TEKS examine properties of soils, including color and texture, capacity to retain water, and ability to support the growth of plants Properties of Soil What Is Soil? Soil may be referred to as dirt, mud, muck, earth, clay, silt, or sand. There are so many words for soil because there are so many different kinds of soil. The soil in one place can be very different than the soil less than a kilometer away. In the United States alone, there are more than 70,000 different kinds of soil! No matter how different soils are, they all have some similarities. Soil is a mixture of small bits of weathered rocks, minerals, water, and the decaying remains of once-living things. One of the main components of soil is humus. Humus is the part of soil made up of dead plants and animals. For example, when a tree loses a leaf, the leaf falls to the ground. As decomposers break the leaf down into smaller parts, it becomes humus. You might not think of soil as a resource, but it is a very important one. Without soil, plants couldn t grow and many animals would not have a place to live. Some soils are used in manufacturing processes. For example, some soils can be used to make bricks or other building materials. Soil Layers and Formation You ve learned that weathering is the process of breaking down rocks into smaller pieces. All soils form from weathered rock. Soil can form from the weathered rock right under it, or it can form from eroded sediment carried from far away. If you dig down into soil, you would find that it is made of up several layers. Each layer has different properties. The top layer is called topsoil. This is the layer in which most biological activity occurs. Animals, plants, fungi, and bacteria all live here. If you dig down far enough, you would find bedrock. Bedrock is solid, unweathered rock that lies beneath the loose surface deposits of soil. Soils form in layers. Each layer has different characteristics. topsoil This pot is made of a type of soil called clay. bedrock and Assessment TEKS Page 1 of 7
Soil Color The most obvious property when looking at soil is its color. The color of soil can tell scientists a lot about it. Geologists officially recognize more than 170 different soil colors. Most of these are shades of black, brown, red, gray, and white. Many types of soil, called loam, have a mixture of sand, silt, and clay particles. Wellbalanced soils are the best ones to support plant growth. Farms are often built on locations with loam. In general, the darker a soil, the more nutrient-rich it is. The darker color often indicates a high amount of humus. Gray soils often have poor drainage, and red soils are usually very undeveloped, poor soils. These general rules about soil colors can, however, be misleading. Under certain conditions, a very poor soil can be dark black, while a rich, healthy soil can look red. Soil Texture Another property of soil is its texture. Pick up a pinch of soil between your fingers, and rub it back and forth. You will quickly notice that the soil is made up of many different-size particles. These particles give the soil its texture. Soil texture refers to the relative proportion of sand, silt, and clay in a sample of soil. These particles differ in size. Sand particles are the largest, silt particles are medium-sized, and clay particles are tiny. Pure clay feels smooth when you rub it between your hands. Particle clay silt sand Particle Size in Soils Particle Size less than 0.002 mm 0.002 mm 0.05 mm greater than 0.05 mm Soil Structure The mixture of particle sizes in a soil affects many other properties, including how much water it can hold and how fast water drains through it. Soils have different amounts of open space, called pores, between particles. Pores can also be filled with water. Soils with larger pore spaces can hold more water. Permeability refers to how quickly water drains into or moves through a soil. This speed depends on how large the soil pores are. A highly permeable soil is one in which water runs though it quite easily. Sandy soils have large, well-connected pore spaces and high permeability. Water does not move through clay soils easily. Nutrients Because soils are made from rocks, they contain minerals. The kinds of minerals found in any soil depend on the kind of rock from which the soil formed. Certain minerals are needed for plant growth. and Assessment TEKS Page 2 of 7
