Module #25

Soil Science and Mining

Module Introduction:

​

• Soil is a combination of geologic and organic material that forms a dynamic membrane over much of the surface of Earth.
• A variety of processes that occur in soil connect the overlying biology with the underlying geology.
• In this module we will explore the weathering of rocks that leads to the formation of soil and the development of specific soil horizons.
• We will discuss physical, chemical, and biological processes that take place in soils.
• Finally, we will examine human activities that degrade soils, including the process of mining.

Module #25: Weathering and Soil Science

Essential Knowledge

4.2 Soil Formation and Erosion (Module 25)

• Soils are formed when parent material is weathered, transported, and deposited.
• Soils are generally categorized by horizons based on their composition and organic material.
• Soils can be eroded by winds or water.
• Protecting soils can protect water quality as soils effectively filter and clean water that moves through them.

​

Essential Knowledge

4.3 Soil Composition and Properties (Module 25)

• Water holding capacity—the total amount of water soil can hold—varies with different soil types. Water retention contributes to land productivity and fertility of soils.
• The particle size and composition of each soil horizon can affect the porosity, permeability, and fertility of the soil.
• There are a variety of methods to test the chemical, physical, and biological properties of soil that can aid in a variety of decisions, such as irrigation and fertilizer requirements.
• A soil texture triangle is a diagram that allows for the identification and comparison of soil types based on their percentage of clay, silt, and sand.

​

Essential Knowledge

5.9 Impacts of Mining (Module 25)

• As the more accessible ores are mined to depletion, mining operations are forced to access lower grade ores. Accessing these ores requires increased use of resources that can cause increased waste and pollution.
• Surface mining is the removal of large portions of soil and rock, called overburden, in order to access the ore underneath. An example is strip mining, which removes the vegetation from an area, making the area more susceptible to erosion.
• Mining wastes include the soil and rocks that are moved to gain access to the ore and the waste, called slag and tailings that remain when the minerals have been removed from the ore. Mining helps to provide low cost energy and material necessary to make products. The mining of coal can destroy habitats, contaminate groundwater, and release dust particles and methane.
• As coal reserves get smaller, due to a lack of easily accessible reserves, it becomes necessary to access coal through subsurface mining, which is very expensive.

​

Essential Knowledge

8.2 Human Impacts on Ecosystems (Modules 25, 41-44, 51-54, 57, 60)

• Organisms have a range of tolerance for various pollutants. Organisms have an optimum range for each factor where they can maintain homeostasis. Outside of this range, organisms may experience physiological stress, limited growth, reduced reproduction, and in extreme cases, death.
• Coral reefs have been suffering damage due to a variety of factors, including increasing ocean temperature, sediment runoff, and destructive fishing practices.
• Oil spills in marine waters cause organisms to die from the hydrocarbons in oil. Oil that floats on the surface of water can coat the feathers of birds and fur of marine mammals. Some components of oil sink to the ocean floor, killing some bottom-dwelling organisms.
• Oil that washes up on the beach can have economic consequences on the fishing and tourism industries.

​

Essential Knowledge

8.2 Human Impacts on Ecosystems (Continued)

• Oceanic dead zones are areas of low oxygen in the world’s oceans caused by increased nutrient pollution.
• An oxygen sag curve is a plot of dissolved oxygen levels versus the distance from a source of pollution, usually excess nutrients and biological refuse.
• Heavy metals used for industry, especially mining and burning of fossil fuels, can reach the groundwater, impacting the drinking water supply.
• Litter that reaches aquatic ecosystems, besides being unsightly, can create intestinal blockage and choking hazards for wildlife and introduce toxic substances to the food chain.
• Increased sediment in waterways can reduce light infiltration, which can affect primary producers and visual predators. Sediment can also settle, disrupting habitats.
• When elemental sources of mercury enter aquatic environments, bacteria in the water convert it to highly toxic methylmercury.

Soil

• Soil is a complex mixture of organic matter, minerals, water, and air that supports life, while dirt is a component of soil that doesn't support life.
• Soil is composed of organic and inorganic components.
• Organic components are supplied by organisms (dead or alive) and their wastes through decomposition, while the inorganic compounds of soil are derived from parent materials (rocks) via weathering.

In a sense, decomposition is a “top-down” process and weathering is a “bottom-up” process. In other words, decomposition tends to occur at the surface with organic components “raining down” into the topsoil (A horizon) while inorganic components migrate upwards into the topsoil from below thanks to the weathering of bedrock.

Weathering

​

• Soil is composed of organic and inorganic components supplied by decomposition and weathering respectively.
• Decomposition: The process by which organic substances are broken down into simpler organic matter.
• Weathering refers to the breakdown of parent material such as rock and minerals into smaller components forming soil.
• Weathering can be physical, chemical or biological in nature.

