Ferralsols, Plinthosols Alisols, Acrisols, Lixisols Nitisols
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Ferralsols, Plinthosols Alisols, Acrisols, Lixisols Nitisols Peter Schad Soil Science Department of Ecology Technische Universität München Ferralsols and Plinthosols occur in the - permanently humid tropics - summer-humid tropics Factors of soil formation climate parent materials biota topography time Which ones differ largely comparing the tropics with Europe? Climate high temperatures and high precipitation: 1. chemical weathering increased 2. decomposition of organic matter increased 3. ion leaching increased Time many parts of the old Gondwana continent: - South American lowlands - most parts of Africa - India - western Australia no glaciars and no tundra in the Pleistocene (Central and Northern Europe: nearly all soils began their formation after the Pleistocene) Main process of soil formation: ferralitization 1. weathering of silicates, mainly by hydrolysis 2. leaching out of base cations and silicon ions 3. formation of kaolinite and oxides Mineral constituents of Ferralsols and Plinthosols 1. quartz (residual): SiO2 2. kaolinite: 1:1 clay mineral 3. iron oxides: hematite: red, high temperatures: Fe2O3 goethite: brown, everywhere: FeOOH 4. aluminium oxides: gibbsite: Al(OH)3 Definition of Ferralsols 1. having a ferralic horizon at some depth between 25 and 200 cm from the soil surface; and 2. lacking a nitic horizon within 100 cm from the soil surface; and 3. lacking a layer which fulfils the requirements of an argic horizon and which has in the upper 30 cm, 10% or more water-dispersible clay (unless the soil material has geric properties or more than 1.4% organic carbon) Definition of the ferralic horizon (1) 1. a sandy loam or finer particle size and less than 90% (by weight) gravel, stones or petroplinthic (iron-manganese) concretions; and 2. a cation exchange capacity (by 1 M NH4OAc) -1 of 16 cmolc kg clay or less and an effective cation exchange capacity (sum of exchangeable bases plus exchangeable -1 acidity in 1 M KCl) of less than 12 cmolc kg clay; and Definition of the ferralic horizon (2) 3. less than 10% water-dispersible clay, unless the soil material has geric properties or more than 1.4% organic carbon; and 4. less than 10% weatherable minerals in the 50 - 200 µm fraction; and 5. no characteristics diagnostic for the andic horizon; and 6. thickness of at least 30 cm Porfile depth often several meters, up to 100 m deeper horizons: saprolite chemical weathering in situ nothing else happens: - little physical weathering - almost no organisms - no translocations -> rock structure remains unchanged -> low bulk density (ions washed out, but volume remains) Physical characteristics (1) microaggregates: kaolinite-oxide-complexes kaolinite: neg. charged (unless the pH is very low) oxides: pos. charged (pH < 6.5) also called: pseudosand, pseudosilt (-> problems with the field detection of texture by feel) stable, little erosion risk fall to pieces if pH rises over 6.5 Physical characteristics (2) pseudosand -> many macropores -> good aeration, good drainage high clay content -> many micropores few mesopores -> low plant available water capacity permanently humid tropics: no problem tropics with dry seasons: may be a problem Chemical characteristics (1) organic matter: 1. rapid decomposition and intensive bioturbation - high rainfall - high temperature - good drainage -> only few organic acids pH only moderately acid (often ca. 5) 2. high biomass production -> many plant residues -> rel. high stocks of org. matter in the min. soil Chemical characteristics (2) mineral constituents: -low CEC - little nutrient release by weathering of primary minerals (only a few primary minerals left) organic constituents: - high CEC - high nutrient release by decomposition Chemical characteristics (3) special problem: phosphate: at pH < 5: bound to oxides problem: there are many oxides in Ferralsols available P: by mineralization of organic matter Land use traditional: shifting cultivation: slash and burn: 2 – 4 years of agriculture 10 – 20 years of forest fallow modern intensive agriculture: possible if: - large amounts of mineral fertilizers (-> expensive) - no tillage (avoid too fast mineralization and erosion) modern alternative: agroforestry systems Ferralsol in Burkina Faso under Tectona grandis Ferralsol in Brazil under soybean Ferralsol in Bolivia with saprolite in greater depth the soils are savanna (above) physically stable forest (below) Formation of Plinthosols (1) 1. ferralitization: relative enrichment of Fe in situ 2. some Plinthosols: absolute enrichment of Fe in depressions: - laterally flowing water - capillary rise of groundwater Formation of Plinthosols (2) 3. iron