Tropical soils

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Tropical soils
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• General comments
• tend to have more oxides of iron and aluminium
  due
• to weathering bringing about desilication of
  clay fraction
• to form laterite (see box)
• because of intensive weathering (and so
  leaching),
• less fertile
• P deficiency is a very common problem - P
  fixation
• because of large amounts of Fe and Al
• Al and Mn toxicity can occur in low pH soils

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Fertility 1 low
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• Oxisols
• Properties
• depth gt 30 cm thick
• NH4 lt 10 meq / 100 g
• CEC1 lt 16 meq / 100 g (but only partly
  saturated)
• old few tracesa of weatherable primary material
• dominently sandy loam gt 15 clay
• water dispersible clay
• gradual boundaries
• rocks lt 5
• clay kaolinitic Fe Al oxides
• exchangeable Al relatively high
• stone lines common

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Problems Physical Very hard when dry at the
beginning of the dry season or after a dry spell
cutivation difficult at some times Chemical N
P particular problems
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Typic Torrox in Hawaii
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• Oxisols in Africa
• dominant soils of equatorial zone where R is high
• low CEC
• low base saturation (lt40)
• relatively infertile
• poorest of the African soils but suitable for
  some perenniel crops
• yields of annual crops low with traditional
  agriculture
• rooting restricted if hardened plinthite

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• Ultisols
• Properties
• thin A1 gt leached (lighter) A2 gt finer
  (illuviation) red to yellow B gt lighter C
• where small fluctuations in T (25 to 28C) R gt
  2000 mm but seasonal deficit
• younger than oxisols
• higher CEC than oxisols - gt 12 meq/100 g in B
• may also have plinthite
• base saturaiton lt 35

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Florida
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Australia
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• Alfisols
• Properties
• sub-humid and semi-arid areas
• argillic B horizons
• B has high base saturation gt 40
• moisture less than wilting point for more than 3
  months
• course structured
• small (not negligible) amounts of weatherable
  material

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• Alfisols in Africa
• dominent soils (60) of W Africa savannahs, some
  on E of L Victoria and in S. Africa
• derived from crystalline acid rocks and basement
  complex
• gravelly horizon
• course textured A
• low in silt
• low CEC
• relatively high base saturation
• dominent soils in dry sub-humid and subhumid areas

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• Vertisols
• black or dark, clay soils mainly in semi-arid or
  sub-humid areas
• high shrink-swell properties
• develop cracks to gt50 cm
• high BD between cracks
• high in montmorillonitic clays
• crumb structure at surface prismatic below
• alternate shrinking drying so surface soils
  falls down the cracks so-called self-mulching
• form gilgais
• very difficult to work when wet (avoid driving)
  hard when dry narrow window
• otherwise very fertile high CEC

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• Vertisols in Africa
• widespread classically in S. Sudan R lt 1000 mm

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• Mollisols
• also cracking clay - higher in OM and have more
  favourable moisture regimes
• Aridisols
• moisture unavaliable for gt 9 months
• surface light
• soft when dry
• Aridisols in Africa
• common in Sahel region S Africa
• usually too dry for crops unless irrigated

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• Andosols
• soils derived from volcanic ash
• very porous, friable, non-plastic and non-sticky
• well structured in A horizon

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Inceptisols and Entisols Entisols (recent) are
soils of recent origin. The central concept is
soils developed in unconsolidated parent material
with usually no genetic horizons except an A
horizon. All soils that do not fit into one of
the other eleven orders are Entisols so order is
characterized by great diversity.
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• Inceptisols (inception) represent the earliest
  horizon development.
• Small amounts of organic matter that darken the
  topmost horizon (sometimes forming an ochric
  epipedon Ochrept) may be all that characterizes
  the inceptisol.
• Weathering is minimal, and amounts of weatherable
  minerals in the profile are high.
• Inceptisols and Entisols are most fertile in
• Africa
• African wetlands are underutilised
• Water management is a problem.

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Lithosols (FAO), Regosols (FAO), Yermosols
(FAO) Lithosols Lithosols are thin soils,
limited in thickness by the presence of a
continuous hard rock substrate within a depth of
10 centimeters. Azonal soils characterized by
an incomplete solum or no clearly expressed soil
morphology and consisting of freshly and
imperfectly weathered rock or rock fragments.
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Regosols Regosols, one of the azonal soils, are
characterized by well to imperfectly drained
mineral soils which lack horizon development or
have minimal A and B horizon development (Soil
Science Glossary, 1976), which indicate
relatively unstable conditions that destroy soil
profile zonation (McGregor, 1984). Regosols are
common over a wide range of ecological
conditions there is low to moderate development
of regosols in shrubs, and low development under
trees, dryland grasses and wetland grasses (Soil
Science Glossary, 1976). The parent materials
are commonly moraine and colluvium, underlain by
bedrock at different depths. Can occur on
relatively young surficial deposits at higher
elevations, such as colluvial slopes and active
fluvial landforms.
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Yermosols Have a very weak ochric A horizon and
an aridic moisture regime (see box for
description of different types of soil horizon
(epipedon))
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See the handout for more details. Please look
for these soil types on the web