Elements of the Nature and Properties of Soils Third Edition

Transcription

1 Instructor's Manual with Test Item File to accompany Elements of the Nature and Properties of Soils Third Edition Nyle C. Brady, Ph.D. Emeritus Professor of Soil Science Cornell University Ithaca, New York Ray R. Weil, Ph.D. Professor of Soil Science University of Maryland College Park, Maryland Prentice Hall Boston Columbus Indianapolis New York San Francisco Upper Saddle River Amsterdam Cape Town Dubai London Madrid Milan Munich Paris Montreal Toronto Delhi Mexico City Sao Paulo Sydney Hong Kong Seoul Singapore Taipei Tokyo

2 Copyright 2010 by Pearson Education, Inc., Upper Saddle River, New Jersey Pearson Prentice Hall. All rights reserved. Printed in the United States of America. This publication is protected by Copyright and permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department. Pearson Prentice Hall is a trademark of Pearson Education, Inc. Pearson is a registered trademark of Pearson plc Prentice Hall is a registered trademark of Pearson Education, Inc. Instructors of classes using Brady and Weil, Elements of the Nature and Properties of Soils, 3 rd edition may reproduce material from the instructor s manual for classroom use ISBN-13: ISBN-10:

3 TABLE OF CONTENTS Preface iv Chapter 1 The Soils Around Us 1 Chapter 2 Formation of Soils from Parent Materials 9 Chapter 3 Soil Classification 19 Chapter 4 Soil Architecture and Physical Properties 30 Chapter 5 Soil Water: Characteristics and Behavior 38 Chapter 6 Soil and the Hydrologic Cycle 46 Chapter 7 Soil Aeration and Soil Temperature 56 Chapter 8 Soil Colloids: Seat of Soil Chemical and Physical Activity 64 Chapter 9 Soil Acidity, Alkalinity, Aridity and Salinity 74 Chapter 10 Organisms and Ecology of the Soil 90 Chapter 11 Soil Organic Matter 99 Chapter 12 Nutrient Cycles and Soil Fertility 108 Chapter 13 Practical Nutrient Management 130 Chapter 14 Soil Erosion and Its Control 144 Chapter 15 Soils and Chemical Pollution 154 iii

4 PREFACE This Instructor's Manual is designed to make it easier to teach soil science classes using the third edition of the textbook, Elements of the Nature and Properties of Soils. The manual brings four basic resources to the instructor: 1) A concise, three to five page summary of the main concepts from each chapter in the textbook. 2) Concise model answers to the study questions found at the end of each chapter. 3) A bank of 40 or more test questions and answers for each chapter of the textbook, over 700 questions in all. These questions are in a format suitable for machine scoring in large classes. About half of the questions are in a multiple choice format and half in a true-false format. A TestGen file with these questions is also available for download from the Instructor s Resource website as an aid in constructing exams and assignments. 4) Downloadable image files for most of the Figures and Tables in the textbook can be found on the Companion Website. These can be easily inserted into presentation software in order to make overhead transparencies, slides or computer presentations for the classroom. Teaching a college level course in the fundamentals of soil science can be one of the most rewarding and satisfying experiences available to the professional educator and scientist. For most students, enrolling in such a course will mark the first time that they have given much thought to soils. In our experience, most students are unaware of the fascinating world that will be opened to them when they learn to truly see the soils around them. For many, the course in soil science will represent their first real opportunity to see practical applications of the principles learned in their basic chemistry, biology, and physics courses. A course in soil science offers an unparalleled opportunity to help students integrate the many concepts and skills they have collected during their years of education. Furthermore, students soon realize that soil science concepts, learned well, will reward them many times over in on-the-job situations. Inspired teaching of soil science will reward the instructor with the light of wonder and discovery in students' eyes. I prepared this Instructor's Manual to accompany our textbook in order that you, the instructor, may be able to spend less time reviewing the material, writing exam questions, and preparing lecture visual aids. Perhaps you will thus have more time to interact with students and help them master, in an integrated way, the broad subject of soil science. I hope that you find both this Instructor's Manual and the third edition of the textbook, Elements of the Nature and Properties of Soils, to be valuable tools in your teaching activities. Please let me know what you think. I have tried to make these books as up-to-date, accurate, comprehensive, and comprehensible as possible. I would gladly welcome your suggestions for improvement in future editions of the manual and the textbook. If you find an error, would like to suggest material for inclusion or deletion, or are aware of particularly useful illustrations for basic concepts, I would enjoy hearing from you. Please fax, , or write to me at the following address: Ray R. Weil Dept. of Environmental Science and Technology Rm H.J. Patterson Hall University of Maryland College Park, MD FAX: rweil@umd.edu iv

