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Soils and Environment

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  1. Soils and Environment

  2. WEATHERING PhysicalandChemicalEffects

  3. WEATHERING, EROSION, TRANSPORTATION • Weathering- Physical disintegration and chemical decomposition of rocks • Erosion- Physical removal • Transportation- Movement of eroded particles • Chemical vs. Physical Weathering • Effects of weathering • Surface alteration of outcrops • Spheroidal weathering • Differential weathering

  4. Mechanical Weathering • Freeze-Thaw Weathering • Salt Weathering • Wetting and Drying • Insolation Weathering • Pressure Release • Stress Corrosion Cracking

  5. CHEMICAL WEATHERING • Decomposition of rock to form new substances • Changes in Equilibrium • Water • Congruent solution (limestones) vs incongruent solution (clay minerals) • Carbon Dioxide- changes in pH change solubility of minerals • Role of Oxygen • Fe in ferromagnesian minerals becomes oxidized • Hematite and Limonite

  6. Chemical Weathering • Solution of ions and molecules • Production of new materials • clay minerals • oxides • hydroxides • Release of residual unweathered materials • quartz and gold

  7. Chemical Weathering of Silicates Interlayer Cations hydrolysis Na, K Ca, Mg Solutions of Na+, K+, Ca2+, Mg2+ Hydration, solution Aluminosilicate sheets (e.g. as part of feldspars) solution Silicic acid (H4SiO4) Al 3+ & Si 4+ Hydrolysis, hydration Secondary minerals, e.g. clays Brucite and alumina sheets and incorporated ions, e.g. Fe2+ Oxidation, hydrolysis Hydrous oxides, e.g. FeO(OH) Brucite Alumina Fe2+ hydration Chelate complexes hydrolysis chelation

  8. Results of Weathering: Clay Mineral • Clay minerals give information about weathering conditions • Kaolinite: humid, acid conditions, alteration of K-Feldspar • Illite: weathering of feldspars and micas under alkaline conditions where leaching of mobile K does not occur • Montmorillonite: weathering of basic igneous rocks under alkaline conditions with a deficit of K+ ions

  9. Clays • Kaolinite • Illite • Montmorillonite • Chlorite • Mixed-layer clays

  10. Soils: Definitions • Loose unconsolidated material composed of regolith and partially decayed organic matter, water and gases • A soil profile is a vertical face of soil that can be exposed and includes all the layers (horizons) from the surface to the parent rock (bedrock) • The solum is that part of the profile that is influenced by plant roots • A pedon is a 3-D representation, the smallest volume that can be called a soil

  11. Soils and Food Production • Roots of Agriculture • Middle East (Iraq) origins • Roman techniques of soil fertility • Terrace building in Meso-America and South East Asia • Increasing world population reliance on pesticides and fertilizers

  12. Useful Properties of Soils • Provides water, nutrients and anchorage for vegetation • Provides habitat for decomposers, essential in carbon cycle and mineral cycling • Acts as a buffer for temperature changes and for the flow of water between atmosphere and groundwater • Because of its cation exchange properties, acts as a pH buffer, retains nutrients and other element loss by leaching and volatilization

  13. Soils as part of the Ecosystem • An ecosystem is a community of interacting organisms and their physical-chemical environment that function as a self sufficient whole • Soils are an essential part of the Carbon cycle due to the effects of microorganisms Atmospheric CO2 Primary Producers Decomposers Organic Compounds

  14. Soils and Geologic Time • Soils could only exist after the colonization of land by organisms, in particular vegetation • First land plants in the Ordovician (450 my) • By the Devonian the land had been colonized (370 my) • By the Carboniferous (300 my) extensive forest habitat generated soils similar to today • Properties of soils determined by climate, organisms, relief, parent material and time, thus we can extrapolate the conditions that formed paleosols

  15. Soils and Humans • Cultivation of soils began about 10,000 BP in Mesopotamia (Tigris and Euphrates rivers of Iraq) • The land was a porous friable silt loam that required irrigation. • The civilization ended due to wars, floods, infilled irrigation channels, erosion (gullying), salinization, loss of food production and famine • Other centers of agriculture in the fertile Nile valley, Indus and the river valleys of China • In Europe soil erosion instigated colonization of other lands and remains the worst problem facing humans. • In other areas terracing became the primary farming technique (Southeast Asia, Peru)

  16. The Green Revolution: An Idea of the 1960’s • Increase world population demands the increase of food production • Increases in land under cultivation, more intensive agriculture (mechanization) or both • The introduction of fertilizers, pesticides, irrigation, varietal seeds (seed banks), Population growth 2% WHILE food growth 4%. • In 1970, Norman Borlaug received the Nobel Peace Price • However, not a panacea >> potential realized, need for irrigation,constant inputs of fertilizers, pesticides, and energy intensive mechanized labor, benefit large land holders, detrimental to most 3rd world countries

  17. Soils • Pebbles, gravel and sand particles • Aggregates (mm to cm) of clays • Roots • Partially decayed to totally decayed vegetation (Humus) • Organisms (earthworms, arthropods) • Pore spaces filled with air and gases • Water

  18. SOIL Texture • Texture: Relative proportions of sand, silt and clay • Dominant size fraction as a descriptor [clay, sandy clay, silty clay] • If no dominant fraction then >> Loam {40% sand; 40% silt; 20% clay} • Clay-sized particles vs. clay minerals • type of clay not just % clay • Texture is an indicator of other properties (ease of cultivation)

