| Podzol | |
|---|---|
| Podsol, Podosol, Spodosol, Espodossolo | |
 The picture is of a stagnic podzol in upland Wales, and shows the typical sequence of organic topsoil with leached grey-white subsoil with iron-rich horizon below. This example has two weakironpans. | |
| Used in | WRB,USDA soil taxonomy, others |
| WRB code | PZ |
| Profile | O(Ah)EBhsC |
| Key process | podzolization |
| Parent material | quartz rich debris and sediment |
| Climate | humid continental,subarctic,oceanic,equatorial |
| H: common O: always, hashumified organic matter mixed with minerals A: absent in most boreal podzols[1] E: common, is ashen grey and leached inFe andAl B: always, receives Fe and Al throughilluviation C: common | |
Podzols, also known aspodosols,spodosols, orespodossolos, are the typicalsoils ofconiferous orboreal forests and also the typical soils ofeucalypt forests andheathlands in southern Australia. In Western Europe, podzols develop on heathland, which is often a construct of human interference through grazing and burning. In some Britishmoorlands with podzolic soils,cambisols are preserved underBronze Age barrows.[2]
Podzol means "under-ash" and is derived from theRussianпод (pod) +зола́ (zola); the full form isподзо́листая по́чва (podzolistaya pochva), meaning "under-ashed soil". The term was first given in mid-1875 byVasily Dokuchaev,[3][4] and over time adopted bysoil science. It refers to the common experience ofRussian peasants of plowing up an apparent under-layer of ash (leached orE horizon) during first plowing of a virgin soil of that type.[5]
Podzols can occur on almost any parent material but generally derive from either quartz-rich sands and sandstone or sedimentary debris from magmatic rocks, provided there is high precipitation.[6] Most Podzols are poor soils foragriculture due to the sandy portion, resulting in a low level of moisture andnutrients. 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-drainedloamy types can be very productive for crops iflime andfertilizer are used.
The E horizon (or Ae in Canadian soil classification system), which is usually 4 to 8 centimetres (1.6 to 3.1 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 asgranite orsandstone, is rich inquartz. It is found under a layer oforganic material in the process ofdecomposition, which is usually 5 to 10 centimetres (2.0 to 3.9 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, which always has a highervalue than the horizons above and below it, 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 (20 to 40 in) progressively to the part of the soil that is mainly not affected by processes; that is the parent material. Thesoil 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 calledUmbrisols orUmbrepts.
Podzols cover about 4,850,000 square kilometres (1,870,000 mi2) worldwide and are usually found undersclerophyllous woody vegetation. By extent Podzols are most common in temperate andboreal zones of theNorthern Hemisphere but they can also be found in other settings including bothtemperate rainforests and tropical areas.[7]
In South America Podzols occur beneathNothofagus betuloides forests inTierra del Fuego.[8]
Podzolization (or Podsolization[9]) is a complexsoil formation process by which dissolvedorganic matter and ions ofiron andaluminium, 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, theeluvial horizon becomes bleached and of ash-grey colour. The complexes move with percolating water further down toilluviated horizons which are commonly coloured brown, red or black as they accumulate and consist of cementedsesquioxides and/or organic compounds. The podzolization is a typical soil formation process in Podzols.[9][10]
Podzolization usually occurs underforest orheath vegetation and is common in cool and humid climates as these climates inhibit the activity ofsoil microbes in the topsoil. Overall, podzolization happens where thedecomposition 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.[10][11] Medium to coarse textured soils with base-poorparent material (usually rich inquartz) also promote podzolization, as they encourage percolating water flow.[11][12]
The soil-forming process of podzolization can be broken down into two main steps:
In the topsoil of acidic soils, organic matter (mostly fromplant litter, the humus layer androot exudates) together with Al- and Fe-ions, form organo-mineral complexes. These solublechelates then relocate with percolating water from theA (orE horizon) to theB horizon. As a result of this, the E horizon (or Ae horizon in theCanadian system of soil classification) is left bleached and ash-grey in colour, while the B horizon becomes enriched with relocated organo-mineral complexes. The colour of B horizon is consequently red, brown or black, depending on the dominance of metal ions or organic matter. Usually, the boundary between the B and eluvial Ae (or E) horizon is very distinct, and sometimes ahardpan (or Ortstein[12]) can form, as the relocated Fe and Al and organic matter increase mineral particles, cementing them into this compacted layer.[10][11][12]
There are several reasons why these organo-mineral complexes immobilize in the B horizon: If during the eluviation process more Al- or Fe-ions bind to the organic compounds, the complex can flocculate as the solubility of it decreases with increasing metal to carbon ratio. Apart from that, a higherpH (or higher Ca content) in the lowersoil horizons can result in the breakdown of metal-humus complexes. In the lower soil layers, the organic complexing agents can be degraded by functioningmicroorganisms. Already established complexes in theB horizon can act as a filter, as they adsorb the traveling complexes from the upper soil horizons. A decreased water conductivity due to higher clay content can also result in the early flocculation of organo-mineral complexes.[10][11]
The relocated substances can sometimes separate in the illuvial horizons. Then, organic substances are mostly enriched in the uppermost part of the illuvial horizon, whereas Fe- and Al-oxides are mostly found in the lower parts of the illuvial horizon.[10]
Podzolization also promotes the relocation of some nutrients (Cu,Fe,Mn,Mo andP) that sometimes brings them closer to plantroots.[10]
The definitions in different soil classification systems are quite different. Especially soils that show pronounced other soil-forming processes in addition to podzolization are handled in different ways. The following correlations refer to soils, which have undergone advanced podzolization but lack prominent other soil-forming processes.
The termPodzols is used in theWorld Reference Base for Soil Resources[15] (WRB) and in many national soil classification systems (in some of them, spelledPodsols).
{{cite book}}: CS1 maint: multiple names: authors list (link)
| International | |
|---|---|
| National | |
| Other | |
