Set #6
Mineral Soils conditioned by a Wet (Sub)Tropical Climate
Major landforms in the (sub-)humid
tropics
Plinthosols
Ferralsols
Alisols
Nitisols
Acrisols
Lixisols
Major landforms
in the (sub-)humid tropics
Large parts of the humid and sub-humid tropics belong to one of three morphostructural units:
Landforms in high mountain areas were discussed in an earlier chapter as were the alluvial lowlands. The present chapter discusses common landforms on Precambrian shields and in the lower ranges of Alpine fold belts (below 3000 meters) in the humid and seasonally dry tropics.
Precambrian shields, or `cratons', constitute the oldest cores of continents; they are remnants of mountains that formed more than 600 million years ago and that have since eroded to undulating plains that rise up to only a few hundred meters above the present sea level. The lithospheric plates on which the shields rest move over the Earth's surface at a rate of several centimetres per year. In places, this movement produces weak stretches that become subject to rifting and subsidence. Such locations are preferential sites for formation of sedimentary river basins (e.g. the Amazon basin) or for deposition of rocks (e.g. during the Mesozoic in South Sweden).
The Precambrian era spans 80 percent of the geological history of the Earth and includes many periods of mountain building, erosion and sedimentation. Igneous, sedimentary and metamorphic rocks of Precambrian age exist in great variety but crystalline (plutonic and metamorphic) rocks predominate.
By and large, Precambrian formations belong to one or more the following:
Tropical shield areas were mainly modified by chemical weathering and by fluvial and marine processes (glacial, periglacial and aeolian processes were insignificant in the recent past). How water could shape the surface in tropical shield areas is largely explained by
In areas under rain forest, most precipitation is intercepted by the canopy from where it trickles down to the forest floor and infiltrates into the soil. There, it promotes rapid chemical weathering of rocks because its low ionic strength and comparatively high temperature promote hydrolysation processes. The `saprolite' (i.e. `rotten rock') under a rain forest may extend down to a depth of tens or even hundreds of metres. The saprolite is usually less thick on granite (say, 10-20 metres) than on metamorphic rock (40-70 metres; data from Surinam). Long periods of strong chemical weathering and little, if any, surface runoff have gradually deepened the weathering front, a process known as `etching'. The saprolite is normally clayey because feldspars and ferromagnesian minerals have weathered to clay minerals and (sesqui-)oxides. The sand content of the saprolite reflects the content of coarse quartz in the original parent rock. Thoroughly weathered saprolites are chemically very poor, despite their lush (rain forest) vegetation cover.
Figure 1
Development of summit levels by differential etching and
stripping.
(Kroonenberg and Melitz, 1983)
Note that the vegetation is less densely spaced in arid areas because each individual plant needs a larger volume of soil for its water supply. A single downpour on such open land can cause torrential `sheet floods'. In the intermediate situation, i.e. in semi-arid savannah and prairie areas, surface runoff and denudation are particularly severe. On sites with sparse vegetation and distinct surface relief, the rate of topsoil erosion may well exceed the rate of weathering. This results in `stripping' of the land; etching is more common under protective vegetation types, e.g. under rain forest.
Figure 1 explains how summit levels were formed after prolonged, differential etching and stripping. Note that the balance between etching and stripping was almost certainly different from the present situation during long periods in the past.
Many shield areas in tropical regions include vast, dissected etch-plains with solitary elevated remnants that are either bare, dome-shaped granite hills (`inselbergs') such as the `sugar loaf' of Rio de Janeiro, or heaps of huge granite boulders known as `tors'. The etch-plains consist of deep, flat or undulating, residual weathering crust dissected by a network of V-shaped valleys that are only a few metres deep. Where the natural drainage pattern is widely spaced, remnants of the original flat surface may still be in place but only rounded, convex hills remain in areas with more densely spaced gullies. Note that natural drainage patterns are normally conditioned by underlying bedrock: low ridges and depressions form upon differential etching and stripping of weathering-resistant rocks.
The occurrence of isolated inselbergs and tors amidst vast expanses of undulating lowland is more common in savannah regions than in areas with rain forest. Valleys in savannah regions (called `dambos' or `vleis' in large parts of Africa) tend to be broad and shallow as a result of colluviation and slope wash. The widespread occurrence of `laterite plateaus' is an indication of climate fluctuations in the past. Strongly weathered saprolite with quartz-rich clays (`plinthite') formed during humid eras. In the (now) semi-arid tropics, much plinthite has subsequently hardened to `ironstone'. Many plateaux are weathering residues protected by an ironstone cap. In places they formed through relief inversion of iron-cemented valley fills.
High mountain areas in tropical regions became glaciated in the Pleistocene but tracts lower than 3,000 meters above the present mean sea level were never reached by descending valley glaciers. In lower mountain areas, the relation between rainfall and land surface transformation is similar to that in shield areas. Rain forest is the preponderant vegetation type and infiltration water reaches great depths. Weathering is rapid and fresh rock is difficult to find, even in deeply dissected terrain. The lower foot slopes of the Andes and Himalayas and uplands in Africa present numerous examples. The dominant geomorphic controls in humid tropical mountain areas are:
Landslides and mudflows have shaped many slope sites in the humid and seasonally dry tropics. These phenomena were triggered by torrential rainfall that saturated the weathering crust with water. Often, seismic events such as earthquakes gave the final stimulus for sliding. Shallow landslides are common in forested mountain areas, e.g. in New Guinea, Sulawesi, Hawaii or the Andes; a provisional chronology can often be established by simply considering degrees of forest regeneration. Note that, contrary to common belief, forest vegetation cannot prevent landslides from happening because the sliding landmass detaches itself at the `weathering front', i.e. the contact plane between saprolite and fresh rock that is beyond the reach of tree roots.
It is generally true that regions with crystalline rocks have symmetrical hills with sharp crests and rectilinear slopes, separated by steep V-shaped valleys. Joints and faults in the underlying rocks determine the drainage pattern.
Weathering mantles tend to be less deep over siliceous sedimentary rocks than over crystalline rocks. The alternation of resistant and less resistant strata is the main controlling factor in folded sedimentary rocks. Nice examples can be seen in areas with alternating limestone and sandstone ridges as extend from India through Burma, Thailand and Laos all the way to Vietnam. Humid tropical areas with calcareous rocks may show abundant `karst' phenomena such as sink holes and caves formed upon dissolution of limestone. `Tower karst' with residual limestone rocks standing in the landscape as towers (e.g. in Guilin, China) formed upon advanced dissolution of limestone. Similarly convincing are the `cockpit' or `mogote' hills at Bohol (Philippines) and the razor-sharp limestone ridges of the `broken-bottle country' of New Guinea. Note that such extreme karstic landscapes can only develop in uplifting areas.
The advanced weathering of rocks in the (sub-)humid tropics produced `typical' tropical soils: red or yellow in colour and strongly leached. Additional features: they are deep, finely textured, contain no more than traces of weatherable minerals, have low-activity clays, less than 5 percent recognisable rock structure and gradual soil boundaries. Differences between soils in the (sub-)humid tropics can often be attributed to differences in lithology and/or (past) moisture regime.
