Semi-arid Tropical Soils and Land Use Problems

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  • Created by: nicola
  • Created on: 11-04-11 21:29


Used for rain fed cropping or irrigation fed crops - can't always rely on rains to provide for crops = very risky

Soil structure - horizons from humic to less humic - sandy soils for top layer = quite humic - therefore less chemical weathering and higher proportion of physical weathering

Movement clay down profile results in hard pan further down - see Med notes


Very sandy - less clay - poorly structured

Found in the drier parts of the humid tropics - chemical weathering more prominant

Low proportion of organic material and poor quality soils = easily erodable

Such facots make it risky to use in farming practices


Desert marginal soils - low rainfall where they exist so extremely poor for agriculture

Need irrigation in order to sustain crops

These are the poorest quality soils which can be used for growing crops - anything lower than these will result in instant crop failure


All soils susceptible to erosion - can occur in long periods of dry or wet seasons - short sharp showers are usually the main problem - increased surface run off, rill and gully formation

Removal of vegetation leads to PONDING - surface compaction from rain drop action leads to soil sealing causing more overland flow (may transform into small streams) and rill/gully formations

GLASOD - identified main causes leading to soil degradation in drylands (in Lal, 1990):

  • Over grazing (1)
  • Deforestation (2)
  • Agricultural (over cultivation) (3)
  • Over exploitation (4)
  • Bad irrigation practices - should also be considered a cause but not included in GLASODs study

Nutrient deficiency - sensitive to nutrient demand in crops e.g. ground nuts - risky

Clearance of natural veg = removal of natural nutrient store held in that environ 

More erosion leads to increasing nutrient deficiency

Sanitisation causes high evaporation rates therefore build up of salt occurs in topsoil - leads to crusting of soil

Drylands - Africa

Highly risky area in which to practice agriculture - many of these areas located near tropics of Cancer and Capricorn (Middleton and Thomas, 1997)

Focus of aid in this area - e.g. Sahel disaster in 60's - widespread drought e.g. in Kenya (Darkoh, 1992)- 

  • Set up as new national Gov - Arid and Semi and Arid Lands Programme (ASAL)
  • Allocated districts in Kenya to different receivers of international aid - UK given Machakos (near Niarobi), Dutch - West Pokot, Germans in East and Norwegians in Turkana

Problem of marginality in drylands- lies at margins of sustainability therefore need system of land classification to say what you can/ can't do with each area of land

FAO set up a system of agro-ecological zones based on  agro-climatic zones e.g. rainfall, temp, moisture availability (evapotranspiration changes moisture availability at different temps)

  • High temps and increased seasonality = low rainfall = reduced moisture availability 

Relationship between rainfall and temps determines the growing season - also topography = higher ground has lower temps therefore slightly lower rates of evapotranspiration

Highlights differences in practices carried out in the hills and those in the plains - also slightly more rainfall in hills due to it being more


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