A reassessment of podzol formation processes |
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| Authors: | H A ANDERSON M L BERROW V C FARMER A HEPBURN J D RUSSELL A D WALKER |
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| Institution: | The Macaulay Institute for Soil Research, Craigiebuckler, Aberdeen, Scotland |
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| Abstract: | Translocated (oxalate-soluble) Al and Fe are present predominantly in inorganic forms in the B2 horizons of the five pcdzol profiles examined: A1 as imogolite and proto-imogolite allophanes, and Fe as a separate oxide phase. Below the top few cm of the B2 horizon, over 75 per cent of the extractable (acid-plus alkali-soluble) organic matter is present as Al-fulvates, largely sorbed on allophanic material. The Bh horizons of the Iron Humus Podzol and Iron Podzol intergrades are distinguished by very high levels of organically bound Fe (soluble in EDTA solution), five to ten times more than in immediately adjacent A2 or B2 horizons, and also by larger humic acid contents than in comparable B2, levels in typical Iron Podzols. Inorganic forms of translocated Al and Fe are probably absent from two of the three Bh horizons examined, and also from the Bhg horizon overlying the thin iron pan in the Peaty Podzol. The organic matter in this Bhg horizon is saturated with Al rather than Fe. Chemical and physical processes which could lead to evolution of a profile along the genetic sequence, Iron Podzol, Iron Humus Podzol, Peaty Podzol, are postulated. During the formation of an Iron Podzol, positively charged inorganic sols carry aluminium, silicon and iron from the A2 and deposit them in the B2 horizon; subsequently, with the development of an H layer, colloidal humus migrates through the A2 and precipitates on the positive colloids at the top of the B2 horizon to form a Bh horizon, in which remobilized ferric species are trapped by the organic matter. In higher rainfall areas, occasional waterlogging above the oxide-impregnated B2 leads to a thin iron pan, separating permanently oxidizing conditions below from seasonally waterlogged and reducing conditions above. |
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