LESSON 1 Soil Guiding Question: What is soil? Explain three processes by which soil forms. Describe the horizons that make up a soil profile. List the four characteristics used to classify soil. Reading Strategy Before you read, create an outline using the dark blue, green, and light blue headings in this lesson. As you read, fill in key phrases about each heading. Vocabulary soil, parent material, bedrock, weathering, soil horizon, soil profile, clay, silt, sand, loam 12.1 LESSON PLAN PREVIEW Real World Students relate concepts of weathering to their local environment. Inquiry Students make a simple model of soil horizons. Differentiated Instruction Less proficient readers create a tool to help them learn about soil characteristics. 12.1 RESOURCES Scientific Method Lab, Testing Soil Properties Lesson 12.1 Worksheets Lesson 12.1 Assessment Chapter 12 Overview Presentation GUIDING QUESTION FOCUS Show students a small jar or plastic bag that contains garden soil or potting soil. Tell students that many people think of this substance as dirt, a material with little or no value. Explain that what you are holding is soil, and it is one of Earth s most valuable resources. Figure 1 Influences on Soil Formation Soil formation processes are influenced by outside factors such as climate, organisms, landforms, parent material, and time. We use about 38 percent of Earth s land surface for agriculture. Our lives depend on agriculture we must grow most of our food and fiber. And agriculture depends on fertile soil. But what, exactly, is soil? You might think of soil as an inconvenience as dirt that you wash off your body and clothes. But soil is much more than dirt. It is a complex, life-filled, life-giving substance. Healthy soil is essential for agriculture, forestry, the cycles of matter, and the flow of energy that keep Earth s ecosystems running. Some soil microorganisms even provide us with medicines, such as antibiotics. Soil is a complex plant-supporting system made up of disintegrated rock, remains and wastes of organisms, water, gases, nutrients, and microorganisms. Figure 1 describes several factors in involved in soil formation. Soil is a renewable resource; once its nutrients are used up, it can renew itself over time. But it may take a very long time hundreds or thousands of years for just one inch of topsoil to form. So if we deplete soil by using up all its nutrients, we ruin it not just for ourselves but for generations to come. Factor Climate Organisms Landforms Parent material Time Factors That Influence Soil Formation Effects Soil forms faster in warm, wet climates. Heat speeds chemical reactions, weathering, decomposition, and growth of organisms. Moisture is required for many biological processes so it speeds weathering. Earthworms and other burrowing animals mix and aerate soil, add organic matter, and speed decomposition. Plants add organic matter and affect a soil s composition and structure. Hills and valleys affect exposure to sun, wind, and water. Steeper slopes promote runoff and erosion; they also slow leaching, accumulation of organic matter, and formation of soil layers. Chemical and physical attributes of parent material influence properties of the soil formed from it. Soil formation takes decades, centuries, or millennia. Adapted from Jenny, H. 1941. Factors of soil formation: A system of quantitative pedology. New York: McGraw-Hill, Inc. Reprinted 1994 by Dover Publications, Mineola, New York. 352 Lesson 1
Soil Formation Soil is a complex substance that forms through weathering, deposition, and decomposition. We often overlook the complexity of soil. Though soil is mostly broken rocks, water, and air, soil also contains a great deal of life (Figure 2). Soil consists of roughly 45 percent mineral matter and 5 percent organic (living or once-living) matter. Water and air in the spaces, or pores, between soil particles make up the other 50 percent. The mineral matter in soil is made up of tiny particles of rock. The organic matter includes decomposing organisms as well as living microorganisms. You might be surprised to find that one teaspoon of soil can contain millions of bacteria and thousands of fungi, algae, and protists! Soil also provides a habitat for larger animals, including hundreds of thousands of invertebrate species, amphibians, reptiles, and burrowing mammals. As you recall, the formation of soil plays a key role in primary succession, which begins when the parent material in an area is exposed. Parent material is the base geological material in a particular location. It can be lava or volcanic ash; rock or sediment deposited by glaciers; sand dunes; sediments deposited by rivers, in lakes, or in the ocean; or bedrock. Bedrock is the continuous mass of solid rock that makes up Earth s crust. After parent material is exposed to the air, the processes that form most soils are weathering, deposition, and the decomposition of organic matter. Reading Checkpoint List five different types of parent material. Litter layer Topsoil Sowbug Soil fungi Snail Reading Checkpoint Any five of the following: lava, volcanic ash, rock or sediment deposited by glaciers, sand dunes, sediments deposited by bodies of water, bedrock Mole Slug Cicada nymph Earthworm Mite Beetle grub Figure 2 Soil Complexity Soil is a complex mixture of abiotic and biotic components, including many organisms whose actions help keep it fertile. Most soil organisms, from bacteria to fungi to insects to earthworms, decompose organic matter. Many, such as moles and earthworms, help to aerate the soil. Entire ecosystems exist in soil. Subsoil Protists Bacteria Soil and Agriculture 353
