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Set #8
Mineral Soils conditioned by a Steppic Climate

Major landforms in steppe regions

Major landforms in steppe regions

Steppes and steppic regions (pampas, prairies) receive between 250 and 500 mm of precipitation annually, i.e. more than twice the quantity that falls in true desert areas where rainfall is insufficient to support a vegetation that could protect the land from erosion.

Dunes and sand plains form where strong winds carry sand grains `in saltation' over short distances. Particles finer than sand are transported `in suspension' and over greater distances until they settle as `loess', predominantly in the steppe regions adjacent to the desert zone.

Figure 1
Climatic history recorded in a brickyard in Czechia. Events of the past 130,000 years are visible in a quarry at Nove Mesto. (Based on the work of G.J. Kukla.)

Landforms in regions with loess

Chinese records make mention of extensive loess deposition between 400 and 600 AD, between 1000 and 1200 AD and between 1500 and 1900 AD (during the `Little Ice Age'). However, the most extensive occurrences of loess on Earth (in the steppe regions of Eastern and Central Europe and the USA) are of Pleistocene age (see Figure 1).

For good understanding of landforms and soils in steppe regions it is important that one understands the relation between Ice Age aridity and loess deposition.

During the Late Pleniglacial, between 20,000 and 13,000 BP, some 25 percent of the land surface became covered with continental ice sheets (versus some 10 percent today). With so much water stored in ice sheets, the sea level dropped to about 120 metres below the present level and large parts of the world became extremely arid. The Amazon rain forest dwindled to isolated refugia, European forests disappeared but for small sheltered areas, and large parts of the globe turned to tundra, steppe, savannah or desert.

Clearly, aeolian processes were much more important at that time than at present. Large parts of the present temperate zone, from the `cover sands' of the Netherlands to the sand dunes in north-east Siberia are Ice Age (aeolian) sands. South and east of this cover sand belt lies a belt of loess deposits, extending from France, across Belgium, the southern Netherlands, Germany and large parts of Eastern Europe into the vast steppes of Russia, and further east to Siberia and China. See Figure 2. A similar east-west loess belt exists in the USA and less extensive areas occur on the Southern Hemisphere, e.g. in the Argentinean pampas.

Loess is a well sorted, usually calcareous, non-stratified, yellowish-grey, aeolian clastic sediment. It consists predominantly of silt-sized particles (2-50 mm), and contains normally less than 20 percent clay and less than 15 percent sand. It covers the land surface as a blanket, which is less than 8 metres thick in the Netherlands (exceptionally 17 metres) but can reach up to 40 metres in Eastern Europe and 330 metres in China.

Figure 2
Distribution of loess in Europe. Source: Flint, 1971

Loess is very porous material and vertical walls remain remarkably stable, but loess slakes easily so that exposed surface areas are prone to water erosion. The loess material itself is produced by abrasion of rock surfaces by glaciers and blown out from glacial outwash plains and alluvium. It is generally difficult to identify the exact source areas of specific loess deposits because the various loess deposits have a surprisingly similar mineralogy. `Typical' loess contains quartz, feldspar, some micas, calcium carbonate and clay minerals. A possible explanation may be that glaciers abrade large surfaces of diverse mineralogy, so that the mineralogical variation between different source areas is averaged out. Further mixing and homogenisation of dust particles from various sources occurs during transport. Loess is absent from regions that were covered by glaciers in the last glacial period, nor does it occur in the humid tropics. The vast areas of loess in China may not have a glacial origin: the loess grades into the sandy loess and sands of the Gobi desert in Mongolia. Deposition is still going on today at a rate of several millimetres per year. Long-distance transport of dust particles from the Gobi desert seems to be responsible for the thick Chinese loess deposits.

The aeolian origin of loess is evidenced by the following facts:

  1. loess occurs as a blanket over a wide range of surfaces, to a large extent independent of topography;
  2. loess blankets are thickest on the leeward sides of obstacles;
  3. there is absolutely no (cor)relation between the mineralogy of the loess blanket and that of subsurface strata (which rules out the possibility of in-situ weathering);
  4. the grain-size distribution of loess is typical of material transported in suspension;
  5. grain sizes show a downwind fining gradient, away from the source;
  6. loess deposits become thinner away from their presumed source;
  7. fossil terrestrial (sic!) snails have been found in loess deposits;
  8. intercalated `paleosols' (fossil soils) are common in most loess belts;
  9. loess deposition still happens around desert areas today.

