Soil-Forming Factors ESS 210

Transcription

1 What should you know? Soil-Forming Factors ESS 210 Chapter 2 pages Weathering processes - physical and chemical The five soil forming factors Types of soil parent materials Types of rocks and minerals Impacts of parent material, climate, organisms, topography, and time on soil formation Minerals Homogeneous, inorganic compounds, with definite chemical formula Primary minerals Formed as molten lava cools and solidifies Not chemically altered by weathering processes Secondary minerals Recrystallization and/or alteration products of primary minerals Primary Minerals Light colored aluminosilicate minerals Quartz [SiO 2 ]: most common, weather very slowly, sand size Feldspars: sand size, weather to soil clays K-feldspars KAlSi 3 O 8 Plagioclase feldspars: Albite NaAlSi 3 O 8 Anorthite CaAl 2 Si 2 O 8 Muscovite mica KAl 3 Si 3 O 10 (OH) 2 A parent of soil clay minerals: weathers to soil clay minerals Thin, translucent sheets (isinglass) Primary Minerals Dark colored, ferro-magnesium minerals Biotite mica KAl(Mg,Fe) 3 Si 3 O 10 (OH) 2 Thin dark sheets Weathers to soil clay minerals Hornblende NaCa 2 Mg 5 Fe 2 AlSi 7 O 22 (OH) Diopside CaMgSi 2 O 6 Hornblende and diopside weather to soil clay minerals Olivine (Mg,Fe,Mn) 2 SiO 4 Ferro-magnesium minerals weather more rapidly than aluminosilicate minerals Secondary minerals Al and Fe (metal) oxides and hydroxides (sesquioxides) Goethite FeOOH Hematite Fe 2 O 3 Gibbsite Al(OH) 3 Very stable soil minerals dominate in OLD soils Aluminosilicate clay minerals several types, common, and chemically complex Salts: calcite [CaCO 3 ], gypsum [CaSO 4 2H 2 O] 1

2 Rocks Mixtures of minerals Randomly dispersed, individual mineral crystals; heterogeneous solid Texture refers to the size of mineral crystals in rock: fine, intermediate, coarse Minerals present and rock texture determine weathering rate Igneous Cooling & Crystallization Weathering Rock Cycle Liquid Magma Heat & Pressure Weathering Sedimentary Heat & Pressure Metamorphic Heat & Pressure Igneous Rocks Formed when molten lava cools Primary minerals Coarse textured: granite Primarily quartz, feldspars, some dark minerals very slow weathering Fine to intermediate texture: basalt hornblende, augite, biotite, and other dark minerals relatively rapid weathering Granite Igneous Rocks Basalt Sedimentary and Metamorphic Rocks Sedimentary: deposition and re-cementation of weathering products from other rocks Sandstone, shale, limestone Metamorphic: igneous or sedimentary rocks transformed by high heat and/or pressure Sedimentary Rocks Granite Gneiss, schist Shale Slate Sandstone Limestone Quartzite Marble Sandstone Limestone 2

3 Metamorphic Rocks Weathering Gneiss Slate The (1) physical disintegration of rock to form smaller rocks or individual mineral particles and the (2) chemical decomposition of minerals to form dissolved substances and new minerals Weathering categories Physical Chemical Physical Weathering A disintegration process that decreases particle size and increase particle surface area. Occurs through the affect of: Temperature Differential heating or cooling of rocks exfoliation Freeze-thaw: water expands upon freezing, exerting tremendous force Abrasion by water and water-borne sediments, windblown particles, and ice in glaciers Organisms Plant roots Soil animals Humans Chemical Weathering Alters the composition of minerals Conversion of primary minerals into secondary minerals, and secondary into other secondary minerals Most rapid with warm temperatures, high precipitation, and small particle size There are geochemical and biochemical agents of change Water is required Chemical Weathering Processes Solutioning (dissolution): mineral dissolves in soil solution; common to soluble salts CaSO 4 2H 2 O (gypsum) Ca 2+ + SO H 2 O CaCO 3 (calcite) Ca 2+ + CO 3 2- Hydrolysis: water acts upon a substance to create a new substance Involves both H 2 O and H + as reactants Often results in release of nutrients from minerals and the formation of sesquioxides KAlSi 3 O 8 (K-feldspar) + 7 H 2 O + H + K + + Al(OH) 3 (gibbsite) + 3 H 4 SiO 4 0 Hydration: addition of water to a mineral structure 5 Fe 2 O 3 (hematite) + 9H 2 O Fe 10 O 15 9H 2 O (ferrihydrite) Chemical Weathering Processes Hydrolysis is an important weathering process Presence of H + (acidity) accelerates weathering Sources of protons CO 2 in rainfall produces carbonic acid: CO 2 + H 2 O H 2 CO 3 H + + HCO 3 (rainfall is naturally acidic; ph ~ 5.6) Plant roots and soil organisms respire and produce carbonic acid Soil organic matter is a proton source Other acidic substances in rainfall: SO x /NO x + H 2 O H 2 SO 4 /HNO 3 Fertilizers (e.g., NH 4+ ) 3

