Humid Tropical Soils and Land Use Problems

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…