subtopic 5.1: Soils

PictureImage from Food Tank
We tend to take the soil around us for granted. It is much more than mud, clay or dirt. All the food that we consume depends ultimately upon soil. Plants grow and depend on the soil. We either eat plants that grow directly in the soil or the animals that eat the plants.
Soils are the part of the lithosphere where life processes and soil forming processes both take place.

In this unit we will look at the soil system, soil water, soil formation and the consequences of soil degradation. 

This unit is a minimum of 3 SL hours
Guiding Questions
  • How do soils play a role in sustaining natural systems?
  • How are human activities affecting the stability of soil systems?
Understanding:

soil systems

5.1.1 Soil is a dynamic system within the larger ecosystem that has its own inputs, outputs, storages and flows.
  • ​​Define what is meant by a soil system 
Picturehttps://www.sciencedirect.com/science
Soils are part of a larger system with dynamic interactions. As with any open system, it exchanges both matter and energy. They are not static. Like other systems you have learned and will learn about, soil is a system with:
  • Inputs and outputs of energy and matter across its system boundary from and to other Earth systems: the biosphere, hydrosphere, atmosphere, and the lithosphere.
  • Storages and flows (transfers and transformations) of water, gases, minerals, and organic matter.

Soils are major components of the world's ecosystems.Soil forms the Earth’s atmosphere, lithosphere (rocks), biosphere (living matter) and hydrosphere (water). Soil is what forms the outermost layer of the Earth’s surface, and comprise weathered bedrock (regolith), organic matter (both dead and alive), air and water. 

The soil interacts with the atmosphere, lithosphere, biosphere and hydrosphere.
  • The water cycle moves through the soil by infiltration and water may evaporate from the surface.
  • The atmosphere may contain particulate matter that is deposited on the soils and particles may blow up into the atmosphere.
  • Rocks in the lithosphere weather to form soils, and soils at depth and pressure may form rocks.
  • Plants in the biosphere may extract nutrients from the soils and dead plants may end up forming parts of the soil.

Soil is a mixture of four basic parts – minerals, organic matter, air and water. It serves four primary functions:
  • Medium for plant growth. Soil supplies nutrients and water as well as anchors roots.
  • Water storage and purification system.
  • Habitat for organisms, such as bacteria, insects and mammals. These organisms modify the soil.
  • Soil modifies the atmosphere through respiration of the soil organisms and plant roots.

Soils are important to humans in many ways:
  • soil is the medium for plant growth, which most of foods for humans are grown in
  • soil stores freshwater, 0.005% of world’s freshwater
  • soil filters materials added to the soil, keeping quality water
  • recycling of nutrients takes place in the soil when dead organic matter is broken down
  • soil is the habitat for billions of micro-organisms, as well as other larger animals
  • soil provides raw material in the forms of peat, clay, sands, gravel and minerals
5.1.2 Soil is made up of inorganic and organic components, water and air.
  • Describe how the composition of soil influences its ability to retain water and nutrients.
  • Explain the role of soil horizons in the distribution of organic matter and minerals in a soil profile
  • Discuss how the processes of weathering and decomposition contribute to soil formation over time.
Soil has matter in all three states:
  • organic and inorganic matter form the solid state
  • soil water(from precipitation, groundwater and seepage) form the liquid state
  • soil atmosphere forms the gaseous state
Pictureimage from www.eoearth.org
Inorganic and Organic Components:
  • Inorganic components consist of minerals like sand, silt, and clay, which affect the soil’s texture and drainage. For example, sandy soils, found in deserts like the Sahara, have larger particles, which means they drain water quickly but do not retain much moisture. In contrast, clay soils, such as those in temperate regions like the Midwest USA, have smaller particles, which retain water but can become waterlogged.
  • Organic components made up of decayed plant and animal matter, form humus, which enhances soil fertility. Soils rich in organic matter, like those in the prairies of North America, are some of the most productive agricultural soils in the world.

​Soil Nutrients:
  • Nutrients in soil come from both organic sources (like decomposed plants and animals) and inorganic minerals. Organic nutrients help create a fertile environment for plant roots to grow. The distribution of these nutrients tends to be concentrated in the upper layers of the soil, known as the organic layer and topsoil  For example, the black soils of Ukraine are rich in organic matter, making them highly fertile. In contrast, desert soils like those in the Atacama Desert contain little organic material and are less suitable for agriculture.

Soil Horizons:
  • Soil forms distinct layers over time, called soil horizons, which together make up the soil profile. The horizons vary in composition, from the nutrient-rich organic layer at the top to the subsoil and bedrock below, which contain fewer nutrients and organic material. These layers have different properties such as water retention and pH levels, and they affect plant growth and soil fertility.

Soil Formation:
  • Soil forms over thousands of years through the processes of weathering (breaking down of rocks) and decomposition (breakdown of organic matter). As organic layers accumulate over time, they compact and form distinct horizons. Soil also evolves through succession, where plants and animals gradually influence the development of the soil profile. For instance, volcanic activity in Iceland produces rich mineral soils that, over time, accumulate organic matter and become fertile.
The composition of soil varies depending on the environment, such as desert soils having less organic matter compared to tropical rainforest soils.
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AI Generated Graphs
  • Rainforest Soil Composition: Contains a higher proportion of organic matter (15%) due to the rich plant and animal life, with relatively balanced levels of minerals, air, and water.
  • Desert Soil Composition: Has much less organic matter (5%) due to limited vegetation, with a higher proportion of air (30%) and minerals (45%).
5.1.3 Soils develop a stable, layered structure known as a profile made up of several horizons, produced by interactions within the system over long periods of time.
  • ​Describe the soil horizon
  • Discuss how the depth and composition of the topsoil can impact agricultural productivity
​Soils develop over time into distinct layers, known as horizons, each with unique characteristics. The combination of these horizons is called a soil profile. These horizons vary in terms of organic content, minerals, and overall composition
Picturehttps://en.wikipedia.org/wiki/Humus