Loess settles when dust-laden winds slow down to speeds between 7 (on dry surfaces) to 14 meters per second (on moist surfaces). The pore distribution of loess lets it quickly be retained by capillary forces if it lands on a moist surface. The presence of a vegetation cover may also enhance the rate of loess deposition, and many authors maintain that the northern limit of loess deposition coincides with the northernmost extent of grass steppes during arid periods in the Pleistocene.

It has already been said that small-scale stratification is usually absent from loess because of the extreme uniformity in grain size. Laminated `loessoid' deposits are the products of post-depositional sheet wash. Larger-scale layering, (deci)metres thick, suggests a certain periodicity in loess deposition. At least two separate loess sequences were identified in the Netherlands, a Weichselian one, and an older, Saalian sequence. The boundary between the two sequences is marked by the occurrence of a relic soil profile that developed in the Saalian loess during the Eemian interglacial period.

Landforms in regions without loess

Vast undulating till plains occur in North America, between the Canadian shield area and the loess belt. This area is either covered with thick tills or with `deglaciation' sediments, lacustrine sediments in particular. The lake areas are level as such but the till landscape has a typical `hummocky' relief. The main characteristic of hummocky tills (40% of the total area) is the predominance of very local drainage patterns (mainly in depressions). Tills and loess have in common that they are internally uniform and that they all date back to deglaciation periods.

The vast loess and till plains are now colonised by grasses and/or forest. They are the home of some of the best soils of the world: the `black earths'. Deep, black Chernozems occupy the central parts of the Eurasian and North American steppe zones. Brown Kastanozems are typical of the drier parts of the steppe zone and border on arid and semi-arid lands. Dusky red Phaeozems occur in slightly more humid areas such as the American prairies and pampas.

Chernozems (CH)

The Reference Soil Group of the Chernozems accommodates soils with a thick black surface layer rich in organic matter. Russian soil scientist Dokuchaev coined the name "Chernozems" in 1883 to denote the typical "zonal" soil of the tall grass steppes in continental Russia. Some international synonyms: `Calcareous Black Soils', `Eluviated Black Soils' (Canada), and (several suborders of) `Mollisols' (USDA Soil Taxonomy).

Definition of Chernozems#

Soils having,

  1. a mollic@ horizon with a moist chroma of 2 or less if the texture is finer than sandy loam, or less than 3.5 if the texture is sandy loam or coarser, both to a depth of at least 20 cm, or having these chromas directly below any plough layer; and
  2. concentrations of secondary carbonates@ starting within 200 cm from the soil surface; and
  3. no petrocalcic@ horizon between 25 and 100 cm from the soil surface; and
  4. no secondary gypsum; and
  5. no uncoated silt and sand grains on structural ped faces.

Common soil units:

Chernic*, Vertic*, Gleyic*, Calcic*, Luvic*, Glossic*, Siltic*, Vermic*, 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 Chernozems

Connotation: black soils rich in organic matter; from R. chern, black, and zemlja, earth or land.

Parent material: mostly aeolian and re-washed aeolian sediments (loess).

Environment: regions with a continental climate with cold winters and hot summers; in flat to undulating plains with tall-grass vegetation (forest in the northern transitional zone).

Profile development: AhBC profiles with a dark brown to black mollic surface horizon over a cambic or argic subsurface horizon, commonly with redistribution of calcium carbonate to a calcic horizon or pockets of secondary carbonates in the subsurface soil.

Use: the high natural fertility of Chernozems and their favourable topography permit a wide range of agricultural uses including arable cropping (with supplemental irrigation in dry summers) and cattle ranging.

Regional distribution of Chernozems

Chernozems cover an estimated 230 million hectares world-wide, mainly in the middle latitude steppes of Eurasia and North America, north of a zone with Kastanozems. Figure 1 presents an overview of their main areas of occurrence.

Figure 1
Chernozems world-wide

Associations with other Reference Soil Groups

Chernozems in Russia (north of the Ural range) and in North America are associated with Luvisols, Albeluvisols and Greyic Phaeozems towards the cool northern border of the steppe zone and grade into Kastanozems towards the warm and dry south. Where the Chernozem belt borders on warm, humid regions, Chernozems may grade into Phaeozems.