Physical Weathering

• Water can work its way into cracks in rock, where it can wash away loose material. When the water freezes and expands, it can widen the cracks.
• The ability of water/ice to physically weather rock and form soil is one more reason water is fundamentally important to life on Earth.

Physical weathering: The mechanical breakdown of rocks and minerals.

Chemical Weathering

• Water that contains carbonic acid wears away limestone, sometimes forming spectacular caves.

Calcium bicarbonate is aqueous and therefore the limestone dissolves away over time.

Chemical weathering: The breakdown of rocks and minerals by chemical reactions, the dissolving of chemical elements from rocks, or both.

Biological Weathering

• Growing plant roots can force rock sections apart. Plants will often release acids to help further degrade rock.

Biological weathering can be physical or chemical in nature, so long as an organism is the one exerting the force or promoting the chemical reaction.

Biological Weathering: The breakdown of rocks and minerals by the exertion of physical forces and chemical reactions by a biological organism.

Erosion and Deposition

• Erosion: The physical removal of soil from a landscape or ecosystem.
• Erosion has a significant impact on soil formation/productivity and is usually the result of two processes:
• Abiotic erosion: Wind, water and ice move materials downslope.
• Biotic erosion: Living organisms burrow under the soil.
• Erosion is complemented by deposition, the process by which sediments/soil is added to a landform/landmass.
• From the perspective of soil quality, erosion is degradative while deposition is additive.

Soil erosion can negatively impact water and air quality. In air, soil contributes to dust pollution and directly harms human health. In water, soil raises turbidity choking wildlife and suppressing photosynthesis. Clay particles are particularly challenging to remove from water because of their small size and negative charge which increase their retention time.

Soil Ecosystem Services

• Supporting: Soil provides habitat for organisms to live in including microorganisms, invertebrates and burrowing mammals.
• Supporting: Soil serves as an important medium for nutrient cycling. In particular soil serves a major reservoir for carbon.
• Regulating: Soil can filter out pollutants. As the water percolates through the soil, the soil acts a physical filtration system. Soil and soil organisms collect and remove the harmful pollutants that settle.
• Regulating: Soil provides a major carbon sink to mitigate climate change.

• Provisioning: Indirectly, soil provides food through the crops we grow in agricultural operations.
• Cultural: Many of our natural spaces and the benefits they provide (e.g. ecotourism, spiritual connection, etc.) would not exist without soil.

PSA: Leaves and Soil Health

Don’t rake your leaves, they provide numerous benefits for the environment, wildlife and soil including:

• Creating a natural mulch that helps to suppress weeds while fertilizing the soil as it breaks down.
• Providing habitat for wildlife including lizards, birds, turtles, frogs, and insects that overwinter in the fallen leaves.
• Supporting the growth of microorganisms that make soil healthier and plants grow stronger.
• As leaves decay, they add organic matter back into the soil, which lessens the need for fertilizer.

Soil Formation

​

Soil is a mixture of organic and inorganic matter:

• The weathering of rock and parent material provides the inorganic matter.
• The organic matter comes from the decomposition of organisms and the biological waste they produce.
• Both of these processes take time to occur and so soils mature as they age.
• Other factors that influence soil formation come from CLORPT.

The Formation of Soil (CLORPT)�

​

Soil Horizons

Over time, soil develop horizons. There are five soil horizons:

• O horizon: The organic horizon at the surface of many soils, composed of organic detritus in various stages of decomposition.
• A horizon: Frequently the top layer of soil, a zone of organic material and minerals that have been mixed together. Also known as Topsoil.
• E horizon: A zone of leaching, or eluviation, found in some acidic soils under the O horizon or, less often, the A horizon.
• B horizon: A soil horizon composed primarily of mineral material with very little organic matter.
• C horizon: The least-weathered soil horizon, which always occurs beneath the B horizon and is similar to the parent material.

​

Horizon: A horizontal layer in a soil defined by distinctive physical features such as texture and color.

The organic matter in soil is called humus.

More precipitation → more leaching in soil (cations dissolve in rainwater and are carried away).

The R layer is technically not a horizon but refers to the bedrock at the base of the soil that provides inorganic components to the soil via weathering.

Soil Horizons

• All soils have horizons, or layers, which vary depending on soil-forming factors such as climate, organisms, and parent material. Most soils have either an O or A horizon and usually not both. Some soils that have an O horizon also have an E horizon.

The R layer refers to the bedrock at the base of the soil that provides inorganic components to the soil via weathering.

Properties of Soil� �

• There are three major properties of soil: 
• Physical
• Chemical
• Biological
• These properties are used to classify soils and better understand their role in ecosystems.
• The United State Department of Agriculture (USDA) uses a 12 order soil taxonomy system centered around these properties (you don’t need to memorize the soil orders).
• Soil degradation: The loss of some or all of a soil’s ability to support plant growth.

