Fig.3. 1: A farmer spraying rice
Key unit Competence
By the end of the unit the learner should be able to evaluate applications of Physics in Agriculture.
• Describe the atmosphere and its constituents.
• Outline variation of atmospheric pressure, air density and water vapour with altitude.
• Evaluate how heat and mass transfers occur in the atmosphere.
• Apply knowledge of physics to illustrate changes in water vapour atmospheric pressure, and air with altitude.
• Evaluate and interpret physical properties of soil (soil Texture and structure).
• Evaluate why air, temperature and rainfall limit economical activities in Agriculture.
• Explaining how mechanical weathering and soil erosion impact economic activities in agriculture.
• Explaining clearly how agrophysics plays an important role in the limitation of hazards to agricultural objects and environment in our country.
Introductory activity: Role of machines in agriculture
It is very important to know the role of Physics in agriculture and environment. Knowledge in physics can contribute more in the limitation of hazards in agriculture (soils, plants, agricultural products and foods) and the environment based on suitable programs of transformation and modernization of agriculture in our country.
Look at the image given in Fig.3.1 above!
a. What do you observe? Is there any application of prior knowledge of Physics learnt before applied on the image? Justify your answer?
b. Can you suggest the role of machines in agriculture? How do they contribute in the rapid development of the country programs of transformation and modernization of agriculture? Knowing different stages of growing plants activities, suggest which stages mostly benefit the use of technology!
Plan it! To get started, brainstorm about your prior knowledge on applications of physics in agriculture and try to suggest answers to questions given above based on your understanding.
3.1 ATMOSPHERE AND ITS CONSTITUENTS
Activity 3.1: How the atmosphere protect life on the earth
a. Brainstorm and write short notes on constituents of atmosphere and explain clearly how the atmosphere of Earth protects life on earth.
b. Why should you care about protecting the atmosphere and minimise the long-term changes in the climate?
c. What can be the use of atmospheric knowledge in evaluating and improving agricultural activities?
The atmosphere of earth is the layer of gases, commonly known as air that surrounds the earth. This layer of gases is retained by Earth’s gravity. The atmosphere of Earth protects life on Earth by absorbing ultraviolet solar radiations that cause cancers and other diseases, warming the surface through heat retention (greenhouse effect) and reducing temperature extremes between day and night. It also contain the oxygen which human beings, animals and plants are using, The atmospheric knowledge can be helpful in evaluating and improving the quality of soils and agricultural products as well as the technological processes.
3.1.2 Composition of the Atmosphere
The atmosphere is composed of a mixture of several gases in differing amounts. The permanent gases whose percentages do not change from day to day are nitrogen, oxygen and argon. By volume, dry air contains 78.0% nitrogen, 21% oxygen, 0.9% argon, 0.04% carbon dioxide, and small amounts of other gases called trace gases 0.1% as shown in Fig.3.2
Gases like carbon dioxide, nitrous oxides, methane, and ozone are trace gases that account for about a tenth of one percent of the atmosphere.
Fig.3. 2Composition of the atmosphere
Air also contains a variable amount of water vapor, on average around 1% at sea level, and 0.4% over the entire atmosphere. Water vapor is unique in that its concentration varies from 0-4% of the atmosphere depending on where you are and what time of the day it is. In the cold, dry Arctic regions water vapor usually accounts for less than 1% of the atmosphere, while in humid, tropical regions water vapor can account for almost 4% of the atmosphere. Water vapor content is very important in predicting weather. Air content and atmospheric pressure vary at different layers, and air suitable for use in photosynthesis by terrestrial plants and breathing of terrestrial animals is found only in earth’s troposphere.
The composition of the atmosphere, among other things, determines its ability to partly absorb and transmit sunlight radiations and trap infrared radiations, leading to potentially minimise the long-term changes in climate.
3.1.3 Layers of the atmosphere.
Activity 3.2 Classifying layers of the atmosphere.
Materials For class demonstration:
• Chalk board or dry erase board mounted on a wall
• Chalk or dry erase marker
• 2-meter piece of string
• 1000 ml (1 litre) graduated cylinder
• Four bags of fish gravel or coloured sand (different colours)
1. Use a model to explore how far the earth’s atmosphere extends above the surface of the earth and learn about the thickness of the different layers of the atmosphere.
2. How far do you think the atmosphere extends above us? Tie a dry eraser marker or a piece of chalk to one end of the string. Standing next to the board, place your foot on the free end of the string and draw an arc on the board with a radius of about 1.2 m. Your foot represents the center of the earth. The arc represents the surface of the Earth.
Fig.3. 3 A person demonstrating the layers of the atmosphere
3. Suggest how far the earth’s atmosphere would extend above the surface in this drawing. Mark your suggestions on the board above the chalk/marker line. Note that it is found that over 90% of the earth’s atmosphere is within about 12km of the earth’s surface. The distance from the centre of the earth to its surface equals about 6361 km.
1. Use a 1000 ml (1 litter) graduated cylinder and represent the layers by using the following amounts of fish gravel or coloured sand found in the photo and table below.
2. Choose what colour you want for each atmospheric layer. Keep in mind these are relative proportions and not exact points of departure for the different layers. In this scale model, each millilitre of volume represents one kilometre of atmosphere layer thickness (for example, the troposphere is 10 km thick and is represented by 10 ml of sand or gravel in the graduated cylinder).