5 Chapter 1. The Soils Around Us Overview Roles of Soil Soils play critical roles in many of the environmental challenges facing the world. Soils are essential for life on earth. People may not realize it, but they are becoming ever more dependent on the ecological functions of soil. Soil plays six key roles in the ecosystem: (1) supporting plant growth, (2) largely controlling the flow of water through the hydrologic cycle, (3) recycling waste products of society and nature, (4) modifying the composition and properties of the atmosphere (5) providing habitat for an enormous diversity of organisms, and (6) functioning in built environments as construction material and support for building foundations. Medium for Plant Growth Most plants depend on the soil for a suitable medium for growth. Soils provide at least six factors for plant growth: (1) physical support, (2) aeration for roots, (3) moisture supply and storage, (4) moderation of root zone and near-ground air temperature, (5) an environment relatively free of phytotoxins, and (6) 13 of the 17 essential nutrient elements. Usually animals and people obtain their mineral nutrients indirectly from the soil via consumption of plant materials, however for various reasons, geophagy- intentional eating of soils-is practiced by millions of people around the world for The Pedosphere Concept The importance of soil as a natural body derives in large part from its roles as an interface between the worlds of rock (the lithosphere), air (the atmosphere), water (the hydrosphere), and living things (the biosphere). Environments where all four of these worlds interact are often the most complex and productive on Earth. The soil, or pedosphere, is an example of such an environment. Soil as A Natural Body A soil is a three-dimensional natural body that exists in the landscape, much as a mountain or river does. The soil forms in the upper part of the regolith. Soil horizons develop as horizontal layers of the regolith become differentiated and a soil profile is formed, typically with O, A, E, B, and C master horizons. Organic matter is added to the surface, clay and salts are translocated, and parent material weathers into new soil material. In the process, topsoil becomes quite different from the subsoil. The nature of the soil at all depths in the profile - from as shallow as the upper 1 cm, to as deep as 5 or 10 meters - can be important for making wise land management decisions. Soil: The Interface of Air, Minerals, Water and Life An ideal soil in good condition for plant growth would have a volume composition of about 50% solid (about 45% mineral and 5% organic matter) and 50% pore space (about half filled with air and half with water). Mineral (Inorganic) Constituents of Soils Mineral soil particles come in all sizes and are grouped by size into coarse fragments (>2 mm), sand (2-.05 mm), silt ( mm) and clay (<0.002). Clay differs, not only in size, but also in mineral make-up, from the sand and silt. Clay consists of both primary and secondary minerals. Soil structure describes the manner in which soil particles are grouped together. Soil Organic Matter Soil organic matter generally accounts for only 1 to 6% of a soil's dry weight, but greatly influences nearly all soil properties and uses. The soil organic matter includes living macro and micro organisms that make up the biomass portion. Humus is the portion that is well decomposed, colloidal, v 1

6 and relatively resistant to further microbial attack. Soil Water: A Dynamic Solution Water in the soil is different from the water encountered in everyday life, for two reasons: (1) the liquid properties of soil water are modified by the attraction between water molecules and soil particles and (2) the water in the soil is never pure, but always has a tremendous variety of substances dissolved in it. The soil solution contains, among other solutes, most of the elements essential for plant growth. Eighteen elements (Table 1.1 lists both the micronutrient and the macronutrients) are now considered to be essential to plant growth (nickel was recently recognized as such), and all but C, H, and O are supplied to plants from the soil solution. The ph of the soil solution may range from very strongly acid (<4) to very strongly alkaline (ph>10), but most soils have ph values between 5 and 8. Soil Air: A Changing Mixture of Gases Soil air is different from atmospheric air because carbon dioxide is produced and oxygen is used up by soil organisms. Soil air is generally several times more concentrated in carbon dioxide than is atmospheric air. The exchange of gases with the atmosphere is of critical importance to plants. Interaction of Four Components to Supply Plant Nutrients Mineral weathering and organic matter decomposition release nutrient elements from unavailable storage forms to more available adsorbed or dissolved forms. The great bulk of most soil nutrients is present in unavailable forms stored in the solid framework of the soil. Nutrient Uptake by Plant Roots Soluble nutrients reach plant roots from the bulk soil by root interception, mass flow, and diffusion. Nutrients are taken up into the root by a biologically active carrier mechanism that transports nutrient ions across cell membranes. Soil Quality, degradation and Resilience Continued growth of the human population forces us to produce more food from the same or smaller area of land. Intensified agriculture on the best soils may help spare land for wildlife habitat and other uses, but only if soil quality can be maintained. Soil quality is a measure of the ability of a soil to carry out ecological functions, such as plant growth, water supply, and nutrient recycling. Soil quality reflects a combination of chemical, physical, and biological properties. Soil erosion, salt accumulation, nutrient depletion and organic matter depletion are some of the processes by which poor management can lead to soil degradation. Some soils are more resilient than other as evidenced by their ability to recover their quality more rapidly after such disturbances. 2

