Laterization of Tropical Soils

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Laterization of Tropical Soils

• Tyson Wine
• Envir 315
• Professor Terry Swanson

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Laterization of Tropical Soils

• Laterization is the weathering process of
  tropical soils in which soil and rocks are
  depleted of silica and bases (acidic) while being
  enhanced with aluminum and iron oxides.
• This process forms what is known as a laterite,
  from the Latin later meaning brick or tile.

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Laterization

• This image is what is known as an ultisol.
• Laterization forms these deep red to bright
  orange-red soils because they are high in iron
  content..
• These soils are often acidic and lack much
  organic matter.

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Laterization

• These soils form in hot, rainy locations,
  especially the tropics, where chemical reactions
  from weathering are rapid.
• Decomposition and leaching occur quickly.
• The heavy rains, porous underlying rock, and hot
  climates make for high weathering and ultimately
  depleted soils which can harden into laterites.

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Laterites

• Also classified as oxisols.
• Oxisols are defined as containing no more than
  10 weatherable minerals (at any depth).
• The weatherable materials have already been
  weathered.

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Oxisol
These oxisols are classified as such by the USDA.
This soil taxonomy refers to soils of tropical
rain forest 15 to 25 degrees from the equator.
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Laterization

• Iron and aluminum rich oxides concentrate at or
  near the topsoil.
• Bases (Ca, Mg, K) are available from the
  vegetation forms a lack of humic acid.
• Iron and aluminum oxides are insoluble without
  humic acid and concentrate.
• When these high concentrations of oxides are
  exposed to air, they harden through desiccation
  to form laterites.

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Laterization in Tropical Rainforests

• Rainforests were originally considered to be high
  in nutrients, but are now realized to survive off
  of the decomposing debris on the forest floor,
  not oxisols.
• Oxisols lack of soluble minerals makes them
  infertile, making it virtually impossible for
  plants or trees to form without the presence of
  plant and tree debris.

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Coping with Laterization
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Coping with Laterization
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Feedbacks

• Tree roots extend laterally up to 25 feet,
  providing structure and preventing erosion.
• The elimination of these roots allows for greater
  runoff and erosion.

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Deforestation

• Since the plants and trees of tropical
  rainforests rely on decomposing organic matter
  for nutrients, deforestation eliminates the
  source of their nutrients.
• Oxisols do not contain high nutrients and do not
  allow for the regrowth of an extensive forest.
• In addition, many hardwood trees depend on
  animals for seed dispersal. Elimination of
  animals thus prevents many rainforest flora from
  germinating.
• Also, the loss of the forest canopy results in
  many seedlings not being able to germinate due to
  temperature or other variations in microclimatic
  conditions.
• Lastly, the loss of mycorrhiza leaves many
  seedlings without the ability to fix nitrogen and
  assist in nutrient transport.

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Deforestation in Madagascar
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Deforestation
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Deforestation
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Deforestation

• In addition, the lack of a canopy allows for the
  soil to be exposed to the sun and wind, allowing
  farther evaporation (desiccation) to harden the
  soils more.
• Another positive feedback that results from this
  occurs due to reduced infill and increased
  runoff, leading to faster removal of water and
  nutrients and increasing the laterization effect.

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Deforestation
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Slash and Burn Agriculture

• The slash and burn method of deforestation
  provides enough nutrients to support agriculture
  for a few years.
• Since the soil contains no nutrients, these soils
  are completely used up within a few years, after
  which they are virtually useless.

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Permanence

• Laterization is irreversible due to feedbacks and
  rapid rate of erosion. The end result is hard
  soils incapable of supporting agriculture, let
  alone the forests that survived for millennia.

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References

• Butler, Rhett A. Soils and Nutrient Cycling.
  ltwww.rainforests.mongabay.comgt. 2008.
• ThinkQuest Team 26634. Impact of Deforestation
  on Laterization. 1999. ltlibrary.thinkquest.orggt.
• Ritter, Michael E. Soil Forming Processes. The
  Physical Environment an Introduction to Physical
  Geography. lthttp//www.uwsp.edu/geo/faculty/ritter
  /geog101/textbook/title_page.htmlgt. 2009.
• Desertification. Wikipedia, the free
  encyclopedia. 2009. ltwww.wikipedia.comgt.
• Butler, Rhett A. Amazon Destruction. Mongabay.
  ltwww.rainforests.mongabay.comgt. 2008.

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