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World Soil Resources Reports 94 ISSN 0532-0488 LECTURE NOTES ON THE MAJOR SOILS OF THE WORLD Catholic University of LeuvenLecture Notes on the Major Soils of the World 3 Introduction Soil is a 3-dimensional body with properties that reflect the impact of (1) climate, (2) vegetation, fauna, Man and (3) topography on the soil’s (4) parent material over a variable (5) time span. The nature and relative importance of each of these five ‘soil forming factors’ vary in time and in space. With few exceptions, soils are still in a process of change; they show in their ‘soil profile’ signs of differentiation or alteration of the soil material incurred in a process of soil formation or ‘pedogenesis’. Unlike plants and animals, which can be identified as separate entities, the world’s soil cover is a continuum. Its components occur in temporal and/or spatial successions. In the early days of soil science, soil classification was based on the (surmised) genesis of the soils. Many ‘traditional’ soil names refer to the soil forming factor considered to be dominant in a particular pedogenetic history, for instance ‘desert soils’ (climate being the dominant factor), ‘plaggen soils’ (human interference), ‘prairie soils’ (vegetation), ‘mountain soils’ (topography), or ‘volcanic ash soils’ (parent material). Alternatively, soil names referred to a prominent single factor, for instance ‘Brown Soils’ (colour), ‘alkali soils’ (chemical characteristic), ‘hydromorphic soils’ (physical characteristic), ‘sandy soils’ (texture) or ‘lithosols’ (depth). The many soil classification schemes developed over the years reflect different views held on concepts of soil formation and mirror differences of opinion about the criteria to be used for classification. In the 1950’s, international communications intensified while the number of soil surveys increased sharply both in temperate regions and in the tropics. The experience gained in those years and the exchange of data between scientists rekindled interest in (the dynamics of) the world’s soil cover. Classification systems were developed, which aimed at embracing the full spectrum of the soil continuum. In addition, emphasis shifted away from the genetic approach, which often contained an element of conjecture, to the use of soil properties as differentiating criteria. By and large, consensus evolved as to the major soil bodies which needed to be distinguished in broad level soil classification although differences in definitions and terminology remained. THE FAO-UNESCO SOIL CLASSIFICATION SYSTEM In 1974, the Food and Agriculture Organization of the United Nations (FAO) published its Soil Map of the World (SMW). Compilation of the SMW was a formidable task involving collection and correlation of soil information from all over the world. Initially, the Legend to the SMW consisted of 26 (‘first level’) “Major Soil Groupings” comprising a total of 106 (‘second level’) ‘Soil Units’. In 1990, a ‘Revised Legend’ was published and a third hierarchical level of ‘Soil Subunits’ was introduced to support soil inventory at larger scales. Soil Subunits were not defined as such but guidelines for their identification and naming were given. De facto this converted the SMW map legend, with a finite number of entries, into an open-ended, globally applicable ‘FAO- Unesco Soil Classification System’.4 Introduction THE WORLD REFERENCE BASE FOR SOIL RESOURCES In 1998, the International Union of Soil Sciences (IUSS) officially adopted the World Reference Base for Soil Resources (WRB) as the Union’s system for soil correlation. The structure, concepts and definitions of the WRB are strongly influenced by (the philosophy behind and experience gained with) the FAO-Unesco Soil Classification System. At the time of its inception, the WRB proposed 30 ‘Soil Reference Groups’ accommodating more than 200 (‘second level’) Soil Units. In the present text, the 30 Reference Soil Groups are aggregated in 10 ‘sets’ composed as follows: 1. First, a separation is made between organic soils and mineral soils; all organic soils are grouped in Set 1. 2. The remaining (mineral) Major Soil Groups are each allocated to one of nine sets on the basis of ‘dominant identifiers’, i.e. those soil forming factor(s) which most clearly conditioned soil formation. Table 1 summarises the 10 sets, their dominant identifiers and the Reference Soil Groups within each set. SET 1 holds all soils with more than a defined quantity of ‘organic soil materials’. These organic soils are brought together in only one Reference Soil Group: the HISTOSOLS. SET 2 contains all man-made soils. These soils vary widely in properties and appearance and can occur in any environment but have in common that their properties are strongly affected by human intervention. They are aggregated to only one Reference Soil Group: the ANTHROSOLS. SET 3 includes mineral soils whose formation is conditioned by the particular properties of their parent material. The set includes three Reference Soil Groups: 1. the ANDOSOLS of volcanic regions, 2. the sandy ARENOSOLS of desert areas, beach ridges, inland dunes, areas with highly weathered sandstone, etc., and 3. the swelling and shrinking heavy clayey VERTISOLS of backswamps, river