The Reference Soil Group of the Plinthosols holds soils that contain `plinthite', i.e. an iron-rich, humus-poor mixture of kaolinitic clay with quartz and other constituents that changes irreversibly to a hardpan or to irregular aggregates on exposure to repeated wetting and drying. Internationally, these soils are known as `Groundwater Laterite Soils', `Lateritas Hydromorficas' (Brazil), `Sols gris latéritiques' (France), `Plinthaquox' (USA, Soil Taxonomy) or as Plinthosols (FAO).
Definition of Plinthosols#
Soils having
a petroplinthic@ horizon starting within 50 cm from the soil surface, or
a plinthic@ horizon starting within 50 cm from the soil surface, or
a plinthic@ horizon starting within 100 cm from the soil surface underlying either an albic@ horizon or a horizon with stagnic@ properties.
Common soil units:
Petric*, Endoduric*, Alic*, Acric*, Umbric*, Geric*, Stagnic*, Abruptic*, Pachic*, Glossic*, Humic*, Albic*, Ferric*, Skeletic*, Vetic*, Alumic*, Endoeutric*, 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 `plinthite'; from Gr, plinthos, brick.
Parent material: plinthite is more common in weathering material from basic rocks than from acidic rocks. In any case it is crucial that sufficient iron is present, originating either from the parent material itself or brought in by seepage water from elsewhere.
Environment: formation of plinthite is associated with level to gently sloping areas with fluctuating groundwater. `Petroplinthic' soils with continuous, hard `ironstone' form where plinthite becomes exposed to the surface, e.g. on erosion surfaces that are above the present drainage base. `Skeletic' soil units having a layer of hardened plinthite concretions occur mostly in colluvial or alluvial deposits. Soft plinthite is associated with rain forest areas; petroplinthic and skeletic soils are more common in the savannah zone.
Profile development: mostly A(E)BC-profiles with segregation of plinthite at the depth of groundwater fluctuation. Hardening of plinthite to petroplinthite takes place upon repeated drying and wetting, commonly after geological uplift of the terrain and/or climate change towards drier conditions.
Use: mostly low volume grazing. Arable cropping is hindered by poor rooting conditions associated with frequent water logging and/or excessive stoniness and low chemical soil fertility.
The global extent of soils with plinthite is estimated at some 60 million hectares. Soft plinthite is most common in the wet tropics, notably in the eastern Amazon basin, the central Congo basin and parts of Southeast Asia. Extensive areas of hardened plinthite occur in the Sudano-Sahelian zone where petroplinthite forms hard caps on top of uplifted/exposed landscape elements. Similar soils occur on the Indian subcontinent, and in drier parts of Southeast Asia and northern Australia. See Figure 1.
Figure 1
Plinthosols world-wide
Plinthosols occur in tropical regions with `red tropical soils' such as Ferralsols, Alisols, Acrisols and Lixisols. Soils with residual `soft' plinthite occur in less well-drained positions in the landscape; they have gleyic or stagnic properties and many of these are linked to Gleysols. Well-drained soils with abundant loose iron concretions (`pisolithes' or `pea iron') in tropical and subtropical regions are commonly formed in plinthitic material that was dislocated, hardened, transported and finally deposited as alluvial or (more commonly) colluvial soil parent material. Such soils are related to Plinthosols but may have to be classified as Plinthic soil units of other Reference Soil Groups. Petric Plinthosols in eroding areas occur together with Leptosols and/or leptic units of other soils.
Areas where the formation of plinthite is still active have a hot and humid climate with a high annual rainfall sum and a short dry season. Buchanan (1807) coined the term `laterite' for an iron-rich, humus-poor mixture of kaolinitic clay and quartz that was used as a building material in western India (Lat. `later' means `brick'). The term `plinthite' was introduced much later to evade confusion created by different interpretations of the term `laterite' and its many derivatives.
Plinthite forms in perennially moist (sub)soil layers. Formation of plinthite involves the following processes:
In its unaltered form, plinthite is firm but can be cut with a spade. If the land becomes drier, e.g. because of a change in base level and/or a change in climate, plinthite hardens irreversibly to petroplinthite. Hardening of plinthite involves the following processes:
Hardening of plinthite is often initiated by removal of the vegetation, especially forest, as this triggers erosion of the surface soil and exposure of plinthite to the open air. Hardened plinthite occurs in many tropical soils, either in a `skeletic' (concretionary) form or as continuous petroplinthite. Plinthosols with soft plinthite are indigenous to the rain forest zone. Soils with petroplinthite are especially abundant in the transition zone from rain forest to savannah, notably in dry areas that were once much wetter, e.g. in sub-Sahelian Africa; plinthite that was once at some depth hardened and became exposed as a thick ironstone cap that resists (further) erosion. This may ultimately lead to inversion of the original relief: depression areas where plinthite formed are shielded against erosion by their ironstone caps and become the highest parts of the landscape. See Figure 2.
Plinthite is red mottled clay but not all red mottled clay is plinthite. It is not always easy to distinguish between `normal' mottled clay, plinthite and ironstone gravel because they grade into each other. Field criteria for identification of plinthite are:
The most obvious distinguishing feature of plinthite is of course, that it hardens irreversibly to petroplinthite upon repeated wetting and drying but this cannot always be ascertained in the field.
Petroplinthite (also referred to as `ironstone', `laterite', `murram' or `ferricrete') can be divided on basis of its morphology into
Figure 2
Inversion of relief in an eroding landscape: hardening of exposed plinthite produces a protective shield against further erosion
Plinthite and petroplinthite have high contents of hydrated Fe- and Al-oxides (`sesquioxides'). Free iron is present as oxide minerals, notably lepidocrocite (FeOOH), goethite (FeOOH) and hematite (Fe2O3); free aluminium occurs in gibbsite (Al2O3.3H2O) and/or boehmite (Al2O3.H2O). Old ironstone crusts contain more hematite and boehmite and less sesquioxides than plinthite. Free silica is present as quartz inherited from the parent material. Easily weatherable primary minerals have disappeared; the dominant clay mineral is well-crystallized kaolinite.
Plinthosols with `soft' plinthite occur in bottomlands, in regions with a distinct annual precipitation surplus over evaporation. Percolating rainwater may cause eluviation symptoms such as an albic subsurface horizon, often under an umbric surface horizon. Plinthosols in bottomlands tend to develop gleyic or stagnic properties.
Soft plinthite is dense and obstructs deep percolation of water and penetration of plant roots. The specific density of petroplinthite ranges from 2.5 to 3.6 Mg m-3 and increases with increasing iron content. Plinthosols with continuous ironstone at shallow depth are generally unsuitable for arable uses on account of their low water storage capacity.
All Plinthosols have high contents of iron and/or aluminium, with proportions varying from more than 80 percent iron oxides with little aluminium to about 40 percent of each. Most Plinthosols have poor cation exchange properties and low base saturation but there are exceptions, e.g. Endoeutric soil units.
