Humid Tropical Soils and Land Use Problems

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  • Created by: nicola
  • Created on: 04-04-11 17:39

Laterization

Dominant soil forming process in humid tropics,

Demineralisation = occurs through leeching from intense rain showers.

Minerals are taken out of the upper soil layers and deposited lower down the profile.

Causes a build up of iron and hence a build up of laterite -> when water table rises, mobile iron is brought up from below and deposited as the levels drop.

Upper layers often eroded exposing lower layers - causes a vegetation recession as they are less able to cope with the drying conditions (reduces diversification)

Soil Profile:

  • 0-10m = FERRUGINOUS ZONE
    • Red soils
    • Rich in iron and aluminium
  • 11-20m = MOTTLED ZONE
    • Soils still red with soft white compounds
  • Up to 50m = PALLID ZONE
    • Very light soils with white compounds
    • Often soils are pale grey in colour

McFarlane (1983)

Humid tropical soils undergo high levels of chemical weathering

Results in SAPROLITIC soils - erosion of soil and pisolith in the upper layers leaves this exposed

This lowering of the soil profile causes minerals to be knocked out allowing a build up of iron and aluminium - pedogenic process.

Also slowly dries out due to climate change - change from forest to drylands and savannahs.

As profile dries, laterite (Pisoliths) hardens - problem for farmers, poor soils to manage


Groundwater Laterite

  • Capillary fringe sits just above water table - allows water to be drawn up from lower depths due to hydraulic processes
  • Causes a build up of iron solution (see above/previous lecture) and deposition of nodules within the profile
  • As water table moves down, solution drains leaving iron precipitate around the nodule
  • Known as PISOLITHS - soft as long as forest exists i.e. there are humid conditions
  • Gradually build up and consolidate within the soil

 

SOIL TYPES IN THE HUMID TROPICS (USDA Classifications)

Oxisols (Furley 1990)

  • Makes up 45% of humid tropic soils
  • Usually have deep soil profiles
  • Well drained soils - saprolitic zones underneath
  • Red colour
  • Have plinthite zones and concretions
  • Highly weathered
  • Don't hold nutrients well -> low levels of available nutrients for plants
  • Mainly found on flat ground on the top and base of slopes
  • Low plant nutrient levels, cation exchange capacity, calcium content, nitrogen levels etc 

Ultisols (Furley 1990)

  • Make up 25% of humid tropic soils
  • Similar to oxisols
  • Not as good as oxisols for agriculture
  • Found on slopes - therefore more prone to erosion
  • Low plant nutrient levels
  • Nitrogen lost through leeching
  • Weak soil structure

Entisols

  • Alluvial soils - found on flood plains
  • More fertile than oxisols and ultisols - however, depends on sediment input material
  • Also have better water storing capacity
  • Only make up 10-15% of humid tropic soils

Vertisols

  • Make up 40% of soils
  • Found in marginal locations around rainforests
  • Products of humid tropical erosion
  • In perennial (wet) times, soils reduced to 1:1 clays e.g. kaolinite
  • In seasonal years not as much weathering occurs therefore soils stay as 2:1 clays - oresmectites
    • These are potentially very fertile

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