basins, lake bottoms, and other areas with a high content of expanding 2:1 lattice clays. SET 4 accommodates mineral soils whose formation was markedly influenced by their topographic/physiographic setting. This set holds soils in low terrain positions associated with recurrent floods and/or prolonged wetness, but also soils in elevated or accidented terrain where soil formation is hindered by low temperatures or erosion. The set holds four Reference Soil Groups: In low terrain positions: 1. Young alluvial FLUVISOLS, which show stratification or other evidence of recent sedimentation, and 2. Non-stratified GLEYSOLS in waterlogged areas that do not receive regular additions of sediment. In elevated and/or eroding areas:Lecture Notes on the Major Soils of the World 5 3. Shallow LEPTOSOLS over hard rock or highly calcareous material, and 4. Deeper REGOSOLS, which occur in unconsolidated materials and which have only surficial profile development, e.g. because of low soil temperatures, prolonged dryness or erosion. SET 5 holds soils that are only moderately developed on account of their limited pedogenetic age or because of rejuvenation of the soil material. Moderately developed soils occur in all environments, from sea level to the highlands, from the equator to the boreal regions, and under all kinds of vegetation. They have not more in common than ‘signs of beginning soil formation’ so that there is considerable diversity among the soils in this set. Yet, they all belong to only one Reference Soil Group: the CAMBISOLS. SET 6 accommodates the ‘typical’ red and yellow soils of wet tropical and subtropical regions. High soil temperatures and (at times) ample moisture promote rock weathering and rapid decay of soil organic matter. The Reference Soil Groups in this set have in common that a long history of dissolution and transport of weathering products has produced deep and genetically mature soils: 1. PLINTHOSOLS on old weathering surfaces; these soils are marked by the presence of a mixture of clay and quartz (‘plinthite’) that hardens irreversibly upon exposure to the open air, 2. deeply weathered FERRALSOLS that have a very low cation exchange capacity and are virtually devoid of weatherable minerals, 3. ALISOLS with high cation exchange capacity and much exchangeable aluminium, 4. deep NITISOLS in relatively rich parent material and marked by shiny, nutty structure elements, 5. strongly leached, red and yellow ACRISOLS on acid parent rock, with a clay accumulation horizon, low cation exchange capacity and low base saturation, and 6. LIXISOLS with a low cation exchange capacity but high base saturation percentage. SET 7 accommodates Reference Soil Groups in arid and semi-arid regions. Redistribu- tion of calcium carbonate and gypsum is an important mechanism of horizon differentiation in soils in the dry zone. Soluble salts may accumulate at some depth or, in areas with shallow groundwater, near the soil surface. The Reference Soil Groups assembled in set 7 are: 1. SOLONCHAKS with a high content of soluble salts, 2. SOLONETZ with a high percentage of adsorbed sodium ions, 3. GYPSISOLS with a horizon of secondary gypsum enrichment, 4. DURISOLS with a layer or nodules of soil material that is cemented by silica, and 5. CALCISOLS with secondary carbonate enrichment. SET 8 holds soils that occur in the steppe zone between the dry climates and the humid Temperate Zone. This transition zone has a climax vegetation of ephemeral grasses and dry forest; its location corresponds roughly with the transition from a dominance of accumulation processes in soil formation to a dominance of leaching processes. Set 8 includes three Reference Soil Groups:6 Introduction 1. CHERNOZEMS with deep, very dark surface soils and carbonate enrichment in the subsoil, 2. KASTANOZEMS with less deep, brownish surface soils and carbonate and/or gypsum accumulation at some depth (these soils occur in the driest parts of the steppe zone), and 3. PHAEOZEMS, the dusky red soils of prairie regions with high base saturation but no visible signs of secondary carbonate accumulation. SET 9 holds the brownish and greyish soils of humid temperate regions. The soils in this set show evidence of redistribution of clay and/or organic matter. The cool climate and short genetic history of most soils in this zone explain why some soils are still relatively rich in bases despite a dominance of eluviation over enrichment processes. Eluviation and illuviation of metal- humus complexes produce the greyish (bleaching) and brown to black (coating) colours of soils of this set. Set 9 contains five Reference Soil Groups: 1. acid PODZOLS with a bleached eluviation horizon over an accumulation horizon of organic matter with aluminium and/or iron, 2. PLANOSOLS with a bleached topsoil over dense, slowly permeable subsoil, 3. base-poor ALBELUVISOLS with a bleached eluviation horizon tonguing into a clay-enriched subsurface horizon, 4. base-rich LUVISOLS with a distinct clay accumulation horizon, and 5. UMBRISOLS with a thick, dark, acid surface horizon that is rich in organic matter. SET 10 holds the soils of permafrost regions. These soils show signs of ‘cryoturbation’ (i.e. disturbance by freeze-thaw sequences and ice segregation) such as irregular or broken soil