Earth Science: 5.2 SOIL Soil Key Concepts for Today . . . . Today students will learn and identify What are the major components of soil What are the most important factors in soil formation How soil varies with depth The three common types of soil How human activities affect the rate of soil erosion Soil Soil, an important product of weathering, covers most land surfaces. Along with air and water, soil is one of our most important resources. All life depends on a dozen or so elements that comes from the Earth’s crust. Once weathering and other processes create soil, plants absorb the elements and make them available to animals, including humans. Characteristics of Soil Soil has four major components Mineral matter (broken down rock) Organic matter (humus- the decayed remains of organisms) Water Air Soil Composition Good soil composition The proportions of these components vary in different soils. 45% mineral matter 5% organic matter In a good quality surface soil, mineral matter and organic matter generally make up half (50%) the mixture. 25% water 25% air m ineral m atter 25% The other half consists of pore spaces where air and water circulate generally in equal amounts (25% each) 45% organic m atter w ater air 25% 5% Soil Composition The percentage of organic matter in soils vary greatly Bog soils are composed almost entirely of organic matter, decayed plant and leaf material Desert soils may contain only a small amount of organic matter In most soils, organic matter or humus is an essential component. It is an important source of plant nutrients and increases the soil’s ability to hold water. Soil Composition The water and air components of soil are also vital for plant growth. Soil water provides the moisture for chemical reactions that sustain life Soil water also provides nutrients in a form that plants can use. Air is the source of carbon dioxide plants use to produce sugar during photosynthesis Soil Texture Most soils contain particles of different sizes Soil texture refers to the proportions of different particle sizes. To classify soil textures, the US Department of Agriculture has established categories based on the percentages of clay, silt, and sand in soil. Soil Texture For example: loam falls in the area between 50% to 25% sand, 25% to 50% silt, and 28% to 40% clay as shown by the chart In soils called loam, neither clay, sand or silt is dominant. Soil Texture Texture strongly influences a soils ability to support plant life. Sandy soils drain very quickly and do not hold water while clay-rich soils drain slowly. Soil Texture Plant roots often have difficulty penetrating soils that contain a high percentage of clay and silt. Loam soils are usually the best for plant growth. They retain water better and store more nutrients than do soils composed mainly of clay or sand. Soil Structure Soil particles usually form clumps that give soils a particular structure. Soil structure determines how easily a soil can be cultivated and how susceptible it is to erosion. Soil structure also affects the ease with which water can penetrate the soil. This in turn influences how much nutrients get to the plant roots. Soil Formation Soil Formation: soil forms through the complex interaction of several factors. The most important factors in soil formation are Parent material Time Climate Organisms slope Soil Formation Parent Material: The source of mineral matter in soil is known as parent material bedrock is the native consolidated rock underlying the planet Soil that forms on bedrock is called residual soil Soil that forms on unconsolidated deposits (areas of rock/minerals built up on bedrock) is called transported soil Soil Formation: Parent Material The nature of the parent material influences soil in two ways: First, because unconsolidated deposits are already partly weathered, they provide more surface area for chemical weathering. Therefore, transported soil usually develops more rapidly than residual soil develops Second, the chemical makeup of the parent material affects the soil’s fertility affecting the types of plants that will grow. Soil Formation: The longer a soil has been forming, the thicker it becomes. The parent material largely determines the characteristics of young soils. As weathering continues, the influence of the parent material can be overshadowed by other factors. Time Soil Formation: Climate Climate has the greatest affect on soil formation. Variations in temperature and precipitation influence the rate, depth, and type of weathering. For example; a hot, wet, climate may produce a thick layer of chemically weathered soil where a cold climate may produce a thin layer of mechanically weathered soil. The amount of precipitation also affects soil fertility, influencing the rate at which nutrients are removed from the soil Soil Formation: Organisms The types of organisms and how many there are in a soil have a major impact on it’s physical and chemical properties. Plants are the main source of organic matter in soil. Animals and microorganisms also contribute. Earthworms , for example, mix soil as they burrow and feed on the organic matter it contains. Microorganisms ( including fungi, bacteria, and protozoans ) play an active role in decomposing dead plants and animals. Because organic matter releases nutrients when it decays, it contributes to soil fertility. Soil Formation: Slope The slope of land can vary greatly in a short distance. This variation can result in many types of soils. Many of these differences are related to the amount of erosion and the water content of the soil. On steep slopes, erosion is accelerated, so soil is thin or nonexistent. On flat areas, erosion is minimal, so soils are typically thicker. The soil tends to be more waterlogged resulting in darker, thicker soil with more organic matter. Soil Profile The processes that form soil work from the surface downward Soil varies in composition, texture, structure, and color at different depths. These variations divide soil into zones called soil horizons. Soil Profile: A Horizon The