Weathering The first process in soil formation is often weathering. Weathering describes the physical and chemical processes that break down rocks and minerals into smaller particles. Physical Weathering Anything that touches a rock can cause physical weathering, which is the natural breakup of rock without a chemical change. Wind and rain are two main causes of physical weathering. Daily and seasonal temperatures also contribute to physical weathering, since parent material and rocks weaken as they repeatedly expand with heat and contract with cold. For this reason, areas with extreme temperature fluctuations undergo rapid physical weathering. Water that freezes and expands in cracks in rocks also causes physical weathering. Living things, such as a tree whose roots break up rocks as they grow, also add to physical weathering. Chemical Weathering When water and other substances chemically break down parent material and rocks, transforming them into different materials, chemical weathering is occurring. Living and once-living things also cause chemical weathering with chemical products of their life processes and decomposition, such as water and carbon dioxide. Warm, wet conditions increase chemical weathering. Deposition Erosion is often viewed as a destructive process. But it frequently plays a part in soil formation. Erosion may help form soil in one area by depositing material eroded from another. Deposition, as you recall, is the drop-off of eroded material at a new location. You will read more about soil erosion and deposition later in this chapter. Figure 3 Humus You can tell a lot about a soil s fertility just by looking at it. Very dark soil has a high proportion of humus, so it is very fertile. Decomposition The activities of living things and the decomposition of formerly living things also help form soil. As plants, animals, and microorganisms deposit waste or die and decompose, nutrients are incorporated into the soil. Deciduous trees, for example, drop their leaves each fall. These dead leaves and other vegetation make up leaf litter. Leaf litter is broken down by decomposers and detritivores, and its nutrients become part of the soil. Partially decomposed organic matter is known as humus, a dark, spongy, crumbly mass of material made up of complex organic compounds. Soils with high humus content, such as that in Figure 3, hold moisture well and contain many plant nutrients. 354 Lesson 1
Soil Horizons A soil profile consists of layers known as horizons. As soon as soil begins to form, wind, water, and organisms move the particles. Eventually, the soil is sorted into distinct layers, or soil horizons. A cross-section of all the soil horizons in a specific soil, from surface to bedrock, is known as a soil profile. The simplest way to categorize soil horizons is by the A, B, and C horizons, which correspond to topsoil, subsoil, and weathered parent material, respectively. However, soil scientists often subdivide those layers. The six major soil horizons are the O, A, E, B, C, and R horizons (Figure 4). Soils from different locations vary, and few soil profiles contain all six of these horizons, but every soil contains at least some of them. Topsoil A crucial horizon for agriculture and ecosystems is the A horizon, or topsoil. Topsoil consists mostly of mineral particles such as weathered parent material mixed with organic matter and humus from the O horizon. Topsoil is the horizon that has the most plant nutrients available. Its loose texture, dark color, and ability to hold water come from its humus content. The O horizon, or litter layer, and the A horizon are home to most of the countless organisms that give life to soil. For all its vitality, topsoil is fragile. Agriculture practiced carelessly can deplete the nutrients in topsoil or erode it, as you will read in the next lesson. O Horizon Litter layer A Horizon Topsoil E Horizon Leaching Layer B Horizon Subsoil C Horizon Weathered parent material R Horizon Parent material Figure 4 Soil Horizons Mature soil consists of horizons that have different compositions and characteristics. The surface layer is the O horizon, or litter layer, which consists mostly of organic matter, such as dead leaves. Below it lies the A horizon, or topsoil, which consists of organic matter mixed with minerals. Minerals and organic matter tend to leach out of the E horizon down into the B horizon, or subsoil, where they accumulate. The C horizon consists largely of weathered parent material unaltered or only slightly altered by the processes of soil formation. The C horizon may overlie an R horizon of parent material. Lower Horizons Generally, as one moves downward through a soil profile, the particle size increases and the concentration of organic matter decreases. Minerals are generally transported downward as a result of leaching, the process whereby solid particles suspended or dissolved in liquid are transported to another location. Soil that undergoes leaching is a bit like the coffee grounds in a filter. When it rains, water filters through the soil, dissolves some soil components, and carries them downward into the lower horizons. Minerals commonly leached from the E horizon include iron, aluminum, and silicate clay. In some soils, minerals may be leached so rapidly that plants are deprived of nutrients. Substances that leach from soils may be carried into groundwater, and some can pose threats to human health. Reading Checkpoint In which horizon would you expect to find soil like that in Figure 3? Reading Checkpoint The A horizon, or topsoil Soil and Agriculture 355