In order for plants to obtain the nutrients they need to survive, nutrients must be carried into them. This happens as nutrients are dissolved into the groundwater that the plants absorb through their roots. As humus decays, it adds organic matter in the form of humus to the soil. Humus improves soil moisture retention while affecting soil chemistry. If a soil has too much acid in it, it is said to be acidic. In this kind of soil, the nutrients in the soil are dissolved too quickly and wash away as the water drains. If a soil is too alkaline if there is not enough acid then nutrients will not dissolve quickly enough. Thus, a neutral soil, which is neither too acidic nor too alkaline, is the best kind of soil for plants. Soils Hold Water There are many properties of soil that you can examine. In this activity, you will measure how well two types of soil hold water. Procedure: Hands-On Activity Materials potting soil sand measuring cup coffee filter 2 funnels 2 ring stands water 2 beakers graduated cylinder 1. Place a funnel in each ring stand, and place a coffee filter inside each funnel. Put a beaker beneath the funnel. 2. Fill one of the filters with potting soil. Measure the volume of soil you add. 3. Add the same amount of sand to the other filter. 4. Put 200 ml of water into a graduated cylinder. Slowly pour the water into the potting soil. Stop as soon as you see water dripping from the bottom of the funnel. Record how much water the soil held. 5. Repeat Step 4 with the sand. Discussion Questions: 1. How do the two types of soil compare? 2. Where can you infer the water poured into the soil went? and Assessment TEKS Page 3 of 7
Hands-On Activity Soils and Plant Growth Are the soils in your area good for growing plants? Does soil type affect how well a plant grows? You will answers these questions in this investigation. Procedure: Materials at least 3 soil samples gloves and goggles measuring cup planting pots water seeds 1. With a trusted adult, collect a sample of soil and label the location where it was collected. Bring it into the class and share the sample with your group. 2. Analyze each soil sample. Record properties of the soil, including the soil s color and texture. Determine texture by rubbing the soil between your fingers and by observing it with a hand lens. 3. Measure enough soil to fill a planting pot half way. Add a few seeds, and cover them with another inch of soil. Label the pot. 4. Repeat Steps 2 and 3 with the soil samples from the other members of your group. Be sure to keep the amount of soil and number of seeds used constant. 5. Over the next few weeks, add water to the pots as they dry out. All of the pots need to receive the same amount of water. Record how well the plants grow. You could record the number of leaves or the height of the plants. 6. Evaluate your results and compare them with your analysis of the soil samples. As a group, come up with a scientific explanation. Discussion Questions: 1. How well did the plants grow in each type of soil? 2. How did you control variables in this experiment? 3. Describe the process your group used to come up with an explanation you all agreed on. and Assessment TEKS Page 4 of 7
Properties of Soil List the three particle sizes that make up soil in order from smallest to largest. What do plants obtain from soil? How does the texture of clay differ from the texture of sand? Draw what the soil near your home looks like through a hand lens. Label any parts you can. and Assessment TEKS Page 5 of 7
TEKS Assessment Fill in the letter of the best choice. Soil is made up of four main ingredients. Choose the correct list of ingredients. tiny pieces of rock; humus; air; sand humus, or dead plant material; clay; water; salts tiny pieces of rock; air; water; humus or dead plant material air; water; tiny pieces of rock; manure Which type of soil dries out quickly after a rainstorm? clay soil loam silty soil sandy soil Use the drawing below to answer questions 5 and 6. Which label points to bedrock? A B C D A B C D Which soil ingredient is made of decayed plants and animals? air humus minerals water Which type of soil feels the smoothest when you rub it between your fingers? Which label points to the layer where you would expect to find the most humus? A B C D loam clay silt sand and Assessment TEKS Page 6 of 7
Answer Key Hands-On Activity (p. 3) 1. Students should find that the potting soil held more water than the sand. 2. Water that was poured into the soil filled the spaces between the pores. Hands-On Activity (p. 4) 1. Answers will vary but should be based on observations and measurements. 2. Sample answer: I controlled variables by ensuring that the only difference between the pots was the type of soil in the pots. All other variables, including the number of seeds, the amount of water added to each pot, and the amount of light the plants received, were controlled (kept constant). 3. Sample answer: We discussed our data and compared our results. We used logic to eliminate explanations that didn t make sense. We discussed what inferences and conclusions we all agreed on, then we debated those we didn t agree on. Student Response Activity (p. 5) 1. clay; silt; sand 2. Plants obtain water and nutrients from soil. 3. Clay feels smooth when you rub it between your fingers. Sand feels gritty. You can feel the individual particles of sand. The clay particles are too small for you to feel them individually. 4. Check students drawings. Provide soil samples for those students who are unable to obtain them at home. TEKS Assessment (p. 6) 1. C 2. D 3. B 4. B 5. D 6. A and Assessment TEKS Page 7 of 7