1 Soil-Forming Factors ESS 210 Chapter 2 pages 31–74 What should you know? • 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]
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Soil-Forming FactorsESS 210
Chapter 2pages 31–74
What should you know?
• 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
• The (1) physical disintegration of rock to form smaller rocks or individual mineral particles and the (2) chemicaldecomposition of minerals to form dissolved substances and new minerals
• Weathering categories– Physical– Chemical
Physical WeatheringA 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– CaSO4•2H2O (gypsum) → Ca2+ + SO4
2- + 2H2O– CaCO3 (calcite) → Ca2+ + CO3
2-
• Hydrolysis: water acts upon a substance to create a new substance– Involves both H2O and H+ as reactants– Often results in release of nutrients from minerals and
the formation of sesquioxides– KAlSi3O8 (K-feldspar) + 7 H2O + H+
→ K+ + Al(OH)3 (gibbsite) + 3 H4SiO40
• Hydration: addition of water to a mineral structure– 5 Fe2O3 (hematite) + 9H2O → Fe10O15•9H2O (ferrihydrite)
Chemical Weathering ProcessesHydrolysis is an important weathering process• Presence of H+ (acidity) accelerates weathering• Sources of protons
– CO2 in rainfall produces carbonic acid: CO2 + H2O →H2CO3 → H+ + HCO3
– (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: SOx/NOx + H2O →
H2SO4/HNO3– Fertilizers (e.g., NH4
+)
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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., Fe2+); electron-poor elements are termed oxidized(e.g., Fe3+)
– O2 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 Fe2+ by O2 (O2 is the oxidant, it will
be reduced during the redox process)• Oxidation half-reaction:
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
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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.
• 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
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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
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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,
• 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 > 6 ºC
• 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