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Download Lecture 3, January 25, 2017 - EPSc 413 Introduction to Soil Science
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Transcript
Lecture 3: Introduction to Soil Formation and Classification Soil Formation One Planet, Different Soils Andisol Spodosol Aridisol Mollisol Alfisol Ultisol Major Soil Forming Processes • Transformations: – Mineral weathering – Degradation of organic matter • Translocations: – Horizontal or vertical movement of materials – Key to forming soil horizons • Additions: – Organic matter from dead plants – Wind-blown dust • Losses: – Erosion and leaching by water – Decomposition of organic matter to CO2 Factors Controlling Soil Formation 1. Parent material: Soil precursor 2. Climate: Temperature and precipitation 3. Biota: Native vegetation, microbes, soil animals, humans 4. Topography: Slope, aspect, landscape position 5. Time: Duration of exposure of parent material to weathering Parent Material Review: Rocks and Sediments • Major classes of rocks: – Igneous: Formed from cooling of molten rock – Sedimentary: Formed from lithification of sediments upon burial at Earth’s surface – Metamorphic: Formed from alternation of igneous or sedimentary rocks • Key types of sediment materials: – Sand & Silt: Typically composed of quartz [SiO2] – Clay: Dominated by microscopic aluminosilicate minerals – Loess: Wind-blown silt produced by glaciers; “rock flour” Weathering: Turning Parent Material into Soil Primary and Secondary Minerals Climate • Climate determines the nature and intensity of weathering of parent material • Temperature and precipitation are the major climate variables that affect soil formation Global Atmospheric Circulation Dry Wet Dry Wet Dry Wet Dry Variation in Soil Depth with Latitude From: Schaetzl and Anderson (2005) Soils: Genesis and Geomorphology Influences of Biota Topography Time (Granite in SE United States) Interaction of Soil Forming Factors Key Concepts in Soil Formation • Five Major Soil Forming Factors: Parent Material, Climate, Biota, Topography, Time – The combined effects of these factors leads to the soil found at a specific location • Four Major Soil Forming Processes: Transformations, Translocations, Additions, Losses Soil Classification Variation in US Soils Andisol Spodosol Aridisol Mollisol Alfisol Ultisol Concept of Individual Soils • Soil varies across a landscape, but soils in specific locations display specific characteristics • The smallest unit to display a set of unique characteristics is called a pedon • A group of similar, closely associated pedons is called a polypedon or soil individual Soil Taxonomy • A comprehensive classification system for soils – Hierarchical structure – System is based on soil properties that can be observed and measured • Diagnostic soil horizons • Clay type and content • Base saturation • Provides a universal means of discussing soils • Well-documented, publicly available method for fully naming any soil on Earth Official Soil Classification System Available at: https://www.nrcs.usda.gov/wps/portal/nrcs/main/soils/survey/class/taxonomy/ Official Instruction Manual Available at: https://www.nrcs.usda.gov/wps/portal/nrcs/main/soils/survey/class/taxonomy/ Categories in Soil Taxonomy 12 Soil Orders • • • • • • Alfisol Andisol Aridisol Entisol Gelisol Histosol • • • • • • Inceptisol Mollisol Oxisol Spodosol Ultisol Vertisol I AM A SUAVE HOG • • • • • • • • • • • • Inceptisol Alfisol Mollisol Andisol Spodosol Ultisol Aridisol Vertisol Entisol Histosol Oxisol Gelisol I GAVE US OMAHA • • • • • • • • • • • • Inceptisol Gelisol Alfisol Vertisol Entisol Ultisol Spodosol Oxisol Mollisol Andisol Histosol Aridisol Argillic Horizon Alfisols • Moderately leached soils with a clay-rich B-horizon • Typically found in forests Andisols • Dominated by short-rangeorder minerals • Form in volcanic ash Aridisols • Dry and often saline soils • Form in arid regions Histic Epipedon Entisols • Little or no evidence of pedogenesis • Often sandy or shallow Gelisols • Contain permafrost • Found in arctic and high alpine regions Histosols • Dominantly organic soil material, no permafrost • Wetlands, bogs, marshes Mollic Epipedon Oxic Horizon Inceptisols • Soils with altered horizons but no accumulations • Young soils, still developing Mollisols • Dark surface layer rich in base cations • Found in grasslands Oxisols • Highly leached soils with iron and aluminum oxides • Found in tropical regions Albic Horizon Spodic Horizon Argillic Horizon Spodosols • Accumulation of Fe/Al and OM below leached horizon • Found in coniferous forests Ultisols • Highly leached soils with a clay-rich B-horizon • Found in warm, humid regions Argillic Horizon Vertisols • High content of swelling clay; deep cracks when dry • Shrink-swell behavior Soil Orders Vary with the Degree of Weathering Hypothetical Soil Development Sequence Entisol Inceptisol Alfisol Ultisol Oxisol Increasing Age, Increasing Soil Development, Increasing Weathering Process of Assigning Soil Orders Official Definitions More Complex Lower Levels of Classification • Suborder: Soils of an order having a unique property (moisture or temperature regime) • Great Group: Soils of a suborder having a type of diagnostic horizon or other feature • Subgroup: Soils of a great group sharing another common feature • Family: Soils of a subgroup sharing common soil properties • Series: Soils of a family sharing all major characteristics, including depth of horizons Order Suborder Great Group Subgroup Family Series Soil Series • All the soil individuals in the world with a common suit of soil profile properties and horizons are called a soil series – Generally unique to a state; rarely global – 25,000 soil series named in US MENFRO SERIES The Menfro series consists of very deep, well drained, moderately permeable soils formed in thick loess deposits on upland ridgetops, backslopes and benches adjacent to the Missouri and Mississippi Rivers and their major tributaries. Slopes range from 2 to 60 percent. Mean annual temperature is 56 degrees F, and mean annual precipitation is 36 inches. TAXONOMIC CLASS: Fine-silty, mixed, superactive, mesic Typic Hapludalfs Taxonomic Class • A soil’s taxonomic class is a descriptive term that covers all of the basic features of the soil • Example: Fine-silty, mixed, superactive, mesic Typic Hapludalfs – – – – – Fine silty texture in the upper mineral surface horizon Mixture of clay types with high cation exchange capacity Average soil temperature of 8 to 15°C “Typic” means a standard Hapludalfs Hapludalfs: A moist (-ud-) Alfisol (-alfs) with no other special features (hapl-) Key Concepts in Soil Classification • A system has been created to objectively classify soils – This system relies of characteristics of the soil, including diagnostic horizons and physical and chemical properties • There are 12 major soil orders: Know these! • There are many lower levels of classification – The soil series is the most important level for understanding soils at a specific location