  19. Soil Textural Classification

  20. Soil Structure • Arrangement of soil particles into cemented aggregates • Aggregates are secondary units or granules composed of many soil particles held together by organic substances, iron oxides, carbonates, clays and/or silica • Natural aggregates are called PEDS • A CLOD is a coherent mass of soil broken apart by artificial means

  21. Bulk Density • Density of soil minerals ranges between 2.6 to 2.7 g/cm3 • When dry, the bulk density is about half the above value, because voids are filled with air • Defined as rb = M/V; • Commonly 1.0-1.6 g/cm3 • Varies over small distances due to weather, cultivation, compression by animals • Increases with depth

  22. Core sampler for determination of bulk density. The sampler yields a core of a fixed volume. The core is dried and weighed The weight divided into volume gives the bulk density of soil

  23. Porosity • Calculated from the dry bulk density and the particle density • e = 1 - (rb / rs) x 100 = % porosity • Where rs is usually between 2.6-2.7 g/cm3 • The pore space is occupied by water and air • Transmission pores >50mm • Storage pores 0.5-50 mm • Residual pores <0.5 mm

  24. Relationship Between Texture Bulk Density, and Porosity

  25. SOIL • Various definitions • Unconsolidated material above bedrock • weathered material & organic matter • supports plant life [air, water, organic matter & mineral material] • Loam {40% sand; 40% silt; 20% clay} • Clay-sized particles vs. clay minerals • Soil Horizons • Residual Soil (on bedrock) • Transported Soil (alluvium)

  26. SOIL • Parent Rock, Time, and Slope • Organic Activity • Soils and Climate • Pedalfer- aluminum and iron rich clays • Pedocal- calcium rich • Hardpan- crusts (Fe and caliche) • Laterites- tropical soils • Bauxite- Principal ore of Al • Buried soils

  27. Soil Horizons • Identified, named, by symbols consisting of upper and lower case letters • Each symbol recognizes a formational property • O- organic material • A (A1)-accumulation of decomposed org. matter • E (A2)- mineral layer, loss of silicate, eluviation (leaching) horizon • B (B2)-Illuviated humus, silicate clay or hydrous oxides • C (C)- mineral horizon above parent • R (R)- Consolidated Bedrock

  28. Soil Structure: Pan Structures • Dense layers or pans • Interference with root and water penetration • Produce shallow soils • Due to compaction, filling of pores with clays or chemical cements • Very firm layers are called hardpans

  29. Types of Pans • Claypan • Dense soil layers produced by downward migration of clay and accumulation in subsoil as a B-horizon material • Duripan • Layers cemented by precipitates of silica, alone or in combination with iron oxides or calcium carbonates • Fragipan • Fragilis (brittle), dense subsoil layers (50-60 cm beneath surface) bonded into a hard, brittle form by clay • Caliche • Hard lime-cemented white crust in arid regions • Plinthite • Laterite, precipitated sesquioxides as cements. Weathered soils of the tropics formed at depth • Plowpan • Artificially produced, due to compaction by plows.

  30. Caliche

  31. Soil Taxonomy • Organization into 11 orders*, 54 suborders, 238 great groups, 1922 subgroups and then families and series, each series subdivided into mapping units called phases of series • * including a tentative order andisols (soils with over 60% volcanic ejecta)

  32. Orders • Most general category • 5 of the orders exist in a wide variety of climates • Histosols (organic soils); Entisols (undeveloped); Inceptisols (slightly developed); Andisols (volcanic); Vertisols (swelling-clay) • 6 are the product of time and the microclimate in which they develop • Mollisols- naturally fertile, slightly leached, semiarid to subhumid, grassland • Alfisols- fertile soils in good moisture regimes • Ultisols-leached, acidic soils, warm climates, low-moderate fertility • Aridisols-arid region soils • Oxisols- infertile, hot humid tropics • Spodosols- cool climate, acidic sandy

  33. Soil Orders • In the US Mollisols cover 25% of the land • Worldwide distribution • Aridisols 19% • Alfisols 13% • Inceptisols 9% • Mollisols 8% • Oxisols 8%

  34. Soil Taxonomy • Classification at 6 different levels • Level of generalization relates to the range in properties allowed in the different classes • Soil Orders • Suborders • Great Groups • Subgroups • Families • Series

  35. Soil Orders

  36. Engineering Properties • Plasticity-water content of soil • Soil Strength-ability to resist deformation • Cohesion-ability of particles to stick together • Friction-fnc. Density, size, shape of soil particles • Sensitivity-measures changes in soil strength, clay soils very sensitive to disturbance (liquifaction) • Compressibility- soils tendency to consolidate (decrease in volume), settling causes foundation cracks • Erodibility- ease of removal by wind and water • Corrosion- function of soil chemistry • Ease of Excavation • Shrink-swell potential- gain or loose water (expansive soils)

  37. Soil Erosion • Soil Erosion: Removal in part or whole of soil by wind or water • Natural process • Erosion is slight from areas covered by dense grasses or forest but increases dramatically in exposed steep, poorly covered soils • Increased by human activity especially poor agricultural practices

  38. Soil Erosion • Soil erosion has been documented as early as 10,000 ybp in Mesopotamia due to agriculture that cleared the land (deforestation), overgrazed the land by herbivores (sheep, goats) • Erosion in Europe is believed to have occurred 5000 ybp with clearing of woodlands • In the us during the 1930’s (Dust Bowl) wind and water erosion left devastating effects • On a positive note, erosion from Ethiopian highlands generated the fertile sediment for Egyptian agriculture for 1000’s of years

  39. Erosion by water in 1961 Kentucky