Typical Soil Horizons:
  • O Horizon (Organic Layer): This layer is rich in organic material from decaying plants and animals. It is usually found in forest ecosystems where plant litter accumulates.
  • A Horizon (Topsoil): The most fertile layer, rich in nutrients and organic matter, this is where most plant roots grow. It is crucial for agricultural productivity, such as in the prairies of North America.
  • B Horizon (Subsoil): Found below the topsoil, this layer contains fewer nutrients but is rich in minerals leached from the upper layers.
  • C Horizon (Parent Material): This layer consists of partially weathered rock and has little organic material. Over time, it breaks down to form the upper layers of the soil profile.
  • R Horizon (Bedrock): The lowest layer, composed of unweathered rock, plays a minimal role in plant growth but serves as the foundation for the development of the other layers.
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The characteristics of soils will dependent on:
  • Climate: Precipitation/evaporation determines the dominant direction of water movement. 
  • Organisms: Breaks down organic matter and mix it into the upper layers of the soil.
  • Relief: The elevation, aspect of the slope (the direction it faces) and the angle of the slope.
  • Parent material: The original material that the soil develops from. This can be either the bedrock or lake or glacial till that has been laid down on top of the bedrock.
  • Time: Soil formation is a  long and slow process, therefore it is considered to be a non-renewable natural capital. The amount of time the soil has had to form will affect its characteristics.
Understanding soil profiles is essential for land management and agriculture. Different plants require different soil horizons to thrive, and land use must consider the thickness and fertility of the topsoil for sustainable farming.
Soil Formation and Profiles:
  • Soil profiles are formed through long-term processes of weathering (breaking down of rocks) and decomposition (breakdown of organic matter). The depth and composition of each horizon depend on factors like climate, vegetation, and land use. For instance, tropical rainforests typically have thin topsoils, while temperate regions like the Midwest USA have deeper, more fertile topsoils.
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https://environmental-geol.pressbooks.tru.ca/chapter/soil-formation/
Activity: Draw and label the different soil horizons (O, A, B, C, R) and annotate the properties of each horizon. They can use colored pencils to differentiate the layers. Provide examples of ecosystems where these horizons might be found., and explain the role of each horizon in supporting plant life.
Activity:  Take a soil core sample from a local site. Students will examine the layers of the soil sample and identify the horizons present. Discuss the differences in color, texture, and composition between the layers.
5.1.4 Soil system inputs include those from dead organic matter and inorganic minerals.
  • Explain the role of dead organic matter in soil fertility 
  • Discuss the importance of inputs like plant litter and manure in maintaining a healthy soil system
Pictureimage from www.gov.mb.ca
Inputs into soil systems include a wide range of organic and inorganic materials, as well as natural and human-made (anthropogenic) inputs. These inputs are essential for maintaining soil fertility and supporting life.
  • Organic Inputs:
    • Dead Organic Matter: Organic material enters the soil system from sources like plant litter, dead animal biomass, and manure. For example, in forest ecosystems, leaf litter from trees contributes significantly to the organic content of the soil.
    • Decomposition: As dead organic matter decomposes, it releases nutrients like nitrogen and phosphorus, which are vital for plant growth. This is especially important in ecosystems such as grasslands where plant litter continuously replenishes the soil.
  • Inorganic Inputs:
    • Weathering and Deposition: Rocks break down through weathering, releasing minerals into the soil. Additionally, dust and particles can be deposited from other regions, often carried by wind or water. For instance, dust from the Sahara Desert is carried by winds and deposited in the Amazon Rainforest, providing nutrients to the soil.
    • Precipitation and Air: Rainwater can bring dissolved minerals into the soil, and gases from the atmosphere, such as nitrogen, can be absorbed into the soil system through nitrogen-fixing bacteria.
  • Anthropogenic Inputs:
    • Managed Soils: Human activity significantly alters soil systems by introducing inputs such as compost, fertilizers, agrochemicals, and irrigation. These inputs aim to enhance soil productivity but can lead to unintended consequences such as salinization from over-irrigation.
    • Agrochemicals: The use of chemicals like pesticides and herbicides in farming introduces synthetic compounds into the soil, which can affect both nutrient availability and soil health in the long term.
  • Natural Inputs: Inputs can originate from both within and outside the ecosystem:
    • Internal Inputs: These include weathering of underlying parent rock, decomposition of organic matter, and litter from vegetation.
    • External Inputs: These can be transported from other ecosystems via wind-blown deposits, waterborne materials, and even guano from migratory birds.
5.1.5 Soil system outputs include losses of dead organic matter due to decomposition, losses of mineral components and loss of energy due to heat loss.
  • Describe how wind and water erosion affect soil fertility
  • Explain how wind and water processes lead to the loss of mineral components
  • Explain the process of leaching 
Soil systems experience various outputs, which can lead to the loss or modification of soil components. These outputs are essential in regulating the balance of soil nutrients, minerals, and water but can also lead to soil degradation if excessive.

Mineral Component Outputs:
  • Wind and Water Erosion: Wind and water can carry away soil particles, particularly the fine mineral components like sand, silt, and clay. This process removes topsoil and nutrients, impacting soil fertility. For example, in semi-arid regions like the Sahel, wind erosion is a significant factor contributing to desertification.
  • Leaching of Dissolved Nutrients: When water percolates through the soil, it can dissolve and carry away important minerals and nutrients, such as nitrogen and potassium. This process, known as leaching, depletes the soil of nutrients essential for plant growth. Leaching is especially common in tropical rainforests with heavy rainfall.
  • Mineral Absorption by Plant Roots: Plants absorb minerals like nitrogen, phosphorus, and potassium from the soil through their roots. Over time, if these nutrients are not replenished, the soil can become nutrient-poor, leading to reduced productivity.
  • Diffusion of Gases and Evaporation of Water: Soil also loses water through evaporation, and gases like carbon dioxide and oxygen diffuse out of the soil. In hot, dry climates, excessive evaporation can reduce soil moisture, making it difficult for plants to survive.

Soil Degradation:
  • These outputs, while natural, can lead to soil degradation if they occur at rates faster than the soil can recover. Erosion, leaching, and excessive nutrient absorption can result in a loss of productivity, making it harder to grow crops or support vegetation.
  • Total Loss of Soil: Severe wind or water erosion can result in the complete loss of soil from an area, as seen in the Dust Bowl of the 1930s in the United States, where poor farming practices combined with drought led to massive soil loss.

​Managing Soil Outputs: Sustainable land management practices, such as crop rotation, cover crops, and contour farming, can help reduce the negative impacts of soil outputs and maintain productive soils.
5.1.6 Transfers occur across soil horizons, into and out of soils.
  • Describe the processes of infiltration and percolation
  • Explain the role of biological mixing and aeration in maintaining soil structure and fertility
  • Discuss how erosion and leaching can lead to soil degradation
Pictureimage from www.senninger.com
Soil systems involve the movement, or transfer, of water, nutrients, gases, and organisms. These transfers occur both horizontally and vertically within the soil and are crucial for maintaining soil health and supporting plant life. They include infiltration, percolation, groundwater flow, biological mixing, aeration, erosion, and leaching.

​Infiltration and Percolation:
  • Infiltration is the process by which water enters the soil from the surface. This is vital for providing moisture to plant roots and recharging groundwater. In regions with sandy soils, like desert ecosystems, water infiltrates quickly, while in clay-rich soils, infiltration can be slower.
  • Percolation refers to the downward movement of water through the soil layers. As water percolates, it can carry dissolved nutrients and minerals with it, a process that is essential for nutrient cycling. However, excessive percolation can lead to leaching, where valuable nutrients are lost from the upper soil layers.
  • Groundwater Flow: Once water infiltrates and percolates through the soil, it may reach the groundwater level, where it can move laterally. This process is important for distributing water across large areas. Groundwater flow supports the water table and can contribute to springs and streams. In arid regions like the Southwestern United States, groundwater flow is essential for sustaining ecosystems during dry periods.

Biological Mixing and Aeration:
  • Biological Mixing occurs when organisms like earthworms and insects burrow through the soil, mixing organic matter and minerals between layers. This process improves soil fertility and structure by distributing nutrients more evenly. Earthworms are particularly important for this, as they create channels that allow water and air to flow more freely.
  • Aeration is the exchange of gases between the soil and the atmosphere. This process is essential for providing oxygen to plant roots and soil organisms. Soils with poor aeration, such as compacted soils in urban areas, can inhibit plant growth due to the lack of oxygen.

