Set #9
Mineral Soils conditioned by a (Sub)Humid Temperate Climate
Major landforms in (sub-)humid temperate
regions
Podzols
Plansols
Albeluvisols
Luvisols
Umbrisols
Major landforms in (Sub-)Humid Temperate Regions
Most of the Earth's temperate regions were covered with continental ice sheets when the Ice Ages had their maximum expanse. Massive glacial and fluvio-glacial deposits were laid down in these regions when the ice melted. See Figure 1.
`Periglacial areas', once adjacent to ice-capped regions, still show evidence of their low temperatures in the past: (older) sediments show characteristic deformation structures and compaction of soil material, incurred in repeated freezing and thawing. Periglacial areas had little vegetation, if any. Strong winds blew sand and silt out of the frozen plains; this material settled again as `cover sands' and sand dunes, and, at greater distance from the source, as `loess blankets'.
Virtually all landforms in temperate regions have in addition to features incurred during past glacial or periglacial periods also some `typical' characteristics that are associated with the present cool (sub)humid climate:
Three broad morphotectonic categories can be distinguished in the temperate zones:
For a discussion of landforms in the third category, the reader is referred to the chapter on the morphology of low-range mountains in eroding uplands.
Figure 1
Sketch map of Europe and northern Asia, showing the (supposed) maximum extent of glaciated areas in the Pleistocene Ice Ages. Source: Flint, 1971
Landforms in fluvial and marine environments were discussed in the chapter on `Landforms in lowlands'; the following paragraphs discuss areas that are underlain by glacial, fluvio-glacial, periglacial or aeolian deposits. Such areas are particularly extensive in the temperate regions of the Northern Hemisphere where continental ice sheets had their greatest expanse during Pleistocene glacial periods. It is generally believed that the Southern Hemisphere had insufficient land at high latitude for extensive ice sheets to develop.
Studies of Pleistocene climate changes suggest that there were at least four advances of continental ice in Eurasia and on the North American continent. Three major advances could be identified in Northwest Europe; they are known as the `Elster', `Saale' and `Weichsel' glacial periods; still older ones have been recognised elsewhere, e.g. in northern parts of the Eurasian continent
Glacial advance and retreat produced some typical landforms. The most common ones are:
The extensive and thick continental ice sheets cooled the air and created a permanent high-pressure area above them. Strong winds blew away from the ice sheets and influenced the climate of regions near the margins of the ice where cold desert conditions prevailed during glacial periods. Arctic tundra vegetation with herbs and (dwarf) shrubs colonised large parts of North America and Northwest Europe. The soil was frozen all-year round (`permafrost') and only during the short summer season would a shallow `active soil layer' thaw. Even on low-angle slopes, this active layer could slide downhill, producing a `cryopediment'.
Repeated thawing and freezing of soil material produced typical `cryoturbation' structures and differential heaving of stones, which ultimately resulted in characteristic landscape features such as stone wedges, stony polygons and `palsas' (small frozen mounds with peat and/or mineral matter pushed into them). `Pingos' i.e. ice lenses that grew year after year into a fractured dome of ice with or without a thin soil cover, can still be traced today because when the ice melted the dome collapsed into a circular depression area. Such `pingo-ruins' are a common feature in former periglacial areas, e.g. in the northern Netherlands. Vast tracts of land in periglacial regions became covered with aeolian deposits. Desert pavements, sand plains and dunes were formed at short distances from the ice front, with loess covers farther away. Note that dune formation was discussed in the paragraph on residual and aeolian sands; loess deposits were treated in the chapter on landforms in steppes and prairie regions.
Most uplifted sedimentary basins in Western Europe and the western USA were formed after the Hercynian orogeny. Their sediments, mostly shallow-water limestone, marls and calcareous sandstone stem from Mesozoic transgressions and regressions. They were never folded but differential subsidence and later uplift have in places resulted in tilting. Elevated flat-topped `cuesta' formations are common landscape elements. A cuesta is a tilted, low-angle dipping sequence of resistant sedimentary rocks, which stand out in the landscape as a non-eroded ridge. The ridge itself is the steep escarpment; the gentle slope towards it is known as a `dipslope'. Remnants of Tertiary soils on cuesta dipslopes, e.g. the `clay-with-flint' on Cretaceous chalk (`limons à silex' in France; `kleefaarde' or `vuursteeneluvium' in the Netherlands) suggest that these basins formed part of extensive peneplains during the Tertiary. Uplift as a result of orogeny in nearby mountain regions such as the Alps or the Rocky Mountains led to formation of river terraces and incised meanders; the `Ile de Paris' and the Green River tributary (Colorado, USA) are just two examples.
Many of these basins became partially covered with glacial deposits or were influenced by periglacial processes. Moraines and fluvioglacial deposits are widespread on top of the East-European Platform; large parts of the Paris basin are covered with loess. Only where such covers are absent, e.g. on cuesta slopes that were too high or too steep to collect a thick loess blanket, did soils form in the original parent material.
Podzols are common soils in fluvioglacial and aeolian sands; parabolic dunes of only 2-3 metres height contain fine podzolic catenas that reflect differences in groundwater depth. Luvisols are among the commonest soils in loess blankets in temperate regions; they grade into Chernozems towards the drier end of their zone. Luvisols occur also in (less rigidly sorted) fluvial deposits. Albeluvisols with bleached tongues extending into a clay-enriched subsurface soil are common in clayey glacial till and fine-textured materials of fluvioglacial or glaciolacustrine origin but also in loess. Planosols occur predominantly in sub-humid and semi-arid regions in the Southern Hemisphere. In some instances they formed through degradation of Albeluvisols.
Podzols are soils with an ash-grey subsurface horizon, bleached by organic acids, on top of a dark accumulation horizon with brown or black illuviated humus and/or reddish iron compounds. Podzols occur in humid areas, in particular in the Boreal and Temperate Zones but locally also in the tropics. The name `Podzol' is used in most national and international soil classification systems; the USDA Soil Taxonomy refers to these soils as `Spodosols'.
Definition of Podzols#
Soils, having a spodic@ horizon starting within 200 cm from the soil surface, underlying an albic@, histic@, umbric@ or ochric@ horizon, or an anthropedogenic@ horizon less than 50 cm thick.
Common soil units:
Densic*, Carbic*, Rustic*, Histic*, Gelic*, Anthric*, Gleyic*, Umbric*, Placic*, Skeletic*, Stagnic*, Lamellic*, Fragic*, Entic*, 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.
Connotation: soils with a `spodic' illuviation horizon under a subsurface horizon that has the appearance of ash; from R. pod, under, and zola, ash.
Parent material: unconsolidated weathering materials of siliceous rock, prominent on glacial till, and alluvial and aeolian deposits of quartzitic sands. Podzols in boreal regions occur on almost any rock.