A horizon is commonly known as the topsoil. It’s upper part consists mostly of organic matter, including loose leaves and partly decomposed plant structures. It is teeming with insects, fungi, and microorganisms. The lower part of A is a mixture of mineral matter and organic matter Soil Profile: B Horizon The B horizon, or subsoil, contains fine clay particles washed out of the A horizon by water that filters through the pore spaces. The B horizon is the lower limit of most plant roots and burrowing animals. Soil Profile: C Horizon Between the B horizon and the unaltered parent material is the C horizon, which contains partially weathered parent material. While the A and B horizons barely resemble the parent material, the C horizon does. Soil Types Recall that climate is the most important factor in soil formation. Climate also has a major affect on the type of soil that forms. Three common types of soil are Pedalfer Pedocal Laterite Pedalfer Soil Pedalfers usually form in temperate areas that receive more than 63 cm of rain each year. This soil type is present in much of the Eastern US, most often in forested areas. The B horizon in pedalfers contains large amounts of iron oxides and aluminum-rich clays giving it a brown-red color. Pedocal Soil Pedocols are found in the dryer western US in areas that have grasses and brush vegetation. Because chemical weathering slower in dry climates, pedocols generally contains less clay than pedalfers. Pedocols contain abundant amounts of calcite or calcium carbonate and are typically a light grey-brown in color. Laterite Soil Laterites form in hot, wet, tropical areas. Chemical weathering is intense in these areas. Laterites go deeper than soils that develop in other areas over the same amount of time. The large amounts of water in these climates filters out the calcite and silica in these soils washing them out when it rains. The iron oxide that is left behind gives the soil a distinctive red color. Laterite contains almost no organic materials and thus few nutrients. Laterite is one of the poorest soils there is for farming. Laterite Soil When dried, laterite becomes hard and virtually waterproof. It is cut and shaped into bricks and used for construction in other parts of the world Temple of Angkor Wat, Cambodia Soil Erosion Because soils are necessary for the growth of rooted plants, they are the foundation of our own food supply. The loss of topsoil in an area can make an area no longer able to sustain crops and grow food. Destruction of topsoil through poor farming practices is an important environmental issue that affects farmers all over Earth Soil Erosion: Water and Soil Erosion How Water Erodes Soil: Every time it rains, raindrops strike the soil with amazing force. Each drop acts like a minute bomb, blasting soil particles off the surface. Water flowing across the ground’s surface in thin sheets than carries these particles away as sediments. Because thin sheets of water move the soil particles away, this process is called sheet erosion. Soil Erosion: Water and Soil Erosion How Water Erodes Soil: After flowing as a thin sheet of water, the water forms tiny streams called rills. As more water enters the rills, they erode further turning into deeper gullies, eventually moving down slope to streams. These streams carry the sediments away until the water eventually slows enough to deposit them. Rates of Erosion: In the past, soil eroded more slowly than it does today because the land was covered with trees, grasses, plants and forests. The roots of all these hold the soil together and prevent erosion. Any human activities that remove plants or vegetation; such as logging, farming, and construction, greatly accelerate erosion. Rates of Erosion: Without plants, soil is more easily carried away by wind and water. Water generally erodes soil much faster than wind does. During a strong drought however, strong winds can remove large quantities of soil when all the plant material has died and the soil is revealed. Rates of Erosion: The rate of soil erosion depends on soil characteristics and on factors such as climate, slope, and type of vegetation. In many regions, including about one third of the world’s farmlands, soil is eroding faster than it is being formed. This results in lower productivity, poorer crop quality, which threatens the world’s food supplies Sediment Deposition Another problem caused by excessive soil erosion is the deposition of sediment. Rivers that accumulate sediments must be dredged to remain open for shipping. As sediments settle in reservoirs they become less useful for storing water, controlling floods or generating electricity Sediment Deposition Some sediments are contaminated with agricultural pesticides. These wash down streams to rivers, lakes and eventually the ocean, contaminating the water supplies and killing fish and plants. Soil also contains soil nutrients. When these nutrients wash into lakes, they stimulate the growth of algae and plants in the lake leading to algae blooms crowding out other life forms in the lake, killing the lake’s ecosystem. Controlling Erosion Although we can not eliminate erosion, we can slow it down using soil conservation measures and erosion control techniques. These measures include Planting rows of trees called windbreaks Plowing along the contours of hills to stop sheet runoff Rotating crops Using erosion barriers to prevent sediments from moving during construction Computer Lab: Review the PowerPoint lecture and use resources off the internet to write a one page report in Word answering one of the following questions: How does climate affects soil formation? Explain with three detailed examples. How can activity such as road construction affect soil erosion? Explain and give three effective erosion control techniques that prevent erosion in this situation. Which activity would cause more sediment to be deposited in a river that flows through a gently sloping valley: cultivating the valley or cultivating the hills that surround the valley? Explain your answer fully.