7 Model Answers to Study Questions 1. See Section 1.1 and Figure Need to transform solar energy into food energy over huge areas of the Earth s surface. -Society s increased appreciation of and continued dependency on the services of natural soilbased ecosystems for water, wildlife, natural cycles and recreation. - Increasing replacement of petroleum by soil-grown plant products as feedstocks for chemical industry (e.g. soy bean based inks).. - Increasing replacement of finite fossil fuels with renewable plant-based biofuels (e.g. ethanol). 2. See Section 1.8. A soil is an organized, three dimensional body that is a component of a landscape. Mere soil is some material from such a body that can be moved around by a shovel or bulldozer. 3. See Section Six soil roles or functions: (1) medium for plant growth; (2) regulator of water supplies; (3) recycler of nutrients and carbon; (4) modifier of the atmosphere; (5) habitat for soil organisms; (6) engineering medium. Examples of interactions between these roles: Plant cover (medium for plant growth) enhances infiltration of rainwater (regulator of water supplies); an abundance of soil organisms (habitat for soil organisms) enhances the release of nutrients (recycler of nutrients) by the decay of plant residues. 4. See Box 1.3. Individual experiences. Examples: observing erosion at a construction site; washing grit from a batch of fresh spinach; watering house plants; planting a tree; observing earthworms on the sidewalk after a rain; eating almost any food that contains nutrients from the soil in which it was grown. 5. See Section Approximate percentages after compaction: air (10%), Water (30%), mineral (54%), organic (6%). Approximate percentages on a weight (mass) basis: air (0%), mineral (80%), water (18%), organic (2%) from the following approximate calculation: Component Volume, cm 3 A Density, g/cm 3 B Mass, g C=A x B Fraction of total mass air water /135.5 =0.18 mineral /135.5=0.80 organic /135.5=0.02 total See Section 1.15 and Figure Most of the soil s nutrient assets are in complex and very slowly available mineral and organic forms (analogous to long term investments), but these release smaller amounts of nutrients into somewhat more available colloid fraction, which in turn help keep up the supply of adsorbed or exchangeable nutrients on colloid surfaces, which in turn resupply the nutrient content of the soil solution (analogous to pocket cash). 3

8 7. See columns 2 and 3 in Table 1.1. Plants derive these essential (or quasi-essential) elements mainly from the soil: N, P, K, S, Ca, Mg, Si, Na, Fe, Mn, B, Zn, Cu, Cl, Co, Mo, Ni. 8. See Section 1.2 and Appendix B. No, plants also take up elements which are not essential to the plant s growth. Examples are silica and sodium (not essential for some plants), selenium, vanadium, lead, etc. Some of these are nutrients for animals eating the plants, others (such as lead) are not, but are of concern because of their potential for animal toxicity. 9. See glossary for definitions. 10. See Section Some soil degrading processes: erosion, salt accumulation (salinization), nutrient depletion, and chemical pollution. 11. See Section 1.8 Pedologists study soils as natural bodies in the landscape without a particular land use in mind. Pedology is therefore closely related to surficial geology. The edapthological approach to soils aims at understanding the soil system as it relates to the suitability of the soil as a medium for plant growth. Edaphology is therefore a kind of ecology. 4

9 Multiple Choice Questions (Circle the single best answer for each question.) 1. Most of the different nutrients essential for growth are supplied to plants directly from the. A. rain water B. soil solution C. atmosphere D. cosmic radiation E. humus 2. In a load of 10 cubic meters of topsoil, approximately how cubic meters of the volume would be solid material? A. 1 B. 2.5 C. 4 D. 5 E Which of the following is (are) essential plant nutrients? A. Cu B. Al C. Sr D. Pb E. all of the above 4. Which of the following is considered to be a plant macronutrient? A. N B. P C. S D. Ca E. all of the above 5. Soil occupies the part of the regolith. A. upper B. lower C. younger D. both B and C 6. The lithosphere is made up of. A. air B. rock C. water D. plants and animals E. all of the above 7. The layers of contrasting material found when one digs a hole in the ground are called. A. pseudoliths B. regoliths C. pedons D. horizons E. soil structure 8. A soil profile consists of. A. the sum of chemical and physical data known about a soil B. the way a soil "feels" C. the spatial boundaries of a particular soil D. the set of layers seen in a vertical cross Section of a soil E. the general outline of a soil or group of soils when viewed from the side 9. "Topsoil" is generally equivalent to which soil horizon? A. A B. B C. C D. D E. E 10. "Subsoil" is generally equivalent to which soil horizon? A. A B. B C. C D. D E. E 11. In a typical mineral soil in optimal condition for plant growth, approximately what percentage of the pore space would be filled with water and what percentage filled with air? A. 10% water and 90% air B. 90% water and 10% air C. 25% water and 25% air D. 50% water and 50% air E. 25% water and 75% air 12. The amount of different sizes of mineral particles in a soil defines the soil. A. structure B. texture C. pore space D. solution E. profile 13. The water in the soil typically differs from pure water because the soil water. 5

10 A. contains organic compounds B. contains mineral nutrients C. is restrained in its flow by attraction to particle surfaces D. all of the above E. none of the above 14. Compared to silt, clay-sized soil particles are characterized by. A. greater attraction for water B. greater proportion of primary minerals C. less tendency to form hard clods when dry D. less capacity to hold nutrients in plant-available forms 15. Which of the following ph values represents a neutral condition? A. 1.0 B. 5.0 C. 6.0 D. 7.0 E Which of the following ph values represents the most acid condition? A. 1.0 B C. 7.0 D. 100 E Most (usually 80% or more) of soil potassium and calcium can be found in the form of. A. dissolved substances B. structural components of minerals C. exchangeable ions D. organic compounds 18. Increasing the organic matter content of a soil is likely to. A. have no effect on water holding capacity B. increase the soil's water holding capacity C. decrease the soil's water holding capacity 19. Hydroxyl ion concentrations are greatest in a soil solution with a ph value of. A. 0.1 B. 4.0 C. 5.0 D In a given soil, the horizon with the highest organic matter content is generally the horizon. A. E B. C C. D D. B E. A 21. Information about conditions at 2 to 4 meters deep in a soil is usually most helpful for understanding. A. how best to design a building foundation B. the diversity of animal life in the soil. C. the proper classification of the soil. D. fertility requirements of most crops 6