1. What atmospheric layers are represented by the different colors?
2. What atmospheric layer do we live in?
3. How much thicker is the stratosphere compared to the troposphere?
4. How much thicker is the thermosphere compared to all the other layers combined?
5. Where in this model would you expect to find clouds?
6. Where in this model would you expect to find mounts Everest and Kalisimbi?
7. Where in this model would you expect to find a satellite?
8. Where in this model would you expect to find the space shuttle?
9. Where in this model is the ozone layer?
Earth’s atmosphere has a series of layers, each with its own specific characteristics and properties. Moving upward from ground level, these layers are named the troposphere, stratosphere, mesosphere, thermosphere and exosphere.
The above activity demonstrates the relative thickness of the thin section of the atmosphere that includes the troposphere and stratosphere. These layers are essential to all life on Earth.
Over 99% of the mass of the Earth’s atmosphere is contained in the two lowest layers: the troposphere and the stratosphere. Most of the Earth’s atmosphere (80 to 90%) is found in the troposphere, the atmospheric layer where we live (Cunningham & William, 2000).
This layer, where the Earth’s weather occurs, is within about 10 km of the Earth’s surface. The stratosphere goes up to about 50 km. Gravity is the reason why atmosphere is more dense closer to the Earth’s surface.
Fig.3. 5: Illustration of layers from the core to the atmosphere
While we may think of the atmosphere as a vast ocean of air around us, it is very thin relative to the size of the earth. The “thickness” of the atmosphere, or the distance between the earth’s surface and the “top” of the atmosphere, is not an exact measure. Although air is considered as a fluid, it does not have the same well-defined surface as water does. The atmosphere just fades away into space with increasing altitude. Description of layers of earth’s atmosphere:
This is the lowest layer of our atmosphere starting at ground level; it extends upward to about 10 km above the sea level. We live in the troposphere, and nearly all weather occurs in this lowest layer. Most clouds appear here, mainly because 99% of the water vapor in the atmosphere is found in the troposphere. Air pressure drops and temperatures get colder, as you climb higher in the troposphere.
The stratosphere extends from the top of the troposphere to about 50 km above the ground. The infamous ozone layer is found within the stratosphere. Ozone molecules in this layer absorb high-energy ultraviolet (UV) light from the sun, converting the ultraviolet energy into heat. Unlike the troposphere, the stratosphere actually gets warmer the higher you go! That trend of rising temperatures with altitude means that air in the stratosphere lacks the turbulence and updrafts of the troposphere beneath. Commercial passenger jets fly in the lower stratosphere, partly because this less-turbulent layer provides a smoother ride.
It extends upward to a height of about 85 km above our planet. Most meteors burn up in the mesosphere. Unlike the stratosphere, temperatures once again grow colder as you rise up through the mesosphere. The coldest temperatures in earth’s atmosphere, about -90° C, are found near the top of this layer. The air in the mesosphere is far too thin to breathe; air pressure at the bottom of this layer is well below 1% of the pressure at sea level, and continues dropping as you go higher.
High-energy X-rays and ultraviolet radiations from the Sun are absorbed in the thermosphere, raising its temperature to hundreds or at times thousands of degrees. Many satellites actually orbit Earth within the thermosphere! Variations in the amount of energy coming from the Sun exert a powerful influence on both the height of the top of this layer and the temperature within it. Because of this, the top of the thermosphere can be found anywhere between 500 km and 1 000 km above the ground. Temperatures in the upper thermosphere can range from about 500 ° C to 2 000 °C or even higher.
Although some experts consider the thermosphere to be the uppermost layer of our atmosphere, others consider the exosphere to be the actual “final frontier” of Earth’s gaseous envelope. As you might imagine, the “air” in the exosphere is very thin, making this layer even more space-like than the thermosphere. In fact, air in the exosphere is constantly “leaking” out of earth’s atmosphere into outer space. There is no clear-cut upper boundary where the exosphere finally fades away into space. Different definitions place the top of the exosphere somewhere between 100 000 km and 190 000 km above the surface of earth. The latter value is about halfway to the moon!
The ionosphere is not a distinct layer like the others mentioned above. Instead, the ionosphere is a series of regions in parts of the mesosphere and thermosphere where high-energy radiation from the sun has ionized atoms and molecules. The ions formed in this way are responsible of the naming of this region as the ionosphere and endowing it with some special properties.
Activity 3.3 Classifying layers of the atmosphere.
Observe scale models of the atmosphere on fig.3.3 below and its layers. Note the height of earth’s atmosphere as compared to the size of the planet overall and the relative thickness of each of the four main layers of the atmosphere. Interpret the graph and Use it to answer questions below:
Fig.3. 6: Layers of the atmosphere.
a. Outline the economic activities taking place in the layers represented above?
b. Why is it very important to have ozone layer in the layers close to the earth surface?
c. Explain why it is advisable to travel in troposphere and stratosphere than in other layers of the atmosphere?
d. Explain clearly why the rocket and aeroplanes decide to move in the corresponding layers of the atmosphere?
3.1.4 Checking my progress
1. Use the fruit question suggested in the procedure and respond in writing or with a picture instead of simply orally in class. Think of the fruit as the size of the Earth and the skin of the fruit represents the thickness of the atmosphere. Make a labelled diagram in your note book illustrating the most important point of the lesson and the reason why the atmosphere is considered as the skin of the fruit?
