Elemental Geosystems, 5th Edition - Chapter 15

PART FOUR: Biogeography Overview Earth is the home of the only known biosphere in the Solar System—a unique, complex, and interactive system of abiotic and biotic components working together to sustain a tremendous diversity of life. Thus we begin Part Four of Elemental Geosystems and an examination of the biogeography of the biosphere–soils, ecosystems, and terrestrial biomes. Remember the description of the biosphere from Chapter 1: The intricate, interconnected web that links all organisms with their physical environment is the biosphere. Sometimes called the ecosphere, the biosphere is the area in which physical and chemical factors form the context of life. The biosphere exists in the overlap among the abiotic spheres, extending from the seafloor to about 8 km (5 mi) into the atmosphere. Life is sustainable within these natural limits. In turn, life processes have powerfully shaped the other three spheres through various interactive processes. The biosphere has evolved, reorganized itself at times, faced extinction, gained new vitality, and managed to flourish overall. Earth’s biosphere is the only one known in the Solar System; thus, life as we know it is unique to Earth. Part Four is a synthesis of many of the elements covered throughout the text. 15 The Geography of Soils Earth's landscape is generally covered with soil. Soil is a dynamic natural body comprises fine materials in which plants grow, and which is composed of both mineral and organic matter. By their diverse nature, soils are a complex subject and pose a challenge for spatial analysis. This chapter presents an overview of the modern system of soil classification used in the United States, with mention of the Canadian system. The Soil Taxonomy system is often misnamed in texts, and anachronistic soil classification terminology is sometimes used. Since the system has been in constant use since 1975 and is integrated into all major pedology textbooks, I present this system in an accurate version true to original source material. The latest revision to the Soil Taxonomy occurred in 1998–99 and Elemental Geosystems presents this, including the newest soil order (as of November 1998), the Gelisols. Please find Table 15.1, and utilize it to simplify this geographic study of soils. The table summarizes the 12 soil orders of the Soil Taxonomy, including general location and climate association, areal coverage estimate, and basic description. Also, you will find small locator maps for most soil orders included with a picture of that soil's profile. These locator maps are derived from the new worldwide distribution map presented in Figure 15.8. Please consult the companion text Geosystems—An Introduction to Physical Geography, sixth edition 2006, for more detail on soil characteristics including “Principal Soil Moisture Regimes,” Table 18.2. A description and analysis of the Canadian System of Soil Classification (CSSC), Appendix B, includes a soils map and descriptive tables. Outline Headings and Key Terms The first-, second-, and third-order headings that divide Chapter 15 serve as an outline for your notes and studies. The key terms and concepts that appear boldface in the text are listed here under their appropriate heading in bold italics. All these highlighted terms appear in the text glossary. Note the check-off box () so you can mark your progress as you master each concept. Your students have this same outline in their Student Study Guide. The icon indicates that there is an accompanying animation or other resource on the CD. The outline headings for Chapter 15: soil soil science Soil Characteristics Soil Profiles pedon polypedon Soil Horizons soil horizon humus eluviation illuviation solum Soil Properties Soil Ion Exchange: Soil Particles and Soil Water Soil Color Soil Texture loam Soil Structure Soil Consistence Soil Porosity Soil Moisture Soil Chemistry Soil Colloids and Mineral Ions soil colloids adsorption cation-exchange capacity (CEC) soil fertility Soil Acidity and Alkalinity Soil Formation Factors and Management Natural Factors The Human Factor Soil Classification Soil Taxonomy Soil Taxonomy Pedogenic Regimes laterization salinization calcification podzolization gleization Diagnostic Soil Horizons epipedon diagnostic subsurface horizon The 12 Soil Orders of the Soil Taxonomy Oxisols Oxisols plinthite Aridisols Aridisols Mollisols Mollisols Alfisols Alfisols Ultisols Ultisols Spodosols Spodosols Entisols Entisols Inceptisols Inceptisols Gelisols Gelisols Andisols Andisols Vertisols Vertisols Histosols Histosols Summary and Review News Report and Focus Study News Report 15.1: Soil Is Slipping through Our Fingers Focus Study 15.1: Selenium Concentration in Western Soils The URLs related to this chapter of Elemental Geosystems can be found at http://www.prenticehall.com/christopherson Key Learning Concepts After reading the chapter and using the Student Study Guide, the student should be able to: • Define soil and soil science and describe a pedon, polypedon, and typical soil profile. • Describe soil properties of color, texture, structure, consistence, porosity, and soil moisture. • Explain basic soil chemistry, including cation-exchange capacity, and relate these concepts to soil fertility. • Evaluate principal soil formation factors, including the human element. • Describe the 12 soil orders of the Soil Taxonomy classification system and explain their general occurrence. Annotated Chapter Review Questions • Define soil and soil science and describe a pedon, polypedon, and typical soil profile. 