Soil Erosion & Desertification (Cambridge (CIE) IGCSE Geography)

Soil Erosion & Desertification

Soil erosion

• Soil erosion is the wearing away of the topsoil by the action of wind or water

• It is a natural process which is made worse by human activities which leave the soil exposed to wind and water

Causes of soil erosion

• Deforestation

  • Removes the trees reducing interception so the soil is more exposed to the rainfall

  • Reduces the amount of roots which reduces infiltration and increases surface runoff

  • The bare soil is more exposed to the wind which would have been reduced by tree cover

• Overgrazing

  • Removes vegetation cover which acts to protect the soil from both wind and water

  • More animals trampling the land leads to compaction and increased surface runoff

  • This happens more in LEDCs where farmers have less and less land to graze

• Over-cultivation

  • Land is not left to rest (fallow) between crops and so the fertility of the soil is reduced

• Population increase

  • More people increase the settlement sizes reducing the areas which can be farmed - increasing over-cultivation and overgrazing

  • Increases the demand for food which leads to over-cultivation and overgrazing

• Over-abstraction of groundwater

  • This leads to the soil moisture content decreasing

  • Drier soil is more easily eroded by wind or water

• Human-induced climate change

  • Changes in climate patterns, increasing floods and increasing droughts all lead to an increase in soil erosion

Salinisation

• Salinisation is also an increasing issue in many semi-arid areas

  • Occurs when high rates of evaporation lead to groundwater being drawn to the surface

  • When the groundwater evaporates salts are left in or on the topsoil

  • These salts make the land toxic to many crops and so the land can no longer be used

• Soil erosion may over time lead to desertification

Desertification

• Desertification is caused by both natural factors and human activities

• Many of the natural causes may be made worse by climate change 

Natural causes

• Soil erosion leads to the loss of nutrients

  • As a result, plants cannot establish and grow

• Less predictable rainfall leads to drought and vegetation dying

• Intense rainfall leads to increased surface runoff

• Reduced vegetation means that fewer nutrients are added to the soil

Human causes

• Overgrazing reduces the amount of vegetation so the soil is bare and no nutrients are added to the soil

• Over-cultivation means that the crops take up all the remaining nutrients

• Deforestation decreases:

  • the nutrients added to the soil

  • infiltration

  • interception

  • the number of roots which bind the soil

• Population growth increases the pressure on the land increasing the number of animals and crops

Sustainable Management of Soil Erosion and Desertification

• Halting and reversing soil erosion and desertification means tackling the causes

• There are a variety of political and social responses 

Education

• Education, including:

  • Sustainable farming methods, including agroforestry and crop rotation, help to keep the soil healthy

  • Family planning to reduce population growth

Agroforestry 

• This combines agriculture with forestry, which means some trees remain, which

  • Decreases deforestation

  • It provides shade as well as increasing infiltration and interception, which reduces soil erosion

  • Provides organic matter from the trees and adds nutrients to the soil

Afforestation

• Tree planting, such as the Great Green Wall across the Sahel, helps to reverse desertification in several ways:

  • The roots help to bind the soil together, reducing soil erosion

  • The canopy offers shade, helping to prevent the soil from drying out and also reducing soil erosion from rainfall landing directly on the soil

  • Nutrients in the soil are replaced by falling leaves and branches

  • The trees increase animal and insect activity, which helps improve soil quality

Contour Stones and Terraces

• These help to reduce soil erosion by:

  • Preventing the soil from being blown or washed away

  • Increasing infiltration of water and reducing overland flow

  • Ensuring that dead organic matter stays in one place and can decompose, adding nutrients to the soil