3.1 Soil profile
Soils vary according to geographical location, climate, etc., but most soils have a distinct profile or sequence of horizontal layers. These horizons result from the
processes of: chemical weathering, eluviation, illuviation and organic decomposition.

Parts of a soil profile

A soil profile

• O = Organic:
  • The topmost layer of most soil – lying on the surface of the soil at various levels of decomposition and humus. Not present in cultivated fields.
• A = Topsoil:
  • Darker (brown or black) than lower layers (but not in arid or dry areas), loose and crumbly, with
    varying amounts of organic matter. Consists of rock material that has been chemically and physically broken down and changed, and mixed with organic materials, particularly plant roots. It is full of plant and animal life. The most productive layer.
• B = Subsoil:
  • Light colour, dense and low in organic matter. Also consists of altered rock material, but contains much less plant life (mainly roots) and living creatures. However, minerals can be broken down, and nutrients released from this layer for use by roots of plants.
• C = Parent material:
  • Usually below half a metre, this layer consists mostly of unconsolidated organic and mineral material, unaltered rock or glacial deposits, the matter from which the soil is formed.
• R = Bedrock:
  • The solid rock that underlies the soil and other unconsolidated material. This soil layer simply consists of unweathered bedrock.

1.3.2 Soil components
Components of soil are grouped into two kinds:

• Inorganic components of soil –
  • The inorganic components form the major part of soil: water, air and mineral materials (small stones, sand, silt and clay) that never decay or rot.
• Organic component of soil –
  • The organic component of soil is made up of decomposed leaves, roots, bones and animal droppings, etc.
  • It is called humus (formed in a process called humification).
  • About 5% of the soil is humus.
  • Our ancestors used decomposed organic matter = source of nutrients in the soil:
    • Guano for nitrogen
    • Wood ash for potash
    • Animal bones and hooves for calcium and phosphorus.

The importance of humus

• Humus is a dark-coloured, loose colloid.
• It is the ‘life-force’ of soil → 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 → 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.

1.3.3 Soil air

• There are pores between soil particles: big (macro-pores) or small (micro-pores), depending upon the type and size of the soil particles.
• Soil air is the volume of air that fills the soil pore spaces where there is no water.
  • Soil air makes up about 25% of the total volume of the soil.
  • The movement of air in the soil pore spaces is called aeration.
  • Soil air contains gases, e.g. oxygen (O₂), nitrogen (N₂) and carbon dioxide (CO₂).

The importance of oxygen in the soil

• It is necessary for the respiration of plants, roots and soil organisms
• It helps to decrease carbon dioxide concentration in the soil
• It is necessary for organic matter decomposition in the soil
• Seeds in the soil require oxygen to sprout
• Some chemical processes, like oxidation, take place in the presence of oxygen
• Soil air prevents the development of fungi in the soil that can harm

Practical ways to improve aeration

• Cultivation or tillage
• Ploughing
• Adding bulky organic material
• Draining waterlogged areas.

1.3.4 Soil water

Concepts related to soil water

• Saturation – all soil pores are filled with water; occurs right after a rain → represents 0
• Field capacity – moisture content of the soil after gravity has removed all the water it can. Usually occurs 1–3 days after rain → represents 1/3
• Wilting point – soil moisture percentage at which plants cannot obtain enough moisture to continue growing → represents 15 bars.
• Oven dry – soil that has been dried in an oven at 105ºC for 12 hours → all soil moisture has been removed. (This point is not important for plant growth, but is important for calculations, because soil moisture percentage is always based on oven-dry weight.)
• Plant available water – the water held in soil at a water potential of between -1/3 and -15 bar
• Soil water makes up about 25% of the total volume of the soil.
• The volume of water found in a soil type depends on:
  • soil structure – soil structure is the arrangement of particles into aggregates. Soil structure also affects air movement and the resistance of the soil to erosion and plant root growth
  • soil texture – the composition of sand, silt and clay in a soil type
  • soil content – this affects soil behaviour, like air movement, water movement and the retention capacity for nutrients and water. Organic matter component of the soil, exposure of the soil surface to solar radiation, the vegetative cover of the soil surface and the topography of the land all influence soil content.