Environment: mainly in temperate and boreal regions of the northern hemisphere, in level to hilly land under heather and/or coniferous forest; in the humid tropics under light forest.
Profile development: mostly O(Ah)EBhsC profiles. Complexes of Al, Fe and organic compounds migrate from the surface soil to the B-horizon with percolating rainwater. The humus complexes precipitate in an illuvial `spodic' horizon; the overlying soil remains behind as a strongly leached Ah-horizon and a bleached `albic' eluvial horizon. Most boreal Podzols lack an Ah-horizon.
Use: severe acidity, high Al-levels, low chemical fertility and unfavourable physical properties make most Podzols unattractive for arable cropping, unless improved, e.g. by deep-plowing and fertilization. Podzols have some potential for forestry and extensive grazing.
Figure 1
Podzols world-wide
Podzols cover an estimated 485 million hectares world-wide, mainly in temperate and boreal regions on the Northern Hemisphere (see Figure 1). They are extensive in Scandinavia, northwest Russia and Canada. Besides `zonal' Podzols, there are smaller occurrences of `intrazonal' Podzols, both in the Temperate Zone and in the tropics.
Tropical Podzols occupy less than 10 million hectares, mainly in residual sandstone weathering in perhumid regions and in alluvial quartz sands, e.g. in uplifted coastal areas. The exact distribution of tropical Podzols is not known; important occurrences are found along the Rio Negro and in the Guianas in South America, in the Malesian region (Kalimantan, Sumatra, Irian), and in northern and southern Australia. They seem to be less common in Africa.
Podzols occur together with soils that have evidence of displacement of organic-Fe/Al complexes but not strong enough to qualify as Podzols. Arenosols, Albeluvisols, Cambisols, Cryosols, Leptosols, Histosols and Gleysols are commonly associated with Podzols but also Andosols, Anthrosols, Ferralsols and Planosols.
Podzol-Histosol-Gleysol combinations are common in plains with quartzitic sand and a shallow water table in the Temperate Zone. Cryosol-Podzol linkages are found at high latitudes and in places also at high altitude. Tropical Podzols are associated with poor quartzitic Arenosols, with Gleysols and with Ferralsols.
`Podzolization' (the formation of a spodic subsurface horizon) is actually a combination of processes, including
Soluble organic substances produced by microbial attack on plant litter move downward with the soil solution and form complexes with Al3+- and Fe3+-ions. The rate of such processes depends strongly on the soil. In poor quartz sands, Podzol morphology is visible after a hundred years of soil formation. Rates are much slower in richer parent materials but the humus fraction of most Podzols appears to have reached equilibrium in 1000-3000 years.
Carboxylic and phenolic groups of dissolved soil organic matter act as `claws' (Gr. chela, hence the term `chelation') and preferentially `grab' polyvalent metal ions such as Al3+ and Fe3+. This process continues until the binding capacity of the organic matter is saturated. Saturation appears to promote precipitation of the complex. It is likely that transfer of bound metals occurs between highly aggressive but easily decomposable Low Molecular Weight (LMW) acids and less acid but more stable `humic compounds'.
It appears that uncharged organic matter is also transported by water. This is explained by `hydrophobic arrangement', or `mycelle behaviour': molecules arrange themselves in such a way that their hydrophobic parts are in contact with the interior of `superstructures', while their charged parts are in contact with water. This causes an apparent solubility of largely hydrophobic units.
In well-drained soils, transport of solutes is restricted to the penetration depth of rainfall. Organic matter, with its bound metal ions, precipitates either through saturation (loss of surface charge), or where the waterfront stops. In most cases accumulation of saturated complexes occurs within one metre from the soil surface. Accumulations of different organic matter in irregular bands that reflect the depth of water penetration and the porosity of the matrix material may occur deeper than this illuviation horizon. However, only in extremely poor parent materials, or after extremely long periods of soil formation, will accumulation horizons reach greater depths.
In hydromorphic Podzols, dissolved organic matter, with its bound Al, can be transported laterally and over considerable distances. Hydromorphic Podzols in areas with lateral water flow are associated with `black water' rivers and lakes in boreal, temperate and tropical areas. The limited depth of the phreatic zone usually restricts vertical transport in the soil. Hydromorphic Podzols tend to have slightly deeper eluviation horizons than well-drained relatives in the same climatic zone; their illuvial horizons extend down to greater depths (1-3 metres) and are more vaguely defined. Many hydromorphic Podzols in stratified materials have well-defined humus bands in the subsoil.
The accumulation process is to some extent reversible. If unsaturated organic substances reach the top of an illuviation horizon, the Al,Fe-humus complexes will re-dissolve. Ultimately, an entire spodic horizon moves slowly to a greater depth. Strongly podzolized soils with a very thick albic eluviation horizon occur on poor quartz sands, notably in the humid tropics. The illuvial horizon of such soils occurs at a depth of several metres (`Giant Podzols') and may even be absent altogether if the mobile humus is removed by lateral groundwater flow.
In terms of soil formation, opposite processes take place in Podzols, where Fe- and Al-oxides dissolve and iron and aluminium are leached out, and in Ferralsols where Fe- and Al-oxides remain stable and increase in content through relative accumulation. The main reason for the difference is that organic acids are the principal weathering agents in Podzols whereas carbonic acid plays this role where organic matter decomposition is more rapid, as in Ferralsols.
Strongly leached Ferralsols, although very low in cations and with a soil-pH of 4.0 or less, show no tendency to develop an eluvial horizon. The production of organic acids is too slow and/or their decomposition too fast and the high content of iron oxides would immediately precipitate such complexes. Such soils will podzolize if iron compounds are removed and the clay is decomposed by ferrolysis under conditions of periodic water stagnation.
In the wet tropics, soil formation will produce a Ferralsol in most well drained parent materials that are rich in iron and not too siliceous. A Podzol will form in imperfectly drained, coarse-textured and quartz-rich materials, which receive organic matter that decomposes slowly under conditions of oligotrophy.
A typical zonal Podzol has an ash-grey, strongly leached eluvial horizon under a dark surface horizon with organic matter, and above a brown to very dark brown, spodic illuviation horizon. Most Podzols have a surface litter layer (an H-horizon) that is 1 to 5 cm thick, loose and spongy, and grades into an Ah-horizon with partly humified organic matter. In the litter layer in particular, most plant fragments are still recognizable and live roots may be beset with mycorrhizae. The Ah-horizon consists of a dark grey mixture of organic matter and mineral material (mainly quartz). The underlying bleached E-horizon has a single grain structure whereas the structure of the brown to black illuviation horizon varies from loose (rare) through firm, subangular blocky to very hard and massive.