Plinthosols come with considerable management problems. Poor natural soil fertility, water logging in bottomlands and drought on shallow and/or skeletal Plinthosols are serious limitations. Many Plinthosols outside the wet tropics have shallow, continuous petroplinthite, which limits their rootable soil volume to the extent that arable farming is no longer possible; such land can at best be used for low volume grazing. The stoniness of many Plinthosols is an added complication. Skeletic soils, many with high contents of pisolithes (up to 80 percent) are still planted to food crops and tree crops (e.g. cocoa in West Africa, cashew in India) but the crops suffer from drought in the dry season.
Civil engineers have a different appreciation of petroplinthite and plinthite than agronomists. To them, plinthite is a valuable material for making bricks (massive petroplinthite can also be cut to building blocks); ironstone gravel can be used in foundations and as surfacing material on roads and airfields. In some instances plinthite is a valuable ore of iron, aluminium, manganese and/or titanium.
The Reference Soil Group of the Ferralsols holds the `classical', deeply weathered, red or yellow soils of the humid tropics. These soils have diffuse horizon boundaries, a clay assemblage dominated by low activity clays (mainly kaolinite) and a high content of sesquioxides. Local names usually refer to the colour of the soil. Internationally, Ferralsols are known as Oxisols (Soil Taxonomy, USA), Latosols (Brazil), Sols ferralitiques (France), Lateritic soils, Ferralitic soils (Russia) and Ferralsols (FAO).
Definition of Ferralsols#
Soils
Common soil units:
Gibbsic*, Geric*, Posic*, Histic*, Gleyic*, Andic*, Plinthic*, Mollic*, Acric*, Lixic*, Umbric*, Arenic*, Endostagnic*, Humic*, Ferric*, Vetic*, Alumic*, Hyperdystric*, Hypereutric*, Rhodic*, Xanthic*, 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: red and yellow tropical soils with a high content of sesquioxides; from L. ferrum, iron and aluminium, alum.
Parent material: strongly weathered material on old, stable geomorphic surfaces; more in weathering material from basic rock than in siliceous material.
Environment: typically in level to undulating land of Pleistocene age or older; less common on younger, easily weathering rocks. Perhumid or humid tropics; minor occurrences elsewhere are considered to be relics from past eras with a wetter climate than today.
Profile development: ABC profiles. Deep and intensive weathering has resulted in a high concentration of residual, resistant primary minerals alongside sesquioxides and well-crystallized kaolinite. This mineralogy and the low pH explain the stable microstructure (pseudo-sand) and yellowish (goethite) or reddish (hematite) soil colours.
Use: Ferralsols have good physical properties but are chemically poor. Their low natural fertility and tendency to `fix' phosphates are serious limitations. In natural systems, the limited stock of plant nutrients is in a constant process of `cycling' with most nutrients contained in the biomass. Many Ferralsols are (still) used for shifting cultivation. Liming and full fertilisation are required for sustainable sedentary agriculture.
The worldwide extent of Ferralsols is estimated at some 750 million hectares, almost exclusively in the humid tropics on the continental shields of South America (Brazil) and Africa (Zaire, southern Central African Republic, Angola, Guinea and eastern Madagascar). Outside the continental shields, Ferralsols are restricted to regions with easily weathering basic rock and a hot and humid climate, e.g. in southeast Asia. See Figure 1.
Figure 1
Ferralsols world-wide
Ferralsols tend to occupy the upper portions of stable land surfaces in the humid tropics where they occur alongside Acrisols (in lower positions or on more acidic parent rock such as gneiss) or Nitisols that evolved on top of more basic rock, e.g. dolerite. Clear zonality of Ferralsols and Acrisols exists on a continental scale. Ferralsols are dominant in (humid) Central Africa with Acrisols occurring in the sub-humid periphery of the Ferralsol area, extending into West and East Africa. In South America, Ferralsols are prevalent in the more humid eastern Amazon Basin and Acrisols in the western Amazon.
Water affects primary minerals through the processes of `hydration' and `hydrolysis'.
`Ferralitization' is hydrolysis in an advanced stage. If the soil temperature is high and percolation intense (humid climate!), all weatherable primary minerals will ultimately dissolve and be removed from the soil mass. Less soluble compounds such as iron and aluminium oxides and hydroxides and coarse quartz grains remain behind. Ferralitization (or `desilication' as it is also called) is furthered by the following conditions:
TABLE 1
Schematic occurrence of gibbsite (Al(OH)3) and kaolinite in strongly weathered soils
with various drainage conditions
Parent material |
Internal drainage very good good moderate poor |
Mafic (`basic') rock Felsic (`acidic') rock |
Gibbsite gibbsite kaolinite 2:1 clays gibbsite kaolinite kaolinite kaolinite |
Ferrihydrite (Fe(OH)3; see also the chapter on Andosols) is a common weathering product of iron-rich parent material. Hematite (Fe2O3, the mineral that gives many tropical soils their bright red colour) forms out of ferrihydrite if:
Goethite (FeOOH, more orange in colour than bright red hematite) is formed when one or more of the above conditions are not (fully) met.
Ferralsols are deep, intensely weathered soils. By and large, Ferralsols have the following characteristic features:
Ferralsols are characterized by relative accumulation of stable primary and secondary minerals; easily weathering primary minerals such as glasses and ferro-magnesian minerals and even the more resistant feldspars and micas have disappeared completely. Quartz is the main primary mineral (if originally present in the parent rock). The clay assemblage is dominated by kaolinite, goethite, hematite and gibbsite in varying amounts, in line with the kind of parent rock and the drainage conditions (see also Table 1).
Most Ferralsols are clayey (a consequence of advanced weathering) and have strong water retention at permanent wilting point while the presence of micro-aggregates reduces moisture storage at field capacity. This explains their rather limited capacity to hold `available' water (i.e. available to most crops); some 10 mm of `available' water per 10 cm soil depth is `typical'. Ferralsols are poorly equipped to supply crops with moisture during periods of drought, particularly those in elevated positions.
Stable micro-aggregates explain the excellent porosity, good permeability and favourable infiltration rates measured on Ferralsols. Soils with high contents of (positively charged) iron oxides and (negatively charged) kaolinite have stable soil structure due to bonding of opposite elements. Ferralsols with low contents of iron and/or organic matter as occur in Surinam and Brazil (Xanthic Ferralsols) have less stable structure elements, especially the sandy ones. Surface sealing and compaction become serious limitations if such soils are taken into cultivation.
The strong cohesion of (micro-)aggregates and rapid (re)flocculation of suspended particles complicate measurements of the particle size distribution of Ferralsol material. The clay content found after the removal of iron and addition of a dispersing chemical is known as the `total clay' content. The clay content found after shaking an aliquot of soil with distilled water (without removal of iron or addition of dispersion agents) is the `natural clay' content. The high degree of aggregation in ferralic subsurface horizons explains the low contents of natural clay (< 10 percent).
