Chapter 5 process of pedogensis Combined 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-horizons Dependent upon vegetation; gains outpace losses Begins to form upon establishment of plant communities chicken vs egg scenario soil first- then plants? or plants 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-horizons Dependent upon vegetation; gains outpace losses Begins to form upon establishment of plant communities chicken vs egg scenario soil first- then plants? or plants 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 Al major 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 Fe 2+ and not Fe 3+ Good internal drainage favors formation Cool climates in Alpine forest ecosystems Translocation of Fe and Al major 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 Fe 2+ and not Fe 3+ 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

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 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 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

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

Lateritic 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 CaCO 3 common in arid and semi-arid climates –Why? CaCO3 is much more mobile in soils than on the surface. –Why? CO 2 + H 2 O --> H 2 CO 3 which in turn leads to CaCO 3 +H 2 CO 3 ===> Ca HCO 3 - 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 CaCO 3 common in arid and semi-arid climates –Why? CaCO3 is much more mobile in soils than on the surface. –Why? CO 2 + H 2 O --> H 2 CO 3 which in turn leads to CaCO 3 +H 2 CO 3 ===> Ca HCO 3 -

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 CaCO 3 –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 CaCO 3 –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 depth related to the mixing