Notes on Soils What are they? - Zoned weathering profiles developed by mechanical, chemical and biological processes acting on rock and/or sediment. Processes affecting soil profile development: Thermal expansion-contraction Frost wedging, heaving, riving Unloading – sheeting and exfoliation Bioturbation – burrowing re-mixes soil horizons Biological reactions in roots – generate organic acids that decrease pH. > acidity = faster reaction rates for mineral weathering. Dissolution – of carbonate minerals by weak acids (precipitation) CaCO 3 + H 2 CO 3 = Ca 2+ + 2HCO 3- in solution Oxidation – of Fe freed by silicate hydrolysis from silicate minerals. Oxides – hematite, Fe 2 O 3 ; 5R-2.5YR, bright red Hydrous oxides – Goethite, FeO(OH); 7.5-10YR, yellowish brown, drained soils Lepidocrocite, FeO(OH); 5-7.5YR, orange, wet soils Ferrihydrite, Fe 5 HO 8 • 4H 2 O; 5-7.5YR, reddish brown Intense formation of hydrous iron oxides in very acidic settings such as acid rock and acid mine drainage can cause ferricrete, a solid mass. Silicate hydrolysis
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