Understanding erosion – Part 2

This week we take a closer look at soil erosion, which reduces productivity and pollutes watercourses, wetlands and lakes.

Understanding erosion – Part 2
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In agriculture, soil erosion means the destruction of the topsoil by water and wind or through farming activities such as tillage. Erosion is influenced in several ways.

The steeper the slope, the greater the potential for erosion. Water speeds up as it flows down steep slopes. The length of the slope is important, because the larger the area, the greater the concentration of the flooding water.

Soil structure
‘Soil structure’ is the grouping of soil particles. Over-cultivation and compaction causes soil to lose its structure and cohesion (ability to stick together), so it erodes more easily.

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Terrain unit
The crest (top of slope) is usually well-drained, as soil moisture moves downhill, leaving air behind in the pore spaces.
Over time, fine (clay) particles are carried downslope, leaving the soil in the higher parts sandy. Plant roots can penetrate easily to deep levels to access soil water. These soils have a lower erosion potential and are normally more stable.
At midslope, soil moisture moving from the crest starts to dam up as a result of the clay-rich soil downhill. These soils are fairly well drained, with a higher erosion potential.

In the footslope, the soil has been waterlogged as a result of the long-term accumulation of clay, which does not allow water to infiltrate. Plants that grow here must adapt their root systems to grow sideways above the hard clay layer. These imperfectly drained soils have a high erosion potential.

Figure 1: The different sections of a slope vary in erosion potential.

Organic material
Organic material is the ‘glue’ that binds soil particles together and plays an important part in preventing soil erosion. Organic matter is the main source of energy for soil organisms, both plant and animal. It also influences the infiltration capacity of the soil, reducing runoff.

Vegetation cover
The loss of protective vegetation through overgrazing, ploughing and burning makes soil vulnerable to erosion. Plants provide protective surface cover and prevent soil erosion in several ways:
– They slow down water as it flows over the land and allow rainfall to soak into the ground.
– Their roots bind the soil in position and prevent it from being washed away.
– They reduce the impact of a raindrop before it hits the soil, reducing soil erosion.
– Plants in wetlands and on riverbanks slow down the flow of water.

Land use
Grass is the best natural protector against soil erosion because of its relatively dense basal cover. Small grains, such as wheat, offer considerable obstruction to surface wash. Row crops, such as maize and potatoes, offer little cover during the early growth stages and erosion occurs here relatively easily. Fallow lands, where no crop is grown and all the residue has been incorporated into the soil, are highly vulnerable to erosion.

Source: Directorate Agricultural Information Services, department of agriculture, in co-operation with Directorate Agricultural Land and Resources Management.