Genesis of Chernozems

The `typical' Chernozem has formed in uniformly textured, silty parent material (loess), under tall-grass vegetation with vigorous growth. The above ground biomass amounts to some 1 to 1.5 tons of dry matter per hectare; the corresponding root mass, already incorporated in the soil, weighs 4 to 6 tons/hectare. The main concentration of roots is in the upper 60 cm of soil, with 80 percent of all roots concentrated in the top 30 to 40 cm.

Deep, humus-rich Chernozems occur in the central part of the steppe zone where the annual precipitation sum is approximately equal to the evaporation sum. Such soils contain 10 to 16 percent organic matter in their surface layers, are neutral in reaction (pH 7.0, and around 7.5 in the subsoil), and highly saturated with bases. Soil fauna is very active in Chernozems, in wet periods predominantly in the upper 50-cm layer but the animals move to deeper strata at the onset of the dry period. Vermic Chernozems consist for the greater part of worm casts, a stable mixture of mineral and organic soil material. Burrowing small vertebrates contribute significantly to intense homogenization (`bioturbation') of the soil. Animal burrows that became filled-in with humus-rich surface soil stand out as black `krotovinas' (from R. krot, a Eurasian mole, Talpa europaea) against the typically cinnamon coloured deeper soil matrix.

The high porosity and favourable structure of the deep, homogenized Ah-horizon explain why deep percolation of rainwater during wet spells is sufficient to flush virtually all readily soluble salts from the soil. There may be some accumulation of gypsum at a depth of 2 to 3 meters from the soil surface (1.5 to 2.5 m in southern Chernozems), and accumulation of lime at a shallower depth, say at about 1 metre from the soil surface. A `dead dry horizon' may be present at a depth of about 4 metres, deeper in the north of the Chernozem belt than in the south. This soil layer receives neither percolation water from above nor capillary rise from below. The dead dry horizon needs not to be continuous; its thickness varies and it can even be absent altogether.

Migration of clay has resulted in slightly increased clay contents between 50 to 200 cm from the surface in many Chernozems in the central steppe zone. This indicates that central Chernozems are exposed to moderately strong leaching during wet periods.

Northern Chernozems are subject to stronger leaching than those of the central steppe zone.

Towards the northern fringe of the Chernozem belt, the surface horizon becomes shallower, more acid (pH 6-6.5) and more greyish until signs of podzolization such as an ash-grey eluvial subsurface horizon and/or horizontal lamellae in the subsoil become evident. In northern Chernozems, the horizon with carbonate accumulation is normally separated from the humus-rich surface layer by a carbonate-free layer of appreciable thickness.

Towards the southern fringe of the steppe zone, the water regime becomes more and more intermittent, with increasingly longer dry periods. Consequently, plants with a long vegetative period disappear and xerophytes and ephemeral grasses move in. Also, the soil's humus undergoes more intense mineralization and there is an increase in the content of readily soluble salts in the surface soil.

Note that the colour of the surface soil has diagnostic value: where the chroma of the upper 20 cm of soil has become more than 2, this is seen as a sign that aridity is so severe that the soils are no longer true Chernozems. They are then classified as Kastanozems.

Characteristics of Chernozems

Morphological characteristics

Virgin Chernozems have a thin leafy litter layer on top of a dark grey to black, crumb, `vermic' Ah-horizon. The surface horizon can be only 20 cm thick but extends down to a depth of more than 2 metres in well-developed Chernozems. Worm casts and krotovinas testify of intense faunal activity.

Calcium carbonate accumulation in the lower part of the surface soil is common, secondary carbonates occur as pseudo-mycelium and/or nodules in a brownish grey to cinnamon subsoil. The subsurface horizon has blocky or weakly prismatic structure.

The grass vegetation grades into deciduous forest towards the north of the Chernozem belt where the Ah-horizon may overly an argic B-horizon (Luvic Chernozems) or even tongue into the B-horizon (Glossic Chernozems). There, Chernozems grade into Luvisols or Albeluvisols. Many Chernozems in wet areas develop signs of hydromorphy (Gleyic Chernozems, known as `Meadow Chernozems' in Russia and most of Eastern Europe).

Mineralogical characteristics

The mineral composition of Chernozems is rather uniform throughout the profile, in line with the high rate of homogenization of the soil material. The SiO2/R2O-ratio is high, at about 2.0.