2. Imagine that you are in an orbit around a planet one half the size and mass of the Earth. Explain how I would expect the atmosphere of the new planet to be different from my planet?
3. What is the structure and composition of the atmosphere?
4. How does solar energy influence the atmosphere?
5. How does the atmosphere interact with land and oceans?
6. Outline two most important layers that are essential to all life on earth?
Explain clearly to support your answer.
3.2 HEAT AND MASS TRANSFER
3.2.1 Modes of Heat Transfer
Activity 3.4: Describing the different modes of heat transfer in the atmosphere
Brainstorm on modes of heat transfer and explain clearly how each affects agricultural activities?
Heat transfer is concerned with the exchange of thermal energy through a body or between bodies which occurs when there is a temperature difference. When two bodies are at different temperatures, thermal energy transfers from the one with higher temperature to the one with lower temperature. Naturally heat transfers from hot to cold.
A small amount of the energy that was directed towards the earth from the sun is absorbed by the atmosphere, a larger amount (about 30%) is reflected back to space by clouds and the Earth’s surface, and the remaining is absorbed at the planet surface and then partially released as heat.
Energy is transferred between the Earth’s surface and the atmosphere in a variety of ways, including radiation, conduction, and convection. The figure below uses a camp stove to summarize the various mechanisms of heat transfer. If you were standing next to the camp stove, you would be warmed by the radiation emitted
by the gas flame. A portion of the radiant energy generated by the gas flame is absorbed by the frying pan and the pot of water.
By the process of conduction, this energy is transferred through the pot and pan. If you reached for the metal handle of the frying pan without using a potholder, you would burn your fingers! As the temperature of the water at the bottom of the pot increases, this layer of water moves upward and is replaced by cool water descending from above. Thus convection currents that redistribute the newly acquired energy throughout the pot are established.
Fig.3. 7: Camp stove to summarize the various mechanisms of heat transfer.
As in this simple example using a camp stove, the heating of the Earth’s atmosphere involves radiation, conduction, and convection, all occurring simultaneously. A basic theory of meteorology is that the Sun warms the ground and the ground warms the air. This activity focuses on radiation, the process by which the Sun warms the ground. Energy from the Sun is the driving force behind weather and climate.
Quick check 3.1
What do trees, snow, cars, horses, rocks, centipedes, oceans, the atmosphere, and you have in common?
Each one is a source of radiation to some degree. Most of this radiation is invisible to humans but that does not make it any less real.
Radiation is the transfer of energy by electromagnetic waves. The transfer of energy from the Sun across nearly empty space is accomplished primarily by radiation. Radiation occurs without the involvement of a physical substance as the medium. The Sun emits many forms of electromagnetic radiation in varying quantities.
Fig.3. 8: The spectrum of electromagnetic radiations emitted by the sun
About 43% of the total radiant energy emitted from the Sun is in the visible part of the spectrum. The bulk of the remainder lies in the near-infrared (49%) and ultraviolet (7%) bands. Less than 1% of solar radiation is emitted as x-rays, gamma rays, and radio waves.
A perfect radiating body emits energy in all possible wavelengths, but the wave energies are not emitted equally in all wavelengths; a spectrum will show a distinct maximum in energy at a particular wavelength depending upon the temperature of the radiating body. As the temperature increases, the maximum radiation occurs at shorter wavelengths.
The hotter the radiating body, the shorter the wavelength of maximum radiation. For example, a very hot metal rod will emit visible radiation and produce a white glow. On cooling, it will emit more of its energy in longer wavelengths and will glow a reddish color. Eventually no light will be given off, but if you place your hand near the rod, the infrared radiation will be detectable as heat.
The amount of energy absorbed by an object depends upon the following:
• The object’s absorptivity, which, in the visible range of wavelengths, is a function of its color
• The intensity of the radiation striking the object
Every surface on Earth absorbs and reflects energy at varying degrees, based on its color and texture. Darker-colored objects absorb more visible radiation, whereas lighter-colored objects reflect more visible radiation. These concepts are clearly discussed in unit two.
3.2.2 Environmental heat energy and mass transfer
Fig.3. 9: Illustration of interactions of solar radiations with different constituents of the atmosphere.
Practically all of the energy that reaches the earth comes from the sun. Intercepted first by the atmosphere, a small part is directly absorbed, particularly by certain gases such as ozone and water vapor. Some energy is also reflected back to space by clouds and the earth’s surface.
In the atmosphere, convection includes large- and small-scale rising and sinking of air masses.. These vertical motions effectively distribute heat and moisture throughout the atmospheric column and contribute to cloud and storm development where rising motion occurs) and dissipation where sinking motion occurs.
3.2.3 Water vapour in the atmosphere.
Activity3.5: Impact of water vapor in agricultural activities
1. Brainstorm on water vapor in the atmosphere and explain clearly how it impacts on agricultural activities?
2. Does water vapor play an important role in the atmosphere? Justify your answer with clear reasons.
When water vapor condenses onto a surface, a net warming occurs on that surface. The water molecule brings heat energy with it. In turn, the temperature of the atmosphere drops slightly. In the atmosphere, condensation produces clouds, fog and precipitation (usually only when facilitated by cloud condensation nuclei).
The role of water vapor in the atmosphere
Water vapor plays a dominant role in the radiative balance and the hydrological cycle. It is a principal element in the thermodynamics of the atmosphere as it transports latent heat and contributes to absorption and emission in a number of bands. It also condenses into clouds that reflect and adsorb solar radiation, thus directly affecting the energy balance.
