Lecture 7, Soil Development

GES175, Science of Soils

Lecture 7, Soil Development

Parent Material → Soil primary minerals

secondary minerals

Rock

Ideal Weathering Series Entisol Inceptisol Aridisol (dry climate) Mollisol

Spodosol (cool and wet)

Vertisol (clay mineralogy) Alfisol Ultisol

Oxisol

Soil Formation Five Soil Forming Factors: ¾Climate ¾Biota ¾Topography ¾Parent Material ¾Time

Soil Formation „

Climate – Arid: Minimal leaching, slow dissolution – Humid: Extensive leaching, rapid

dissolution – Cool: Active physical weathering, slow chemical weathering. Buildup of OM. – Warm: Strong chemical weathering, rapid OM decay (nutrient cycling)

Soil Formation „

Biota – Diversity and activity – Plant canopy Root exudates promote weathering „ Grasslands: Base cations, humus-mineral interaction (strong A horizon). „ Forest: Lower base saturation, less humus „

– Confers: low B.S., less incorporated OM – Deciduous: Higher B.S., more OM

Soil Formation Topography: Slope and Aspect N Warmer Drier Less Developed

Cold Wet

Shallow Soils

More Developed More Clay

Deep Soils Finer Texture

Soil Formation „

Time – Recent „

Little profile development – Weakly weathered – Little clay movement

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Primary minerals present

– Old Well-developed horizons „ Secondary minerals „

Soil Formation „

Parent Material – Solid Rock „

Basalt (mafic) – High base saturation

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Granite (felsic) – Lower base saturation

– Transported Material Alluvium „ Colluvium „ Eolian „

Parent Material: Transported Agent Deposit gravity ------------- Colluvial water --------------- Alluvial Marine Lacustrine ice ----------------- Glacial (till, moraine) wind ----------------- Eolian  Wide diversity within groups

Alluvial deposits

-Energy of water determines size of particles transported -Flood plains, terraces (old flood plains), deltas are rich, fertile soils (typically) -Alluvial fans are coarser in texture but remain productive

Boise Valley

First Terrace looking north from second terrace

Pleistocene Ice Age 1. Major influence on soil materials 2. Glacial till and moraines (a) highly variable and heterogeneous (b) properties reflect source

3. Glacial outwash and lacustrine deposits (a) streams from glacial melt = alluvium (b) lakes formed, when dried = lacustrine

Process of Soil Formation „

Transformations – Mineral weathering, OM decomp

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Translocations – Transport of inorganic and organic material

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Additions – OM, particles (eolian, aluvial, colluvial)

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Losses – Erosion – Leaching

Translocations (Soil Mixing) Impact of Macro-organisms

Natural Soil Mixing

Earthworms, Gophers, Moles, Ants, Termites, Cicadas

Channeling Water

‘piping’

Transformations

Parent Material → Soil primary minerals

secondary minerals

4.38 Smectite Soil

Iron and Al-oxide Rich

?

NE Montana Vertisol

Hawaiian Oxisol

Mineralogy reflects weathering processes Young, weakly weathered soils = fine-grained mica, chlorite, vermiculite (Entisol, Inceptisol)

Intermediate weathering = vermiculite, smectite, kaolinite (Mollisol, Alfisol, Ultisol)

Strong weathering = kaolinite, hydrous oxides (Ultisol--> Oxisol)

Weathering of Parent Material „ Physical

– Thermal expansion/contraction „ Freeze/thaw

– Erosion (water, ice, wind) – Biological (plant roots…)

Weathering of Parent Material „

Chemical – Hydration – Hydrolysis – Acidic Decomposition „

Water, inorganic and organic acids

– Complexation Decomposition „

Organic acids

– Reductive/oxidative dissolution

Gains Alluvial Deposits

Flood Plain (First Terrace) Boise Valley

Organic Matter

4.40

- reactive functional groups: carboxyl, hydroxyl, phenolic * Humus, Humic Acid, Fulvic Acid

Volcanic ash over alluvium

Bonner Series

Losses -Erosion -Leaching