Soil. I INTRODUCTION Soil, the loose material that covers the land surfaces of

Soil. I INTRODUCTION Soil, the loose material that covers the land surfaces of Earth and supports the growth of plants. In general, soil is an unconsolidated, or loose, combination of inorganic and organic materials. The inorganic components of soil are principally the products of rocks and minerals that have been gradually broken down by weather, chemical action, and other natural processes. The organic materials are composed of debris from plants and from the decomposition of the many tiny life forms that inhabit the soil. Soils vary widely from place to place. Many factors determine the chemical composition and physical structure of the soil at any given location. The different kinds of rocks, minerals, and other geologic materials from which the soil originally formed play a role. The kinds of plants or other vegetation that grow in the soil are also important. Topography--that is, whether the terrain is steep, flat, or some combination--is another factor. In some cases, human activity such as farming or building has caused disruption. Soils also differ in color, texture, chemical makeup, and the kinds of plants they can support. Soil actually constitutes a living system, combining with air, water, and sunlight to sustain plant life. The essential process of photosynthesis, in which plants convert sunlight into energy, depends on exchanges that take place within the soil. Plants, in turn, serve as a vital part of the food chain for living things, including humans. Without soil there would be no vegetation--no crops for food, no forests, flowers, or grasslands. To a great extent, life on Earth depends on soil. The study of different soil types and their properties is called soil science or pedology. Soil science plays a key role in agriculture, helping farmers to select and support the crops on their land and to maintain fertile, healthy ground for planting. Understanding soil is also important in engineering and construction. Soil engineers carry out detailed analysis of the soil prior to building roads, houses, industrial and retail complexes, and other structures. Soil takes a great deal of time to develop--thousands or even millions of years. As such, it is effectively a nonrenewable resource. Yet even now, in many areas of the world, soil is under siege. Deforestation, over-development, and pollution from humanmade chemicals are just a few of the consequences of human activity and carelessness. As the human population grows, its demand for food from crops increases, making soil conservation crucial. II COMPOSITION OF SOILS Soils comprise a mixture of inorganic and organic components: minerals, air, water, and plant and animal material. Mineral and organic particles generally compose roughly 50 percent of a soil's volume. The other 50 percent consists of pores--open areas of various shapes and sizes. Networks of pores hold water within the soil and also provide a means of water transport. Oxygen and other gases move through pore spaces in soil. Pores also serve as passageways for small animals and provide room for the growth of plant roots. A Inorganic Material The mineral component of soil is made up of an arrangement of particles that are less than 2.0 mm (0.08in) in diameter. Soil scientists divide soil particles, also known as soil separates, into three main size groups: sand, silt, and clay. According to the classification scheme used by the United States Department of Agriculture (USDA), the size designations are: sand, 0.05 to 2.00 mm (0.002 to 0.08 in); silt 0.002 to 0.05 mm (0.00008 to 0.002 in); and clay, less than 0.002 mm (0.00008 in). Depending upon the rock materials from which they were derived, these assorted mineral particles ultimately release the chemicals on which plants depend for survival, such as potassium, calcium, magnesium, phosphorus, sulfur, iron, and manganese. B Organic Material Organic materials constitute another essential component of soils. Some of this material comes from the residue of plants--for example, the remains of plant roots deep within the soil, or materials that fall on the ground, such as leaves on a forest floor. These materials become part of a cycle of decomposition and decay, a cycle that provides important nutrients to the soil. In general, soil fertility depends on a high content of organic materials. Even a small area of soil holds a universe of living things, ranging in size from the fairly large to the microscopic: earthworms, mites, millipedes, centipedes, grubs, termites, lice, springtails, and more. And even a gram of soil might contain as many as a billion microbes--bacteria and fungi too small to be seen with the naked eye. All these living things form a complex chain: Larger creatures eat organic debris and excrete waste into the soil, predators consume living prey, and microbes feed on the bodies of dead animals. Bacteria and fungi, in particular, digest the complex organic compounds