11 True or False Questions (Write T or F after each question.) 22. Except for some kinds of foods, modern industry has made human dependence on soils a thing of the past. 23. Most of the water in our rivers and lakes has come in contact with and has been affected by soils. 24. Soil air usually has a higher carbon dioxide content than the air in the atmosphere. 25. Plants can be grown without any soil. 26. Hydroponics will likely be a key element in enabling the world to feed and clothe its increasing human population in the next few decades. 27. Practices that tend to increase the amount of organic matter in soils would be expected to reduce the global greenhouse effect. 28. Soil, like concrete and steel, is a standard construction material. Its properties are well characterized and predictable so that standard building foundation designs can be used uniformly at all building sites of a given topography. 29. Although subsoil is more difficult to obtain, it is generally equally as good as topsoil for landscaping purposes. 30. Subsoil is typically equivalent to the O horizon. 31. The mineral particles in soil consist of sand, silt, and clay. 32. Where organic matter constitutes only 1 or 2 percent of the soil by weight, it has only negligible influence on soil properties. 33. The dark brown and black humus found in many soils does not mix well with clay minerals so there is very little contact between these two soil components. 34. Soil horizons, like alluvial sediments, generally have a horizontal orientation, regardless of the slope of the land. 35. A, B, C, and E horizons can be found in any true soil. 36. For any soil in which it is present, the C horizon is the parent material for the B horizon. 37. While many organisms depend on the soil for nutrients and water, only a few very specialized organisms live in the soil itself. 38. If supplied with a suitable nutrient solution, plants can grow normally without any soil at all. 39. Natural soils (as opposed to modern farm soils) can recycle organic compounds, but not inorganic elements. 40. Most of the water flowing in rivers passed through a soil profile or over soil surfaces before reaching the river. 7

12 41. Most, if not all, of the nutrient supply stored in a fertile soil is in forms readily available to plants. 42. In humid regions most rainwater that soaks into the soil and is not used by plants eventually flows into rivers and streams. Chapter 1 Answers 1. A 2. D 3. A 4. E 5. A 6. B 7. D 8. D 9. A 10. B 11. D 12. B 13. D 14. A 15. D 16. A 17. B 18. B 19. D 20. E 21. A 22. F 23. T 24. T 25. T 26. F 27. T 28. F 29. F 30. F 31. T 32. F 33. F 34. F 35. F 36. F 37. F 38. T 39. F 40. T 41. F 42. T 8

13 Chapter 2. Formation of Soils from Parent Materials Overview Soils vary greatly from one location to another around the world. This variation can largely be understood in terms of the way in which certain environmental factors of soil formation affect the four basic processes by which geologic parent materials are changed into soils. Weathering of Rocks and Minerals The earth's mantle was originally covered by water and solidified magma, the latter being comprised of igneous rocks such as granite and gabbro. These rocks in turn were made up of mixtures of specific primary minerals such as quartz, feldspars, micas, hornblende, and biotite. Weathering broke down the rocks and minerals, destroying some in the process, but also synthesizing new ones (secondary minerals). Some of the weathering products were transported by water and wind to new locations where they become sediments in lake or ocean bottoms. These sediments were later recemented into sedimentary rocks such as sandstone and shale. Some igneous and sedimentary rocks were later subjected to high pressures and temperatures bringing about a metamorphosis (change) in form and structure, and creating metamorphic rocks. Gneiss, schists, and slate are examples of metamorphic rocks. Weathering Processes (1) Physical weathering (disintegration) results in physical breakdown of the rock/minerals into smaller particles without significant change in chemical composition. Disintegration is enhanced by temperature changes and differential expansion of minerals, frost action and peeling of layers from the parent mass (exfoliation). Erosion, transport, and deposition by wind, water, and ice are physical processes that grind and breakdown the rock and mineral particles. (2) Biogeochemical weathering (decomposition) results in the destruction of the primary minerals and in some cases the simultaneous generation of secondary minerals such as silicate clays. The latter are formed either by alteration of primary minerals or by recrystallization of decay products into new minerals. Biogeochemical breakdown is enhanced by the (a) hydrolysis, or the destruction of a mineral by reaction with H + and OH - ions in water and (b) hydration or the chemical combining of the mineral with intact water molecules, [c] complexation of metal cations by organic ligands. Oxidation of elements such as iron or manganese in minerals helps breakdown rocks and releases secondary minerals. Carbonation and other reactions utilizing acids formed by plants and microbes (e.g. H 2 CO 3, HNO 3, and H 2 SO 4 and some organic acids) result in the formation of secondary minerals such as silicate clays. All these reactions require or are enhanced by the presence of water, and many of them may lead to the synthesis of secondary minerals. Five Factors of Soil Formation 1. Parent material 2. Climate 3. Living organisms 4. Topography 5. Time Parent materials are the starting point for soil development, and their nature profoundly influence soil properties, especially in areas where chemical weathering has not destroyed or greatly modified the original minerals. Soil texture is often determined by the nature of the parent materials as are the rate and nature of some weathering processes. For example, limestones resist the acidifying processes while shales resist the rapid downward movement of water. Also, the type of silicate clays formed in a given area are commonly related to the nature of the parent materials. Parent materials may have been formed in place from the native rock (residual or sedentary), or they may have been transported from one area to another by water, ice, wind or gravity. Residual 9

