PEDALFERS

example 4 sandy, mixed, frigid, ortstein Typic Durorthod (Wallace series) These soils are found on dunes and sandy outwash plains in northern Michigan and New York. Mean annual precipitation is 27-34 inches, and mean annual temperature ranges from 41-45 degrees F. This soil has a very distinct spodic horizon, which is strongly cemented and referred to as an ortstein. Note the irregular topography of the E/Bhsm and Bhsm/Bsm horizon boundaries. Wallace soils are used almost entirely for second-growth woodlands. For more information on the Wallace soil, click

In forested, moist-temperate climatic zones, soils form by hydrolysis weathering of the underlying bedrock. Hydrolysis involves the reaction of silicate minerals and water (which near silicate crystal surfaces is torn into hydrogen cations and hydroxide anions). The products of hydrolysis are clays and salts in solution. Hydrolysis proceeds wherever water can penetrate the underlying bedrock: along joint fractures but more importantly along cleavage openings in the silicate minerals. As the clay takes up more space than the mineral replaced, the principle of the lever results in stresses that cause the cleavage cracks to propagate and others to open. The soil is youngest at the bottom where the bedrock is beginning to be altered by hydrolysis and is oldest at the top where only quartz, which has no cleavage and is resistant to chemical change, survives.

Soils that form in forested, moist-temperate climatic zones are pedalfers. Leaf litter on the forest floor is a sedimentary layer (O horizon) on top of the soil. Under the moist-temperate climatic conditions, this composts and becomes turned in by earth worm activity or is carried in by down filtering water to form a dark black, humus rich, part (A horizon) of the top soil. The top soil is otherwise a quartz sand which, below the worked in, or above the washed down, organic rich black part, is seen as greyish white layer (E horizon). Below the top soil is a clayey, yellow brown to red in color, subsoil (B horizon). The name pedalfer—soil (Gk. pedon), clay (aluminosilicate), and rust (ferric oxide)—refers to the subsoil. Below the subsoil is a layer of saprolite— "rotten rock" (Gk. sapros means rotten) that hydrolysis has made easy to crumble. This youngest part of the soil (C horizon) rests on, as yet, little altered solid bedrock.

In the pedalfer A horizon, the products of hydrolysis are not present and the inference is that they have been removed. The B horizon has an excess of clay and iron oxide over that which hydrolysis could have produced in place from the weathering of the bedrock. The inference is that the extra clay and iron oxide content has been added. An accepted explanation is that the additional materials in the B horizon were eluviated (leached) from the A horizon (leaving there only quartz) and illuviated (deposited) in the B horizon (which is otherwise rotting bedrock). How material is transferred from the A to the B horizon can be explained by following what happens when rainwater that soaks into the ground percolates down though the soil and flows away laterally through the C horizon to exit at seepages and springs. Hydrolysis produces clays such as kaolinite (O. Si, Al)*, illite (O. Si, Al, K) and montmorillonite (O. Si, Al, Fe, Mg) that crystallize. Hydrolysis releases colloidal silica and, also, alkali metal (K⁺, Na⁺), alkaline earth (Ca⁺⁺, Mg⁺⁺), and ferrous iron (Fe⁺⁺) ions. These move with the through-flowing groundwater. Pedalfers are well flushed soils.

Rainwater contains, dissolved in it, gasses of the air. So rainwater is both well oxygenated and is naturally slightly acidic (dissolved carbon dioxide is carbonic acid, H₂CO₃⁰). On soaking through the O horizon the percolating rainwater, now groundwater, becomes more acidic (its pH decreases to about 4) as organic acids (for example: oxalic acid) from composing humus are added to it. In its passage though the E horizon, the rainwater pH is such that any clay produced by hydrolysis will disperse in it and be carried with it in colloidal suspension (that is, like a salt in solution). Iron released by hydrolysis into slightly acidic water, even though this is well oxygenated, is held in solution in the ferrous state (Fe⁺⁺). So both clay and soluble salts (including iron) are leached (eluviated) from the A horizon. In the B horizon, the pH of  the down percolating ground water increases as dissolved K and Na hydroxides and Ca and Mg carbonate salts released by hydrolysis are added to it. In the B horizon, at some depth (which varies according to how much, or little, flushing has just before occurred) the pH increases beyond 7 (neutral) and the ground water is from there on down alkaline (pH greater than 7). In slightly alkaline water, 1) dispersed (colloidal) clay clumps (flocculates) and stops moving with the ground water, and 2) in the presence of dissolved oxygen, dissolved ferrous iron oxidizes to ferric (Fe⁺⁺⁺) iron and precipitates as insoluble limonite or hematite (rust).