that make up living matter and reduce them to simpler compounds that plants can use for food. A typical example of bacterial action is the formation of ammonia from animal and vegetable proteins. Other bacteria oxidize the ammonia to form nitrogen compounds called nitrites, and still other bacteria act on the nitrites to form nitrates, another type of nitrogen compound that can be used by plants. Some types of bacteria are able to fix, or extract, nitrogen directly from the air and make it available in the soil. Ultimately, the decay of plant and animal material results in the formation of a dark-colored organic matter known as humus. Humus, unlike plant residues, is generally resistant to further decomposition. C Water Soil scientists also characterize soils according to how effectively they retain and transport water. Once water enters the soil from rain or irrigation, gravity comes into play, causing water to trickle downward. Water is also taken up in great quantities by the roots of plants: Plants use anywhere from 200 to 1,000 kg (440 to 2,200 lb) of water in the formation of 1 kg (2.2 lb) of dry matter. Soils differ in their capacity to retain moisture against the pull exerted by gravity and by plant roots. Coarse soils, such as those consisting of mostly of sand, tend to hold less water than do soils with finer textures, such as those with a greater proportion of clays. Water also moves through soil pores independently of gravity. This movement can occur via capillary action, in which water molecules move because they are more attracted to the pore walls than to one another. Such movement tends to occur from wetter to drier areas of the soil. The movement from soil to plant roots can also depend on how tightly water molecules are bound to soil particles. The attraction of water molecules to each other is an example of cohesion. The attraction of water molecules to other materials, such as soil or plant roots, is a type of adhesion. These effects, which determine the so-called matric potential of the soil, depend largely on the size and arrangement of the soil particles. Another factor that can affect water movement is referred to as the osmotic potential. The osmotic potential hinges on the amount of dissolved salts in the soil. Soils high in soluble salt tend to reduce uptake of water by plant roots and seeds. The sum of the matric and osmotic potentials is called the total water potential. In soil, water carries out the essential function of bringing mineral nutrients to plants. But the balance between water and air in the soil can be delicate. An overabundance of water will saturate the soil and fill pore spaces needed for the transport of oxygen. The resulting oxygen deficiency can kill plants. Fertile soils permit an exchange between plants and the atmosphere, as oxygen diffuses into the soil and is used by roots for respiration. In turn, the resulting carbon dioxide diffuses through pore spaces and returns to the atmosphere. This exchange is most efficient in soils with a high degree of porosity. For farmers, gardeners, landscapers, and others with a professional interest in soil health, the process of aeration--making holes in the soil surface to permit the exchange of air--is a crucial activity. The burrowing of earthworms and other soil inhabitants provides a natural and beneficial form of aeration. III SOIL FORMATION Soil formation is an ongoing process that proceeds through the combined effects of five soil-forming factors: parent material, climate, living organisms, topography, and time. Each combination of the five factors produces a unique type of soil that can be identified by its characteristic layers, called horizons. Soil formation is also known as pedogenesis (from the Greek words pedon, for "ground," and genesis, meaning "birth" or "origin"). A Parent Material The first step in pedogenesis is the formation of parent material from which the soil itself forms. Roughly 99 percent of the world's soils derive from mineral-based parent materials that are the result of weathering, the physical disintegration and chemical decomposition of exposed bedrock. The small percentage of remaining soils derives from organic parent materials, which are the product of environments where organic matter accumulates faster than it decomposes. This accumulation can occur in marshes, bogs, and wetlands. Bedrock itself does not directly give rise to soil. Rather, the gradual weathering of bedrock, through physical and chemical processes, produces a layer of rock debris called regolith. Further weathering of this debris, leading to increasingly smaller and finer particles, ultimately results in the creation of soil. In some instances, the weathering of bedrock creates parent materials that remain in one place. In other cases, rock materials are transported far from their source--blown by wind, carried by moving water, and borne inside glaciers. B Climate Climate directly affects soil formation. Water, ice, wind, heat, and cold cause