In the lower atmosphere, the water vapor concentrations can vary by orders of magnitude from place to place.
3.2.4 Variation in Atmospheric Pressure
Variation with height or vertical variation: The pressure depends on the density or mass of the air. The density of air depends on its temperature, composition and force of gravity. It is observed that the density of air decreases with increase in height so the pressure also decreases with increase in height.
Horizontal variation of pressure: The horizontal variation of atmospheric pressure depends on temperature, extent of water vapor, latitude and land and water relationship.
Factors affecting atmospheric pressure:
1. Temperature of air
3. Water vapor in air
4. Gravity of the earth.
Effect of atmospheric pressure in agricultural activities
The pressure exerted by the atmosphere of the earth’s surface is called atmospheric pressure. Generally, in areas of higher temperature, the atmospheric pressure is low and in areas of low-temperature the pressure is high. Atmospheric pressure has no direct influence on crop growth. It is, however an important parameter in weather forecasting.
3.2.5 Air density and water vapour with altitude
Activity 3.7: Effect of air density in the atmosphere
Brainstorm on air density and explain clearly its affects in the earth’s atmosphere?
The density of air (air density) is the mass per unit volume of earth’s atmosphere. Air density, like air pressure, decreases with increasing altitude. It also changes with variation in temperature and humidity. At sea level and at 15 °C air has a density of approximately 1.225 kg/m3.
Air density and the water vapor content of the air have an important effect upon engine performance and the takeoff characteristics of air-craft. Some of the effects these two factors have upon engine takeoff, and the methods for computing these elements from a meteorological standpoint. Pressure altitude, density altitude, vapor pressure, and specific humidity in the atmosphere are determined using a Density Altitude Computer.
Pressure altitude: Pressure altitude is defined as the altitude of a given atmospheric pressure in the standard atmosphere. The pressure altitude of a given pressure is, therefore, usually a fictitious altitude, since it is equal to true altitude only rarely, when atmospheric conditions between sea level and the altimeter in the aircraft correspond to those of the standard atmosphere. Aircraft altimeters are constructed for the pressure-height relationship that exists in the standard atmosphere.
3.2.6 Checking my progress
1. Why does atmospheric pressure change with altitude?
2. The graph below gives an indication of how pressure varies non-linearly with altitude. Use the graph to answer the following questions:
Fig.3. 10: A graph of altitude vs pressure
a. Explain what happens to pressure if the altitude reduces?
b. Estimate the atmospheric pressure when someone is at an altitude of 40 km above sea level.
3.3 PHYSICAL PROPERTIES OF SOIL
Practical Activity 3.8: How the surface of earth reflects and absorbs heat
Perform the activity to investigate how different surfaces of the earth reflect and absorb heat and apply this knowledge to real-world situations. It justifies that the physical characteristics of the Earth’s surface affect the way that surface absorbs and releases heat from the Sun.
Materials needed in demonstration
1. a. Brainstorm on the already known concepts about how the color and type of material affects how hot it gets in the sunshine. Try to think about these questions. When it is a hot day, what color shirt would you wear to keep cool and why? During the summer, what would it feel like to walk on gravel with no shoes?
b. In performing activity, explore how different types of surfaces found at the earth’s surface (such as sand, soil, and water) heat up when the sun’s energy reaches them, and how they cool down when out of the sunshine.
c. Note that this experiment uses materials to model sunshine and earth materials. Observe the materials and realize how each material relates to the earth system. (The lamp represents the sun in this model. The sand represents beaches, sand dunes, and rocks. The potting soil represents large areas of soil outdoors. And the water represents lakes, rivers, and the ocean.)
2. Fill the pie pans to the same level, one with dark soil, one with light sand, and one with water.
3. Place the pie pans on a table or desk and position the lamp about 30.48 cm above them. (Do not turn on the lamp yet.)
Fig.3. 11: Arrangement of pie pans to investigate the absorption of solar radiation.
Checking skills Questions
1. Which material absorbed more heat in the first ten minutes?
2. Which material lost the most heat in the last ten minutes?
3. Imagine that it is summer and that the Sun is shining on the ocean and on a stretch of land.
a. Which one will heat up more during the day?
b. Which one will cool more slowly at night? Explain.
4. Imagine three cities in the desert, all at about the same altitude and latitude. Which city would likely have the highest average summer air temperature and why?
• One city (A) is surrounded by a dark-colored rocky surface
• Another city (B) is surrounded by a light-colored sandy surface.
• The third city (C) is built on the edge of a large man-made desert lake.
5. The Earth’s surface tends to lose heat in winter. Which of the above cities would have the warmest average winter temperature? Why?
6. Since the Sun is approximately 93 million miles from the Earth and space has no temperature, how do we get heat from the Sun?
Physical properties of a soil that affect a plant’s ability to grow include: Soil texture, which affects the soil’s ability to hold onto nutrients (cation exchange capacity) and water. Texture refers to the relative distribution of the different sized particles in the soil. It is a stable property of soils and, hence, is used in soil classification and description.
Soil structure, which affects aeration, water-holding capacity, drainage, and penetration by roots and seedlings, among other things. Soil structure refers to the arrangement of soil particles into aggregates and the distribution of pores in between. It is not a stable property and is greatly influenced by soil management practices.