14 parent materials are common in upland areas that have not been covered by materials transported from elsewhere. They are the most extensive of all parent materials. Water-transported materials may have been deposited in former lake bottoms (lacustrine), alongside streams (alluvial), or in ocean bottoms that have since been elevated (marine). If deposited near the seashore or river bank they are generally coarse textured, but if deposited further from the shore and in still water, they are commonly fine in texture giving rise to soils high in silt and clay. Parent materials transported by glaciers (ice) that once covered much of the world's northern latitudes include a mixture of rocks and minerals. Deposited by the melting glaciers, these heterogeneous materials are known collectively as glacial till. Some till is found in irregular deposits known as moraines. Coarse textured glacial outwash materials are another locally important parent material. Wind blown (aeolian) materials have resulted from the action of strong winds that carried finetextured materials from one area to another. These include, in order of decreasing particle size, dune sands, loess, and aerosolic dusts. The latter can move across oceans such that soils in North America may contain dust from China and those in South American contain dust from Africa. Parent materials that move down hill slopes by gravity (colluvium) are not very extensive, but are locally important. Another type of locally important parent material is the accumulated organic debris of partially decomposed plant tissues in which organic soils form, mainly in bogs and other wet areas. Climate largely determines the nature of the weathering that occurs and profoundly influences the living organisms found in an area. Temperature and precipitation affect the rates of chemical, physical and biological processes responsible for weathering and for soil profile development. For every 10 o C rise in temperature the rates of biochemical reactions double. Also, weathering reactions involve water. Effective precipitation in soil formation is the water that moves through the regolith. The greater the effective precipitation, the greater is the development of the soil profile. As a result, soils in desert areas are generally relatively shallow, being influenced mostly by mechanical weathering and by less intensive chemical reactions. Many soils of the humid tropics are deep, and in them most primary minerals have been destroyed by intensive weathering. Climate also has a profound influence on the natural vegetation. By its effects on temperature, moisture, natural vegetation, and soil organisms, climate plays a major role in determining weathering patterns and in influencing the kinds of soils that develop. Living organisms, especially natural vegetation, influence the development of soil characteristics in several ways. First, they are sources of organic matter essential for soil chemical and physical properties. The accumulation of organic matter in the upper layers of soil is one of the first steps in the development of soil profiles. Also, living organisms facilitate the cycling of essential plant nutrients. The nutrients are absorbed from the soil by plants, are returned to the soil surface in plant residues, and then back to the soil when soil organisms decompose the plant residues. Earthworms, termites, and rodents act as mixing agents, moving weathered materials and organic matter up and down the soil profile. Natural vegetation also helps stabilize the soil and protect it from the ravages of soil erosion. Soils of grasslands tend to have higher organic matter contents than forested soils. This results in generally more stable soil aggregates in areas with natural grassland vegetation. Soils developed under pine forests are generally more acid than those formed under deciduous forests since pine tree needles are lower in non-acid cations such as Ca 2+, Mg 2+, and K +. Topography influences the rate of runoff and erosion from the soil surface and consequently the infiltration of the water. Consequently, soils on hillsides are commonly not as deep as those on flat terrain, but the removal of excess water is more difficult in the flat lands. Excess water in depressed areas leads to the formation of peat bogs and in turn organic soils. Soils under shallow coastal waters are called subaqueous soils. The amount of time that parent materials have been subjected to weathering and soil forming processes influences soil properties. Residual parent materials have generally been subjected to soil forming processes longer than transported parent materials. Recently deposited alluvium alongside a stream has had too little time for significant soil profile development to take place. Likewise, soils developed from glacial parent materials and coastal plain areas are generally less weathered than 10

15 those developed from residual materials. Four major soil forming processes 1. Transformations such as rock weathering and organic matter decomposition that destroy some soil constituents and synthesize others. 2. Translocations of organic and inorganic materials out of the profile or from one horizon up or down to another. Often translocations result in accumulations of materials in a particular horizon, such as the accumulation of carbonates in the lower horizons of a semi-arid region soil. 3. Additions of materials such as plant residues, dust, and salts, to the soil profile from outside sources. 4. Losses of materials, such as water, eroded particles, oxidized organic matter, and leached salts from the soil profile. The combined action of these processes brings about vertical differentiation in the regolith, and leads to the formation of distinct horizontal layers called soil horizons. The Soil Profile The layering described above gradually gives rise to natural bodies called soils, each of which has a characteristic sequence of soil horizons that is termed a soil profile. The master horizons of these profiles include the following: O horizons: Organic horizons that form above the mineral soil. A horizons; Topmost mineral horizons, darkened somewhat by organic matter accumulation. Some finer materials have moved downward. E horizons: Zones of maximum leaching (eluviation) of clay and of oxides of Fe and Al, generally lighter in color than A horizon. B horizons: Zones of accumulation (illuviation) of materials such as silicate clays and oxides of Fe and Al, and sulfates and carbonates of Ca, and Mg. Materials may have formed in place or may have moved in from other horizons. C horizons: Material underlying the A, E, and B horizons that is generally little affected by the processes that formed the horizons above it. It may or may not be the same as the material from which the upper horizon formed. R layers: Underlying consolidated rock. Transition horizons: Transitional horizons between master horizons, designated by using both capital letters, the dominant horizon being listed first (e.g. AE, EB, BE, and BC). Subordinate horizon designations help clarify the specific nature of a soil horizon. For example a Bt horizon has accumulated silicate clays, while a Bk horizon has accumulated carbonates. 11