At the drier end of the climatic range for zonal Podzols, the illuviation horizon has commonly a high chroma signifying accumulation of iron oxides (together with aluminium oxides). In more humid regions, the Bhs-horizon is darker and has a higher content of translocated organic matter.
The profile of a typical intrazonal tropical Podzol has a surface layer of poorly decomposed (`raw'), acid humus with a high C/N-ratio. The underlying humus-stained A-horizon is poorly developed and rests on top of a light grey to white eluvial E-horizon of sand texture that can be from 20 cm to several metres thick (`Giant Podzols'). The still deeper illuvial horizon is commonly dark brown and irregular in depth. Rarely, one finds mottles or soft concretions of iron and aluminium oxides, and/or slightly more clay in the illuvial horizon than higher in the profile. Brightly coloured B(h)s-horizons with sesquioxides accumulation as occur in the temperate zone (not in `groundwater Podzols') are uncommon in the tropics where podzolization is largely restricted to iron-poor parent materials under the influence of groundwater.
The mineralogy of Podzols is somewhat variable but is nearly always marked by a predominance of quartz. In cool, humid climates where leaching is intense, the parent material may originally have been of intermediate or even basic composition.
Iron and aluminium maxima may occur at different depths in the B-horizon, depending on the genetic history of a particular soil. Podzols in the USA tend to have the maximum iron content above the Al-maximum. Well-developed intrazonal Podzols in Western Europe normally have their maximum Al-contents in the top of the B-horizon, with the Fe- maximum at greater depth.
Weathering processes in the A- and E-horizons of well-developed Podzols in clay-poor materials transform clay to smectite (beidellite), and sometimes kaolinite whereas clay in the B-horizon may be Al-interstratified. Allophane (amorphous Al-silicate) appears to accumulate in B-horizons in rich parent material.
Hydromorphic Podzols are structurally wet because of climate and/or terrain conditions. Water movement through the soil may be impaired even in upland areas if the soil has a dense illuviation horizon or an indurate layer at some depth. A thin iron-pan can form where there is periodic water stagnation in the soil, either in the B-horizon or below it (e.g. in Densic and Placic Podzols). Even though Podzols are associated with regions that have an annual precipitation surplus, their low water holding capacity may still cause drought stress in dry periods.
Most Podzols have a sandy texture and weak aggregation to structural elements; the bleached eluviation horizon contains normally less than 10 percent clay but the clay content could be slightly higher in the underlying illuvial horizon.
The organic matter profile of Podzols shows two areas of concentration, viz. one at the surface and one in the spodic horizon. The C/N-ratio is typically between 20 and 50 in the surface horizon, decreasing to 10 to 15 in the bleached horizon and then increasing again to 15 to 25 in the spodic horizon. Nutrient levels in Podzols are low as a consequence of the high degree of leaching. Plant nutrients are concentrated in the surface horizon(s) where cycling elements are released by decomposing organic debris but phosphates may accumulate in the B-horizon (as Fe or Al-phosphates). The surface horizons are normally acid, with pH(H2O,1:1) values between 3.5 and 4.5. The pH-value of zonal Podzols increases with depth to a maximum of about 5.5 in the deep subsoil, whereas soil-pH in intrazonal Podzols tends to be lowest in the upper B-horizon.
In boreal and temperate regions, `large' soil animals such as earthworms are scarce in most Podzols; decomposition of organic matter and surface soil homogenization are slow and are mainly done by fungi, small arthropods and insects. Many Australian Podzols show signs of earthworm activity. The activity of moles and earthworms increases sharply when Podzols are fertilized.
Zonal Podzols occur in regions with unattractive climatic conditions for most arable land uses. Intrazonal Podzols are more frequently reclaimed for arable uses than zonal Podzols, particularly those in temperate climates. The low nutrient status, low level of available moisture and low soil-pH make Podzols unattractive soils for arable farming. Aluminium toxicity and phosphorus deficiency are common problems. Deep ploughing, to improve the moisture storage capacity of the soil and/or to eliminate a dense illuviation horizon or hardpan, liming and fertilization are the main ameliorative measures taken.
Most zonal Podzols are under forest; intrazonal Podzols in temperate regions are mostly under forest or shrubs (heath). Most tropical Podzols sustain a light forest that recovers only slowly after cutting/burning. By and large, mature Podzols are best used for extensive (sheep) grazing or left idle under their natural (climax) vegetation.
The Reference Soil Group of the Planosols holds soils with bleached, light-coloured, eluvial surface horizons that show signs of periodic water stagnation and abruptly overly dense, slowly permeable subsoil with significantly more clay than the surface horizon. These soils were formerly regarded as `pseudogley soils' but are now recognized as `Planosols' by most soil classification systems. The US Soil Classification coined the name `Planosols' in 1938; its successor, USDA Soil Taxonomy, includes most of the original Planosols in the Great Soil Groups of the Albaqualfs, Albaquults and Argialbolls.
Definition of Planosols#
Soils having
Common soil units:
Thionic*, Histic*, Gelic*, Vertic*, Endosalic*, Gleyic*, Plinthic*, Mollic*, Gypsic*, Calcic*, Alic*, Luvic*, Umbric*, Arenic*, Geric*, Calcaric*, Albic*, Ferric*, Alcalic*, Sodic*, Alumic*, Dystric*, Eutric*, Rhodic*, 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.
Connotation: soils with a degraded, eluvial surface horizon abruptly over dense subsoil, typically in seasonally waterlogged flat lands; from L. planus, flat.
Parent material: mostly clayey alluvial and colluvial deposits.
Environment: seasonally or periodically wet, level (plateau) areas, mainly in sub-tropical and temperate, semi-arid and sub-humid regions with light forest or grass vegetation.
Profile development: AEBC profiles. Destruction and/or removal of clay produced relatively coarse-textured bleached surface soil abruptly overlying finer subsoil. Impeded downward water percolation accounts for stagnic soil properties in the bleached horizon.
Use: Planosols are poor soils. In regions with a warm summer season they are mostly under wetland rice. Elsewhere, Planosols are sown to dryland (e.g. fodder) crops or used for extensive grazing. Many Planosol areas are not used for agriculture.
The world's major Planosol areas lie in subtropical and temperate regions with clearly alternating wet and dry seasons, in Latin America (southern Brazil, Paraguay, Argentina), southern and eastern Africa (Sahelian zone, East and southern Africa), the eastern United States, southeast Asia (Bangladesh, Thailand) and in Australia. Their total extent is estimated at some 130 million hectares world-wide. See Figure 1.
Figure 1
Planosols world-wide
Planosols occur predominantly in flat lands but can also be found in the lower stretches of slopes, in a strip intermediate between uplands, e.g. with Acrisols or Luvisols, and lowland (plain or basin) areas, e.g. with Vertisols. Planosols occur also on terraces or somewhat higher up, together with Acrisols or other soils with an argic subsurface horizon. In the Ethiopian Highlands, Planosols occur in association with Vertisols in lower parts of the landscape and with Nitisols in higher reaches.