horizons and organic matter in the subsurface soil, often concentrated along the top of the permafrost table. Cryoturbation also results in oriented stones in the soil and sorted and non- sorted patterned ground features at the surface. All ‘permafrost soils’ are assembled in one Reference Soil Group: the CRYOSOLS. Note that the Reference Soil Groups in sets 6 through 10 represent soils, which occur predominantly in specific climate zones. Such soils are known as ‘zonal soils’. Be aware, however, that not all soils in sets 6 through 10 are zonal soils, nor are soils in other sets always non-zonal. Podzols, for instance, are most common in (sub)humid temperate climates (set 9) but they are also found in the humid tropics; Planosols may equally occur in subtropical and steppe climates and Ferralsols may occur as remnants outside the humid tropics. Soils whose characteristics result from the strong local dominance of a soil forming factor other than ‘climate’ are not ‘zonal soils’. They are ‘intrazonal soils’. In other words there are zonal and intrazonal Podzols, zonal and intrazonal Gleysols, zonal and intrazonal Histosols, and many more. Some soils are too young to reflect the influence of site-specific conditions in their profile characteristics; these are ‘azonal soils’. Young alluvial soils (Fluvisols) and soils in recent hillwash (e.g. Cambisols) are examples of azonal soils. The zonality concept helps to understand (some of) the diversity of the global soil cover but is a poor basis for soil classification. The sets of Reference Soil Groups presented in this text may therefore not be seen as high level classification units but merely as an illustration how basic principles of soil formation manifest themselves in prominent global soil patterns.Lecture Notes on the Major Soils of the World 7 TABLE 1 All Reference Soil Groups of the WRB assembled in 10 sets SET 1 Organic soils HISTOSOLS SET 2 Mineral soils whose formation was conditioned by ANTHROSOLS human influences (not confined to any particular region) SET 3 Mineral soils whose formation was conditioned by their parent material - Soils developed in volcanic material ANDOSOLS - Soils developed in residual and shifting sands ARENOSOLS - Soils developed in expanding clays VERTISOLS SET 4 Mineral soils whose formation was conditioned by the topography/physiography of the terrain - Soils in lowlands (wetlands) with level topography FLUVISOLS GLEYSOLS - Soils in elevated regions with non-level topography LEPTOSOLS REGOSOLS SET 5 Mineral soils whose formation is conditioned by their CAMBISOLS limited age (not confined to any particular region) SET 6 Mineral soils whose formation was conditioned by PLINTHOSOLS climate: (sub-)humid tropics FERRALSOLS NITISOLS ACRISOLS ALISOLS LIXISOLS SET 7 Mineral soils whose formation was conditioned by SOLONCHAKS climate: arid and semi-arid regions SOLONETZ GYPSISOLS DURISOLS CALCISOLS SET 8 Mineral soils whose formation was conditioned by KASTANOZEMS climate: steppes and steppic regions CHERNOZEMS PHAEOZEMS SET 9 Mineral soils whose formation was conditioned by PODZOLS climate: (sub-)humid temperate regions PLANOSOLS ALBELUVISOLS LUVISOLS UMBRISOLS SET 10 Mineral soils whose formation was conditioned by CRYOSOLS climate: permafrost regions DIAGNOSTIC HORIZONS, PROPERTIES AND MATERIALS The taxonomic units of the WRB are defined in terms of measurable and observable ‘diagnostic horizons’, the basic identifiers in soil classification. Diagnostic horizons are defined by (combinations of) characteristic ‘soil properties’ and/or ‘soil materials’. The diagnostic horizons, properties and materials used by the WRB to differentiate between Reference Soil Groups are described hereafter in Tables 2, 3 and 4; their full definitions can be found in Annex 2 to this text. Note that a distinction must be made between the soil horizon designations used in soil profile descriptions and diagnostic horizons as used in soil classification. The former belong to a nomencla- ture in which master horizon codes (H, O, A, E, B, C and R) are assigned to the various soil horizons in a soil profile when it is described and interpreted in the field. The choice of horizon code is by personal judgement of the soil surveyor. Diagnostic horizons, on the other hand, are rigidly defined and their presence or absence can be ascertained on the basis of unambiguous field and/or laboratory measurements. Some of the diagnostic horizons in the WRB soil correlation system are special forms of A- or B-horizons, e.g. a ‘mollic’ A-horizon, or a ‘ferralic’ B-horizon. Other diagnostic horizons are not necessarily A- or B-horizons, e.g. a ‘calcic’ or a ‘gypsic’ horizon.8 Introduction TABLE 2 Descriptive overview of diagnostic horizons (see Annex 2 for full definitions) Surface horizons and subsurface horizons at shallow depth anthropogenic horizons surface and subsurface horizons resulting from long-continued ‘anthropedogenic processes’ , notably deep working, intensive fertilisation, addition of earthy materials, irrigation or wet cultivation. chernic horizon deep, well-structured, blackish surface horizon with a high base saturation, high organic matter content, strong biological activity and well-developed, usually granular, structure. Its carbon content is intermediate