Go Outside Classifying Soil 1 While wearing safety gloves, collect a small handful of soil from an area chosen by your teacher. 2 Close your fist around the soil as tightly as you can. Put the clump on a sheet of white paper. Observe the speed at which the clump falls apart. 3 Using the tips of two pencils, separate some of the soil into groups of similar-sized particles. Analyze and Conclude 1. Observe Describe the color of the soil. 2. Classify Based on particle sizes in your sample, describe the relative amounts of clay, silt, and sand. Which is most common? Would you describe your sample as loam? 3. Classify Given the behavior of the soil after you released it, describe its structure. (Hint: How clumpy was it?) 4. Predict How do you think a farmer would describe the soil s workability? Support your answer with your observations. Go Outside For Questions 1 4, answers will vary, but should be consistent with students soil samples and the content in the text. Soil Characteristics Soils can be classified by their color, texture, structure, and ph. The six horizons presented on the previous page make up a common soil profile, but soils display great variety. The characteristics of a region s soil can have as much influence on the region s ecosystems as do the climate, latitude, and elevation. U.S. soil scientists classify soils into 12 major groups, based largely on the processes that form the soils. Scientists further classify soils using properties such as color, texture, structure, and ph. Color The color of a soil reveals details about its composition and fertility. Dark soils are usually rich in humus and therefore nutrients, whereas pale soils often have less humus and nutrients. Long before scientific tests of soil content were developed, farmers and ranchers often used the color of topsoil as an indicator of a soil s fertility. Texture Soil texture is based on particle size (Figure 5). Clay consists of particles less than 0.002 millimeter in diameter, silt consists of particles 0.002 to 0.05 millimeter in diameter, and sand consists mostly of particles 0.05 to 2 millimeters in diameter. Sand particles are large enough to see individually and do not adhere to one another. Clay particles, in contrast, adhere easily to one another. Most soils are a combination of clay, silt, and sand. Soil with a relatively even mixture of the three particle sizes is known as loam. Figure 5 Classifying Soil Texture Scientists use this triangular diagram to classify soil texture. After determining the percentages of sand, silt, and clay particles in a soil sample, you can trace a line inward from each side of the triangle: horizontal for clay, diagonally downward for silt, and diagonally upward to the left for sand. The intersection of the lines reveals the soil texture. Percent clay 60 70 50 Sandy 40 clay 80 90 100 0 Clay 10 Clay loam 20 30 40 Silty clay Percent silt 50 Silty clay loam 30 Sandy 70 clay loam 20 Loam 80 Silt loam 10 Sandy loam 90 Loamy Silt Sand 0 sand 100 100 90 80 70 60 50 40 30 20 10 0 Percent sand 60 Sand (0.05 2 mm diameter) Clay (less than 0.002 mm diameter) Silt (0.002 0.05 mm diameter) 356 Lesson 1
Soil texture influences a soil s workability essentially, how easy it is to plant in and harvest from. Soil texture also indicates how porous a soil is, or the size of the spaces between its particles. In general, the finer the particles, the smaller the spaces and the harder it is for water and air to travel through the soil to roots and soil-nourishing microorganisms. Clay has the finest particles, so it is the least porous soil. Sandy soils are the most porous. Silty soils with medium-sized pores, or loamy soils with a mixture of pore sizes, are generally best for plant growth. Reading Checkpoint Using Figure 5, classify the texture of a soil that is 40 percent sand, 40 percent silt, and 20 percent clay. Structure Soil structure describes the arrangement of soil particles. You can see it in the clumpiness of soil. Clumpy soil may have a great deal of humus, indicating that the soil is rich in nutrients and able to hold water. However, soil clumps that become too large or densely compacted, from heavy equipment (Figure 6) or grazing cattle, for example, can prevent plant roots from growing. ph A soil s acidity or alkalinity affects its ability to support plant growth. Different plants require different ph levels, and plants die in soils that are too acidic or alkaline for them. Soil ph varies naturally, but acid precipitation and the subsequent leaching of minerals from the soil can also affect the ph of soil. Few plants can grow in extremely acidic or alkaline soil. Reading Checkpoint Loam Lesson 1 Assessment 1. Weathering, the physical and chemical processes that break down rocks and minerals; deposition, the depositing of soil eroded from other areas; decomposition, the breakdown of dead organisms and wastes 2. A cross-section of all soil horizons in a specific soil; the A horizon is topsoil, B is subsoil, and C is weathered parent material. 3. Color indicates the amount of organic matter in soil; texture indicates a soil s workability and porosity; structure indicates the arrangement of soil particles and how easily roots can penetrate; ph determines which plants can survive very few survive if the ph is very low or very high. 4. The soil coffee would consist of water and soil components such as minerals, nutrients, and other substances dissolved by water. Figure 6 Compacted Soil Clumpy soil with a great deal of humus is good for most plants. However, clumpy soil can easily become compacted by heavy machinery, as in this photo, or by animals. Compacted soil may be too dense for plants to grow in. 1 1. Explain Describe three major processes that contribute to the formation of most soils. 2. Review What is a soil profile? Describe the A, B, and C horizons. 3. Classify What do each of the four characteristics of soil indicate about its ability to support plant life? 4. Recall the analogy between soil and coffee grounds in the section called Lower Horizons. In this analogy, what do you think the soil coffee consists of? Soil and Agriculture 357