1. Soils provide the foundation for animal and plant life and therefore are critical to Earth's ecosystems. Why is this true? Soil is a dynamic natural body comprised of fine materials in which plants grow, and which is composed of both mineral and organic matter. Specific soil conditions determine soil fertility, which is the ability of soil to support plant productivity. Plants capture sunlight and fix carbon in organic compounds that sustain the biosphere. 2. What are the differences among soil science, pedology, and edaphology? Soil science is interdisciplinary, involving physics, chemistry, biology, mineralogy, hydrology, taxonomy, climatology, and cartography. Pedology concerns the origin, classification, distribution, and description of soil. Pedology is at the center of learning about soils, yet is does not dwell on its practical uses. Edaphology focuses on soil as a medium for sustaining higher plants. Edaphology emphasizes plant growth, fertility, and the differences in productivity among soils. Pedology gives us a general understanding of soils and their classification, whereas edaphology reflects society's concern for food and fiber production and the management of soils to increase fertility and reduce soil losses. 3. Define polypedon and pedon, the basic units of soil. A soil profile selected for study should extend from the surface to the lowest extent of plant roots, or to the point where regolith or bedrock is encountered. Such a profile, known as a pedon, is imagined as a hexagonal column encompassing from 1 m2 to 10 m2 in surface area (Figure 15.1). At the sides of the pedon, the various layers of the soil profile are visible in cross section. A pedon is the basic sampling unit in soil surveys. Many pedons together in one area comprise a polypedon, which has distinctive characteristics differentiating it from surrounding polypedons. These polypedons are the essential soil individuals, constituting an identifiable series of soils in an area. A polypedon has a minimum dimension of about 1 m2 and no specified maximum size. It is the soil unit used in preparing local soil maps. 4. Characterize the principal aspects of each soil horizon. Where does the main accumulation of organic material occur? Where does humus form? Explain the difference between the eluviated layer and the illuviated layer. Which horizons constitute the solum? Each layer exposed in a pedon is a soil horizon. A horizon is roughly parallel to the pedon's surface and has characteristics distinctly different from horizons directly above or below. The boundary between horizons usually is visible in the field, using the properties of color, texture, structure, consistency porosity, the presence or absence of certain minerals, moisture, and chemical processes. At the top of the soil profile is the O horizon, composed of organic material derived from plant and animal litter that was deposited on the surface and transformed into humus. Humus is a mixture of decomposed organic materials in the soil and is usually dark in color. At the bottom of the soil profile is the R horizon, representing either unconsolidated material or consolidated bedrock of granite, sandstone, limestone, or other rock. The A, B, and C horizons mark differing mineral strata between O and R; these middle layers are composed of sand, silt, clay, and other weathered by-products. In the A horizon, the presence of humus and clay particles is particularly important, for they provide essential chemical links between soil nutrients and plants. The lower portion of the A horizon grades into the E horizon, which is a bit more pale and is made up of coarse sand, silt, and resistant minerals. Clays and oxides of aluminum and iron are leached (removed) from the E horizon and migrate to lower horizons with water as it percolates through the soil. This process of rinsing through upper horizons and removing finer particles and minerals is termed eluviation; thus the designation E for this horizon. The greater the precipitation in an area, the higher the rate of eluviation that occurs in the E horizon. Materials are translocated to lower horizons by internal washing in the soil. This process of rinsing through upper horizons and removing finer particles and minerals is termed eluviation—an erosional process. In contrast to A horizons, B horizons demonstrate an accumulation of clays, aluminum, iron, and possibly humus. These horizons are dominated by illuviation—a depositional process. The C horizon is weathered bedrock or weathered parent material, excluding the bedrock itself. This zone is identified as regolith. The combination of A horizon with its eluviation removals and the B horizon with its illuviation accumulations is designated the solum, considered the true soil of the pedon. A and B horizons are most representative of active soil processes. • Describe soil properties of color, texture, structure, consistence, porosity, and soil moisture. 