Module 3: Soil Topics addressed in this module include: introducing soil, identifying soil as distinct from dirt, identifying the various types of soils and soil horizons, and understanding soil formation and the nutrient cycle. Updated Oct. 29, 2019 TREE: Module 3: Soil Page 2 Table of Contents Section 3.1: Soil 101 .......................................................................................................................3 What is Soil? .......................................................................................................................................................... 4 How is Soil Different from Dirt? ............................................................................................................................. 5 How Does Soil Form? ............................................................................................................................................. 5 What Factors Influence Soil Formation? ............................................................................................................... 6 Are All Soils the Same? .......................................................................................................................................... 8 Section 3.2: Soil Horizons .............................................................................................................. 10 What is a Soil Profile? .......................................................................................................................................... 11 What is a Soil Horizon? ........................................................................................................................................ 11 How do Soil Horizons Form? ................................................................................................................................ 12 How do Soil Profiles Compare Across Canada? ................................................................................................... 13 Labelling Soil Horizons ......................................................................................................................................... 14 While Sampling .................................................................................................................................................... 15 Section 3.3 The Nutrient Web ....................................................................................................... 17 What is the Nutrient Web? .................................................................................................................................. 18 How Does the Nutrient Web Work? .................................................................................................................... 18 What Factors Influence the Nutrient Web? ........................................................................................................ 20 Glossary ....................................................................................................................................... 28 TREE: Module 3: Soil Page 3 Section 3.1: Soil 101 Original image by Mesaytesegaye TREE: Module 3: Soil Page 4 Section Overview 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 Dirt Parent Material Texture Acidification Eluviation Pedosphere Topography Addition Erosion pH Transfer Bioavailable Groundwater Porosity Transformation Climate Humans Removal Water Table Colloids Leaching Solutes Weathering Colour Organisms Structure 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. TREE: Module 3: Soil Page 5 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 Figure 2 shows the interconnectedness of Earths many spheres. gases), and the biosphere (Earth’s living creatures). Original image by Jojndon. 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. Figure 3 shows weathering and erosion of a boulder. Image by UCL Mathematical & Physical Sciences. 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 4 shows iron leaching out from a stone wall. Image by Ashley Van Haeften. TREE: Module 3: Soil Page 6 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. o 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 Figure 6 shows a pothole full of water. Image by Matt RF TheusiNo. 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