Ferralsols are chemically poor soils. The types and quantities of clay minerals, oxides and organic matter condition the exchange properties of soils. The total exchange capacity is composed of a permanent and a variable component:
The CEC-clay of a ferralic horizon may, by definition, not exceed 16 cmol(+)/kg clay. Note that CEC is determined in a 1M NH4OAc solution buffered to pH 7; the field-pH of Ferralsols is normally much less than 7.
The net negative charge of the exchange complex is neutralized by exchangeable bases (Na+, K+, Ca2+, Mg2+) plus `exchangeable acidity' (Al3+ + H+). The `Effective CEC' (ECEC), i.e. the sum of bases and exchangeable acidity, is thought to represent the soil's cation exchange capacity at field conditions.
Note that the ECEC of Ferralsols is much less than the CEC; actual cation adsorption is often a mere 3 or 4 cmol(+) per kg soil.
Protonation of hydroxylic groups at low pH-values may boost the soil's `Anion Exchange Capacity' (AEC) to the extent that the AEC equals or exceeds the CEC. This can be detected by comparing pH-values of two samples of the same soil, one in suspension in H2O and the other in 1M KCl. pH(KCl) is less than pH(H2O) in soils with net negative charge (the `normal' situation); the reverse is true in soils with net positive charge.
The following terminology is used in publications on the exchange properties of strongly weathered tropical soils:
Figure 2 presents a schematic outline of the exchange characteristics of strongly weathered tropical soils in relation to soil-pH.
Intense termite activity is, according to some, at least partly accountable for the typical diffuse horizon boundaries of Ferralsols. Termites destroy (remnants of) stratification/rock structure; they increase the depth of the solum and their nests, tunnels and ventilation shafts increase the permeability of the soil. As termites preferentially move fine and medium sized particles and leave coarse sand, gravel and stones behind, they are thought to contribute to `stoneline' formation. The depth of the stoneline would then indicate the depth of termite activity.
Note that stonelines may also occur where termites are absent, e.g. formed by soil creep in sloping terrain.
Figure 2
Schematic relation between exchangeable aluminium level, AEC, CEC,net surface charge and soil-pH(H2O)
Most Ferralsols have good physical properties. Great soil depth, good permeability and stable microstructure make Ferralsols less susceptible to erosion than most other intensely weathered red tropical soils. Moist Ferralsols are friable and easy to work. They are well drained but may in times be droughty because of their low water storage capacity.
The chemical fertility of Ferralsols is poor; weatherable minerals are absent and cation retention by the mineral soil fraction is weak. Under natural vegetation, nutrient elements that are taken up by the roots are eventually returned to the surface soil with falling leaves and other plant debris. The bulk of all cycling plant nutrients is contained in the biomass; `available' plant nutrients in the soil (and all living plant roots) are concentrated in the upper 10 to 50 cm soil layer. If the process of `nutrient cycling' is interrupted, e.g. after introduction of low input sedentary subsistence farming, the root zone will rapidly become depleted of plant nutrients. Maintaining soil fertility by manuring, mulching and/or adequate (i.e. long enough) fallow periods and prevention of surface soil erosion are important management requirements.
Strong retention (`fixing') of phosphorus is a problem of Ferralsols (and several other soils, e.g. Andosols). Ferralsols are normally also low in nitrogen, potassium, secondary nutrients (calcium, magnesium, sulphur) and a score of micro-nutrients. Even silica deficiency is possible if silica-demanding crops (e.g. grasses) are grown. Manganese and zinc, which are very soluble at low pH, may at some time reach toxic levels in the soil or become deficient after intense leaching of the soil.
Liming is a means to raise the pH-value of the rooted surface soil. Liming combats aluminium toxicity and raises the CEC. On the other hand, it lowers the AEC, which might lead to collapse of structure elements and slaking at the soil surface. Frequent small doses of lime or basic slag are therefore preferable over one massive application; 0.5 - 2 tons/ha of lime, or dolomite, are normally enough to supply calcium as a nutrient and to buffer the low soil-pH of Ferralsols.
Fertilizer selection and the mode/timing of fertilizer application determine to a great extent the success of agriculture on Ferralsols. Slow-release (rock) phosphate applied at a rate of several tons per hectare eliminates phosphorus deficiency for a number of years. For a quick fix, much more soluble (Double or Triple) Super Phosphate is used, needed in much smaller quantities, especially if placed in the direct vicinity of the roots.
Sedentary subsistence farmers and shifting cultivators on Ferralsols grow a variety of annual and perennial crops. Low volume grazing is also common and considerable areas of Ferralsols are not used for agriculture at all. The good physical properties of Ferralsols and the often level topography would encourage more intensive forms of land use if problems caused by the poor chemical soil properties could be overcome.
ALISOLS (AL)1
The Reference Soil Group of the Alisols consists of strongly acid soils with accumulated high activity clays in their subsoils. They occur in humid (sub-)tropical and warm temperate regions, on parent materials that contain a substantial amount of unstable Al-bearing minerals. Ongoing hydrolysis of these minerals releases aluminium, which occupies more than half of the cation exchange sites. Hence, Alisols are unproductive soils under all but acid-tolerant crops. Internationally, Alisols correlate with `Red Yellow Podzolic Soils' that have high-activity clays (Brazil), `Ultisols' with high-activity clays (USA, Soil Taxonomy) and with `Fersialsols' and `sols fersiallitiques très lessivés (France).
Definition of Alisols#
Soils having
Common soil units:
Vertic*, Gleyic*, Andic*, Plinthic*, Nitic*, Umbric*, Arenic*, Stagnic*, Abruptic*, Humic*, Albic*, Profondic*, Lamellic*, Ferric*, Skeletic*, Hyperdystric*, 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: strongly acid soils with subsurface accumulation of high activity clays that have more than 50 percent Al3+ saturation; from L. aluminium, alum.
Parent material: Alisols can form in a wide variety of parent materials having high-activity clay minerals such as vermiculite or smectite. Most occurrences of Alisols reported so far are on weathering products of basic rocks.
Environment: most common in old land surfaces with a hilly or undulating topography, in humid (sub-)tropical and monsoon climates.
Profile development: ABtC profiles. Variations among Alisols are mostly related to truncation of A-horizons in eroded lands.
Use: Alisols contain low levels of plant nutrients (except for Mg2+ in some cases) whereas soluble inorganic Al is present in toxic quantities. If liming and full fertilization is no option, use of these soils is generally restricted to crops, which accommodate with low nutrient contents and tolerate high levels of free Al. Alisols are traditionally used in shifting cultivation and for low volume production of undemanding crops. In the past decades, Alisols have increasingly been planted to Al-tolerant estate crops such as tea and rubber, and also to oil palm.
Figure 1
Alisols world-wide
Major occurrences of Alisols are found in Latin America (Ecuador, Nicaragua, Venezuela, Colombia, Peru, Brazil), in the West Indies (Jamaica, Martinique, St. Lucia), in West Africa, the highlands of Eastern Africa, Madagascar and in southeast Asia and northern Australia. See Figure 1. Driessen and Dudal (1991) tentatively estimate that about 100 million ha of these soils are used for agriculture in the tropics.
