Podzolization: Translocation of oxidized Fe, Al and/or humus to Bs, Bh, or Bhs horizons (Fe is NOT moving due to REDOX processes) RUSSIAN school of thought: "Podzolization" includes not only Fe+Al translocation, but also clay destruction in the upper profile. (creates coarser textures and thus enhances podzolization)

Main school of thought: Cheluviation organic acids (anions) combine with (oxidized) Fe+++, Al+++ cations (that is, they CHELATE them), to render them soluble and mobile Forms an E-Bs horizon sequence OC illuviates mainly into the UPPER Bs, eventually leading to an E-Bhs-Bs sequence

A E Bhs Bs BC Typic Haplorthod

Organic acids: humic (large ones, high-pH environs) fulvic (medium ones, low-pH environs) low molecular weight ones

Humic vs Fulvic acids FULVIC HUMIC C Small-ish molecules Large molecules Acidic soils Basic soils Forest vegetation Grass vegetation Fungi decomposition product Bacterial decomposition product Indicative of low biological activity High biological activity Chelates and renders mobile Fe & Al Forms stable Ca-humates Cool, moist climates Warm, moist-dry climates Accelerates weathering via chelation Inhibits weathering: an organic "paint"

Chelation – how does it work? A +3 metal cation, like Fe+++ (not soluble) Chelated metal cation, now soluble

Fulvic acid chain molecule | |--COOH |--OH |--COOH Carboxyl group (Structure before |--OH Hydroxyl group dissociation of H+ and | linkage to metal cation) |--COOH + Fe+++ + 3H2O |--COO--\___Fe(H2O)3 + H+ (Structure after metal |--O------/ | cation (Fe+++) is | OH chelated and one H+ is | released, acidifying soil soln.)

From the top.......the podzolization process (cheluviation) is: (Ch1a) Organic matter accumulates on the soil surface and decomposes, releasing acidic organic compounds. 

TOO MUCH CLAY INHIBITS PODZOLIZATION Acidification occurs (Ch1b) Rainfall adds carbonic acid (CA) to the soil system Additional CA is formed by oxidation of organic matter and microbial respiration.  Acidifies the surface horizons and initiates weathering. (Ch2) Acids dissolve carbonate and other minerals (carbonation), --byproducts are leached.  pH gradient is established (acid in upper profile, non-acid below). (Ch2a) If clays present, initiate lessivage. (If sandy, proceed to step Ch3.) TOO MUCH CLAY INHIBITS PODZOLIZATION

TOO MUCH CLAY INHIBITS PODZOLIZATION Primary mineral weathering Fe+++ Adsorbed to a clay mineral Chelated by organic acids Aint goin’ nowhere! Can be translocated (PODZOLIZATION) RUBIFICATION (reddening in place)

Fe oxides adsorbed to clay minerals Lessivage dominant here E horizon Red colors here due to Fe oxides adsorbed to clay minerals in Bt horizon Bt horizon

Rubification: -reddening in place -forms Bw horizons

(Ch3) When upper profile pH drops <5.0, ferromagnesian minerals really start to weather rapidly, releasing lots of Fe, Mg, Al, and K cations. (Ch4) Any “free” metal cations get chelated (as long as clay doesn’t interfere, and as long as the “correct” acids are present) Chelate complexes migrate with percolating water, until immobilization and/or precipitation occurs, below.

(Ch5) Precipitation of chelate occurs due to: (a) microbial breakdown of chelate (b) loss of energy: dessication (cessation of wetting front movement), or loss of energy at water table, or loss of energy at lith discontinuity (c) increase in pH (d) *saturation of chelate complex ratio of > 0.1 for metal/acid wt will saturate it Al max is deeper than the Fe max, due to greater solubility of Al at low pH.

(Ch6) Aluminum remobilized as pH lowers thru time Al combines and reprecipitates with Si in the lower solum to form Bsm horizons BELOW Bh and Bhs horizons.

A E Al Fe Bs Depth Bsm BC C

E Bsm Bs BC

Guess what I am thinking!

Imogolite school (Anderson, Farmer and others) … the inability of cheluviation to account for:  (1) Al and Fe in Bs horizons that are predominantly in inorganic forms (2) significant amounts of imogolite clay in Bs horizon Where are the organics in the B?

Imogolite-type materials (ITM) theory: Al (and to a lesser extent, Fe) migrates as hydroxyaluminum silcate (proto-imogolite) sols (Al2O3-Fe2O3-SiO2-H2O), Chelate complexes form LATER as Bh horizons, in situ, when colloidal organics precipitate on the inorganic colloids already present in the B horizon

The Imogolite Theory/Process Stages Im1-Im3 same as Ch1-Ch3 (Im4) Sols and sesquioxides form due to liberation from PM by organic acids. They are electrostatically positive and migrate to the B horizon. (Silica moves first because it is more mobile at higher pHs) (Im5) pH increase in lower solum causes precipitation of sols.

(Im6) Organic colloids (electrostatically negative) move from the E and flocculate on the positive sols in the B, forming a Bhs horizon (Im7) Continued acidification remobilizes Al from the Bs/Bhs horizon. Al moves to greater depths, while Fe stays in the Bs/Bhs (Im8) Additional humus is added to the Bs by root decomposition and by continued illuviation of humus colloids. Bhs forms above a Bs.

A E Bhs Bs BC

Si-rich cutans surrounded by OM- and Al+Fe-rich cutans in B horizons EVIDENCE in support? Si-rich cutans surrounded by OM- and Al+Fe-rich cutans in B horizons Quartz grain May be most common in soils/environments with little OM, and in soils developed in volcanic parent materials Not found in soils that have thick O horizons or that have developed in pure quartz sand.

Wet Spodosols: Aquods Form by cheluviation, BUT Fe in B horizon gets reduced and leaves in groundwtaer

Very little Fe in B horizon