3.3.1 Soil texture
Practical Activity3.9: Soil texture is determined by three proportions of the soil.
Brainstorm and try to answer questions using knowledge gained.
a. Outline three proportions of the soil?
b. What does the underlined word mean?
Soil texture, or the ‘feel’ of a soil, is determined by the proportions of sand, silt, and clay in the soil. When they are wet, sandy soils feel gritty, silky soils feel smooth and silky, and clayey soils feel sticky and plastic, or capable of being moulded. Soils with a high proportion of sand are referred to as ‘light’, and those with a high proportion of clay are referred to as ‘heavy’.
The names of soil texture classes are intended to give you an idea of their textural make-up and physical properties. The three basic groups of texture classes are sands, clays and loams.
A soil in the sand group contains at least 70% by weight of sand. A soil in the clay group must contain at least 35% clay and, in most cases, not less than 40%. A loam soil is, ideally, a mixture of sand, silt and clay particles that exhibit light and heavy properties in about equal proportions, so a soil in the loam group will start from this point and then include greater or lesser amounts of sand, silt or clay.
3.3.2 Soil structure
a. It is known that soil structure contains soil particles and pores and is classified under physical properties of soil. Brainstorm and write short notes on soil structure. Use the knowledge gained in part (a) above to answer questions in part (b).
b. (i) List the elements found in soil particles.
I. What do the underlined words mean?
II. Explain the role of pores in soil structure that improves capillary action in plant growth.
Structure is the arrangement of primary sand, silt and clay particles into secondary aggregates called peds or structural units which have distinct shapes and are easy to recognize. These differently shaped aggregates are called the structural type.
There are 5 basic types of structural units:
• Platy: Plate-like aggregates that form parallel to the horizons like pages in a book. This type of structure may reduce air, water and root movement.
• Blocky: Two types--angular blocky and sub angular blocky. These types of structures are commonly seen in the B horizon. Angular is cube-like with sharp corners while sub angular blocky has rounded corners.
• Prismatic: Vertical axis is longer than the horizontal axis. If the top is flat, it is referred to as prismatic. If the top is rounded, it is called columnar.
• Granular: Peds are round and pourous, spheroidal. This is usually the structure of A horizons.
• Structureless: No observable aggregation or structural units.
Good soil structure means the presence of aggregations which has positive benefits for plant growth. These benefits arise from the wider range of pore sizes which result from aggregation.
The nature of the pore spaces of a soil controls to a large extent the behavior of the soil water and the soil atmosphere. It influences the soil temperature. All these affect root growth, as does the presence of soil pores of appropriate size to permit root elongation. Favorable soil structure is therefore crucial for successful crop development. The destruction of soil structure may result in a reduction in soil porosity and/or change to the pore size distribution.
Soil structure refers to the arrangement of soil particles (sand, silt and clay) and pores in the soil and to the ability of the particles to form aggregates.
Aggregates are groups of soil particles held together by organic matter or chemical forces. Pores are the spaces in the soil.
The pores between the aggregates are usually large (macro pores), and their large size allows good aeration, rapid infiltration of water, easy plant root penetration, and good water drainage, as well as providing good conditions for soil micro-organisms to thrive. The smaller pores within the aggregates or between soil’s particles (micro pores) hold water against gravity (capillary action) but not necessarily so tightly that plant cannot extract the water.
A well-structured soil forms stable aggregates and has many pores (Fig.3.12 A). it is friable, easily worked and allows germinating seedlings to emerge and to quickly establish a strong root system. A poorly structured soil has either few or unstable aggregates and few pore spaces (Fig.3.12 B). This type of soil can result in unproductive compacted or waterlogged soils that have poor drainage and aeration. Poorly structured soil is also more likely to slake and to become eroded.
Fig.3. 12: Different soil structures: well structured and poorly structured soil.
3.3.3 Checking my progress:
1. (a) Explain the physical properties of Soil and explain clearly how each impact agricultural activities?
2. (a) Explain how the weathering of rocks contributes to soil formation.
(b) Explain the following terms as used in the context of soil and plant growth.
I. Well structured soil
II. Poor structured soil
(c) The following table shows the water content of three soil samples. Use the table to answer questions that follows:
I. What is the percentage of available water in sample A?
II. Which sample would be the most suitable for a crop suffering a drought during the growing season?
III. Which sample would be the most suitable for a crop growing during a rainy season?
(d) Describe an experiment to compare the capillarity of two contrasting soils.
3.4 MECHANICAL WEATHERING
3.4.1 Concepts of mechanical weathering
Laboratory Activity 3.11: Exploring Mechanical Weathering
Mechanical rock weathering is an important part of the formation of both soils and new rocks, and an important part of the entire rock cycle. The activity explores what causes rocks to break down.
• Coffee can with lid
• Dark-coloured construction paper
Fig.3. 13 Coffee can with lid
Place a handful of rocks on a piece of dark-coloured construction paper. Observe the rocks and take notes on their appearance. Place the rocks in a coffee can. Put the lid on the can and shake the can forcefully for 2 minutes, holding the lid tightly shut. Pour the rocks onto the construction paper. Observe them and take notes on any changes in their appearance.
a. Use the skills gained above to answer the following questions:
b. What happened to the rocks and why?
c. What forces in nature might affect rocks in similar ways?
Briefly explain what causes mechanical weathering?
