Section 3.1: Soil 101

Outcomes

Students will:

  • Describe what soil consists of and its four main functions.

  • Investigate factors and processes that influence soil formation (i.e. addition, removal, transfer, and transformation).

  • Differentiate soils based on their characteristics (i.e. texture, structure, porosity, and colour).

Key Terms

Acid Rain / Acidification / Addition / Bioavailable / Climate / Colloids / Colour / Dirt / Eluviation / Erosion / Groundwater / Humans / Leaching / Organisms / Parent Material / Pedosphere / pH / Porosity / Removal / Solutes / Structure / Texture / Topography / Transfer / Transformation / Water Table / Weathering

See content or Module Glossary for definitions

What is Soil?

To most, soil is just the ground beneath our feet, but soil is a complex mixture of minerals, organic matter, and organisms. With the TREE program, you will need to provide samples of soil near the trembling aspen you are sampling from. Soil is collected because it helps correlate data from the tree cores because as we know, soil is composed of many nutrients and minerals that find their way into the tree. By analyzing the soil, we are able to potentially make these connections and see the impact the environment has on the tree.

Soil contains the three common states of matter: solid, liquid, and gas. Roughly 50% of soil is solid and is composed of minerals (~45%) and organic matter (~5%). Soil is porous though which allows for water and gases to seep through (see Figure 1).

Figure 1 shows the pores or spacing between sand grains. Image by klaber.
Figure 1 shows the pores or spacing between sand grains. Image by klaber.
Figure 2 shows the interconnectedness of Earths many spheres. Original image by Jojndon.
Figure 2 shows the interconnectedness of Earths many spheres. Original image by Jojndon.

Soil has four main functions. It acts as a medium for plant growth. It also stores and filters water. It modifies the atmosphere by emitting and absorbing various gases and dust and finally, soil is also a habitat for countless organisms. These functions continuously alter the composition and structure of soil.

All of the soil on Earth is collectively called the pedosphere. As outlined in Figure 2, the pedosphere connects to the four other spheres: the lithosphere (Earth's crust and upper mantle), the hydrosphere (Earth's water and ice), the atmosphere (Earth's gases), and the biosphere (Earth's living creatures).

How is Soil Different from Dirt?

The terms soil and dirt are used interchangeably but they are, in fact, different and should be used separately. Dirt is distinct from soil in that dirt is soil that has been displaced from its native environment in a way that is unusable. Dirt is incapable of supporting plant growth, so it is sometimes referred to as dead soil. You can identify dirt from soil by adding water to it. If the mixture compacts well together, it is soil. If the mixture does not pack together, then it is most likely dirt.

How Does Soil Form?

Soil formation, or pedogenesis, happens through a variety of complex processes but simply put, it is formed by the breakdown and movement of rocks and sediments called parent material. The processes impacting the parent material can be simplified to just their effects: addition, removal, transfer, and transformation.

  • Addition - This effect refers to adding material into soil. Anything or anyone can add material to soil. For example, adding compost from your kitchen or through weathering of larger rocks will impact soil formation (Figure 3).

  • Removal - With removal, this effect takes material out of soil. Anything can remove material from soil, such as simply digging a hole in the ground, or through erosion from wind or water.

  • Transfer - In this process, soil materials are moved between locations. Anything can transfer material between locations, such as simply tilling the ground or through leaching and eluviation of minerals (Figure 4).

  • Transformation - Transformation refers to altering the chemical composition of soil materials. Many things can cause soil materials to change their chemical composition, such as simply adding fertilizer to the soil or through acidification from acid rain.

Figure 3 shows weathering and erosion of a boulder. Image by UCL Mathematical & Physical Sciences.
Figure 3 shows weathering and erosion of a boulder. Image by UCL Mathematical & Physical Sciences.
Figure 4 shows iron leaching out from a stone wall. Image by Ashley Van Haeften.
Figure 4 shows iron leaching out from a stone wall. Image by Ashley Van Haeften.

Examples of Soil Forming Processes

  • Weathering is the breakdown of soil, rocks, minerals, wood, and man-made materials through contact with wind, water, ice, and organisms. Products of weathering remain in the same place.

  • Erosion is the breakdown and movement of soil, rocks, minerals, wood, and man-made materials by wind, water, ice, gravity, and organisms. Products of erosion do not remain in the same place.

  • Leaching is the loss of dissolved material due to flowing water.

  • Eluviation is the loss of solid material due to floating on flowing water.

  • Acidification is the build-up of hydrogen in the soil, decreasing the pH or the measure of the acidity of a solution. This build up can be from adding in acidic materials or solutions, or removing basic ones.

What Factors Influence Soil Formation?

Soil is the product of parent material, climate, organisms, topography, groundwater, and humans all interacting with each other. Most importantly, soil is the product of time.

Parent material refers to the rocks and sediments that are broken down to form soil (see Figure 5).

  • The composition and structure of soil is highly dependent on the composition of the parent material. Iron-rich parent material will produce iron-rich soil.

  • The rate at which soil forms is also dependent on the parent material. Easily weathered parent material will produce soil faster than stronger, more resistant material.

Figure 5 shows layers of parent sediment. Image by TheusiNo.
Figure 5 shows layers of parent sediment. Image by TheusiNo.
Figure 6 shows potholes full of water. Image by Matt RF Webb.
Figure 6 shows potholes full of water. Image by Matt RF Webb.

The climate refers to the long-term trend in weather.

  • The rate at which soil forms is dependent on the local climate.

