Mineral Soils conditioned by Parent Material
Major landforms in volcanic
Major landforms in landscapes with sands
Major landforms in landscapes with expanding clays
Major landforms in volcanic landscapes
Volcanism is not randomly distributed over the world. It is concentrated near plate boundaries where plate subduction or seafloor spreading takes place. Other occurrences are linked to deep mantle plumes that reach the Earth's surface at distinct `hotspots'. Figure 1 shows the geographic distribution of major volcanic regions.
Landforms in volcanic regions are strongly influenced by the chemical and mineralogical composition of the materials that were deposited during eruptive phases. Volcanic rocks and magmas are grouped according to their silica contents in three main categories labeled `Rhyolite' (65-75% SiO2), `Andesite' (65-55% SiO2) and `Basalt' (55-45% SiO2). The mineralogical properties and chemical composition (notably the contents of K2O, Na2O and CaO) distinguish individual rock types. See Figure 2.
The broad division of volcanic rocks and magmas on the basis of silica content makes sense because the SiO2 content correlates with the viscosity of magmas and hence with the type of volcanism. A rule of thumb: the higher the silica content of magma is, the more acid and viscous the magma is and the more explosive volcanic eruptions are. This influences profoundly the character and morphology of volcanic phenomena.
In the following, major landforms of volcanic regions will be discussed taking magma composition as a reference point.
Basaltic volcanism occurs where basic mantle material reaches the surface, notably
Re 1: The best-known divergent plate margin is the mid-oceanic ridge or rise. The highest parts of the ridge may reach the surface of the ocean and form islands, e.g. Iceland and the Canary Islands. It is not surprising that, like all ocean floors, Iceland consists mainly of basaltic rock.
Re 2: A fine example of basaltic hot-spot volcanism is Hawaii, which constitutes the top of the largest `shield volcano' in the world, with a diameter of 250 km at the base (on the ocean floor) and a total height of 9 km. Basaltic magma is little viscous and gases escape easily. Eruptions are therefore relatively quiet and produce low-viscosity lava flows, lava lakes and lava fountains, but little ash. The fluid magma can flow over large distances and the resulting shield volcanoes are comparatively flat. Most eruptions are `fissure eruptions' that take place along extensional cracks in the Earth's crust. The fissures may be several kilometers in length; the historical `Laki' eruption on Iceland happened along a 24-km long fissure. Much bigger fissure eruptions have taken place in the past. They produced enormous masses of `flood basalt' that covered hundreds of square kilometers. The Paraña plateau in South America is made up of 1 million km3 basalt, which was extruded within 10 million years. Other examples of large occurrences of flood basalt are in Ethiopia, Siberia, Greenland, Antarctica, India (the `Deccan Traps') and in the western USA (Colombia River). See Figure 3.
Re 3. Many hotspots that are situated below a continental crust are associated with mantle plumes that push the crust up (`updoming') and cause large-scale dilation cracks. The latter become manifest as elongated tectonic depressions: the rift valleys. Both basic (SiO2 poor) and acid (SiO2 rich) volcanism occur in and along rift valleys. Basaltic volcanism in continental rift valleys (e.g. the East-African rift valley, the Baikal graben, or the Rhine-Rhone graben) is associated with `strombolian' scoria cones and with `maar' craters (i.e. steam-explosion craters now filled with water). Here too, ash deposits seldom extend beyond the volcanic areas themselves. Where ash blankets are extensive, as in some rift valleys, they are usually more acidic.
The comparatively fluid basaltic lava flows tend to follow river valleys and can flow over considerable distances into the rift valley. Subsequent erosion of soft sediments adjacent to the lava bodies results in `relief inversion', with the former basaltic valley fills extending as elongated plateaus in the eroded landscape.
Andesitic volcanism is a characteristic element at convergent plate boundaries where plate subduction takes place. Typical settings are
The classic volcano type associated with andesitic volcanism is the `stratovolcano'. Literally, the term means `stratified' volcano, which is misleading in the sense that all volcanoes are built up of layers, be it of basalt flows, as in the Hawaiian shield volcanoes, or of pyroclastics, as the scoria cones of the Eifel. What the term indicates, actually, is that this type of volcano is composed of alternating layers of lava and pyroclastic rock, mostly of andesitic composition. Most stratovolcanoes are much larger than scoria cones and have a long history of alternating lava and pyroclastic rock eruptions.
Andesitic magmas hold an intermediate position between basaltic and rhyolitic magmas with respect to their SiO2 content, viscosity and gas content. Whereas basaltic, low viscosity magmas hardly produce pyroclastics (`tephra'), and high-viscosity rhyolites hardly produce lavas, andesitic magmas will normally produce both. Because of the greater viscosity of the magma, greater pressure must build up before an eruption can occur; eruptions are less frequent and more violent than in basaltic volcanism.
Lava flows emitted by stratovolcanoes are more viscous than those of basaltic shield volcanoes, and do not extend as far from the point of emission, usually only a few kilometers. This explains why stratovolcanoes have steeper slopes than shield volcanoes and the `classical' cone shape.
Active, large and high stratovolcanoes are likely to produce devastating volcanic mudflows (also called `lahars'). Lahars can form in several ways:
`Pyroclastic flows' are frothy masses of ash and pumice. They evolve when an extrusive dome collapses, generating a fast moving avalanche of hot gases, ash and pumice. The resulting rocks are known as `ignimbrites' and can have a variety of structures depending on the flow conditions during emplacement and on the degree of post-depositional welding.
`Volcanic ash fall-out' often spreads far beyond the direct vicinity of the erupting volcano. Lava and pyroclastic flows are normally confined to the immediate vicinity of volcanoes but ashes can be blown into the troposphere and stratosphere, and can travel hundreds of kilometers. The thickness of the ash deposits decreases with increasing distance from the point of origin. It may be difficult to recognize the presence of volcanic ash in soils because it is incorporated in the solum, overgrown by vegetation and it weathers rapidly. Nonetheless, `rejuvenation' of soil material with fresh volcanic ash is often of great importance as it restores or improves soil fertility and promotes physical soil stability.
Acid `rhyolitic' magmas are produced by partial melting of the continental crust, e.g. in cordilleran mountain ranges and rift valleys. Rhyolitic magmas are viscous and withstand very high gas pressures. As a result, rhyolitic eruptions are rare, but also extremely violent. If a rhyolitic magma chamber is present below a stratovolcano, tremendous gas pressures build up so that, once a vent for eruption is opened, the magma chamber empties itself completely, leaving a cavity in the Earth's crust in which the entire stratovolcano collapses. Craters of several kilometres in diameter are formed in this way: the `calderas' (e.g. Krakatoa in Indonesia; Ngorongoro in Tanzania; Crater Lake in the USA and the Laacher See in Germany). Only occasionally do more quiet eruptions take place. The high viscosity of the lava precludes lava flow; a lava dome is formed instead (e.g. Obsidian Dome, USA).
The main extrusive products of rhyolitic volcanism are:
Both ashes and ignimbrites consist for the greater part of volcanic glass and weather easily. Crystals of (mainly) quartz and/or feldspars, biotite and hornblende (`phenocrysts'; Du.: `eerstelingen') make up less than 20 percent of the ash. The only historic ignimbrite-forming eruption was that of the Katmai in Alaska in 1912. The largest eruption in comparatively recent times took place some 40,000 years ago and led to the formation of Lake Toba on Sumatra, Indonesia.
Volcanic rocks especially pyroclastic rocks, contain volcanic glass that weathers easily and accounts for the remarkable properties that soils in most volcanic regions have in common. Translocations of weathering products and accumulation of short-range-order minerals and of stable organo-mineral complexes are essential processes in the formation of the characteristic soils of volcanic regions: Andosols.
