Plinthosol

{{short description|Iron-rich soil type}}

Plinthosols are a reference soil group in the World Reference Base for Soil Resources (WRB), developed by the Food and Agriculture Organization (FAO). They are characterized by the presence of plinthite, [https://link.springer.com/article/10.1007/s11368-014-0896-2 petroplinthite], or pisoliths—concretions formed through pedogenic processes involving the accumulation of iron oxides. A key diagnostic feature of these soils is the plinthic horizon, which begins within 50 cm of the soil surface, or 100 cm if it lies beneath an albic horizon or a horizon with [https://ees.kuleuven.be/soil-monoliths/wrb-documentation-centre/wrb-reference-soil-groups/STAGNOSOLS-Historical-Review stagnic] properties.{{Cite book |title=World reference base for soil resources 2014: international soil classification system for naming soils and creating legends for soil maps |date=2014 |publisher=FAO |isbn=978-92-5-108369-7 |editor-last=FAO |series=World soil resources reports |location=Rome}} Plinthosols typically form in tropical environments with alternating wet and dry conditions, where fluctuating water tables promote the transformation of iron and clay minerals. Plinthite, a red, clay-rich material, hardens irreversibly into ironstone when exposed to repeated moisture cycles. Though it may resemble a mineral when hardened, plinthite is a soil-forming (pedogenic) material rather than a true mineral.{{Cite book |last=Driessen |first=P |url=https://research.utwente.nl/en/publications/lecture-notes-on-the-major-soils-of-the-world |title=Lecture notes on the major soils of the world |last2=Deckers |first2=J |last3=Spaargaren |first3=O |last4=Nachtergaele |first4=F |date=2001 |publisher=Food and Agriculture Organization of the United Nations |isbn=978-92-5-104637-1 |language=English}}

Two main processes contribute to plinthite development. First, iron and aluminum accumulate through removing silica and base cations by hydrolysis and leaching, resulting in quartz–kaolinite mineral assemblage or through enrichment from external sources. Second, alternating reduction and oxidation cycles lead to iron segregation within the soil. Under saturated conditions, iron is reduced and becomes mobile in its ferrous form. Upon drying, it oxidizes and precipitates as ferric oxides and hydroxides. Over time, repeated wetting and drying cause these iron compounds to accumulate in mottled patterns, often appearing as platy, polygonal, or reticulate structures. This segregation and accumulation of iron are central to the development of plinthite and, ultimately, to the formation of Plinthosols{{Citation |last=Blake |first=George R. |title=Plinthosols |date=2008 |work=Encyclopedia of Soil Science |pages=579–580 |editor-last=Chesworth |editor-first=Ward |url=http://link.springer.com/10.1007/978-1-4020-3995-9_447 |access-date=2025-04-22 |place=Dordrecht |publisher=Springer Netherlands |language=en |doi=10.1007/978-1-4020-3995-9_447 |isbn=978-1-4020-3994-2 |last2=Steinhardt |first2=Gary C. |last3=Pombal |first3=X. Pontevedra |last4=Muñoz |first4=J. C. Nóvoa |last5=Cortizas |first5=A. Martínez |last6=Arnold |first6=R. W. |last7=Schaetzl |first7=Randall J. |last8=Stagnitti |first8=F. |last9=Parlange |first9=J.‐Y.}}{{Infobox soil

|name=Plinthosols

|alternative_name= Groundwater Laterite, Plinthaquox (USDA)

|image=Plinthosol.JPG

|image_size=200px

|image_caption=

|classification_system=WRB

|code=PT

|profile=

|process=Accumulation of Fe under hydromorphic conditions

|parent_material=basic rock, Fe

|climate=wet tropics

}}

= Characteristics of Plinthosols =

Morphological Characteristics

Differentiating plinthite from typical mottled clay or ironstone gravel can be challenging, as these materials often transition gradually into one another. In the field, plinthite is recognized by several key features: the red mottles are notably firm or very firm when moist and become hard or very hard upon drying. They can be cut with a knife, but not easily. Plinthite has well-defined boundaries, does not readily stain the fingers when rubbed, and does not disintegrate or slake in water.{{Cite book |last=Driessen |first=P |url=https://research.utwente.nl/en/publications/lecture-notes-on-the-major-soils-of-the-world |title=Lecture notes on the major soils of the world |last2=Deckers |first2=J |last3=Spaargaren |first3=O |last4=Nachtergaele |first4=F |date=2001 |publisher=Food and Agriculture Organization of the United Nations |isbn=978-92-5-104637-1 |language=English}}

Mineralogical Characteristics

Plinthite and petroplinthite are rich in hydrated iron and aluminium oxides, with iron oxide content occasionally exceeding 80%, though some units may contain about equal parts of each. Iron is primarily present in the form of oxide minerals such as lepidocrocite (FeOOH), goethite (FeOOH), and hematite (Fe₂O₃), while aluminium occurs as gibbsite (Al₂O₃·3H₂O) and/or boehmite (Al₂O₃·H₂O). Older ironstone crusts generally contain more hematite and boehmite than plinthite but lower overall sesquioxide content. Quartz is often found as inherited silica from the parent material, while easily weathered primary minerals are typically absent. The dominant clay mineral is well-crystallized kaolinite.

Hydrological Characteristics

Plinthosols containing soft plinthite are typically found in bottomlands of regions where annual precipitation exceeds evaporation. These soils often show signs of eluviation, including an albic horizon beneath an umbric surface layer. Due to poor drainage, such soils frequently develop gleyic or stagnic properties.