Factors Determining Soil Erosion

There are various factors determining soil erodibility, of which the following are the most important:

• Slopes in relation to soil conservation.
• If the elevation of a slope is too steep (above 30 degrees), then our natural resources might be in danger.
• Any water run-off will cause erosion.
• Vehicles such as tractors will find it difficult to drive up the steep slopes and might cause deep ruts due to the strain to get up the hill.
• As one travels higher up a mountain slope the shallower the top soils will be – because the soils are newly-formed and, due to the slope, the run-off will lead to higher erosion. It is therefore recommended that the slopes are not exposed to erosion and are kept covered to reduce the erosion. It is important to encourage veld vegetation to cover the area permanently; trying to cultivate the slopes will cause erosion.
• If these slopes must be cultivated, and if you irrigate constantly on a steep slope, the nutrients might be washed down to the valley bottom, causing problems with infertile soil on the top of the slope and poor water quality (brackish water) in the lower areas. Use conservation structures to reduce run-off.
• The steeper the slope, the greater the erosion, because of the increased velocity (swiftness) of water flow. The length of the slope is very important, because the greater the size of the sloping area, the greater the concentration of the flooding water.

Soil Texture

Soil texture is the size distribution of soil particles. The size of particles never changes. A sandy soil, therefore, remains sandy, and a clayey soil remains clayey (unless it is chemically altered).

Soil Structure

The term soil structure means the grouping or arrangement of soil particles. Over-cultivation and compaction cause the soil to lose its structure and cohesion (ability to stick together) and it erodes more easily.

Terrain Unit

The crest (top of the slope) is usually well-drained as soil moisture moves downhill, leaving the air in the pore spaces most of the time. Over time, the fine (clay) particles are carried down-slope, leaving the soil sandy. Plant roots can easily penetrate to deep levels and withdraw enough soil water from there. These soils have a lower erosion potential and are normally more stable.

In the mid-slope, soil moisture moving from the crest starts to dam up because the clay-rich soil is just downhill. The soils are moderately well-drained, with a higher erosion potential.

In the foot-slope, the soil has been waterlogged (saturated with water) because of the long-term accumulation of clay which does not allow water to infiltrate. Plants that grow on these soils are limited to those that can adapt their root systems to grow laterally above the hard-clayey layer. These imperfectly-drained soils have a high erosion potential.

Organic Material

Organic material is the "glue" that binds the 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, therefore reducing run-off.

Vegetation Cover

The loss of protective vegetation through overgrazing, ploughing, and fire, makes soil vulnerable to being swept away by wind and water.

Plants provide protective cover on the land and prevent soil erosion for the following reasons:

• Plants slow water down as it flows over the land and this allows much of the rain to soak into the ground.
• Plant roots hold the soil in position and prevent it from being blown or washed away.
• Vegetative cover breaks the impact of a raindrop before it hits the soil, reducing the soil's ability to erode.
• Plants in wetlands and on the banks of rivers are important as they slow down the flow of the water, and their roots bind the soil, preventing erosion.

Land Use

The grass is the best natural soil protector against soil erosion because of its relatively dense 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 thereby encourage erosion. Fallow areas, where no crop is grown, and the entire residue that has been incorporated into the soil, are most subject to erosion because land becomes exposed to the elements of wind and water.