The Reference Soil Group of the Andosols holds soils developed in volcanic materials. Common international names are `Andosols' (FAO, Soil Map of the World), `Andisols' (USDA Soil Taxonomy), `Andosols' and `Vitrisols' (France) and `volcanic ash soils'.
Definition of Andosols#
Common soil units:
Vitric*, Silandic*, Aluandic*, Eutrisilic*, Melanic*, Fulvic*, Hydric*, Histic*, Leptic*, Gleyic*, Mollic*, Duric*, Luvic*, Umbric*, Arenic*, Placic*, Pachic*, Calcaric*, Skeletic*, Acroxic*, Vetic* , Sodic*, Dystric*, Eutric*, Haplic*.
# See Annex 1 for key to all Reference Soil Groups.
@ Diagnostic horizon, property or material; see Annex 2 for full definition.
* Qualifier for naming soil units; see Annex 3 for full definition.
Connotation: black soils of volcanic landscapes; from Jap. an, black, and do, soil.
Parent material: mainly volcanic ash, but also tuff, pumice, cinders and other volcanic ejecta.
Environment: undulating to mountainous, humid, arctic to tropical regions with a wide range of vegetation types.
Profile development: AC- or ABC-profile. Rapid weathering of porous volcanic material resulted in accumulation of stable organo-mineral complexes and short-range-order minerals such as allophane, imogolite and ferrihydrite.
Use: many Andosols are intensively cultivated and planted to a variety of crops, their major limitation being their considerable capacity to render phosphorus unavailable to plants. In places, steep topography is a serious constraint.
Andosols occur in volcanic regions all over the earth. Major concentrations are found around the Pacific rim: on the west coast of South America, in Central America, the Rocky Mountains, Alaska, Japan, the Philippine Archipelago, Indonesia, Papua New Guinea and New Zealand.
Andosols are also prominent on many islands in the Pacific: Fiji, Vanuatu, New Hebrides, New Caledonia, Samoa and Hawaii.
In Africa, Andosols are prominent along the Rift Valley, in Kenya, Rwanda and Ethiopia and on Madagascar. In Europe, Andosols occur in Italy, France, Germany and Iceland. The total Andosol area is estimated at some 110 million hectares or less than 1 percent of the global land surface. More than half of this is situated in the tropics. Figure 1 presents the world-wide distribution of Andosols. Note that the small scale of this map permits to show only the largest Andosol areas.
Andosols are azonal soils found in all climates and at all altitudes. Consequently they occur together with almost any other Reference Soil Group. A typical configuration on mountain slopes would have Andosols at the higher end of the slope, Cambisols and Luvisols at mid-slope positions and Vertisols (basic volcanic materials) or Acrisols (acidic materials) near the foot of the slope. In tropical highlands, e.g. in Kenya and Ethiopia, Andosols are often associated with Nitisols.
Andosols are characterised by the presence of either an `andic' horizon or a `vitric' horizon. An andic horizon is rich in `allophanes'1 (and similar minerals) or aluminium-humus complexes whereas a vitric horizon contains an abundance of `volcanic glass'.
Andosol formation depends essentially on rapid chemical weathering of porous, permeable, fine-grained mineral material in the presence of organic matter. Hydrolysis of the primary minerals `microcline' and `augite' may serve to illustrate this type of weathering (`glass' is actually an amorphous mixture but reacts in the same way):
KAlSi3O8 + 2 H2O = K+ + Al3+ + 3 SiO2 + 4 OH-
CaFeSi2O6 + 2 H2O = Ca2+ + Fe2+ + 2 SiO2 + 4 OH-
The liberated Fe2+ and (particularly) Al3+ ions are tied up in stable complexes with humus. The ferrous iron is first oxidised to the ferric state after which it precipitates for the greater part as ferrihydrite2:
Fe2+ = Fe3+ + e-
Fe3+ + 3 H2O = Fe(OH)3 + 3 H+
(or: 2 Fe2+ + 1/2 O2 + 5 H2O = 2 Fe(OH)3 + 4 H+ )
Aluminium protects the organic part of Al-humus complexes against bio-degradation. The mobility of these complexes is rather limited because rapid weathering yields sufficient Al and Fe to produce complexes with a high metal/organic ratio that are only sparingly soluble. This combination of low mobility and high resistance against biological attack promotes accumulation of organic matter in the topsoil culminating in the formation of a `melanic' surface horizon that has an intense dark colour and a high content of organic matter.
The fate of the liberated silica is largely conditioned by the extent to which aluminium is tied up in Al-humus complexes. If most or all aluminium is `fixed', the silica concentration of the soil solution increases and while part of the silica is washed out, another part precipitates as opaline silica. If not all aluminium is tied up in complexes, the remainder may co-precipitate with silicon to form allophanes of varying composition, often in association with imogolite3.
Note that formation of Al-humus complexes and formation of allophane associations are mutually competitive. This is known as the `binary composition' of Andosols. It seems that allophane (and imogolite) is stable under mildly acid to neutral conditions (pH>5) whereas Al-humus complexes prevail in more acid environments. If there is still (excess) aluminium available under such acid conditions, this may combine with silicon to form 2:1 and 2:1:1 type phyllosilicate clay minerals (e.g. chlorite) that are often found in association with Al-humus complexes. The stability conferred on the organic matter by aluminium is no less in the presence of allophane. This suggests that the activity of aluminium in allophane is high enough to interact with organic molecules and prevent bio-degradation and leaching.
The competition between humus and silica for Al is influenced by environmental factors:
The total pore fraction of the soil material increases greatly in the course of weathering, typically from some 50 percent to more than 75 percent (by volume). This is caused by leaching losses and stabilisation of the residual material by organic matter and weathering products (silica, allophane, imogolite, ferrihydrite).
The genesis of Andosols is further complicated if there is repeated deposition of fresh ash. Thin ash layers may just rejuvenate the surface soil material but thicker layers bury the soil. A new profile will then develop in the fresh ash layer while soil formation in the buried A-horizon takes a different course in response to the suddenly decreased organic matter supply and the different composition of the soil moisture.
The clay assemblage of Andosols changes over time, particularly that of the subsoil, as allophane and imogolite are transformed to halloysite, kaolinite or gibbsite (depending on the silica concentration of the soil solution). Aluminium from the Al-humus complexes will gradually become available and ferrihydrite will eventually turn into goethite. All these processes are strongly influenced by such factors as the rate of rejuvenation, the depth and composition of the overburden, the composition of the remaining material and the moisture regime. Eventually, an Andosol may grade into a `normal' soil, e.g. a podzolized soil, or a soil with ferric properties, or with clay illuviation.
The `typical' Andosol has an AC or ABC profile with a dark Ah-horizon, 20 to 50 cm thick (thinner or thicker occurs) on top of a brown B- or C-horizon. Topsoil and subsoil colours are distinctly different; colours are generally darker in humid, cool regions than in tropical climates. The average organic matter content of the surface horizon is about 8 percent but the darkest profiles may contain as much as 30 percent organic matter. The surface horizon is very porous, very friable, and has a crumb or granular structure. In some Andosols the surface soil material is smeary and feels greasy or unctuous; it may become almost liquid when rubbed, presumably because of sol-gel transformations under pressure (`thixotropy').
Most Andosols have excellent internal drainage because of their high porosity and their occurrence in predominantly high terrain positions. Gleyic soil properties develop where groundwater occurs at shallow depth; stagnic properties are particularly prominent in paddy fields on terraced volcanic slopes, e.g. on Java and Bali (Indonesia).
