Chapter 5 process of pedogensisCombined effects of

Additions to ground surface

colluvial, alluvial, eolian, vegetative

Transformations within the soil

in situ clay or oxide formation

Translocation

Vertical (down or up) movement of materials*

Removals form the soil

Erosion or leaching

Processes vary form one soil to the next in accordance with the 5 factors of soil formation

* Translocations

• Animals, Plants, physical and chemical processes all contribute to translocation of materials

• Animals move more more physical material than do plants

• Animal rankings– Worms, ants, vertebrates, termites,

other invertebrates

Generalized ranking scheme for processes related to soil orders

A-horizonsDependent upon vegetation; gains outpace lossesBegins to form upon establishment of plant communities

chicken vs egg scenariosoil first- then plants? orplants first- then soil.

Decomposition of vegetation is important

carbon is lost over time

nitrogen is sequestered (stored)

Resultant interactions between plants and soil can result in depletion in certain zones and enrichment in other zones

A-horizonsDependent upon vegetation; gains outpace lossesBegins to form upon establishment of plant communities

chicken vs egg scenariosoil first- then plants? orplants first- then soil.

Decomposition of vegetation is important

carbon is lost over time

nitrogen is sequestered (stored)

Resultant interactions between plants and soil can result in depletion in certain zones and enrichment in other zones

Translocation of Fe and Almajor contributor to podzolization

-eluviation and development of E-horizon

-illuviation and translocation of iron, aluminum, and organics

Creates a spodic subhorizon

How does this happen?

pH must be in correct range, conditions should favor Al and Fe being in the Fe2+ and not Fe3+

Good internal drainage favors formation

Cool climates in Alpine forest ecosystems

Translocation of Fe and Almajor contributor to podzolization

-eluviation and development of E-horizon

-illuviation and translocation of iron, aluminum, and organics

Creates a spodic subhorizon

How does this happen?

pH must be in correct range, conditions should favor Al and Fe being in the Fe2+ and not Fe3+

Good internal drainage favors formation

Cool climates in Alpine forest ecosystems

Chelation-• A commonly described means to get Al

and Fe to move in instances when the environmental conditions and chemical conditions warrant some method other than a strictly chemical process– For example, oxidizing conditions inhibit

podzolization- so how do spodic subhorizons develop in places that have oxidizing conditions?

– Chelation- it is difficult to catch in the act as the process is commonly terminated by biological interaction

Translocation of clays• Causes enrichment of clay content in B-

horizon• 3 potential sources

– In situ disintegration of of minerals into clay mineral byproducts- not really translocation

– Solution of chemical components from upper horizons, downward movement and precipitation of clay minerals- a form of translocation

– Physical transport in suspension of clay minerals derived from upper horizons

Distinguishing clay origins

• No way to differentiate precipitated clay from mechanically transported clay particles

• Clays films are commonly used to distinguish translocation from in situ– Must be careful and may require thin

section work– Clays will be oriented

• If circumstances are favorable, clays can form discrete thin layers (lamellae)

• No way to differentiate precipitated clay from mechanically transported clay particles

• Clays films are commonly used to distinguish translocation from in situ– Must be careful and may require thin

section work– Clays will be oriented

• If circumstances are favorable, clays can form discrete thin layers (lamellae)

Clay lamellae developing in sandy parent material

Plots of isolated partitions in the total clay in soil profiles

Interpreting Clay Sources

• What is the parent material?– Granite?- no clay to begin with– Till?- what did glacier move over and

grind up to make the till– Shale?- clay develop develops fast

because of the rocks physical make up– What is the environment?-

Arid- likely eolian contributions to clay input

- How does the clay move - Flocculation (charges in clay minerals)

inhibits clay movement and contributes to cohesiveness

• What is the parent material?– Granite?- no clay to begin with– Till?- what did glacier move over and

grind up to make the till– Shale?- clay develop develops fast

because of the rocks physical make up– What is the environment?-

Arid- likely eolian contributions to clay input

- How does the clay move - Flocculation (charges in clay minerals)

inhibits clay movement and contributes to cohesiveness

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Influence of chemical content• Certain clays are more mobile than

others– Related to chemical composition– Calcium clays don’t like to move due to

flocculation and creation of larger “net” particles

– Sodium clays do like to move, and rapidly accumulate to create natric horizons

• Upper limit to clay mobilization is about 40%. Why?