between a mollic horizon and a histic horizon. folic horizon surface horizon, or subsurface horizon at shallow depth, consisting of well-aerated organic soil material . fulvic horizon thick, black surface horizon low bulk density high organic having a and carbon content conditioned by short-range-order minerals (usually allophane) and/or organo-aluminium complexes. histic horizon (peaty) surface horizon, or subsurface horizon occurring at shallow depth, consisting of organic soil material. melanic horizon thick, black surface horizon conditioned by short-range-order minerals (usually allophane) and/or organo-aluminium complexes. Similar to the 1 ‘melanic index ’ fulvic horizon except for a of 1.70 or less throughout. mollic horizon dark high base saturation well-structured, surface horizon with and moderate to high organic carbon content. takyric horizon finely textured surface horizon consisting of a dense surface crust and a platy lower part; formed under arid conditions in periodically flooded soils. umbric horizon dark low base saturation well-structured, surface horizon with and moderate to high organic matter content. ochric horizon surface horizon without stratification, which is either light coloured, or thin, or has a low organic carbon content, or is massive and (very) hard when dry. vitric horizon surface or subsurface horizon rich in volcanic glass and other primary minerals associated with volcanic ejecta. yermic horizon surface horizon of rock fragments (‘desert pavement’) usually, but not in a vesicular crust always, embedded and covered by a thin aeolian sand or loess layer. Subsurface horizons albic horizon bleached eluviation horizon with the colour of uncoated soil material, usually overlying an illuviation horizon. andic horizon weathering of mainly pyroclastic deposits horizon evolved during ; mineral assemblage dominated by short-range-order minerals such as allophane. argic horizon subsurface horizon having distinctly more clay than the overlying horizon as a result of illuvial accumulation of clay and/or pedogenetic formation of clay in the subsoil and/or destruction or selective erosion of clay in the surface soil. cambic horizon genetically young subsurface horizon showing evidence of alteration modified colour, removal of carbonates or relative to underlying horizons: presence of soil structure. cryic horizon perennially frozen horizon in mineral or organic soil materials. calcic horizon horizon with distinct calcium carbonate enrichment. duric horizon subsurface horizon with weakly cemented to indurated nodules cemented by silica (SiO ) ‘durinodes’ 2 known as . ferralic horizon strongly weathered low horizon in which the clay fraction is dominated by activity clays and the sand fraction by resistant materials such as iron-, aluminium-, manganese- and titanium oxides. 1 The melanic index (MI) is the ratio of absorbance of NaOH-extractable humus at 450 and 520 nm. See: Honna T., S. Yamamoto and K. Matsui. 1988. A simple procedure to determine the melanic index that is useful for differentiating Melanic from Fulvic Andisols. Pedologist, Vol.32 No 1, 69-75.Lecture Notes on the Major Soils of the World 9 TABLE 2 (continued) Descriptive overview of diagnostic horizons (see Annex 2 for full definitions) ferric horizon subsurface horizon in which segregation of iron has taken place to the extent that large mottles or concretions have formed in a matrix that is largely depleted of iron. fragic horizon dense, non-cemented subsurface horizon that can only be penetrated by roots and water along natural cracks and streaks. gypsic horizon calcium sulphate enrichment horizon with distinct . natric horizon subsurface horizon with more clay than any overlying horizon(s) and high exchangeable sodium percentage columnar or ; usually dense, with prismatic structure. nitic horizon clay-rich subsurface horizon with a moderate to strong polyhedric or nutty structure with shiny ped faces . petrocalcic horizon continuous, cemented or indurated calcic horizon. petroduric horizon continuous subsurface horizon cemented mainly by secondary silica (SiO ), also known as a ‘duripan’. 2 petrogypsic horizon cemented secondary accumulations of gypsum horizon containing (CaSO .2H O). 4 2 petroplinthic horizon continuous layer indurated by iron compounds and without more than traces of organic matter. plinthic horizon subsurface horizon consisting of an iron-rich, humus-poor mixture of kaolinitic clay with quartz and other constituents, and which changes irreversibly to a hardpan or to irregular aggregates on exposure to repeated wetting and drying with free access of oxygen. salic horizon surface or shallow subsurface horizon containing 1 percent of readily soluble salts or more. spodic horizon dark coloured subsurface horizon with illuvial amorphous substances organic matter and aluminium, with or without iron composed of . sulfuric horizon extremely acid subsurface horizon in which sulphuric acid has formed through oxidation of sulphides. vertic horizon subsurface horizon rich in expanding clays and having polished and grooved ped surfaces (‘slickensides’), or wedge-shaped or parallelepiped structural aggregates formed upon repeated swelling and shrinking. TABLE 3 Descriptive summary of diagnostic properties (see