Quantities of volcanic glass, ferromagnesian minerals (olivine, pyroxenes, amphiboles), feldspars and quartz in the silt and sand fractions of Andosol material differ between sites. Some of the mineral grains may have acquired a coating of volcanic glass when the temperature was still high. The mineral composition of the clay fraction of Andosols varies with such factors as `genetic age' of the soil, composition of the parent material, pH, base status, moisture regime, thickness of overburden ash deposits, and content and composition of soil organic matter. The clay fraction of Andosols contains typical `X-ray amorphous materials' such as allophane and imogolite, and/or humus complexes of Al and Fe together with opaline silica. Allophane/imogolite and Al-humus complexes may occur together even though the two groups have conflicting conditions of formation. Besides primary minerals, ferrihydrite, (disordered) halloysite and kaolinite, gibbsite and various 2:1 and 2:1:1 layer silicates and intergrades can be present.
The good aggregate stability of Andosols and their high permeability to water make these soils (relatively) resistant to water erosion. Exceptions to this rule are highly hydrated types of Andosol that dried out strongly, e.g. after deforestation. The surface soil material of such Andosols crumbles to hard granules (`high mountain granulation') that are easily removed with surface run-off water. The difficulty to disperse Andosol material gives problems in texture analysis; caution should be taken when interpreting such data.
The bulk density of Andosols is low, not just in the surface soil; it is typically less than 0.9 Mg/m3 but values as low as 0.3 Mg/m3 have been recorded in highly hydrated Andosols. The bulk density does not change much over a suction range of 1500 kPa (limited shrink and swell). Therefore, values determined on field-moist soil material can in practice be substituted for the bulk density at `field capacity', which is diagnostic for identifying an `andic' horizon.
The moisture content at 1500 kPa suction (`permanent wilting point') is high in most Andosols; the quantity of `available water' is generally greater than in other mineral soils. Excessive air-drying of Andosol material will irreversibly deteriorate water holding properties, ion exchange capacity, soil volume, and ultimately the cohesion of soil particles. In the extreme case these fall apart to a fine dust that is very susceptible to wind erosion.
Andosols have highly variable exchange properties: the charge is strongly dependent on pH and electrolyte concentration. This is also the case with some other soils, e.g. Ferralsols, but the negative charge of Andosols can reach much higher values because of the high contents of soil organic matter and allophane.
Figure 2 shows, for some Andosol components, the variation in charge as a function of pH (the clay minerals halloysite and montmorillonite, having a dominantly permanent charge, are included for comparison).
With charge properties variable, base saturation values are also variable. Base saturation values are generally low because of strong leaching, except in some very young Andosols and in Andosols in dry regions.
The strong chemical reactivity of Andosols has long been attributed to X-ray amorphous compounds. It is more appropriate, however, to ascribe this Andosol characteristic to the presence of `active aluminium' which may occur in various forms:
The role of active iron may not be ignored but is generally considered of less importance than that of active aluminium.
Andosols have a high potential for agricultural production but many are not used to their capacity. By and large, Andosols are fertile soils, particularly Andosols in intermediate or basic volcanic ash and not exposed to excessive leaching. The strong phosphate fixation of Andosols is a problem. Ameliorative measures to reduce this effect (caused by active Al) include application of lime, silica, organic material and `phosphate' fertilizer.
Andosols are easy to till and have good rootability and water storage properties. Strongly hydrated Andosols may pose problems on account of their low bearing capacity and their stickiness.
Andosols in the tropics are planted to a wide variety of crops including sugarcane, tobacco, sweet potato (tolerant of low phosphate levels), tea, vegetables, wheat and orchard crops. Andosols on steep slopes are perhaps best kept under forest. Paddy rice cultivation is a major landuse on Andosols in lowlands with shallow groundwater. Elsewhere, continued paddy rice production has resulted in formation of a dense hardpan over accumulation layers of iron and manganese oxides; these hardpans reduce percolation losses of (irrigation) water.
Major landforms in landscapes with sands
Parent material can decisively influence soil formation: soils in (almost) pure quartz sands are normally `poor'. Extensive regions with such quartz-rich sands exist on earth. By and large, these can be divided into three broad categories:
Extensive, horizontal sandstone plateaux occur in tropical shield areas. Well-known examples are the Precambrian Roraima sandstone formations on the Guiana Shield and the Voltaian sandstone formations in Western Africa. Major occurrences of consolidated sands are found in Northern Africa, in Guyana and Surinam, eastern Peru, northeastern Brazil and in Liberia (western Africa). These sandstone formations have a history of tropical weathering in common; they all have a deep weathering mantle of bleached, white sands that are very rich in quartz, poor in clay and excessively drained. Electrolyte contents differ by region:
Sandy parent materials are also abundant in areas where sand accumulates after selective transportation of weathering material by wind or water. Aeolian (wind-borne) sands will be discussed in this paragraph.
During transport, selection of particles (sorting and winnowing) occurs; the momentary wind speed and the size, shape and density of minerals determine how far a particular grain will be transported. Fine gravel travels by creep and sand-sized particles by saltation. Silt-sized particles can be carried over great distances (Saharan `dust' settles regularly in central Europe and, in the past, loess formations have formed extensive blankets far from the source areas). Fine, plate-shaped clay minerals and micas are blown out and travel even farther (which explains why wind-borne sediments are normally poor in micas). This sorting of grains results in deposits that consist of pure sand with a uniform particle size. Many aeolian sand deposits show characteristic large-scale cross bedding, indicative of sand deposition on the slip faces of dunes. See Figure 1.
`Fixed dunes' are formed when transported sand settles in the lee of an obstacle such as a brush or a piece of rock. The obstacle thus grows in size and more sand settles: the dune grows. The transport capacity of the wind decreases as it drives the sand grains to the top of the dune, causing an increasing part of the transported sand to settle before reaching the dune crest. This steepens the angle of the slope, particularly near the crest. Once the slope angle exceeds the angle of rest of the deposited sand (typically 34o for dry sand), shearing sets in along a slightly less steep plane. Thus, a slip face is formed on the leeward side of the dune. Vegetation growing on (in particular) the lower part of dunes may eventually keep most of the sand in place. Dunes along coasts are often fixed by vegetation (natural or planted by man); `parabolic dunes' may develop by landward migration of beach sand.
`Free dunes' have no fixed position, but migrate downwind by erosion on the gently inclined windward side and deposition on the leeward side (slip face) in the same way as described for fixed dunes. The smallest free dunes are common wind ripples that measure only a few centimetres in height. Large dunes are found in extensive dune areas in deserts, in sand seas known as `ergs'.
Coastal dunes occur along beaches or sand-flats that form part of a non-erosional sandy or deltaic coast. The source areas of the sand will eventually lose all sand, silt and clay particles; some become wet (groundwater) depressions whereas others acquire a rocky or boulder-strewn surface known as a `desert pavement'.
Two main types of free dunes are distinguished, viz. `crescentic' dunes and `linear' dunes.
The area of actual `erg' and dune formation is delimited by the 150 mm/yr isohyet. This precipitation boundary appears to have shifted strongly in the recent past. Between 20,000 and 13,000 yr. BP, the southern limit of active dune formation in the Sahara desert was 800 km south of its present position and most of the now sparsely vegetated Sahelian zone was an area of active dune formation at that time. These dunes, mostly of the longitudinal type, are now fixed by vegetation, but their aeolian parentage is still obvious from their well-sorted material. A similar story can be told for the Kalahari sands. Overgrazing in recent times has reactivated aeolian transport in many regions with sands.
The cover sands (sheet deposits and associated parabolic dunes) of the temperate climate zone were mainly formed under `periglacial' (= polar desert) conditions during the arid interval between 20,000 and 13,000 years BP. River dunes in north-west Europe have for the greater part formed by local deflation of sand from the plains of braided rivers during the cold Younger Dryas period (10,500 to 10,150 years BP). Forests, notably pine forests, re-established on most of these sands, but overgrazing by sheep in medieval times sparked renewed (wind) erosion. Young `anthropogenic' dunes with Arenosols are common in many parts of west and central Europe. See Figure 3.