Planosols have typically a weakly structured ochric or umbric surface horizon over an albic horizon with `stagnic soil properties'. The texture of these horizons is markedly coarser than that of deeper soil layers; the transition is sharp and conforms to the requirements of an `abrupt textural change'. The finer textured subsurface soil may show signs of clay illuviation; it is only slowly permeable to water. Periodic stagnation of water directly above the denser subsurface soil produced typical stagnic soil properties in the bleached, eluvial horizon. (And in many soils also to mottling in the upper part of the clayey subsoil). The `abrupt textural change' from coarse textured surface soil to finer subsoil can be caused by:
Ferrolysis is thought to proceed as follows:
In the absence of oxygen (e.g. in water-saturated soils with reducing organic matter), ferric oxides and hydroxides are reduced to Fe2+-compounds which go into solution:
CH2O + 4 Fe(OH)3 + 7 H+ = 4 Fe2+ + HCO3- + 10 H2O
During this `reduction phase', H+-ions are consumed and the soil-pH rises. Fe2+-ions replace adsorbed basic cations and aluminium at the exchange complex; the replaced ions are partly leached out (together with some of the Fe2+). Once the soil-pH has risen to about pH 5 to 5.5, Al3+-ions and OH--ions polymerize to hydroxy-Al-polymers with ring structures. It is surmised that part of the polynuclear Al-polymers are `trapped' in the interlayer spaces of lattice clays thereby changing the properties of the clay (lower CEC, water content, swell-shrink properties). Remaining polymers are leached out of the soil.
When air re-enters the soil in a subsequent dry period, an `oxidation phase' sets in; exchangeable Fe2+ is oxidized again to insoluble Fe3+-hydroxide. This produces two H+-ions for each Fe2+-ion oxidized:
4 Fe2+ + O2 + 10 H2O = 4 Fe(OH)3 + 8 H+
The clay turns into hydrogen clay, which converts to aluminium-magnesium clay as adsorbed hydrogen ions are replaced by aluminium and basic cations dissolved from the clay structure. Silica is dissolved from the clay lattices at the same time; it may partly be removed and partly re-precipitate in amorphous form when the eluvial horizon dries out.
During the next wet season, a new cycle starts with another `reduction phase'.
Note that the abrupt change in clay content and, in some Planosols, in the nature of the clay, can only develop and persist if there is little homogenization of the soil. There are reports of established Planosols that were later transformed to Phaeozems because of intense soil homogenization by termites.
A typical horizon sequence of Planosols consists of an ochric or umbric surface horizon over an albic subsurface horizon, directly on top of an argic B-horizon. In very wet locations, the surface horizon may even be a dystric histic horizon whereas surface soils contain very little organic matter in more arid regions. The albic eluviation horizon is invariably greyish and has a sandy or loamy texture and a weak structure of low stability.
The most prominent feature of Planosols is the marked increase in clay content on passing from the degraded eluvial horizon to the deeper soil. The latter may be a slowly permeable argic illuviation horizon, mottled and with coarse angular blocky or prismatic structural elements, or massive and structureless. In most Planosols however, the abrupt change in texture appears to be due to geogenetic differentiation or strong weathering in situ in combination with clay destruction in the topsoil.
Destruction of clay reduced both the cation exchange capacity of the clay fraction and the soil's moisture retention capacity.
Planosols are subject to water saturation in wet periods because of stagnation of rain or floodwater. Stagnic soil properties directly above the slowly permeable subsurface layer are telltale signs even in the dry season.
The upper soil horizons of Planosols have weakly expressed and unstable structural elements; silty soils in particular become hard as concrete in the dry season and turn to heavy mud when they become waterlogged in the wet season. Sandy surface soil material becomes hard when dry but not cemented. The poor structure stability of the topsoil, the compactness of the subsoil and the abrupt transition from topsoil to subsoil all impair the rooting of crops.
Mature Planosols are chemically strongly degraded. The surface soil has become acidic and lost (much of) its clay; ion exchange properties have deteriorated as a consequence.
The natural vegetation of areas with Planosols is light forest and/or herbs or grasses. Where trees grow, it concerns species with extensive, shallow root systems that are capable of withstanding both severe drought and seasonal or occasional water logging. The soil fauna is not very diverse and population densities are low.
Natural Planosol areas support a sparse grass vegetation, often with scattered shrubs and trees that have shallow root systems and can cope with temporary water logging. Land use on Planosols is normally less intensive than on most other soils under the same climatic conditions. Vast areas of Planosols are used for extensive grazing. Wood production on Planosols is much less than on other soils under the same conditions.
Planosols in the Temperate Zone are mainly in grass or they are planted to arable crops such as wheat and sugar beet. Yields are only modest, even on drained and deeply loosened soils. Root development on natural, unmodified Planosols is severely hindered by oxygen deficiency in wet periods, dense soil at shallow depth and toxic levels of aluminum in the root zone. The low hydraulic conductivity of the dense subsurface soil makes narrow drain spacing inevitable.
Many Planosols in Southeast Asia are planted to a single crop of paddy rice, produced on bunded fields that are inundated in the rainy season. Efforts to produce dryland crops on the same land during the dry season have often met with little success; the soils seemed better suited to a second crop of rice with supplemental irrigation. Fertilizers are needed for good yields. Paddy fields should be allowed to dry out at least once a year to prevent or minimize microelement deficiencies or toxicity associated with prolonged soil reduction. Some Planosols require application of more than just NPK fertilizers and their poor fertility level can prove difficult to correct. Where the temperature permits paddy rice cultivation, this is probably superior to any other kind of land use.
Grasslands with supplemental irrigation in the dry season are a good land use in climates with long dry periods and short infrequent wet spells. Strongly developed Planosols with a very silty or sandy surface soil are perhaps best left untouched.
Albeluvisols are soils that have, within 1 metre from the surface, a clay illuviation horizon with an irregular or broken upper boundary resulting from deep tonguing of bleached soil material into the illuviation horizon. Common international names are Podzoluvisols (FAO), Derno-podzolic or Ortho-podzolic soils (Russia) and several suborders of the Alfisols (USDA Soil Taxonomy).
Definition of Albeluvisols#
Soils having, within 100 cm from the surface, an argic@ horizon with an irregular upper boundary resulting from albeluvic tonguing@ into the argic horizon.
Common soil units:
Histic*, Gleyic*, Alic*, Umbric*, Arenic*, Gelic*, Stagnic*, Abruptic*, Ferric*, Fragic*, Siltic*, Alumic*, Endoeutric*, Haplic*.
