Processes of Soil Formation and Weathering

In the beginning ... All soil on the earth originates from the consolidated mineral (bedrock) formed as the planet cooled Q The effects of solar energy, atmosphere and the oceans resulting in “weather”, began the process of breaking down the rocks; of “physical weathering” Q

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Soil Formation The process of weathering rock is slow and is still ongoing however… Q The processes of turning the geological substrate into “soil” occur at a more rapid rate and are of greater concern to resource managers and applied soil scientists… Q

Soil Formation Q

The process of soil formation can be summarizes as a function of 5 factors – climate – organisms – relief / topography – parent material – time

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Soil Formation Furthermore… Soil Development can be broadly described in two phases Q - The accumulation of parent materials Q - The development of soil horizons Q

Soil Formation Q

- there are four primary processes of soil formation (pedogenic processes) which occur mainly in the second phase (horizon development) of soil development Q Q Q Q

additions losses translocations transformations

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Soil Formation Once the geological substrate is colonized by plants and animals the processes of soil horizon development accelerates Q Colonization of geological substrates by plants occurs randomly however over relatively short periods specific plant communities tend dominate based on favorable or non-limiting soil conditions Q

Soil Formation Forest soils and forest or other vegetation interact with one another and both succeed towards a “climax” condition Q A stable, relationship of forest communities and forest soil is rarely achieved in the boreal forest due to the frequency of fire and other stand replacing disturbances Q

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Soil Formation Q

Concept of Parent Material ~ “not-soil” – true parent material is virtually unaltered geological material. – It is sometimes termed “not-soil” indicating its lack of development and the fact that it does not support plant or animal life. – Parent material is the starting point from which soil is formed.

Soil Formation Due to the phenomenon of multiple layered (stratified) soil depositions it may occur that the apparent parent material is not the original geological material from which upper soil horizons have developed. Q Several different layers of “parent material” may occur hence the preference for the term “not-soil” Q

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Pedogenic Processes Q

Additions

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Losses

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Translocations

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Transformations

Additions Organic Material Q Atmospheric Elements Q Colluvial - Gravity Q Alluvial - Resulting from flooding and/or erosion Q Eolian - The addition of wind blown geological elements Q

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Losses Q

Erosion – by water, wind, gravity

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Leaching – movement of material in suspension or solution from the solum to deeper “notsoil” layers – leachable materials Fe, Al, Si, clay, organic, calcium carbonate

Translocation Translocation is the movement of parts of the soil within the Solum Q BY WATER Q - leaching Q

– material in suspension or solution moves from upper horizons to lower horizons Q Q

Eluviation (where it came from … Illuviation (where it went to.

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Translocation Q

The degree of leaching is often related to the amount, pH, and patterns of the soil water movement through the soil profile

Translocation chemical reduction of Iron, Aluminum in a permanent water table can make these elements mobile; results in gleyzation “Gley colours” Q Upward movement and accumulation of salts where evapo-transpiration exceeds precipitation for significant parts of a year Q

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Translocation Q

Drying and Wetting of Fine Textured Soils – Causes cracks; other material get washed into cracks

Translocation BY ICE Q Freeze, Thaw (expansion/contraction) Q

– results in movement of soil particles – may result in the movement of stones towards the surface (especially of finer texture soils)

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Translocation Q

Biotic – by earthworms – by ants, termites – by windthrow – by excavation, larger fauna – anthropogenic Q

construction, agriculture, forestry...

Transformation Q

Changes of mineral and organic material – Organic, decomposition – polyphenol extracts from leaf/needle litter help to mobilize iron and increase its leaching. – Cheluviation; complexing organic and iron and aluminum and leaching

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Transformation Q

Clay – translocation and adherance to other particles and peds as clay skins – new clay particles may be weathered from certain forms such as shale

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Calcium – moved through profile as soluble compounds making soil more acidic in upper horizons and more basic in lower

Transformation Q

Fe (iron), Al (Aluminum), Si (Silicon) – Silicon is translocated within, or lost from, the pedon in solution – Sites in clay latices where silicon was removed by weathering may be replaced by iron and aluminum and other cations

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Transformation – Iron and Aluminum cations bond with organic materials to coat other soil particles (ie sand grains) where they oxidize forming reddish B horizons – In tropical regions where soils are older and the climate is wetter and warmer this phenomenon is exaggerated – exposure of these soils by shifting cultivation may result in irreversible hardpan layers forming

Transformation – A similar processes occurs in the boreal forest although at slower rates resulting in the formation of podzolized soils

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Common Soil Horizons – Broadly soil horizons are classified as: Q

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A horizons ~ surface mineral soil horizons most influenced by the interaction with plants, animals and weather B horizons ~ sub surface weathered soils which are impacted by internal illuviation, chemical changes, and subsurface flow of water and water table fluctuation C horizons ~ virtually unaltered mineral substrate, limited changes may include mottling or gleying related to site hydrology

Common Soil Horizons – Common horizon suffix modifiers include: Q

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e ~ eluviated, some mineral components or soil fractions have been translocated out of the horizon by leaching h ~ a horizon (normally an A horizon) which has been enriched with decomposed organic material

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Common Soil Horizons – Common horizon modifiers include: Q

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f ~ an oxidized mineral rich horizon which is reddish in colour formed by the transformation of mixtures of Iron and Aluminum with organic material in a coating around mineral soil particles g ~ a mottled layer or a layer with gley colours

Common Soil Horizons – Common horizon modifiers include: Q

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k ~ a horizon where carbonate is present as indicated by effervescence when treated with dilute Hydrochloric Acid t ~ an illuvial horizon enriched with silicate clay which has leached from higher in the profile.

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Common Soil Horizons – Common horizon modifiers include: Q

Q

m ~ a slightly altered, weathered horizon where the expressed soil characteristics are insufficent to qualify for another specific type (e.g., a reddish horizon which is not rich enough in iron oxide to be called a Bf) p ~ a horizon modified by human caused events; typically a plow layer characterized by organic enrichms

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