Annex 2 for full definitions) abrupt textural change very sharp increase in clay content within a limited vertical distance. albeluvic tonguing iron-depleted material penetrating into an argic horizon along ped surfaces. alic properties very acid soil material with a high level of exchangeable aluminium. aridic properties refer to soil material low in organic matter, with evidence of aeolian activity, light in colour and (virtually) base-saturated. continuous hard rock material which is sufficiently coherent and hard when moist to make digging with a spade impracticable. ferralic properties indicate that the (mineral) soil material has a ‘low’ cation exchange capacity ferralic horizon if it had been less or would have qualified for a coarsely textured. geric properties mark soil material of very low effective cation exchange capacity or even acting as anion exchanger. gleyic properties waterlogging by shallow groundwater visible evidence of prolonged . o permafrost soil temperature is perennially at or below 0 C indicates that the for at least two consecutive years. secondary carbonates significant quantities of translocated lime, soft enough to be readily cut with a finger nail, precipitated from the soil solution rather than being inherited from the soil parent material. stagnic properties visible evidence of prolonged waterlogging by a perched water table. strongly humic properties indicative of a high content of organic carbon in the upper metre of the soil. 10 Introduction TABLE 4 Descriptive summary of diagnostic materials (see Annex 2 for full definitions) anthropogenic soil material unconsolidated mineral or organic material produced largely by human activities and not significantly altered by pedogenetic processes. calcaric soil material soil material, which contains more than 2 percent calcium carbonate equivalent and shows strong effervescence with 10 percent HCl in most of the fine earth. fluvic soil material fluviatile, marine and lacustrine sediments, which show stratification in at least 25 percent of the soil volume over a specified depth and/or have an organic carbon content decreasing irregularly with depth. gypsiric soil material mineral soil material, which contains 5 percent or more gypsum (by volume) . organic soil material organic debris at the surface , which accumulates and in which the mineral component does not significantly influence soil properties. sulfidic soil material waterlogged deposit containing sulphur, mostly sulphides, and not more than moderate amounts of calcium carbonate. tephric soil material products of volcanic unconsolidated, non or only slightly weathered eruptions, with or without admixtures of material from other sources. Note that the generalised descriptions of diagnostic horizons, properties and soil materials given in Tables 2, 3 and 4 are solely meant as a first introduction to WRB terminology. The exact concepts and full definitions presented in Annex 2 must be used for identifying diagnostic horizons, properties and materials in practical taxon identification.Lecture Notes on the Major Soils of the World 11 The World Reference Base for Soil Resources The WRB as a soil correlation system Rules for identifying Soil Units Ranking qualifiers in Soil Unit names Polygenetic and buried soilsLecture Notes on the Major Soils of the World 13 The World Reference Base for Soil Resources THE WRB AS A SOIL CORRELATION SYSTEM The objectives of the World Reference Base are twofold. On the one hand the WRB is intended to be a reference system for users interested in a broad division of soils, at the highest level of generalisation and explained in non-technical terms. On the other hand, the WRB must facilitate soil correlation across a wide range of national soil classification systems. To best reconcile such conflicting requirements, it was decided to design the WRB as a flexible system, with maximum use of ‘morphometric’ (from Gr. morphos ‘shape’ and L. metrum ‘size’) soil profile information, but with rigidly standardised definitions. Using standardised diagnostic criteria and qualifiers facilitates soil correlation and technology transfer between countries and regions, which helps to better understand (relations between) soil resources and facilitates regional application of soil information, e.g. in land use planning. Reference Soil Groups are distinguished by the presence (or absence) of specific diagnostic horizons, properties and/or materials. A limited number of ‘qualifiers’, with unique definitions, describe individual Soil Units within Reference Soil Groups. Annex 1 to this text presents the full key for identifying WRB Reference Soil Groups; Annex 2 defines the diagnostic horizons, properties and materials used to define the various Reference Soil Groups. Note that the number of Reference Soil Groups in the WRB is fixed (30) but the number of Soil Units is not. Soil Units are distinguished on the basis of distinct ‘Rules for identifying Soil Units’ (see hereafter); qualifiers used to identify Soil Units are presented in Annex 3. RULES FOR IDENTIFYING SOIL UNITS 1. Soil units are defined, and named, on the basis of WRB-approved ‘qualifiers’. See Annex 3. 