Hydrological characteristics

Although it is widely accepted that Chernozems formed under conditions of good drainage, there are also (Russian) soil scientists who maintain that certain Chernozems passed through a boggy phase of soil formation. Today's Chernozems are well drained, apart from soils in depressions with occasional shallow groundwater. By and large, there is approximate equity between the annual precipitation sum and evaporation, with a slight precipitation surplus in the north of the steppe zone and a slight deficit in the south. Table 1 presents an overview of the occurrence of Eurasian steppe soils in relation with the annual precipitation sum and the type of vegetation.

Typical Reference Soil Groups in the Eurasian steppe zone




Reference Soil Group/Unit



>550 mm
500 mm
500 mm
450 mm
200-400 mm

< 200 mm

deciduous forest
steppe and forest
tall grass steppe
tall grass steppe
medium height
grass steppe
open vegetation

Luvisols, Albeluvisols, Phaeozems
Luvic Chernozem
Haplic Chernozem
Calcic Chernozem

Physical characteristics

Chernozems possess favourable physical properties. The total pore volume of the Ah-horizon amounts to 55 to 60 volume percent and that of the subsoil lies between 45 and 55 percent. Chernozems have good moisture holding properties; reported soil moisture contents of some 33 percent at `field capacity' and 13 percent at `permanent wilting point' suggest an `available water capacity' (AWC) of some 20 volume percent. The stable micro-aggregate structure (`crumb') of the humus-rich Ah-horizon represents a favourable combination of capillary and non-capillary porosity and makes these soils highly suitable for irrigated farming.

Chemical characteristics

Chernozem surface soils contain between 5 and 15 percent of `mild' humus with a high proportion of humic acids and a C/N-ratio that is typically around 10. The surface horizon is neutral in reaction (pH 6.5-7.5) but the pH may reach a value of 7.5-8.5 in the subsoil, particularly where there is accumulation of lime. Chernozems have good natural fertility; the surface soil contains 0.2-0.5 percent nitrogen and 0.1 to >0.2 percent phosphorus. This phosphorus is only partly `available'; crops on Chernozems tend to respond favourably to application of P-fertilizers. In southern Chernozems, humus contents are lower (4-5 percent) and consequently also the cation exchange capacity: 20-35 cmol(+)/kg dry soil, versus 40-55 cmol(+) per kg in central Chernozems. Normally, the base saturation percentage is close to 95 percent with Ca2+ and Mg2+ as the main adsorbed cations but sodium adsorption may be high in southern Chernozems.

Management and use of Chernozems

Russian soil scientists rank the deep, central Chernozems among the best soils in the world. With less than half of all Chernozems in Eurasia being used for arable cropping, these soils constitute a formidable resource for the future.

Preservation of the favourable soil structure through timely cultivation and careful irrigation at low watering rates prevents ablation and erosion. Application of P-fertilizers is required for high yields. Wheat, barley and maize are the principal crops grown, alongside other food crops and vegetables. Part of the Chernozem area is used for livestock rearing. In the northern temperate climatic belt, the possible growing period is short and the principal crops grown are wheat and barley, in places in rotation with vegetables. Maize is widely grown in the warm temperate belt. Maize production tends to stagnate in drier years unless the crop is adequately irrigated.

Kastanozems (KS)

The Reference Soil Group of the Kastanozems holds the `zonal' soils of the short grass steppe belt, south of the Eurasian tall grass steppe belt with Chernozems. Kastanozems have a brownish humus-rich surface horizon (less deep and less black than that of the Chernozems) and they show prominent accumulation of secondary carbonates in the sub(surface) soil. The chestnut-brown colour of the surface soil gave these soils their name `Kastanozem'; common international synonyms are `(Dark) Chestnut Soils' (Russia), (Dark) Brown Soils (Canada), and Ustolls and Borolls in the Order of the Mollisols (USDA Soil Taxonomy).

Definition of Kastanozems#

Soils having,

  1. a mollic@ horizon with a moist chroma of more than 2 to a depth of at least 20 cm, or having this chroma directly below the depth of any plough layer, and
  2. concentrations of secondary carbonates@ within 100 cm from the soil surface, and
  3. no diagnostic horizons other than an argic@, calcic@, cambic@, gypsic@, petrocalcic@, petrogypsic@ or vertic@ horizon.