Earth’s surface is constantly changing. Rock is integrated and decomposed, moved to lower elevations by gravity, and carried away by water, wind, or ice. When a rock undergoes mechanical weathering, it is broken into smaller and smaller pieces of sediment and dissolved minerals; each retaining the characteristics of the original material. The result is many small pieces from a single large one.
Weathering takes place in two ways: physical weathering and chemical weathering. Physical and chemical weathering can go on at the same time. Weathering is thus the response of Earth’s materials to change environment. Weathering is the first step in the breakdown of rock into smaller fragments. This process is critical to the formation of landscapes and many other geological processes. Our discussion will focus on mechanical weathering.
Mechanical weathering is the physical breaking up of rocks into smaller pieces.
3.4.2 Causes of mechanical weathering
a. Temperature change
Activity 3.12: Effects of temperature on mechanical weathering
a. Brainstorm on the effects of temperature in mechanical weathering and explain clearly its impacts on soil formation and agricultural activities?
b. Explain how thermal expansion and contraction affect mineral composition?
As the water evaporates, the salt is left behind. Over time, these salt deposits build up, creating pressure that can cause rocks to split and weaken. Temperature changes also affect mechanical weathering. As temperatures heat up, the rocks themselves expand.
Fig.3. 2 Illustration of mechanical weathering
Temperature is an essential part of rock creation, modification and destruction. Heating a rock causes it to expand, and cooling causes it to contract. Repeated swelling and shrinking of minerals that have different expansion and contraction rates should exert some stress on the rock’s outer shell.
Thermal expansion and contraction of minerals
Thermal expansion is the tendency of matter to change in shape, area, and volume in response to a change in temperature. Thermal expansion due to the extreme range of temperatures can shatter rocks in desert environments. Temperature is a monotonic function of the average molecular kinetic energy of a substance. When a substance is heated, the kinetic energy of its molecules increases. Thus, the molecules begin vibrating more and usually maintain a greater average separation.
Materials which contract with increasing temperature are unusual; this effect is limited in size, and only occurs within limited temperature ranges. The relative expansion (also called strain) divided by the change in temperature is called the material’s coefficient of thermal expansion and generally varies with temperature. Materials expand or contract when subjected to changes in temperature. Most materials expand when they are heated, and contract when they are cooled.
When free to deform, concrete will expand or contract due to fluctuations in temperature. Concrete expands slightly as temperature rises and contracts as temperature falls. Temperature changes may be caused by environmental conditions or by cement hydration.
Thermal expansion and contraction of concrete varies primarily with aggregate type (shale, limestone, siliceous gravel and granite), cementitious material content, water cement ratio, temperature range, concrete age, and ambient relative humidity. Of these factors, aggregate type has the greatest influence on the expansion and contraction of concrete.
Quick Check 3.2
a. How does climate affect the rate of weathering?
b. What is the process that breaks down rocks?
Effects of temperature and moisture changes on weathering
At high elevations, cold night time temperatures during much of the year can produce relentless freeze-thaw cycles. This process explains the presence of broken boulders and stony fragments that litter mountaintops. And, the minerals in volcanic rock that formed at the highest temperatures and pressures are the most vulnerable to chemical weathering at Earth’s surface.
In many locations, changes in temperature and moisture content of the environment cause significant physical weathering. When rock is warmed, it expands; when it cools, it contracts. In some regions, rocks are heated to relatively high temperatures during the day and then cooled to much lower temperatures during the night. The constant expansion and contraction of the rocks may result in pieces being broken off.
Temperature also affects the land as the cool nights and hot days always cause things to expand and contract. That movement can cause rocks to crack and break apart. The most common type of mechanical weathering is the constant freezing, and thawing of water. In liquid form, water is capable of penetrating holes, joints, and fissures within a rock. As the temperature drops below zero celcius, this water freezes. Frozen water expands compared to its liquid form. The result is that the holes and cracks in rocks are pushed outward. Even the strongest rocks are no match for this force.
As temperatures heat up, the rocks themselves expand. As the temperatures cool down, rocks contract slightly. The effect can be the weakening of the rock itself which induces mechanical weathering. It breaks rock into smaller pieces. These smaller pieces are just like the bigger rock, but smaller. That means the rock has changed physically without changing its composition. The smaller pieces have the same minerals, in just the same proportions as the original rock.
b. Ice wedging
There are many ways that rocks can be broken apart into smaller pieces. Ice wedging is the main form of mechanical weathering in any climate that regularly cycles above and below the freezing point works quickly, breaking apart rocks in areas with temperatures that cycle above and below freezing in the day and night
(Fig.3.13). Ice wedging
Explanation of figure 3.14:
(A) water seeps into cracks and fractures in rock, (B) when the water freezes, it expands about 9% in volume, which wedges apart the rock, (C) with repeated freeze cycles, rock breaks into pieces.
Ice wedging breaks apart so much, rocks with large piles of broken rock are seen at the base of a hillside, as rock fragments separate and tumble down. Ice wedging is common in Earth’s polar regions and mid latitudes, and also at higher elevations in the mountains.
Water has the unique property of expanding about 9% when it freezes. This increase in volume occurs because, as ice form, the water molecules arrange themselves into a very open crystalline structure. As a result, when water freezes, it expands and exerts a tremendous outward force. This can be verified by completely filling a container with water and freezing it.
