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Students will recognize soil changes in relationship to depth and understand factors associated with soil formation.
Interest Approach — Engagement:
Activity 1: Soil Profiles
Activity 2: Making Soil
bedrock: a solid, anchored rock; may be beneath the soil or exposed at the surface (exposed bedrock can be seen in places like Zion National Park)
horizon: soil layer parallel to the soil surface that is different (visibly, chemically, and/or physically) from the layers above and below
infiltration: the movement of water into the soil surface
parent material: the soil horizon just above bedrock that contains broken up pieces of rock and will eventually break down into soil
percolation: the movement of water within and through the soil
pore space: the spaces between soil particles and between soil aggregates; pores can be filled with air or water
soil profile: vertical cross section of the soil from the ground surface to the underlying bedrock; composed of horizons
subsoil: the soil horizon between topsoil and parent material that is low in organic matter and poorly suited to growing crops
topsoil: the upper layer of soil that is rich in organic matter and best suited to growing healthy crops
If you were to dig deeply enough, you would hit solid rock. This is called bedrock. But first, you would have to dig through three or four different layers of soil. These layers are called horizons, and their arrangement in the soil is called the soil profile. Examining the horizons in a soil profile can reveal a lot about how water will move through the soil, what nutrients are in the soil, and what kinds of plants will grow well there. The top layer in the profile is mostly organic matter in varying states of decay. Organic matter comes from plants and animals and their wastes. In general, the arid western United States has soils low in organic matter, and in some dry, rocky places this layer may be nonexistent. You will find deeper organic layers in regions with more moisture that support more plant life.
The first soil horizon under the organic layer is called topsoil. The topsoil is where plants spread their roots. In this layer, roots can most readily absorb water, nutrients (minerals), and air (carbon dioxide and oxygen). If enough of the topsoil blows or washes away we are left with subsoil, the next horizon below the topsoil. Subsoil is usually lighter in color and less productive than topsoil. Minerals here are not in a form that is easy for plants to use. The subsoil is mostly made up of sand, silt, or clay and has very little organic material. Plants grow poorly in subsoil. That’s why farmers must work hard to conserve their topsoil. Between the subsoil and the bedrock is the last horizon in the profile. This layer is composed of small rocks that have started to break off the bedrock, and it is called the parent material of the soil. That is because most of what makes up the soil was once part of the rock.
It can be difficult to imagine how solid rock turns into soil, but rocks are the parent material of every soil. Both physical forces and chemical weathering play a role in soil formation. Water from rain flows into the cracks of rocks. Water expands when it freezes, forcing the cracks in the rocks to get bigger and little bits of the rock break off. Sometimes the roots of plants will grow into cracks in rock causing them to break. Rocks can be broken apart by lichens—tiny crusty coral-like organisms (green, orange, gray, etc.) that grow on rocks and secrete an acid that dissolves some minerals. Plant roots and many microorganisms also secrete organic acids that break down rocks in soil, like the vinegar in Activity 3. These processes take many years and are further affected by relief in the landscape, which directs where water flows and affects exposure to sun and wind. Overall, soil formation is driven by five factors: type of parent material, passage of time, climate, activity of plants and animals, and lay of the land.
In many parts of the United States these factors combined favorably thousands of years ago, and left us with fertile soils well-suited to growing crops. Topsoil is the foundation of agriculture, and it is just a thin layer that sustains life. Topsoils range from several feet deep in grasslands to 12 inches or less in many Western states. Because our topsoils are so important, farmers, ranchers, and others who are charged with caring for the land must use practices that conserve topsoils and hold them in their place. Soil is considered a nonrenewable resource because it takes so long to form.
Activity 1: Soil Profiles
Note: Prepare by asking students to bring in a sample of cereal from home (in a sealable plastic bag), or purchase three to five different kinds of bagged bulk cereal. Other food or nonfood items may be substituted for the cereals.
Activity 2: Making Soil
Note: For this activity, you will need some sandstone. Sandstone is relatively easy to find; however, if you have difficulty finding sandstone, pieces of brick or concrete can be substituted.
Concept Elaboration and Evaluation
After conducting these activities, review and summarize the following key concepts:
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Utah Agriculture in the Classroom