Common soil units:

Anthric*, Vertic*, Petrogypsic*, Gypsic*, Petrocalcic*, Calcic*, Luvic*, Hyposodic*, Siltic*, Chromic*, 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 Kastanozems

Connotation: (dark) brown soils rich in organic matter; from L. castanea, chestnut, and from R. zemlja, earth, land.

Parent material: a wide range of unconsolidated materials. A large part of all Kastanozems have developed in loess.

Environment: dry and warm; flat to undulating grasslands with ephemeral short grasses.

Profile development: mostly AhBC profiles with a brown Ah-horizon of medium depth over a brown to cinnamon cambic or argic B-horizon and with lime and/or gypsum accumulation in or below the B-horizon.

Use: the principal arable land use is the production of small grains and (irrigated) food and vegetable crops. Many Kastanozem areas are used for extensive grazing. Drought and (wind and water) erosion are serious limitations.

Regional distribution of Kastanozems

The total extent of Kastanozems is estimated at about 465 million hectares. Major areas are in the Eurasian short-grass-steppe belt (southern Ukraine, southern Russia, and Mongolia), in the Great Plains of the USA, and in Mexico, southwestern Brazil, and the pampas of Northern Argentina, Uruguay and Paraguay. Figure 1 shows the world-wide occurrence of Kastanozems.

Figure 1
Kastanozems world-wide

Associations with other Reference Soil Groups

Kastanozems on the Northern Hemisphere border on the Chernozem belt in the cooler and less arid north, and on areas with Calcisols and Gypsisols in the warmer and drier south (where they may also occur adjacent to Solonchaks and Solonetz). In the warmer and less arid subtropics, Kastanozems are associated with Phaeozems.

Genesis of Kastanozems

The climax vegetation of the Kastanozem belt is a short grass vegetation, scanty, poor in species and dominated by ephemera (early ripening species). The aboveground dry biomass amounts to only 0.8-1 tons/hectare, whereas the dry root mass reaches 3-4 tons/hectare. More than 50 percent of all roots are concentrated in the upper 25 cm of the soil and there are few roots that extend down to deeper than 1 metre. The greater part of the grass vegetation dies each summer. This specific vegetation type conditioned Kastanozem formation. Kastanozem surface soils are less deep than those of Chernozems (under tall grasses) and are brown rather than black. The organic matter content of the Ah-horizon of Kastanozems is typically 2 to 4 percent and seldom exceeds 5 percent.

Downward percolation of water in spring leaches solutes from the surface to subsurface and subsoil layers. Lime accumulates at a depth of approximately 1 metre; gypsum accumulation is common in drier regions, mostly at a depth between 150 and 200 cm, and in the driest Kastanozems there may be a layer of salt accumulation deeper than 200 cm below the surface.

A clay illuviation horizon may be present as deep as 250-300 cm below the soil surface. The occurrence of argic B-horizons in Kastanozems is still ill understood. They may be fossil, as claimed by some Russian soil scientists, but there are also theories of a more recent formation, through `normal' translocation of clay, or by destruction of clay or fine earth near the surface and reformation at greater depth.

Climatic gradients in the Kastanozem belt are visible from pedogenic features. In Russia, the darkest surface horizons occur in the north of the Kastanozem belt (bordering on the Chernozems) whereas soils with shallower and lighter coloured horizons are more abundant in the south. The differentiation between horizons is clearer in the north than in the south in line with decreasing length and intensity of soil formation as conditions become more arid.

Characteristics of Kastanozems

Morphological characteristics

The morphology of dark Kastanozems is not very different from that of the southern (drier) Chernozems whereas the light Kastanozems of the south grade into Calcisols. The northern Eurasian Kastanozems have Ah-horizons of some 50 cm thick, dark brown and with a granular or fine blocky structure, grading into cinnamon or pale yellow massive to coarse prismatic B-horizons. In the drier south, the Ah-horizon is only 25 cm thick and colours are lighter throughout the profile.

Argic B-horizons are reported to have "more intense coloration" in Luvic Kastanozems. Accumulations of lime and/or gypsum separate Kastanozems (and Chernozems) from Phaeozems and are particularly prominent in Kastanozems of the southern dry steppes. Krotovinas occur in almost all Kastanozems but are less abundant than in Chernozems.