distribution by redox processes: groundwater (gleyic properties): Fe oxides accumulate at the aggregate surfaces rainwater or floodwater (stagnic properties): Fe oxides accumulate in the centres of the aggregates Formation of Plinthosols (3) groundwater: processes 2 and 3 together groundwater: under the enriched horizon: pallid zone: white (pure kaolinite, Fe-depleted) former horizon with permanently reduced cond. most Plinthosols: redox processes actually not going on Hardening in Plinthosols (1) first result: soft horizon with special colour pattern: plinthite then: may harden: - hard concretions: pisoliths possible if originated by stagnic properties - continuously cemented horizon: petroplinthite possible in both cases (originated by stagnic or gleyic properties) Hardening in Plinthosols (2) hardening: if enough crystalline iron oxides accumulate may harden under water-logged conditions may harden deep in the soil but it always hardens : - when exposed near to the surface - and subject to alternating drying and wetting over a long time Definition of Plinthosols having either 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 when underlying either an albic horizon or a horizon with stagnic properties Definition of the plinthic horizon (1) 1. 25% (by volume) or more of an iron-rich, humus-poor mixture of kaolinitic clay with quartz and other diluents, which changes irreversibly to a hardpan or to irregular aggregates on exposure to repeated wetting and drying with free access of oxygen; and Definition of the plinthic horizon (2) 2. a. 2.5% (by weight) or more citrate-dithionite extractable iron in the fine earth fraction, especially in the upper part of the horizon, or 10% in the mottles or concretions; and b. ratio between acid oxalate (pH3) extractable iron and citrate-dithionite extractable iron of less than 0.1; and 3. less than 0.6% (by weight) organic carbon; and 4. thickness of 15 cm or more Definition of the petroplinthic horizon (1) 1. a. 10% (by weight) or more citrate-dithionite extractable iron, at least in the upper part of the horizon; and b. ratio between acid oxalate (pH3) extractable iron and citrate-dithionite extractable iron of less than 0.1; and Definition of the petroplinthic horizon (2) 2. less than 0.6% (by weight) organic carbon; and 3. cementation to the extent that dry fragments do not slake in water and it cannot be penetrated by roots 4. thickness of 10 cm or more We regard as plinthic horizon - plinthite (soft) - horizon with pisoliths - petroplinthite missing the iron oxide requirements (criterion 1) of the petroplinthic horizon (diagnostic criteria of the plinthic horizon have to be met) Characteristics physical characteristics: dominated by plinthite, petroplinthite or pisoliths chemical characteristics: like Ferralsols land use: do not expose the plinthite to the surface -> risk of hardening Plinthosol in Bolivia with a soft plinthic horizon Soil in Bolivia with pisoliths Soil in Bolivia with a petroplinthic horizon Soil in South Africa with a petroplinthic horizon and a pallid zone underneath Petroplinthic horizons at the surface Alisols, Lixisols, Acrisols characterized by an argic horizon (like Luvisols and Albeluvisols) Definition of the argic horizon (1) 1. sandy loam or finer and at least 8% clay; and 2. more total clay than the overlying horizon: a. if the overlying horizon has less than 15% clay, the argic horizon must have at least 3% more (abs.); b. if the overlying horizon has 15 - 40% clay, the ratio of clay in the argic horizon to that of the overlying horizon must be at least 1.2; c. if the overlying horizon has more than 40% clay, the argic horizon must have at least 8% more (abs.); and Definition of the argic horizon (2) 3. if formed by clay illuviation: increase in clay content within 30 cm; if formed by any other processes: increase in clay content within 15 cm; and 4. autochthonous rock structure absent in at least half of the volume; and 5. thickness of at least 7.5 cm (shortened) Genesis of the argic horizon 1. transport of clay minerals from the topsoil to the subsoil 2. in the subsoil: higher weathering of primary silicates and higher clay mineral synthesis 3. in the topsoil: higher weathering of clay minerals 4. selective erosion of clay minerals from the topsoil Distinguish: Luvisol, Alisol, Lixisol, Acrisol BS >= 50 % BS < 50 %, alic properties in 25 – 100 cm no alic prop. in 25 – 100 cm in the major part in 25 – 100 cm in the major part in the major part CEC >= 24 Luvisol Alisol -1 cmolc kg clay in the argic throughout CEC < 24 Lixisol Acrisol -1 cmolc kg clay in the argic in some part Definition of alic properties (1) 1. a cation exchange capacity (by 1 M NH4OAc) -1 of at least 24 cmolc kg clay; and 2. a. a total reserve in