|Podsol, Podosol, Spodosol, Espodossolo|
The picture is of a stagnopodzol in upland Wales, and shows the typical sequence of organic topsoil with leached grey-white subsoil with iron-rich horizon below. The example has two weak ironpans.
|Used in:||WRB, USDA soil taxonomy, others|
|Parent material:||quartz rich debris and sediments|
|Climate:||humid continental, subarctic, oceanic, equatorial|
O: always, has humified organic matter mixed with minerals
A: absent in most boreal podzols
E: common, is ashen grey and leached in Fe and Al
B: always, receives Fe and Al through illuviation
In soil science, podzols (known as spodosols in China and the United States of America and podosols in Australia) are the typical soils of coniferous, or boreal forests. They are also the typical soils of eucalypt forests and heathlands in southern Australia, while in Western Europe podzols develop on heathland, which is often a construct of human interference through grazing and burning. Many podzols in this region may have developed over the past 3000 years in response to vegetation and climatic changes. In some British moorlands with podzolic soils there are brown earths 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"). It likely 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 aluminium 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, 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 layer of 0.5 to 1 centimetre (0.2 to 0.4 in). The bleached soil goes over into a red or redbrown horizon called rusty soil. 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.
The main process in the formation of podzols is podzolisation. Podzolisation is a complex process (or number of sub-processes) in which organic material and soluble minerals (commonly iron and aluminium) are leached from the A and E horizons to the B horizon. It involves mobilization and precipitation of dissolved organic matter, together with aluminum and iron as they leach down from the A and E horizons to the B horizon. Through this process the overlying eluvial horizons are bleached. The complexes move to the brown, red or black horizon, which consist of cemented sesquioxides and/or organic compounds. Podzolisation occurs under low pH values.
In podzols, translocation has meant the eluviation of clays, humic acids, iron, and other soluble constituents from the A and E horizons. These constituents may then accumulate to form a spodic illuvial horizon and in some cases a placic horizon or iron band. Podzolization occurs when severe leaching leaves the upper horizon virtually depleted of all soil constituents except quartz grains. Clay minerals in the A horizon decompose by reaction with humic acids and form soluble salts. The leached material from the A horizon is deposited in the B horizon as a humus-rich horizon band, a hard layer of sesquioxides or a combination of the two.
These sub-processes include mobilisation, eluviation and illuviation. Mobilisation and eluviation both move organic materials and minerals through the A horizon into the B horizon. During this, they react with the water (illuviation) to become oxidised. This process of podzolisation results in the characteristic soil profile of spodosols, in which the E horizon is usually an ashen grey or white colour without structure and there is a distinctive hardpan oxide layer in the B horizon (which is always darker than the E horizon). The E horizon can be dark grey in profiles which are high in organic matter, but in such cases the underlying B is very dark. Spodosols over mafic rock may have a medium brown E horizon underlain by a darker B.
However, as conifers allelopathically reduce competition by producing a thick O horizon of mor (acidic and poisonous leaf litter that is slow to decompose), the primary form of plant-soil interactions is that of the conifers themselves. The acidic O horizon, along with rainfall patterns that are similar to that of the moister grasslands, also promotes the illuviation of oxides of aluminium and iron.
Spodosols are rare as paleosols. Though they are known from as far back as the Carboniferous, there are few examples surviving from before the first Pleistocene glaciation, and some of these may not be true spodosols.
Dimbleby GW (1962). The development of British heathlands and their soils. Oxford Forestry Memoirs 23.
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