​Erosion and Leaching:
  • Erosion is the movement of soil particles, typically due to wind or water. It removes topsoil, which is rich in nutrients and organic matter, leading to reduced soil fertility. In areas like the Great Plains, erosion has been a major issue due to farming practices that leave soil exposed.
  • Leaching occurs when water moving through the soil carries dissolved nutrients away from the root zone. This can lead to a loss of fertility, especially in tropical regions with heavy rainfall, where important nutrients like nitrogen and potassium are quickly lost from the soil.
5.1.7 Transformations within soils can change the components or the whole soil system.
  • Explain the process of decomposition and its role in maintaining soil fertility.
  • Describe how weathering contributes to soil formation,
  • Discuss the differences between physical and chemical weathering.
  • Discuss how nutrient cycling and salinization can affect soil productivity
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Transformations in soil systems involve chemical, physical, and biological processes that change the composition and structure of soils. These transformations are critical for maintaining soil fertility and supporting plant life. Key processes include decomposition, weathering, nutrient cycling, and salinization.

​Decomposition:
  • Decomposition is the breakdown of dead organic matter (DOM) by decomposers such as bacteria, fungi, and detritivores like earthworms. This process releases essential nutrients into the soil, which can then be used by plants.
  • In ecosystems like tropical rainforests, high temperatures and moisture levels accelerate decomposition, leading to rapid nutrient cycling. In contrast, colder environments like tundras have slower decomposition rates, resulting in nutrient-poor soils.

Weathering:
  • Weathering is the breakdown of rocks and minerals into smaller particles, contributing to the formation of soil. There are two main types:
    • Physical weathering involves the mechanical breakdown of rocks through processes like freezing and thawing or abrasion by wind and water. For example, in mountainous regions, freeze-thaw cycles create small rock particles that mix with organic matter to form soil.
    • Chemical weathering occurs when rocks react with water, gases, or organic acids, breaking down into minerals like clay, silt, and sand. In humid regions like tropical rainforests, chemical weathering is a dominant force, contributing to the rich mineral content of soils.

Nutrient Cycling:
  • Nutrient cycling refers to the movement of nutrients through the soil, plants, and organisms. Decomposed organic matter releases nutrients like nitrogen, phosphorus, and potassium, which plants take up through their roots. These nutrients are then returned to the soil when plants die or shed leaves.
  • In agricultural systems, nutrient cycling is often disrupted due to harvesting, where nutrients are removed with the crops. This can be managed through practices such as composting and the use of organic fertilizers.

​Salinization:
  • Salinization is the accumulation of salts in the soil, often as a result of irrigation in arid or semi-arid regions. When water evaporates, it leaves behind salts that accumulate in the upper layers of the soil, which can be harmful to plants by interfering with water uptake.
  • Salinization is a significant problem in regions like the Murray-Darling Basin in Australia, where irrigation has led to high salinity levels, reducing agricultural productivity.
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image from earthonline.com
5.1.8 Systems flow diagrams show flows into, out of and within the soil ecosystem.
  • Describe how soils contribute to the water cycle
  • Explain the role of soils in the carbon cycle
  • Discuss how soils support the nitrogen cycle,
Soils play a critical role in the water, carbon, and nitrogen cycles. These cycles are essential for maintaining life on Earth, and soils act as key reservoirs and facilitators for the movement of water, nutrients, and gases between the atmosphere, biosphere, and lithosphere.
​
Soil in the Water Cycle:
  • Soils are central to the water cycle, influencing the movement and storage of water. Through processes like infiltration and percolation, water enters the soil, recharges groundwater, and is later taken up by plants or released into the atmosphere through evapotranspiration.
  • Soils also help regulate water flow by storing moisture, which plants rely on during dry periods. In ecosystems like wetlands, soils retain large amounts of water, supporting biodiversity and acting as natural flood buffers.

Soil in the Carbon Cycle:
  • Soils are significant carbon sinks, storing large amounts of organic carbon in the form of decomposed plant and animal material. Decomposition releases carbon dioxide (CO₂) back into the atmosphere, while plants absorb CO₂ through photosynthesis, creating a cycle of carbon exchange.
  • In peatlands and forests, large quantities of carbon are stored in the soil. However, human activities like deforestation and soil degradation can release stored carbon, contributing to climate change.

Soil in the Nitrogen Cycle:
  • Soils are vital for the nitrogen cycle, as they provide the environment for nitrogen-fixing bacteria to convert atmospheric nitrogen (N₂) into forms usable by plants, such as nitrates and ammonium.
  • Decomposition of organic matter releases nitrogen compounds into the soil, which plants take up for growth. Nitrogen is then returned to the soil when plants die and decompose. In agricultural systems, nitrogen is often added to soils through fertilizers, but excessive use can lead to issues like eutrophication when nitrogen leaches into water bodies.

​Soil as a Regulator:
  • Soils regulate these cycles by controlling the availability of water, carbon, and nitrogen to plants and microorganisms. Healthy soils with a balanced mix of minerals, organic matter, and water are crucial for the efficient functioning of these cycles.
​Application of skills: Create a systems flow diagram representing the soil system.

foundation of Terrestrial Ecosystems

5.1.9 Soils provide the foundation of terrestrial ecosystems as a medium for plant growth (a seed bank, a store of water and almost all essential plant nutrients). Carbon is an exception; it is obtained by plants from the atmosphere.
  • Explain how soils store essential nutrients such as nitrogen, phosphorus, and potassium
  • Describe the process of nutrient cycling in soils
  • Discuss how nutrient depletion in soils can lead to degradation
 Soils act as a critical storehouse for the nutrients that plants need to grow. The most important nutrients stored in soils include nitrogen (N), phosphorus (P), and potassium (K). These nutrients are essential for plant growth, development, and reproduction.

​Nitrogen (N):
  • Nitrogen is a key component of proteins and nucleic acids, making it essential for plant growth. In the soil, nitrogen is stored in both organic and inorganic forms, such as nitrates (NO₃⁻) and ammonium (NH₄⁺), which plants absorb through their roots.
  • Nitrogen-fixing bacteria in the soil convert atmospheric nitrogen (N₂) into forms that plants can use, making soils a crucial player in the nitrogen cycle. However, the loss of nitrogen through leaching can deplete soil fertility, especially in areas with heavy rainfall.

Phosphorus (P):
  • Phosphorus is important for energy transfer within plants and is a critical component of DNA, RNA, and ATP. In soils, phosphorus is stored in the form of phosphate ions (PO₄³⁻), which plants take up from the soil solution.
  • Phosphorus is often tightly bound to soil particles, making it less available to plants in some soil types. Soils in tropical regions tend to be phosphorus-poor due to leaching, while volcanic soils are typically rich in phosphorus.

Potassium (K):
  • Potassium regulates water uptake and enzyme activation in plants, playing a critical role in photosynthesis and water management. Potassium is stored in the soil as potassium ions (K⁺) and is typically found in greater abundance than nitrogen or phosphorus.
  • Potassium is not as prone to leaching as nitrogen, but soils that are heavily cultivated can become deficient in potassium if not replenished.