Categories of soil water

• Hygroscopic water = a very thin layer of soil water, when the soil is about air dry, that attaches firmly to the soil It is not accessible to plant roots.
• Capillary water = the quantity of water held in the soil macro- and micro-pores (basically =the water absorbed by plant roots)
• Gravitational water = the excess water that moves freely after the capillary pores are saturated → moves down deep into the soil as a result of the Earth’s gravitational pull.

How to conserve soil water

• Introduce organic matter into the soil
• Practise mulching in dry areas
• Control weeds regularly to reduce transpiration
• Adopt farm practices, such as regular stirring of the topsoil → encourages water infiltration and discourages soil erosion.

Unit 2: Minerals (Primary and Secondary)

2.1 The concept of minerals

• Minerals are the inorganic substances that occur in the crust of the Earth, in soils.
• Minerals originate from mother rocks through rock weathering.
• Rock weathering forms soil. The soil contains the same elements as the mother rock.

2.2 Rock minerals

• Rock minerals are also called the inorganic soil fraction → consists of various soil particles (e.g. sand, silt clay stones and stones) → help determine soil type, texture and characteristics (e.g. water holding capacity).
• Minerals are naturally occurring solid substances that occur in the Earth’s crust.
• They have definite chemical composition, crystalline structure, colour and hardness. Some are good for commercial activities. Examples of minerals found in soil include:
  • gold and diamond
  • ore, e.g. iron ore and copper ore
  • rock forming minerals, e.g. sand and clay.

2.2.1 Classification of rock minerals

• Primary minerals: Occur in their original form during the formation of rocks.
  • When rocks are formed, they contain minerals that are in their original forms with their basic properties like shape, colour and elements.
  • Primary minerals are found in soil but are not formed in soil.
  • Examples = apatite, calcite, dolomite, feldspars and quartz.
• Secondary minerals are formed in soils: When the primary minerals undergo physical and chemical changes (e.g. oxidation and temperature variations), they lose some of their original properties.
  • They regroup to form new minerals that are different from their original forms.
  • Secondary minerals are all the minerals that have undergone chemical changes from their original forms.
  • Examples = mica clays, kaolinite clay, and candites.

2.2.2 Characteristics used in mineral identification

• Colour = the first characteristics you notice. (Streak = the colour revealed below the surface of a mineral when you scratch it.)
• 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.

Unit 3: Rocks and Their Formation

3.1 The concept: mother rock
The Earth is built up of layers → from the inner core → then the outer core → followed by the mantle → and finally the crust.

• The crust of the Earth consists of different combinations of minerals found in different types of rocks.
• The quantity and quality of minerals found in the different types of rocks differ.
• All soils are formed when rocks break down.
• The rocks that break down to form soil are called the mother rock or the parent material.
• Soils formed from the mother rocks or parent rocks have the physical and chemical properties of the parent or mother rocks.

The Earth’s structure

3.2 Types of rocks
We get three main types of rocks:

• Igneous rocks
• Sedimentary rocks
• Metamorphic rocks.

3.2.1 Igneous rocks

• Formed when molten magma or lava cools and solidifies → usually during volcanic activity.
• May or may not form crystals below the surface of the Earth:
  • If it is formed by the crystallisation of magma below the surface of the Earth, igneous rocks = 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.
    • Examples = dolerite, granite and gabbro.
  • If the crystallisation of the magma takes place on the surface of the Earth, the igneous rocks = extrusive or volcanic rocks.
  • Extrusive rocks are fine-grained in texture, because they cool down faster on the surface and there is not time for large crystals to develop.
  • Example = basalt.
• Igneous rocks have various properties and mineral deposits, for example, tin and uranium are commonly associated with granites, while ores of chromium and platinum are obtained from gabbros.
• Igneous rocks are generally opaque, rough and dark in colour. Most of the crushed stones on our tarred roads are from igneous rocks.