4 Chemical Weathering Processes Oxidation/reduction (redox) reactions (the second most important weathering process) Addition or loss of electrons (e ) from atom in a mineral Oxidation = loss of e ; reduction = gain of e Electron-rich elements are termed reduced (e.g., Fe 2+ ); electron-poor elements are termed oxidized (e.g., Fe 3+ ) O 2 is most common oxidizing agent Elements in primary minerals commonly exist in a reduced state Oxidation and reduction occur together; they are coupled Redox Reactions Oxidation of Fe 2+ by O 2 (O 2 is the oxidant, it will be reduced during the redox process) Oxidation half-reaction: Fe 2+ Fe 3+ + e Reduction half-reaction: ¼O 2 + e + H + ½H 2 O Complete redox reaction: Fe 2+ + ¼O 2 + H + Fe 3+ + ½H 2 O Complexation Reactions Microorganisms and plant roots exude organic acid anions, e.g., citrate, oxalate, and malate These organic acids bond with (chelate) metals, e.g., Al 3+ and Fe 3+, to form soluble complexes The metal-organic complex is stable and much more soluble than the metal ion alone Complexation Reactions Example: Al 3+ complexation by ketogluconate Al(OH) 3 (gibbsite) + 3H + Al H 2 O Al 3+ + C 5 O 5 H 9 COO C 5 O 5 H 8 COOAl + + H + Soil Formation Processes Soil is an open system Additions - movement into profile Organic matter Rainfall Sediments Chemicals: natural and anthropogenic Losses - movement out of profile Evapotranspiration Erosion Leaching of water and chemicals Gaseous losses of nutrients Removal by vegetation Soil Formation Processes Translocations: movement within the soil profile Eluvial processes Illuvial processes Transformations: a change in form Physical weathering Chemical weathering Microbial degradation 4

5 Five Soil Forming Factors Soil is a dynamic natural body formed by the combined effects of climate and biota, as moderated by topography, acting on parent materials over time. Soil = ƒ(climate, biota, topography, parent materials, time) Factor One: Parent Material Parent material impacts Soil textural class Innate soil fertility Types of clay minerals Soil ph Classes of parent materials based on placement Residual Transported (six types of transported materials) Residual Parent Materials Soils develop from underlying bedrock Igneous, sedimentary, metamorphic Type of rock strongly influences type of soil Limestone clayey soils Sandstone coarse, acidic soils Granite coarse, acidic soils Slate, shale clayey soils Transported Parent Materials Colluvial debris Alluvial deposits Marine sediments Lacustrine sediments Eolian deposits Glacial deposits Colluvial Debris Poorly sorted fragments on steep slopes or at the foot of slopes, carried by gravity Small geographical areas Usually rocky and stony, no layering Physical weathering processes dominate relative to chemical weathering processes Well-drained but unstable 5

6 Alluvial Deposits Floodplains During flooding, water spreads and slows, and fine sediment is deposited. Horizontal and vertical stratification Terraces are old floodplains above the current floodplain Usually very fertile soils and important for agriculture, forestry, wildlife Poor choice for homes and other urban development Alluvial Deposits Alluvial fans Usually gravelly/stony in mountainous regions, can have finer material as well. Stream leaves narrow upland channel, descends to broad valley below Alluvial Deposits Delta deposits The continuation/terminus of the floodplain Rivers carry much clay/fine silt to lake or ocean Very slow water = deposition of fine particles Very clayey, swampy, poorly drained Example: Mississippi River delta in Louisiana 6

7 Marine and Lacustrine Sediments Marine - Coastal Plains Ocean sediments build up over time Exposed by changes in elevation of earth s crust Materials are gravely, sandy, clayey depending on area Atlantic and Gulf Coastal areas, ~ 10% of US Lacustrine Lake sediments build up over time Clayey soils formed as lakes dried Major areas of lacustrine soils in glaciated areas Eolian Deposits Loess deposits Common in central United States Wind carried silts (coarse clays to fine sands) from glaciated areas Cover other soils or parent materials Western one-third of Tennessee is loessial Very thick (8+ m) at Mississippi River to non-existent at Tennessee River Blankets much of Iowa, thick at the Missouri River, thin on eastern side Others - sand dunes (sand-size), aerosolic dust (clay-size), volcanic ash (allophanic soils) Glacial Till As glacier advances, grinds up rock and carries it Till is unsorted, unconsolidated material Deposited as glacier melts and recedes Till deposits called moraines Ground moraine - material deposited in relatively uniform layer during retreat Terminal or end moraine - material left pushed up in ridge at southern-most edge of advancing glacier Recessional moraine terminal moraines from more than one advance Ground moraines Terminal moraines Glacial Outwash As glaciers melt, glacial rivers and streams form and carry sediments Coarse materials drop first Fine materials carried furthest Deposits are sorted Factor Two: Climate Influences soil formation three ways: 1. Precipitation 2. Temperature 3. Native Vegetation 7

