Plinthosols (Pt)

PLINTHOSOLS (PT) The Reference Soil Group of the Plinthosols accommodates soils that contain ‘plinthite’, i.e. an iron- rich, humus-poor mixture of kaolinitic clay with quartz and other constituents that changes irreversibly to a hardpan or to irregular aggregates on exposure to repeated wetting and drying. In Plinthosols, plinthite is present either as an indurate layer at shallow depth (‘petroplinthite’) or as soft plinthite. Inter- nationally, these soils are known as ‘Groundwater Laterite Soils’, ‘’Lateritas Hydromorficas’ (Brazil), ‘Sols gris latéritiques’ (France), ‘Plinthaquox’ (USA, Soil Taxonomy) or as Plinthosols (FAO). Definition of Plinthosols Soils having: 1 a petroplinthic horizon starting within 50 cm from the soil surface; or 2 a plinthic horizon starting within 50 cm from the soil surface; or 3 a plinthic horizon starting within 100 cm from the soil surface underlying either an albic horizon or a horizon with stagnic properties Common soil units: Petric, Endoduric, Alic, Acric, Umbric, Geric, Stagnic, Abruptic, Pachic, Glossic, Humic, Albic, Ferric, Skeletic, Vetic, Alumic, Endoeutric, Haplic. Summary description of Plinthosols Connotation: soils with ‘plinthite’; from Gr, plinthos, brick. Parent material: plinthite is more common in weathering material from basic rock than in acidic rock weathering. In any case it is crucial that sufficient iron is present, originating either from the parent material itself or brought in by seepage water from elsewhere. Environment: formation of plinthite is associated with level to gently sloping areas with fluctuating groundwater. ‘Petroplinthic’ soils with continuous, hard ‘ironstone’ form where plinthite becomes exposed to the surface, e.g. on erosion surfaces that are above the present drainage base. ‘Skeletic’ soils having a layer of hardened plinthite concretions occur mostly in colluvial or alluvial deposits. Soft plinthite is associated with rain forest areas; petroplinthic and skeletic soils are more common in the savanna zone. Profile development: mostly A(E)BC-profiles with segregation of plinthite at the depth of groundwater fluctuation. Hardening of plinthite to petroplinthite takes place upon repeated drying and wetting, commonly after geological uplift of the terrain and/or climate change towards drier conditions. Use: mostly low volume grazing. Arable cropping is hindered by poor rooting conditions associated with frequent water logging and/or excessive stoniness and low chemical soil fertility. Regional distribution of Plinthosols The global extent of soils with plinthite is estimated at some 60 million hectares. Soft plinthite is most common in the wet tropics, notably in the eastern Amazon basin, the central Congo basin and parts of Southeast Asia. Extensive areas of hardened plinthite occur in the Sudano-Sahelian zone where petroplinthite forms hard caps on top of uplifted/exposed landscape elements. Similar soils occur on the Indian subcontinent, and in drier parts of Southeast Asia and northern Australia. See Figure 1. Dominant Associated Inclusions Miscellaneous lands Figure 1. Plinthosols worldwide. Associations with other Reference Soil Groups Plinthosols occur in tropical regions with extensive occurrences of ‘red tropical soils’ such as Ferralsols, Alisols, Acrisols and Lixisols. Soils with residual ‘soft’ plinthite occur in less well-drained positions in the landscape; they exhibit gleyic and/or stagnic properties and are linked to Gleysols. Well drained soils with abundant loose iron concretions (‘pisolithes’ or ‘pea iron’) in tropical and subtropical regions are commonly formed in plinthitic material that was dislocated, hardened, transported and fi- nally deposited as alluvial or (more commonly) colluvial soil parent material. Such soils are related to Plinthosols but may have to be classified as plinthic soil units of other Reference Soil Groups. Petric Plinthosols in eroding areas occur together with Leptosols and/or leptic units of other soils. Genesis of Plinthosols Areas where the formation of plinthite is still active have a hot and humid climate with a high annual rainfall sum and a short dry season. Buchanan (1807) coined the term ‘laterite’ for an iron-rich, humus- poor mixture of kaolinitic clay and quartz that was used as a building material in western India (Lat. ‘later’ means ‘brick’). The term ‘plinthite’ was introduced much later to evade the confusion caused by widely differing interpretations of the term ‘laterite’ and its many derivatives. Formation of plinthite. Plinthite forms in perennially moist (sub)soil layers. Formation of plinthite involves the following processes: 1 accumulation of sesquioxides through * removal of silica and bases through advanced hydrolysis and discharge of dissolved weathering products. This