​

Physical Properties of Soil�

The physical properties of soil refer to physical characteristics such as texture and permeability.

• The texture of a soil is determined by its percentage of sand, silt, and clay. Clay is smaller than silt, silt is smaller than sand.
• Soil permeability: The ability of water and air to move through a soil. Soil permeability depends on its texture which determines porosity (the percentage of open space in a soil).
• Larger particles like sand fit together loosely, while smaller particles like clay pack together tightly.
• As such, clay reduces water flow and lowers the permeability of the soil.

​

(top) Soils consist of a mixture of clay, silt, and sand. The relative proportions of these particles determine the texture of the soil.

(bottom) The relative sizes of sand, silt, and clay.

Waterlogging

• As such, water logging is closely related with permeability (ie. percolation).
• Soil texture composed of 20% or more clay tend to become waterlogged due to low permeability.
• By contrast, soils with a high percolation rate (such as ones dominated by sand) are subject to nutrient leaching.
• While water is necessary for plant growth, waterlogged soils are problematic for most plants especially in agriculture.
• Waterlogged soils will present anaerobic conditions and limit gas exchange with the atmosphere. Left untreated, this can result in the asphyxiation of roots, the development of harmful fungal growth and ultimately the death of the plant.

• When soil is saturated, all available pore space is filled with water and the soil is said to be waterlogged.

Soil Texture Chart

• Each side of the triangle act as an axis for the % of sand, silt and clay.
• If you know the percentages of each soil particle in your sample, you can use the chart to determine the texture.
• Example: a sample with 50% sand, 20% silt and 30% clay is considered a “sandy clay loam”.

Density Gradient Tubes

• A density gradient tube is used to evaluate the composition of sand, silt and clay in a soil sample before determining the texture of the soil using the soil texture chart.
• The soil sample is shaken and then allowed to settle.
• The heavier particles (sand) will settle out first followed by the lighter silt and clay particles.
• Measuring the thickness of each band allows you to determine the percent composition of each soil particle.

​

We use a density gradient tube in Activity #4 of the Soil Productivity Study.

Soil Permeability

The permeability of a soil depends on its texture:

• Sand, with its large, loosely packed particles, drains quickly.
• Clay, with it's smaller and more tightly packed particles, drains much more slowly.

Water-Holding Capacity (WHC)

• Water-Holding Capacity: the ability of a certain soil texture to physically hold water against the force of gravity.
• Water molecules adhere to the surface of the soil particles (e.g. sand, silt and clay).
• Finer textured soils (such as clay) can hold more water because they have the greatest surface area relative to their volume compared to the other soil particles (silt and sand).

Water-holding capacity is important for agriculture:

• Soils that can hold more water require less rainfall or irrigation.
• But, soils with excessive water-holding capacity can also flood more easily, promoting the growth of fungi that can cause root rot.

WHC Visualized

Chemical Properties of Soil�

• The chemical properties of soil help determine how a soil functions.
• Cation exchange capacity (CEC): The ability of a particular soil to absorb and release cations (positively charged ions).
• Cations are important nutrients for plants and include micronutrients like calcium (Ca²⁺), magnesium (Mg²⁺), sodium (Na⁺) as well as macronutrients like nitrogen, phosphorus and potassium (NPK).
• Clay and organic material have negative charges and provide locations for cations to bind in soil, increasing CEC.
• H⁺ and aluminum (Al³⁺) compete for these locations, lowering base saturation.
• Most plants prefer alkaline/basic (as opposed to acidic) soils with a high CEC since they can hold more cation nutrients.
• Base saturation: The percentage of total CEC is occupied by Mg²⁺, Ca²⁺ and K⁺ (nutrient cations) compared to H⁺ and Al³⁺; higher base saturation → higher fertility.

As plants carry out metabolism they produce acids which enter the soil. The acids release H⁺ and displace nutrient cations and lower base saturation.

Cation Exchange Capacity Visualized

• Think of CEC like the number of seats in a stadium.
• The more seats (CEC), the more places for people to sit (nutrients the soil can hold).
• Base saturation refers to what % of the “seats” are filled with nutrient ions (e.g. NPK).

Base Saturation Visualized

• Consider a room full of 100 chairs (CEC).
• If 35 of those chairs are occupied by Mg²⁺, Ca²⁺ and K⁺, the base saturation is 35%.
• The remaining 65 chairs are occupied by H⁺ and Al³⁺ which “box out” the nutrient cations. This is why plants typically prefer more alkaline soils.
• CEC Video Resource

Biological Properties of Soil�

• The biological properties of soil refer to the activities of the many organisms living in soil. 
• Fungi, bacteria and protozoans account for most of the soil organisms (80-90%) and therefore most of the biological activity in soil, especially decomposition. 
• Organisms such as snails, slugs, insects, earthworms and rodents burrow in soil, helping to mix it and perform decomposition reactions that add organic nutrients to the soil.
• Decomposition: The process by which organic substances are broken down into simpler organic matter.