Alisols occur also in subtropical and Mediterranean regions: they are found in China, Japan and the South Eastern USA and minor occurrences have been reported from around the Mediterranean Sea (Italy, France, and Greece).
Alisols have their argic horizon in common with Acrisols, Lixisols, Luvisols and Albeluvisols. They differ from Acrisols and Lixisols because these soils lack high activity clays and from Luvisols because Luvisols lack `alic' soil properties. Alisols are less weathered than `typical red tropical soils' such as Ferralsols and Nitisols. In the landscape, Alisols can be associated with Gleysols and all the Reference Soil Groups mentioned above, except Albeluvisols.
In the humid tropics, Alisols are found on slopes where smectitic saprolithes outcrop; Acrisols or Lixisols and possibly Nitisols or Ferralsols are dominant on plateaux (West Africa, West Indies). Alisols in sloping land can also be seen alongside Cambisols (e.g. in the foothills of the Andes). Alisol-Acrisol patterns in flat and level terrain reflect the lithological composition of the dominant parent material (Amazon region, Colombia).
In tropical and subtropical regions with distinct wet and dry seasons, Alisols occur alongside Luvisols in sloping areas and together with Vertisols (Kenya, Somalia) or Gleysols (Southeast USA) in depression areas. Alisols in regions with warm, wet summers and cold, dry winters are associated with Cambisols, notably on eroding steep slopes in hilly areas (South Eastern China).
In Mediterranean areas, Alisols have been found in old river terraces; their occurrence is probably associated with wetter climate conditions in a distant past. Alisols may also occur on slopes that are exposed to frequent rain bearing winds.
Alisols form where ongoing hydrolysis of secondary high-activity clay minerals such as vermiculite and smectite releases much aluminium. In practice, Alisol formation is confined to environments where most weatherable primary minerals have disappeared and secondary high-activity clays dominate the clay complex. Where these materials outcrop, for instance in hilly topography, the secondary high-activity minerals weather under humid conditions with intense leaching of silica and alkaline and alkaline-earth cations. Alisol formation involves three distinct steps:
Figure 2 shows the relation between KCl-extractable Al and the cation exchange capacity (CEC), both expressed in cmol(+) kg-1 clay. The data were collected from ferralic and argic subsurface horizons in acid (pHKCl < 4.0) Ferralsols, Acrisols and Alisols from Indonesia, the Caribbean region, Rwanda, Cameroon, Peru and Colombia.
The correlation of KCl-extractable Al and CECclay documented by Figure 2 supports the following statements.
Weathering processes affect the clay mineralogy of soils; the mineral assemblage of Alisols appears to be in a state of transition from `bisiallitic' high-activity clays to `fermonosiallitic' material that is rich in iron oxides and kaolinite. The content of iron oxides is largely dictated by the composition of the parent rock, notably its content of primary ferromagnesium silicates. The transitional character of Alisol clay mineralogy is further evidenced by:
Al-hydroxy-interlayered 2:1 clay minerals resemble the mineralogical structure of 1:1 clay minerals in the sense that their structure is of the 1:1:1:1 type, with alternating octahedral and tetrahedral sheets. Their stability in soil environments is indeed similar to that of kaolinite.
Figure 3 illustrates also how release of octahedral Al from 2:1 layer silicates contributes to the development of alic soil properties. Figure 3 is an extended version of Figure 2; it considers additional data from strongly acid soils with smectites. These acid soils developed in weathering material of basic rock and are rich in octahedral Mg. The `red montmorillonitic soils' (from Martinique) contain much smectite clay and free iron oxides (hematite) and are red indeed: Munsell hues of 2.5 YR and 10R are common. They contain less than 12 cmol(+) KCl-extractable Al per kg clay even though the CEC-clay exceeds 55 cmol(+) kg-1. However, the relation between (Al + Mg) and CEC-clay produced the strong positive correlation of Figure 2. This suggests that Al and Mg released from weathering high-activity clay caused saturation of the exchange complex in these soils. Note that strongly acid soils with considerable Mg-saturation are not Alisols; they would rather key out as Eutric Cambisols.
Figure 2
Consistent, strong and positive correlation of KCl-extractable Al and CEC-clay is found in acid Ferralsols, Acrisols and Alisols from all over the world
Alic soil properties may have an impact on the transformation of humus. It was observed in forest areas with Alisols in Kalimantan (Indonesia) that humus readily accumulates, possibly because biological activity is retarded by Al having a toxic effect on soil organisms.
Figure 3
KCl-extractable Al and/or [KCl-extractable Al + exchangeable Mg] correlate with CECclay in tropical soils with pH-KCl values less than 4.0
Most Alisols have an ochric surface horizon but darker umbric horizons can be expected under forest. Soil structure is rather weak in the surface horizon because biological activity is hindered by the strong acidity. The surface horizon overlies a dense argic subsurface horizon that may hinder deep percolation of water. The structure of the argic horizon is clearly more stable than that of the surface soil. The expression of soil structure varies between soils with the relative contents of high-activity clays and free iron.
Secondary clay minerals dominate the mineral assemblage of Alisols. Note however that the proportions of low- and high-activity clays vary between soils or between soil horizons because the clay is in a state of transition. Weathering high-activity clays release considerable quantities of Al; at the same time the content of kaolinite increases and CEC decreases. Strong adsorption of Al3+ by high-activity clays counteracts the formation of gibbsite (Al2O3.3H2O). The content of free iron oxides varies between Alisols depending on the nature and weathering stage of the parent material. The sand fraction consists of (weathering-resistant) quartz and the silt content is small.
The physical characteristics of Alisols are directly related to the relative contents of high-activity clays, low-activity clays and iron oxides. Where swelling and shrinking clays dominate the mineral assemblage, specific physical features may develop that resemble elements of `vertic' horizons. Telltale signs are: distinct cracks, rapid bypass flow of water in dry soil and slow infiltration of water in wet soil, shining faces of structural peds, prismatic structure elements in the subsurface horizon and generally few macro-pores. Such Alisols have CEC-clay values in excess of above 50 cmol(+) kg-1. Alisols in weathering materials from basic rock tend to have more iron oxides and more stable structures, particularly in the subsurface horizon.
In many Alisols, textural differentiation between surface and subsurface horizons imparts different physical properties. Surface horizons tend to have an unstable structure (slaking!) and reduced permeability, in particular where the subsurface horizon is dense and massive as is the case in Alisols that have relatively low contents of high activity clays and iron oxides. This restricts internal soil drainage and increases the danger of erosion in sloping lands. In cropped lands, the low level of biological activity, a direct consequence of the acid and nutrient-poor environment, further enhances the adverse physical properties of the surface horizon.