2. Qualifier names can be used in combination with indicators of depth, thickness or intensity. For instance, an Epi-Dystric Luvisol is a soil unit name in which ‘Epi-’ signifies shallow depth whereas ‘Dystric’ is a qualifier indicative of a low base status. If more than two qualifiers are needed, these are listed behind the Reference Soil Group name (between brackets), e.g. Acri-Geric Ferralsol (Abruptic and Xanthic). 3. Names of soil units must not overlap or conflict with names of other soil units or with Reference Soil Group definitions. For example, a “Dystri-Petric Calcisol” is unacceptable because it contains a contradiction (‘Dystri-’ is incompatible with ‘Calcisol’) and a “Eutri- Petric Calcisol” is rejected because the qualifier “Eutri-” overlaps with information inherent to the Reference Soil Group name “Calcisol”.14 The World Reference Base for Soil Resources 4. New units can only be established if documented by a soil profile description and supporting laboratory analyses. Qualifiers are defined by unique sets of diagnostic criteria. Most diagnostic criteria in qualifier definitions are derived from already established Reference Soil Group criteria such as diagnostic horizons, properties and materials. Weak or incomplete occurrences of features are generally not considered to be differentiating. Attributes referring to climate, parent material, vegetation or to physiographic features such as slope, geomorphology or erosion, are not used to differentiate between soil units. Neither are soil-water related attributes such as depth of water table or drainage, substratum specifications, nor specifications of thickness and/or morphology of the solum or individual horizons. RANKING QUALIFIERS IN SOIL UNIT NAMES It is widely felt that indiscriminate use of qualifiers would create confusion but the precise ranking of qualifiers in Soil Unit names is currently still under discussion. Annex 4 presents tentative ranking orders suggested for common qualifiers within each Reference Soil Group. An example: Within the Reference Soil Group of the Vertisols, the following qualifiers are considered to be ‘common’ (See Annex 4): Intergrades: 1. Thionic intergrade to acid sulphate Gleysols, Fluvisols and Cambisols 2. Salic intergrade to the Reference Soil Group of the Solonchaks 3. Natric intergrade to the Reference Soil Group of the Solonetz 4. Gypsic intergrade to the Reference Soil Group of the Gypsisols 5. Duric intergrade to the Reference Soil Group of the Durisols 6. Calcic intergrade to the Reference Soil Group of the Calcisols 7. Alic intergrade to the Reference Soil Group of the Alisols Extragrades: 8. Gypsiric containing gypsum 9. Grumic having a mulched surface horizon 10. Mazic having a very hard surface horizon; workability problems 11. Mesotrophic having less than 75 percent base saturation 12. Hyposodic having an ESP of 6 to 15 13. Eutric having 50 percent or more base saturation 14. Pellic dark coloured, often poorly drained 15. Chromic reddish coloured 16. Haplic no specific characteristics A reddish coloured Soil Unit within the Reference Soil Group of the Vertisols, having a calcic horizon, would be classified as a Calci-Chromic Vertisol because qualifiers 6 and 15 apply. If information on depth and intensity of the calcic horizon is available, e.g. occurring near the surface, one would classify the soil as an EpiCalci-Chromic Vertisol (indicating that the calcic horizon occurs within 50 cm from the surface).Lecture Notes on the Major Soils of the World 15 If more than two qualifiers are needed, these are added behind the Reference Soil Group name. If, for instance, the Vertisol discussed would also feature a very hard surface horizon (qualifier 10), the soil would be named a Calci-Chromic Vertisol (Mazic). POLYGENETIC AND BURIED SOILS Soils have vertical and horizontal dimensions that evolved over time. The vertical dimension is for practical purposes limited to a “control section” with a depth of 100 cm or, exceptionally, 200 cm below the surface. The qualifier bathic can be used to refer to horizons, properties or characteristics that occur below the control section. Most soil profiles can be named without difficulty but some, more complex situations require additional classification guidelines. The WRB prefers to name soils as they occur, i.e. with present-day characteristics and functional behaviour, rather than emphasising their (supposed) genetic history. It is realised however that few soils have completely evolved in situ and that it may be useful in certain cases to indicate this. Some soils show signs of polygenetic development i.e. a different soil has evolved prior to the present one (often under different environmental conditions) and both soils can be classified. A qualifier thapto- indicates the presence of a buried soil or a buried horizon. This would be the case if a soil has a surface mantel of new material that is 50 cm thick or more. The surface mantel is named in the normal way (e.g. as a Regosol, Andosol or Arenosol) and the buried soil would be classified with a prefix qualifier ‘thapto-‘. If the surface mantle is less than 50 cm thick, it is ignored in the soil name but the soil may be marked on the soil map by a phase indicator. Note that it is not recommended to systematically include the qualifier thapto- if this adds no information that has practical implications for