Similar periglacial dune fields with Arenosols are found in (more continental) parts of North America (Canada) and Russia.
The Reference Soil Group of the Arenosols consists of sandy soils, both soils developed in residual sands, in situ after weathering of old, usually quartz-rich soil material or rock, and soils developed in recently deposited sands as occur in deserts and beach lands. Many Arenosols correlate with Psamments and Psammaquents of the USDA Soil Taxonomy. Deep sandy soils with an argic or a spodic horizon within 200 cm from the surface are `Grossarenic' subgroups within the Alfisol, Ultisol and Spodosol orders. In the French classification system (CPCS, 1967), Arenosols correlate with taxa within the "Classe des sols minéraux bruts" and the "Classe des sols peu évolués". Other international soil names to indicate Arenosols are `siliceous, earthy and calcareous sands' and various `podsolic soils' (Australia), `red and yellow sands' (Brazil) and the Arenosols of the FAO Soil Map of the World.
Definition of Arenosols#
Common soil units:
Gelic*, Hyposalic*, Gleyic*, Hyperalbic*, Plinthic*, Hypoferralic*, Hypoluvic*, Tephric*, Gypsiric*, Calcaric*, Albic*, Lamellic*, Fragic*, Yermic*, Aridic*, Protic*, Dystric*, Eutric*, Rubic*, 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: sandy soils; from L. arena, sand.
Parent material: unconsolidated, in places calcareous, translocated sand; relatively small areas of Arenosols occur on residual sandstone or siliceous rock weathering.
Environment: from arid to (per)humid and from extremely cold to extremely hot; landforms vary from recent dunes, beach ridges and sandy plains under scattered (mostly grassy) vegetation, to very old plateaus under light forest.
Profile development: A(E)C profiles. In the dry zone, an ochric surface horizon is the only diagnostic horizon. Arenosols in the perhumid tropics tend to develop thick albic eluviation horizons; most Arenosols of the humid temperate zone show signs of alteration or transport of humus, iron or clay, but too weak to be diagnostic.
Use: most Arenosols in the dry zone are used for little more than extensive grazing but they could be used for arable cropping if irrigated. Arenosols in temperate regions are used for mixed arable cropping and grazing; supplemental (sprinkler) irrigation is needed during dry spells. Arenosols in the perhumid tropics are chemically exhausted and highly sensitive to erosion. They are best left untouched.
Arenosols are among the most extensive soils in the world, covering about 900 million ha or 7 percent of the land surface. If shifting sands and active dunes (`non-soils') were included, the coverage would be about 10 percent. Vast expanses of deep aeolian sands are found on the central African plateau between the equator and 30o southern latitude. These `Kalahari Sands' form the largest body of sands on earth. Other areas of Arenosols occur in the Sahelian region of Africa, various parts of the Sahara desert, central and western Australia, the Middle East and China. Sandy coastal plains and coastal dune areas are of smaller geographic extent.
Although most Arenosols occur in arid and semi-arid regions, they are typical azonal soils; they are found in the widest possible range of climates, from very arid to perhumid and from cold to hot. Arenosols are widespread in aeolian landscapes but occur also in marine, littoral and lacustrine sands and in coarse-grained weathering mantles of siliceous rocks, mainly sandstone, quartzite and granite.
There is no limitation as to age or period in which soil formation took place. Arenosols occur on very old surfaces as well as in very recent landforms, and may be associated with almost any type of vegetation. Figure 1 presents a sketch map of the main occurrences of Arenosols world-wide.
Arenosols have linkages with almost any other Reference Soil Group. With some groups the surmised linkage is rather theoretical and probably rare, with others the linkage is obvious and well documented. A broad division can be made between Arenic units of other Reference Soil Groups and soil units of Arenosols.
The qualifier `Arenic', indicating a texture of loamy sand or coarser throughout the upper 50 cm of the soil, is recognised for all Reference Soil Groups except Histosols, Cryosols, Leptosols, Vertisols, Solonchaks, Podzols, Plinthosols, Solonetz, Chernozems, Kastanozems, Phaeozems, Gypsisols, Calcisols, Nitisols and Cambisols. It is possible that Arenic units exist within these Reference Soil Groups but that they have not (yet) been sufficiently documented. If so, they are probably rare. Because of "opposed" textural requirements no linkage exists e.g. with Vertisols.
Most Podzols have a sandy texture and therefore it would not make sense to use the qualifier `Arenic'; Leptosols are excluded because of the shallowness requirement.
Lower level units of the Arenosol group are linked with Cryosols (Gelic Arenosols), Solonchaks (Hyposalic Arenosols), Gleysols (Gleyic Arenosols), Andosols (Tephric Arenosols), Podzols (Albic or Hyperalbic Arenosols), Plinthosols (Plinthic Arenosols), Ferralsols (Hypoferralic Arenosols), Gypsisols (Gypsiric Arenosols), Durisols (Hypoduric Arenosols) and Calcisols (Calcaric Arenosols).
Special relationships exist with soils having thick sandy layers over an argic, ferralic or spodic subsurface horizon. In Alisols, Acrisols, Luvisols and Lixisols loamy sand or coarser textures are permitted if the argic horizon (by definition sandy loam or finer) occurs within 200 cm of the surface. In addition the qualifier "Hypoluvic" links Arenosols to Luvisols.
Soils with a ferralic horizon starting within 170 cm of the surface qualify as Ferralsols, irrespective of the texture of the overlying horizons. Similarly, soils with a spodic horizon starting within 200 cm of the surface are Podzols.
Planosols and Albeluvisols may have sandy textures in the upper part of the solum, but the presence of an abrupt textural change or of an argic horizon within 100 cm excludes qualifiers linking these soils to Arenosols.
The development of Arenosols of the dry zone is distinctly different from that of Arenosols in the wet tropics. Arenosols in the dry zone show minimal profile development because soil forming processes are at a standstill during long periods of drought and/or because the parent material is of young age. Arenosols in the wet tropics formed in young sandy deposits or, the other extreme, constitute the thick albic E-horizon of a Giant Podzol and represent the ultimate in soil formation.
Most Arenosols in the dry zone are associated with areas of (shifting) sand dunes. Evidently, soil formation in such dune sand is minimal until the dune is colonized by vegetation and held in place. Then, some humus can accumulate in the surface soil and a shallow, ochric surface horizon can develop; `Aridic' Arenosols contain less than 0.2 percent organic carbon and show evidence of aeolian activity. The sand grains of Arenosols in the dry zone may acquire a coating of (brownish) clay and/or carbonates or gypsum. In places, desert sand is deep red by coatings of goethite (`ferrugination', a relic feature according to some). Where the parent material is gravelly, sand is blown out of the surface layer and the coarser constituents remain behind at the soil surface as a `desert pavement' of polished pebbles and stones. `Yermic' Arenosols may be found in such situations. Depending on parent material and topographical situation, `Gypsiric', `Calcaric', `Hyposalic' and `Hypoduric' Arenosols, or combinations of these, occur as intergrades to Gypsisols, Calcisols, Solonchaks and Durisols. High permeability, low water storage capacity and low biological activity all promote decalcification of the surface layer(s) of Arenosols in the dry zone, even though the annual precipitation sum is extremely low.