Ongoing weathering of high-activity, Al-bearing clay leads to severe chemical infertility: Al and possibly Mn are present in toxic quantities whereas levels of other plant nutrients are low and unbalanced. However, the favourable cation exchange properties make some Alisols productive under intensive management with adequate liming and application of manure and fertilizers. The mineral reserves of Alisols are conditioned by the clay fraction and depend largely on the composition of high-activity clays that act as weatherable minerals in the system. In most Alisols, these reserves are low in Ca and K. Low pH and presence of large quantities of iron oxide are conducive to P-immobilization but much less than in Acrisols and Ferralsols. The organic matter content of cultivated Alisols is usually modest, in contrast with Alisols under natural forest.
Alisols occur predominantly on old land surfaces with hilly or undulating topography. The generally unstable surface soil of cultivated Alisols makes them susceptible to erosion; truncated soils are quite common. Toxic levels of aluminium at shallow depth and poor natural soil fertility are added constraints. As a consequence, many Alisols allow only cultivation of shallow-rooting crops and these suffer from drought stress during the dry season. By and large, Alisols are unproductive soils. Their use is generally restricted to acidity-tolerant crops or low volume grazing. The productivity of Alisols in subsistence agriculture is generally low as these soils have a limited capacity to recover from chemical exhaustion. If fully limed and fertilized, crops on Alisols may benefit from the considerable cation exchange capacity and rather good water holding capacity. Alisols are increasingly planted to aluminium-tolerant estate crops such tea and rubber but also to oil palm and in places to coffee and sugar cane.
The Reference Soil Group of the Nitisols accommodates deep, well-drained, red, tropical soils with diffuse horizon boundaries and a subsurface horizon with more than 30 percent clay and moderate to strong angular blocky structure elements that easily fall apart into characteristic shiny, polyhedric (`nutty') elements. Nitisols are strongly weathered soils but far more productive than most other red tropical soils. Nitisols correlate with `Terra roxa estruturada' (Brazil), kandic groups of Alfisols and Ultisols (Soil Taxonomy, USA), `Sols Fersialitiques' or `Ferrisols' (France) and with the `Red Earths'.
Definition of Nitisols#
Soils,
Common soil units:
Andic*, Ferralic*, Mollic*, Alic*, Umbric*, Humic*, Vetic*, Alumic*, Dystric*, Eutric*, Rhodic*, 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: deep, red, well-drained tropical soils with a clayey `nitic' subsurface horizon that has typical `nutty', polyhedric, blocky structure elements with shiny ped faces; from L. nitidus, shiny.
Parent material: finely textured weathering products of intermediate to basic parent rock, possibly rejuvenated by recent admixtures of volcanic ash. The clay assemblage of Nitisols is dominated by kaolinite/(meta)halloysite. Nitisols are rich in iron and have little water-dispersible (`natural') clay.
Environment: Nitisols are predominantly found in level to hilly land under tropical rain forest or savannah vegetation.
Profile development: AB(t)C-profiles. Red or reddish brown clayey soils with a `nitic' subsurface horizon of high aggregate stability.
Use: Nitisols are planted to farm and plantation crops. They are generally considered to be `fertile' soils in spite of their low level of `available' phosphorus and their normally low base status. Nitisols are deep, stable soils with favourable physical properties.
There are approximately 200 million hectares of Nitisols world-wide. More than half of all Nitisols are found in tropical Africa, notably in the highlands (>1000 m.) of Ethiopia, Kenya, Congo and Cameroon. Elsewhere, Nitisols are well represented at lower altitudes, e.g. in tropical Asia, South America, Central America and Australia. See Figure 1.
Figure 1
Nitisols world-wide
Relationships between Nitisols and other Reference Soil Groups are quite diverse because they are conditioned by a score of (localized) factors. Figure 2 presents common lateral linkages.
Figure 2
Some common lateral linkages between Nitisols and other Reference Soil Groups
Nitisol formation involves the following processes:
Nitisols are normally deeper than 150 cm and dusky red or dark red in colour. They are well-drained soils with a clayey subsurface horizon that is deeply stretched and has nutty or polyhedric blocky structure elements with shiny ped faces. Reticular manganese segregation on ped faces is common in the lower parts of the `nitic' subsurface horizon. The relative decrease of the clay content of the nitic horizon is gradual (less than 20 percent from its maximum at 150 cm below the surface). Horizon boundaries are typically gradual or diffuse. Laterally, the nitic horizon may wedge out or decrease in thickness, or dip below a ferralic or argic horizon. It may replace either one of these or change into a cambic horizon. It also may acquire properties found in vertic or ferric horizons. Such lateral transitions are gradual and hardly perceptible within distances of 5 to 10 metres.
The clay assemblage of Nitisols is dominated by kaolinite and (meta)halloysite. Minor quantities of illite, chloritized vermiculite and randomly interstratified clay minerals may be present, alongside hematite, goethite and gibbsite. Nitisols contain 4.0 percent or more `free' iron (Fe2O3 by dithionite-citrate extraction) in the fine earth fraction and more than 0.2 percent `active' iron (by acid oxalate extraction at pH 3). The ratio of `active' to `free' iron is 0.05 or more. The mineralogical composition of the sand fraction depends strongly on the nature of the parent material. Although weathering-resistant minerals (notably quartz) predominate, minor quantities of more easily weathering minerals, e.g. feldspars, volcanic glass, apatite, or amphiboles, may (still) be present indicating that Nitisols are less strongly weathered than associated Ferralsols.
Nitisols are free-draining soils and permeable to water (50-60 percent pores). Their retention of `plant-available' moisture is only fair (5-15 percent by volume) but their total moisture storage is nonetheless satisfactory because the rootable soil layer extends to great depth, commonly deeper than 2 m.. Most Nitisols can be tilled within 24 hours after wetting without serious deterioration of the soil structure.
Nitisols are hard when dry, very friable to firm when moist and sticky and plastic when wet. Gravel or stones are rare but fine iron-manganese concretions (`shot') may be present.
The cation exchange capacity of Nitisols is high if compared to that of other tropical soils such as Ferralsols, Lixisols and Acrisols. The reasons are:
Intense faunal activity is accountable for the typical gradual horizon boundaries of Nitisols. Termites are particularly effective in homogenizing soil; volcanic glass deposited on the (present) surface was found back at a depth of 7 meters in Nitisols in Kenya.
Nitisols are among the most productive soils of the humid tropics. The deep and porous solum and the stable soil structure of Nitisols permit deep rooting and make these soils quite resistant to erosion. The good workability of Nitisols, their good internal drainage and fair water holding properties are complemented by chemical (fertility) properties that compare favourably to those of most other tropical soils. Nitisols have relatively high contents of weathering minerals and surface soils may contain several percent of organic matter, in particular under forest or tree crops. Nitisols are planted to plantation crops such as cocoa, coffee, rubber and pineapple, and are also widely used for food crop production on smallholdings. High P-sorption calls for application of P-fertilizer, usually provided as slow release, low-grade `rock phosphate' (several tons/ha with maintenance doses every few years) in combination with smaller applications of better soluble `super phosphate' for short-term response by the crop.