3.2.2 Sedimentary rocks

• The Earth’s crust is constantly being eroded by rivers, runoff, glaciations and wind.
• The eroded pieces are carried along by water, wind and glaciers (agents of erosion) and are eventually deposited as layers of sediment = bedding.
• Rocks form from this sediment (bedding), e.g. shale, sandstone and siltstone.
• Some sedimentary rocks are organically formed from the remains of plants, animals and trees, e.g. limestone = shells of animals and coal = layers of carbonised trees and plants.

3.2.3 Metamorphic rocks

• Metamorphic rocks have transformed physically and chemically as a result of heat (150–2 000ºC).
• Any type of rock can undergo metamorphosis, e.g. sandstone to quartzite, limestone to marble.

3.2.4 Properties of minerals
The table below shows the properties of some common minerals.

The properties of some common minerals

3.3 The cultivation properties and suitability of soil from rocks

• The physical and chemical properties of a soil depend on the rocks from which the soil is formed.
• Soil = rock that has broken down after millions of years of weathering (involving water, wind, movement, chemical action and varying temperatures).
• Cultivation properties of soil and its suitability for agricultural purposes depend on the composition of the original rock and the processes that have acted on it.

3.3.1 Soil from igneous rocks
Igneous rocks are intrusive or extrusive, according to how they are formed. So, the suitability of soil formed from igneous rocks depends on whether the parent material is of plutonic origin (extrusive) or volcanic origin (intrusive).

• When plutonic igneous rocks (e.g. granite) disintegrate → the soil particles that form are large and course. They are generally not suitable for crop production because they:
  • make the soil loose
  • have poor water holding capacity
  • are usually light in colour
  • are generally not fertile because they may be acidic with no nutritious value to plants.
• Volcanic or extrusive igneous rocks, for example, basalt, weather to form fine grains consisting of clay.
• The soil from fine grains is fertile and suitable for many crops, because it:
  • is smooth and plastic
  • has good water holding capacity
  • has good cation exchange capacity and so is very rich
  • is dark in colour:
    • dark soil colour is good for heat conduction and retention
    • is ideal for soil microbe activities in the soil (makes soil fertile).

3.3.2 Soil from sedimentary rocks
Soils formed from sedimentary rocks that contain organic deposits are suitable for crop cultivation. This is because:

• Sedimentary rocks erode easily.
• When sedimentary rocks disintegrate, different soil particles (clay, silt, sand) come loose.
• Often, organic residue, like humus, may be a constituent of the sediments that break loose.
• A good combination of soil particles (loamy soil) is formed from the sediments.
• The dark colour from sedimentary rocks comes from organic matter inclusions in the sedimentary rocks.
• Soils from most sedimentary rocks, like alluvial deposits, contain humus → are very fertile (high mineral content).

3.3.3 Soil from metamorphic rocks
The suitability of the soil formed from metamorphic rocks depends upon the rock type that went through metamorphosis to form the soil type. However, metamorphic rocks are hard and resist erosion → so they are mostly not suitable for crop cultivation because:

• They produce weak soils that contain few minerals good for plant growth.
• They usually produce red soils due to the way the parent materials are formed from warmth, temperature changes and pressure.
• They weather slowly to resist acidification of soil.
  • Soils that develop from rocks with low amounts of weatherable minerals (ferromagnesian) and low iron content (quartz-mica schist), for example, calcite and dolomite are:
    • reddish yellow
    • have silty clay textures and blocky structures
    • low iron oxide content.

Unit 4: Weathering of Rocks

4.1 Concept of weathering

• Soils are formed from mother rock or parent material.
• Rocks are subjected to soil forming factors (e.g. weathering) over a period → the outer layer of the rock loosens and crumbles to form soils.
• Rock weathering takes place through:
  • mechanical or physical weathering
  • chemical weathering
  • biological weathering.
• Rock weathering forms soil → it releases biochemical elements, such as calcium, potassium, iron and phosphorus into the soil as nutrients.

