SOIL FORMATION Chapter 2. Processes of Soil Formation

SOIL FORMATION

Chapter 2

Rough Outline

Processes of soil formation

Rock is ultimate parent material of soil

Factors of soil formation

More on horizon designations

There are 4 processes of soil formation

and

There are 5 factors of soil formation

Processes form soilFactors control processes

And these 4 processes are

Transformation

Weathering / synthesis of minerals

Decomposition / synthesis of organicmatter

Processes of Soil Formation

Translocation

Movement of mineral and organic particlesin the developing profile


Addition

Organic matter produced

Material deposited on top


Loss

And how do losses occur?


How about an example of soilformation?

Start with uniform parent material

1 Organic matter added by plants

2 Microbes biochemically transform it

You get A horizon 50 years


3 Salts leached translocatedlost

Minerals weather transformed

Clay moves down translocated

You get weak B horizon 2500 years


4 With more accumulation of clay, structuredevelops

You get for real B horizon 10,000 years


TransformationTranslocationAdditionLoss

These processes are controlled by thefactors of soil formation which are

Do you think climate might be one?

What about organisms, especiallytype of vegetation?

grass

Same type of soil expectedregardless of whether on hilltop, side or bottom? Topography?

Do soils form overnight?

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

→

→

Five Factors of Soil Formation

Parent materialClimateOrganismsTopographyTime

We know that

Soils either

Formed in place from rock

Formed in weathered materialstransported from elsewhere

But ultimately

Unconsolidated mineral matter at thesurface of the earth came from rock

Rocks

Mixtures of minerals

IgneousMetamorphicSedimentary

Rock

Igneous

From molten magmaContain primary minerals

Rock

Sedimentary

By deposition and cementation ofweathered products

Sandstone sand particlesShale clay particles

Rock

Metamorphic

From igneous or sedimentary

High pressure and temperature

Quartzite from sandstoneSlate shale

Rock

Weathering

Physical disintegrationChemical decomposition

Physical decreases size

Chemical alters composition

Rock

Thermal stress

Ice expansion

Abrasion by water and wind

Rock

Chemical reactions

HydrolysisHydration

Al2O3 Al2O3 · 3H2O

2KAlSi3O8 + 13H2O →

Al2O3 + 6H4SiO4 + 2K+ + 2OH-

Rock

Acid dissolution

CaCO3 + H2CO3 Ca2+ + 2HCO3-

Oxidation

Particularly, Fe2+ Fe3+

Rock

→

→ Fe3+

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 ↑

Parent Material

Geologic material in which a soil forms

Residual

Transported

Factors of Soil Formation

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.

Residual parent material comes fromunderlying rock

Soil properties tend to reflect parentmaterial


For example,

Limestone sand or clay impurities

Sandstone shallow if SiO2 cementsdeep if CaCO3

Shale clay minerals


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.

Colluvial is coarse material carrieddownslope by gravity


Soils formed in colluvial parent material are foundin landscapes like to right.

Alluvial from streams and rivers

Alluvial fanFlood plainDelta


These are the three types of alluvial deposits.

Alluvial fan occurs at the discharge of anupland stream into a broader valley below


These not all that common.

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


Common and often important in agriculture.

Coarse sediment deposited near to oraway from channel

Fine sediment where?

Deposition forms a natural levee


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.


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.


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.

Changes in course produce complex patternof coarse and fine sediments


Apparent oldlevee, thusearlier course.

A delta occurs at mouth of river

End of flood plain


Marine sediments

Uplifted / exposed

Variable from sandy to clayey


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.

Soils in East Baton Rouge Parishformed in residual parent material,right?

Obviously, no.

Near the river channel you’ve got claysoil but in the backswamp it’s moresandy (True / False).

Advancing icesheet accumulatedunconsolidatedmaterial

Deposited whenglacier meltedand retreated

Glacial deposits


Partially melted in summer, giving sedimentdeposited at leading edge or further away.

See various topographic featuresin a glaciated landscape.

In low areas, lakes formed with lacustrinedeposits

Material directly deposited calledglacial till

Streams from glacier producedoutwash plains


None of this in La., however, there are soils in eolian parent material that came from glacial melt. See next slide.

Eolian deposits from outwash areas

Silt and some fine sand plus clay (loess)

Along Mississippi and Missouri Rivers


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.

So, eolian deposits are more or loess silt.

Well, not necessarily because there are deposits that are more sandy.

Organic soils

Wet places

Plant growth faster than residuedecomposition

Deposits called peat

Factors of Soil Formation, Parent Material, Misfits

Sedimentary(aquaticplants)

Herbaceous(sedges andso forth)

Woody(trees)

Factors of Soil Formation, Parent Material, Misfits

This is the general progression.

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.


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.


High rain (increase / decrease) clay translocation

High rain and high temperature (increase / decrease) mineral weathering

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.

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

Climate indirectly influences soil formationby its effect on vegetation

Like (match these)

Trees SemiaridGrasses AridBrush Humid


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.

Let’s compare soils formed under forest andprairie vegetation. Big contrast.

OrganismsFactors of Soil Formation, Organisms

Thick, organic matter rich A under grass

Thinner A but E over clayey B in forest

Factors of Soil Formation, Organisms

Deciduousversus coniferousaffects soil development

Nutrient cyclingfaster under deciduous

Slows leaching of base cations

Slows acidification


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.

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?


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.

Soil Calhoun Jeanerette

Cover forest prairie

Solum 175 cm 125 cmClay weathered less weatheredpH 4.5 6.5


So there you see. The effect of trees has been to cause deeper profiledevelopment, greater mineral weathering and greater soil acidification.

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.

Profile development slowed by erosion andless water infiltration


Surface

Does the water table do this or that?

Topography affects soil development byaffecting depth to water table


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.

Water table impedes drainage

Does slow drainage speed up or slow down profile development?


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.

More organic matter on N or S slopes?

More organic matter here or there?


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.

Time

It takes time for soils to form

Effect of time seen in chronosequences

Factors of Soil Formation, Time

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


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?


Gallion on abandonedlevees andSevern oncurrent levees.

Roxanna onboth.

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


Soils defined –dynamic natural bodieshaving properties derived from thecombined effects of climate and bioticactivities, as modified by topography,acting on parent material over time.

More on Horizon Designations

Five master horizons

Can you still name them?

Here’s a mnemonic device

Horizon Designations

O CEBA Building, How Dear Thou Art to Me

Patrick F. Taylor Hall, alas


Some vocabulary for E and B horizons

Eluvial washed out of E

Illuvial washed into B


Ap Plowed

Bt Clay

Bg Gley

Bx Fragipan (dense and brittle)

Transitional horizons such as AE, EB, BE andBC


The t is for translocated, of course.

Not clearly an A or an E butmore like an A than an E, etc.

Documents

SOIL FORMATION Chapter 2. Processes of Soil Formation