the user.Lecture Notes on the Major Soils of the World 17 Reference Soil GroupsLecture Notes on the Major Soils of the World 19 Set 1 ORGANIC SOILS HistosolsLecture Notes on the Major Soils of the World 21 HISTOSOLS (HS) The Reference Soil Group of the Histosols comprises soils formed in ‘organic soil material ’. These vary from soils developed in (predominantly) moss peat in boreal, arctic and subarctic regions, via moss peat, reeds/sedge peat and forest peat in temperate regions to mangrove peat and swamp forest peat in the humid tropics. Histosols are found at all altitudes but the vast majority occurs in lowlands. Common international names are ‘peat soils’, ‘muck soils’, ‘bog soils’ and ‘organic soils’. Definition of Histosols Soils, 1. having a histic or folic horizon, either 10 cm or more thick from the soil surface to a lithic or paralithic contact, or 40 cm or more thick and starting within 30 cm from the soil surface; and 2. having no andic or vitric horizon starting within 30 cm from the soil surface. Common soil units: Glacic, Thionic, Cryic, Gelic, Salic, Folic, Fibric, Sapric, Ombric, Rheic, Alcalic, Toxic, Dystric, Eutric, Haplic. See Annex 1 for key to all Reference Soil Groups Diagnostic horizon, property or material; see Annex 2 for full definition. Qualifier for naming soil units; see Annex 3 for full definition. SUMMARY DESCRIPTION OF HISTOSOLS Connotation: peat and muck soils; from Gr. histos, tissue. Parent material: incompletely decomposed plant remains, with or without admixtures of sand, silt or clay. Environment: Histosols occur extensively in boreal, arctic and subarctic regions. Elsewhere, they are confined to poorly drained basins, depressions, swamps and marshlands with shallow groundwater, and highland areas with a high precipitation/evapotranspiration ratio. Profile development: Transformation of plant remains through biochemical disintegration and formation of humic substances creates a surface layer of mould. Translocated organic material may accumulate in deeper tiers but is more often leached from the soil. Use: Sustainable use of peat lands is limited to extensive forms of forestry or grazing. If carefully managed, Histosols can be very productive under capital-intensive forms of arable cropping/horticulture, at the cost of sharply increased mineralization losses. Deep peat formations and peat in northern regions are best left untouched. In places, peat bogs are mined, e.g. for production of growth substrate for horticulture, or to fuel power stations.22 Set 1 – Organic Soils Figure 1 Histosols world-wide REGIONAL DISTRIBUTION OF HISTOSOLS The total extent of Histosols in the world is estimated at some 325 - 375 million ha, of which the majority are located in the boreal, subarctic and low arctic regions of the Northern Hemisphere. Most of the remaining Histosols occur in temperate lowlands and cool mountain areas; only one-tenth of all Histosols are found in the tropics. Extensive peat areas occur in the USA and Canada, Western Europe and northern Scandinavia, and in northern regions east of the Ural mountain range. Some 20 million hectares of tropical forest peat border the Sunda shelf in Southeast Asia. Smaller areas of tropical Histosols are found in river deltas, e.g. in the Orinoco delta and the delta of the Mekong River, and in depression areas at some altitude. Figure 1 presents a sketch map of the main occurrences of Histosols world-wide. ASSOCIATIONS WITH OTHER REFERENCE SOIL GROUPS Organic soil materials in northern regions could accumulate there because decay of organic debris is retarded by frost in the cold season and by prolonged water-saturation of the thawed surface soil during summer. Permafrost-affected Histosols are associated with Cryosols and with soils that have gleyic or stagnic properties, e.g. Gleysols in Alaska and in the northern part of the former USSR. Where the (sub)arctic region grades into the cool Temperate Zone, associations with Podzols can be expected. Histosols that formed in organic soil material under the permanent influence of groundwater (‘low moor peat’) occupy the lower parts of fluvial, lacustrine and marine landscapes, mainly in temperate regions. Other soils in the same environment are Fluvisols, Gleysols and, in coastal regions, Solonchaks (e.g. adjacent to coastal mangrove peat). Histosols in lacustrine landforms are commonly associated with Vertisols.Lecture Notes on the Major Soils of the World 23 Rain-dependent Histosols are found in environments with sufficiently high and evenly spread rainfall, e.g. in raised ‘dome’ peat formations (‘high moor peat’) in lowland areas and in upland areas with blanket peat, where paucity of nutrient elements, acidity and near-permanent wetness retard decay of organic debris. Lateral linkages exist with a variety of Reference Soil Groups, including Andosols, Podzols, Fluvisols, Gleysols, Cambisols and Regosols. GENESIS OF HISTOSOLS Histosols are unlike all other soils in that they are formed in ‘organic soil material’ with physical, chemical and mechanical properties that differ strongly from those of mineral soil materials. ‘Organic soil material’ is soil material that contains more than 20 percent organic matter by weight, roughly equivalent to 30 – 35 percent by volume. Organic soil