Soil Nutrient Depletion:
  • When soils are overused or improperly managed, nutrient stores can become depleted. For instance, repeated cropping without the addition of fertilizers can deplete nitrogen, phosphorus, and potassium, leading to soil degradation.
  • Sustainable soil management practices, such as crop rotation, cover cropping, and the use of organic fertilizers, help maintain nutrient stores in the soil, ensuring long-term productivity.

​Nutrient Cycling:
  • The nutrients stored in soils are cycled through plants, animals, and microorganisms. Dead organic matter, such as fallen leaves and animal waste, decomposes and returns nutrients to the soil, where they are reabsorbed by plants. This process is essential for maintaining soil fertility.
5.1.10 Soils contribute to biodiversity by providing a habitat and a niche for many species.
  • Describe the role of microorganisms in maintaining soil fertility
  • Explain the importance of soil fungi, such as mycorrhizal fungi, in nutrient uptake by plants
 Soil systems are home to a wide range of organisms, including microorganisms, fungi, and animals. These organisms play vital roles in maintaining soil health, contributing to processes like decomposition, nutrient cycling, and aeration. The biodiversity found in soils is immense, with many species still unknown and yet to be studied.
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Microorganisms in Soil:
  • Bacteria and archaea are the most abundant microorganisms in soil, responsible for key processes like nitrogen fixation, decomposition, and the breakdown of organic matter. Certain bacteria, such as nitrogen-fixing bacteria, convert atmospheric nitrogen into a form that plants can absorb.
  • Actinomycetes, a group of filamentous bacteria, help decompose complex organic materials like cellulose and chitin, releasing nutrients into the soil. They are also responsible for the characteristic “earthy” smell of soil.
  • Protozoa and algae are also part of the soil microbial community, contributing to nutrient cycling and soil structure.

Fungi in Soil:
  • Fungi play an essential role in decomposing organic material and recycling nutrients. Mycorrhizal fungi form symbiotic relationships with plant roots, helping plants absorb nutrients like phosphorus while receiving carbohydrates in return.
  • Saprophytic fungi break down dead organic matter, releasing nutrients back into the soil. Fungi are crucial for maintaining soil structure by producing substances that bind soil particles together.

Soil Animals:
  • Soil animals include invertebrates like earthworms, nematodes, and arthropods (e.g., ants, beetles, mites). These animals contribute to biological mixing, aeration, and the breakdown of organic material.
  • Earthworms are particularly important for soil health. They aerate the soil by burrowing and create channels that allow water and air to move more freely through the soil. Their digestion of organic matter produces nutrient-rich castings, which enhance soil fertility.

Unexplored Biodiversity:
  • The diversity of soil life is vast, and many species are still undiscovered or poorly understood. Scientists estimate that only a fraction of soil organisms have been identified, especially in remote and unique ecosystems like tropical rainforests and deep soil layers.

​Soil Biodiversity and Ecosystem Health:
  • The biodiversity of soil communities is crucial for maintaining soil functions like nutrient cycling, carbon storage, and water retention. A diverse soil community ensures that multiple processes are supported, contributing to the resilience of ecosystems.
  • Soil degradation, such as erosion, compaction, and pollution, can reduce soil biodiversity, leading to a decline in soil health and productivity
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image from asknature
5.1.11 Soils have an important role in the recycling of elements as a part of biogeochemical cycles.
  • Describe the role of detritivores, such as earthworms, in the decomposition of organic matter
  • Explain how saprotrophs, including fungi and bacteria, decompose organic matter and discuss their importance in nutrient cycling.
Decomposition is a fundamental process in soil systems, where dead organic matter (DOM), such as leaf litter, is broken down into simpler substances, releasing nutrients back into the soil. This process is primarily carried out by detritivores, saprotrophs, and microorganisms, which transform organic material into humus.
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Major Inputs to Soil:
  • The main input of organic matter into soil systems is dead plant material, including leaf litter, fallen branches, and roots.
  • In forest ecosystems, leaf litter accumulates on the soil surface, providing a rich source of nutrients for decomposition. In agricultural fields, crop residues and manure contribute to the organic matter input.

Role of Detritivores:
  • Detritivores, such as earthworms, millipedes, and woodlice, are responsible for breaking down large pieces of organic material into smaller fragments. They physically break down leaf litter and other organic matter, making it easier for microorganisms to further decompose the material.
  • Earthworms are particularly important in this process, as their burrowing activities aerate the soil and mix organic matter with minerals. Earthworm digestion also results in nutrient-rich castings that enhance soil fertility.

Role of Saprotrophs (Decomposers):
  • After detritivores break down organic matter, saprotrophs, such as fungi and bacteria, take over the decomposition process. These microorganisms release enzymes that break down complex organic compounds into simpler molecules, such as sugars, amino acids, and nutrients.
  • Fungi are critical for breaking down tough plant materials like cellulose and lignin, which other organisms cannot decompose. Bacteria play a key role in the final stages of decomposition, converting organic matter into humus, which improves soil structure and nutrient retention.

Nutrient Cycling:
  • The decomposition of organic matter releases important nutrients, including nitrogen, phosphorus, and potassium, which plants absorb through their roots. This nutrient cycling is essential for maintaining soil fertility and supporting plant growth.
  • Soils in temperate forests typically have a slower decomposition rate due to cooler temperatures, resulting in a thick layer of organic matter. In contrast, tropical rainforests experience rapid decomposition due to warm, moist conditions, leading to fast nutrient cycling.

​Decomposition and Soil Health:
  • Efficient decomposition maintains healthy soils by replenishing nutrients and preventing the buildup of undecomposed material. A healthy community of detritivores and decomposers ensures that dead organic matter is continually recycled, maintaining a balance in nutrient availability.

soil texture

5.1.12 Soil texture defines the physical make-up of the mineral soil. It depends on the relative proportions of sand, silt, clay and humus.
  • ​​Explain how the texture of soil affects water retention and plant growth
  • Discuss how soil texture influences land use and agricultural productivity
Soil texture refers to the relative proportions of different particle sizes in the soil, specifically sand, silt, and clay. The texture of the soil influences its properties, including water retention, drainage, aeration, and nutrient availability. Understanding soil texture is essential for determining how soil can support plant life.

​Soil Particles:
  • Sand particles are the largest and have the greatest permeability, allowing water to flow through quickly. However, sandy soils tend to have poor water retention.
  • Silt particles are medium-sized and retain more water than sand. Silt soils are often found in floodplains and support rich agricultural growth.
  • Clay particles are the smallest and have excellent water retention capabilities, but they can become compacted and poorly aerated.

Soil Texture Classes:
  • Soils are classified based on the percentage of sand, silt, and clay. There are several texture classes, such as sandy loam, silty clay, and clay loam, which describe the predominant particle type.
  • Loamy soils, which have a balanced mix of sand, silt, and clay, are generally considered the best for agriculture because they retain moisture and nutrients well while still allowing for good drainage.

Determining Soil Texture:
  • Key Test: A soil key can be used to classify the texture based on particle size and feel. This involves rubbing the soil between your fingers and comparing the texture to a chart.
  • Feel Test: The feel test is a simple method where soil is moistened and rubbed between the fingers to determine whether it feels gritty (sand), smooth (silt), or sticky (clay).
  • Water Separation Test: In a laboratory setting, soil can be mixed with water and shaken in a jar. As the particles settle, they separate into layers, with sand at the bottom, silt in the middle, and clay on top. This visual method helps students understand the proportions of each particle size in the soil sample.

