Learning Unit 12: Soil Erosion

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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

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.

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.

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)

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.

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

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

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

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’)

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

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

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 (p334-335)