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.