soil formation chapter 2. processes of soil formation
TRANSCRIPT
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SOIL FORMATION
Chapter 2
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Rough Outline
Processes of soil formation
Rock is ultimate parent material of soil
Factors of soil formation
More on horizon designations
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There are 4 processes of soil formation
and
There are 5 factors of soil formation
Processes form soilFactors control processes
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And these 4 processes are
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Transformation
Weathering / synthesis of minerals
Decomposition / synthesis of organicmatter
Processes of Soil Formation
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Translocation
Movement of mineral and organic particlesin the developing profile
Processes of Soil Formation
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Addition
Organic matter produced
Material deposited on top
Processes of Soil Formation
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Loss
And how do losses occur?
Processes of Soil Formation
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How about an example of soilformation?
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Start with uniform parent material
1 Organic matter added by plants
2 Microbes biochemically transform it
You get A horizon 50 years
Processes of Soil Formation
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3 Salts leached translocatedlost
Minerals weather transformed
Clay moves down translocated
You get weak B horizon 2500 years
Processes of Soil Formation
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4 With more accumulation of clay, structuredevelops
You get for real B horizon 10,000 years
Processes of Soil Formation
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TransformationTranslocationAdditionLoss
These processes are controlled by thefactors of soil formation which are
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Do you think climate might be one?
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What about organisms, especiallytype of vegetation?
grass
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Same type of soil expectedregardless of whether on hilltop, side or bottom? Topography?
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Do soils form overnight?
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A B
C D
A and C are different starting materialsandB and D are different end products
So does starting material also controlsoil formation? Parent material
→
→
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Five Factors of Soil Formation
Parent materialClimateOrganismsTopographyTime
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We know that
Soils either
Formed in place from rock
Formed in weathered materialstransported from elsewhere
But ultimately
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Unconsolidated mineral matter at thesurface of the earth came from rock
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Rocks
Mixtures of minerals
IgneousMetamorphicSedimentary
Rock
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Igneous
From molten magmaContain primary minerals
Rock
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Sedimentary
By deposition and cementation ofweathered products
Sandstone sand particlesShale clay particles
Rock
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Metamorphic
From igneous or sedimentary
High pressure and temperature
Quartzite from sandstoneSlate shale
Rock
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Weathering
Physical disintegrationChemical decomposition
Physical decreases size
Chemical alters composition
Rock
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Thermal stress
Ice expansion
Abrasion by water and wind
Rock
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Chemical reactions
HydrolysisHydration
Al2O3 Al2O3 · 3H2O
2KAlSi3O8 + 13H2O →
Al2O3 + 6H4SiO4 + 2K+ + 2OH-
Rock
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Acid dissolution
CaCO3 + H2CO3 Ca2+ + 2HCO3-
Oxidation
Particularly, Fe2+ Fe3+
Rock
→
→ Fe3+
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What happens when you drop a little 3M HCl on some CaCO3?
By the way, what happens is a test forcarbonate in soil.
Rock
It fizzes because
CaCO3 + 2HCl → Ca2+ + 2Cl- + H2CO3 H2CO3 → H2O + CO2 ↑
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Parent Material
Geologic material in which a soil forms
Residual
Transported
Factors of Soil Formation
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Transported
ColluvialAlluvialMarineLacustrineGlacialEolian
Organic parent materialresidual or transported?
Factors of Soil Formation, Parent Material
These are the types of transported parent material.Residual parent material is residual, i.e., weatheredrock. Organic soils don’t fit into this dichotomy.
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Residual parent material comes fromunderlying rock
Soil properties tend to reflect parentmaterial
Factors of Soil Formation, Parent Material
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For example,
Limestone sand or clay impurities
Sandstone shallow if SiO2 cementsdeep if CaCO3
Shale clay minerals
Factors of Soil Formation, Parent Material
As the cementing agent (carbonate) dissolves, limestone residue can be anysize particulate, from sand to clay. Sandstone give sandy soils but how deepthese are depends on how fast the cementing agent dissolves. Shale givesclayey soils.
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Colluvial is coarse material carrieddownslope by gravity
Factors of Soil Formation, Parent Material
Soils formed in colluvial parent material are foundin landscapes like to right.
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Alluvial from streams and rivers
Alluvial fanFlood plainDelta
Factors of Soil Formation, Parent Material
These are the three types of alluvial deposits.
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Alluvial fan occurs at the discharge of anupland stream into a broader valley below
Factors of Soil Formation, Parent Material
These not all that common.
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Flood plain next to river
What happens during a flood?
Water with sediment spills over the banks,and then what?
Think particle size sedimentation rate
Factors of Soil Formation, Parent Material
Common and often important in agriculture.
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Coarse sediment deposited near to oraway from channel
Fine sediment where?
Deposition forms a natural levee
Factors of Soil Formation, Parent Material
Deposition is greatest near the bank leading to highest elevation. Furthermore,this area of deposition is comparatively high in sand + silt. Thus, since the coarserparticles are deposited quickly, deposition from the flood water further away islow in sand + silt and high in clay.
