Introduction

The FAO-Unesco Soil Classification System
The World Reference Base for soil resources
Diagnostic horizons, properties and materialsg

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'.

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 Arenosolsof 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:

1. Shallow LEPTOSOLS over hard rock or highly calcareous material, and
2. 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. Redistribution 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:

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.

TABLE 1
All Reference Soil Groups of the WRB assembled in 10 sets

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 nomenclature 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.

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 having a low bulk density and high organic 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 fulvic horizon except for a `melanic index1' of 1.70 or less throughout.
mollic horizon	well-structured, dark surface horizon with high base saturation 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	well-structured, dark surface horizon with low base saturation 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 always, embedded in a vesicular crust 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	horizon evolved during weathering of mainly pyroclastic deposits; 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 relative to underlying horizons: modified colour, removal of carbonates or 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 (SiO2) known as `durinodes'.
ferralic horizon	strongly weathered horizon in which the clay fraction is dominated by low activity clays and the sand fraction by resistant materials such as iron-, aluminium-, manganese- and titanium oxides.
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	horizon with distinct calcium sulphate enrichment.
natric horizon	subsurface horizon with more clay than any overlying horizon(s) and high exchangeable sodium percentage; usually dense, with columnar or 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 (SiO2), also known as a `duripan'.
petrogypsic horizon	cemented horizon containing secondary accumulations of gypsum (CaSO4.2H2O).
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 composed of organic matter and aluminium, with or without iron.
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 or would have qualified for a ferralic horizon if it had been less coarsely textured.
geric properties	mark soil material of very low effective cation exchange capacity or even acting as anion exchanger.
gleyic properties	visible evidence of prolonged waterlogging by shallow groundwater.
permafrost	indicates that the soil temperature is perennially at or below 0 oC 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.

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, which accumulates at the surface 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	unconsolidated, non or only slightly weathered products of volcanic 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.

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.