16 Model Answers to Study Questions 1. See Section 2.1 and Figure 2.3 During weathering the original rocks and minerals are destroyed or broken down into smaller particles or simpler compounds, but these products may also be combined so that new minerals are synthesized. Example: orthoclase breaks down to release potassium, aluminum and silicon into the weathering solution and from these components new minerals, perhaps silicate clays, are formed. 2. See Sections 2.1, 2.4 and Figure 2.3 (which show roles for chemical weathering reactions). Water participates in weathering reaction by several processes outline in Figure 2.3. During hydration reactions intact water molecules bind to the hydrating mineral. During hydrolysis water molecules split into H + and OH - ions, with the H + often replacing a metal cation from a mineral structure. During dissolution, water molecules surround ions from the soluble mineral until they become dissociated from each other. During carbonation, water dissolves carbon dioxide to from carbonic acid. 3. See Figure 2.3 (also refer to Figure 8.10) As the weathering of alumina-silicate minerals proceeds, the ratio of silicon to aluminum tends to decrease because silicon dissolves in the weathering solution (shown in Figure 2.3 as release of silicic acid) much more readily than does aluminum (which remains in mineral structures or as aluminum hydroxyl-oxides. Therefore, a low ratio of silicon to aluminum in a soil usually indicates a highly weathered stage. 4. See Figures 2.5, 2.7, 2.8, 2.9, 2.10, 2.13 and Example: Loess deposits cover thousands of km 2 in the central United States, but in a given landscape in this loess deposit, geologic erosion may cut down through the loess into underlying glacial till layers, exposing the latter as the main soil parent material in lower slope positions. See also Figure 3.23 for an example of the latter. Figure 2.10 illustrates how coastal plain layering can cause sandy soils to lie adjacent to clayey ones. 5. See Sections One possible set of comparisons is given here in tabular form. Factors of Soil Formation Forested mountain Grassland plain parent material residual sedimentary and igneous rocks sedimentary rock, outwash, other climate cooler, more humid - more effective precipitation more arid, warmer - less effective precipitation biota coniferous trees, under story ferns, shrubs, grasses mainly perennial grasses, forbs, herds of grazing animals, prairie dogs topography steep relatively level time relatively young due to rapid erosion losses relatively old surfaces 6. See Section 2.3. Colluvium is material transported downhill mainly by gravity. It includes an unsorted jumble of all sizes of particles, often with sharp edges. Glacial till is material transported and dropped by glaciers 12

17 and also includes an unsorted mixture of all particle sizes, but generally with rounded shapes. Alluvium is material transported by flowing river water and is usually sorted by particle size in difference horizontal layers with particles generally quite rounded in shape. 7. See Section 2.3, pp and Figures Loess is material transported and deposited by wind and consisting primarily of silt-sized particles. As a parent material, loess has a high capacity to hold water, but is generally quite permeable. Commonly, plant productivity is high and so is the rate of soil formation. 8. See Section 2.8. Transformations might include weathering of mica to form clay and the decomposition of plant residues to form humus. Translocations might include the upward movement of salts from groundwater to the soil surface in arid regions, and the downward illuviation of clay from the surface horizons to the B horizons. Additions might include calcium added with the deposition of dust and organic matter added with the annual shedding of deciduous tree leaves. Losses might include cations leached down out of the profile and into the groundwater and clay particles from the soil surface eroded away by runoff water. 9. Information from all parts of the chapter may be applied to this answer. See also Figure If we assume that the age (time factor) is the same in both cases, then the soil differences will be mainly due to the biota and climate factors of soil formation. The soil under the cool humid pine forest will likely be acid, relatively deeply weathered, with a thick O horizon and probably a thin A horizon. The soil under the semiarid grassland will likely be more shallow, neutral to alkaline in reaction, with little O horizon, but a thick A horizon. Both soils are likely to be relatively coarse in texture. 10. See Figures 2.16 and 2.22 The soil on the forested mountainside might include these horizons: Oi-Oa-Oe-A-E-Bw-C. The soil on the semiarid grassland plain might include these horizons: Oi-A1-A2-Bk-C. Many other answers would be reasonable. 11. See Section 2.6. The answer will vary with each location, but should include such changes as increasing thickness of A horizon, differences on development of the B horizon, color changes in the subsurface horizons (usually from bright reds and yellows in the upper members to grays and black in the lower ones), changes in parent material (e.g. colluvium near the bottom of the sequence). Changes in vegetation might be from trees to sedges, etc. 13