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Factors of Soil Formation, Parent Material
A laterally compressed cross section of the Mississippi River natural levee.Notice any familiar names to the soils? The name comes from some place orgeographic feature nearby where the soil was first described.
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Factors of Soil Formation, Parent Material
Another depiction of the Mississippi River natural levee. The course has followeda low position in the landscape, and river sediments have been deposited in the channel and outside it, the latter creating the levee.
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Changes in course produce complex patternof coarse and fine sediments
Factors of Soil Formation, Parent Material
Apparent oldlevee, thusearlier course.
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A delta occurs at mouth of river
End of flood plain
Factors of Soil Formation, Parent Material
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Marine sediments
Uplifted / exposed
Variable from sandy to clayey
Factors of Soil Formation, Parent Material
These old materials vary in composition depending on their ancient source. Due to uplift or sea level drop, these became exposed and have undergone pedogensis. As with alluvial parent materials (floodplain and deltaic deposits),there are a lot of soils in Louisiana that have formed in marine sediments.
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Soils in East Baton Rouge Parishformed in residual parent material,right?
Obviously, no.
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Near the river channel you’ve got claysoil but in the backswamp it’s moresandy (True / False).
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Advancing icesheet accumulatedunconsolidatedmaterial
Deposited whenglacier meltedand retreated
Glacial deposits
Factors of Soil Formation, Parent Material
Partially melted in summer, giving sedimentdeposited at leading edge or further away.
See various topographic featuresin a glaciated landscape.
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In low areas, lakes formed with lacustrinedeposits
Material directly deposited calledglacial till
Streams from glacier producedoutwash plains
Factors of Soil Formation, Parent Material
None of this in La., however, there are soils in eolian parent material that came from glacial melt. See next slide.
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Eolian deposits from outwash areas
Silt and some fine sand plus clay (loess)
Along Mississippi and Missouri Rivers
Factors of Soil Formation, Parent Material
This figure showsloess only to eastof river, however,there is loess to the west, just not as much. Also,not nearly as deepin Baton Rougeas further north, like Natchez.
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So, eolian deposits are more or loess silt.
Well, not necessarily because there are deposits that are more sandy.
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Organic soils
Wet places
Plant growth faster than residuedecomposition
Deposits called peat
Factors of Soil Formation, Parent Material, Misfits
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Sedimentary(aquaticplants)
Herbaceous(sedges andso forth)
Woody(trees)
Factors of Soil Formation, Parent Material, Misfits
This is the general progression.
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Climate
Precipitation and temperature
Affect rates of soil formation processes
Factors of Soil Formation, Climate
Clearly, parent material is the most important of the 5 factors of soil formation.Climate comes in next. Without precipitation (water) and warm temperature,there will not be a lot of plant growth, mineral weathering nor translocation of particulate matter in the profile –thus, pedogenesis is slow.
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Factors of Soil Formation, Climate
Example effects
High rain and low temperature (increase / decrease) organic matter
High rain (increase / decrease) salt leaching
The reason why low temperature leads to accumulation of organic matteris the effect it has on slowing microbial decomposition of organic matter.
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Factors of Soil Formation, Climate
High rain (increase / decrease) clay translocation
High rain and high temperature (increase / decrease) mineral weathering
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Soils in Utah likely
A) Contain few weatherable mineralsB) Are leached of soluble saltsC) Both of the aboveD) Neither of the above
represents generally cool and dry climate
D is the right choice. If it’s cool and dry there, there hasn’t been a lot ofweathering and leaching going on. The opposite would be true for Panamain the next slide.
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Soils in Panama likely
A) Contain few weatherable mineralsB) Are leached of soluble saltsC) Both of the aboveD) Neither of the above
represents generally warm and wet climate
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Climate indirectly influences soil formationby its effect on vegetation
Like (match these)
Trees SemiaridGrasses AridBrush Humid
Factors of Soil Formation, Climate
The 5 factors of soil formation are interrelated to varying extent. In this example,ample water is needed for forest vegetation and jointly due to greater rainfall andspecific effect of trees, forest soils are different from prairie soils.
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Let’s compare soils formed under forest andprairie vegetation. Big contrast.
OrganismsFactors of Soil Formation, Organisms
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Thick, organic matter rich A under grass
Thinner A but E over clayey B in forest
Factors of Soil Formation, Organisms
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Deciduousversus coniferousaffects soil development
Nutrient cyclingfaster under deciduous
Slows leaching of base cations
Slows acidification
Factors of Soil Formation, Organisms
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The preceding slide was omitted earlier because the explanation is long. It goeslike this.