Silt translocationSilt translocation• Common coarse-grained parent material

– Must have a source of silt however, so must be poorly sorted, have source for in situ creation, or have an external silt source• Loess or other eolian silt

– Should have fairly good drainage• Allows water to move particles through the ground

– Abundance of silt creates silt caps and coats• Similar in appearance to clay films only coarser

grained

• Common coarse-grained parent material– Must have a source of silt however, so must

be poorly sorted, have source for in situ creation, or have an external silt source• Loess or other eolian silt

– Should have fairly good drainage• Allows water to move particles through the ground

– Abundance of silt creates silt caps and coats• Similar in appearance to clay films only coarser

grained

Fragipan originsFragipan origins• Weird horizon• How it forms is debatable

– Translocation of and accumulation of clays in lower B-horizon, below eluvial horizon

– Drying of larger massive wet soils•Physical ripening creates the prismatic

structure

– Weakly binding together of clays and silts with silica cement•Not easily detectable and not very strong-

slaking in water

• Weird horizon• How it forms is debatable

– Translocation of and accumulation of clays in lower B-horizon, below eluvial horizon

– Drying of larger massive wet soils•Physical ripening creates the prismatic

structure

– Weakly binding together of clays and silts with silica cement•Not easily detectable and not very strong-

slaking in water

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Laterites (Oxisols)• Extreme weathering soil profile

– Warm and wet

• Highly concentrated Fe, Al• Depleted in Si and Bases• Commonly hard, especially if dried• Sometimes occurs as relict soils• Generally nutrient poor• Can occur in all rock types and parent materials• They are old stable landscapes

– - millions of years in most cases

• Can have high clay content• Often produces pisolitic-like structures of Al and

Fe

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Lateritic soilLateritic soil

Schematic of lateritic soil profile

Origins of salts and Carbonates• Salts like halite or gypsum occur in location

with arid climates and periodic influxes of water.– Also common near oceans or other salt sources

• E.g., salt pans

• CaCO3 common in arid and semi-arid climates– Why?

• CaCO3 is much more mobile in soils than on the surface.– Why?

• CO2 + H2O --> H2CO3 which in turn leads to• CaCO3 +H2CO3 ===> Ca2+ + 2HCO3

-

• Salts like halite or gypsum occur in location with arid climates and periodic influxes of water.– Also common near oceans or other salt sources

• E.g., salt pans

• CaCO3 common in arid and semi-arid climates– Why?

• CaCO3 is much more mobile in soils than on the surface.– Why?

• CO2 + H2O --> H2CO3 which in turn leads to• CaCO3 +H2CO3 ===> Ca2+ + 2HCO3

-

Stages of Caliche accumulation• Pedogenic carbonate is termed

caliche• Stages of accumulation are given by

roman numerals with larger number having more carbonate– I, II, III, IV, V and VI

• In coarse sediments it accumulates on the bottom of clasts

• In fine sediments it occurs as filaments or fine strands

• Pedogenic carbonate is termed caliche

• Stages of accumulation are given by roman numerals with larger number having more carbonate– I, II, III, IV, V and VI

• In coarse sediments it accumulates on the bottom of clasts

• In fine sediments it occurs as filaments or fine strands

Sources of carbonate• Parent material

– If derived from limestone

• Eolian– Dust provides both Ca2+ and CaCO3

– Can continually accumulate in small amounts that are easily translocated downward by water

• Rainwater– Eolian materials dissolved or collected in

rainwater contribute as well

• Groundwater– Raising and lowering of water table causes

capillary rise

• Parent material– If derived from limestone

• Eolian– Dust provides both Ca2+ and CaCO3

– Can continually accumulate in small amounts that are easily translocated downward by water

• Rainwater– Eolian materials dissolved or collected in

rainwater contribute as well

• Groundwater– Raising and lowering of water table causes

capillary rise

Radiocarbon dating of soils• Open system in terms of carbon

A lot of mixing of old carbon sources (e.g., from limestone as eolian dust) and new carbon sources (e.g., roots and worm poo.

Often a progressively older date is produced with depthrelated to the mixing