Arenosols in the Temperate Zone show signs of more advanced soil formation than Arenosols in arid regions. They occur predominantly in fluvio-glacial, alluvial, lacustrine, marine or aeolian quartzitic sands of very young to Tertiary age. In young fluvio-glacial or marine sandy deposits, pedogenesis would most likely proceed as follows: in geomorphologically stable conditions a plant cover establishes itself and calcareous sands are deeply decalcified. An ochric surface horizon forms, which contains humus of the `moder' type, consisting for the greater part of excrements. Soluble organic substances produced in the ochric surface horizon percolate downward while forming complexes with iron and aluminium (`cheluviation', see under Podzols). At this stage the soils show signs of beginning `podzolization' with accumulation of Fe- and Al-humus complexes in thin lamellae. If the process continues until a true spodic subsurface horizon has formed, the soil has become a Podzol. In very poor sands (low in clay, silt and weatherable minerals), the incipient spodic horizon consists almost entirely of humus (Bh) whereas in richer materials it also contains amorphous, dispersible, humus-sequioxide complexes (Bhs). Human intervention can result in formation of an anthric horizon (e.g. `plaggic horizon'). Once the thickness of the anthric horizon reaches 50 cm or more the soil becomes an Anthrosol, otherwise the qualifier `Anthric' (or `Plaggic') applies.
Lamellae may be of different origin and composition. Lamellae in geomorphologically unstable aeolian or fluvio-glacial deposits are mere markers of short periods of stability and vegetative cover alternating with periods of wind erosion and deposition. In more stable situations, lamellae are formed by vertical transport, after decalcification, of fine components over short distances. Humus and/or humus-iron complexes precipitate as the ratio of sequioxides to organic carbon increases in the course of cheluviation or upon saturation after evaporation at the depth of water penetration. Clay lamellae commonly follow visible stratification and are correlated with differences in pore size. Pores slightly larger than those in the next deeper layer cause water to `hang' (unsaturated flow). If this water is withdrawn by plants or as vapour, any suspended clay is left behind and the difference in size of pores is accentuated. Thus, once the process has begun, clay continues to accumulate in the same place. The process may take place at several depths. Once the combined thickness of clay lamellae exceeds 15 cm within 100 cm from the surface, the qualifier `lamellic' applies. The effect of lamellae on the water-holding capacity of the soil can be significant because water hangs on each lamella.
Biological homogenization may counteract the transport of metal-humus complexes or suspended clay in loamy sands that are relatively rich and deep. In this case homogeneous, brown or reddish profiles develop; many with an orange-red colour under the ochric surface horizon, indicative of thin (<10-5 m) iron coatings on the sand grains.
Arenosols in the humid tropics are either young soils in coarsely textured alluvial, lacustrine or aeolian deposits, or they are very old soils in residual acid rock weathering that lost all primary minerals other than (coarse grained) quartz in the course of an impressive pedogenetic history.
The young Arenosols of beach ridges and coastal plains, are azonal soils; they merely have a thin brown ochric surface horizon over a deep subsoil that may have gleyic properties and/or show signs of beginning horizon differentiation that are taxonomically insignificant.
The old (Albic) Arenosols constitute the deep, bleached surface soils of Giant Podzols whose albic horizon extends downward to a depth below 100 cm from the surface (`Hyperalbic'). If the underlying spodic horizon starts within 200 cm, the soil is classified as a Podzol but where the spodic horizon starts deeper (beyond the taxonomic control section) the soil is back among the Arenosols. These Arenosols are zonal soils; they result from intense and prolonged dissociation of weatherable minerals and translocation of the weathering products.
Arenosols in the arid zone have a beginning A-horizon with weak single grain or crumb structure over a massive C-horizon. Arenosols in the temperate zone have better developed but still ochric surface horizons over a substratum that may have thin iron coatings throughout, or contain lamellae of illuviated humus, clay or iron compounds that are too thin, too few or contain too little humus to qualify as a diagnostic horizon. Young tropical Arenosols are morphologically not very different from those in temperate regions. Old tropical Arenosols under forest on residual quartzitic rock weathering or sandy deposits have a dark brown O-horizon over a shallow greyish brown mineral surface horizon that tops a deep, grey to white, coarse sandy eluvial horizon (e.g. "Giant Podzols"). A shallow mini-Podzol may form in the A-horizon; it remains intact because biological activity is virtually absent.
Coarsely textured soils hold a much greater proportion of their `available' water at low suctions than finer soils. Since most of the pores are relatively large, much of the retained moisture is lost at a soil suction of only 100 kPa. Depending on the grain size distribution and organic matter content, the `Available Water (storage) Capacity' (AWC) may be as low as 3 to 4 percent or as high as 15 to 17 percent.
Arenosols are permeable to water; saturated hydraulic conductivity varies with the packing density of the sand and can assume any value between 300 and 30,000 cm/day. Infiltration of water in sandy soils varies between 2.5 and 25 cm/hour and may be 250 times faster than in clay soils (0.01 - 0.1 cm/hr). Note that under unsaturated flow conditions water moves more slowly in sandy soils than in clayey soils on account of their lower moisture content and lower unsaturated hydraulic conductivity. Understanding these relations is important for proper irrigation and drainage practices.
The principal minerals found in the sand and silt fractions of Arenosols are quartz and feldspars and, to a lesser extent, micas, ferromagnesian minerals (pyroxenes, amphiboles, olivines) and `heavy' minerals (zircon, garnet, tourmaline, ilmenite, magnetite, rutile, etc). The nature of the clay fraction is conditioned by weathering conditions and parent rock. Aggregates of certain clay minerals (e.g. vermiculite, chlorite and kaolin) may be large enough to belong to the sand or silt fraction of the soil.
Arenosols have relatively high bulk density values that are typically between 1.5 and 1.7 kg dm-3; somewhat lower or higher values are not uncommon. With the specific gravity of quartz close to 2.65 g dm-3, the calculated total porosity of Arenosols amounts to 36 to 46 volume-percent, less than that of most finely textured soils. Arenosols have a high proportion of large pores that account for their good aeration, rapid drainage and low moisture holding capacity.
Most sands and loamy sands are non-coherent, `single grain' materials, especially in the absence of organic matter or other cementing agents. Arenosols are predominantly `structureless'; they are `non-sticky' and `non-plastic' when wet and `loose' when dry. A cemented or indurated layer may occur at some depth.
Static loads produce very little compaction of Arenosols but vibration does; fine sand in a loose state and saturated with water is a very unstable material, especially in embankments.
Arenosols occur in vastly different environments and possibilities to use them for agriculture vary accordingly. All Arenosols have a coarse texture, accountable for the generally high permeability and low water and nutrient storage capacity. Arenosols are further marked by ease of cultivation, rooting and harvesting of root and tuber crops.
Arenosols in the humid tropics are best left under their natural vegetation, particularly so the deeply weathered Albic Arenosols. As nutrient elements are all concentrated in the biomass and in the top 20 cm of the soil, removal of the vegetation inevitably results in infertile badlands without ecological or economic value. Under forest, the land can still produce some timber (e.g. Agathis spp.) and wood for the pulp and paper industry. Permanent cultivation of annual crops would require management inputs that are usually not economically justifiable. In places, Arenosols have been planted to perennial crops such as rubber and pepper; coastal sands are widely planted to estate crops such as coconut, cashew, casuarina and pine, especially where good quality groundwater is within reach of the root system. Root and tuber crops benefit from the ease of harvesting, notably cassava, with its tolerance of low nutrient levels. Groundnut and bambara groundnut can be found on the better soils.
Major landforms in landscapes with smectites
Soil materials whose properties are dominated by an abundance of expanding 2:1 lattice clays are associated with specific soils that show signs of seasonal swelling (wet) and shrinking (dry). Such soils can occur in many landscape elements. They are particularly extensive in:
Sedimentary lowlands with expanding `smectitic' clays cover large areas, e.g. along the southern border of the Sahara desert where lakes and floodplains were abundant between 12,000 and 8,000 years BP, when the climate was more humid than at present. The Saharan lakes (notably Lake Chad), the inland delta of the river Niger and the alluvial plains of the Nile were much larger then than at present. The level of Lake Chad was until 40 metres higher than today and the lake had the size of the present Caspian Sea. Most rivers in the Sahelian zone, even those that are intermittent `wadis' in our time, flowed continuously in meandering channels and deposited finely textured sediments.
