A reassessment of podzol formation processes

Abstract

Summary Translocated (oxalate‐soluble) Al and Fe are present predominantly in inorganic forms in the B 2 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 B 2 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 A 2 or B 2 horizons, and also by larger humic acid contents than in comparable B 2 , 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 A 2 and deposit them in the B 2 horizon; subsequently, with the development of an H layer, colloidal humus migrates through the A 2 and precipitates on the positive colloids at the top of the B 2 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 B 2 leads to a thin iron pan, separating permanently oxidizing conditions below from seasonally waterlogged and reducing conditions above.