18 Multiple Choice Questions (Circle the single best answer for each question.) 1. Igneous rocks can best be characterized as: A. rocks formed when molten magma solidifies B. rocks containing both feldspars and micas C. rocks formed from the recrystallization of sedimentary material D. rocks containing a mixture of primary and secondary minerals E. rocks found primarily near volcanoes. 2. Which mineral is most resistant to weathering under humid temperate conditions? A. dolomite B. muscovite C. gypsum D. gibbsite E. biotite. 3. Which of the following is not a secondary mineral? A. silicate clay B. microcline C. calcite D. hematite E. gypsum. 4. Mechanical weathering processes result in: A. the decomposition of primary minerals B. the hydrolysis of minerals through frost action C. the disintegration of rocks due to differential expansion of minerals D. the oxidation of iron and manganese compounds 5. Which of the following is not considered one of the five major factors influencing soil formation? A. native parent materials B. living organisms C. climate D. valence state E. topography 6. Residual parent materials are best described as. A. materials formed under organic residues. B. materials formed by weathering of rocks and minerals in place. C. materials transported from one location to another by water, ice or wind. D. materials more dominant in Iowa than in the Southern United States. E. upland materials formed with relatively little chemical weathering. 7. Glacial till is a term used to describe parent materials that: A. were transported by water gushing from glacial fronts. B. were laid down in the bottom of former glacial lakes. C. were transported by high winds during glacial periods. D. are sorted by rapidly flowing melt waters. E. contain a heterogeneous mixture of mineral debris dropped by receding glaciers. 8. If you wanted to find a soil where physical weathering dominated over chemical breakdown you would be most apt to find it in. A. a desert region of Arizona B. a humid region in Brazil C. the hill lands of Georgia D. a lacustrine deposit in Minnesota E. a coastal plain area of Delaware 9. Igneous, sedimentary, and metamorphic are three. A. types of rocks B. basic classes of soils C. master horizon names D. forms of minerals E. processes of weathering 10. The element most often involved in oxidation reactions as minerals weather is. A. copper B. silicon C. aluminum D. magnesium E. iron 11. In which of the following horizons has the process of illuviation most likely occurred? 14

19 A. O horizon B. C horizon C. A horizon D. E horizon E. B horizon 12. Organic matter accumulation is most pronounced in the. A. O horizon B. A horizon C. E horizon D. B horizon E. C horizon 13. Silicate clay accumulation is most common in the. A. A horizon B. B horizon C. C horizon D. O Horizon E. E horizon 14. Which of the following statements is not correct? A. Grasslands are found in semi-arid and sub-humid areas. B. Coniferous forests are found mostly in cool humid areas. C. The type of native vegetation is controlled primarily by climate. D. Dense forests are found soil profiles have prominent O horizons. E. Tropical forests protect the soil from excessive weathering. 15. Which of the following statements is correct? A. Soils on hillsides tend to be deeper than those on level lands. B. Lacustrine parent materials have been subject to weathering for shorter periods of time than residual parent materials nearby. C. Limestone parent materials enhance the process of acidification. D. Nutrient cycling in forested areas contributes little to soil formation. E. Calcium carbonate accumulation is more prominent in humid than in arid regions. 16. Secondary minerals are most prominent in the fraction of soils. A. organic B. sand C. silt D. clay 17. The presence of rocks such as shale and sandstone indicate the existence of. A. a high water Table B. ancient seas C. old mountain ranges D. iron-rich minerals E. highly weathered soils 18. "Biotite-->clay-->iron oxide" represents a. A. catena B. weathering sequence C. silicate mineral sequence D. both A and B E. none of the above 19. Granite is an example of a(n). A. primary mineral B. sedimentary rock C. secondary mineral D. igneous rock E. eolian parent material 20. The transformation of gneiss into mica, quartz, and feldspar crystals is an example of: A. physical weathering B. chemical weathering C. disintegration D. hydrolysis E. both A and C 21. The reaction: mica + H 2 O K + + OH - + acid clay is an example of. A. chemical weathering B. physical weathering C. exfoliation D. carbonation E. both A and C 22. Exfoliation is caused by changes in. A. hydration B. oxidation C. temperature D. carbon dioxide dissolution E. all of the above 23. The mixed angular gravel, rock, and soil found at the foot of a slope is typical of what type of parent material? 15

20 A. eolian B. colluvial C. fluvial D. glacial E. lacustrine 24. Alluvial fans are usually characterized by soils. A. sandy and gravelly B. clay textured C. poorly drained D. nearly level True or False Questions (Write T or F after each question.) 25. Igneous rocks are formed when molten magma cools and solidifies. 26. Sandstones are good examples of metamorphic rocks. 27. Secondary minerals are recrystallized products of the chemical breakdown and/or alteration of primary minerals. 28. Iron and aluminum oxides are major components of igneous rocks. 29. Chemical weathering is accelerated by water, oxygen, and organic and inorganic acids moving down through the regolith. 30. Hydrolysis involves the splitting of water into its H + and OH - components while hydration attaches intact water molecules to a compound. 31. The presence of iron in a mineral generally increases its resistance to chemical breakdown. 32. Alluvial parent materials are those that have been laid down in former lake bottoms. 33. Residual parent materials have formed in place and have not been transported from one area to another. 34. Glacial till is laid down by melt waters gushing out from the front of glaciers. 35. Marine sediments are typical parent materials in coastal plain areas. 36. Organic deposits are most common in areas where water flow over the soil surface is restricted. 37. Climate influences not only the rate of weathering but the type of native vegetation dominant in an area. 38. Living organisms affect soil formation primarily by their constraining the level of oxygen in the soil. 39. Residual parent materials have generally been subjected to weathering for a longer period of time than have lacustrine or alluvial parent materials. 40. The O horizons of a soil are dominantly organic horizons occurring above mineral horizons. 41. The A horizons are more apt to be cultivated than the E horizons. 42. In most B horizons one of the dominant processes of soil formation has been eluviation. 16