5. How can soil color be an indication of soil qualities? Give a couple of examples. Color is important, for it sometimes reflects composition and chemical makeup. Soil scientists describe a soil's color by comparing it with a Munsell Color Chart (from the Munsell Color Company). These charts are in a loose-leaf binder, with 175 different colors arranged by hue (the dominant spectral color, such as red), value (degree of darkness or lightness), and chroma (purity and strength of the color saturation, which increases with decreasing grayness). Color is identified by a name and a Munsell notation, and checked at various depths within a pedon. 6. Define a soil separate. What are the various sizes of particles in soil? What is loam? Why is loam regarded so highly by agriculturalists? Individual mineral particles are called soil separates; those smaller than 2 mm in diameter (0.08 in.), such as very coarse sand, are considered part of the soil, whereas larger particles are identified as pebbles, gravels, or cobbles. Figure 15.3 shows a diagram of soil textures with sand, silt, and clay concentrations. The figure includes the common designation loam, which is a mixture of sand, silt, and clay in almost equal shares. The water-holding characteristics and ease of cultivation make a sandy loam soil, with clay content below 30%, ideal for farmers. 7. What is a quick, hands-on method for determining soil consistence? A corollary to texture and structure is soil consistency, which is the cohesion in soil and its resistance to mechanical stress and manipulation under varying moisture conditions. Wet soils are variably sticky when held between the thumb and forefinger, ranging from a little adherence to either finger, to sticking to both fingers, to stretching when the fingers are moved apart. Plasticity, the quality of being molded, is roughly measured by rolling a piece of soil between your fingers and thumb to see whether it rolls into a thin strand. Moist soil implies that it is filled to about half of field capacity, and its consistence grades from loose (noncoherent), to friable (easily pulverized), to firm (not crushable between thumb and forefinger). 8. Summarize the role of soil moisture in mature soils. Soil moisture regimes and their associated climate types shape the biotic and abiotic properties of the soil more than any other factor. Based on Thornthwaite's water-balance principles (Chapter 7, Figures 7.7 and 7.8), the U.S. Soil Conservation Service recognizes five soil moisture regimes. • Explain basic soil chemistry, including cation-exchange capacity, and relate these concepts to soil fertility. 9. What are soil colloids? How do they relate to cations and anions in the soil? Explain cation-exchange capacity. Soil colloids are important for retention of ions in soil. These tiny clay and organic particles carry a negative electrical charge and consequently are attracted to any positively charged ions in the soil. Clay colloids and organic colloids exhibit different levels of chemical activity. Individual clay colloids are thin and platelike, with parallel surfaces that are negatively charged. Colloids can exchange cations between their surfaces and the soil solution, a measured ability called cation-exchange capacity (CEC). A high CEC means that the soil colloids can store or exchange more cations from the soil solution, an indication of good soil fertility (unless there is a complicating factor, such as the soil being too acid). Cations attach to the surfaces of the colloids by adsorption, that is, the metallic cations are adsorbed by the soil colloids. A cation is a positively charged ion and an anion is a negatively charged ion. 10. What is meant by the concept of soil fertility? Soil fertility is the ability of soil to sustain plants. Soil is fertile when it contains organic substances and clay mineral that absorb certain elements needed by plants. • Evaluate principal soil formation factors, including the human element. 11. Briefly describe the contribution of the following factors and their effect on soil formation: parent material, climate, vegetation, landforms, time, and humans. The role of parent material in providing weathered minerals to form soils is important in establishing the basic mineral structure and character of the developing soil. At the bottom of the soil profile is the R horizon, representing either unconsolidated material or consolidated bedrock of granite, sandstone, limestone, or other rock. When bedrock weathers to form regolith, it may or may not contribute to overlying soil horizons. Worldwide, soil types show a close correlation to climate types. The moisture, evaporation, and temperature regimes associated with varying climates determine the chemical reactions, organic activity, and eluviation rates of soils. Not only is the present climate important, but many soils also exhibit the imprint of past climates, sometimes over thousands of years. The organic content of soil is determined in part by the vegetation growing in that soil, as well as by animal and bacterial activity. The chemical makeup of vegetation contributes to acidity or alkalinity in the soil solution. For example, broadleaf trees tend to