​

Just as weathering adds inorganic nutrients to soil, decomposition supplies organic nutrients.

Environmental Themes and Soil

​

• Soil is a shared resource and can be overexploited → degradation and tragedy of the commons.
• When soils are degraded because they are depleted of their nutrients (such as by intensive farming) or become inundated with salts (such as from salinization caused by irrigation) they can no longer support plant growth.
• Erosion increases as a result of the loss of plant roots to hold soil in place.
• This is what happened during the Dust Bowl of the 1930s.

Salinization occurs when the salts in groundwater remain in the soil after the water evaporates.

Mining

• The distribution of minerals on Earth has social and environmental consequences particularly for geopolitics and energy independence.
• Rocks, minerals, metals and even fossil fuels are all finite resources that we collect through mining. 
• Some resources are abundant but others are rare.
• Ore: A concentrated accumulation of minerals from which economically valuable materials can be extracted.
• Metal: An element with properties that allow it to conduct electricity and heat energy, and to perform other important functions.
• Reserve: In resource management, the known quantity of a resource that can be economically recovered.�

Abundance of Ores and Metals

Oxygen is the most abundant element in the crust. Silicon, aluminum, and iron are the next three most abundant elements.

Surface Mining Techniques�

• Strip mining: The removal of strips of soil and rock to expose ore such as coal.
• Open-pit mining: A mining technique that uses a large visible pit or hole in the ground.
• Mountaintop removal: A mining technique in which the entire top of a mountain is removed with explosives.
• Placer mining: The process of looking for minerals, metals, and precious stones in river sediments.

Surface mining is a broad category of mining in which soil and rock overlying the mineral deposit are removed, in contrast to underground mining, in which the overlying rock is left in place, and the mineral is removed through shafts or tunnels.

Mining Operations

• A major challenge with mining is the production of tailings that can alter and pollute the landscape.
• Above: tailings ponds are used to hold disturbed soil and overburden during mining operations.
• Below: an illustration of overburden in surface and subsurface mining.
• Mining tailings: Unwanted waste material created during mining
• Overburden: rock or soil overlying a mineral deposit, or other underground feature.

The Impacts of Tailings

​

Tailings Have Poor Soil Stability

Tailing piles lack vegetation to hold soil particles in place and the recently disturbed soil is loose and aerated. This leads to numerous issues including:

• Increased sedimentation and turbidity in streams → damage of aquatic ecosystems.
• Increased erosion and particulate matter (dust) pollution.
• Increased landslides.

​

Tailings Generate Pollution

Tailing piles often contain harmful materials and pollutants. This leads to numerous issues including:

• Release of toxic heavy metals (e.g. mercury) into the environment.
• Increased human health problems due to inhalation of dust/particulate matter.
• Release of radioactivity from copper or uranium mining.
• Release of acids used in the mining process into local aquifers and aquatic ecosystems.

Subsurface Mining Techniques

• Subsurface mining: Mining techniques used when the desired resource is more than 100 m (328 feet) below the surface of Earth.
• Diamonds, and gold are some of the materials extracted by subsurface mining.

​

Subsurface mining is generally more expensive than surface mining because of higher costs including: wages, healthcare and safety concerns (e.g. insurance and workman’s compensation for injuries), and complexity (e.g. more powerful, expensive drills and tools.

Mining Impacts and Important College Board Terms

• Like many other anthropogenic impacts, mining causes habitat destruction and fragmentation which leads to reductions in biodiversity and ecosystem services.
• Habitat Fragmentation: The process by which large and contiguous habitats get divided into smaller, isolated patches of habitats often by road building, deforestation, mining, etc. Learn more about habitat fragmentation here.
• Habitat Destruction: The process by which a natural habitat, such as a forest or wetland, is altered so dramatically that it no longer supports the species it originally sustained. This is often generated by filling in wetlands, mountaintop removal, agriculture/land development, etc. learn more about habitat loss (including habitat fragmentation and destruction) here.

The Environmental and Health Impacts of Mining

Mining Reclamation Methods

​

Acid Mine Drainage: the outflow of acidic water from metal mines or coal mines.

​

Reclamation: the process of restoring land that has been mined to a natural or economically usable state.

Module Review:

​

​

• In this module, we saw that rocks and minerals undergo physical and chemical weathering and become products that are precursors for soil.
• Weathered materials are subject to erosion, which is a natural process that can be enhanced by human activity. Erosion also influences the precursors to soil.
• Soil forms from geologic materials as well as biological material. Soil properties result from physical, chemical and biological processes that are influenced by five soil forming factors.
• Concentrated accumulations of elements and minerals in and below soils that are economically valuable are called ores.
• When ores are extracted, a variety of consequences affect humans and the environment.