physical weathering by loosening and breaking up rocks. Water in rock crevices expands when it freezes, causing the rocks to crack. Rocks are worn down by water and wind and ground to bits by the slow movement of glaciers. Climate also determines the speed at which parent materials undergo chemical weathering, a process in which existing minerals are broken down into new mineral components. Chemical weathering is fastest in hot, moist climates and slowest in cold, dry climates. Climate also influences the developing soil by determining the types of plant growth that occur. Low rainfall or recurring drought often discourage the growth of trees but allow the growth of grass. Soils that develop in cool rainy areas suited to pines and other needle-leaf trees are low in humus. C Living Organisms As the parent material accumulates, living things gradually gain a foothold in it. The arrival of living organisms marks the beginning of the formation of true soil. Mosses, lichens, and lower plant forms appear first. As they die, their remains add to the developing soil until a thin layer of humus is built up. Animals' waste materials add nutrients that are used by plants. Higher forms of plants are eventually able to establish themselves as more and more humus accumulates. The presence of humus in the upper layers of a soil is important because humus contains large amounts of the elements needed by plants. Living organisms also contribute to the development of soils in other ways. Plants build soils by catching dust from volcanoes and deserts, and plants' growing roots break up rocks and stir the developing soil. Animals also mix soils by tunneling in them. D Topography Topography, or relief, is another important factor in soil formation. The degree of slope on which a soil forms helps to determine how much rainfall will run off the surface and how much will be retained by the soil. Relief may also affect the average temperature of a soil, depending on whether or not the slope faces the sun most of the day. E Time The amount of time a soil requires to develop varies widely according to the action of the other soil-forming factors. Young soils may develop in a few days from the alluvium (sediments left by floods) or from the ash from volcanic eruptions. Other soils may take hundreds of thousands of years to form. In some areas, the soils may be more than a million years old. F Horizons Most soils, as they develop, become arranged in a series of layers, known as horizons. These horizons, starting at the soil surface and proceeding deeper into the ground, reflect different properties and different degrees of weathering. Soil scientists have designated several main types of horizons. The surface horizon is usually referred to as the O layer; it consists of loose organic matter such as fallen leaves and other biomass. Below that is the A horizon, containing a mixture of inorganic mineral materials and organic matter. Next is the E horizon, a layer from which clay, iron, and aluminum oxides have been lost by a process known as leaching (when water carries materials in solution down from one soil level to another). Removal of materials in this manner is known as eluviation, the process that gives the E horizon its name. Below E horizon is the B horizon, in which most of the iron, clays, and other leached materials have accumulated. The influx of such materials is called illuviation. Under that layer is the C horizon, consisting of partially weather bedrock, and last, the R horizon of hard bedrock. Along with these primary designations, soil scientists use many subordinate names to describe the transitional areas between the main horizons, such as Bt horizon or BX2 horizon. Soil scientists refer to this arrangement of layers atop one another as a soil profile. Soil profiles change constantly but usually very slowly. Under normal conditions, soil at the surface is slowly eroded but is constantly replaced by new soil that is created from the parent material in the C horizon. IV SOIL CHARACTERISTICS Scientists can learn a lot about a soil's composition and origin by examining various features of the soil. Color, texture, aggregation, porosity, ion content, and pH are all important soil characteristics. A Color Soils come in a wide range of colors--shades of brown, red, orange, yellow, gray, and even blue or green. Color alone does not affect a soil, but it is often a reliable indicator of other soil properties. In the surface soil horizons, a dark color usually indicates the presence of organic matter. Soils with significant organic material content appear dark brown or black. The most common soil hues are in the red-to-yellow range, getting their color from iron oxide minerals coating soil particles. Red iron oxides dominate highly weathered soils. Soils frequently saturated by water appear gray, blue, or green because the minerals that give them the red and yellow colors have been leached away. B Texture A soil's texture depends on its content of the