increase alkalinity, whereas needleleaf trees tend to produce higher acidity. Landforms also affect soil formation, mainly through slope and orientation. Slopes that are too steep do not have full soil development, but slopes that are slight may inhibit soil drainage. As for orientation, in the Northern Hemisphere, a southern slope exposure is warmest (slope faces the southern Sun), which affects water balance relationships. All of these identified factors require time to operate. A few centimeters' thickness of prime farmland soil may require 500 years for maturation. Yet these same soils are being lost at a few centimeters per year to sheetwash and gullying produced by human abuse of the soil resource. The GAO estimates that between 3 and 5 million acres of prime farmland are being lost each year in the United States directly because of poor practices. 12. Explain some of the details that support concern over loss of our most fertile soils. What cost estimates have been placed on soil erosion? Much effort and many dollars are expended to create fertile soil conditions, yet we live in an era when the future of Earth's most fertile soils is threatened. Soil erosion is increasing worldwide. Some 35% of farmland is losing soil faster than it can form–a loss exceeding 22.75 billion metric tons per year (25 billion tons). Increases in production resulting from artificial fertilizers and new crop designs partially mask this effect, but such compensations for soil loss are nearing an end. Soil depletion and loss are at record levels from Iowa to China, Peru to Ethiopia, and the Middle East to the Americas. The impact on society could be significant. One 1995 study tabulated the market value of lost nutrients and other variables at over $25 billion a year in the United States and hundreds of billion dollars worldwide. The cost to bring soil erosion under control in the United States is estimated at approximately $8.5 billion, or about 30 cents on every dollar of damage and loss. • Describe the 12 soil orders of the Soil Taxonomy classification system and explain their general occurrence. 13. Summarize what led soil scientists to develop the new Soil Taxonomy classification system? Taxonomy is defined in Webster's as “Classification according to natural relationships...the systematic distinguishing, ordering, and naming of type groups within a subject field....a formal system of nomenclature.” Additional background is added to this section concerning the history of soil classification efforts in Canada. In 1883, the Russian soil scientist V. V. Dukuchaev published a monograph that organized soils into rough groups based on observable properties, most of which resulted from climatic and biological soil-forming processes. The Russians contributed greatly to modern soil classification because they were first to consider soil an independent natural body with a definite individual soil genesis that was recognizable in an orderly global pattern. They were first to demonstrate broad interrelationships among the physical environment, vegetation, and soils, at a time when a scientific revolution was beginning to sweep their country in various fields. By contrast, American and European scientists were still considering soil as a geologic product, or simply a mixture of chemical compounds. The first formal American system is credited to Dr. Curtis F. Marbut, who derived his approach from Dukuchaev. Published in the 1935 Atlas of American Agriculture and the Yearbook of Agriculture–1938, Marbut's classification of great soil groups recognized the importance of soils as products of dynamic natural processes. The inadequacies of the 1938 system were obvious to soil scientists at about this time. As the number of identified soil series grew, it became increasingly problematic to associate all the soil series to soil families, groups, and higher categories. The 1938 system simply did not contain detailed criteria for the higher categories of classification. The work of developing a new classification system began about 1950 with a series of approximations, each tested in-progress by the Soil Survey Staff of the Soil Conservation Service in the United States. International cooperation was important, as evidenced by conferences at Ghent, Leopoldville, Paris, London, and Washington. The 3rd Approximation included many of the European suggestions. By 1955, the 4th Approximation was introduced with efforts to group soil series for possible identification of patterns in categories above the soil-family level. This produced the 5th Approximation at the 1956 6th Congress of the International Society of Soil Science in Paris. By 1957 the 6th Approximation was developed to provide the Soil Survey Staff with a guide for better grouping of subgroups and families. Also, through this stage the nomenclature was developed in consultation with specialists in language, the classics, and with particular assistance from Belgian scientists. By 1960, the 7th Approximation was completed and distributed at the 7th International Congress of Soil Science held at Madison, Wisconsin. At this time we were approaching some 10,000 soil series in active use in the United States. Many texts still use this 30-year old outdated name to