Soils are open and leaky systems, at least in humid climates. Thus, soluble substances are leached from them, including nutrient cations, like Ca2+, Mg2+
and K+. To some extent these are replaced by deposition from the atmospherebut this is not sufficient to offset natural leaching. To make this matter worse,there is continuous generation of H+ in the soil. It comes from CO2 released inrespiration (microbes, roots, etc.) which forms H2CO3, a weak but abundantacid. Furthermore, the decomposition of organic matter in soil results in releaseof various organic acids (R-COOH) and small amounts of the strong acids, nitric(HNO3) and sulfuric (H2SO4). The H+ from these acids tends to replace basecations like Ca2+, Mg2+ and K+ that are adsorbed onto the negatively chargedsoil colloidal particles (clay and humus), thus acidifying them, and with the basesin solution, they are subject to loss in water that drains through the soil. The neteffect is long-term acidification of soil. This is the natural course of things. However, if the base cations, which are nutrients, are taken up in large quantitiesby plant roots, the acidification process is slowed. Ca2+ etc. taken up by plantsis returned to the soil in litter so that these nutrients are cycled between soil and plant. Some plants, like the deciduous trees compared with coniferous trees,cycle nutrients quickly, thereby more effectively slow soil acidification.
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Biosequence in Louisiana loess
Soil Calhoun Jeanerette
Cover forest prairie
Solum Deeper development in which?Clay More weathered minerals in
which?pH Lower pH in which?
Factors of Soil Formation, Organisms
So how can a soils guy say that observed differences between two soils are due to one or another of parent material, climate, organisms, topography or time? If thefocus is effect of organisms, find 2+ soils that come from the same parent material, formed under the same climate, on the same type of landscape and are about the same age. These 2+ soils would constitute a biosequence of soils.
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Soil Calhoun Jeanerette
Cover forest prairie
Solum 175 cm 125 cmClay weathered less weatheredpH 4.5 6.5
Factors of Soil Formation, Organisms
So there you see. The effect of trees has been to cause deeper profiledevelopment, greater mineral weathering and greater soil acidification.
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Why?
Topography
Modifies effects of organisms and climate
Thinner soil and less mature profile on sideslopes or top / bottom?
Factors of Soil Formation, Topography
Erosion takes away topsoil. Furthermore, less water infiltrates sothere is less translocation of clay to form a clayey B horizon.
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Profile development slowed by erosion andless water infiltration
Factors of Soil Formation, Topography
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Surface
Does the water table do this or that?
Topography affects soil development byaffecting depth to water table
Factors of Soil Formation, Topography
It does the this, not the that. Where there is a shallow groundwater table, it tendsto roughly parallel the soil surface, but not exactly. The effect of a shallow water tableis to impede drainage, thus minimize translocation of particulates through the profile.
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Water table impedes drainage
Does slow drainage speed up or slow down profile development?
Factors of Soil Formation, Topography
To the left, there is deepersoil development at the topof the hillock but in the lowerscenario there may be deeper development on thesides. However, erosionprobably comes into play,complicating interpretations.
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More organic matter on N or S slopes?
More organic matter here or there?
Factors of Soil Formation, Topography
Probably more on side away fromheat source, lower temperature soslower rate of microbial activityand organic matter decomposition.
The low wet spot is prime foraccumulation of organic mattersince poorer aeration slowsoverall microbial activity.
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Time
It takes time for soils to form
Effect of time seen in chronosequences
Factors of Soil Formation, Time
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Like in Red and Mississippi River alluvia
Severn, Roxanna and Gallion on naturallevees of the Red River
Soil Channel Age CaCO3 Depth
Severn recent / old ? < 50 cmRoxanna recent / old > 50 cmGallion recent / old leached
Factors of Soil Formation, Time
Red River soils are somewhat special in that they can have carbonate (from theWest Texas, New Mexico, Oklahoma source area) in them. However, it tends to beneutralized and leached with time. So which is oldest and on what levees is it found?
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Factors of Soil Formation, Time
Gallion on abandonedlevees andSevern oncurrent levees.
Roxanna onboth.
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Match soil depth with right soil
15 cm 45 cm 60 cm
Convent, Bruin and Dundee on Mississippinatural levees
Soil Belt Age Solumdepth
Convent 5 < 3,000Bruin 5 - 3 > 3,000Dundee 4 - 2 > 4,000
Factors of Soil Formation, Time
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Soils defined –dynamic natural bodieshaving properties derived from thecombined effects of climate and bioticactivities, as modified by topography,acting on parent material over time.
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More on Horizon Designations
Five master horizons
Can you still name them?
Here’s a mnemonic device
Horizon Designations
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O CEBA Building, How Dear Thou Art to Me
Patrick F. Taylor Hall, alas
Horizon Designations
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Some vocabulary for E and B horizons
Eluvial washed out of E
Illuvial washed into B
Horizon Designations
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Ap Plowed
Bt Clay
Bg Gley
Bx Fragipan (dense and brittle)
Transitional horizons such as AE, EB, BE andBC
Horizon Designations
The t is for translocated, of course.
Not clearly an A or an E butmore like an A than an E, etc.