​Impact of Soil Texture:
  • Water Retention and Drainage: Sandy soils drain water quickly and have poor nutrient retention, making them less suitable for crops that require consistent moisture. Clay soils, on the other hand, retain water but can become waterlogged.
  • Plant Growth: The texture of soil affects root penetration, water uptake, and nutrient availability. Loamy soils provide the best conditions for plant growth due to their balance of water retention and aeration
Activity: Determine the texture of various soil samples using the feel test.
  • Collect soil samples from different locations (garden, forest, agricultural field).
  • After moistening the soil, they will rub it between their fingers and classify it as sandy, silty, or clayey using a soil texture key.
Activity:  Place soil samples in a clear jar filled with water and shake vigorously.
  • Allow the soil to settle and observe the separation into layers.
  • Measure the thickness of each layer to calculate the proportion of sand, silt, and clay.
5.1.13 Soil texture affects primary productivity through the differing influences of sand, silt, clay and dead organic matter, including humus.
  • Describe the characteristics of humus
  • Explain how humus helps in retaining nutrients and water in the soil
  • Explain how humus influences soil texture and fertility.
Humus is the dark brown or black substance found in soils, lying just beneath the leaf litter. It forms from the partial decay of dead plant material, including leaves, roots, and branches. This organic matter undergoes decomposition by detritivores and saprotrophs, resulting in humus, which contributes significantly to soil structure and fertility.


Characteristics of Humus:
  • Texture: Humus has a loose, crumbly texture that improves soil structure by preventing compaction and enhancing aeration. Soils rich in humus tend to be more productive because of their favorable physical properties.
  • Color: The dark color of humus helps in absorbing heat, which can slightly raise the temperature of the soil, promoting faster root growth and microbial activity.
  • Location: Humus is typically found in the A horizon of the soil profile, where it interacts with mineral particles and plant roots.

Role of Humus in Soil Systems:
  • Nutrient Retention vs. Leaching: Humus helps retain essential nutrients such as nitrogen, phosphorus, and potassium, preventing them from leaching out of the soil. This retention is vital in maintaining soil fertility, especially in regions with heavy rainfall.
  • Water Retention vs. Drainage: Humus increases the water-holding capacity of soils, allowing them to retain moisture during dry periods. At the same time, its loose texture promotes good drainage, preventing waterlogging. This balance is crucial for plant growth in a variety of ecosystems, from wetlands to grasslands.
  • Aeration vs. Compaction: Humus improves soil aeration by creating spaces between soil particles, allowing air to circulate and preventing compaction. This promotes healthy root growth and supports soil organisms like earthworms and microbes.

Humus and Primary Productivity:
  • Soils with a high humus content are generally more fertile and can support higher primary productivity (the rate at which plants produce biomass). By improving nutrient availability, water retention, and aeration, humus creates optimal conditions for plant growth.
  • In ecosystems like temperate forests, where there is abundant leaf litter and organic matter, humus-rich soils promote high biodiversity and productivity. In contrast, in areas with little organic input, such as deserts, soils lack humus and are less productive.

​Humus Depletion:
  • Human activities such as deforestation, overgrazing, and intensive agriculture can deplete humus levels in the soil. When organic matter is removed from the system and not replenished, soil becomes less fertile and more prone to erosion and nutrient leaching.
Activity:  
  • Collect soil samples from various locations (forest, garden, agricultural field) and examine the presence of humus.
  • Students can look for its dark color, crumbly texture, and loose structure. Discuss how the presence of humus differs across ecosystems.

soil as a carbon sink

5.1.14 Soils can act as carbon sinks, stores or sources, depending on the relative rates of input of dead organic matter and decomposition.
  • Explain why soils in tropical forests store relatively little carbon compared to soils in tundra and wetland ecosystems
  • Discuss how temperature and moisture conditions influence carbon storage in soils
 Soils are one of the largest carbon sinks on Earth, storing carbon in the form of organic matter (such as plant roots, dead leaves, and decomposing organisms) and inorganic carbon (such as carbonates). The carbon stored in soils plays a crucial role in regulating the global carbon cycle and mitigating climate change.


Carbon Storage in Soils:
  • Organic Carbon: Soils store organic carbon primarily through the accumulation of dead plant material and its slow decomposition. The amount of carbon stored in soil depends on factors like temperature, moisture, and the rate of decomposition.
  • Inorganic Carbon: In arid and semi-arid regions, soils may store carbon as calcium carbonate (CaCO₃), contributing to long-term carbon storage.

Carbon Storage by Ecosystem:
  • Tropical Forests: Despite the high productivity of tropical forests, their soils store relatively little carbon. This is because the warm, moist conditions lead to rapid decomposition of organic matter, releasing carbon dioxide back into the atmosphere. Most carbon in tropical forests is stored in biomass, such as trees, rather than in the soil.
  • Tundra and Permafrost: Soils in the tundra store large amounts of carbon due to the cold temperatures, which slow down decomposition. Organic matter accumulates in frozen soils (permafrost), trapping carbon for long periods. Climate change poses a threat to these carbon stores, as thawing permafrost releases stored carbon into the atmosphere.
  • Wetlands: Wetland soils are one of the most significant carbon stores due to the anaerobic (oxygen-poor) conditions that slow decomposition. These conditions prevent the rapid breakdown of organic matter, allowing carbon to accumulate over time.
  • Temperate Grasslands: Grassland soils store a considerable amount of carbon due to the accumulation of plant roots and slow decomposition rates. Unlike forests, where carbon is stored mainly in biomass, most of the carbon in grasslands is found in the soil itself.

​Factors Affecting Soil Carbon Storage:
  • Temperature and Moisture: Warm, wet conditions like those in tropical rainforests lead to rapid decomposition and carbon release, while cold, dry, or waterlogged conditions like in tundras and wetlands slow down decomposition, allowing carbon to accumulate in the soil.
  • Land Use and Soil Management: Human activities such as deforestation, agriculture, and land conversion can reduce soil carbon storage by disrupting natural cycles and accelerating the release of carbon. In contrast, sustainable land management practices, such as reforestation and no-till farming, can help increase carbon sequestration in soils.
Picture
https://www.visualcapitalist.com/sp/visualizing-carbon-storage-in-earths-ecosystems/

hl only

This unit is a minimum of 2 SL hours

soil classification

5.1.15 Soils are classified and mapped by appearance of the whole soil profile.
A soil profile represents a vertical section of the soil, showing different layers, or horizons, that have developed over time through various transfer and transformation processes. The profile is useful for understanding how water, nutrients, and organic matter move through the soil, as well as how soils evolve and interact with ecosystems.

