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

OC illuviates mainly into the UPPER Bs, eventually leading to an E-Bhs-Bs sequence

Forms an E-Bs horizon sequence

A

E

Bhs

Bs

BC

TypicHaplorthod

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

FULVIC HUMIC C

Small-ish molecules Large moleculesAcidic soils Basic soilsForest vegetation Grass vegetationFungi decomposition product Bacterial decomposition productIndicative of low biological activity High biological activityChelates and renders mobile Fe & Al Forms stable Ca-humatesCool, moist climates Warm, moist-dry climatesAccelerates weathering via chelation Inhibits weathering: an organic "paint"

Humic vs Fulvic acids

Chelation – how does it work?

A +3 metal cation, like Fe+++ (not soluble)

Chelated metal cation, now soluble

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

Fulvic acid chain molecule

From the top.......the podzolization process (cheluviation) is:

(Ch1a) Organic matter accumulates on the soil surface and decomposes,releasing acidic organic compounds. 

(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

Acidification occurs

Adsorbed to a clay mineral

Chelated byorganic acids

TOO MUCH CLAY INHIBITS PODZOLIZATION

Primary mineral

Fe+++

weathering

Can be translocated(PODZOLIZATION)

Aint goin’ nowhere!

RUBIFICATION(reddening in place)

Red colors here due toFe oxides adsorbed to clay minerals

in Bt horizon

E horizon

Bt horizon

Lessivage dominant here

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, orloss 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.

Al Fe

Dep

thA

E

Bs

Bsm

BC

C

E

Bsm

BC

Bs

Guess whatI 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 arethe 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.

(Im5) pH increase in lower solum causes precipitation of sols.

They are electrostatically positive and migrate to the B horizon. (Silica moves first because it is more mobile at higher pHs)

(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

May be most common in soils/environments with little OM, and in soils developed in volcanic parent materials

EVIDENCE in support?Si-rich cutans surrounded by OM- and Al+Fe-rich cutans in B horizons

Not found in soils that have thick O horizons or that have developed in pure quartz sand.

Quartz grain

Wet Spodosols: Aquods

Form by cheluviation, BUTFe in B horizon gets reduced and leaves in groundwtaer

Very little Fe in B horizon