lecture 7, soil development - stanford...

Lecture 7, Soil Development

GES175, Science of Soils

Parent Material → Soil

primaryprimary secondarysecondarymineralsminerals mineralsminerals

Rock

Entisol

Inceptisol

Alfisol

Ultisol

Oxisol

Mollisol

Spodosol(cool and wet)

Aridisol (dry climate)Vertisol (clay mineralogy)

Ideal Weathering Series

Soil Formation

Five Soil Forming Factors:

Climate

Biota

Topography

Parent Material

Time

Soil FormationSoil Formation

ClimateClimate–– Arid: Arid: Minimal leaching, slow dissolutionMinimal leaching, slow dissolution–– Humid: Humid: Extensive leaching, rapid Extensive leaching, rapid

dissolutiondissolution–– Cool: Cool: Active physical weathering, slow Active physical weathering, slow

chemical weathering. Buildup of OM.chemical weathering. Buildup of OM.–– Warm: Warm: Strong chemical weathering, rapid Strong chemical weathering, rapid

OM decay (nutrient cycling)OM decay (nutrient cycling)

Soil FormationSoil FormationBiotaBiota–– Diversity and activityDiversity and activity–– Plant canopyPlant canopy

Root exudates promote weathering Root exudates promote weathering Grasslands: Base Grasslands: Base cationscations, humus, humus--mineral mineral interaction (strong A horizon).interaction (strong A horizon).Forest: Lower base saturation, less humusForest: Lower base saturation, less humus

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

Soil FormationSoil FormationTopography: Slope and Aspect

NColdWet

WarmerDrier

Shallow Soils

Deep Soils

Finer Texture

Less Developed

More Developed

More Clay

Soil FormationSoil FormationTimeTime–– RecentRecent

Little profile developmentLittle profile development–– Weakly weatheredWeakly weathered–– Little clay movementLittle clay movement

Primary minerals presentPrimary minerals present

–– OldOldWellWell--developed horizonsdeveloped horizonsSecondary mineralsSecondary minerals

Soil FormationSoil FormationParent MaterialParent Material–– Solid RockSolid Rock

Basalt (Basalt (maficmafic))–– High base saturationHigh base saturation

Granite (Granite (felsicfelsic) ) –– Lower base saturationLower base saturation

–– Transported MaterialTransported MaterialAlluviumAlluviumColluviumColluviumEolianEolian

Parent Material: Transported Parent Material: Transported AgentAgent DepositDepositgravity gravity -------------------------- ColluvialColluvialwater water ------------------------------ AlluvialAlluvial

MarineMarineLacustrineLacustrine

ice ice ---------------------------------- Glacial (till, moraine)Glacial (till, moraine)wind wind ---------------------------------- EolianEolian

Wide diversity within Wide diversity within

groupsgroups

-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

Alluvial depositsAlluvial deposits

Boise Valley

First Terracelooking north from second terrace

Pleistocene Ice AgePleistocene Ice Age1. Major influence on soil materials1. Major influence on soil materials

2. Glacial till and moraines2. Glacial till and moraines(a) highly variable and heterogeneous(a) highly variable and heterogeneous(b) properties reflect source(b) properties reflect source

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

Process of Soil FormationProcess of Soil Formation

TransformationsTransformations–– Mineral weathering, OM Mineral weathering, OM decompdecompTranslocationsTranslocations–– Transport of inorganic and organic materialTransport of inorganic and organic materialAdditionsAdditions–– OM, particles (OM, particles (eolianeolian, , aluvialaluvial, , colluvialcolluvial))LossesLosses–– ErosionErosion–– LeachingLeaching

Impact of MacroImpact of Macro--organismsorganisms

Translocations (Soil Mixing)

Natural Soil Mixing

Earthworms, Gophers, Moles,Ants, Termites, Cicadas

Channeling Water

‘piping’

Parent Material → Soil

primaryprimary secondarysecondarymineralsminerals mineralsminerals

Transformations

Smectite Soil

NE Montana VertisolHawaiian Oxisol

Iron and Al-oxide Rich

4.38

?

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)

Mineralogy reflects weathering processes

Weathering of Parent MaterialWeathering of Parent Material

PhysicalPhysical–– Thermal expansion/contractionThermal expansion/contraction

Freeze/thawFreeze/thaw–– Erosion (water, ice, wind)Erosion (water, ice, wind)–– Biological (plant roots…)Biological (plant roots…)

Weathering of Parent MaterialWeathering of Parent MaterialChemical Chemical –– HydrationHydration–– HydrolysisHydrolysis–– Acidic DecompositionAcidic Decomposition

Water, inorganic and organic acidsWater, inorganic and organic acids

–– Complexation Complexation DecompositionDecompositionOrganic acidsOrganic acids

–– Reductive/oxidative dissolutionReductive/oxidative dissolution

Flood Plain (First Terrace)Boise Valley

Alluvial Deposits

Gains

Organic Matter4.40

- reactive functional groups: carboxyl, hydroxyl, phenolic

* Humus, Humic Acid, Fulvic Acid

Volcanic ashover alluvium

Bonner Series

Losses

-Erosion-Leaching