4.2 The importance of weathering
Weathering is important for the following reasons:

• Soil formation:
  • Rock weathering is the basis of soil formation. It is important for the release of biochemical elements that have no gaseous form, for example, calcium (Ca), potassium (K) iron (Fe) and phosphorus (P).
• Nutrient cycling:
  • Weathering of parent materials contributes to nutrient cycle formation in soil.
• Duricrust formation:
  • Duricrust is a hard, thin layer near the surface of soil, usually a few millimetres thick → formed by the accumulation of soluble minerals deposited by mineral- bearing waters that move upwards, downwards or laterally by capillary action. Minerals often found in duricrust include silica, iron, calcium and gypsum.
• Ore deposits:
  • An ore is a type of rock that contains minerals with important elements including metals. Ores are extracted through mining.
• Clay mineral formation:
  • Clay minerals are formed over long periods of time by the gradual chemical weathering of rocks. End product of rock weathering = clay mineral formation. Clay is an important soil component because of its cation absorption capacity.
• Coastal landforms:
  • The combined effect of waves, currents and tides results in rock disintegration. Beach drifting transports sand grains along the beach as the waves strike the shore. The continuous deposit of weathered materials over a long period of time → leads to the formation of landforms at the coast.
• Salt from the ocean:
  • Salt in rocks = released from weathered rocks → drained into oceans →form salts.

4.3 Weathering agents and processes

• The agents that are responsible for the breaking down of rocks (weathering) are physical/mechanical, chemical and biological agents.
• So, soil formation always involves one or all of the processes below to bring about soil formation.

4.3.1 Physical/mechanical weathering

• Physical or mechanical weathering causes the disintegration of rocks without chemical change.
• The primary process in physical weathering is abrasion.
• All physical weathering agents involve energy in the breaking down processes.
• Physical weathering agents are:
  • wind
  • water (rain, rivers, ocean, and lakes)
  • temperature.

Wind
Strong winds have much energy → can carry soil particles in their way.

• These soils can hit the surfaces of exposed rocks with a strong force → can further remove particles from the rock surfaces.
• Over many years, a considerable pile of soil may be formed at the base of the rocks.
• This type of weathering is common in desert areas where evaporation is higher than precipitation.

Water
Water has a dual role in rock weathering → causes physical and chemical weathering.

• Running water (e.g. streams) 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. The stones rub against each other → causes weathering.

Temperature changes

• Can cause weathering, e.g., with daily temperature variation → repeated cooling and heating of a rock surface → weakens surface and it breaks in smaller pieces.

Note:
Chemical and physical weathering often go hand in hand, for example, 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.

4.3.2 Chemical weathering

• The agents of chemical weathering are water, oxygen and carbon dioxide.
• The five main chemical processes involved in rock weathering are: dissolution/ solution (water); hydration (water); hydrolysis (water); carbonation (air); oxidation (air).

Water as an agent of chemical weathering

• Dissolution/solution:
  • Some rocks (e.g. gypsum and rock salt), dissolve in water when they stay in water for years, because materials that bond the soil particles dissolve.
• Hydration:
  • Water combines with rocks in rivers, ponds and in the sea. The rocks absorb water gradually to become saturated. The hydrogen and oxygen atoms in the water → cause the rocks to chemically change their original status to form a weaker rock. The newly formed rock weakens → disintegrates to form soil.
• Hydrolysis:
  • When a chemical disintegration of rocks takes place in the presence of water, hydrolysis has taken place: H+ or OH- replaces an ion in the mineral.

Air as an agent of chemical weathering

• Carbonation:
• During rainfall, the raindrops collect carbon dioxide in the atmosphere. When the carbonated water combines with rocks → they form dilute carbonic acid (weak acid). The acid dissolves rocks (e.g. limestone) → releases colloids and granules from the limestone.
  H₂0    +          CO₂              →                      H₂C0₃                            →              H⁺ + HCO₃
  water + carbon dioxide                 carbonic acid hydrogen ion                         bicarbonate ion
• Oxidation:
  • During rainfall the raindrops combine with the oxygen in the atmosphere. The oxygen reacts with rock minerals that contain iron → forms iron oxides → causes rocks to break apart.