material accumulates in conditions where plant matter is produced by an adapted (‘climax’) vegetation, and where decomposition of plant debris is slowed by: • low temperatures, • persistent water saturation of the soil body, • extreme acidity or paucity of nutrient elements (‘oligotrophy’), and/or • high levels of electrolytes or organic toxins. Figure 2 indicates that a surplus of organic soil material can build up in cold and temperate regions, and under swamp conditions even in the tropics. Organic soil materials that formed in different environments are generally of different botanical composition; degrees of decomposition and contents of mineral admixtures are equally varied. Figure 2 Comparative rates of production (A) and decomposition (B1 in aerated soil; B2 under water) of organic matter as influenced by the temperature and aeration status of the soil body. (Mohr, Van Baren & Van Schuylenborg, 1972)24 Set 1 – Organic Soils Note: In view of the limited agricultural significance of the (extensive) northern Histosols, and because Histosols in temperate and tropical climates are under much stronger attack, the following discussion will focus on Histosol development and Histosol deterioration in temperate and tropical climates. The majority of all peat bogs in the Temperate Zone and in the tropics are found in lowland areas, e.g. in coastal plains and deltas and in fluvial and lacustrine inland areas. Local depressions/ pools in such wetlands are gradually filled in with reeds and sedges and with the remains of aquatic plants that accumulate in the deeper parts. The margins of a depression area are the first to become ‘dry’. This prompts the vegetation, differentiated in floral belts adapted to different degrees of wetness, to shift toward the center of the depression. Eventually, the entire depression is filled with ‘topogenous peat’ (i.e. ‘low moor peat’, formed under the influence of groundwater). The transition from the mineral substrata to the overlying peat body may be gradual but a thin transitional layer of black, smeary, completely decomposed organic sediment (‘gyttja’) is not uncommon (see Figure 3). Topogenous peat is shallow by nature. Only where its accumulation coincides with gradual tectonic lowering of the land surface can it reach a great depth. Topogenous peat deposits in the Drama Plain, Greece, for instance, are in places deeper than 300 meters. In upland areas where temperatures are ‘low’ and rainfall/fog is evenly spread over the year, rain-dependent ‘ombrogenous peat’ (or ‘high moor peat’) forms where microbial activity is depressed by severe acidity, oligotrophy and/or organic toxins. The ‘blanket peat’ in Scotland and Wales is an example of (shallow) ombrogenous peat that lies directly on top of hard bedrock. Ombrogenous peat formations in lowland areas normally overlie topogenous peat. After a depression is completely filled with topogenous peat, accumulation of organic soil material may continue. This happens where rainfall is high and evenly spread over the year, and microbial activity is suppressed by low temperature, wetness, acidity/oligotrophy and/or salt or organic toxins. The then formed peat mass rises over the mean water level and becomes increasingly ombrogenous in character as it is ever less enriched with mineral material (clay, nutrients) carried on with floods. As long as a peat body is still shallow, the vegetation can draw nutrients from the underlying mineral base. Once the peat has grown to a depth that puts the subsoil out of the reach of living roots, uptake of nutrient elements from outside stops while losses of nutrients (e.g. through leaching) continue; the vegetation must survive on a gradually decreasing quantity of cycling nutrients. The (climax) vegetation adapts to this gradual change by becoming poorer in quality and species composition. An initially heavy mixed swamp forest degrades slowly into light monotonous forest with only few tree species, and ultimately into stunted forest which produces insufficient organic material for further vertical growth of the bog. The net rate of vertical peat accumulation seems to decrease over time according to a roughly exponential pattern. Carbon dating of deep (8 to 12 m) dome peat formations under swamp forest in Sarawak and Indonesia suggest an initial accumulation rate of 0.25 to 0.45 cm/year that decreased in the course of 3 to 5 millennia to 0.05 cm/year and less (Anderson, 1964). As the peat mass rises over its surroundings, the continual precipitation surplus (a precondition for the formation of ombrogenous peat) drains away to the fringes of the bog where it creates a wet peripheral zone. Topogenous peat can grow there and become covered with ombrogenous peat later on. Peat bodies from several nuclei (depressions) in a plain will eventually merge into one coherent peat body. The limit to vertical growth, in combination with continuing lateral expansion, explains the characteristic dome shape of ombrogenous ‘raised bogs’.Lecture Notes on the Major Soils of the World 25 Figure 3 A depression is gradually filled in with topogenous peat, which is then overgrown by a laterally expanding ombrogenous peat mass. Note the changing composition of the vegetation

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