Hydrological characteristics

Kastanozems have an intermittent water regime. The soils dry out to great depth in the dry season and are often incompletely moistened in wet periods. The low total precipitation sum and low non-capillary porosity of Kastanozems explain why run-off (losses) during and after heavy showers can be considerable. A `dead dry horizon' occurs below the limit of wetting; this horizon receives neither percolation water from above nor capillary rise from below and is `physiologically dead'.

Physical characteristics

The physical properties of Kastanozems are slightly less favourable than of Chernozems but otherwise comparable. The lower humus content of the surface layer, particularly in the lighter Kastanozems, is associated with weaker micro-aggregation, which manifests itself in less total pore volume (40-55 percent), less moisture storage capacity, denser packing of the soil and lower permeability to water.

Chemical characteristics

Kastanozems are chemically rich soils with a cation exchange capacity of 25-30 cmol(+)/kg dry soil, and typically 95 percent base saturation percentage or more. The majority of all adsorbed cations are Ca2+ and Mg2+-ions; ESP-values of 4 to 20 have been reported.

The C/N-ratio of the organic soil fraction of the surface horizon is around 10, as in Chernozems. The soil-pH is slightly above 7.0 but may increase to a value around 8.5 at some depth. Accumulations of lime and gypsum are common; the accumulation horizon contains 10 to 20 percent more secondary carbonates than the deeper solum. More easily soluble salts may have accumulated deeper down, deeper in dark Kastanozems than in the lighter soils of the drier steppe. The salt content of the accumulation layer is commonly between 0.05 and 0.1 percent and does not seriously inhibit the growth of crops. In places, salt levels may reach 0.4 percent and more.

Management and use of Kastanozems

Kastanozems are potentially rich soils; periodic lack of soil moisture is the main obstacle to high yields. Irrigation is nearly always necessary for high yields; care must be taken to avoid secondary salinization of the surface soil. Small grains and (irrigated) food and vegetable crops are the principal crops grown. Wind erosion is a problem on Kastanozems, especially on fallow lands.

Extensive grazing is another important land use but the sparsely vegetated grazing lands are inferior to the tall grass steppes on Chernozems and overgrazing is a serious problem.

Phaeozems (PH)

The Reference Soil Group of the Phaeozems accommodates soils of wet steppe (prairie) regions. Phaeozems are much like Chernozems and Kastanozems but are more intensively leached in wet seasons. Consequently, they have dark, humous surface soils that are less rich in bases than surface soils of Chernozems and Kastanozems and Phaeozems have no (signs of) secondary carbonates in the upper metre of soil. Commonly used international names are `Brunizems' (Argentina, France), `Degraded Chernozems' (former USSR), `Parabraunerde-Tsjernozems' (Germany), `Dusky red prairie soils' (USA) or `Udolls' and `Aquolls' in the order of the Mollisols (USDA Soil Taxonomy).

Definition of Phaeozems#

Soils having

  1. a mollic@ horizon, and
  2. a base saturation (in 1 M NH4OAc at pH 7.0) of 50 percent or more and having no secondary carbonates to at least a depth of 100 cm from the soil surface, or to a contrasting layer (lithic@ or paralithic@ contact, petrocalcic@ horizon) between 25 and 100 cm, and
  3. no diagnostic horizons other than an albic@, argic@, cambic@ or vertic@ horizon.

Common soil units:

Chernic*, Leptic*, Vertic*, Gleyic*, Vitric*, Andic*, Luvic*, Tephric*, Stagnic*, Abruptic*, Greyic*, Pachic*, Glossic*, Calcaric*, Albic*, Skeletic*, Sodic*, Siltic*, Vermic*, Dystric*, Chromic*, 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 Phaeozems

Connotation: dark soils rich in organic matter; from Gr. phaios, dusky, and R. zemlja, earth, land.

Parent material: aeolian (loess), glacial till and other unconsolidated, predominantly basic materials.

Environment: flat to undulating land in warm to cool (e.g. tropical highland) regions, humid enough that there is some percolation of the soil in most years but also with periods in which the soil dries out. The natural vegetation is tall grass steppe and/or forest.

Profile development: mostly AhBC profiles with a mollic surface horizon (thinner and somewhat less dark than in Chernozems) over a cambic or argic subsurface horizon.