describe the Soil Taxonomy system—a practice not duplicated in any soil science or pedology text! In preparing Elemental Geosystems I spoke to Dr. Henry D. Foth of Michigan State (Fundamentals of Soil Science, 8th ed., Wiley, 1990). He commented on this use of misnomers for Soil Taxonomy and that we should get in line with the related field of soil science. I have made an effort to do this. Various supplements and updates were subsequently prepared to improve the system especially with regards to improving tropical soil classification. The effort is clearly international in scope and reflects work from many scientists worldwide. All this led to the publication of Agricultural Handbook No. 436 in 1975, the Soil Taxonomy—A Basic System of Soil Classification for Making and Interpreting Soil Surveys. Many aspects of Soil Taxonomy overlap with the world soil map published by the Food and Agriculture Organization. If it is not already on your shelf you might want to obtain a copy of this 750-page volume, along with Agricultural Handbook No. 18 the Soil Survey Manual. Also useful is the Soil Management Support Services Technical Monograph No. 6, the Keys to Soil Taxonomy by the Soil Survey Staff (3rd ed., 1987). The latest 1998, 1999 revision is described in the text. Over the years various revisions and clarifications in the system were published in Keys to the Soil Taxonomy, now in its 8th edition (1998) which include all the revisions to the 1975 Soil Taxonomy. Major revisions include the addition of two new soil orders: Andisols (volcanic soils) in 1990 and Gelisols (cold and frozen soils) in 1998. Canadian efforts at soil classification began in 1914 with the partial mapping of soils in Ontario by A.J. Galbraith. Efforts to develop a taxonomic system spread across the country, anchored by academic departments at universities in each province. Regional differences emerged, hampered by a lack of specific soil details. Only 1.7% of Canadian soil, totaling 15 million hectares, was surveyed by 1936. The Canadian System of Soil Classification (CSSC) in Appendix B provides taxa for all soils presently recognized in Canada and is adapted to Canada's expanses of forest, tundra, prairie, frozen ground, and colder climates. As in the U.S. Soil Taxonomy system, the CSSC classifications are based on observable and measurable properties found in real soils rather than idealized soils that may result from the interactions of genetic processes. The system is flexible in that its framework can accept new findings and information in step with progressive developments in the soil sciences. The system is arranged in a nested, hierarchical pattern to allow generalization at several levels of detail. Elements of the horizon suffix descriptions are derived from the Soil Taxonomy although adapted to conditions in the Canadian environment. Appendix B, Table 1, is a summary derived from several Canadian publications. I tried to summarize in an admittedly succinct form a large volume of information. Note that the percentage of land area classified under each soil order and the Soil Taxonomy equivalent is given with each characteristics section. 14. What is the basis of the Soil Taxonomy system? How many orders, suborders, great groups, subgroups, families, and soil series are there? The original version of the Soil Taxonomy system was published in 1975: Soil Taxonomy—A Basic System of Soil Classification for Making and Interpreting Soil Surveys, is generally called simply Soil Taxonomy through to its current revision. Much of the information in this chapter is derived from that keystone publication. The classification system divides soils into six categories, creating a hierarchical sorting system. Each soil series (the smallest, most detailed category) ideally includes only one polypedon, but may include portions continuous with adjoining polypedons in the field. Soil orders = 12; suborders = 47; great groups = 230; subgroups = 1200; families = 6000; and soil series = 15,000. 15. Define an epipedon and a subsurface diagnostic horizon. Give a simple example of each. Two diagnostic horizons may be identified in the solum: the epipedon and the subsurface. The epipedon (literally, over the soil) is the diagnostic horizon that forms at the surface and may extend through the A horizon, even including all or part of an illuviated B horizon. The epipedon is visibly darkened by organic matter and sometimes is leached of minerals. There are six recognized epipedons. A diagnostic horizon often reflects a physical property (color, texture, structure, consistence, porosity, moisture), or a dominant soil process in a pedon. The second type of diagnostic horizon is the subsurface diagnostic horizon. It originates below the surface at varying depths and may include part of the A and/or B horizons. Many subsurface diagnostic horizons have been identified In the Canadian system soil horizons are named and standardized as diagnostic in the classification process. Several mineral and organic horizons and layers are used in the CSSC. Three mineral horizons are recognized by capital letter designation, followed by lowercase suffixes for further description. Principal soil-mineral horizons and suffixes are presented in Appendix