Podzol

In soil science, Podzols are the typical soils of coniferous or boreal forests. They are also the typical soils of eucalypt forests and heathlands in southern Australia. In Western Europe, Podzols develop on heathland, which is often a construct of human interference through grazing and burning. In some British moorlands with Podzolic soils, Cambisols are preserved under Bronze Age barrows (Dimbleby, 1962). In soil science, Podzols are the typical soils of coniferous or boreal forests. They are also the typical soils of eucalypt forests and heathlands in southern Australia. In Western Europe, Podzols develop on heathland, which is often a construct of human interference through grazing and burning. In some British moorlands with Podzolic soils, Cambisols are preserved under Bronze Age barrows (Dimbleby, 1962). Podzol means 'under-ash' and is derived from the Russian под (pod) + зола́ (zola); the full form is 'подзо́листая по́чва' (podzolistaya pochva, 'under-ashed soil'). The term was introduced in 1875 by Vasily Dokuchaev. It refers to the common experience of Russian peasants of plowing up an apparent under-layer of ash (leached or E horizon) during first plowing of a virgin soil of this type. Podzols are able to occur on almost any parent material but generally derive from either quartz-rich sands and sandstones or sedimentary debris from magmatic rocks, provided there is high precipitation. Most Podzols are poor soils for agriculture due to the sandy portion, resulting in a low level of moisture and nutrients. Some are sandy and excessively drained. Others have shallow rooting zones and poor drainage due to subsoil cementation. A low pH further compounds issues, along with phosphate deficiencies and aluminum toxicity. The best agricultural use of Podzols is for grazing, although well-drained loamy types can be very productive for crops if lime and fertilizer are used. The E horizon (or Ae in Canadian soil classification system), which is usually 4 to 8 centimetres (1.57 to 3.15 in) thick, is low in Fe and Al oxides and humus. It is formed under moist, cool and acidic conditions, especially where the parent material, such as granite or sandstone, is rich in quartz. It is found under a layer of organic material in the process of decomposition, which is usually 5 to 10 centimetres (1.97 to 3.94 in) thick. In the middle, there is often a thin horizon of 0.5 to 1 centimetre (0.2 to 0.4 in). The bleached soil horizon goes over into a red or red-brown horizon (so-called Podzolic B). The colour is strongest in the upper part, and change at a depth of 50 to 100 centimetres (19.7 to 39.4 in) progressively to the part of the soil that is mainly not affected by processes; that is the parent material. The soil profiles are designated by the letters A (topsoil), E (eluviated soil), B (subsoil) and C (parent material). In some Podzols, the E horizon is absent—either masked by biological activity or obliterated by disturbance. Podzols with little or no E horizon development are often classified as brown Podzolic soils, also called Umbrisols or Umbrepts. Podzols cover about 4,850,000 square kilometres (1,870,000 sq mi) worldwide and are usually found under sclerophyllous woody vegetation. By extent Podzols are most common in temperate and boreal zones of the northern hemisphere but they can also be found in other settings including both temperate rainforests and tropical areas. In South America Podzols occur beneath Nothofagus betuloides forests in Tierra del Fuego. Podzolization (or Podsolization) is complex soil formation process by which dissolved organic matter and ions of iron and aluminium, released through weathering of various minerals, form organo-mineral complexes (chelates) and are moved from the upper parts of the soil profile and deposit in the deeper parts of soil. Through this process, the eluvial horizon becomes bleached and of ash-grey colour. The complexes move with percolating water further down to illuviated horizons which are commonly coloured brown, red or black as they accumulate and consist of cemented sesquioxides and/or organic compounds. The podzolization is a typical soil formation process in Podzols. Podzolization usually occurs under forest or heath vegetation and is common in cool and humid climates as these climates inhibit the activity of soil microbes in the topsoil. Overall, podzolization happens where the decomposition of organic matter is inhibited and as a result, acidic organic surface (mor) layers build up. Under these, typically acidic conditions, nutrient deficiency further hampers the microbial degradation of organic complexing agents. Medium to coarse textured soils with base-poor parent material (usually rich in quartz) also promote podzolization, as they encourage percolating water flow.