Use: Untouched Phaeozems (of which there are few left) carry a grass or forest vegetation. Phaeozems are fertile soils; they are planted to irrigated cereals and pulses or are used for cattle rearing and fattening on improved pastures. Periodic drought and wind and water erosion are the main limitations.

Regional distribution of Phaeozems

Phaeozems cover an estimated 190 million hectares world-wide. Some 70 million hectares of Phaeozems are found in the USA, in the (sub-)humid Central Lowlands and easternmost parts of the Great Plains. Another 50 million hectares of Phaeozems are in the subtropical pampas of Argentina and Uruguay and the third largest distribution area of Phaeozems (18 million hectares) is in northeastern China. Smaller, mostly discontinuous areas are found in Central Europe, notably in the Danube region of Hungary and adjacent parts of Yugoslavia and in elevated areas in the tropics. Figure 1 presents the main Phaeozem areas.

Figure 1
Phaeozems worldwide

Associations with other Reference Soil Groups

Phaeozems occur in steppe, forest-steppe or forest-prairie areas that border on the humid side of the Chernozem belt in the temperate climatic zone and on the humid border of the Kastanozem belt in the subtropics. Phaeozems north of the Eurasian and North American Chernozems may occur together with Albeluvisols; they may even develop uncoated silt and sand grains on structural ped surfaces. South American Phaeozems are associated with Planosols, Solonchaks and Kastanozems.

Genesis of Phaeozems

By and large, Phaeozems occur on fine-textured, basic parent material in more humid environments than Chernozems or Kastanozems. The rates of weathering and leaching of bases are higher in Phaeozems than in Chernozems and Kastanozems. Calcium carbonate is absent from the upper metre of the soil profile but leaching is not so intense that the soils have become depleted of bases and/or plant nutrients. Biomass and faunal activity are high; earthworms and burrowing mammals homogenize the soil. In places, faunal activity is so intense that the mollic A-horizon is thickened and wormholes and krotovinas extend into the C-horizon.

Phaeozem formation appears to be conditioned by an annual precipitation surplus (which infiltrates into the soil). The North American Phaeozem belt extends from Canada, with an annual precipitation sum of only 400 mm and an average temperature of 2 oC, to Missouri in the south, with 1200 mm rainfall/year and an average temperature of 18 oC. The precipitation surplus over (temperature-dependent) evapotranspiration is about the same from north to south, despite the considerable increase in precipitation sum.

Argic B-horizons do occur in Phaeozems but they are widely regarded as relics from an earlier development towards Luvisols (in eras with a more humid climate).

Characteristics of Phaeozems

Morphological characteristics

Phaeozems have a brown to grey, mollic surface horizon of 30-50 cm over a brown cambic horizon or a yellowish brown C-horizon, or over a brown or reddish brown argic horizon. A-horizons of Phaeozems are thinner than of Chernozems and somewhat less dark. Where the water table is at shallow depth or a perched water table occurs (e.g. on top of an argic horizon), the surface soil may be mottled and/or dark. Luvic soil units, polygenetic or not, represent a more advanced stage of soil formation and tend to have more reddish colours than other Phaeozems.

Hydrological characteristics

Phaeozems with clay accumulation have even better water storage properties than other Phaeozems but may still be short of water in the dry season.

Physical characteristics

Phaeozems are porous, well-aerated soils with moderate to strong, very stable, crumb to blocky structures. Where clay illuviation occurs, the illuviation layer contains commonly 10-20 percent more clay than the overlying horizon.

Chemical characteristics

The organic matter content of the surface layer of Phaeozems is typically around 5 percent; the C/N-ratio of the organic matter is 10-12; pH-values are between 5 and 7 and increase towards the C-horizon. The Cation Exchange Capacity of Phaeozems is 25-30 cmol(+) per kg dry soil or somewhat less; the base saturation percentage lies between 65 and 100 percent, with the higher values in the deeper subsoil.

Management and use of Phaeozems

Phaeozems are porous, fertile soils and make excellent farmland. In the USA and Argentina, Phaeozems are in use for the production of soybean and wheat (and other small grains). Phaeozems on the High Plains of Texas produce good yields of irrigated cotton. Phaeozems in the temperate climatic belt are planted to wheat, barley and vegetables alongside other crops. Wind and water erosion are serious hazards. Vast areas of Phaeozems are used for cattle rearing and fattening on improved pastures.

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