B. 16. Locate each soil order on the world map and on the U.S. map as you give a general description of it. Utilize Table 15.1, for an integrated overview of the twelve major soil orders and their descriptions, characteristics, areal distribution, former equivalent name and Canadian equivalent, and pronunciation. Figure 15.8 presents the worldwide distribution of all 12 soil orders. Small locator maps are also presented for eight of the orders as follows: Figure 15.9 (Oxisols), Figure 15.12 (Aridisols), Figure 15.14 (Mollisols), Figure 15.18 (Alfisols), Figure 15.19 (Ultisols), Figure 15.20 (Spodosols), Figure 15.22 (Gelisols, the newest order), Figure 15.24 (Vertisols), and Figure 15.25 (Histosols). These smaller maps were prepared from Figure 15.8 so that students will be able to relate the individual soil order depicted back to the worldwide pattern. 17. How was slash-and-burn shifting cultivation, as practiced in the past, a form of crop and soil rotation and conservation of soil properties? Earlier slash-and-burn shifting cultivation practices were adapted to equatorial and tropical soil conditions and formed a unique style of crop rotation. The scenario went like this: people in the tropics cut down (slashed) and burned the rain forest in small tracts, cultivated the land with stick and hoe, and planted maize (corn), beans, and squash. After several years the soil lost fertility, and the people moved on to the next tract to repeat the process. After many years of movement from tract to tract, the group returned to the first patch to begin the cycle again. This practice protected the limited fertility of the soils somewhat, allowing periods of recovery to follow active production. However, the invasion of foreign plantation interests, development by local governments, vastly increased population pressures, and conversion of vast new tracts to pasturage halted this orderly native pattern of land rotation. 18. Describe the salinization process in arid and semiarid soils. What associated soil horizons develop? A soil process that occurs in Aridisols and nearby soil orders is salinization. Salinization results from excessive POTET rates in the deserts and semiarid regions of the world. Salts dissolved in soil water are brought to surface horizons and deposited there as surface water evaporates. These deposits appear as subsurface salic horizons, which will damage and kill plants when they occur near the root zone. Obviously, salinization complicates farming in Aridisols. The introduction of irrigation water may either water log poorly drained soils or lead to salinization. Nonetheless, vegetation does grow where soils are better drained and lower in salt content. 19. Which of the soil orders are associated with Earth's most productive agricultural areas? Mollisols (grassland soils) are some of Earth's most significant agricultural soils. There are seven recognized suborders, not all of which bear the same degree of fertility. The dominant diagnostic horizon is called the mollic epipedon, which is a dark, organic surface layer some 25 cm (10 in.) thick. As the Latin name implies, Mollisols are soft, even when dry, with granular or crumbly peds, loosely arranged when dry. These humus-rich organic soils are high in base cations (calcium, magnesium, and potassium) and have a high CEC. 20. What is the significance to plants of the 51 cm (20 in.) isohyet in the Midwest relative to soils, pH, and lime content? In North America, the Great Plains straddle the 98th meridian, which is coincident with the 51 cm (20 in.) isohyet of annual precipitation—wetter to the east and drier to the west. The Mollisols here mark the historic division between the short- and tall-grass prairies. The relationship among Mollisols, Aridisols (to the west), and Alfisols (to the east) is shown in Figure 15.15. The illustration also presents some of the important graduated changes that denote these different soil regions, including the level of pH concentration and the depth of available lime. 21. Describe the podzolization process associated with northern coniferous forest soils. What characteristics are associated with the surface horizons? What strategies might enhance these soils? The Spodosols (northern coniferous forest soils) and their four related suborders occur generally to the north and east of the Alfisols. Spodosols lack humus and clay in the A horizons. An eluviated albic horizon, sandy and leached of clays and irons, lies in the A horizon instead and overlies a spodic horizon of illuviated organic matter and iron and aluminum oxides. The surface horizon receives organic litter from base-poor, acid-rich trees, which contribute to acid accumulations in the soil. The low pH (acid) soil solution effectively removes clays, iron, and aluminum, which are passed to the upper diagnostic horizon. An ashen-gray color is common in these subarctic forest soils and is characteristic of a formation process called podzolization. The low base-cation content of Spodosols requires the addition of nitrogen, phosphate, and potash, and perhaps crop rotation as well, if agriculture is to be attempted. A soil amendment such as limestone can significantly increase crop production in these acidic soils. Podzolic (Russian, podzol) (25 subgroups) Soils formed in association with the conditions of coniferous forests and sometimes heath. Leaching of overlying horizons occurs in response to moist, cool-to-cold climates. Iron, aluminum, and organic matter from L, F, and H horizons are redeposited in podzolic B horizon. 