Much of what is known about the former expansion of the Saharan lakes was revealed by palynological studies of diatoms and pollen contained in lacustrine sediments. It is believed that large parts of the present Sahara desert were once colonised by savannah vegetation. Rock drawings in the area suggest that ostriches, rhinoceroses, crocodiles and giraffes once lived there. The (then) more humid climate is attributed to southward penetration of polar air masses when subtropical high-pressure cells were weaker than at present. Later in the Holocene, notably after 5000 years BP, the climate became drier again; lake levels dropped and rivers became intermittent. Under this regime of alternating dry and wet spells, Vertisols could form in the alluvial deposits.
In North America, large ice-dammed lakes (e.g. Lake Agassiz in central Canada and Lake Bonneville in Utah, USA) were formed during de-glaciation of the `Laurentide' ice sheet (14,000 - 9000 years BP). Smectitic clays accumulated in the more central parts of these lakes. The North American landmass experienced alternating wetter and drier periods during the Holocene. Soils with `vertic properties' are found in low landscape positions in regions that are currently semi-arid.
Vertisols in marine clays can be found in coastal zones with active crustal uplift, e.g. on plateaux (coastal terraces) that once were lagoon areas. This is particularly common along the western (pacific) coast of Central America.
Denudation plains with smectitic clays occur in the same (semi-arid) climate zone but are restricted to areas where the parent rock is rich in Ca, Mg and Na. Most denudation plains are underlain by basic volcanic rock such as the flood basalts of the Deccan Traps in India, or by basic basement rocks, e.g. amphibolites and greenschists. Vertisol formation is especially plausible where shallow groundwater held `bases' (Ca, Mg and Na) in solution and neo-formation of smectites could occur, e.g. in plains and on extensive, poorly drained plateaux.
Poorly consolidated clays, marl or shale have become exposed at the surface in many landscapes with actively incising rivers, usually in uplifting settings. In contrast with sedimentary lowlands and denudation plains, there is no direct relationship between current conditions and the environment in which the smectitic clays were deposited. The clays originate from a marine environment or were once incorporated in limestone or marl. Uplift and renewed denudation of the landscape brought the strata to the surface again. After limestone or marl became exposed to chemical weathering, the clastic residues were transported to lower positions in the landscape. Local hydrological conditions determine whether can Vertisols form or not. If the clay accumulates in wet depressions, Vertisols can form provided that there is a dry season that is long and dry enough for the clay to shrink and crack and develop vertic properties in a subsequent wet spell. If the clay is on well-drained slopes, its swell and shrink may induce mass wasting such as landslides and slumps.
Vertisols are churning heavy clay soils with a high proportion of swelling 2:1 lattice clays. These soils form deep wide cracks from the surface downward when they dry out, which happens in most years. The name Vertisols (from L. vertere, to turn) refers to the constant internal turnover of soil material. Some of the many local names became internationally known, e.g. `black cotton soils' (USA), `regur' (India), `vlei soils' (South Africa), `margalites (Indonesia), and `gilgai' (Australia).
Definition of Vertisols#
Common soil units:
Thionic*, Salic*, Natric*, Gypsic*, Duric*, Calcic*, Alic*, Gypsiric*, Grumic*, Mazic*, Mesotrophic*, Hyposodic*, Eutric*, Pellic*, Chromic*, Haplic*.
# See Annex 1 for key to all Reference Soil Groups
@ Diagnostic horizon, property or material; see Annex 2 for full definition.
* Qualifiers for naming soil units; see Annex 3 for full definition.
Connotation: churning heavy clay soils; from L. vertere, to turn.
Parent material: sediments that contain a high proportion of smectitic clay, or products of rock weathering that have the characteristics of smectitic clay.
Environment: depressions and level to undulating areas, mainly in tropical, semi-arid to (sub)humid and Mediterranean climates with an alternation of distinct wet and dry seasons. The climax vegetation is savanna, natural grassland and/or woodland.
Profile development: A(B)C-profiles. Alternate swelling and shrinking of expanding clay results in deep cracks during the dry season, and formation of `slickensides' and wedge-shaped structural elements in the subsurface soil.
Use: Vertisols become very hard in the dry season and are sticky in the wet season. Tillage is difficult, except for a short period at the transition between the wet and dry seasons. Vertisols are productive soils if properly managed.
Vertisols cover 335 million hectares world-wide. An estimated 150 million hectares is potential cropland. Vertisols in the tropics cover some 200 million hectares; a quarter of this is considered to be `useful'. Most Vertisols occur in the semi-arid tropics, with an average annual rainfall sum between 500 and 1000 mm but Vertisols are also found in the wet tropics, e.g. in Trinidad where the annual rainfall sum amounts to 3000 mm. The largest Vertisol areas are on sediments that have a high content of smectitic clays or produce such clays upon post-depositional weathering (e.g. in the Sudan) and on extensive basalt plateaux (e.g. in India and Ethiopia). Vertisols are also prominent in Australia, southwestern USA (Texas), Uruguay, Paraguay and Argentina. Vertisols are typically found in lower landscape positions such as dry lake bottoms, river basins, lower river terraces and other lowlands that are periodically wet in their natural state. Depending on parent rock and environmental conditions, Vertisols occur only in bottomlands or also on contiguous lower foot slopes or, as residual soils, even on (gently) sloping hillsides. Figure 1 gives an overview of the world-wide occurrence of Vertisols.
Vertisols stand apart from other soils by having a vertic horizon, with high clay content, typical wedge-shaped or parallelepiped structural aggregates, and intersecting `slickensides'. They form deep, wide cracks upon drying. Other soils may show one or more of these properties, but not to the extent characteristic of Vertisols. Such soils form intergrades and extragrades to Vertisols and normally occur together with Vertisols. They may have cracks that are not sufficiently wide, or slickensides or wedge-shaped aggregates only, or a vertic horizon underlying a coarser textured surface layer, or they may be clayey with a beginning vertic horizon that has not yet become sufficiently deep. Most associated vertic intergrades (e.g. Vertic Calcisols, Luvisols, Cambisols) occur in higher landscape positions than Vertisols, e.g. on gently sloping or moderately steep plateaux, on mesas and on pediment surfaces. Figure 2 presents a Vertisol landscape with associated soils.
In the same topographic position, Vertisols on the arid side of the climatic spectrum grade into soils with accumulated soluble compounds (Calcisols, Gypsisols, Solonchaks), a consequence of the high evaporation surplus. On the humid side, intergrades to Vertisols have stronger accumulation of organic matter because of more luxuriant vegetation (e.g. Phaeozems and Chernozems). Toposequences with Nitisols and/or Luvisols (on slopes) and Vertisols/Planosols (in low-lying positions) are common in tropical and subtropical regions with basic rocks. Areas with sodium-rich parent materials may develop combinations of Vertisols and Solonetz, with the latter in a transitional position between upland soils (often Luvisols) and Vertisols. In river areas, depositional patterns play a role in the lateral linkages with other soils. Vertisols in backswamps are commonly associated with Solonetz and/or Planosols in more elevated positions, and with Fluvisols, Gleysols (and even Histosols) in central backswamp areas. Vertisols in marine deposition areas may occur alongside Solonchaks.
The environmental conditions that lead to the formation of a vertic soil structure are also conducive to the formation of suitable parent materials.
The formation of Vertisol parent materials and Vertisol profiles becomes evident if one examines `red-black' soil catenas2, as abundant in Africa, on the Indian subcontinent and in Australia (Blokhuis, 1982). The typical configuration features red soils (Luvisols) on crest and upper slope, shallow or moderately deep red soils (Leptosols and Cambisols) on steeper sections of the slope, and black Vertisols in lower positions.