# See Annex 1 for the 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.
Connotation: from L. albus, white, and L. eluere, to wash out.
Parent material: mostly unconsolidated glacial till, materials of lacustrine or fluvial origin and of aeolian deposits (loess).
Environment: flat to undulating plains under boreal taiga, coniferous forest or mixed forest. The climate is temperate to boreal with cold winters, short and cool summers, and an average annual precipitation sum of 500 to 1000 mm. Precipitation is evenly distributed over the year or, in the continental part of the Albeluvisol belt, has a peak in early summer.
Profile development: mostly AEBtC profiles with a dark, thin ochric surface horizon over an albic subsurface horizon that tongues into an underlying brown clay illuviation horizon. Stagnic soil properties are common in boreal Albeluvisols.
Use: short growing season (frost!), acidity, low nutrient status, tillage and drainage problems are serious limitations of Albeluvisols. Most Albeluvisols are under forest; livestock farming ranks second; arable cropping plays a minor role. In Russia, the share of arable cropping increases towards the south and west of the Albeluvisol belt, especially on relatively nutrient-rich Endoeutric Albeluvisols.
Albeluvisols cover an estimated 320 million hectares in Europe, North Asia and Central Asia, with minor occurrences in North America. Figure 1 shows that Albeluvisols are concentrated in two regions, each having a particular set of climatic conditions:
Figure 1
Albeluvisols world-wide
Albeluvisols have diagnostic horizons and properties in common with Luvisols and Podzols. They differ from Luvisols by having `albeluvic tonguing'. Luvisols may have small penetrations of the overlying horizon into the argic subsurface horizon (`interfingering') but these do not have the dimensions of the tongues in Albeluvisols. Podzols differ from Albeluvisols by their spodic subsurface horizon. Some Albeluvisols have an eluvial horizon with sub-horizons that show characteristics of a spodic horizon. If these features become so pronounced that the sub-horizon qualifies as a spodic horizon, the soil is classified as a Podzol.
Albeluvisols in cold continental areas may border on Podzols to their north. At the interface between both soil groups podzolization of the strongly clay and iron-depleted eluvial horizon is common. Such soils are `bisequum soils', i.e. polygenetic soils with a recent A/E1/Bh solum overlying an older E2/Bt solum.
Albeluvisols in temperate regions occur also together with Podzols, particularly where the latter developed in sandy aeolian deposits. Large parts of the original Albeluvisol belt of Western Europe have now become Luvisols as a result of ploughing and man-induced erosion of the upper decimetres of the soil. The upper 50-80 cm of the original Albeluvisol have changed and its albeluvic tonguing has disappeared after centuries of human intervention. Agricultural activities, notably liming/manuring, have also increased the numbers of burrowing animals such as earthworms and moles and have raised the base saturation of the soils to the extent that they key out as Luvisols. It is common to find Luvisols under agriculture adjacent to Albeluvisols under forest.
The genesis of Albeluvisols has elements of `argilluviation' (i.e. translocation of clay as discussed in the chapter on Luvisols) and elements of present-day or paleo-periglacial (soil) climatic factors. The typical albeluvic tongues, which penetrate into the compacted top of the argic horizon, are the result of periglacial freeze-thaw sequencing during last glacial period.
Albeluvisols occur in regions that had or still have a harsh climate, which explains the little biological activity in their surface horizons. The sudden change in texture from the eluviation horizon to the illuviation horizon hinders internal drainage. Periodic saturation of the surface soil and reduction of iron compounds (enhanced by dissolved organic compounds) cause strong bleaching of the eluvial horizon. The eluvial horizon extends into the underlying argic horizon along root channels and cracks (the characteristic `tonguing'). This penetration of clay and iron-depleted material into the underlying horizon is distinctly different from the tonguing in (some) Chernozems or Podzols.
Albeluvic tongues have the colour and the coarser texture of the eluvial horizon from which they extend. Tongues must be wider than 5 mm in clayey argic horizons, 10 mm or wider in loamy and silty argic horizons and 15 mm or wider in coarser (silt, loam or sandy loam) argic horizons. The tongues must be deeper than wide and occupy more than 10 % of the volume of the upper 10 cm or the upper quarter (whichever is less) of the argic horizon, both in vertical and horizontal sections.
Albeluvisols are closely related to Albic Luvisols. The main difference is that the eluvial horizon of Albic Luvisols does not extend so prominently into the argic horizon. In most instances the tongues have the same colour as the argic horizon and are less easily detected in the soil profile. (Their lower penetration resistance can be ascertained by piercing them with a knife.)
Periodic saturation with water causes segregation of iron compounds in mottles or concretions of iron (hydr)oxides. Vertical transport of iron compounds may lead to accumulation of iron compounds in a deeper horizon or the iron may be discharged to the subsoil, leaving the soil matrix increasingly depleted. Stagnic properties are present in many Albeluvisols; gleyic properties are much less common.
In the absence of percolation, iron will remain in the soil where it accumulates in `discrete nodules'. These nodules form upon repeated drying-wetting of the soil with hysteresis between the rates of precipitation of iron compounds in the oxidative phase and of (re)dissolution when the soil is reduced again.
Repeated saturation and leaching of the eluvial horizon cause acidification of the horizon and loss of bases. Ultimately loss of clay and sesquioxides from the eluvial horizon may become so pronounced that only a sandy surface layer remains in which even a micro-Podzol may form. The low organic matter and iron contents of the leached surface soil explain why this layer has low structure stability and low resistance to mechanical stress and why it is normally somewhat compacted. Alternate wetting and drying promotes clay decomposition. In the extreme case, an acid, seasonally wet Planosol may be formed.
At the interface between the eluvial and the illuvial horizons, a `fragipan' (from L. fragere, to break) can form, commonly overlapping with the argic horizon. A `fragipan' is a natural, non-cemented subsurface horizon through which roots and percolating water can pass only along preferential paths, e.g. along ped faces. The natural character of the fragipan excludes plough pans and surface traffic pans. The penetration resistance of a fragipan, measured at field capacity, exceeds the reach of most field instruments (50 kN/m).
Most untouched Albeluvisols are under forest vegetation. A raw litter layer tops a dark, thin A(h)-horizon over a distinctly bleached eluvial E-horizon that extends into a brown argic illuvial horizon. The top of the argic horizon is normally dense. Clay coatings in the upper half of the argic horizon are invisible to the naked eye. Microscopic examination of thin sections will normally reveal disturbed clay coatings and clay papules within structure elements.
Where the eluvial horizon is not periodically saturated with (ground)water, the eluvial horizon has a brown to yellowish brown colour and contains a fair amount of roots. In the more temperate part of the distribution belt, the eluvial horizon may meet the diagnostic requirements of a cambic horizon. Such a horizon is sometimes referred to as a "biologically active B-horizon" because thin sections show that the entire soil mass is composed of pellets and earthy excrements of soil (micro)fauna. In the colder part of the distribution belt, a thin layer with all features of a spodic horizon may be found in the upper part of the eluvial horizon.