  • Climates with a trend for rain will experience rock and soil erosion which will create and relocate soil faster than soil in dry climates. Climates with a trend for freezing and thawing will experience rock and soil weathering, creating soil faster than soil in climates without the freezing and thawing.

  • The effects of freezing and thawing can be seen every spring when potholes on the road start to appear. As shown in Figure 6, potholes are due, in part, to spring melt-water making its way into road cracks during the day, where the water then freezes and expands as it becomes ice overnight.

The term organism refers to any living creature: plants, fungi, animals, insects, and microbes alike.

  • The rate at which soil forms and the composition and structure of the soil are dependent on the organisms inhabiting the soil.

  • Burrowing animals are able to erode rocks and mix around the soil, creating soil faster than soil without these animals nearby (Figure 7).

  • Smaller microbes consume and concentrate nutrients from the gases and water within the soil. They also mobilize these nutrients as they move and when the microbes die, the nutrients are deposited to the new location.

  • Nitrogen, for example, is not readily available in the soil. Microbes must consume nitrogen, convert it to be bioavailable (able to be absorbed by a body), and then die off to deposit it for plants.

  • Plants alter the composition of soil by absorbing and storing nutrients within it. Some plant roots are capable of splitting boulders in half, eroding them into soil over time.

Figure 7 show how Cathedral Termites will remove large amounts of soil to create their homes, called termite mounds. Image by J Brew.
Figure 7 show how Cathedral Termites will remove large amounts of soil to create their homes, called termite mounds. Image by J Brew.

Topography is used to describe the elevation, slope, and orientation of the terrain.

  • The topography of the terrain impacts the rate at which soil forms and the composition and structure of the soil.

  • Elevation is the height above or below a fixed reference point when talking of a geographical location. Elevation will impact how water travels through the landscape.

  • Slopes, a type of elevation, encourage water to flow quickly. This occurrence causes landslides that erode rocks into soil and mixes the soil up. Depending on the direction the slopes face as well, the soil may receive more sunlight causing weathering and/or drier conditions.

Groundwater refers to the water stored within the ground or soil. The depth where the pores in soil become saturated with water is called the water table (see Figure 8).

  • Groundwater affects the rate at which soil forms by weathering and eroding parent material.

  • Fast flowing groundwater will erode the parent material faster than still groundwater.

Humans have a diverse effect on the composition and structure of the soil. Human activity is another factor that influences soil formation.

  • Industrial and agricultural activities (Figure 9) change the features of soil either to have the properties needed for a specific task or as part of the resource extraction process.

  • Gardening even alters the composition of the soil through fertilizers and the plants absorbing nutrients.

Time is the last main factor that impacts soil formation. Time refers to the duration that the above factors have to act on the parent material. It is by far the most important factor in soil formation. With enough time, even mountains can be eroded into pebbles.

Figure 8 shows where the water table is defined. Image by Geoff Ruth.
Figure 8 shows where the water table is defined. Image by Geoff Ruth.
Figure 9 shows a farmer tending to his crops. Image by Jean Beaufort.
Figure 9 shows a farmer tending to his crops. Image by Jean Beaufort.

Are All Soils the Same?

No. Soils are characterized by many factors. The main ones include: texture, structure, porosity, and colour. These factors are intricately related as changing one factor will change many others.

  • Texture - Texture refers to the feel, appearance, and consistency of the soil. Soil's texture is determined by the ratio of mineral components of soil (the ratio between sand, silt, and clay).

  • Structure - Structure refers to the clumping of the textural components. Small clumps are called aggregates, while clumps of aggregates are called peds. The structure of soil can make it more or less erosion resistant.

  • Porosity - Porosity is a measure of the porous space in a material. It is the ratio between the volume of pores to the total volume. Porous soil allows materials to flow easier, making it susceptible to weathering. Non-porous soil withstands weathering better, but inhibits plant root growth.

  • Colour - Colour refers to the different colour of the soil horizons (see Figure 10). Colours give a visual indication of the elements present and the processes acting on the soil. Soils from different location can be completely different colour or be completely identical. Soil can change colours as you increase depth, making it easy to identify soil horizons which is outlined in the next section.

Figure 10 shows differently coloured soil horizons. Image by HolgerK.
Figure 10 shows differently coloured soil horizons. Image by HolgerK.

Additional Resources & References for Section 3.1

Resources

  • Soils of Canada: https://www.soilsofcanada.ca/
    Website created through the help of soil scientists from across Canada that provides information on Canadian soil, how it forms, what it is made of, the different types of soil, and much more. Also has helpful links for further exploration.

  • Soil vs. Dirt, You'll be Surprised to Know the Difference: https://www.tinyplantation.com/soil-fertilizers/soil-vs-dirt
    An online blog post that covers the difference between soil and dirt, how soil becomes dirt, and the different types within each. Informative read to help students distinguish between these two commonly associated words.

  • Virtual Soil Science Learning Resources: http://soilweb.ca/
    A website full of resources for soil science courses and national resource programs across Canadian and other Northern American universities. This webpage has a lot of soil resources, lessons, activities, research papers, and other resources that can be adapted for the classroom.

References

Annenberg Learner. (2017). Earth & space science: Session 1. Retrieved from https://www.learner.org/series/essential-science-for-teachers-earth-and-space-science/

National Research Council of Canada. (1998). The Canadian system of soil classification (3rd ed.). Ottawa: Research Branch, Canada Dept. of Agriculture.

Soils of Canada. (n.d.). Welcome. Retrieved from https://www.soilsofcanada.ca/index.php

Soil Science Society of America. (n.d.). Soils overview. Retrieved from https://www.soils.org/files/about-soils/soils-overview.pdf