Smectite is the first secondary mineral to form upon rock weathering in the semi-arid to sub-humid tropics. Smectitic clay retains most of the ions, notably Ca2+ and Mg2+, liberated from weathering primary silicates. Iron, present as Fe2+ in primary minerals, is preserved in the smectite crystal lattice as Fe3+. The smectites become unstable as weathering proceeds and basic cations and silica are removed by leaching. Fe3+-compounds however remain in the soil, lending it a reddish colour; aluminium is retained in kaolinite and Al-oxides. Leached soil components accumulate at poorly drained, lower terrain positions where they precipitate and form new smectitic clays that remain stable as long as the pH is above neutral.
There are more reasons why there is a relative dominance of smectite in the lower members of the catena:
The combined processes of rock weathering, breakdown of primary minerals and formation of secondary minerals, and transport of soil components produce the typical catenary differentiation with reddish, well-drained soils on higher positions, and black, poorly drained soils in depressions (see Table 1).
Colour differences between Vertisols are often indicative of differences in drainage status. The more reddish hue or stronger chroma of relatively better-drained Vertisols reflects higher contents of free iron-oxides. Poorly drained Vertisols are low in kaolinite and have less free ferric iron; their hues are less red and their chromas are weaker.
Some analytical data of the highest (Luvisol) and the lowest member (Vertisol)
of a 'red-black' soil catena in the Sudan
The formation of characteristic structural aggregates (`vertic structure') is the principal genetic process in Vertisols. This typical structure may occur in most of the solum but has its strongest expression in the `vertic horizon'; the grade of development and the sizes of peds change only gradually with depth. In the following, the processes at work will be explained for a level plain with (smectitic) clayey sediments and a semi-arid tropical climate with a distinct rainy season.
Picture this: the clay plain is flooded at the end of the rainy season, but most of the standing water evaporates eventually. When the saturated surface soil starts to dry out, shrinkage of the clayey topsoil is initially one-dimensional and the soil surface subsides without cracking. Upon further drying, the soil loses its plasticity and tension builds up until the tensile strength of the soil material is locally exceeded and the soil cracks. Cracks are formed in a pattern that becomes finer as desiccation proceeds. In most Vertisols, the surface soil turns into a `surface mulch' with a granular or crumb structure. Vertisols, which develop surface mulch, are called `self-mulching'. See Figure 3.
Granules or crumbs of the mulch fall into cracks. Upon re-wetting, part of the space that the soil requires for its increased volume is occupied by mulch material. Continued water uptake generates pressures that result in shearing: the sliding of soil masses against each other.
Shearing occurs as soon as the `shear stress' that acts upon a given volume of soil exceeds its `shear strength'. The swelling pressure acts in all directions. Mass movement along oblique planes at an angle of 20 to 30 degrees with the horizontal plane resolves this pressure
The shear planes are known as `slickensides', polished surfaces that are grooved in the direction of shear. Such ped surfaces are known as `pressure faces'. Intersecting shear planes define wedge-shaped angular blocky peds. Although the structure conforms to the definition of an angular blocky structure, the specific shape of the peds has prompted authors to coin special names such as `lentils', `wedge-shaped peds', `tilted wedges', `parallelepipeds' and `bicuneate peds'. The type of structure is also called `lenticular' or `bicuneate' but shall be referred to as `vertic' in this text. See also Figure 4.
The size of the peds increases with depth. In uniform soil material this is attributable to:
The characteristic vertic horizon extends from some 15 or 20 cm below the surface mulch down to the transition from solum to substratum, i.e. just below the depth of cracking. Where there are no seasonal moisture changes in the substratum, the vertic structure is fossil. Vertisols with very deep, fossil, vertic horizons are common where sedimentation has alternated with periods of geogenetic standstill.
Sliding of crumb surface soil into cracks and the resultant shearing have important consequences:
Note that not all Vertisols develop a surface mulch; some develop a hard surface crust. Cracks in such soils are sharp-edged, remain open throughout the dry season, and little surface soil falls into them. Swelling pressures will still build up because of differential wetting between adjoining parts of soil. Therefore these soils do have a vertic structure but the grade of the structure is weaker than in self-mulching Vertisols.
Crusty Vertisols are but one example of the variation in structure formation among Vertisols. Fine peds or, alternatively, cracks at close intervals, are generally formed in soil materials that have low tensile and shear strengths, whereas large peds (cracks at wider intervals) are formed in soil materials with high tensile and shear strengths. Vertisols that are rich in sodium have greater tensile and shear strengths than soils with lower sodium saturation; many of such soils have a surface crust rather than a mulch. If the exchangeable sodium percentage (ESP) is low and there is much finely divided lime, surface mulching is maximal and peds are fine. The processes that lead to a vertic structure become stronger with increasing clay content and with a higher proportion of swelling clay minerals. Sandy Vertisols have limited swell and shrink; they develop narrow cracks and a surface crust.
A typical self-mulching Vertisol has an uneven surface topography: the edges of crack-bounded soil prisms crumble, whereas the centres are pushed upward. The scale of this surface irregularity is that of the cracking pattern, usually a few decimeters. `Gilgai' however represents micro-relief at a larger scale, superimposed on this unevenness. Gilgai on level terrain consists of small mounds in a continuous pattern of small depressions, or depressions surrounded by a continuous network of narrow ridges.
Several hypotheses have been put forward to explain the gilgai micro-relief. These have in common that they relate gilgai to mass movement in swell/shrink soils. Gilgai is sometimes seen as the result of sloughing of surface mulch into cracks and upward thrust of soil between cracks upon subsurface soil swelling. However, gilgai is clearly superimposed over the cracking pattern; it originates in the subsurface soil and substratum. For gilgai to form, the soil must have sufficient cohesion to transfer pressures all the way to the soil surface.
There are two observations to support a subsurface origin of gilgai micro-relief:
The commonest form of gilgai is the `normal' or `round' gilgai. On slightly sloping terrain (0.5 to 2 percent slope) `wavy' or `linear' gilgai occurs; `lattice' gilgai is a transitional form on very slight slopes. Wavy gilgai consists of parallel micro-ridges and micro-valleys that run with the slope, i.e. at right angles to the contours. The wavelength (from centre of mound to centre of depression) is between 2 and 8 m in most gilgais; the vertical interval or `amplitudo' is normally between 15 and 50 cm. Figure 6 presents some common forms of gilgai.
Most gilgaied areas have Vertisols, but not all Vertisols develop a gilgai micro-relief. In the Sudan, Vertisols occur in a more or less continuous clay plain over a distance of some 700 km from north to south. The annual rainfall sum increases in that direction from 150 to 1000 mm. Gilgai micro-relief occurs only in the 500-1000 mm rainfall zone. Gilgaied Vertisols in the south have thinner and less clearly expressed surface mulch and are less calcareous than non-gilgaied Vertisols in the north.
The morphology of gilgaied Vertisols differs between mound and depression areas. The A-horizon is thin on mounds whereas profiles in depression areas have a deeper (thickened) and usually darker A-horizon. Coarse components of substratum material that reach the soil surface at the mound site, e.g. quartz gravel and carbonate concretions, remain at the surface whereas finer soil material is washed down to the depressions.
Note: `High gilgais', with wavelengths up to 120 m and amplitudes of up to 240 cm, occur in Australia. These high gilgais may well have formed in an entirely different way.
Vertisols have A(B)C-profiles; the A-horizon comprises both the surface mulch (or crust) and the underlying structured horizon that changes only gradually with depth. The subsurface soil with its distinct vertic structure conforms to the definition of a vertic horizon but it is not always clear where the A-horizon ends and the B-horizon begins. Important morphological characteristics such as soil colour, texture, element composition, etc are all uniform throughout the solum. There is hardly any movement of soluble or colloidal soil components. (If such transport occurs, pedoturbation counteracts it.) A calcic horizon or a concentration of soft powdery lime may be present in or below the vertic horizon. Gypsum can occur as well, either uniformly distributed over the matrix or in nests of gypsum crystals.