22. What former Inceptisols now form a new soil order named in 1998? Describe these soils as to location, nature, and formation processes. Why do you think they were separated into their own order? Andisols formerly were considered under Inceptisols and Entisols, but in 1990 they were placed in this new order. Andisols are derived from volcanic ash and glass. Previous soil horizons frequently are found buried by ejecta from repeated eruptions. Volcanic soils are unique in their mineral content and in their recharge by eruptions. Weathering and mineral transformations are important in this soil order. Volcanic glass weathers readily into allophane (a noncrystalline aluminum silicate clay mineral that acts as a colloid) and oxides of aluminum and iron. Andisols feature a high CEC, high water-holding ability, and develop moderate fertility, although phosphorus availability is an occasional problem. In November 1998 a new soil order, formerly classified under Inceptisols, Entisols, and Histosols, the Gelisols were designated (Figure 15.22). These are cold and frozen soils associated with high latitude and high elevation conditions. 23. Why has a selenium contamination problem arisen in western soils? Explain the impact of agricultural practices, and tell why you think this is or is not a serious problem. See Focus Study 15.1. About 95% of the irrigated acreage in the United States is west of the 98th meridian. This region is increasingly troubled by salinization and water-logging problems. But at least nine sites in the West, particularly in California's western San Joaquin valley, are experiencing related contamination of a more serious nature—increasing selenium concentrations. The soils in these areas were derived from former marine sediments that formed shales in the adjoining Coast Ranges. As parent materials weathered, selenium-rich alluvium washed into the semiarid valley, forming the soils that needed only irrigation water to become productive. After 1960, large-scale irrigation efforts intensified, resulting in subtle initial increases in selenium concentrations in soil and water. In trace amounts, selenium is a dietary requirement for animals and humans, but in higher amounts it is toxic to both. Toxic effects were reported during the 1980s in some domestic animals grazing on grasses grown in selenium-rich soils in the Great Plains. According to U.S. Fish and Wildlife Service scientists, the toxicity moves through the food chain and genetically damages and kills wildlife. For example, birth defects and death were widely reported in all varieties of birds that nested at selenium-contaminated Kesterson Wildlife Refuge; approximately 90% of the exposed birds perished or were injured. Such damage to wildlife presents a real warning to human populations at the top of the food chain. At the very least we must acknowledge that soil processes are complex. Certainly, much remains to be learned to avoid environmental tragedies such as the one at Kesterson—remember that there are nine such threatened sites in the West, and Kesterson was only the first catastrophe. 24. Examine the peat cutting and drying shown in the chapter-opening photo and Figure 15.25c. Explain the soil-forming processes at work to produce these organic soils. How are the peat blocks used? Into which Soil Taxonomy soil order do these soils fall? Peat , a Histosol organic soil, is cut into blocks by hand with a spade, which then are set out to dry (see this chapter’s opening photo). As layer after layer of fibrous material compresses and chemically breaks down, peat is formed. Poor drainage and high-water content is important to the process. In Figure 15.25c, note the fibrous texture of the sphagnum moss growing on the surface and the darkening layers with depth in the soil profile as the peat is compressed and chemically altered. Such beds can be more than 2-m thick. Once dried, the peat blocks burn hot and smoky. Peat is the first stage in the formation of lignite, an intermediate step toward coal. Imagine such soils forming in plant-lush swamp environments in the Carboniferous Period (359 to 299 m.y.a.), only to go through coalification to become coal deposits. Overhead Transparencies 273. 15.1 Soil sampling and mapping units 274. 15.3 a and b Soil texture triangle 275. 15.4 Soil structure types 276. 15.5 and 15.6 Soil colloids and CEC (top); soil pH scale and labels (bottom) 277. 15.8 Worldwide distribution of the 12 soil orders and color legend 278. 15.10, 15.17, 15.20c Laterization, calcification, podzolization 279. 15.16 a, b, and c A soil continuum in north central U.S. 280. Appendix B, Fig. 1 Principal soil regions of the Canadian System of Soil Classification 210 • The Geography of Soils The Geography of Soils • 210 • The Geography of Soils The Geography of Soils • • The Geography of Soils The Geography of Soils • • The Geography of Soils The Geography of Soils • • The Geography of Soils The Geography of Soils •