As clay, spongelike, is able to hold between its weakly bonded sheet silicate layers, ions of soluble salts that would otherwise be flushed away, it can be thought of as a warehouse of inorganic nutrients needed by plants. Roots passing though, and near, clay minerals can extract the clay adhering salts. For this reason the forest that allows for the development of pedalfers is of trees that have roots that penetrate the B horizon. The same trees block light that would allow groundcover plants to grow. The forest floor is devoid of vegetation except where a tree has died or fire has removed the shading trees. In cleared patches of the forest, a succession of weeds with shallow root that can gain nutrients directly from the organic rich top soil thrive for a while until forest seedlings grow to restore the forest and close off the light below their canopies.
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* 

NaAlSi₃O₈(albite) + CO₂(g) + 11/2H₂O(l)    
<—>
Na⁺   +  HCO₃^-  +  2H₄SiO₄⁰   +  1/2Al₂Si₂O₅(OH)₄(kaolinite)

 

Illustration of a pedalfer in Essentials by Lutgens/Tarbuck

 

United States Soil Classification System  Ref:    http://www.geog.ouc.bc.ca/physgeog/contents/11f.html

The first formal system of soil classification was introduced in the United States by Curtis F. Marbut in the 1930s. This system, however, had some serious limitations, and by the early 1950s the United States Soil Conservation Service began the development of a new method of soil classification. The process of development of the new system took nearly a decade to complete. By 1960, the review process was completed and the Seventh Approximation Soil Classification System was introduced. Since 1960, this soil classification system has undergone numerous minor modifications and is now under the control of Natural Resources Conservation Service (NRCS), which is a branch of the Department of Agriculture. The current version of the system has six levels of classification in its hierarchical structure. The major divisions in this classification system, from general to specific, are: orders, suborders, great groups, subgroups, families, and series. At its lowest level of organization, the U.S. system of soil classification recognizes approximately 15,000 different soil series.

USA Department of Agriculture 12 soil orders:   MAP

Gelisols - soils with permafrost within 2 m of the surface
Histosols - organic soils
Spodosols - acid soils with a subsurface accumulation of metal-humus complexes
Andisols - soils formed in volcanic ash
Oxisols - intensely weathered soils of tropical and subtropical environments
Vertisols - clayey soils with high shrink/swell capacity
Aridisols - CaCO₃-containing soils of arid environments with subsurface horizon development
Ultisols - soils with a subsurface zone of silicate clay accumulation and <35% base saturation
Mollisols - grassland soils with high base status
Alfisols - soils with a subsurface zone of silicate clay accumulation and >35% base saturation
Inceptisols - soils with weakly developed subsurface horizons
Entisols - soils with little or no morphological development

Spodosols are soils that develop under coniferous vegetation and as a result are modified by podzolization. Parent materials of these soils tend to be rich in sand. The litter of the coniferous vegetation is low in base cations and contributes to acid accumulations in the soil. In these soils, mixtures of organic matter and aluminum, with or without iron, accumulate in the B horizon. The A horizon of these soils normally has an eluvial layer that has the color of more or less quartz sand. Most spodosols have little silicate clay and only small quantities of humus in their A horizon.

Mollisols are soils common to grassland environments. In the United States most of the natural grasslands have been converted into agricultural fields for crop growth. Mollisols have a dark colored surface horizon, tend to be base rich, and are quite fertile. The dark color of the A horizon is the result of humus enrichment from the decomposition of litterfall. Mollisols found in more arid environments often exhibit calcification.

Alfisols form under forest vegetation where the parent material has undergone significant weathering. These soils are quite widespread in their distribution and are found from southern Florida to northern Minnesota. The most distinguishing characteristics of this soil type are the illuviation of clay in the B horizon, moderate to high concentrations of base cations, and light colored surface horizons.

Ultisols are soils common to the southeastern United States. This region receives high amounts of precipitation because of summer thunderstorms and the winter dominance of the mid-latitude cyclone. Warm temperatures and the abundant availability of moisture enhances the weathering process and increases the rate of leaching in these soils. Enhanced weathering causes mineral alteration and the dominance of iron and aluminum oxides. The presence of the iron oxides causes the A horizon of these soils to be stained red. Leaching causes these soils to have low quantities of base cations.

Oxisols develop in tropical and subtropical latitudes that experience an environment with high precipitation and temperature. The profiles of oxisols contain mixtures of quartz, kaolin clay, iron and aluminum oxides, and organic matter. For the most part they have a nearly featureless soil profile without clearly marked horizons. The abundance of iron and aluminum oxides found in these soils results from strong chemical weathering and heavy leaching. Many oxisols contain laterite layers because of a seasonally fluctuating water table.

Aridsols are soils that develop in very dry environments. The main characteristic of this soil is poor and shallow soil horizon development. Aridsols also tend to be light colored because of limited humus additions from vegetation. The hot climate under which these soils develop tends to restrict vegetation growth. Because of limited rain and high temperatures soil water tends to migrate in these soils in an upward direction. This condition causes the deposition of salts carried by the water at or near the ground surface because of evaporation. This soil process is of course called salinization.

Entisols are immature soils that lack the vertical development of horizons. These soils are often associated with recently deposited sediments from wind, water, or ice erosion. Given more time, these soils will develop into another soil type.

Inceptisols are young soils that are more developed than entisols. These soils are found in arctic tundra environments, glacial deposits, and relatively recent deposits of stream alluvium. Common characteristics of recognition include immature development of eluviation in the A horizon and illuviation in the B horizon, and evidence of the beginning of weathering processes on parent material sediments.

Vertisols are heavy clay soils that show significant expansion and contraction due to the presence or absence of moisture. Vertisols are common in areas that have shale parent material and heavy precipitation. The location of these soils in the United States is primarily found in Texas where they are used to grow cotton.

Andisols develop from volcanic parent materials. Volcanic deposits have a unique process of weathering that causes the accumulation of allophane and oxides of iron and aluminum in developing soils.

Histosols are organic soils that form in areas of poor drainage. Their profile consists of thick accumulations of organic matter at various stages of decomposition.