Vertisols with strong pedoturbation have a uniform particle size distribution throughout the solum but texture may change sharply where the substratum is reached. Dry Vertisols have a very hard consistence; wet Vertisols are (very) plastic and sticky. It is generally true that Vertisols are friable only over a narrow moisture range but their physical properties are greatly influenced by soluble salts and/or adsorbed sodium.
Infiltration of water in dry (cracked) Vertisols with surface mulch or a fine tilth is initially rapid. However, once the surface soil is thoroughly wetted and cracks have closed, the rate of water infiltration becomes almost zero. (The very process of swell/shrink implies that pores are discontinuous and non-permanent.) If, at this stage, the rains continue (or irrigation is prolonged), Vertisols flood readily. The highest infiltration rates are measured on Vertisols that have a considerable shrink/swell capacity, but maintain a relatively fine class of structure. Not only the cracks transmit water from the (first) rains but also the open spaces between slickensided ped surfaces that developed as the peds shrunk.
Data on the water holding capacity of Vertisols vary widely, which may be attributed to the complex pore space dynamics. Water is adsobed at the clay surfaces and retained between crystal lattice layers. By and large, Vertisols are soils with good water holding properties. However, a large proportion of all water in Vertisols, and notably the water held between the basic crystal units, is not available to plants. Investigations in the Sudan Gezira have shown that the soil moisture content midway between large cracks changes very little, if at all, when the clay plain is flooded for several days or even several weeks. The soil's moisture content decreases gradually from more than 50 percent in the upper 20 cm layer to 30 percent at 50 cm depth. Deeper than 100 cm, the soil moisture content remains almost invariant throughout the year.
Most Vertisols have a high cation exchange capacity (CEC) and a high base saturation percentage (BS). The soil reaction varies from weakly acid to weakly alkaline; pH-values are in the range 6.0 to 8.0. Higher pH values (8.0-9.5) were measured on Vertisols with much exchangeable sodium. The CEC of the soil material (in 1 M NH4OAc at pH 7.0) is commonly between 30 and 80 cmol(+)/kg of dry soil; the CEC of the clay is of the order of 50 to 100 cmol(+)/kg clay. The base saturation percentage is greater than 50 and often close to 100 percent with Ca2+ and Mg2+ occupying more than 90 percent of the exchange sites; the Ca/Mg-ratio is normally between 3 and 1.
Salic and Natric Vertisols are common in the more arid parts of the Vertisol coverage. In places, sodicity occurs also in higher-rainfall areas, e.g. in depressions without outlet. The effect of sodicity on the physical properties of Vertisols is still a subject of debate. As stated earlier, Na-clays have greater tensile and shear strengths than Ca-clays, and a high exchangeable sodium percentage (ESP) is associated with soil structure of a relatively coarse class.
The effect that a high ESP has on the diffuse double layer (wide double layer, hence low structure stability) is offset by the high ionic strength of the soil solution in Vertisols that are both saline and sodic. Clay dispersion accompanied by clay movement, the normal consequence of high sodium saturation in clay soils, cannot take place on account of the low hydraulic conductivity and low volume of soil that ever becomes saturated with water. Salinity in Vertisols may be inherited from the parent material or may be caused by irrigation. Leaching of excess salt is hardly possible. It is, however, possible to flush salts that have precipitated on the walls of cracks. Surface leaching of salts from rice paddies in India was achieved by evacuating the standing water at regular intervals. There are strong indications that the fallow year observed in rotations in the Gezira/Manaqil irrigation scheme in Sudan, is indispensable for maintaining a low salinity level in the surface soil.
Large areas of Vertisols in the semi-arid tropics are still unused or are used only for extensive grazing, wood chopping, charcoal burning and the like. These soils form a considerable agricultural potential but adapted management is a precondition for sustained production. The comparatively good chemical fertility and their occurrence in extensive level plains where reclamation and mechanical cultivation can be envisaged are assets of Vertisols. Their physical soil characteristics and notably their difficult water management cause problems.
The agricultural use of Vertisols ranges from very extensive (grazing, collection of fire wood, charcoal burning) through smallholder post-rainy season crop production (millet, sorghum, cotton, chick peas) to small-scale (rice) and large-scale irrigated agriculture (cotton, wheat, barley, sorghum, chickpeas, flax, noug (Guzotia Abessynica) and sugar cane). Cotton is known to perform well on Vertisols allegedly because cotton has a vertical root system that is not severely damaged by cracking of the soil. Tree crops are generally less successful because tree roots find it difficult to establish themselves in the subsoil and are damaged as the soil shrinks and swells. Management practices for crop production ought to be primarily directed at water control in combination with conservation or improvement of the soil's fertility level.
The physical properties and the soil moisture regime of Vertisols represent serious management constraints. The heavy soil texture and domination of expanding clay minerals result in a narrow soil moisture range between moisture stress and water excess. Tillage is hindered by stickiness when the soil is wet and by hardness when it is dry. The susceptibility of Vertisols to waterlogging is the single most important factor that reduces the actual growing period (below estimates based on climatic data). Excess water during the rainy season must be stored for post-rainy season use (`water harvesting') on Vertisols with very slow infiltration rates.
Several management practises have been devised to improve the water regime:
Vertisols are considered to be among the most fertile soils of the seasonally dry tropics. The soils are rich in bases, with calcium and magnesium prevailing on the exchange complex. Many traditional farming systems observed a fallow period of 1 - 4 years in which Vertisols could restore the organic matter content of the surface soil after a period of intensive use. Increased population pressure has now reduced the proportion of fallow land (read: the fallow period) and many areas are left in fallow only when completely degraded. Trials have shown that continuous cropping can be sustainable provided that soil and water conservation and fertiliser management are adequate.
Many Vertisols are deficient in nitrogen, in line with their low organic matter content. Nitrogen fertilisers have to be applied in such a way that excessive volatilisation of ammoniacal nitrogen or leaching of nitrate ions are avoided. Placement of nitrate fertiliser in the root zone is best in dry regions whereas split banded application is preferred in wet conditions. If nitrogen is supplied in the ammonium form, the exchange complex of Vertisols, which curbs (leaching) losses, retains it. Many Vertisols have a low content of available phosphorus. In the East-African highlands, Vertisols on weathered basalt showed little response to application of phosphate under low-intensity farming but phosphorus became strongly limiting if farming was intensified (and yields went up). Acidic Alic Vertisols and Chromic Vertisols may contain much exchangeable aluminium and are notorious for inactivating fertiliser phosphate. In places Vertisols are low on sulphur and/or zinc.
It is generally believed that application of animal manure would improve soil organic matter and soil physical properties but trials remained largely inconclusive. Crop residues should be returned to the land but are used instead as animal feed, fuel and building materials. Trials with green manure (legumes) showed a remarkable increase of the yields of cereals and increased efficiency of mineral fertiliser uptake. Combining broad beds and furrows with application of phosphorus fertiliser and inter-cropping of cereals and legumes takes full benefit of crop-livestock interactions. The legumes overgrow the cereal stover after harvest (Jutzi et al., 1987; Gryseels, 1988).
1 a crack is a separation between gross polyhedrons. If the surface soil is strongly self-mulching (`grumic'*), or if the soil is cultivated while cracks are open, the cracks may be filled with granular material from the surface but they remain `open' in the sense that polyhedrons are separated. Vertisols develop cracks from the soil surface downward at some period in most years unless the soil is irrigated.
2 a catena is a succession of soils developed from the same parent material and extending from a high position in the landscape to a low position.