Soil Horizons:
  • O Horizon (Organic Layer): Consists of organic material like leaf litter, dead plants, and animals, which decomposes to form humus.
  • A Horizon (Topsoil): Rich in humus and minerals, this layer supports plant growth and is where most biological activity occurs.
  • B Horizon (Subsoil): This layer contains minerals leached from the upper layers and has less organic material. It often shows signs of clay accumulation.
  • C Horizon (Parent Material): Contains weathered rock from which the soil develops, providing the base material for the formation of the soil.
  • R Horizon (Bedrock): The solid rock layer beneath the soil profile that plays little to no role in plant growth.

Processes in Soil Profiles:
  • Transfers: The movement of water, nutrients, gases, and particles within the soil is essential for the development of a soil profile. Transfers include infiltration, where water enters the soil, and percolation, where water moves downward through the horizons, carrying nutrients.
  • Transformations: These involve the breakdown of organic and inorganic materials into simpler forms. Decomposition, weathering, and humification (the formation of humus) are key transformation processes.
  • Soil Profile Diagram: Drawing a soil profile diagram is a valuable way to visually represent these processes. It allows students to see how organic material in the O horizon is broken down by detritivores, how water and nutrients percolate through the A and B horizons, and how the transformation of parent material in the C horizon affects soil formation.

​Understanding Transfers and Transformations:
  • Water Movement: Water infiltrates the topsoil and percolates down through the profile. Along the way, it leaches nutrients and minerals from the top layers to the subsoil.
  • Nutrient Cycling: Decomposition of organic matter releases nutrients in the O horizon that are absorbed by plant roots in the A horizon. Some nutrients are carried deeper by water into the B horizon, affecting soil fertility.
  • Soil Formation: Over time, the breakdown of rock in the C horizon through weathering provides minerals that are slowly transformed into the upper soil layers, contributing to soil texture and fertility
​Application of skills: Use soil profile diagrams to classify examples of soils that can be linked to the biomes
studied, for example, brown earths to temperate deciduous forests, or oxisols to rainforests.
5.1.16 Horizons are horizontal strata that are distinctive to the soil type. The key horizons are organic layer, mixed layer, mineral soil and parent rock (O, A, B and C horizons).
  • Draw and compare the soil profiles of a natural ecosystem and an intensively farmed agricultural system.
  • Explain how the loss of the O and A horizons affects soil fertility.
  • Describe the processes of soil erosion and nutrient depletion in agricultural systems
 Intensive agricultural systems often lead to soil degradation, where the natural soil structure is altered or lost. In natural soil systems, there are distinct O, A, B, and C horizons. However, in heavily farmed soils, the O horizon (organic layer) and A horizon (topsoil) may be stripped away, leaving only the B horizon (subsoil) and C horizon (parent material). This degradation impacts soil fertility and structure, leading to long-term consequences for agricultural productivity.
​
Natural Soil Profiles vs. Degraded Agricultural Soils:
  • Natural Soil Systems: In a healthy, natural soil system, the O horizon contains decomposing organic matter like leaf litter, which provides essential nutrients. The A horizon or topsoil is rich in humus and nutrients, supporting plant growth. Below these layers, the B horizon contains minerals leached from the upper layers, and the C horizon consists of weathered parent material.
  • Degraded Agricultural Systems: In areas of intensive agriculture, soil degradation leads to the loss of the O horizon and the depletion of the A horizon. Erosion, over-tilling, and excessive use of agrochemicals can strip the topsoil, leaving behind the less fertile B horizon and C horizon, which are poor in organic matter and nutrients.

Causes of Soil Degradation:
  • Erosion: Intensive farming practices, such as over-plowing and leaving fields bare between crops, can accelerate wind and water erosion, stripping away the topsoil.
  • Nutrient Depletion: Repeated planting of the same crops without crop rotation or the addition of organic matter depletes the soil's nutrient reserves, leading to the degradation of the topsoil.
  • Compaction: Heavy machinery used in agriculture compacts the soil, reducing its ability to retain water and air, which affects root growth and microbial activity.
  • Salinization: Excessive irrigation in arid regions can lead to the build-up of salts in the soil, further degrading its structure and fertility.

​Impact of Soil Degradation:
  • Loss of Fertility: The removal of the topsoil (O and A horizons) reduces the availability of nutrients for plants, leading to lower agricultural yields. This can be seen in regions like the Great Plains of the United States, where poor farming practices led to soil erosion and the Dust Bowl of the 1930s.
  • Reduced Water Retention: Soils without an organic-rich topsoil lose their ability to retain water, leading to increased runoff and reduced moisture availability for crops. This is common in arid and semi-arid regions where over-irrigation and poor land management have degraded the soil.
  • Biodiversity Loss: Healthy soil supports a diverse range of organisms, including earthworms, fungi, and bacteria. As soils degrade, the biodiversity of the soil community declines, affecting the entire ecosystem.
Picture
https://temperategrasslandsbiomes.weebly.com/soillandform.html
Picture
created by AI
Activity: 
  • Set up two trays with soil, one covered with vegetation and the other bare.
  • Simulate rainfall and observe how the vegetated soil retains water and resists erosion, while the bare soil loses topsoil to runoff.
5.1.17 The A horizon is the layer of soil just beneath the uppermost organic humus layer, where present. It is rich in organic matter and is also known as the mixed layer or topsoil. This is the most valuable for plant growth but, along with the O horizon, is also the most vulnerable to erosion and degradation, with implications for sustainable management of soil.
  • Explain the characteristics of topsoil
  • Discuss how these characteristics contribute to soil fertility and plant growth.
  • Describe the impact of intensive farming on topsoil 
Picture
The topsoil is the uppermost layer of soil, rich in organic matter, nutrients, and microorganisms. It is crucial for plant growth and is where most root activity occurs. Topsoil typically contains more oxygen and organic matter than the lower soil horizons, making it the most biologically active layer in the soil profile.
​
Characteristics of Topsoil:
  • High Organic Matter Content: Topsoil contains decaying plant and animal material (humus), which provides essential nutrients like nitrogen, phosphorus, and potassium. This organic matter improves soil structure, increases water retention, and supports healthy root growth.
  • Oxygen Levels: Topsoil has a loose, well-aerated structure that allows for better gas exchange. Oxygen is vital for root respiration and supports the activity of soil organisms, including bacteria and fungi, which help break down organic matter.
  • Microorganisms and Nutrient Recycling: Topsoil is home to a vast array of microorganisms, including bacteria, fungi, and earthworms, that play a critical role in nutrient cycling. These organisms break down organic matter, releasing nutrients that plants absorb through their roots.

Role of Topsoil in Plant Growth:
  • Root Growth: Most plant roots are concentrated in the topsoil because it is rich in nutrients and has good water retention. The loose, crumbly texture of topsoil allows roots to spread and access nutrients and water easily.
  • Biological Activity: The biological activity in topsoil, including the breakdown of organic matter and the release of nutrients, supports the growth of plants. This makes topsoil essential for maintaining high levels of primary productivity in ecosystems.