three main mineral components of the soil: sand, silt, and clay. Texture is the relative percentage of each particle size in a soil. Texture differences can affect many other physical and chemical properties and are therefore important in measures such as soil productivity. Soils with predominantly large particles tend to drain quickly and have lower fertility. Very fine-textured soils may be poorly drained, tend to become waterlogged, and are therefore not well-suited for agriculture. Soils with a medium texture and a relatively even proportion of all particle sizes are most versatile. A combination of 10 to 20 percent clay, along with sand and silt in roughly equal amounts, and a good quantity of organic materials, is considered an ideal mixture for productive soil. C Aggregation Individual soil particles tend to be bound together into larger units referred to as aggregates or soil peds. Aggregation occurs as a result of complex chemical forces acting on small soil components or when organisms and organic matter in soil act as glue binding particles together. Soil aggregates form soil structure, defined by the shape, size, and strength of the aggregates. There are three main soil shapes: platelike, in which the aggregates are flat and mostly horizontal; prismlike, meaning greater in vertical than in horizontal dimension; and blocklike, roughly equal in horizontal and vertical dimensions and either angular or rounded. Soil peds range in size from very fine--less than 1 mm (0.04 in)--to very coarse--greater than 10 mm (0.4 in). The measure of strength or grade refers to the stability of the structural unit and is ranked as weak, moderate, or strong. Very young or sandy soils may have no discernible structure. D Porosity The part of the soil that is not solid is made up of pores of various sizes and shapes--sometimes small and separate, sometimes consisting of continuous tubes. Soil scientists refer to the size, number, and arrangement of these pores as the soil's porosity. Porosity greatly affects water movement and gas exchange. Well-aggregated soils have numerous pores, which are important for organisms that live in the soil and require water and oxygen to survive. The transport of nutrients and contaminants will also be affected by soil structure and porosity. E Ion Content Soils also have key chemical characteristics. The surfaces of certain soil particles, particularly the clays, hold groupings of atoms known as ions. These ions carry a negative charge. Like magnets, these negative ions (called anions) attract positive ions (called cations). Cations, including those from calcium, magnesium, and potassium, then become attached to the soil particles, in a process known as cation exchange. The chemical reactions in cation exchange make it possible for calcium and the other elements to be changed into water-soluble forms that plants can use for food. Therefore, a soil's cation exchange capacity is an important measure of its fertility. F pH Another important chemical measure is soil pH, which refers to the soil's acidity or alkalinity. This property hinges on the concentration of hydrogen ions in solution. A greater concentration of hydrogen results in a lower pH, meaning greater acidity. Scientists consider pure water, with a pH of 7, neutral. The pH of a soil will often determine whether certain plants can be grown successfully. Blueberry plants, for example, require acidic soils with a pH of roughly 4 to 4.5. Alfalfa and many grasses, on the other hand, require a neutral or slightly alkaline soil. In agriculture, farmers add limestone to acid soils to neutralize them. V SOIL CLASSIFICATION As yet there is no worldwide, unified classification scheme for soil. Since the birth of the modern discipline of soil science roughly 100 years ago, scientists in different countries have used many systems to organize the various types of soils into groups. For much of the 20th century in the United States, for example, soil scientists at the USDA used a classification scheme patterned after an earlier Russian method. This system recognized some three dozen Great Soil Groups. In 1975 a new classification scheme known as soil taxonomy was published in the United States and is now used by the USDA. Unlike earlier systems, which organized soils according to various soil formation factors, the new system emphasizes characteristics that can be precisely measured, including diagnostic horizons (which give clues to soil formation), soil moisture, and soil temperature. In a manner similar to the kingdom, phylum, class, order, family, genus, species system used to classify living things, the USDA soil taxonomy employs six categories. From the general to the more specific, its categories are order, suborder, great group, subgroup, family, and series. This system has classified more than 17,000 types of soil in the United States. The top level of the system consists of 12 orders: alfisols, andisols, aridisols, entisols, gelisols, histosols, inceptisols, mollisols, oxisols, spodosols, ultisols, and