4.3.3 Biological weathering

• In biological weathering, living organisms (e.g. plants and animals) → cause rock to decompose. For example: plant and tree roots can work their way into the crevices of a rock → forces the rock apart → causes it to fracture.
• Some creatures (e.g. worms and termites) can be responsible for biologically weathering rocks and rock particles → they physically break rocks apart during physical activities such as boring.
• When plants and animals decay → they release carbon dioxide (CO2) into the air. When the CO2 mixes with water → it forms carbonic acid → can break down the minerals in rocks.

Unit 5: Soil Forming Factors

Soil formation factors
Natural processes of soil formation take thousands of years to create very small amounts of soil.

• The process of soil formation and development by soil forming factors is called pedogenesis.
• There are five natural factors that combine with the activities of people to contribute towards the formation of soils:
• Soil formation can be represented by the equation:
  • S = F (P, R, Cl, O, T), where
    • S = Soil
    • F = factors
    • P = Parent material
    • R = Relief/topography
    • Cl = Climate
    • O = Organisms
    • T = Time

5.1 Topography/relief in soil formation
Topography/relief (geographical factors) = the appearance of the area, e.g. mountains, valleys, water patterns, cliffs. Can affect the rate of soil formation in the following ways:

• Soil forming materials drop from high altitudes and pile up in valleys to form more soil over the years than could be formed on the upper land surfaces. Temperature differences between high and low altitudes causes rock disintegration.
• The effect of solar radiation, the impact of rainfall and wind action on soil formation is greater on high lands than in valleys.
• Runoff erodes soil particles and debris, and deposits it lower down → causes more soils to form in low-lying areas.

5.2 Climatic factors contributing to the formation of soil
Climatic factors or elements of climate → break down parent materials → form soils.

5.2.1 Sunlight and temperature

• The influence of sunlight and temperature are inter-related:
  • the temperature of the parent material is the result of solar radiation.
• Hot / cold temperature variations → cause expansion and contraction → as a result, rocks disintegrate.
• High temperatures speed up chemical processes (e.g. oxidation).
  • This action → causes the elements in the soil to be released causing lines of weaknesses and eventual disintegration.
• Water stored in the pore spaces of rocks freeze in very cold regions.
  • When water freezes, it expands → causes gradual disintegration.

5.2.2 Wind

• The effect of wind as a soil-forming agent is more visible in the areas with less or no vegetation (e.g. deserts).
• When strong winds move over bare soil surface → the wind removes loose soil particles. This is called abrasion.
• The soil particles deposited by wind pile up over years to form a sheet of soil.

5.2.3 Rain

• Torrential rain on sand stone removes particles of sand to form sandy soil.
• Swift and high volumes of runoff after torrential rains run over rock surfaces.
  • The runoff removes loose soil particles as it moves down the slope.
  • Friction between rolling stones → causes disintegrating.

5.3 Biological factors contribute to the formation of soil
5.3.1 Plants

• Roots penetrate layers of rocks. As the roots grow thicker → they cause disintegration and crumbling.
• Decomposition of plants form humus.
• During respiration of plant roots, carbon dioxide is released into the soil.
  • CO₂ dissolves in soil water to form carbonic acid → causes rocks to disintegrate.

5.3.2 Animals

• 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.

5.3.3 Human activities
Human activities, such as road construction, terracing, rock quarrying, excavation for new settlements and mining, break down rocks.

5.4 Parent material in soil formation
All types of soils are formed from a source. Soils are formed from hard rocks (mother rock), unconsolidated sediments and decomposed plants and animals (humus).