21 43. The C horizons are generally more completely weathered than the other horizons. 44. Even if all the glaciers present today in the world were to melt, the melt water would have no measurable effect on the level of the world s oceans. 45. Glacial till can be recognized by the distinct layering of different kinds of particles. 46. Soils developed in wind-blown parent materials such as loess are generally of little agricultural value. 47. Sapric and fibric are terms used to describe peat parent materials. 48. A soil developed in residual parent materials will likely have properties related to the properties of the rock below the C horizon. 49. A soil developed in transported parent materials will likely have properties related to the properties of the rock below the C horizon. 50. The topmost horizon in most humid region forest soils is the A horizon. 51. Eluviation of clay, iron, and other materials is the principal process responsible for the formation of an E horizon. 52. Weathering of rocks usually is most intense in the center of a rock fragment, and gradually decreases toward the outside. 53. The parent materials for most coastal plain soils are residual in nature. 17

22 1. A 2. D 3. B 4. C 5. D 6.B 7. E 8. A 9. A 10. E 11. E 12. A 13. B 14. E 15. B 16. D 17. B 18. B 19. D 20. E 21. A 22. C 23. B 24. A 25. T 26. F 27. T 28. F 29. T 30. T 31. F 32. F 33. T 34. F Chapter 2 Answers 35. T 36. T 37. T 38. F 39. T 40. T 41. T 42. F 43. F 44. F 45. F 46. F 47. T 48. T 49. F 50. F 51. T 52. F 53. F 18

23 Chapter 3 Soil Classification Overview To make proper land-use decisions we need to know what kinds of soils we are dealing with, and how these soils differ from those at another location. Classification systems make it possible for us to communicate with each other about these different soils and their management. Comprehensive Classification System: Soil Taxonomy There are many systems of classifying soils in use today, each meeting the needs of the country or countries in which it is used. In this text we focus on Soil Taxonomy, a system used in the United States and, at least in part, in some 55 other countries. Soil Taxonomy utilizes the concept of soils as natural bodies and is based on observable and measurable soil properties. The system also employs a unique nomenclature that connotes the major characteristics of the soils in question. Soil Taxonomy utilizes many chemical, physical, and biological properties including soil moisture and soil temperature status. In addition, the presence or absence of certain diagnostic horizons in the soil profile helps ascertain the soil's classification category. A diagnostic horizon is a layer or soil zone whose properties meet certain criteria specified for the purposes of classification. A diagnostic horizon may be comprised of one or more of the genetic horizons discussed in Chapter 2. Seven of these diagnostic horizons, called epipedons, are surface horizons. Eighteen diagnostic subsurface horizons are used to characterize the different soils in Soil Taxonomy. In addition, five soil moisture regimes and ten soil temperature regimes help identify the category in which a soil belongs. Categories of Soil Taxonomy In the Soil Taxonomy all the world's soils are classed in the following categories (from the most general grouping to the most specific soil): orders (12), suborders (63), great groups (about 250), subgroups (about 1,400), families (about 8,000), and series (> 20,000). All soils belong to one of the 12 orders which are differentiated from each other primarily by the presence or absence of specific diagnostic horizons. In each order are found suborders that are distinguished from each other by different soil moisture and soil temperature regimes, and by dominant chemical and physical properties. In turn, each suborder is subdivided into great groups differing in the presence or absence of diagnostic horizons (including impervious pans), and in levels of certain chemicals such as clays and salts. Each great group is comprised of a number of subgroups that are characterized by a central (typic) member and by other members that are intergrades toward other orders, suborders or great groups, or that have characteristics not shared with the typic member. Within each subgroup are soil families that vary in properties that are important for plant growth or for engineering uses of the soil. Finally, within each family are soil series, the most specific category of the system. Soil series are identified in localized surveys and are given names of local significance. The two extremes of most soil classification systems are the soil that represents all soils collectively around the world, and a soil that is a specific natural body with characteristics that distinguish it from other such natural bodies. A soil can be characterized by a small three dimensional hypothetical unit called a pedon that is only about 10 square meters on the surface. This area is too small to serve as a practical field classification unit. However, a number of contiguous and closely related pedons that constitute polypedon make up what is termed a soil individual. Such a soil individual is approximated by the soil series described above. Terminology for Soil Taxonomy The names applied to the different soil classes in Soil Taxonomy connote much about the soil's properties. The nomenclature is logical and relatively simple. For example, the root of most of the names applied to the different soil orders is coined from words from Latin, Greek, or one of the modern languages. To this root is added a vowel and the letters sols. Thus, the root of Mollisols stems from 19