The Reference Soil Group of the Acrisols holds soils that are characterized by accumulation of low activity clays in an argic subsurface horizon and by a low base saturation level. Acrisols correlate with `Red-Yellow Podzolic soils' (e.g. Indonesia), `Podzolicos vermelho-amarello distroficos a argila de atividade baixa' (Brazil), `Sols ferralitiques fortement ou moyennement désaturés' (France), `Red and Yellow Earths' and with several subgroups of Alfisols and Ultisols (Soil Taxonomy, USA).
Definition of Acrisols#
Soils,
Common soil units:
Leptic*, Gleyic*, Vitric*, Andic*, Plinthic*, Umbric*, Arenic*, Stagnic*, Abruptic*, Geric*, Humic*, Albic*, Profondic*, Lamellic*, Ferric*, Hyperochric*, Skeletic*, Vetic*, Alumic*, Hyperdystric*, 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: strongly weathered acid soils with low base saturation; from L. acris, very acid.
Parent material: most extensive on acid rock weathering, notably in strongly weathered clays, which are undergoing further degradation.
Environment: mostly old land surfaces with hilly or undulating topography, in regions with a wet tropical/monsoonal, subtropical or warm temperate climate. Light forest is the natural vegetation type.
Profile development: AEBtC-profiles. Variations in Acrisols will normally correlate with variations in terrain conditions (drainage, seepage). A shallow A-horizon with dark, raw and acid organic matter grades into a yellowish E-horizon. The underlying argic Bt-horizon has stronger reddish or yellowish colour than the E-horizon.
Use: a general paucity of plant nutrients, aluminium toxicity, strong phosphorus sorption, slaking/crusting and high susceptibility to erosion impose severe restrictions on arable land uses. Large areas of Acrisols are used for subsistence farming, partly in a system of shifting cultivation. By and large, Acrisols are not very productive soils; they perform best under undemanding, acidity-tolerant crops such as pineapple, cashew, oil palm or rubber.
Acrisols are found on acid rocks, mostly of Pleistocene age or older. They are most extensive in Southeast Asia, the southern fringes of the Amazon Basin, the southeastern USA and in both east and west Africa. There are approximately 1000 million hectares of Acrisols world-wide. See Figure 1.
Figure 1
Acrisols world-wide
Acrisols are often the dominant soil group on old erosional or depositional surfaces and in piedmont areas in humic tropical regions where they are associated and alternating with Nitisols, Ferralsols and Lixisols. Acrisols are also well represented on ancient shield landscapes in the humid tropics, often alongside Ferralsols in less eroded, flatter areas or in areas that receive weathering material from adjacent uplands. A typical setting would have Acrisols on the eroding slopes of low hills and Ferralsols on nearby stable pediments or uplands. In mountain areas, Acrisols can be found on stable ridge tops, with Regosols and Cambisols on steeper and less stable slopes. In valleys, Acrisols are to be expected on the higher terraces with Luvisols or Cambisols on lower terraces. Old alluvial fans in the humid tropics may have Acrisols on higher parts with Plinthosols in adjacent depression areas.
Acrisols are characterized by their argic B-horizon, dominance of stable low activity clays and general paucity of bases. Formation of an argic illuviation horizon involves
These processes are discussed in some detail in the chapter on Luvisols. Note that some authors dismiss all clay illuviation horizons in highly weathered soils in the wet tropics as relics from a distant past.
The process of `ferralitization' by which sesquioxides accumulate in the soil profile as a result of advanced hydrolysis of weatherable primary minerals was discussed in the chapter on Ferralsols. Subsequent redistribution of iron compounds by `cheluviation' and `chilluviation' (see under Podzols) is accountable for colour differentiation directly under the A(h)-horizon where an eluviation horizon with yellowish colours overlies a more reddish coloured Bst-horizon (hence the name `Red-Yellow Podzolics' as used e.g. in southeast Asia).
Most Acrisols have a thin, brown, ochric surface horizon, particularly in regions with pronounced dry seasons; darker colours are found where (periodic) waterlogging retards mineralization of soil organic matter. The underlying albic subsurface horizon has weakly developed structure elements and may even be massive; it is normally whitish to yellow and overlies a stronger coloured yellow to red argic subsurface horizon. The structure of this sesquioxide-rich illuviation horizon is more stable than that of the eluviation horizon. Gleyic soil properties and/or plinthite are common in Acrisols in low terrain positions.
Acrisols have little weatherable minerals left. The contents of Fe-, Al- and Ti-oxides are comparable to those of Ferralsols or somewhat lower; the SiO2/Al2O3 ratio is 2 or less. The clay fraction consists almost entirely of well-crystallized kaolinite and some gibbsite.
Acrisols under a protective forest cover have porous surface soils. If the forest is cleared, the valuable A-horizon degrades and slakes to form a hard surface crust. The crust allows insufficient penetration of water during rain showers with devastating surface erosion (low structure stability!) as an inevitable consequence. Many Acrisols in low landscape positions show signs of periodic water saturation; their surface horizons are almost black whereas matrix colours are close to white in the eluvial albic horizon.
Most Acrisols have weak microstructure and massive macrostructure, especially in the surface and shallow subsurface soil that have become depleted of sesquioxides. Bonding between sesquioxides and negatively charged low activity clays is less strong than in Ferralsols. Consequently, the ratio of water-dispersible `natural clay' over `total clay' (see under Ferralsols) is higher than in Ferralsols.
Acrisols have poor chemical properties. Levels of plant nutrients are low and aluminium toxicity and P-sorption are strong limitations. As biological activity is low in Acrisols, natural regeneration, e.g. of surface soil that was degraded by mechanical operations, is very slow.
Preservation of the surface soil with its all-important organic matter is a precondition for farming on Acrisols. Mechanical clearing of natural forest by extraction of root balls and filling of the holes with surrounding surface soil produces land that is largely sterile because toxic levels of aluminium (the former subsoil) kill any seedlings planted outside the filled-in spots.
Adapted cropping systems with complete fertilization and careful management are required if sedentary farming is to be practiced on Acrisols. The widely used `slash and burn' agriculture (`shifting cultivation') may seem primitive at first sight but is really a well adapted form of land use, developed over centuries of trial and error. If occupation periods are short (one or a few years only) and followed by a sufficiently long regeneration period (up to several decades), this system probably makes the best use of the limited resources of Acrisols.
Low-input farming on Acrisols is not very rewarding. Undemanding, acidity-tolerant cash crops such as pineapple, cashew or rubber can be grown with some success. Increasing areas of Acrisols are planted to oil palm (e.g. in Malaysia and on Sumatra). Large areas of Acrisols are (still) under forest, ranging from high, dense rain forest to open woodland. Most of the tree roots are concentrated in the humous surface horizon with only few tap roots extending down into the subsoil. In South America, Acrisols are also found under savannah. Acrisols are suitable for production of rain-fed and irrigated crops only after liming and full fertilization. Rotation of annual crops with improved pasture maintains the organic matter content.
