1. Learning Unit 12: Soil Erosion

2. Disclaimer
Please note that the following PowerPoint representation DOES NOT replace the official Study Material. The purpose of this additional resource is to support, assist and enrich your learning experience

3. Learning Outcomes
Explain what soil degradation is and the processes through which it occurs
Describe the causes and results of soil degradation
Explain how soil degradation is interwoven with socioeconomic conditions
Evaluate the complexities involved in lessening the impact thereof

4. Factors affecting soil erosion
Pages 319–321 in Middleton
Soils are an integral part of the support system for ecosystems and human communities. Soils are important to society in several ways.
Wind and water as the two main agents of erosion.
Wind erosion transfers soil particles by means of surface creep, saltation and suspension.
In the case of suspension, the source area and the area of deposition may be thousands of kilometres apart.
Erosion by water is caused by both the impact that falling raindrops have on soil particles (rain splash) and water runoff.

5. Factors affecting soil erosion
When a water droplet strikes the soil, soil particles are disturbed and set in motion. If the rainfall rate (intensity) exceeds the infiltration capacity of the soil, the water starts flowing over the land surface as overland flow. Overland flow consists of narrow, shallow layers of water that converge only periodically in places.
Overland flow also initiates erosion known as rain-wash. Lower down along the runoff slope, the overland flow converges. The greater erosive force leads to the formation of runoff grooves or rills with steep sides that are only a few centimetres deep.
Where and when soil erosion takes place, is determined by the interaction between the force of the eroding agent (water or wind) and the erodibility of the soil surface.
The erosivity factor refers to the ability of the agent of erosion to erode the soil surface.
Erodibility, on the other hand, concerns the degree to which the soil surface can resist this erosive onslaught or otherwise.

6. Factors affecting soil erosion
Variables that play a role in soil erosion by water
List of variables
How variable influence erosion
Rainfall
Drop size
Velocity
Distribution
Angle and direction
Rain intensity
Frequency
Duration
Runoff
Supply rate
Flow depth
Magnitude
Sediment content
Soil
Particle size
Clod-forming properties
Cohesiveness
Aggregates
Infiltration capacity
Vegetation
Ground cover
Vegetation type
Degree of protection
Topography
Slope inclination and length
Surface roughness
Flow convergence or divergence
Land-use practices
Contour ploughing
Gully stabilization
Rotations
Cover cropping
Terracing
Mulching
Organic content (p321)

7. Impacts of erosion Pages 324-328 in Middleton
Erosion has an impact on the environment at the place where soil particles are lifted and carried (entrained), along the entire route over which they are transported, and at the place where the transported material is deposited.
Lifting and carrying or entrainment is the first phase in the erosion process. This is where the soil particles are disturbed and set in motion. The entrainment of material has on-site effects affecting the land surface in the source area.
The reduction of an area's capacity to support plant growth is certainly the most important on-site effect of soil erosion.
Seriously eroded areas are known as badlands and are frequently characterised by deep erosion gullies. The removal of the topsoil exposes the deeper-lying hardpans and duricrusts in the soil. Water cannot infiltrate these dense, impervious layers, and plant roots cannot penetrate them, with the result that plants are unable to grow there.

8. Impacts of erosion
Impacts of the transportation and deposition of material are experienced beyond the source area, and are known as off-site effects. In fact, deposition could occur thousands of kilometres from the source area.
Sediment deposition may occur on flat, low-lying areas such as floodplains and valley bottoms
Wind-deposited dust or loess may provide fertile agricultural land – the loess plateau in China
Saharan dust provides nutrients to tropical rain forests in Ghana and the Amazon

9. Negative effects on agriculture
Impacts of erosion
Negative effects on agriculture
Deformation of terrain due to the uneven displacement of soil can result in rills, gullies, mass movements, hollows, hummocks or dunes
Physical changes can present problems for the use of machinery and the absolute loss of cultivable land
Deposition of soil within a field may also result in burial of plants and seedlings, loss of soil may expose roots and sand-blasting by wind-eroded material can damage plants and break down soil cods, impoverishing soil structure and rendering soil more erodible
Splash erosion can cause compaction and crusting of the soil surface which may hinder germination and the establishment of seedlings while exposure of hardpans and duricrusts presents a barrier to root penetration
When the top layer of the soil (A horizon) is depleted, organic material, soil nutrients and seeds are lost and this may reduce the capacity of the soil for holding water and nutrients

10. Accelerated erosion Pages 328-334 in Middleton
Five reasons why the natural erosion rates for two different areas might differ:
Climate
Vegetation
Soils
Bedrock
Landforms
Two most common human activities that give rise to accelerated erosion:
Modifications to or the removal of vegetation
Destabilization of natural surfaces

11. Accelerated erosion
Deforestation removes the protection from raindrop impact offered to soil by the tree canopy and reduces the high permeability humus cover of forest floors.
Example: Deforestation and subsistence agriculture lead to extensive erosion on hillslopes in Madagascar
Cultivation removes the natural vegetation cover from the soil, which is particularly susceptible to erosion when bare after harvests. Mechanical disturbance and compaction of the soil by ploughing and tilling can enhance its erodibility
Example: The introduction of goats and cattle to the Bolivian Andes lead to the formation of severely eroded terrain by steep-sided gullies (‘badlands’)

12. History of accelerated erosion in parts of the Caribbean
Introduction of plantation monoculture in the early 18th century
Large-scale deforestation and fires enhanced soil erosion
Population increase put pressure on resources
Agricultural lands became fragmented
In Haiti, large areas of marginal land were irreversibly degraded
Driving forces of the large-scale soil degradation in Haiti:
Population growth
Unequal distribution of resources
Land tenure
Terms of trade and subsidies
Colonial attitudes and legacies
Class struggles

13. History of accelerated erosion in parts of the Caribbean
The link between deforestation, erosion and food production was shown in Haiti when a hookworm infection increased in the town of Leogane
The increase of the infection was linked to deforestation, erosion and silting of a local river, flooding and the saturation of soils
The enhanced soil moisture, conducive to transmission of the infection, allowed hookworm to re-emerge as a health issue

14. Soil protection techniques Soil management techniques
Soil conservation
Pages 334–337 in Middleton
Soil protection techniques
Agronomic measures
Soil management techniques
Mechanical methods
Manipulate vegetation to minimize erosion by protection of the soil surface
Crop rotation (shifting cultivation)
Mulching – leaving some crop material such as leaves, stalks and roots on or near the surface
Focus on ways of preparing the soil to promote good vegetative growth and improve soil structure in order to increase resistance to erosion
Different methods of soil tillage – providing a suitable seed bed for plant growth and helps to control weeds
Strip tillage, minimum tillage and zero-tillage practices
Manipulate the surface topography to control the flow of water or wind
Techniques such as building of terraces and creation of windbreaks
Check dams are constructed to control gully erosion – the dam impedes the flow of water which causes the deposition of sediment. A small check dam might then support a single fruit tree (p )