Teacher Notes

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:

• topsoil
• subsoil
• 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.

Soil Types

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.