The Reference Soil Group of the Lixisols consists of strongly weathered soils in which clay has washed out of an eluvial horizon (L. lixivia is washed-out substances) down to an argic subsurface horizon that has low activity clays and a moderate to high base saturation level. Lixisols were formerly included in the `Red-Yellow Podzolic soils' (e.g. Indonesia), `Podzolicos vermelho-amarello eutroficos a argila de atividade baixa' (Brazil), `Sols ferralitiques faiblement désaturés appauvris' and `Sols ferrugineux tropicaux lessivés' (France), `Red and Yellow Earths', `Latosols' or classified as oxic subgroups of Alfisols (Soil Taxonomy, USA).
Definition of Lixisols#
Soils,
having an argic horizon starting within 100cm from the soil surface, or within 200 cm from the soil surface if the argic horizon is overlain by loamy sand or coarser textures throughout.
Common soil units:
Leptic*, Gleyic*, Vitric*, Andic*, Plinthic*, Calcic*, Arenic*, Geric*, Stagnic*, Abruptic*, Humic*, Albic*, Profondic*, Lamellic*, Ferric*, Hyperochric*, Vetic*, 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: strongly weathered soils in which clay is washed down from the surface soil to an accumulation horizon at some depth; from L. lixivia, washed-out substances.
Parent material: unconsolidated, strongly weathered and strongly leached, finely textured materials.
Environment: regions with a tropical, subtropical or warm temperate climate with a pronounced dry season, notably on old erosional or depositional surfaces. Many Lixisols are (surmised to be) polygenetic soils with characteristics formed under a more humid climate in the past.
Profile development: ABtC-profiles. On slopes and on other surfaces subject to erosion, the argic accumulation horizon may be exposed or at shallow depth.
Use: most `unreclaimed' Lixisols are under savannah or open woodland vegetation. Such areas are often used for low volume grazing. Perennial crops or forestry are suitable land uses; arable farming requires recurrent inputs of fertilizers and/or lime. The unstable surface soil structure makes Lixisols prone to slaking and erosion in sloping land.
Lixisols are found in seasonally dry tropical, subtropical and warm temperate regions, on Pleistocene and older surfaces. These soils cover a total area of about 435 million hectares, of which more than half in Sub-Sahelian and East Africa, about one quarter in South and Central America and the remainder on the Indian subcontinent and in southeast Asia and Australia. As Lixisols are a recent introduction in soil classification, their total extent is not accurately known. See Figure 1.
Figure 1
Lixisols world-wide
Lixisols are found together with other soils that have an argic subsurface horizon such as Alisols, Acrisols and Luvisols. Differences between these Reference Soil Groups and Lixisols are entirely based on analytical properties and separation may be problematic in the field. The situation is further complicated by the fact that most Lixisols are probably polygenetic; they are particularly well represented on old erosional or depositional surfaces where arid and humid periods have alternated in Pleistocene times. Lixisols in areas with basic rocks occur together with Nitisols or with Vertisols, Planosols, Plinthosols and Gleysols in depression areas and on plains. Lixisols in ancient shield areas in the wet tropics are found together with Ferralsols, generally with Lixisols on slopes and other surfaces that are subject to erosion and Ferralsols in flatter, less erodable terrain. Lixisols in valleys are mostly restricted to the higher (= older) terraces whereas lower terraces have Luvisols or Cambisols. Lixisols on old alluvial fans in tropical regions can occur alongside Plinthosols in wet depression areas.
It is widely felt that (many) Lixisols started their development under a wetter climate than the present. Strong weathering during the early stages of soil formation could have been followed by chemical enrichment in more recent times, i.e. after the climate had changed towards an annual evaporation surplus. Fossil plinthite and/or coarse reddish mottles or indurate iron nodules in the subsurface soil of many Lixisols also hint at wetter conditions in the past. There are indications that base-rich aeolian deposits enriched (some) Lixisols whereas others could have been improved by biological activity (import of bases from the deeper subsoil) or by lateral seepage of water. The reddish or yellow colours of many Lixisols (notably in argic horizons) are the result of `rubefaction' brought about by dehydration of iron compounds in long dry seasons.
Most Lixisols have a thin, brown, ochric surface horizon over a brown or reddish brown argic Bt-horizon that often lacks clear evidence of clay illuviation other than a sharp increase in clay content over a short vertical distance. The argic horizon has a somewhat stronger structure than normally observed in Acrisols (higher base saturation!). The overlying eluvial E-horizon, when still present, is commonly massive and very hard when dry (referred to as `hard setting'). Stone lines are not uncommon in the subsoil.
Advanced weathering is commensurate with a low silt-to-clay ratio, dominance of 1:1 clays (leaching of silica) and higher Fe-, Al- and Ti-oxide contents than are normal in less weathered soils. The SiO2/Al2O3 ratio of Lixisol material is 2 or less; gibbsite contents are only slightly below those found in most Ferralsols.
Most Lixisols are free-draining and lack evidence of water saturation. However, Lixisols with redoximorphic features in the upper metre of the profile are not rare; they are either Stagnic Lixisols that show evidence of a perched water table (above the argic B-horizon) in periods of wetness or Gleyic Lixisols in depression areas with shallow groundwater.
Lixisols have higher base saturation and accordingly somewhat stronger structure than normally found in Acrisols but slaking and caking of the surface soil are still serious problems. The moisture holding properties of Lixisols are slightly better than of Ferralsols or Acrisols with the same contents of clay and organic matter.
Lixisols are strongly weathered soils with low levels of available nutrients and low nutrient reserves. However the chemical properties of Lixisols are generally better than of Ferralsols and Acrisols because of their higher soil-pH and the absence of serious Al-toxicity. The absolute amount of exchangeable bases is generally not more than 2 cmol(+) kg-1 fine earth on account of the low cation exchange capacity of Lixisols.
Areas with Lixisols that are still under natural savannah or open woodland vegetation are widely used for low volume grazing. Preservation of the surface soil with its all-important organic matter is of utmost importance. Degraded surface soils have low aggregate stability and are prone to slaking and/or erosion if exposed to the direct impact of raindrops. Tillage of wet soil or use of (too) heavy machinery will compact the soil and cause serious structure deterioration. Tillage and erosion control measures such as terracing, contour ploughing, mulching and use of cover crops help to conserve the soil. The low absolute level of plant nutrients and the low cation retention by Lixisols makes recurrent inputs of fertilizers and/or lime a precondition for continuous cultivation. Chemically and/or physically deteriorated Lixisols regenerate very slowly if not actively reclaimed.
By and large, perennial crops are to be preferred over annual crops, particularly on sloping land. Cultivation of tuber crops (cassava, sweet potato) or groundnut increases the danger of soil deterioration and erosion. Rotation of annual crops with improved pasture has been recommended to maintain or improve the soil's organic matter content (Deckers et al, 1998).
1 This chapter was contributed by Messrs B. Delvaux and V. Brahy,, Université Catholique de Louvain (UCL), Unité Sciences du Sol, Place Croix du Sud, 2/10 B-1348 Louvain-la-Neuve, Belgium.