The configuration of an eluvial horizon on top of an illuvial horizon is, as such, indicative of downward water flow through the soil during at least part of the year. The diagnostic features of an Albeluvisol (viz. iron depletion and tonguing or, alternatively, iron nodules in an eluvial horizon) may or may not be strong enough to meet the specifications of `stagnic properties' (gley-like features caused by perched water on top of a slowly permeable subsurface horizon).
Most Albeluvisols have formed in quartz-rich parent material. The sandier the soil material is, the more pronounced the albeluvic tonguing. Many of these parent materials were once calcareous but the upper limit of the calcareous subsoil has since shifted to more than 2 metres below the surface, if it can be found at all. The (clay) mineralogical assembly, which was originally mixed, shows pedogenic differentiation: smectites and interstratified smectites have disappeared from the eluvial horizon and from the albeluvic tongues, where chloritic or degraded chloritic clay minerals have formed. The proportion of smectitic minerals is higher in the argic horizon than in the original parent material.
The eluvial horizon is normally sandy. The horizon is typically somewhat compacted; many eluvial horizons have a platy structure. The low organic matter content of the surface soil and its high susceptibility to structure deterioration demand that tillage is done at the proper soil moisture content. The dense argic horizon and/or permafrost may hinder rooting and uptake of water, either directly or indirectly because of its poor internal drainage and inadequate aeration.
The surface horizon of Albeluvisols contains typically between 1 and 10 percent organic carbon; the C/N ratio of the accumulated organic matter is greater than 15. The eluvial subsurface horizon contains rarely more than 1 percent organic C and a similar amount is present in the illuvial horizon. Natural, not cultivated, Albeluvisols are moderately to strongly acid; pH(1M KCl) values range from less than 4 to 5.5 or slightly higher. The Cation Exchange Capacity is typically of the order of 10 to 20 cmol(+)/kg, exclusive of the contribution by organic matter. Base saturation varies from a mere 10 percent in Haplic Albeluvisols with much exchangeable aluminium and Al-interlayered clays to values between 60 and 90 percent in cultivated Endoeutric Albeluvisols with little Al-interlayering.
Note that the distinction between Endoeutric and Haplic Albeluvisols is based on the base saturation of the argic horizon; the eluvial horizon is always very low in bases.
Burrowing animals of the macro- and meso-fauna are scarce in Albeluvisols or absent altogether. Biological activity is accordingly slow and it takes several years before leaves in the litter layer are decomposed to the extent that the original plant tissue is no longer recognisable (i.e. until a mor or moder type of terrestrial humus has formed). Fungi and actinomycetes account for most of the organic matter decomposition. Another consequence of the low rate of biological activity is that mixing of organic colloids with the mineral soil is slow and the humiferous surface horizon of Albeluvisols is normally only a few centimetres thick. Many Albeluvisols in forest areas in Western Europe, where little or no cattle grazing is practised, have a fragipan overlapping with the argic horizon. In such soils root penetration and water percolation are limited to the albeluvic tongues. If such soils are taken into cultivation, `bioturbation' sets in and this can remove the fragipan in a few centuries.
The agricultural suitability of Albeluvisols is limited by their acidity, low nutrient levels, tillage and drainage problems and because the climate dictates a short growing season followed by frost during the long winter. The Albeluvisols of the northern taiga are almost exclusively under forest; small areas are used as pastureland or hay fields. In the southern taiga zone, less than 10 percent of the non-forested area is used for agricultural production. Livestock farming is the main agricultural land use on Albeluvisols (dairy production and cattle rearing); arable cropping (cereals, potatoes, sugar beet, forage maize) plays a minor role.
In Russia, the share of arable farming increases in southern and western directions, especially on Endoeutric Albeluvisols. With careful tillage, liming and application of fertilisers, Albeluvisols can produce 25-30 tons of potatoes per hectare, 2-5 tons of winter wheat or 5-10 tons of dry herbage.
NOTES
The Reference Soil Group of the Luvisols holds soils whose dominant characteristic is a marked textural differentiation within the soil profile, with the surface horizon being depleted of clay and with accumulation of clay in a subsurface `argic' horizon. Luvisols have high activity clays and lack the abrupt textural change of Planosols, albeluvic tonguing as in Albeluvisols, a mollic surface horizon as in steppe soils, and the alic properties of Alisols. The name `Luvisols' is already used in the legend to the FAO Soil Map of the World; local names for these soils include `Pseudo-podzolic soils' (Russia), `sols lessivés' (France), `Parabraunerde' (Germany), `Grey Brown Podzolic soils' (earlier USA terminology) and `Alfisols' (USDA Soil Taxonomy).
Definition of Luvisols#
Soils having an argic@ horizon with a cation exchange capacity (in 1 M NH4OAc at pH 7.0) equal to or greater than 24 cmol(+) kg-1 clay, either starting within 100 cm from the soil surface or within 200 cm from the soil surface if the argic horizon is overlain by material that is loamy sand or coarser throughout.
Common soil units
Leptic*, Vertic*, Gleyic*, Vitric*, Andic*, Calcic*, Arenic*, Stagnic*, Abruptic*, Albic*, Profondic*, Lamellic*, Cutanic*, Ferric*, Hyperochric*, Skeletic*, Hyposodic*, Dystric*, Rhodic*, 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.
Connotation: soils in which clay is washed down from the surface soil to an accumulation horizon at some depth; from L. luere, to wash.
Parent material: a wide variety of unconsolidated materials including glacial till, and aeolian, alluvial and colluvial deposits.
Environment: most common in flat or gently sloping land in cool temperate regions and in warm (e.g. Mediterranean) regions with distinct dry and wet seasons.
Profile development: ABtC profiles; intergrades to Albeluvisols having an albic eluviation horizon above the argic subsurface horizon are not rare. The wide range of parent materials and environmental conditions led to a great diversity of soils in this Reference Soil Group.
Use: Luvisols with a good internal drainage are potentially suitable for a wide range of agricultural uses because of their moderate stage of weathering and high base saturation.
Luvisols extend over 500 to 600 million hectares world-wide, for the greater part in temperate regions such as west/central Russia, the USA and Europe but also in the Mediterranean and in southern Australia. Most Luvisols in subtropical and tropical regions occur on young land surfaces. Figure 1 gives an indication of the major concentrations of Luvisols.
