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Transcript
Soils As A Resource Weathering • Mechanical – physical breakup of rocks – Tree roots – Frost wedging – Abrasion • Chemical – transformation of minerals in rock – Solution – Hydrolysis – Oxidation Mechanical Weathering Frost Wedging and Talus Slope Abrasion Tree Roots 1 Hydrolysis • Example • Weathering of feldspar to kaolinite 2KALSi3O8 + 2H+ +H2O >>>>> Al2Si2O5(OH)4 + 2K+ + 4SiO2 Oxidation 4FeO + 2H2O + O2 >>>>> 4FeOOH SOILS AND SOIL EROSION Five factors of soil formation: Climate Topography (slope/exposure) Parent material (geology) Biota Time Soil horizons = Soil development = weathering, accumulation, translocation, and soil structure (soil vs. raw dirt) topsoil vs. subsoil (“tilth” and productivity decline with depth) Dissolution CO2 +H2O >>>> H2CO3 >>> H+ + HCO3CACO3 + H+ >>>>> Ca2+ + HCO3- Idealized Soil Profile Horizons O (organic) A (dark, humus) E (eluviation) B (illuviation) C (weathered rock) R (rock) 2 Parent Material • Some rocks and minerals are more resistant – example quartz takes longer to break down than feldspar Time • Greater time = greater soil development Climate • This is the most important factor – Humid tropical climates result in intense chemical weathering. • End result is soil depleted of most minerals except iron oxides and aluminum oxides – Humid mid-latitudes • Dissolved species recombine to form stable clay minerals – Hot Arid Climates • Little water for chemical weathering • Little water for leaching • Clays formed may be removed by deflation – Cold climates • Very dry • Less biological activity • Mechanical weathering is more dominant 3 Topography • Slope and exposure • Greater slope = less moisture retention • Northern exposure in northern hemisphere = less intense summer sun Photo By Werner Schellmann, 1982 Biota Soil Characteristics • Plant roots – open avenues for water, air and other chemicals to reach deeper in solid • Earthworms – stir soil, increase water infiltration • Rodents, stir soil • Deer, cattle, etc. – add nutrients • • • • • • Texture Color Structure Porosity Chemistry Moisture content Soil Texture Triangle Types of Soil Structure Figure 15.3 Figure 15.4 4 SOIL CLASSIFICATION (a Linnean system): SIX CATEGORIES Soil Color • Determined by composition and degree of soil formation NUMBER OF SOILS IN EACH CATEGORY order suborder great group subgroup family series 12 worldwide 47 230 1,200 6,000 15,000 in the USA SOIL ORDERS: • Provides evidence about how the soil formed – Organic rich, poorly drained – dark – Gray – wet most of the time – Yellow, brown, tan – good drainage Alfisols: Andisols: Aridisols: Entisols: Gelisols: Histosols: Inceptisols: Mollisols: Oxisols: Spodosols: Ultisols: Vertisols: world breadbasket, temperate forest soils volcanic soils arid soils new soils frozen soils with permafrost organic soils young, weakly developed soils grassland soils, most naturally fertile on earth tropical soils, extremely leached Northern conifer forest soils highly leached forest soils, low natural fertility heavy clay soils, high shrink/swell world breadbasket temperate forest soils grassland soils most naturally fertile on earth highly leached forest soils low natural fertility heavy clay soils high shrink/swell new soils No humus/salinization tropical soils extremely leached coniferous forest soils Aridisols Figures 15.12, 15.13 Spodosols Figure 15.20 5 Expandable clay soils (peat/swamp) Vertisols Figures 15.24, 15.25 Histosols Figure 15.26 Where Does it Come From? • • • • • • Farming Off road vehicles Human trails Horse trails Construction Deforestation Measurement of Soil Erosion • Measure directly on each slope • Not realistic • Too many variables • • • • Estimate depletion of reservoir capacity Sediment yield/yr to reservoir Size of basin Can calculate m3/acre or tons/km2 6 Natural Resource Conservation Service (NRCS) • Use an empirical equation to estimate • Universal Soil Loss Equation water sediment Example: Initial Volume of Reservoir = 100,000 m3 Volume of Reservoir after 1 year = 99,000 m3 – A=RxKxLxSxCxP – Where: A = average annual soil loss R = long term rainfall-runoff factor K = soil erodibility index L = hill slope length S = hill slope gradient C = soil cover factor P = erosion control practices Size of Basin = 5,000 km2 Yield = 0.2 m3/km2/yr 7