results in relative accumulation of weathering resistant sesquioxides, quartz and kaolinite, or * absolute accumulation of sesquioxides through enrichment from outside, and 2 segregation of iron (mottles) by alternating reduction and oxidation. Under conditions of water saturation, much of the iron is in the ferrous form and ‘mobile’. This iron precipitates as ferric oxide when/where conditions become drier and will not or only partially re-dissolve when conditions become wetter again. This explains why plinthite shows a typical redistribution pattern with red mottles in a platy, polygonal or reticulate configuration. Hardening of plinthite to petroplinthite In its unaltered form, plinthite is firm but can be cut with a spade. If the land becomes drier, e.g. because of a change in base level and/or a change in climate, plinthite hardens irreversibly to petroplinthite. Hardening of plinthite involves the following processes: • crystallization of amorphous iron compounds to continuous aggregates of iron oxide minerals, especially goethite, and • dehydration of goethite (FeOOH) to hematite (Fe2O3) and, if present, of gibbsite (Al2O3.3H2O) to boemite (Al2O3.H2O). Hardening of plinthite is often initiated by removal of the vegetation, especially forest, as this triggers erosion of the surface soil and exposure of plinthite to the open air. Hardened plinthite occurs in many tropical soils, either in a ‘skeletic’(concretionary) form or as continuous petroplinthite. Plinthosols with soft plinthite are indigenous to the rain forest zone. Soils with petroplinthite are especially abundant in the transition zone from rain forest to savanna. It is particularly prominent in dry areas that were once much wetter, e.g. in sub-Sahelian Africa; plinthite that was once at some depth hardened and became exposed as a thick ironstone cap that resists (further) erosion. This may ultimately lead to inversion of the original relief: former depression areas where plinthite formed are shielded against erosion by their ironstone caps and become the highest parts of the landscape. See Figure 2. Figure 2. Inversion of relief in an eroding landscape. Hardening of exposed plinthite produces a protective shield against further erosion. After Buringh & Som- broek, 1971. Characteristics of Plinthosols Morphological characteristics Plinthite is red mottled clay but not every red mottled clay is plinthite. It is not always easy to distin- guish between ‘normal’ mottled clay, plinthite and ironstone gravel because they grade into each other. Field criteria for identification of plinthite are: • red mottles are firm or very firm when moist and hard or very hard when dry • they can be cut with a knife but only with difficulty • they have sharp boundaries • they hardly stain the fingers when rubbed, and • they do not slake in water. The most obvious distinguishing feature of plinthite is, of course, that it hardens irreversibly to petroplinthite upon repeated wetting and drying but this cannot always be ascertained in the field. Petroplinthite (also referred to as ‘ironstone’, ‘laterite’, ‘murram’ or ‘ferricrete’) can be divided on basis of its morphology into 1 hyperplinthic massive iron pans that are either * residual: a continuous layer of indurate plinthite. Iron oxides (generally > 30%) are the main cementing agent; organic matter is absent or present only in traces. The layer may be massive or show a reticulate or interconnected platy or columnar pattern that encloses lighter coloured, non-indurate material, or * secondary: (mostly) colluvial ironstone gravel, stones and boulders formed by disintegra- tion of a massive iron pan, that are re-cemented together. 2 orthiplinthic discontinuous petroplinthite either * residual: gravel formed by in situ hardening of plinthite. Red mottles constitute a consid- erable part of the soil volume and are not interconnected. The gravels are rounded (‘pisolithes’ or ‘pea iron’) or irregular nodules, or * secondary: (mostly) colluvial ironstone gravel, stones and boulders that are not cemented together. Mineralogical characteristics Plinthite and petroplinthite have high contents of hydrated Fe- and Al-oxides (‘sesquioxides’). Free iron is present as oxide minerals, notably lepidocrocite (FeOOH), goethite (FeOOH) and hematite (Fe2O3); free aluminium occurs in gibbsite (Al2O3.3H2O) and/or boehmite (Al2O3.H 2O). Old ironstone crusts contain more hematite and boehmite and less sesquioxides than plinthite. Free silica is present as quartz inherited from the parent material. Easily weatherable primary minerals have disappeared; the dominant clay mineral is well-crystallized kaolinite. Hydrological characteristics Plinthosols with ‘soft’ plinthite occur in bottomlands in regions with

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