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 microorganisms. Already established complexes in the B 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. 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. In different soil classification systems The term Podzols is used in the World Reference Base for Soil Resources (WRB) and in many national soil classification systems (in some of them, spelled Podsols). The USDA soil taxonomy and the Chinese soil taxonomy call these soils Spodosols. The Canadian system of soil classification matches Podzols with soils under the Podzolic order (e.g. Humo-Ferric Podzol). The Australian Soil Classification uses the term Podosols. The Brazilian Soil Classification System calls them Espodossolos. See also Soil type References Further reading W. Zech, P. Schad, G. Hintermaier-Erhard: Soils of the World. Springer, Berlin 2022, Chapter 3.3.3. External links The Podzolic Order Podzol http://classification.soilweb.ca/podzol/ http://edafologia.ugr.es/revista/tomo9b/a107text.pdf http://www.bauing.uni-wuppertal.de/boden/downloads_save/Podsol_Boden%20des%20Jahres_2007_JPNSS.pdf profile photos (with classification) WRB homepage profile photos (with classification) IUSS World of Soils Pedology Types of soil