Environmental Engineering Reference
In-Depth Information
illu-viation
(in-washing) (Plate 19.3 and see Colour Plate 16 between pp. 400 and 401).
Podzols are the result of intensive leaching and the translocation of sesquioxides. These
processes occur typically under coniferous woodland, mixed forest vegetation and heath,
especially in subarctic and temperate climates. These vegetation types produce an acid
humus layer on the soil surface. This acid humus or mor decomposes only slowly and
only partially, releasing organic acids (fulvic acids) and reactive organic chemicals
(hydroquinones) which can form complexes with the iron and aluminium cations in the
soil. The chemical complex is often in the form of a chelate, as illustrated in Figure 19.8.
In this form the normally immobile iron and aluminium can migrate easily downwards in
percolating water. These sesquioxides are deposited in the B horizon together with any
humic colloids. The resulting soil is strongly acid at the surface (pH 3ยท0), with a bleached
subsurface horizon (Ea) above B horizons where humus, iron and aluminium have been
precipitated. As one might expect, the most distinctive podzol profiles are found where
drainage is good and the parent material is acid (e.g. acid igneous and metamorphic
rocks, sandstones, depositional acid sands).
Most soil classifications recognize different types of podzol, depending on local
conditions. Three varieties are illustrated in Figure 19.9. The typical humo-ferric podzol
(FAO: Orthic Podzol) has accumulations of organic material (Bh or Bhs) overlying the
horizon of iron accumulation (Bs) (Plate 19.4). This is the normal profile found under
coniferous and mixed forest in Russia and North America where the parent materials
have a reasonable content of iron. They are common on coarse, non-calcareous materials
or on materials like glacial tills from which free lime has been removed. By contrast,
humic podzols have strong Bh horizons but lack a horizon of iron accumulation. They
form under cool, moist conditions and usually on parent materials with a low iron
content.
Figure 19.8
A chelate structure holding an atom of iron.
The third type is the iron pan stagnopodzol (FAO Placic Podzol), where
placic
signifies
the presence of a thin ironpan (Bf) above or within the podzolic Bs horizon. Frequently
the Bf is so impermeable that it blocks the downward percola-tion of water, causing
gleying in the bleached horizon (Eag). The reducing conditions so produced lead to the
reduction of ferric iron (Fe
3+
) to the more mobile ferrous iron (Fe
2+
), which causes further
iron removal from the Eag. In Britain placic podzols are also common in uplands, where