5.4.1 Parent materials and geology

• The rate of soil formation, the structure of a soil, the texture of a soil and all other physical and chemical characteristics of a soil depend on the geology of the area. The geological processes provide the parent material.
  • Melted rock flows away from inside the earth through volcanicity (the activity of volcanoes) and eventually cools and hardens.
    • During the process, minerals crystallise and new rock types are formed.
    • These types of rocks are called igneous rocks.
    • Igneous rocks are the original parent material rocks formed on the earth.
  • All soil types formed from rocks on the earth depend upon the processes of their development from the igneous rocks.
• The parent material also determines the minerals in soil.
  • The structure, texture, aeration and drainage status of all soils are the result of the mineral particles that originate from the parent material.

5.4.2 Parent materials and mineral soils (mineralogy)

• The minerals found in soils, but not formed in the soil, are primary minerals.
  • Primary minerals come from igneous rocks and have not undergone any changes since they were formed.
  • Secondary minerals are formed when the primary minerals undergo physical or chemical changes.
  • Mineral parent material is the particle sizes that make the soil = sand, silt or clay.
  • Mineral particles are classified according to the mode of deposition:
    • Colluvium deposits → mineral materials are transported and deposited at the base of cliffs and mountains by the force of gravity from cliffs.
    • Alluvium mineral materials are transported by water during floods → they have wide ranges in particle sizes.
    • Outwash mineral materials are glacial materials that have been carried by water after glacial melting → it usually is made up of a range of particle sizes, from gravel to sand.
    • Beach deposits are sandy = the result of wave action.

5.5 Time as a soil-forming factor
It takes time for soils to form from rocks → from the formation of the bedrock to the period when soil particles are derived from the parent material:

• magma from volcanic eruptions takes time to settle and cool down.
• it takes hundreds of years for the agents of rock weathering to act on the parent rocks to disintegrate or erode to form soils.
• the rate of disintegration and erosion depends upon the type of parent material and the prevailing weathering agents.
• organic matter in the soil takes some time to decompose completely to form humus.

Unit 6: Soil-forming Processes

6.1 Soil-forming processes active in soils
6.1.1 Mineralisation
Mineralisation = the release of organic compounds during decomposition of organic residues by oxidation to form soluble or gaseous chemical compounds.

• The chemical compounds may then take part in further soil processes or be utilised by plant life.
• Mineralisation is an essential process in the formation of humus.

6.1.2 Humification

• The process whereby the carbon of organic residues is transformed and converted into humic substances (humus) through biochemical processes.

6.1.3 Leaching

• The removal of soluble nutrients from an upper soil horizon to a lower soil zone beyond the reach of plant roots.
• Leached nutrients are not available to plants.

6.1.4 Luviation
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.

6.1.5 Gley soil
Gley soils are sticky and difficult to cultivate.

• Formed in waterlogged areas where there is little oxygen in the soil.
• Greenish-blue-grey or mottled colour.
  • The grey colour is the result of the reduction, under anaerobic conditions, of ferric iron to the ferrous state.

6.1.6 Plinthite formations

• Plinthite soils contain high iron and low humus content, and do not have most essential elements → therefore poor for crop growth.
  • The high iron content causes phosphorus to be fixed and made unavailable to crops.
    • It is a highly weathered mixture of clay with quartz and other diluents.
    • It commonly appears as red mottles, usually in platy or rectangular patterns.

6.1.7 Inversion
Human activities, e.g. ploughing and tilling, contribute to soil inversion = the topsoil is fully turned upside down.

• Inversion is used to bury weeds deep in the soil to prepare land crops.
• Advantages are:
  • weed seeds are buried deep in the soil to prevent sprouting
  • it controls plant diseases and pests
  • it encourages mineralisation of nitrogen
  • microbial activities are encouraged
  • it enhances even mixture of soil nutrients for both deep rooted and shallow crops
  • green manure and organic matter are incorporated into the soil.

6.1.8 Bioturbation

• Bioturbation = the churning of soil by organisms and plants roots organisms (e.g. earthworms) and burrowers (e.g. moles, rats, rabbits) that dig deep into the soil and push subsoil to the soil surface
• → this leads to a change in the composition of the soil.
• Bioturbation has similar advantages to soil inversion.

Topic 4: Questions

• 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]