After many freezing cycle, the rock is broken into pieces. This process is called frost wedging also as known as Freeze-thaw weathering as shown in Fig.3.13. This occurs when water gets into the small holes and gaps in rocks. If the water in the gap freezes, it expands, splitting the existing gaps into wider cracks. When the water thaws, the wider gaps allow even more water to enter the rock and freeze. Frost wedging can repeat over months or years, turning microscopic gaps in the rock into large cracks.
Ice has more volume than liquid water, so the cracks are forced wider. Then, more
water accumulates in the cracks the next day, which freeze at night to widen the cracks further. When this happens repeatedly, the rock eventually breaks apart along the crevices.
Frost heaving, a similar process to frost wedging, occurs when a layer of ice forms under loose rock or soil during the winter, causing the ground surface to bulge upward. When it melts in the spring, the ground surface collapses.
Activity 3.14: Importance of abrasion in real life situations
With the help of knowledge gained in the concepts above, explain how abrasion is formed and suggest its importance in real life situations
The word ‘abrasion’ literally means scraping of the surface of an object. This is exactly what happens with abrasion of rocks. Weathering by abrasion is responsible for the creation of some of the largest deserts in the world. The rock’s surface is exposed to blown sands - high velocity winds which blow throughout the day while carrying large sand particles.
The sand blasts against the surfaces of the rocks, undercutting and deflating them. As a result, smaller rock particles are formed, which when exposed to further sand abrasion become sand particles themselves.
Abrasion makes rocks with sharp or jagged edges smooth and round. If you have ever collected beach glass or cobbles from a stream, you have witnessed the work of abrasion (Fig.3.14 below). Rocks on a beach are worn down by abrasion as passing waves cause them to strike each other.
Fig.3. 14 Smooth round rocks
In abrasion, one rock bumps against another rock. The following are the causes of abrasion;
• Gravity causes abrasion as a rock tumbles down a mountainside or cliff.
• Moving water causes abrasion as particles in the water collide and bump against one another.
• Strong winds carrying pieces of sand can sandblast surfaces.
• Ice in glaciers carries many bits and pieces of rock. Rocks embedded at the bottom of the glacier scrape against the rocks.
In abrasion, one rock bumps against another rock. The following are the causes of abrasion; • Gravity causes abrasion as a rock tumbles down a mountainside or cliff. • Moving water causes abrasion as particles in the water collide and bump against one another. • Strong winds carrying pieces of sand can sandblast surfaces. • Ice in glaciers carries many bits and pieces of rock. Rocks embedded at the bottom of the glacier scrape against the rocks.
d. Biological activity
Weathering is also accomplished by the activities of organisms, including plants, burrowing animals, and humans. Plant roots in search of minerals and water grow into fractures, and as the roots grow they wedge the rock apart .
Burrowing animals further break down the rock by moving fresh material to the surface, where physical and chemical process can more effectively attack it. Decaying organisms also produce acids, which contribute to chemical weathering .
3.4.3 Factors influencing the type and rate of rock weathering
Brainstorm and classify clearly the factors affecting the rate of weathering?
Several factors influence the type and rate of rock weathering. By breaking a rock into smaller pieces, the amount of surface area exposed to chemical weathering is increased. The presence or absence of joints can be significant because they influence the ability of water to penetrate the rock. Other important factors include the mineral makeup of rocks and climate.
The amount of water in the air and the temperature of an area are both part of an area’s climate. Moisture speeds up chemical weathering. Weathering occurs fastest in hot, wet climates. It occurs very slowly in hot and dry climates. Without temperature changes, ice wedging cannot occur. In very cold, dry areas, there is little weathering.
area Most weathering occurs on exposed surfaces of rocks and minerals. The more surface area a rock has, the more quickly it will weather. When a block is cut into smaller pieces, it has more surface area. So, therefore, the smaller pieces of a rock will weather faster than a large block of rock.
composition Headstones of granite, which is composed of silicate minerals, are relatively resistant to chemical weathering. The minerals that crystallize first form under much higher temperatures than those which crystallize last. Consequently, the early formed minerals are not as stable at Earth’s surface, where the temperatures are different from the environment in which they formed. Olivine crystalizes first and is therefore the least resistant to chemical weathering, whereas quartz, which crystallizes last, is the most resistant.
speeds up weathering Factories and cars release carbon dioxide and other gases into the air. These gases dissolve in the rainwater, causing acid rain to form. Acid rain contains nitric and sulfuric acid, causing rocks and minerals to dissolve faster.
e. Soil erosion and soil deposition
Activity 3.17: Soil erosion
Look at the figure below that represents soil erosion. Carefully study the figure and answer the following questions:
Fig.3. 15 The erosive force of wind on an open field.
I. What does the term soil erosion mean?
II. Write down the two kinds of soil erosion illustrated in the figure above? Which kind of soil erosion mostly occurs in your home area? III. Explain clearly how the kinds of soil erosion outlined in ii) above affects on agricultural activities?
Soil covers most land surfaces. Along with air and water, it is one of our most indispensableresources. Soil is a combination of mineral and organic matter.
Soil erosion is a naturally occurring process that affects all landforms. In agriculture, soil erosion refers to the wearing away of a field’s topsoil by the natural physical forces of water and wind or through forces associated with farming activities.
Erosion is incorporation and transportation of material by a mobile agent, usually water, wind, or ice.