vertisols. Each term employs a Latin or Greek word root to describe a range of soil characteristics. Mollisols, for example (from the Latin mollis, for "soft") are soils with thick, dark surface horizons that have a high proportion of organic matter. Such soils can be found in the midwestern United States stretching up into Canada and in portions of northwestern North America. Regions in New England and the eastern portion of Canada, meanwhile, contain spodosols (from the Greek spodos, meaning "wood ash"), which are characterized by a light-colored, grayish topsoil and subsoil accumulation of aluminum, organic matter, and iron. Soil scientists classify soils in many of the southern United States as ultisols (from the Latin for "last"), heavily weathered soils with high concentrations of aluminum. In the southwest, meanwhile, aridisols (from the Latin aridus, for "dry"), featuring little organic matter, are found, as their name implies, in arid lands with little plant growth. The suborder and great group names of the soil taxonomy provide increasing levels of detail. The suborder aqualf, for example, combines aqu from the Latin aqua, for "water," and alf from alfisol to describe wet soils. Using assorted roots and combining them in different ways, scientists describe soils in a highly specialized and specific language. Aeric fragiaqualfs, for example, are wet, well-developed soils with aerated surface layers and restrictive subsoils. VI SOIL USE For most of human history, soil has not been treated as the valuable and essentially nonrenewable resource that it is. Erosion has devastated soils worldwide as a result of overuse and misuse. In recent years, however, farmers and agricultural experts have become increasingly concerned with soil management. A Erosion Erosion is the wearing away of material on the surface of the land by wind, water, or gravity. In nature, erosion occurs very slowly, as natural weathering and geologic processes remove rock, parent material, or soil from the land surface. Human activity, on the other hand, greatly increases the rate of erosion. In the United States, the farming of crops accounts for the loss of over 3 billion metric tons of soil each year. In a cultivated field from which crops have been harvested, the soil is often left bare, without protection from the elements, particularly water. Raindrops smash into the soil, dislodging soil particles. Water then carries these particles away. This movement may take the form of broad overland flows known as sheet erosion. More often, the eroding soil is concentrated into small channels, or rills, producing so-called rill erosion. Gravity intensifies water erosion. Landslides, in which large masses of waterloosened soil slide down an incline, are a particularly extreme example. Wind erosion occurs where soils are dry, bare, and exposed to winds. Very small soil particles can be suspended in the air and carried away with the wind. Larger particles bounce along the ground in a process called saltation. B Soil Management To prevent exposure of bare soil, farmers can use techniques such as leaving crop residue in the soil after harvesting or planting temporary growths, such as grasses, to protect the soil from rain between crop-growing seasons. Farmers can also control water runoff by planting crops along the slope of a hill (on the contour) instead of in rows that go up and down. Soil faces many threats throughout the world. Deforestation, overgrazing by livestock, and agricultural practices that fail to conserve soil are three main causes of accelerated soil loss. Other acts of human carelessness also damage soil. These include pollution from agricultural pesticides, chemical spills, liquid and solid wastes, and acidification from the fall of acid rain. Loss of green spaces, such as grassland and forested areas, in favor of impermeable surfaces, such as pavement, buildings, and developed land, reduces the amount of soil and increases pressure on what soil remains. Soil is also compacted by heavy machinery and off-road vehicles. Compaction rearranges soil particles, increasing the density of the soil and reducing porosity. Crusts form on compacted soils, preventing water movement into the soil and increasing runoff and erosion. With the world's population now numbering upwards of 6 billion people--a figure that may rise to 10 billion or more within three decades--humans will depend more than ever on soil for the growth of food crops. Yet the rapidly increasing population, the intensity of agriculture, and the replacement of soil with concrete and buildings all reduce the capacity of the soil to fulfill this need. As a result of an increased awareness of soil's importance, many changes are being made to protect soil. Recent interest in soil conservation holds the promise that humanity will take better care of this precious resource. Contributed By: Christopher King Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.