​Impact of Intensive Farming on Topsoil:
  • Topsoil Loss: Intensive agricultural practices, such as plowing and overgrazing, remove the organic-rich topsoil layer. Without the topsoil, soil fertility declines, and the remaining soil horizons are less able to support plant growth.
  • Dependence on Fertilizers: When the topsoil is lost or degraded, farmers often rely on synthetic fertilizers to replenish nutrients. However, these fertilizers cannot replace the organic matter that topsoil provides, leading to long-term soil degradation and reduced resilience.
  • Soil Erosion: Without the protective layer of topsoil, soils are more vulnerable to wind and water erosion, which further degrades the soil and reduces agricultural productivity.
Activity: :
  • Students will collect soil samples from two different depths: the topsoil and the subsoil (below 30 cm).
  • They will observe the color, texture, and organic matter content of each sample and record their findings.
Activity: 
  • Set up two plant pots: one with healthy topsoil and one with only subsoil (from deeper layers).
  • Plant the same seeds in both pots and observe plant growth over time.

soil formation

5.1.18 Factors that influence soil formation include climate, organisms, geomorphology (landscape), geology (parent material) and time.
  • Explain how geomorphological factors, such as slope and drainage, influence soil formation
  • Describe how parent rock influences soil characteristics and evaluate the role of volcanic and calcareous rocks in soil fertility.
  • Discuss the role of climate and time in soil formation
Picturehttps://compass.rauias.com/geography/soils-classification-factors-formation-processes/
Soils develop over long periods through the interaction of various factors, including climate, organisms, topography, geology, and time. These factors influence the physical, chemical, and biological processes that shape the soil profile and determine its characteristics, such as texture, fertility, and drainage.

Geomorphological Factors:
  • Slope: The steepness of the land affects soil depth and stability. On steep slopes, soils are often shallow due to erosion, while flat or gently sloping areas allow for deeper soil formation. Soils on slopes are more prone to erosion, resulting in nutrient loss.
  • Aspect: The direction that a slope faces (its aspect) influences exposure to sunlight and wind, which in turn affects temperature and moisture levels in the soil. For example, south-facing slopes in the northern hemisphere receive more sunlight, leading to drier soils, while north-facing slopes retain more moisture.
  • Drainage: The natural drainage of an area impacts soil formation. Poorly drained areas, such as valleys or depressions, often experience waterlogging, which slows down decomposition and leads to the accumulation of organic matter. In contrast, well-drained soils develop in areas where water percolates quickly through the soil.

Geological and Time Factors:
  • Parent Rock: The type of rock from which the soil forms, called the parent rock, significantly influences soil characteristics.
    • Calcareous Rocks: Soils formed from limestone or calcareous rocks tend to be rich in calcium carbonate (CaCO₃), which makes them more alkaline. These soils are typically fertile and support agriculture but may have drainage issues due to the fine particles.
    • Volcanic Rocks: Soils derived from volcanic rocks are often rich in minerals such as iron, magnesium, and potassium. Volcanic soils, such as those found in regions like Iceland or the Andes, are highly fertile and retain moisture well, making them ideal for farming.
  • Weathering: The breakdown of rocks into smaller particles through weathering processes, such as physical weathering (freeze-thaw cycles) and chemical weathering (acid rain), contributes to soil formation. Over time, these processes create soil particles that mix with organic matter to form soil horizons.
  • Time: Soil formation is a slow process that can take thousands of years. Over time, soils develop distinct horizons as organic matter accumulates, minerals are leached, and chemical reactions alter the parent material. Older soils tend to be more weathered and may be less fertile unless organic material is regularly replenished.
  • Polar Regions: In polar regions, soil formation is extremely slow due to cold temperatures and limited biological activity. Permafrost (permanently frozen ground) prevents deep soil formation, and the topsoil is often thin and nutrient-poor.

​Influence of Climate:
  • Tropical Climates: In warm, humid climates, like those of tropical rainforests, soils form quickly due to rapid weathering and decomposition. However, these soils are often leached of nutrients, making them less fertile despite their rapid formation.
  • Arid Climates: In deserts, soil formation is slow due to limited moisture and vegetation. The lack of organic matter results in poor soil structure and low fertility

soil texture

5.1.19 Differences between soils rich in sand, silt or clay include particle size and chemical properties.
  • Explain the relationship between soil texture and cation-exchange capacity (CEC)
  • Describe how sand, silt, and clay contribute to the overall fertility of soil
  • Discuss how the cation-exchange capacity of soils can be enhanced through the addition of organic matter
Soil texture refers to the proportion of sand, silt, and clay in the soil, which directly influences the soil's cation-exchange capacity (CEC). CEC is a measure of how well soil particles can hold onto positively charged ions (cations), such as calcium (Ca²⁺), magnesium (Mg²⁺), and potassium (K⁺), which are essential for plant growth.

Sand and Silt:
  • Sand particles are the largest in soil and are derived mostly from quartz. Sand has a low CEC, meaning it has a limited ability to retain cations, leading to a low capacity to hold onto nutrients like calcium, magnesium, and potassium.
  • Silt particles are smaller than sand but larger than clay. Silt is also derived mainly from quartz and has a moderate CEC, but its ability to hold nutrients is still lower than that of clay.

​Clay:
  • Clay particles are the smallest and have a complex structure made up of silicates. Clay has a high CEC, which allows it to hold onto positively charged nutrients more effectively. This makes clay-rich soils better at storing essential nutrients and providing them to plants over time.
  • The high CEC of clay increases the availability of key nutrients such as calcium, magnesium, and potassium. These nutrients are held by clay particles and slowly released into the soil solution, where plants can absorb them.
Picture
soil texture and percolation image from soils.ifas.ufl.edu

Importance of CEC:
  • Soils with a higher CEC are generally more fertile because they can retain and supply more nutrients to plants. Clay soils, with their high CEC, tend to be more fertile than sandy soils, which lose nutrients through leaching due to their low CEC.
  • The presence of organic matter (humus) can also increase CEC, as humus has a high ability to hold cations and further improve soil fertility.

​Soil Texture and Nutrient Availability:
  • Soils with a balanced mix of sand, silt, and clay (known as loam) have a good balance between drainage, aeration, and nutrient retention. Loamy soils, due to their combination of particle sizes, typically have moderate to high CEC and support high agricultural productivity.
  • Sandy soils, due to their low CEC, are less able to retain nutrients, making them more reliant on regular fertilizer applications to maintain soil fertility.
  • Clay soils retain nutrients well but can become compacted and poorly drained, requiring careful management to ensure proper aeration and root penetration.
5.1.20 Soil properties can be determined from analysing the sand, silt and clay percentages, percentage organic matter, percentage water, infiltration, bulk density, colour and pH.
  • Describe how the soil texture triangle is used to classify soils,
  • Explain how the proportions of sand, silt, and clay affect soil properties such as water retention and drainage.
  • Explain how soil texture data can inform land management decisions 
 Soil properties, such as texture, pH, organic matter content, and moisture, can be measured and analyzed to assess soil health and suitability for different uses. Soil texture is one of the most important factors in determining how soil retains water and nutrients, and how well it supports plant growth.


​Soil Texture: Soil texture refers to the proportions of sand, silt, and clay in a given soil sample. These particles differ in size:
  • Sand: Largest particles (0.05–2.0 mm)
  • Silt: Medium-sized particles (0.002–0.05 mm)
  • Clay: Smallest particles (<0.002 mm)
Picture

The Soil Texture Triangle:
  • The soil texture triangle is a diagram used to classify soils based on the percentage of sand, silt, and clay they contain. Each side of the triangle represents a percentage of one of the three soil components.
  • By plotting the proportions of sand, silt, and clay on the triangle, you can determine the texture class of a soil, such as loam, sandy clay, or silt loam.
  • Loamy soils, which are a balanced mix of sand, silt, and clay, are ideal for agriculture because they provide good water retention and drainage, as well as high nutrient availability.