8 Climate: Precipitation As rainfall increases, chemical and physical weathering rates increase Profile depth increases Nutrient status changes Loss of base cations Ca 2+, Mg 2+, K +, Na + Al 3+, Fe 3+, Mn 2+, H + increase Soil acidity increases Soil Moisture Regimes Aquic: saturated with reducing conditions most of the year Udic: soil moisture control section is dry for < 90 cumulative days per year Ustic: is dry for > 90 cumulative days per year Aridic: dry in all parts for > half the year Xeric: moist winters, dry summers (Mediterranean, California) Soil Moisture Regimes Aquic = wet = tile needed for row crops Udic = enough precipitation for corn Ustic = enough precipitation for wheat Aridic = cacti without irrigation Xeric = precipitation when not needed for production of most crops winter Climate: Temperature Chemical and biological reaction rates double for every 10 ºC increase Climates with extreme T, physical weathering (e.g., freeze-thaw) more significant than chemical weathering Evapotranspiration increases with increasing T Soil Temperature Regimes Cryic mean annual T < 8 ºC Frigid mean annual T < 8 ºC; difference between mean summer and mean winter T is > 6ºC Mesic mean annual T > 8 ºC and < 15 ºC; difference between mean summer and mean winter T is > 6 ºC Thermic mean annual T > 15 ºC and < 22 ºC; difference between mean summer and mean winter T is > 6 ºC Hyperthermic mean annual T > 22 ºC; difference between mean summer and mean winter T is > 6 ºC Climate: Type of vegetation Humid = forest Sub-humid, semi-arid = grasslands Arid = shrubs, brush, succulents 8

9 Factor Three: Biota Plants, animals, microorganisms Important for MANY processes in soil formation Chemical weathering Organic acid anions, carbonic acid, oxidationreduction Organic matter accumulation (humification) Water holding, nutrient holding Aggregation Polysaccharides, gelatinous materials Biota Nutrient cycling Base recycling Ca, Mg, K Nitrogen addition Microbial N-fixation N 2 NH 4 + Profile mixing bioturbation earthworms, insects, etc. Impact of Native Vegetation Grasslands High OM below surface Continuous root production, high interception of rain Coniferous Forests Vegetation low base cations (Ca, Mg, K) Low recycling Highly leached, acidic soils Impact of Native Vegetation Deciduous forests High in basic cations High base cycling Slightly to moderately acid Forest soils are usually more developed with more horizons, etc... Grassland vs. Forest Soils Grassland Deciduous Coniferous Factor Four: Topography Affects amount of water soil sees (yellow arrows): concept of effective precipitation Slope aspect affects soil temperature Upland stable Sideslope active erosion Footslope active deposition Floodplain active deposition Mollisol Alfisol Spodosol Terrace/Fan stable 9

10 Landscape Positions Upland Soil developed in residuum or in stable, unconsolidated materials (loess, glacial till) Rocks angular (except in till) Well-developed soils Highly-dissected Footslope Bottom of slope, colluvial and alluvial deposits Partly rounded rock, immature/younger soils Landscape Positions Terrace (second bottom, bench land) Old alluvium, higher elevation than current Floodplain Round stones, rocks - indicates water worked Mature soils, some dissection Bottomland (floodplain) Deposited by present stream action Rounded stones Immature soils, little dissection Topography: Catena or Toposequence Soils with same parent material, differ primarily in topographic location Typical pattern of soils and underlying material in the Hawthorne- Dellrose-Mimosa general soil map unit (Marshall Co., TN) Hawthorne-Dellrose-Mimosa Alfisol A Bt1 Bt2 Bt3 Bt4 BC C R Mimosa Ultisol A BA Bt1 Bt2 2Bt3 Dellrose Inceptisol A AE Bw C Cr Hawthorne Factor Five: Time Pretty obvious! Works in concert with other factors Chronologically old soil may be developmentally young, e.g., arid region soils which have very little development Soil age is a relative thing! Old soils = high water throughput (Ultisols & Oxisols) Young soils = low water throughput (Aridisols) Physiography of Tennessee Mississippi River Plateau Slope Central Basin Highland Rim Great Valley Cumberland Plateau Unaka Range Modified from "Geography of Tennessee", published by Ginn and Co. 10

11 Physiographic Regions Regions and their soils Mississippi River floodplain Loess Coastal Plain Highland Rim Central Basin Cumberland Plateau Valley and Ridge Smoky Mountains Unaka Range Generally young (developmentally), shallow soils. Parent materials are metamorphic and igneous rock Inceptisols very common - weak horizonation Ultisols in valleys, low elevations Valley and Ridge region (Knoxville) Well-developed soils Ultisols and Alfisols in limestone, sandstone, shale Regions and their soils Cumberland Plateau Generally loamy soils Sandstone is dominant parent material Ultisols dominant Highland Rim Generally clayey soils, many cherty Limestone is dominant parent material Ultisols and Alfisols Central Basin Clayey, often shallow soils Alfisols, Ultisols, Mollisols, Inceptisols Regions and their soils Coastal Plain Ultisols & Alfisols Clayey soils from fine sediments Loamy soils in coarse sediments Fine-loamy soils in loess over sediments West Tennessee Loess Region - Alfisols Fine-loamy soils in loess deposits, many fragipans Erosion is major risk Mississippi River Floodplain Entisols, Inceptisols, Alfisols, Mollisols Young, productive soils 11