Lesson Two: Sustainable Ag Starts With Soil
What is soil? Begin this activity by asking your students this question and having them record their responses in their journals.
After they’ve had time respond to this question, have them share their responses and discuss as a class. Then, share the following background information to enable the students to have a basis of understanding of the types of soil prior to participating in the activity.
Why Is Soil So Important?
Although soil is a renewable natural resource, it is only renewable over long periods of time, measured not in days or years but in decades and even centuries. The livelihood of farmers worldwide is dependent upon their management and nurturing of the soil. In turn, so too is the world's supply of food and fiber, and, ultimately, the well-being of the planet's population. The production of food and fiber for both humans and animals requires an underground plant environment that is favorable for plant growth. Soil affects the growth and development of all plants, whether for human or animal consumption. Soil provides an anchor, or medium, for plant roots to take in oxygen, moisture, and minerals, which are all vital to plant life.
What Is Soil?
Soil is made up of sand, silt, clay, organic matter, living organisms, and pore spaces which hold water and air. The percentage of sand, silt, and clay determines how soils are categorized. Soil particles vary greatly in size. A sand particle is much larger than a silt particle. Clay particles are by far the smallest and hold moisture and plant food elements much more effectively than larger particles. A certain amount of clay in all soil is important for this reason.
Depending on their origin, soils vary greatly in general composition. Some soils were formed as a result of rock breaking down over thousands of years; others developed as certain materials were deposited by water. A normal soil profile consists of three layers:
- soil bedrock (or if rock is not present, lower subsoil)
The depth of topsoil represents that soil which is normally plowed or tilled and contains the most organic matter or decaying plant parts. Deep-rooting plants send roots down into the subsoil, which is a well-defined layer immediately below the topsoil. If the soil is well drained, roots penetrate deeper into subsoil since oxygen is available at greater depths.
An ideal soil is about 50 percent solid material, consisting mainly of minerals and a small percentage of organic matter. The other 50 percent of this ideal soil is pore space, which consists of small holes between soil particles that are filled with water and air in different amounts. After rain or irrigation, the pores may be nearly filled with water and the air is pushed out. As the soil dries, the amount of water decreases and the pores gradually fill with air again. The ideal water-to-air ratio in the pores is about half and half, 50 percent air to 50 percent water.
The amount of moisture and air a soil holds depends on the soil structure and the type of soil. Sandy soils with large particles have large pore spaces. Pore space can be illustrated by comparing a door or window screen to nylon stockings. The screen represents the large pore spaces found in sandy soil, and the nylon represents clay soil with small pore spaces. If you dip each into water, you can observe the differences in the two as to how they hold water. Water is lost more quickly from these large pores as the force of gravity drains the water out; these are well-drained soils. As the content of clay in the soil increases, more water is held. If soils contain too much clay, they may not drain well enough to allow enough oxygen in the pore space for good plant growth.
Note: Collect and bring in samples for you students to see, touch, smell, and compare.
SANDY SOIL: Sandy (or light) soils are soils in which silt and clay make up less than 20 percent of the material by weight. These soils drain well, but have little capacity to hold moisture and plant food. Sandy soils have comparatively large particles that permit good aeration, quick passage of water, and quick warming.
CLAYEY SOIL: A clayey soil must contain at least 30 percent clay and is known as a heavy soil. Heavy soils have relatively poor drainage and aeration capabilities. Because of this, heavy soils tend to hold more moisture than is good for plants. However, this type of soil also holds fertilizer and plant food well, which can be beneficial to plant growth.
LOAMY SOIL: This is the most desirable soil for agricultural use. Loam is a mixture of approximately equal parts of sand, silt, and clay. If loamy soil has more sand than silt or clay, it is known as a sandy loam; more clay, it is known as a clayey loam; more silt, a silty loam.Download lesson as PDF
- To analyze the different properties of the varying types of soils and how each type responds to the presence of water
- 1 cup sugar/salt
- 1 cup flour
- Modeling clay
- 1 cup water
- Recording sheet/science journal
- This activity is best suited for a partnership of two students.
- Instruct your students to rub sugar or salt between their thumbs and fingers to give them an idea of what sand feels like. Have them write down a description of the texture of the material that they are using in their journals or on the recording sheets.
- Next, have your students review the Soil Sizes - Some Surprises!, an online lesson plan on the NASA website that helps students differentiate between types of soil. This will further reinforce what they are about to do in the activity.
- Have them add a few drops of water to the sugar or salt. Ask them to write down a description of this material in their journals or on their recording sheets. Then have them respond to the following questions: Do the crystals stick together? Why?
- Instruct them to feel the flour between their fingers and ask them to record a description of how it feels. Explain to them that the flour represents how silt feels.
- Next, have them add a few drops of water to the flour. How does it feel now? Does it stick together? Have your students respond to these questions in their journals or on their recording sheets.
- Repeat this same experience again, except this time, have your students feel the modeling clay, as you explain to them that this is how clay in the soil feels.
- Then, have them add a few drops of water to the clay. How does it feel now? Is there any difference? Again,ask them to respond to these questions in their journals or on their recording sheets.
- To end the lesson, lead a class discussion regarding the responses that your students had for the previous questions. Guide the discussion to the topic of how each material reacted to the presence of water, noting the differences in the texture and form. Raise the point that the varying surfaces of the Earth react to precipitation, the presence of water sources, etc. differently, causing constant changes in the Earth’s surface.
Lesson Plan Assets
- An online lesson plan on the EPA website that will reinforce the idea that soil is made up of many things and that time is important to the formation of soil
- An online activity that helps students understand the various horizon levels in a soil sample
Project 2061 Benchmarks
- 4C/2 Change is something that happens to many things.
- 4C/E1 Waves, wind, water, and ice shape and reshape the earth’s land surface by eroding rock and soil in some areas and depositing them in other areas, sometimes in seasonal layers.
- 4D/1 Objects can be described in terms of the materials they are made of and their physical properties.
- recording data
- Generation of Ideas