Erosion whether it is by water and wind, involves three distinct actions – soil detachment, movement and deposition. Topsoil, which is high in organic matter, fertility and soil life, is relocated elsewhere “on-site” where it builds up over time or is carried “off-site” where it fills in drainage channels. Soil erosion reduces cropland productivity and contributes to the pollution of adjacent watercourses, wetlands and lakes.
Soil erosion can be a slow process that continues relatively unnoticed or can occur at an alarming rate, causing serious loss of topsoil.
Soil compaction, low organic matter, loss of soil structure, poor internal drainage and soil acidity problems are other serious soil degradation conditions that can accelerate the soil erosion process.
Fig.3. 16 The erosive force of water from concentrated surface water runoff.
Deposition is the geological process in which sediments, soil and rocks are added to a landform or land mass. Wind, ice, and water, as well as sediment flowing via gravity, transport previously eroded sediment, which, at the loss of enough kinetic energy in the fluid, is deposited, building up layers of sediment.
3.4.4 Checking my progress
1. The pictures A and B are of two geographical features. Look and carefully study the pictures to answer questions below.
Fig.3. 17 Illustration of geographical features
a. Interpret the images above and Use your observation to suggest names of the corresponding geographical features in the image above?
b. Do you have such geographical features in your district or neighboring districts? Use your observation to explain clearly the two geographical features occurring in the image above?
c. Explain the causes for each geographical feature occurring above?
d. Can the geographical features identified above impact agriculture in our communities? Explain with clear facts to support your decision.
e. What are moral and ethical issues associated with the geographical features given above?
3.5 END UNIT ASSESSMENT
3.5.1 Multiple choices questions
For question 1 to 5, choose the letter of the best answer
1. It is known that earth’s atmosphere has a series of layers, each with its own specific characteristics and properties? The following is the appropriate layer where we live.
2. Consider the following statements:
I. The atmosphere of Earth protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through greenhouse effect and reducing temperature extremes between day and night.
II. X-rays and ultraviolet radiation from the Sun are absorbed in the thermosphere.
III. The stratosphere extends from the top of the thermosphere to about 50 km above the ground.
Of these statements:
a. I, II, and III are correct.
b. I, II and III are wrong
c. I and II are correct but III is wrong d. I and III are wrong but II is correct
3. Agrophysics is defined as
a. The branch of science dealing with study of matter and energy and their mutual relation.
b. The branch of science dealing with communication devices to measure and collect information about physical conditions in agricultural and natural environments.
c. The branch of natural sciences dealing with the application of physics in agriculture and environment.
d. None of these
3.5.2 Structured Questions
1. Write the missing word or words on the space before each number. For items 1-9
a. ___speeds up chemical weathering.
b. Weathering happens ______ in hot, wet (humid) climates.
c. Weathering occurs very slowly in _______ and ______ climates.
d. Without ________ changes, ice wedging cannot occur e. In very ________ and ________ areas, there is little weathering.
f. Most weathering occurs on ____________________of rocks and minerals
g. The ________ surface area a rock has, the quicker it will weather. h. Some minerals resist weathering. _________________ is a mineral that weathers slowly.
i. Rocks made up of minerals such as feldspar, ______, and iron, weather more quickly.
2. If the statement is true, write true. If it is false, change the underlined word or words to make the statement true.
a. Water vapor is very important in predicting weather.
b. Temperature is a reason why atmosphere is more dense close to the earth’s surface.
c. Agrophysics plays an important role in the limitation of hazards to agricultural objects and environment.
d. Energy is transferred between the earth surface and planet in a variety of ways.
e. As the temperature increases in the atmosphere, the minimum radiation occurs at short wavelengths.
3. Write a sentence describing the relationship between each pair of terms.
I. Gravity, atmosphere
II. Temperature, rocks.
4. Marry wants to make agrophysics journal. She says, “My journal will be focused on advances in sensors and communication devices to measure and collect information about physical conditions in agricultural and natural environments”. Evaluate Marry’s statement.
5. With the help of two clear examples on each, explain clearly how temperature and water vapor impact agricultural activities using the table. Terms
6. Complete the chart below. If the left column is blank, give the correct term. If the right column is blank, give an example of economic activities taking place in the corresponding layer if possible.
7. How do climate impact agricultural activities?
8. Explain briefly the role of machines in agriculture in rapid development of the country towards suitable programs of transformation and modernization of agriculture?
9. Knowing different stages of growing plants in our daily agriculture activities, explain clearly which stages mostly benefit the use of technology?
10. Cracks in rocks widen as water in them freezes and thaws. How does this affect the surface of Earth?
11. Name the four factors that can hasten or speed up the process of weathering.
12. How is weathering different from erosion? Think!
13. How can increasing the surface area of rock hasten or speed up the process of weathering Think!
14. Human activities are responsible for enormous amounts of mechanical weathering, by digging or blasting into rock to build homes, roads, and subways or to quarry stone. Suggest measures that can be taken to minimize mechanical weathering caused by human activities?
3.5.3 Essay type questions
15. Design and conduct your own research into the influence of surfaces on temperature comparing earth surfaces that interest them (such as colored soils, dry and wet soils, grass, dry leaves, or different types of coverings such as plastic or aluminum foil). Compare the data with these new surfaces compared to the given surfaces (water, light soil, dark soil). Note that the data may not be comparable due to variations in experimental design, such as differences in light bulb temperature and height of the lamp.