Figure 1
Luvisols world-wide
Luvisols in upland areas are commonly associated with Cambisols; those in lowlands with Gleysols or Solonetz. Note however that there are many other Reference Soil Groups with which Luvisols share common properties. For example, Luvisols occurring together with Vertisols may have slickensides in the argic horizon but not meet the other criteria of a vertic horizon. Luvisols may be associated with Gypsisols and Calcisols that have an argic subsurface horizon; the dominant presence of gypsum and/or calcium carbonate in the argic horizon sets these soils apart from Luvisols. Steppe soils with a dark, base-rich surface horizon that (just) does not qualify as a mollic horizon may be Luvisols and occur in association with Chernozems or Phaeozems. Luvisols with an umbric surface horizon may well grade into Umbrisols. Land use history can affect lateral linkages. Well-known examples are the Albeluvisols under forest in the Belgian loess belt that lie adjacent to enriched Luvisols under agriculture. The latter evolved from Albeluvisols but lost albeluvic tonguing (through erosion of the surface soil and increased bioturbation) and acquired a higher base saturation in the argic horizon after years of liming and fertilization.
The dominant characteristic of Luvisols is their argic illuviation horizon formed by translocation of clay from the surface soil to the depth of accumulation. The process knows three essential phases:
Normally, clay in soil is not present as individual particles but is clustered to aggregates that consist wholly of clay or of a mixture of clay and other mineral and/or organic soil material. Mass transport of soil material along cracks and pores, common in cracking soils in regions with alternating wet and dry periods, does not necessarily enrich the subsoil horizons with clay.
For an argic horizon to form, the (coagulated) clay must disperse in the horizon of eluviation before it is transported to the depth of accumulation by percolating water.
Mobilization of clay can take place if the thickness of the electric `double layer', i.e. the shell around individual clay particles that is influenced by the charged sides of the clay plates, becomes sufficiently wide. If the double layers increase in width, the bonds between negatively charged sides and positive charges at the edges of clay plates become weaker until individual clay particles are no longer held together in aggregates. The strength of aggregation is influenced by:
At high electrolyte concentrations of the soil solution, the double layer is compressed so that clay remains flocculated. A decrease in ion concentration, e.g. as a result of dilution by percolating rain water, can result in dispersion of clay and collapse of aggregates. If the exchange complex is dominated by polyvalent ions, the double layer may remain narrow even at low electrolyte concentrations and consequently aggregates remain intact.
Soil-pH may influence both the concentrations of ions in the soil solution and the charge characteristics of the clay. Dispersion of clays is thus, to some extent, a pH-dependent process. At soil-pH(H2O,1:1) values below 5, the aluminium concentration of the soil solution is normally sufficiently high to keep clay flocculated (Al3+ is preferentially adsorbed over divalent and monovalent ions in the soil solution). Between pH 5.5 and 7.0, the content of exchangeable aluminium is `low'. If concentrations of divalent ions are low, clay can disperse. At still higher pH values, divalent bases will normally keep the clay flocculated unless there is a strong dominance of Na+-ions in the soil solution.
Certain organic compounds, especially polyphenols, stimulate mobilization of clay by neutralizing positive charges at the edges of clay minerals. As iron-saturated organic complexes are insoluble, this process might be of little importance in Fe-rich Luvisols (particularly common in the subtropics).
Transport of peptized clay particles requires downward percolation of water through wide (>20 um) pores and voids. Clay translocation is particularly prominent in soils that shrink and crack in the dry season but become wet during occasional downpours.
Note that `smectite' clays disperse more easily than non-swelling clays; smectite clays are a common constituent of Luvisols.
Precipitation of clay particles takes place at some depth in the soil as a result of
Flocculation can be initiated by an increase in the electrolyte concentration of the soil solution or by an increase of the content of divalent cations (e.g. in a CaCO3-rich subsurface horizon).
Filtration occurs where a clay suspension percolates through relatively dry soil; it forces the clay plates against the faces of peds or against the walls of (bio)pores where skins of strongly `oriented' clay (`cutans') are formed. With time, the cutans may wholly or partly disappear through homogenization of the soil by soil fauna, or the cutans may be destroyed mechanically in soils with a high content of swelling clays. This explains why there is often less oriented clay in the argic subsurface horizon than one would expect on the basis of a budget analysis of the clay profile. There could also be more illuviated clay than expected viz. if (part of) the eluviated surface soil is lost through erosion.
Luvisols have typically a brown to dark brown surface horizon over a (greyish) brown to strong brown or red argic subsurface horizon. In subtropical Luvisols in particular, a calcic horizon may be present or pockets of soft powdery lime occur in and below a reddish brown argic horizon. Soil colours are less reddish in Luvisols in cool regions than in warmer climates. In wet environments, the surface soil may become depleted of clay and free iron oxides to the extent that a greyish eluviation horizon forms under a dark but thin A-horizon. Many Luvisols in Western Europe have evolved from Albeluvisols that underwent substantial morphological changes when they were taken into cultivation. Some causes:
Consequently many intergrades exist between Luvisols and Albeluvisols; they reflect the time and intensity of agricultural land use. Examples are Luvisols with a compact argic horizon or with remnants of albeluvic tonguing, or Luvisols with an acid soil reaction in the argic horizon.
Luvisols are moderately weathered soils; they contain less Al-, Fe- and Ti-oxides than their tropical counterparts, the Lixisols, and have an SiO2/Al2O3 ratio in excess of 2.0. Luvisols tend to become richer in swelling and shrinking clays towards the dry end of their climatic zone. As a consequence, pressure faces and parallelepiped structure elements become more and more prominent.
By and large, Luvisols have favourable physical properties; they have granular or crumb surface soils that are porous and well aerated. The `available' moisture storage capacity is highest in the argic horizon (15 to 20 volume percent). The argic horizon has a stable blocky structure but surface soils with a high silt content may be sensitive to slaking and erosion.
Most Luvisols are well drained but Luvisols in depression areas with shallow groundwater may develop gleyic soil properties in and below the argic horizon. Stagnic properties are found where a dense illuviation horizon obstructs downward percolation and the surface soil becomes saturated with water for extended periods of time
The chemical properties of Luvisols vary with parent material and pedogenetic history. Surface soils are normally wholly or partly de-calcified and slightly acid in reaction; they contain a few percent organic matter with a C/N ratio of 10 to 15. Subsurface soils tend to have a neutral reaction and may contain some calcium carbonate.
With the possible exception of Leptic, Gleyic, Vitric, Albic, Ferric and Dystric soil units, Luvisols are fertile soils and suitable for a wide range of agricultural uses. Luvisols with a high silt content are susceptible to structure deterioration if tilled in wet condition and/or with heavy machinery. Luvisols on steep slopes require erosion control measures.
The eluvial horizons of some Luvisols are depleted to the extent that an unfavourable platy structure formed with `pseudogley' (stagnic properties) as a result. This is the reason why truncated Luvisols are in many instances better soils for farming than the original, non-eroded soils.