Using the Soil Texture Triangle:
  • To use the soil texture triangle, soil data is needed, specifically the percentage of sand, silt, and clay in a sample. Once these values are known, they are plotted on the triangle to determine the soil’s texture class.
  • For example, if a soil sample is 30% clay, 40% silt, and 30% sand, the point where these values intersect on the triangle will show that the soil is classified as a clay loam.

​Importance of Soil Texture Data:
  • Soil texture data is essential for understanding how soil behaves in terms of water retention, drainage, and nutrient availability. Sandy soils drain water quickly but hold fewer nutrients, while clay soils retain water and nutrients but can become compacted and poorly drained.
  • Knowing the texture class helps land managers and farmers make decisions about irrigation, fertilization, and crop selection.
Activity: Students will measure the proportions of sand, silt, and clay in soil samples using the soil separation method (mixing soil with water and allowing it to settle).
  • They will then plot these percentages on the soil texture triangle to classify the soils.
Activity: Provide students with a set of soil data that includes percentages of sand, silt, and clay from different locations (e.g., forest, farm, desert).
  • They will classify each sample using the soil texture triangle and discuss the implications for water retention, nutrient availability, and land management.

agricultural impact on soils

5.1.21 Carbon is released from soils as methane or carbon dioxide.
  • Explain how global warming can accelerate the release of carbon from soils 
  • Describe how human activities, such as agriculture and wetland drainage, affect carbon release from soils
 Soils are one of the largest global carbon reservoirs, storing carbon in both organic matter (like plant and animal remains) and inorganic forms (like carbonates). However, under certain conditions, this carbon can be released into the atmosphere, contributing to the greenhouse effect and climate change.

Carbon Release from Soils:
  • Decomposition: As organic matter decomposes in the soil, carbon dioxide (CO₂) is released as a byproduct. Under normal conditions, this process is part of the carbon cycle, but human activities and climate change can accelerate decomposition, increasing the release of CO₂.
  • Global Warming: Increased temperatures from global warming speed up the decomposition of organic matter in soils, releasing more CO₂. This is particularly concerning in tundra and permafrost regions, where large amounts of carbon have been stored for thousands of years. When permafrost thaws, it releases both carbon dioxide and methane (CH₄), a potent greenhouse gas.
  • Agricultural Practices: Intensive farming can lead to the loss of carbon stored in soils. Plowing and tilling expose soil to oxygen, speeding up the decomposition of organic matter and releasing CO₂. Practices like deforestation and land conversion also disturb soils, leading to higher carbon emissions.
  • Drainage of Wetlands: Wetland soils store vast amounts of carbon due to anaerobic (oxygen-poor) conditions, which slow decomposition. When wetlands are drained for agriculture or development, the exposure to oxygen accelerates decomposition, releasing stored carbon.

​Tipping Points:
  • A tipping point occurs when an ecosystem or system undergoes a significant and often irreversible change. In the context of soils, increasing temperatures can push ecosystems past a tipping point, where the release of carbon accelerates, amplifying global warming.
  • One potential tipping point involves the breakdown of methane clathrates. These are ice-like compounds found in permafrost and beneath the ocean floor that trap methane. As temperatures rise, these clathrates can break down, releasing large amounts of methane into the atmosphere. Methane is a much more potent greenhouse gas than CO₂, and its sudden release could significantly accelerate climate change.
Human Activities and Soil Carbon:
  • Land use change: Activities like deforestation, draining of wetlands, and the conversion of grasslands to cropland disturb soil carbon stores and increase emissions.
  • Sustainable Practices: To mitigate carbon release, sustainable agricultural practices like no-till farming, reforestation, and wetland conservation can help preserve soil carbon stores and slow down decomposition.
Key Terms
Soil system
Organic matter
Mineral components
Soil texture
Infiltration


HL ONLY

Cation-exchange capacity
​Methane clathrates
Tipping points
Methanogenesis
Sustainable agriculture
Soil carbon sequestration
Carbon fluxes
Carbon sink dynamics
Climate feedback loops
Erosion control
Nutrient recycling
Organic farming
Climate mitigation
No-till farming
Soil compaction
Desertification
Deforestation impacts
Saline soils
Soil degradation
Soil productivity
Waterlogging
Leaching
Decomposition
Weathering
Nutrient cycling
Salinization
Organic carbon
Inorganic carbon
Methane (CH₄)
Loam
​​Carbon storage
Permafrost
Wetlands​
​Percolation
Groundwater flow
Erosion
Parent rock
Volcanic soils
Calcareous soils​
​Topsoil
​Humus
​Soil texture triangle
Classroom Materials

Soils article 
Simulation Lab: Soils
Soil Types Comparison International Assessment
​
​​​Correct use of terminology is a key skill in ESS. It is essential to use key terms correctly when communicating your understanding, particularly in assessments. Use the quizlet flashcards or other tools such as learn, scatter, space race, speller and test to help you master the vocabulary.
Useful Links
Soils of the World
Smithsonian: The Secrets of Soil
From the Ground Up: The Science of Soil
Soil Basics
Soil Types
The Dirt on Soil - Student Science
Soil Horizons animation - Wiley
Basic Properties of the Soil  - eHow
This animation shows chemical weathering - North Carolina State University
This animation describes soil horizons’ - Wiley
Click here for an interactive soil triangle - North Carolina State University,
Down on Dirt - Discovery Education
Secret of Soil - National Museum of Natural History
Animated Soil Pyramid - North Carolina State University
Soil Formation Animation - North Carolina State University
International Year of Soil
In The News
Here's How Earth's Soil Can Save The Planet From Global Warming - April 2016
Our Good Earth - National Geographic

International-mindedness:
  • Significant differences exist in arable (potential to promote primary productivity) soil availability around the world. These differences have socio-political, economic and ecological influences.
Theory of knowledge:
  • The soil system may be represented by a soil profile—since a model is, strictly speaking, not real, how can it lead to knowledge?
Videos
Julia Roberts, Harrison Ford, Edward Norton, Penélope Cruz, Robert Redford and Ian Somerhalder all join forces to give nature a voice.
​A new, seven-part video series explores how an increasing number of farmers throughout the country are creating a new hope in healthy soil by regenerating our nation’s living and life-giving soil.
Excursion into the realms of soil, the number 1 lifegiver on Earth...It's fundamentally important ecological functions, our poor understanding of it as well as our destructive relationship with it. They are so essential to life and thus to us, that our survival is directly dependent on their understanding and protection!
In "Soil Stories", our protagonist, Francine, embarks on a journey of discovery that begins with her realization that soil is alive and that without soil, life as we know would not exist.
Fred Kirschenmann has been involved in sustainable agriculture and food issues for most of his life. He currently serves as both a Distinguished Fellow at the Leopold Center for Sustainable Agriculture at Iowa State University, and as President of the Stone Barns Center for Food and Agriculture in Pocantico Hills, New York.