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= Podzolization = 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.

Preconditions
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.

Key steps of podzolization
The soil-forming process of podzolization can be broken down into two main steps:


 * 1) Mobilization and translocation of organic matter, Fe and Al from the surface horizon, and
 * 2) Immobilization and stabilization of organic matter, Fe and Al into the subsoil.

In the topsoil of acidic soils, organic matter (mostly from plant litter, the humus layer and root exudates) together with Al- and Fe-ions, form organo-mineral complexes. These soluble chelates then relocate with percolating water from the A (or E horizon) to the B-horizon. As a result of this, the E horizon (or Ae horizon in the Canadian 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 a hardpan (or Ortstein ) can form, as the relocated Fe and Al and organic matter increase mineral particles, cementing them into this compacted layer.

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 higher pH (or higher Ca content) in the lower soil horizons can result in the breakdown of metal-humus complexes. In the lower soil layers, the organic complexing agents can be degraded by functioning microorganism. Already established complexes in the B horizon can act as 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.

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.

Podzolization also promotes the relocation of some nutrients (Cu, Fe, Mn, Mo and P) that sometimes brings them closer to plant roots.

Podzolization in different soil classification systems
In the World Reference Base for Soil Resources, Podzols are characterized by cheluviation (the movement of Al/Fe- humus complexes through soil ) and chilluviation (accumulation of Al/Fe- humus complexes in lower soil horizons ), which describe the same processes happening during podzolization.

The USDA soil taxonomy describes podzolization in Spodosols.

The Canadian system of soil classification matches podzolization with soils under the Podzolic order (e.g. Humo-Ferric Podzol).

The Australian Soil Classification depicts the process of podzolization in Podosols.