Luvisols in the Temperate Zone are widely grown to small grains, sugar beet and fodder; in sloping areas they are used for orchards and/or grazing. In the Mediterranean region, where Chromic, Calcic and Vertic Luvisols are common in colluvial deposits of limestone weathering, the lower slopes are commonly sown to wheat and/or sugar beet while (eroded) upper slopes are in use for extensive grazing or planted to tree crops.
The Reference Soil Group of the Umbrisols contains soils in which organic matter of low base saturation has accumulated at the surface to the extent that it significantly affects the properties and utilization of the soil. Umbrisols are the logical pendant of soils with a mollic horizon (e.g. Chernozems, Kastanozems and Phaeozems). Not previously recognized at such high taxonomic level, these soils are classified in other systems as Umbrepts and Humitropepts (USA Soil Taxonomy), Humic Cambisols and Umbric Regosols (FAO), Sombric Brunisols and Humic Regosols (France) or `Brown Podzolic soils' (e.g. Indonesia).
Definition of Umbrisols#
Soils, having
Common soil units:
Thionic*, Gelic*, Anthric*, Leptic*, Gleyic*, Ferralic*, Arenic*, Stagnic*, Humic*, Albic*, Skeletic*, 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.
Connotation: soils with dark topsoil; from L., umbra, shade.
Parent material: weathering material of siliceous rock; predominantly in late Pleistocene and Holocene deposits.
Environment: cool and humid climates, e.g. in mountain regions with little or no moisture deficit.
Profile development: AC and A(B)C profiles.
Use: predominantly forestry and extensive grazing. Under adequate management, Umbrisols may be planted to cash crops such as cereals, root crops, tea and coffee.
Umbrisols occur in cool, humid regions, mostly mountainous and with little or no soil moisture deficit. They occupy about 100 million hectares throughout the world. In South America, Umbrisols are common in the Andean ranges of Columbia, Ecuador and, to a lesser extent, in Venezuela, Bolivia and Peru. They occur also in Brazil, e.g. in the Serra do Mar. Umbrisols in North America are largely confined to the north western Pacific seaboard. In Europe, Umbrisols occur along the north western Atlantic seaboard, e.g. in Iceland, on the British Isles and in northwest Portugal and Spain. In Asia, they are found in the mountain ranges east and west of Lake Baikal, and on fringes of the Himalayas, notably in India, Nepal, China and Burma. Umbrisols occur at lower altitudes in Manipur (eastern India), in the Chin Hills (western Burma) and in Sumatra (Barisan range). In Australasia, Umbrisols are found in the mountain ranges of New Guinea and Southeast Australia and in the eastern parts of South Island, New Zealand.
Figure 1
Umbrisols world-wide
Umbrisols are associated with Reference Soil Groups that occur under cool-temperate, moist, free-draining conditions. Linkages vary with the age of the landscape and local conditions.
Umbrisols in cool and/or wet areas are associated with Regosols and Leptosols, and in places with Histosols. In low-lying areas with a fluctuating water table, Umbrisols on lower slopes are found adjacent to Gleysols and Histosols (in depressions) and Cambisols, Podzols, Regosols and Leptosols (at higher elevation).
In places, this general pattern was compromised by human intervention. Where Umbrisols are being cultivated, lime is normally applied in appreciable quantities. This increases the soil's base saturation level, in places to the extent that the umbric horizon comes to resemble a mollic horizon. Ultimately, the Umbrisol changes into a Phaeozem. In other cases, notably in Western Europe, Umbrisols under cultivation have received bulk quantities of organic manure or earthy materials for several centuries. Here, the umbric horizon gradually transformed to a plaggic horizon or a terric horizon. In such areas, a complex mosaic of Umbrisols, Phaeozems and Anthrosols can be found.
Vegetation and climate influence the development of an umbric horizon. In some instances, an umbric horizon may form quite rapidly while concurrent development of an incipient, non-diagnostic, spodic or argic horizon is slow. This explains why umbric horizons are found in young, relatively undeveloped soils that lack any other diagnostic horizon, or have only a weak cambic horizon. Profile development is strongly dependent on deposition of (significant quantities of) organic material with low base saturation at the soil surface.
The organic material that characterises Umbrisols can comprise a variety of humus forms that have been variously described as `acid or oligitrophic mull', `moder', `raw humus' and `mor'. It could accumulate because of slow biological turnover of organic matter under conditions of acidity, low temperature, surface wetness, or a combination of these. However, Umbrisols were never cold and/or wet for sufficiently long periods to have developed a diagnostic histic horizon.
Most Umbrisols have AC or A(B)C profiles. The central concept of Umbrisols is that of deeply drained, medium-textured soils with a dark, acid surface horizon rich in organic matter as the distinguishing feature. Umbrisols may have an albic horizon provided that there are no other diagnostic horizons present within 200 cm of the surface. In the absence of an albic horizon a cambic horizon may be present as evidence of incipient soil formation. Umbrisols that were modified by Man may have a thickened surface horizon (less than 50 cm thick), which is classified as an anthropedogenic horizon.
Umbrisols do not have particular hydrological characteristics as soil texture and soil depth can vary widely.
Most Umbrisols are moderately deep to deep, medium-textured, permeable and well-drained soils. Gravel, stones and boulders can occur throughout the profile. Base saturation is less than 50 percent in the umbric horizon and normally also deeper down. Umbrisols have good physical properties and a moderate natural fertility level, largely on account of the high organic matter content of the umbric surface horizon. Umbrisols on slopes are susceptible to erosion if exposed to torrential rains.
Many Umbrisols are (still) under a natural or near-natural vegetation cover. Umbrisols above the tree line in the Andean, Himalayan and central Asian mountain ranges, or at lower altitudes in northern and western Europe where the former forest vegetation has been largely cleared, carry a cover of short grasses of low nutritional value. Coniferous forest predominates in Brazil (e.g. Araucaria spp.) and in the USA (mainly Thuja, Tsuga and Pseudotsuga species). Umbrisols in tropical mountain areas in south Asia and Australasia are under montane evergreen forest.
The predominance of sloping land and wet and cold climatic conditions restrict land use on most Umbrisols to extensive grazing. Management focuses on introduction of improved grasses and correction of the soil-pH by liming. Many Umbrisols are susceptible to erosion. Planting of perennial crops and bench or contour terracing offer possibilities for permanent agriculture on gentler slopes. Where conditions are suitable, cash crops may be grown, e.g. cereals and root crops in the USA, Europe and South America, or tea and cinchona in south Asia (e.g. Indonesia). Highland coffee on Umbrisols demands high management inputs to meet the stringent nutrient requirements of coffee. In New Zealand, Umbrisols have been transformed into highly productive soils, used for intensive sheep and dairy farming, and production of cash crops.
