Plinthosol

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, 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 stagnic properties. 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.

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